Effects of naloxone on hemodynamics and sympathetic activity after exercise KAZUHIRO HARA AND JOHN S. FLORAS Division of Cardiology and the Centre fur Cardiovascular Research, The Toronto Hospital, Toronto, Ontario M5G 2C4, Canada HARA,
KAZUHIRO,
on hemodynamics
AND JOHN S. FLORAS. Effects of mdoxone and sympathetic activity after exercise. J.
Appl. Physiol. 73(5): 2028-2035, 1992.-The effects of highdose naloxone (0.4 mgkg iv) on hemodynamics and muscle sympathetic nerve activity (MSNA) after exercisewere studied in nine normotensive young men randomly allocated the opioid antagonist or vehicle 30 min before treadmill exerciseat 70%of resting heart rate reserve. Mean arterial pressure (MAP) was lower after exercise;cardiac output wasincreased.Mean values for MSNA and plasmanorepinephrine were similar before and after exercise, but in individual subjects changes in resting MAP 60 min after exercisewere inversely related to changesin sympathetic activity, suggestingthat arterial baroreflex regulation of MSNA had been shifted to a lower set point. Naloxone did not prevent postexercise hypotension but transformed theseinverse correlations into positive relationships. Naloxone attenuated both calf and systemicvasodilation without altering meanvalues for MSNA, indicating a peripheral effect of opioid antagonism.In normotensivesubjects,naloxone alters the regulation of sympathetic outflow and vascular resistanceduring recovery from exercise but doesnot prevent the fall in MAP. atria1 natriuretic factor; baroreceptor reflexes; Doppler cardiac output; endorphins; musclesympathetic nerve activity; opioids
endurance exercise is often followed by a persistent decrease in blood pressure (1, 2,16). The absence of reflex tachycardia and the altered vasoconstrictor responses to orthostatic stimuli seen in conjunction with the fall in blood pressure in these studies suggest that neuroregulatory mechanisms may be reset after submaximal or exhaustive exercise. Indeed, postexercise hypotension tends not to be accompanied by anticipated reflex increases in efferent muscle sympathetic nerve activity (MSNA) (14). The blood pressure and heart rate of spontaneously hypertensive rats (SHR) are also lower after exercise. This depressor response can be reversed by naloxone (lo-20 mg/kg iv) (37). Also, sustained and parallel reductions in blood pressure may be seen after exercise is simulated by electrical stimulation of the sciatic nerve or gastrocnemius muscle (37). Reductions in blood pressure after such stimulation is not accompanied, as might be expected, by baroreflex-mediated tachycardia and sympathoexcitation; indeed, splanchnic sympathetic nerve activity (SNA) is reduced in parallel with blood pressure. Both the fall in blood pressure and the reduction in efferent nerve activity in these preparations can be abolished by pretreatment with naloxone (lo-15 mg/kg iv) (37).
A SINGLE
2028
BOUT
OF PROLONGED
These observations, and supporting evidence from other experimental and human studies, led to the hypothesis that prolonged rhythmic exercise can reset the arterial baroreceptor centrally by activating sympathoinhibitory opioid systems (37). Evidence that central opioid pathways are involved in the autonomic adjustments to exercise in humans is sparse. Only some of the experimental evidence to date supports the concept that circulatory, metabolic, endocrine and thermoregulatory adjustments to submaximal and maximal exercise are influenced by pretreatment with high doses of naloxone (5,17,21,33,35), a selective and specific opioid antagonist with high affinity for p-receptors (30). The effects of naloxone on hemodynamics, MSNA, and plasma norepinephrine concentrations during the recovery period after exercise have not been defined. Our aims in this study were 1) to determine hemodynamic and neurohumoral sequelae to treadmill exercise in young untrained normal men and 2) to determine if hemodynamics and sympathetic activity after exercise can be altered by prior administration of high-dose naloxone. MATERIALS
AND
METHODS
Subjects
We studied nine young untrained normotensive subjects. Their mean age was 27 t 2 (SE) yr, their mean weight was 78 t 4 kg, and their mean body surface area was 1.93 t 0.05 m2. A medical history, physical examination, and laboratory investigations excluded hypertension, concurrent illness, and use of medication. All subjects gave an informed written consent. The study was approved by the Human Subjects Review Committee of the University of Toronto. Procedures Microneurogruphy. Multiunit recordings of postganglionic SNA were obtained with a unipolar tungsten electrode inserted selectively into muscle-nerve fascicles of
the peroneal nerve, posterior to the fibular head (39). The raw neurogram was amplified (by 20,000~50,000), filtered (at a bandwidth of 700-2,000 Hz), rectified, and integrated (time constant 0.1 s) to obtain a mean voltage display of MSNA. Bursts were differentiated from those arising from skin sympathetic nerves using previously accepted criteria (14,39). MSNA was quantitated both in
0161-7567/92 $2.00 Copyright 0 1992 the AmericanPhysiological Society
