Ventilatory and metabolic responses to cold and hypoxia in intact and carotid body-denervated rats HENRY Laboratoire
GAUTIER
AND
MONIQUE
de Physiologie Respiratoire,
BONORA Faculte’ de Mbdecine Saint-Antoine,
GAUTIER, HENRY, AND MONIQUE BONORA. Ventilatory and metabolic responses to cold and hypoxia in intact and carotid body-denervated rats. J. Appl. Physiol. 73(3): 847-854, 1992.The effects of hypoxia on thermoregulation and ventilatory control were studied in conscious rats before and after carotid denervation (CD). Measurements of metabolic rate (VO,), ventilation (V), shivering intensity (SI), and colonic temperature (T,) were made in groups of eight rats subjected to three protocols. In protocols 1 and 2, at ambient temperature (T,) of 25 and 5”C, respectively, rats were exposed to normoxia and hypoxia [inspired 0, fraction (FI,,) O.l3-ON]. In protocol 3, T, was decreased from 25 to 5°C in 30-min steps of 5OC. Recordings were made in normoxia and hypoxia (FI,, 0.12). The results show that in both intact and CD rats 1) in normoxia, cold exposure increased vo2, V, and SI, and these increases were proportional to the decrease in T,; 2) hypoxia induced only a transient de.crease in SI, and, for a given T,, VO, was. reduced whereas V and SI were increased; and 3) in CD rats, V increased less during cold exposure in both normoxia and hypoxia; VO, and T, were more depressed during hypoxia. It is concluded that 1) the interaction between T, and FI,, in the control of V is partly dependent on the carotid body afferents, 2) shivering thermogenesis may be transiently affected by hypoxia independently of the carotid body afferents, and 3) nonshivering thermogenesis may be directly inhibited by hypoxia, especially during cold exposure. shivering; thermogenesis; body temperature; metabolism; control of ventilation
DURING EXPOSURE of homeotherms to cold in normoxia, thermogenesis is increased to maintain constant body temperature. However, under hypoxic conditions, thermoregulation is altered, and a decrease of body temperature is observed (3,33). Since the original observations of Legallois (23), this hypoxic hypothermia has been described in many animal species, including the rat, and seems to result from a transient inhibition of shivering thermogenesis (ST) and a more sustained inhibition of nonshivering thermogenesis (NST) (15). The role of arterial chemoreflex afferents in the mediation of hypoxic hypothermia is not clear. Pharmacological stimulation of the carotid body induces inhibition of shivering in anesthetized animals (11, 16, 28). However, in conscious newborn rabbits (2) and adult cats (18), the inhibitory effect of hypoxia on thermogenesis persists after carotid denervation. In the present study, we report the effects of hypoxia on the metabolic and ventilatory responses to cold in intact and carotid body-denervated conscious rats. 0161-7567/92
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These investigations were done to clarify the role of the chemoreflex afferents in hypoxic hypothermia and to describe the interactions of increased metabolism resulting from cold exposure and changes in level of oxygenation on the control of breathing. METHODS Animals
Experiments were performed in groups of eight female Wistar rats -2 mo of age, with a body weight of -250 g at the beginning of the study. They were caged in an animal room in groups of four at a temperature of 2325OC for 21 wk before they were used for experiments. They were provided with a commercial rat chow and tap water ad libitum. Measurements
Metabolic rate (VO,), shivering activity, and colonic temperature (Tc) were measured as previously reported (15). Briefly, VO, was computed using a closed-circuit method from the time spent by the rat to consume 10 or 20 ml of 0,, expressed in ml/min STPD. Shivering activity was recorded from two electrodes inserted in the thigh musculature, amplified (Grass P511 amplifier, band width 30-3,000 Hz), and integrated (integrator reset to zero when a fixed output was reached). The number of resets per minute was used to quantify shivering intensity. Colonic temperature (T,) was monitored with a thermistor probe (model 402, Yellow Springs Instruments) inserted 6 cm into the colon. Ventilation (V) was monitored using the plethysmographic technique initially described and validated in the rat (1, 9). The inlet and outlet parts of the metabolic chamber were closed, and the chamber was connected to a Validyne DP 45 pressure transducer. Pressure changes associated with the breathing cycle were recorded on an oscillograph for -10 s. Data analysis [tidal volume (VT),breathing frequency (f), and V] was subsequently performed using a graphics tablet connected to a computer. During all studies, Vo2, calorimeter (ambient) temperature (T,), T,, and V were recorded at 5min intervals. Shivering activity was continuously monitored and averaged over 5-min periods. In each of the following protocols, rats were studied before (intact) and after carotid body denervation (CD). Under halothane anesthesia, carotid sinus nerves were exposed under an operating microscope, as described by
0 1992 the American
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VENTILATION
DURING
Sapru and Krieger (32), and subsequently sectioned at their junctions with the glossopharyngeal nerves. Experiments were carried out within 3-8 days after the surgery. Body weight of all CD animals (248 t 7 g) was the same as that in intact animals (250 t 7 g). Protocol 1: Ventilatory and Metabolic to Hypoxia at Thermoneutrality
Responses
In T, 25”C, the animals were studied for 20 min in normoxia [inspired 0, fraction (FQ 0.211, 20 min in hypoxia (FI,, 0.13), another 20 min in hypoxia with 0.11 FI,, in intact animals and 0.12 FIEF in CD animals, and a final 20 min in normoxia. Protocol 2: Ventilatory and Metabolic Responses to Hypoxia in a Cold T,
T, was 5OC. Measurements were made, in succession, for 30 min in normoxia, 45 min in hypoxia (0.12 FI&, and again for 30 min in normoxia. Protocol 3: Ventilatory and Metabolic Responses to Sequential Decreases in Ta in Normoxia and Hypoxia
Rats were exposed in steps of 30-min duration to T, of 25,20,15,10, and 5OC. Measurements were made one day with 0.21 FI,, and two days later with 0.12 FIEF. In all experiments, 0, concentration in the calorimeter was monitored with a Beckman OM14 0, analyzer every 10 (protocol I) or 5 min (protocols 2 and 3) and adjusted to the desired level. Statistics
Results are means t SE. Analysis of variance and, when appropriate, the Dunnett test for multiple comparisons were used to evaluate the significance of changes induced by hypoxia, T,, or CD. Other tests used are indicated in the text. RESULTS
Protocol 1: Ventilatory and Metabolic to Hypoxia at Thermoneutrality
Responses
Intact animals. As shown in Fig. 1, hypoxia induced a decrease in T, that, after 20 min at 0.11 FI,, , reached a level 0.93 t 0.17”C below the initial normoxic level. This was associated with a slight decrease of ~8% in VO,. Furthermore, hypoxia produced a marked increase in f that was almost entirely responsible for the augmentation in V. During the recovery period in normoxia, a small recovery in T, (0.47 t 0.22”C) was observed. This was associated with an increase of 16% in VO, over the initial control value in normoxia, whereas f and V returned progressively to control values. CD animals. Hypoxia induced a decrease in T, of 1.97 t 0.20°C after 20 min at 0.12 FIEF, significantly larger than before CD (P < 0.01). This was associated with a decrease in VO, that reached 29% with 0.12 FI,,. Furthermore, hypoxia produced a significant increase in f and, despite the small decrease observed in VT, V was slightly increased. During the recovery period in normoxia, T, increased significantly (0.67 t O.l7”C, P < 0.01). This
HYPOXIA
AND
COLD
was associated with a marked increase in did not change significantly. Protocol 2: Ventilatory and Metabolic to Hypoxia in a Cold T,
iTo,,
whereas V
Responses
Intact animals. As shown in Fig. 2, exposure to hypoxia induced a progressive increase in f that accounted for the increase in V inasmuch as VT did not change significantly. Meanwhile, T, showed a progressive decrease that amounted to 3.27 t 0.38OC after 45 min of hypoxia. After 5 min of hypoxia, shivering intensity was decreased by 57%, but a complete recovery was observed after 3545 min of hypoxia. In contrast, VO,, which was decreased by 26% after 5 min of hypoxia, remained decreased by 18% after 45 min of hypoxia. During recovery in normoxia, V returned progressively to the control normoxic value. T, increased progressively by 1.77 t 0.14OC over the last measurement in hypoxia. This was associated with a transient overshoot in shivering intensity and VO,. CD animals. As shown in Fig. 2, exposure to hypoxia induced no significant change in V. In contrast, T, showed a progressive decrease that amounted to 3.99 t 0.18OC during the hypoxic exposure. Shivering intensity, which was decreased by 74% after 5 min of hypoxia, markedly exceeded (+32%) the initial normoxic control value after 35-45 min of hypoxia. Finally, Vo2, which was decreased by 40% after 5 min of hypoxia, was still diminished by 21% at the end of the hypoxic exposure. During recovery in normoxia, a marked overshoot was observed in V, shivering intensity, and VO,. T, increased progressively by 2.66 t 0.27”C, an increase greater than that observed before CD (1.77 t O.l4”C, P < 0.02). Protocol 3: Ventilatory and Metabolic Responses to Sequential Decreases in T, in Normoxia and Hypoxia Intact animals. In normoxia, exposure to progressively lower T, induced an increase in both f a-nd VT that accounted-for the progressive increase in V from 119 t 5 ml/min at 25OC to 296 t 12 ml/min at 5OC (Fig. 3). T, did not change significantly, and shivering that was sometimes apparent at T, - of 20°C increased progressively at lower T,‘s. Finally, VO, increased linearly with the decrease in T,. Exposure to hypoxia at 25°C caused an increase in f that accounted for the increase in V. Exposure to lower T, induced a progressive increase in V from 208 t 6 ml/ min at 25OC to 448 ml/min at 5OC. T, decreased markedly from 37.56 t 0.14”C at 25°C to 34.33 t 0.63OC at 5OC. Shivering intensity increased as T, decreased and was on average greater than in normoxia for a given T,. Finally, VO, increased linearly with the decrease in T, but at all Ta’s was lower than in normoxia. CD animals. In normoxia, exposure to progressively lower T, induced a progressive increase in V; however, for a given T,, this increase remained lower in CD than in intact animals because of a lower f (Fig. 3). T, did not change significantly. Shivering intensity and VO, increased when T, decreased and were generally greater in CD than in intact animals. In hypoxia, V was much lower than in intact animals.
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FIG. 1. Ventilation (V), tidal volume (VT), breathing frequency (f), metabolic rate (VO,), and colonic temperature (T,) in 8 rats before (intact, closed symbols) and after carotid denervation (CD, open symbols). In T, of 25”C, animals were exposed to normoxia (NX, circles), then to hypoxia (HX, triangles), and again to normoxia. FI,~, inspired O2 fraction. Values significantly different from average initial values in normoxia: *P < 0.05 (protocol I).
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T,, which was 25”C, continued 5OC. As in intact sity was greater normoxia.
already decreased to 36.64 t 0.23OC at to decrease, reaching 33.74 t 0.25OC at animals, for a given T,, shivering intenand Vo2 was lower in hypoxia than in
Relationships between ventilation, shivering intensity, and VOW.As shown Fig. 4, in all conditions V was highly correlated with VO,. In intact animals (left), v = 17.6 VO, + 8 (r = 0.998, P < 0.001) in normoxia and V = 29.8 vo2 + 33 (r = 0.997, P < 0.001) in hypoxia, with a slope significantly steeper (P < O.Ol) in hyp.oxia than in normoxia. In CD animals (right), V = 14.5 VO, + 7 (r = 0.984, P < 0.01) in normoxia, with a slope significantly lower (P < 0.01) than in intact animals. Finally, in CD animals in hypoxia, V = 23.6 VO, + 4 (r = 0.975, P < O.Ol), with a slope again significantly lower (P < 0.01) than in intact
animals. As shown in Fig. 5, irO, was closely correlated with shivering intensity. In intact animals, r = 0.995 (P < 0.001) in normoxia and 0.998 (P < 0.001) in hypoxia, but the relationship was significantly shifted (P < 0.01) to lower values of voz relative to normoxia. This indicates that, for a given shivering intensity, Vo2 was lower in hypoxia than in normoxia, presumably due to inhibition of the NST component. In CD animals, Vo2 was again
highly correlated with shivering intensity: r = 0.996 (P < 0.001)in normoxia and 0.993 (P < 0.001) in hypoxia. The relationship in CD rats in hypoxia was significantly lower than in CD rats in normoxia (P < 0.01) and in intact rats in hypoxia (P < 0.05). DISCUSSION
Role of the Carotid Body Afferents Response to Hypoxia
in the A4etabolic
Thermoneutrality. At T, of 25°C decreases in VO, and T, during the exposure to hypoxia were greater in CD than in intact animals. The depressant effects of hypoxia on metabolism in small animals are well known. The magnitude of the decrease in metabolism depends on T,, because VO, would be unaltered by hypoxia above the thermoneutral point, according to Hill (21). However, values of thermoneutral zone reported in the literature for the rat are quite variable (from 22 to 34”C), according to the review of Gordon (20). In addition, it appears from our previous study in rats (15) that the thermoneutral zone may be lower in hypoxia than in normoxia. This led us to select 25OC as the higher T, for the present study. The greater decrease in VO, and T, observed in CD animals exposed to hypoxia is in agreement with our pre-
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I I I I 1 I 25 10 20 0 5 15 FIG. 2. VO,, shivering intensity (in percent of average values observed in initial before (closed symbols) and after CD (open symbols). In T, of 5”C, animals were then to hypoxia (HX, 0.12 FIEF, triangles, broken lines), and again to normoxia. average last 10 min of initial normoxia: *P < 0.05 (protocol 2).