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ENDORPHINS terms of burst frequency (burstsfmin) and (to adjust for the pulse synchronous nature of MSNA) burst incidence (bursts/l00 cardiac cycles). Plethysmography. Calf blood flow (ml min-’ 100 ml of calf volume-‘) was estimated by venous occlusion plethysmography. A Whitney strain gauge was applied to the midportion of the calf. Circulation to the foot was interrupted by an ankle cuff inflated to 180 mmHg. A second cuff was applied above the popliteal crease. Sequential 50-mmHg inflations and subsequent deflations of the proximal cuff were timed to give four estimates of calf blood flow per minute. Echocardiography. With subjects in the left lateral position, the parasternal long axis view was used to measure the aortic ring diameter (II) as well as left ventricular and left atria1 dimensions (~-MHZ mechanical scan transducer, Ultramark 8, Advanced Technology Laboratories, Bothell, WA). Cardiac output was calculated from the product of stroke volume and heart rate. Stroke volume was calculated from the product of the mean timevelocity integral and the cross-sectional area of the aortic orifice (A) calculated as A = r X (D/2)2 (18). With subjects supine we measured instantaneous flow velocity in the ascending aorta using continuous wave Doppler directed through the suprasternal window (2.25MHz CW Doppler transducer, Advanced Technology Laboratories). The continuous Doppler beam was initially directed to the aortic valve and then adjusted to obtain the maximum aortic flow velocity. Doppler signals were recorded onto super VHS videotape simultaneously with the electrocardiogram and the phonocardiogram. Timevelocity integrals recorded during held expiration were planimetered (Nova Microsonics, Mahwah, NJ). At least 10 consecutive cardiac cycles were averaged to derive a mean value for time-velocity integrals for these calculations. l
l
Protocol Study design. High-dose naloxone (0.4 mg/kg iv) or vehicle was administered 30 min before exercise in a random double-blind placebo controlled study design. Experiments were performed on two occasions with an interval of 1 mo, in the same laboratory at constant room temperature and humidity, and at the same time of day. Subjects were instructed to maintain their usual diet before the study, to avoid exercise for 48 h before the study, and to avoid alcohol, caffeine-containing beverages, and tobacco during the 12 h preceding each study day. Subjects were studied supine. An indwelling salinelock catheter (20 gauge, Deseret Medical, Sandy, UT) was introduced into a right arm antecubital vein for blood sampling. Blood pressure was measured by an automatic cuff recorder (Physio-Control Lifestat 200, Redmond, WA). Respiratory fluctuations were measured continuously using a respiratory belt connected to a Statham P23 ID pressure transducer (Gould, Cleveland, OH). Preexercise. After ZO-min supine rest, blood pressure was measured every minute, and heart rate, the electrocardiogram, respiratory excursions, and the mean voltage neurogram (left peroneal nerve) of muscle sympathetic nerve activity were recorded continuously (Gould
AND EXERCISE
2029
model 2800s ink recorder) over a 15-min baseline period. The last 5 min were used to represent preexercise baseline values. Calf blood flow (right leg) was measured over the last 2 min of this period. The microelectrode was removed from the peroneal nerve; venous blood was withdrawn for the determination of plasma norepinephrine by high-performance liquid chromatography with electrochemical detection (l3), atria1 natriuretic factor (ANF) by radioimmunoassay of extracted plasma samples (13), and luteinizing hormone concentrations by radioimmunoassay; and Doppler cardiac output was determined. We then gave either naloxone (0.4 mg/kg in a concentration of 10 mg/dl; Narcan, DuPont Pharmaceuticals) or vehicle (DuPont Pharmaceuticals) by intravenous bolus over 10 min. Blood pressure and heart rate were monitored for a further 20 min. Subjects then exercised at their assigned target heart rate for 45 min. Exercise. Exercise was performed on a commercially available treadmill (T2000, Complex Engineering, Markham, Ontario, Canada) with a speed and grade that could be adjusted to maintain subjects’ heart rates at 70% of their resting heart rate (RHR) reserve (11). The latter was calculated as 0.7 [ (220 - age) - RHR] + RHR. Cooled by a nearby fan, subjects exercised for 45 min. Small amounts of water were provided ad libitum. Korotkoff phase V was used to determine diastolic blood pressure during exercise. Postexercise. Subjects lay supine after exercise. Blood pressure, heart rate, the electrocardiogram, respirations, and MSNA (right peroneal nerve) were recorded continuously until 60 min after exercise. As in our previous protocol (14), data obtained over the last 5 min of this period were used to represent postexercise values. Calf blood flow (left leg) was measured over the last 2 min of this hour, after which the microelectrode was removed, a second series of blood samples was obtained, the left leg was lowered, and cardiac output was calculated. The latter measurements were acquired -90 min after exercise. Derived
Values
Mean arterial pressure (MAP) was calculated as the diastolic pressure plus one-third of the pulse pressure. Total peripheral resistance (TPR) was estimated by the formula TPR (dyn se cmm5) = 80 X MAP (mmHg)/CO l
where CO is cardiac output. Calf vascular resistance units) was calculated as the pressure divided by the mean calf blood flow. Plasma volume was derived tration and hematocrit (12).