vious study showing that conscious CD cats show a significant decrease in T, even at thermoneutral T, (18). Because CD rats hyperventilate less than intact animals for a given FIEF, their arterial PO, (Pa,,) would be expected to be lower than that of intact animals. However, because we used a higher FI,, in the second hypoxic exposure in CD rats and because their VO, was markedly reduced, we may assume that their Pa,, was not too different from that of intact rats. Arterial PCO, (Pat,,) was also probably higher in CD than in intact rats (12, 30), and this factor may also affect thermogenesis (17,31). These caveats notwithstanding, the present results suggest that, in agreement with our previous conclusion in cats, carotid sinus nerve afferents act to maintain constant body temperature during hypoxia by stimulation of NST (18). Cold T,. In intact animals, the results of protocols 2 and 3 concerning the effects of normoxia and hypoxia on
I I 1 I I 35 0 10 20 30 min normoxia), T,, V, VT, and fin 8 rats exposed to normoxia (NX, circles), Values significantly different from
body temperature regulation are in agreement with those in our previous study using similar protocols (15). During normoxia in protocol 3, VO, and shivering intensity were higher in CD rats. The same was observed for VO, at the beginning of protocol 2. At T, of 5”C, this resulted in a higher T, in CD rats. This increase in ST observed in CD rats may be ascribed to the removal of the tonic inhibition of shivering by the carotid chemoreceptors that was initially demonstrated by pharmacological stimulation (11, 16, 28). Similarly, the greater recovery in shivering intensity, TO,, and T, observed after hypoxia in protocol 2 in CD rats may involve the same mechanism. During hypoxia, CD rats still display a severe inhibition of 60, and hypothermia, which appear greater than in intact rats. In agreement with previous studies (2,18), this suggests that hypoxic inhibition of thermogenesis is
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VENTILATION
Nx INT CD Hx INT CD
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DURING
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10
15
20
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FIG. 3. V, VT, f (left), VOW,shivering intensity, and T, (right) in 8 rats before (INT, closed symbols) and after CD (open symbols) exposed to sequential decreases in T,. Animals were exposed to normoxia (Nx, circles, solid lines) or hypoxia (HX, 0.12 FI,, , triangles, broken lines). Values significantly different from those observed at T, of 25°C: *P c 8.05. In all conditions (Nx, Hx, INT, CD), VT was significantly different at T, of 15,10, and 5OC from T, of 25°C; VO, was significantly different at T, of 20, 15, 10, and 5OC from T, of 25°C; and shivering intensity was significantly different at T, of 10 and 5°C from T, of 20°C (protocol 3).
mediated independently of the carotid chemoreceptors’ activity, via central mechanisms or other peripheral chemoreceptors that remain to be elucidated. The greater inhibition of VO, observed in CD rats may be caused by their relatively lower Paoz. In addition, the “protective” effect of carotid body afferents on thermoregulation, which has been suggested above as operating during hypoxia at thermoneutrality, has been removed in CD rats. Finally, it appears that hypoxia shifts the balance of thermogenic output away from NST and toward ST even more in CD than in intact rats. This is shown after 30-45 min of hypoxia in protocol 2 when VO, is the same in CD and intact rats, whereas shivering is greater in CD rats. Similarly, as indicated by the shivering-Vo, relationships shown in Fig. 5, shivering in CD rats during hypoxia is generally greater than in intact rats, even though their VO, is lower. The enhanced intensity of shivering during prolonged hypoxia observed in CD compared with intact rats may be accounted for by several independent factors: first, removal of the inhibition of shivering from carotid chemoreceptors discussed above; second, a compensatory increase in shivering when NST has been inhibited (2, 6, 13); and third, the lower T, observed during hypoxia in CD rats representing a potent stimulus for thermogenesis.
This study confirms that hypoxia may markedly affect the body temperature regulation (3). This effect may be mediated reflexly, through activation of carotid chemoreceptors, and directly, through an action on the central nervous system, possibly the thermoregulatory centers. The comparison of the results obtained at thermoneutrality and with a cold T,, on the one hand, and before and after CD, on the other, suggests that 1) NST may be strongly inhibited directly by hypoxia, especially in hypothermia, and this in turn may lead to a compensatory increase in ST; 2) NST may be stimulated by carotid body afferents and, in hypoxia at thermoneutrality, this will counterbalance the direct inhibitory effect of hypoxia on thermogenesis, such that body temperature will remain unaffected in the intact animal; and 3) ST may be transiently inhibited by hypoxia, directly or through the mediation of the carotid body activity. Interactions Between Changes in Fro2 and T,., in the Control of Breathing Intact animals. Those studies that have reported the ventilatory response to changes in T, in normoxia have mainly been carried out in humans (4,7,19,29) and anesthetized animals (16, 22, 24). However, in intact conscious rats exposed to T, of 24-25°C Maskrey (26)
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852
VENTILATION I
DURING
I
I
10
1s
HYPOXIA
AND COLD I
I
I
5
10
15
INTACT
0
5
0
20
\;(Oz , ml. mid FIG. 4. V vs. corresponding values of VO, in 8 rats intact (left) or after CD (right) exposed to sequential decreases in T, from 25 to 5OC in normoxia (Nx, circles) or hypoxia (Hx, 0.12 FI, , triangles). Regression lines are shown for normoxia (solid lines) and hypoxia (dashed lines) in protocol 3. For T, of%“C, transient changes in V and VO, are also shown with results obtained in protocol 2.
recently showed that when T, was experimentally decreased by -3OC by means of an abdominal heat exchanger, V was increased ~2.4 times. In the present work, V was increased by the same order of magnitude (2.5 times) during the exposure to Ta)s of 5 and 25°C. Because VO, and shivering activity were not measured in the Maskrey study, we can conclude only that, in normoxia, exposure to a T, of 5°C (peripheral cold stimulation) induces changes in V similar to an experimental decrease of T, of 3°C (central cold stimulation). The present study indicates, in addition, that during cold exposure, V increases at the same rate as metabolism, in agreement with previous studies in rats (9) and other animal species (16,19,22,24), suggesting that regulation of breathing is precisely adjusted to the metabolic needs as in muscular exercise of moderate intensity (29). During hypoxia at thermoneutrality, V was increased as expected, and this was caused essentially by an increase in f (10, 12,25). In cold T,, hypoxia has two opposing effects on the control of breathing: enhanced chemoreflex hypoxic drive and metabolic drive (VOJ, which has been reduced by hypoxia. It was observed that V increased proportionately with VO, and that V/Voz was higher in hypoxia than in normoxia (Fig. 4). For any given T,, V,Z was lower and V was higher in hypoxia. This indicates that the hypoxic chemoreflex drive remains operative during cold exposure and appears even higher when T, decreases, suggesting a multiplicative interaction between VO, and hypoxia in the control of breathing. CD animals. In normoxia, at thermoneutrality, V was decreased compared with intact animals, in agreement with previous reports (12, 25,30). During cold exposure, V increased proportionately with VO,, as in intact ani-
mals, but V/Vo, was lower after CD. We are not aware of any study reporting the ventilatory response to cold after CD in conscious animals. However, during muscular exercise, V and WVO, in dogs were decreased by ~20% after CD (5). It may therefore be concluded that the integrity of the chemoreflex drive of breathing is essential in determining the normal level of V at rest as well as during exercise or exposure to cold. In hypoxia at thermoneutrality, the ventilatory response was reduced but not abolished after CD, in agreement with previous studies performed in rats (8, 12, 25, 30). The reduction is, however, moderate if one takes into account the fact that Vo2 has been inhibited by hypoxia. The residual hyperventilation was caused by an increase in f despite a diminished VT, as in conscious CD cats exposed to hypoxia (27). In hypoxia at low T,, a reduction of 30-40s of the ventilatory response to hypoxia at constant VO, was observed in CD compared with intact rats (Fig. 4), but such a comparison is not exactly reflective of chemoreflex drive, because the ventilatory response was not measured at a constant Pa,,. In CD rats, the residual effects of hypoxia on V may originate in peripheral extracarotid chemoreceptors (8, 25). In the cat, hypoxia may also affect breathing through a direct action on the central nervous system (27), and interactions between PO, and PCO, on the control of breathing are still observed in CD cats (14). Such an interaction may also be operative as far as the control of breathing is concerned during hypoxia and hypothermia in CD rats. The role of the arterial chemoreceptors in the interaction between FI,, and V,Z may also be studied during transient changes in the level of oxygenation as in protocol2. At the onset of hypoxia, iTog decreases in both intact and CD rats, but the prominent chemoreflex drive
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VENTILATION
DURING
HYPOXIA
853
AND COLD
2. BLA?TEIS, C. M. Hypoxia and the metabolic response to cold in new-born rabbits. J. Physiol. Lond. 172: 358-368, 1964. 3. BLA’ITEIS, C. M. Shivering and nonshivering thermogenesis during hypoxia. Proc. Int. Symp. Physiol. Bioenerg. Wash. DC 1972, p. 151-160. 4. BLA?TEIS,