(l/min)
(expressed as resistance quotient of mean arterial of four to six measures of from hemoglobin
concen-
Statistics
Data were expressed as means +- SE. We used both paired t test and two-way analysis of variance (ANOVA) for comparisons between pre- and postexercise values. P values of ~0~05 were considered significant. To quantitate the effect of naloxone on the relationship between blood pressure and (direct and indirect)
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2030
ENDORPHINS
AND EXERCISE
FIG. 1. Systolic and diastolic blood pressures and heart rates (means t SE; n = 9) at baseline, during and after injection of either vehicle or intravenous naloxone (IV), during exercise, and in recovery period. Open circles, on vehicle day; solid circles, on naloxone day. (Diastolic blood pressure is deleted from exercise component.)
200 n E CL
150
P CD z
a
+& L
100
z x
.-
Treadmill
0
Exercise
Supine
.
rIl’~~~“~‘*‘5 10 15
5 10 15 M
25 30 5 10 15 20 25 30 35 40 45 5 10 15 20 25 30 35 10 45 50 55 60
indexes of sympathetic activity, we employed a permutation test based on a randomized reference distribution (4). Under each drug condition, we obtained the Pearson correlation between the percent change in the two variables of interest and calculated an absolute difference between these two correlation coefficients obtained on the naloxone and placebo days. Assuming the null hypothesis that naloxone has no effect on the correlation between blood pressure and that particular measure of sympathetic outflow, and assuming random allocation of vehicle or naloxone, this difference should be independent of the order that each drug was allocated to the subjects. We calculated the 2n possible correlations between these variables based on the 212potential placebodrug allocations for the n subjects and counted the number of correlations in which such absolute differences were at least as large as we observed in our study. The P value was derived from the ratio of this number to the total number of possibilities. P values of ~0.05 were considered significant. RESULTS
Preexercise. There were no symptoms such as nausea, drowsiness, or dysphoria as clues to whether subjects had received intravenous naloxone or vehicle, and neither blood pressure nor heart rate increased over the 30-min observation period after this injection (Fig. 1). Exercise data. All nine subjects completed the exercise protocol. Mean work loads during exercise were comparable on the 2 study days (treadmill grade 13.9 & 1.2 vs.
14.7 t 0.9%; speed 7.4 & 0.3 vs. 7.4 t 0.4 km/h). Mean work distance was also similar on the 2 study days (5.5 t 0.2 km after vehicle; 5.5 t 0.2 km after naloxone). Although average achieved heart rates at this work load were higher on the vehicle day (146 t 5 beats/min) than on the naloxone day (139 t 6 beats/min; P < 0.05), the ratios of achieved heart rate to target heart rate were similar (92 t 2% after vehicle; 87 t 3% after naloxone) (Fig. 1). Naloxone had no effect on blood pressure during exercise (2-way ANOVA). Neurohumoral and hemodynamic variables after exer-
cise and naloxone. The effects of exercise and naloxone on these variables appear in Tables 1 and 2. Mean and diastolic blood pressures were lower and heart rate higher 60 min after exercise on both study days. This postexercise hypotension was caused by a reduction in peripheral resistance rather than cardiac output. Although this modest postexercise hypotension had dissipated by 90 min after exercise, cardiac output remained above resting values (P < 0.002 after vehicle; P < 0.05 after naloxone), because of significantly higher heart rates during the recovery period (P < 0.005 on both days). Total peripheral resistance was lower after exercise on the vehicle (placebo) day (P < 0.004), but not after naloxone, because of a trend to higher mean arterial pressure and lower cardiac output after naloxone compared with vehicle (Table 2). Calf vascular resistance was lower 60 min after exercise on the vehicle (placebo) day (P < 0.02). Calf vasodilation was absent after naloxone (Table 1, Fig. 2). Exercise had minor nonsignificant effects on plasma volume on both study days (vehicle -2.3 t 3.6%, n = 7;
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ENDORPHINS TABLE
1. Hemodynumic
und neurohumorul
2031
AND EXERCISE
vuriubles before and 60 min after treadmill
exercise
Vehicle Variable
Preexercise