C. M., AND L. 0. LIJTHERER. Effects of altitude exposure on thermoregulatory response of man to cold. J. Appl. Physiol.
41: 848-858,1976. 5. BOWEROT, P., R. COLLIN, R. FAVIER, BERT. Carotid chemoreceptor function
R. FLANDROIS, AND P. SEin ventilatory and circulatory 0, convection of exercising dogs at low and high altitude. Re-
spir. Physiol. 43: 147-167, 1981. 6. BRUCK, K., AND W. WUNNENBERG.
“Meshed” control of two effecand shivering thermogenesis. In: Physiological and Behavioral Temperature Regulation, edited by J. D. Hardy, A. P. Gagge, and A. J. Stolwijk. Springfield, IL: Thomas, 1970, p. 562-580. 7. CABANAC, M., A. LACAISSE, P. PASQUIS, AND P. DEJOURS. Caract&es et mecanisme des reactions ventilatoires au frisson thermique chez l’homme. C. R. Sot. Biol. 158: 80-84, 1964. 8. CARDENAS, H., AND P. ZAPATA. Ventilatory reflexes originated from carotid and extracarotid chemoreceptors in rats. Am. J. Physiol. 244 (Regulatory Integrative Comp. Physiol. 13): Rll9-R125, 1983. 9. CHASSAIN, A., AND D. BARGETON. Ventilation au froid chez le rat. tor systems: nonshivering
.’
/ /’ ‘+?
20
Ta,OC
Nx INT CD Hx INT CD
-
0
2.5
5.0
-ae -+-*-
J. Physiol. Paris 53: 543-560, 1961. 10. DE SANCTIS, G. T., F. H. Y. GREEN,
7.5 10 Shivering Intensity
5. VO, vs. corresponding values of shivering intensity (resets/ min) in 8 rats intact (closed symbols) or after CD (open symbols) exposed to sequential decreases in T, from 20 to 5°C in normoxia (circles) or hypoxia (0.12 FIN,, triangles). Regression lines are shown for normoxia (solid lines) and hypoxia (dashed lines) in protocol 3. FIG.
exceeds the restrained metabolic stimulus leading to an increase in V in intact rats. In contrast, in CD rats, the decreased metabolic stimulus associated with an attenuated chemoreflex drive will result in reduced V (Fig. 4). Similar mechanisms may explain the changes in V observed during the recovery in normoxia. In conclusion, this study confirms in the conscious rat that V is precisely adjusted to the level of metabolism during cold exposure in normoxia. In hypoxia, interaction between the chemoreflex drive and the level of metabolism results in a marked increase in V in the intact animal. The attenuation of the chemoreflex drive in CD rats and the relatively direct depression of metabolism by hypoxia result in a weaker interaction between FI,, and VO, than in intact rats. The residual effects of hypoxia on V in the CD rat may originate in extracarotid chemoreceptors and/or through direct action on the central nervous system. The authors thank P. Fontanges for building the electronic equipment, J. Chandellier for making the illustrations, M. Gras for typing the manuscript, and Dr. S. Parot for help in the statistical analysis. This research was supported by Contrat de Recherche Externe Grant 895007 from Institut National de la Sante et de la Recherche Medicale. Address for reprint requests: H. Gautier, Faculte de Medecine SaintAntoine, 27, rue Chaligny, 75012 Paris, France. Received 21 November 1991; accepted in final form 23 March 1992.
AND J. E. REMMERS. Ventilatory responses to hypoxia and hypercapnia in awake rats pretreated with capsaicin. J. Appl. Physiol. 70: 1168-1174, 1991. 11. EULER, C. VON, AND U. S~DERBERG. Co-ordinated changes in temperature thresholds for thermoregulatory reflexes. Acta Physiol.
Stand. 42: 112-129, 1958. 12. FAVIER, R., AND A. LACAISSE. Stimulus oxygene de la chez le rat eveill& J. Physiol. Paris 74: 411-417, 1978. 13. FULLER, C. A., B. A. HORWITZ, AND J. M. HOROWITZ.
and nonshivering thermogenic hypothalamic warming. Am. J. 14. GAUTIER, H., AND M. BONOFU. stimulants in conscious intact Eur. Physiopathol. Respir. 15. GAIJTIER, H., M. BONORA,
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D., JR., AND S. M. TENNEY. Control anemia. Respir. Ph-ysiol. 10: 384-395,
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S. A. SCHULTZ, AND J. E. REMMERS. changes in shivering and body temperature. J.
Appl. Physiol. 62: 2477-2484, 1987. 19. GLICKMAN, N., H. H. MITCHELL, R. W. KEETON, AND E. H. LAMBERT. Shivering and heat production in men exposed to intense cold. J. Appl. Physiol. 22: l-8, 1967. 20. GORDON, C. J. Thermal biology of the laboratory rat. Physiol. Behuv. 47: 963-991,199O. 21. HILL, J. R. The oxygen consumption of new-born and adult mam-
mals. Its dependence on the oxygen tension in inspired air and on the environmental temperature. J. Pnysiol. Lond. 149: 346-373, 1959. 22. KAO,
F. F., AND B. B. SCHLIG. Impairment of gas transport and gas exchange in dogs during acute hypothermia. J. Appl. Physiol. 9:
387-394, 1956. 23. LEGALLOIS, C. Deuxieme memoire Oeuvres. Paris: Le Rouge, 1813, vol. 24. LIM, T. P. K. Central and peripheral
ing and its effects on respiration. 25.
REFERENCES 1. BARTLETT,
ventilation
in 26.
sur la chaleur animale. In: II, p. 21-66. control mechanisms of shiverJ. Appl.
Physiol.
15: 567-574,
1960. MARTIN-BODY,
R. L., G. J. ROBSON, AND J. D. SINCLAIR. Respiratory effects of sectioning the carotid sinus glossopharyngeal and abdominal vagal nerves in the awake rat. J. Physiol. Lond. 361: 35-45,1985. MASKREY,
M. Bodv temperature
effects on hypoxic and hypercap-
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854
VENTILATION
DURING
nit responses in awake rats. Am. J. Physiol. 259 (Regulatory Integrative Comp. Physiol. 28): R492-R498, 1990. 27. MILLER, M. J., AND S. M. TENNEY. Hypoxia-induced tachypnea in carotid-deafferented cats. Respir. Physiol. 23: 31-39, 1975. 28. MOTT, J. C. The effect of baroreceptor and chemoreceptor stimulation on shivering. J. Physiol. Lond. 166: 563-586, 1963. 29. NEWSTEAD, C. G. The relationship between ventilation and oxygen consumption in man is the same during both moderate exercise and shivering. J. Physiol. Lond. 383: 455-459, 1987.
HYPOXIA
AND
COLD
30. OLSON, E. B., JR., E. H. VIDRUK, AND J. A. DEMPSEY. Carotid body excision significantly changes ventilatory control in awake rats. J. Appl. Physiol. 64: 666-671,1988. 31. PAPPENHEIMER, J. R. Sleep and respiration of rats during hypoxia. J. Physiol. Lord. 266: 191-207, 1977. 32. SAPRU, H. N., AND A. J. KRIEGER. Carotid and aortic function in the rat. J. Appl. Physiol. 42: 344-348, 1977. 33. WOOD, S. C. Interactions between hypoxia and hypothermia. Annu. Rev. Physiol. 53: 71-85, 1991.
Downloaded from www.physiology.org/journal/jappl by ${individualUser.givenNames} ${individualUser.surname} (132.210.236.020) on January 14, 2019.
GAUTIER
AND
MONIQUE
de Physiologie Respiratoire,
BONORA Faculte’ de Mbdecine Saint-Antoine,
GAUTIER, HENRY, AND MONIQUE BONORA. Ventilatory and metabolic responses to cold and hypoxia in intact and carotid body-denervated rats. J. Appl. Physiol. 73(3): 847-854, 1992.The effects of hypoxia on thermoregulation and ventilatory control were studied in conscious rats before and after carotid denervation (CD). Measurements of metabolic rate (VO,), ventilation (V), shivering intensity (SI), and colonic temperature (T,) were made in groups of eight rats subjected to three protocols. In protocols 1 and 2, at ambient temperature (T,) of 25 and 5”C, respectively, rats were exposed to normoxia and hypoxia [inspired 0, fraction (FI,,) O.l3-ON]. In protocol 3, T, was decreased from 25 to 5°C in 30-min steps of 5OC. Recordings were made in normoxia and hypoxia (FI,, 0.12). The results show that in both intact and CD rats 1) in normoxia, cold exposure increased vo2, V, and SI, and these increases were proportional to the decrease in T,; 2) hypoxia induced only a transient de.crease in SI, and, for a given T,, VO, was. reduced whereas V and SI were increased; and 3) in CD rats, V increased less during cold exposure in both normoxia and hypoxia; VO, and T, were more depressed during hypoxia. It is concluded that 1) the interaction between T, and FI,, in the control of V is partly dependent on the carotid body afferents, 2) shivering thermogenesis may be transiently affected by hypoxia independently of the carotid body afferents, and 3) nonshivering thermogenesis may be directly inhibited by hypoxia, especially during cold exposure. shivering; thermogenesis; body temperature; metabolism; control of ventilation
DURING EXPOSURE of homeotherms to cold in normoxia, thermogenesis is increased to maintain constant body temperature. However, under hypoxic conditions, thermoregulation is altered, and a decrease of body temperature is observed (3,33). Since the original observations of Legallois (23), this hypoxic hypothermia has been described in many animal species, including the rat, and seems to result from a transient inhibition of shivering thermogenesis (ST) and a more sustained inhibition of nonshivering thermogenesis (NST) (15). The role of arterial chemoreflex afferents in the mediation of hypoxic hypothermia is not clear. Pharmacological stimulation of the carotid body induces inhibition of shivering in anesthetized animals (11, 16, 28). However, in conscious newborn rabbits (2) and adult cats (18), the inhibitory effect of hypoxia on thermogenesis persists after carotid denervation. In the present study, we report the effects of hypoxia on the metabolic and ventilatory responses to cold in intact and carotid body-denervated conscious rats. 0161-7567/92
$2.00
Copyright
75012 Paris, France
These investigations were done to clarify the role of the chemoreflex afferents in hypoxic hypothermia and to describe the interactions of increased metabolism resulting from cold exposure and changes in level of oxygenation on the control of breathing. METHODS Animals
Experiments were performed in groups of eight female Wistar rats -2 mo of age, with a body weight of -250 g at the beginning of the study. They were caged in an animal room in groups of four at a temperature of 2325OC for 21 wk before they were used for experiments. They were provided with a commercial rat chow and tap water ad libitum. Measurements
Metabolic rate (VO,), shivering activity, and colonic temperature (Tc) were measured as previously reported (15). Briefly, VO, was computed using a closed-circuit method from the time spent by the rat to consume 10 or 20 ml of 0,, expressed in ml/min STPD. Shivering activity was recorded from two electrodes inserted in the thigh musculature, amplified (Grass P511 amplifier, band width 30-3,000 Hz), and integrated (integrator reset to zero when a fixed output was reached). The number of resets per minute was used to quantify shivering intensity. Colonic temperature (T,) was monitored with a thermistor probe (model 402, Yellow Springs Instruments) inserted 6 cm into the colon. Ventilation (V) was monitored using the plethysmographic technique initially described and validated in the rat (1, 9). The inlet and outlet parts of the metabolic chamber were closed, and the chamber was connected to a Validyne DP 45 pressure transducer. Pressure changes associated with the breathing cycle were recorded on an oscillograph for -10 s. Data analysis [tidal volume (VT),breathing frequency (f), and V] was subsequently performed using a graphics tablet connected to a computer. During all studies, Vo2, calorimeter (ambient) temperature (T,), T,, and V were recorded at 5min intervals. Shivering activity was continuously monitored and averaged over 5-min periods. In each of the following protocols, rats were studied before (intact) and after carotid body denervation (CD). Under halothane anesthesia, carotid sinus nerves were exposed under an operating microscope, as described by
0 1992 the American
Physiological
Society