Systolic blood pressure, mmHg Mean arterial pressure, mmHg Diastolic blood pressure, mmHg Heart rate, beatsfmin Calf blood flow, ml. min-’ 100 ml-’ Calf vascular resistance, units Muscle sympathetic nerve activity, burstslmin Muscle sympathetic nerve activity, bursts/IO0 cardiac cycles Plasma norepinephrine, nmol/l Atria1 natriuretic factor, pg/ml Luteinizing hormone, W/l Plasma volume, ml/100 ml blood l
Naloxone Postexercise
12lt3
Preexercise
Postexercise
117t3
121k3
119+2
7813-f
80+2*
67+4"f 4.5t0.6 2Ok3*
85t3 67t3 57k4 3.0t0.4 31t4
3.OkO.4 (8) 30t4 (8)
19+3
21t5
23~13
27k5
34s 1.2~0*2 24t3 3.1+0.5 60tl
32t6 1.6t0.3
38t3
38+6 (8) 1.2kO.2 18+2* 4.o-to.4* 59+2 (7)
83t3 64t3 57-t4 3.2t0.3 28+3
59+3t
(8)
60t3t 65t4t
0.9t0.2
19t2*
22+3 2.4kO.5 61+1
2.4tU.5 58t2 (7)
(8)
Values are means t SE in 9 subjects except where indicated in parentheses. * P < 0.05; “f P < 0.005.
naloxone -2.4 t 2.7%, II = 7). Plasma ANF was lower after exercise on the both study days; this decrease occurred in the absence of changes in either left atria1 diameter index (e.g., 14.4 t 0.5 vs. 14.4 t 0.7 mm/m2 before exercise; n = 8 on the naloxone day) or left ventricular end-diastolic diameter index (22.7 t 3.7 vs. 25.6 t 1.1 mm/m2 before exercise, n = 6). No naloxone-specific effects were detected, but twoway ANOVA documented significant naloxone-exercise interactions for plasma luteinizing hormone concentration (P = 0.023) and a trend toward similar interactions for calf blood flow (P = 0.078) and calf vascular resistance (P = 0.068). High-quality mean voltage neurograms were obtained on all four sessions (i.e., both study sessions on both study days) in eight of these nine subjects, and paired data for plasma norepinephrine on both study days were obtained in eight of the nine subjects. These data also appear in Table 1. Despite the fall in mean arterial blood pressure 60 min after exercise, average mean values for MSNA and plasma norepinephrine concentration at this time were not increased reflexively above baseline on either day. Analysis of individual relationships between changes in blood pressure and changes in indexes of sympathetic outflow on the vehicle day revealed an inverse correlation between changes in plasma norepinephrine concentration and changes in mean arterial pressure (r = -0.76; n = 8; P < 0.005) and an inverse correlation between changes in MSNA and changes in mean arterial TABLE
2. Hemodynumics
before and 90 min after treadmill
pressure (r = -0.66; rz = 9; P = 0.05) after exercise (Fig. 3). The inverse relationship between plasma norepinephrine concentration and changes in mean arterial pressure was transformed into a positive relationship by naloxone (from -0.76 to +O.Sl; n = 8; P < 0.05 by the permutation test) (Fig. 3). In contrast to the inverse relationship documented on the vehicle day, there was a significant positive correlation between changes in sympathetic nerve activity and changes in mean blood pressure (r = 0.77; rz = 8; P < 0.05) (Fig. 3) after exercise and naloxone. Naloxone abolished calf vasodilation after exercise without affecting mean values for peroneal nerve muscle sympathetic activity (Fig. 2, Table 1). Naloxone appeared to transform an inverse correlation between changes in plasma norepinephrine and changes in calf vascular resistance on the vehicle day (r = -0.46; n = 8; P = 0.25) into a positive relationship (r = +0.71; n = 8; P < 0.05) (Fig 4). DISCUSSfON In this study of healthy untrained young normotensive subjects, 45 min of treadmill exercise at ~60% maximal oxygen consumption (11) were followed by a reduction in blood pressure. Postexercise hypotension was caused by a fall in peripheral and calf vascular resistance; cardiac output was increased. Mean values for MSNA and plasma norepinephrine concentration were similar beexercise
Vehicle
Naloxone
Variable
Preexercise
Postexercise
Preexercise
Postexercise
Systolic blood pressure, mmHg Mean arterial pressure, mmHg Diastolic blood pressure, mmHg Heart rate, beatslmin Stroke volume, ml Cardiac output, l/min Total peripheral resistance, dyn s cmm5
118t3 81t3
116t2
116t2 79t3
118t2
62t3 56t3 7Ok-5
59t3
l
l
3.9kO.3 1,739+164
78t3 66+4$ 67t5
4.4+0.3t 1,516*178$
6Ozk3 57t4
69k7 3.8-+0.3 1,764,168
80-r-3 61t4 63-t4$ 67+6 4.2t0.4* 1,659*234
Values are means + SE for 9 subjects. * P < 0.05; t P < 0.01; $ P < 0.005. Downloaded from www.physiology.org/journal/jappl by ${individualUser.givenNames} ${individualUser.surname} (129.186.138.035) on January 16, 2019.