847
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848
VENTILATION
DURING
Sapru and Krieger (32), and subsequently sectioned at their junctions with the glossopharyngeal nerves. Experiments were carried out within 3-8 days after the surgery. Body weight of all CD animals (248 t 7 g) was the same as that in intact animals (250 t 7 g). Protocol 1: Ventilatory and Metabolic to Hypoxia at Thermoneutrality
Responses
In T, 25”C, the animals were studied for 20 min in normoxia [inspired 0, fraction (FQ 0.211, 20 min in hypoxia (FI,, 0.13), another 20 min in hypoxia with 0.11 FI,, in intact animals and 0.12 FIEF in CD animals, and a final 20 min in normoxia. Protocol 2: Ventilatory and Metabolic Responses to Hypoxia in a Cold T,
T, was 5OC. Measurements were made, in succession, for 30 min in normoxia, 45 min in hypoxia (0.12 FI&, and again for 30 min in normoxia. Protocol 3: Ventilatory and Metabolic Responses to Sequential Decreases in Ta in Normoxia and Hypoxia
Rats were exposed in steps of 30-min duration to T, of 25,20,15,10, and 5OC. Measurements were made one day with 0.21 FI,, and two days later with 0.12 FIEF. In all experiments, 0, concentration in the calorimeter was monitored with a Beckman OM14 0, analyzer every 10 (protocol I) or 5 min (protocols 2 and 3) and adjusted to the desired level. Statistics
Results are means t SE. Analysis of variance and, when appropriate, the Dunnett test for multiple comparisons were used to evaluate the significance of changes induced by hypoxia, T,, or CD. Other tests used are indicated in the text. RESULTS
Protocol 1: Ventilatory and Metabolic to Hypoxia at Thermoneutrality
Responses
Intact animals. As shown in Fig. 1, hypoxia induced a decrease in T, that, after 20 min at 0.11 FI,, , reached a level 0.93 t 0.17”C below the initial normoxic level. This was associated with a slight decrease of ~8% in VO,. Furthermore, hypoxia produced a marked increase in f that was almost entirely responsible for the augmentation in V. During the recovery period in normoxia, a small recovery in T, (0.47 t 0.22”C) was observed. This was associated with an increase of 16% in VO, over the initial control value in normoxia, whereas f and V returned progressively to control values. CD animals. Hypoxia induced a decrease in T, of 1.97 t 0.20°C after 20 min at 0.12 FIEF, significantly larger than before CD (P < 0.01). This was associated with a decrease in VO, that reached 29% with 0.12 FI,,. Furthermore, hypoxia produced a significant increase in f and, despite the small decrease observed in VT, V was slightly increased. During the recovery period in normoxia, T, increased significantly (0.67 t O.l7”C, P < 0.01). This
HYPOXIA
AND
COLD
was associated with a marked increase in did not change significantly. Protocol 2: Ventilatory and Metabolic to Hypoxia in a Cold T,
iTo,,
whereas V
Responses
Intact animals. As shown in Fig. 2, exposure to hypoxia induced a progressive increase in f that accounted for the increase in V inasmuch as VT did not change significantly. Meanwhile, T, showed a progressive decrease that amounted to 3.27 t 0.38OC after 45 min of hypoxia. After 5 min of hypoxia, shivering intensity was decreased by 57%, but a complete recovery was observed after 3545 min of hypoxia. In contrast, VO,, which was decreased by 26% after 5 min of hypoxia, remained decreased by 18% after 45 min of hypoxia. During recovery in normoxia, V returned progressively to the control normoxic value. T, increased progressively by 1.77 t 0.14OC over the last measurement in hypoxia. This was associated with a transient overshoot in shivering intensity and VO,. CD animals. As shown in Fig. 2, exposure to hypoxia induced no significant change in V. In contrast, T, showed a progressive decrease that amounted to 3.99 t 0.18OC during the hypoxic exposure. Shivering intensity, which was decreased by 74% after 5 min of hypoxia, markedly exceeded (+32%) the initial normoxic control value after 35-45 min of hypoxia. Finally, Vo2, which was decreased by 40% after 5 min of hypoxia, was still diminished by 21% at the end of the hypoxic exposure. During recovery in normoxia, a marked overshoot was observed in V, shivering intensity, and VO,. T, increased progressively by 2.66 t 0.27”C, an increase greater than that observed before CD (1.77 t O.l4”C, P < 0.02). Protocol 3: Ventilatory and Metabolic Responses to Sequential Decreases in T, in Normoxia and Hypoxia Intact animals. In normoxia, exposure to progressively lower T, induced an increase in both f a-nd VT that accounted-for the progressive increase in V from 119 t 5 ml/min at 25OC to 296 t 12 ml/min at 5OC (Fig. 3). T, did not change significantly, and shivering that was sometimes apparent at T, - of 20°C increased progressively at lower T,‘s. Finally, VO, increased linearly with the decrease in T,. Exposure to hypoxia at 25°C caused an increase in f that accounted for the increase in V. Exposure to lower T, induced a progressive increase in V from 208 t 6 ml/ min at 25OC to 448 ml/min at 5OC. T, decreased markedly from 37.56 t 0.14”C at 25°C to 34.33 t 0.63OC at 5OC. Shivering intensity increased as T, decreased and was on average greater than in normoxia for a given T,. Finally, VO, increased linearly with the decrease in T, but at all Ta’s was lower than in normoxia. CD animals. In normoxia, exposure to progressively lower T, induced a progressive increase in V; however, for a given T,, this increase remained lower in CD than in intact animals because of a lower f (Fig. 3). T, did not change significantly. Shivering intensity and VO, increased when T, decreased and were generally greater in CD than in intact animals. In hypoxia, V was much lower than in intact animals.
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VENTILATION
39 Y0 I-
37
NX
::
B=e,-&*-*
35
7t .E E
:: :: :
8
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:: :: ::: :: i:
::
DURING
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HX ho2 O-13
0.11
HYPOXIA
NX
:
6
4
200
FIG. 1. Ventilation (V), tidal volume (VT), breathing frequency (f), metabolic rate (VO,), and colonic temperature (T,) in 8 rats before (intact, closed symbols) and after carotid denervation (CD, open symbols). In T, of 25”C, animals were exposed to normoxia (NX, circles), then to hypoxia (HX, triangles), and again to normoxia. FI,~, inspired O2 fraction. Values significantly different from average initial values in normoxia: *P < 0.05 (protocol I).
7
.r150
E.
e100 50
849
COLD
1 1 1
i .
-+L*
:
AND
:
i
:
1 .3
-I .-c E. -
.2
l,
A
~ min
T,, which was 25”C, continued 5OC. As in intact sity was greater normoxia.
already decreased to 36.64 t 0.23OC at to decrease, reaching 33.74 t 0.25OC at animals, for a given T,, shivering intenand Vo2 was lower in hypoxia than in
Relationships between ventilation, shivering intensity, and VOW.As shown Fig. 4, in all conditions V was highly correlated with VO,. In intact animals (left), v = 17.6 VO, + 8 (r = 0.998, P < 0.001) in normoxia and V = 29.8 vo2 + 33 (r = 0.997, P < 0.001) in hypoxia, with a slope significantly steeper (P < O.Ol) in hyp.oxia than in normoxia. In CD animals (right), V = 14.5 VO, + 7 (r = 0.984, P < 0.01) in normoxia, with a slope significantly lower (P < 0.01) than in intact animals. Finally, in CD animals in hypoxia, V = 23.6 VO, + 4 (r = 0.975, P < O.Ol), with a slope again significantly lower (P < 0.01) than in intact
animals. As shown in Fig. 5, irO, was closely correlated with shivering intensity. In intact animals, r = 0.995 (P < 0.001) in normoxia and 0.998 (P < 0.001) in hypoxia, but the relationship was significantly shifted (P < 0.01) to lower values of voz relative to normoxia. This indicates that, for a given shivering intensity, Vo2 was lower in hypoxia than in normoxia, presumably due to inhibition of the NST component. In CD animals, Vo2 was again
highly correlated with shivering intensity: r = 0.996 (P < 0.001)in normoxia and 0.993 (P < 0.001) in hypoxia. The relationship in CD rats in hypoxia was significantly lower than in CD rats in normoxia (P < 0.01) and in intact rats in hypoxia (P < 0.05). DISCUSSION
Role of the Carotid Body Afferents Response to Hypoxia
in the A4etabolic
Thermoneutrality. At T, of 25°C decreases in VO, and T, during the exposure to hypoxia were greater in CD than in intact animals. The depressant effects of hypoxia on metabolism in small animals are well known. The magnitude of the decrease in metabolism depends on T,, because VO, would be unaltered by hypoxia above the thermoneutral point, according to Hill (21). However, values of thermoneutral zone reported in the literature for the rat are quite variable (from 22 to 34”C), according to the review of Gordon (20). In addition, it appears from our previous study in rats (15) that the thermoneutral zone may be lower in hypoxia than in normoxia. This led us to select 25OC as the higher T, for the present study. The greater decrease in VO, and T, observed in CD animals exposed to hypoxia is in agreement with our pre-
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850
VENTILATION NX
DURING
1; :
200
HYPOXIA
AND COLD :: :
HX
NX
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I I I I 1 I 25 10 20 0 5 15 FIG. 2. VO,, shivering intensity (in percent of average values observed in initial before (closed symbols) and after CD (open symbols). In T, of 5”C, animals were then to hypoxia (HX, 0.12 FIEF, triangles, broken lines), and again to normoxia. average last 10 min of initial normoxia: *P < 0.05 (protocol 2).
vious study showing that conscious CD cats show a significant decrease in T, even at thermoneutral T, (18). Because CD rats hyperventilate less than intact animals for a given FIEF, their arterial PO, (Pa,,) would be expected to be lower than that of intact animals. However, because we used a higher FI,, in the second hypoxic exposure in CD rats and because their VO, was markedly reduced, we may assume that their Pa,, was not too different from that of intact rats. Arterial PCO, (Pat,,) was also probably higher in CD than in intact rats (12, 30), and this factor may also affect thermogenesis (17,31). These caveats notwithstanding, the present results suggest that, in agreement with our previous conclusion in cats, carotid sinus nerve afferents act to maintain constant body temperature during hypoxia by stimulation of NST (18). Cold T,. In intact animals, the results of protocols 2 and 3 concerning the effects of normoxia and hypoxia on
I I 1 I I 35 0 10 20 30 min normoxia), T,, V, VT, and fin 8 rats exposed to normoxia (NX, circles), Values significantly different from
body temperature regulation are in agreement with those in our previous study using similar protocols (15). During normoxia in protocol 3, VO, and shivering intensity were higher in CD rats. The same was observed for VO, at the beginning of protocol 2. At T, of 5”C, this resulted in a higher T, in CD rats. This increase in ST observed in CD rats may be ascribed to the removal of the tonic inhibition of shivering by the carotid chemoreceptors that was initially demonstrated by pharmacological stimulation (11, 16, 28). Similarly, the greater recovery in shivering intensity, TO,, and T, observed after hypoxia in protocol 2 in CD rats may involve the same mechanism. During hypoxia, CD rats still display a severe inhibition of 60, and hypothermia, which appear greater than in intact rats. In agreement with previous studies (2,18), this suggests that hypoxic inhibition of thermogenesis is
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VENTILATION
Nx INT CD Hx INT CD
r4 4 -9 --9% -4
I 5
-9 -9
-L,
DURING
HYPOXIA
AND
851
COLD
4 -A--*-
I 15
Ta,“C
5
10
15
20
25
Ta,“C
FIG. 3. V, VT, f (left), VOW,shivering intensity, and T, (right) in 8 rats before (INT, closed symbols) and after CD (open symbols) exposed to sequential decreases in T,. Animals were exposed to normoxia (Nx, circles, solid lines) or hypoxia (HX, 0.12 FI,, , triangles, broken lines). Values significantly different from those observed at T, of 25°C: *P c 8.05. In all conditions (Nx, Hx, INT, CD), VT was significantly different at T, of 15,10, and 5OC from T, of 25°C; VO, was significantly different at T, of 20, 15, 10, and 5OC from T, of 25°C; and shivering intensity was significantly different at T, of 10 and 5°C from T, of 20°C (protocol 3).
mediated independently of the carotid chemoreceptors’ activity, via central mechanisms or other peripheral chemoreceptors that remain to be elucidated. The greater inhibition of VO, observed in CD rats may be caused by their relatively lower Paoz. In addition, the “protective” effect of carotid body afferents on thermoregulation, which has been suggested above as operating during hypoxia at thermoneutrality, has been removed in CD rats. Finally, it appears that hypoxia shifts the balance of thermogenic output away from NST and toward ST even more in CD than in intact rats. This is shown after 30-45 min of hypoxia in protocol 2 when VO, is the same in CD and intact rats, whereas shivering is greater in CD rats. Similarly, as indicated by the shivering-Vo, relationships shown in Fig. 5, shivering in CD rats during hypoxia is generally greater than in intact rats, even though their VO, is lower. The enhanced intensity of shivering during prolonged hypoxia observed in CD compared with intact rats may be accounted for by several independent factors: first, removal of the inhibition of shivering from carotid chemoreceptors discussed above; second, a compensatory increase in shivering when NST has been inhibited (2, 6, 13); and third, the lower T, observed during hypoxia in CD rats representing a potent stimulus for thermogenesis.
This study confirms that hypoxia may markedly affect the body temperature regulation (3). This effect may be mediated reflexly, through activation of carotid chemoreceptors, and directly, through an action on the central nervous system, possibly the thermoregulatory centers. The comparison of the results obtained at thermoneutrality and with a cold T,, on the one hand, and before and after CD, on the other, suggests that 1) NST may be strongly inhibited directly by hypoxia, especially in hypothermia, and this in turn may lead to a compensatory increase in ST; 2) NST may be stimulated by carotid body afferents and, in hypoxia at thermoneutrality, this will counterbalance the direct inhibitory effect of hypoxia on thermogenesis, such that body temperature will remain unaffected in the intact animal; and 3) ST may be transiently inhibited by hypoxia, directly or through the mediation of the carotid body activity. Interactions Between Changes in Fro2 and T,., in the Control of Breathing Intact animals. Those studies that have reported the ventilatory response to changes in T, in normoxia have mainly been carried out in humans (4,7,19,29) and anesthetized animals (16, 22, 24). However, in intact conscious rats exposed to T, of 24-25°C Maskrey (26)
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852
VENTILATION I
DURING
I
I
10
1s
HYPOXIA
AND COLD I
I
I
5
10
15
INTACT
0
5
0
20
\;(Oz , ml. mid FIG. 4. V vs. corresponding values of VO, in 8 rats intact (left) or after CD (right) exposed to sequential decreases in T, from 25 to 5OC in normoxia (Nx, circles) or hypoxia (Hx, 0.12 FI, , triangles). Regression lines are shown for normoxia (solid lines) and hypoxia (dashed lines) in protocol 3. For T, of%“C, transient changes in V and VO, are also shown with results obtained in protocol 2.
recently showed that when T, was experimentally decreased by -3OC by means of an abdominal heat exchanger, V was increased ~2.4 times. In the present work, V was increased by the same order of magnitude (2.5 times) during the exposure to Ta)s of 5 and 25°C. Because VO, and shivering activity were not measured in the Maskrey study, we can conclude only that, in normoxia, exposure to a T, of 5°C (peripheral cold stimulation) induces changes in V similar to an experimental decrease of T, of 3°C (central cold stimulation). The present study indicates, in addition, that during cold exposure, V increases at the same rate as metabolism, in agreement with previous studies in rats (9) and other animal species (16,19,22,24), suggesting that regulation of breathing is precisely adjusted to the metabolic needs as in muscular exercise of moderate intensity (29). During hypoxia at thermoneutrality, V was increased as expected, and this was caused essentially by an increase in f (10, 12,25). In cold T,, hypoxia has two opposing effects on the control of breathing: enhanced chemoreflex hypoxic drive and metabolic drive (VOJ, which has been reduced by hypoxia. It was observed that V increased proportionately with VO, and that V/Voz was higher in hypoxia than in normoxia (Fig. 4). For any given T,, V,Z was lower and V was higher in hypoxia. This indicates that the hypoxic chemoreflex drive remains operative during cold exposure and appears even higher when T, decreases, suggesting a multiplicative interaction between VO, and hypoxia in the control of breathing. CD animals. In normoxia, at thermoneutrality, V was decreased compared with intact animals, in agreement with previous reports (12, 25,30). During cold exposure, V increased proportionately with VO,, as in intact ani-
mals, but V/Vo, was lower after CD. We are not aware of any study reporting the ventilatory response to cold after CD in conscious animals. However, during muscular exercise, V and WVO, in dogs were decreased by ~20% after CD (5). It may therefore be concluded that the integrity of the chemoreflex drive of breathing is essential in determining the normal level of V at rest as well as during exercise or exposure to cold. In hypoxia at thermoneutrality, the ventilatory response was reduced but not abolished after CD, in agreement with previous studies performed in rats (8, 12, 25, 30). The reduction is, however, moderate if one takes into account the fact that Vo2 has been inhibited by hypoxia. The residual hyperventilation was caused by an increase in f despite a diminished VT, as in conscious CD cats exposed to hypoxia (27). In hypoxia at low T,, a reduction of 30-40s of the ventilatory response to hypoxia at constant VO, was observed in CD compared with intact rats (Fig. 4), but such a comparison is not exactly reflective of chemoreflex drive, because the ventilatory response was not measured at a constant Pa,,. In CD rats, the residual effects of hypoxia on V may originate in peripheral extracarotid chemoreceptors (8, 25). In the cat, hypoxia may also affect breathing through a direct action on the central nervous system (27), and interactions between PO, and PCO, on the control of breathing are still observed in CD cats (14). Such an interaction may also be operative as far as the control of breathing is concerned during hypoxia and hypothermia in CD rats. The role of the arterial chemoreceptors in the interaction between FI,, and V,Z may also be studied during transient changes in the level of oxygenation as in protocol2. At the onset of hypoxia, iTog decreases in both intact and CD rats, but the prominent chemoreflex drive
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VENTILATION
DURING
HYPOXIA
853
AND COLD
2. BLA?TEIS, C. M. Hypoxia and the metabolic response to cold in new-born rabbits. J. Physiol. Lond. 172: 358-368, 1964. 3. BLA’ITEIS, C. M. Shivering and nonshivering thermogenesis during hypoxia. Proc. Int. Symp. Physiol. Bioenerg. Wash. DC 1972, p. 151-160. 4. BLA?TEIS,
C. M., AND L. 0. LIJTHERER. Effects of altitude exposure on thermoregulatory response of man to cold. J. Appl. Physiol.
41: 848-858,1976. 5. BOWEROT, P., R. COLLIN, R. FAVIER, BERT. Carotid chemoreceptor function
R. FLANDROIS, AND P. SEin ventilatory and circulatory 0, convection of exercising dogs at low and high altitude. Re-
spir. Physiol. 43: 147-167, 1981. 6. BRUCK, K., AND W. WUNNENBERG.
“Meshed” control of two effecand shivering thermogenesis. In: Physiological and Behavioral Temperature Regulation, edited by J. D. Hardy, A. P. Gagge, and A. J. Stolwijk. Springfield, IL: Thomas, 1970, p. 562-580. 7. CABANAC, M., A. LACAISSE, P. PASQUIS, AND P. DEJOURS. Caract&es et mecanisme des reactions ventilatoires au frisson thermique chez l’homme. C. R. Sot. Biol. 158: 80-84, 1964. 8. CARDENAS, H., AND P. ZAPATA. Ventilatory reflexes originated from carotid and extracarotid chemoreceptors in rats. Am. J. Physiol. 244 (Regulatory Integrative Comp. Physiol. 13): Rll9-R125, 1983. 9. CHASSAIN, A., AND D. BARGETON. Ventilation au froid chez le rat. tor systems: nonshivering
.’
/ /’ ‘+?
20
Ta,OC
Nx INT CD Hx INT CD
-
0
2.5
5.0
-ae -+-*-
J. Physiol. Paris 53: 543-560, 1961. 10. DE SANCTIS, G. T., F. H. Y. GREEN,
7.5 10 Shivering Intensity
5. VO, vs. corresponding values of shivering intensity (resets/ min) in 8 rats intact (closed symbols) or after CD (open symbols) exposed to sequential decreases in T, from 20 to 5°C in normoxia (circles) or hypoxia (0.12 FIN,, triangles). Regression lines are shown for normoxia (solid lines) and hypoxia (dashed lines) in protocol 3. FIG.
exceeds the restrained metabolic stimulus leading to an increase in V in intact rats. In contrast, in CD rats, the decreased metabolic stimulus associated with an attenuated chemoreflex drive will result in reduced V (Fig. 4). Similar mechanisms may explain the changes in V observed during the recovery in normoxia. In conclusion, this study confirms in the conscious rat that V is precisely adjusted to the level of metabolism during cold exposure in normoxia. In hypoxia, interaction between the chemoreflex drive and the level of metabolism results in a marked increase in V in the intact animal. The attenuation of the chemoreflex drive in CD rats and the relatively direct depression of metabolism by hypoxia result in a weaker interaction between FI,, and VO, than in intact rats. The residual effects of hypoxia on V in the CD rat may originate in extracarotid chemoreceptors and/or through direct action on the central nervous system. The authors thank P. Fontanges for building the electronic equipment, J. Chandellier for making the illustrations, M. Gras for typing the manuscript, and Dr. S. Parot for help in the statistical analysis. This research was supported by Contrat de Recherche Externe Grant 895007 from Institut National de la Sante et de la Recherche Medicale. Address for reprint requests: H. Gautier, Faculte de Medecine SaintAntoine, 27, rue Chaligny, 75012 Paris, France. Received 21 November 1991; accepted in final form 23 March 1992.
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