2032
ENDORPHINS
). 40 .r: > .zi a 30 Q1 LF S-E .-*g z;20ra dV cu e 57 lo-
ia
AND EXERCISE
lation would therefore appear to be a peripheral effect of naloxone. Reductions in blood pressure after running exercise have been observed in humans and SHR (1, 14, 16, 37). Withdrawal of sympathetic vasoconstrictor tone, as a result of activation of central opioidergic systems during exercise, may mediate this response in SHR (37), but mechanisms of postexercise hypotension in humans appear to be more complex. In our subjects the vasodilation during recovery from exercise could not be attributed to withdrawal of vasoconstrictor tone, as significant reductions in calf vascular resistance were not paralleled by decreases in peroneal MSNA, Several indirect observations suggest that neural mechanisms might contribute to postexercise hypotension in humans (1, 2, 7, 8, 10, 14, 16). In normotensive subjects, baroreflex control of heart rate is enhanced for up to 24 h after exercise to exhaustion (lo), whereas the effects of prior exercise on the arterial baroreflex control of forearm vascular resistance have not been clearly established. Bennett et al. (1) reported a fivefold increase in reflex forearm vasoconstriction (in response to -50 mmHg of lower body negative pressure) 30 min after exercise, Cleroux et al. (7) found forearm vascular responses to arterial baroreceptor unloading to be unaffected by prior exercise, and Bjurstedt et al. (2) observed marked impairment of orthostatic tolerance during the first 0.5 h of recovery from exhaustive exercise of short duration. The microneurographic technique used in our study allows direct quantitation of central sympathetic outflow
----I---
Post-exercise
Pre-exercise
2. Calf vasodilation after exercise and vehicle (open circles) (* P < 0.05) was abolished by naloxone (solid circles), but mean values for sympathetic nerve activity were unchanged. (Data are from 7 subjects in whom complete data on calf vascular resistance and muscle sympathetic nerve activity were available.) FIG.
fore and after exercise, but in individual subjects changes in resting mean arterial pressure 60 min after exercise were inversely related to changes in sympathetic activity. Naloxone, given before exercise, did not prevent postexercise hypotension but transformed these inverse correlations into positive relationships and attenuated both calf and peripheral vasodilation after exercise without altering mean values for MSNA. Attenuation of vasodiVehicle
(n=9; I= -0.66;
Naloxone
pdI.05)
(nd;
I
Vehicle
(n=8; I= -0.76; p
KAZUHIRO,
on hemodynamics
AND JOHN S. FLORAS. Effects of mdoxone and sympathetic activity after exercise. J.
Appl. Physiol. 73(5): 2028-2035, 1992.-The effects of highdose naloxone (0.4 mgkg iv) on hemodynamics and muscle sympathetic nerve activity (MSNA) after exercisewere studied in nine normotensive young men randomly allocated the opioid antagonist or vehicle 30 min before treadmill exerciseat 70%of resting heart rate reserve. Mean arterial pressure (MAP) was lower after exercise;cardiac output wasincreased.Mean values for MSNA and plasmanorepinephrine were similar before and after exercise, but in individual subjects changes in resting MAP 60 min after exercisewere inversely related to changesin sympathetic activity, suggestingthat arterial baroreflex regulation of MSNA had been shifted to a lower set point. Naloxone did not prevent postexercise hypotension but transformed theseinverse correlations into positive relationships. Naloxone attenuated both calf and systemicvasodilation without altering meanvalues for MSNA, indicating a peripheral effect of opioid antagonism.In normotensivesubjects,naloxone alters the regulation of sympathetic outflow and vascular resistanceduring recovery from exercise but doesnot prevent the fall in MAP. atria1 natriuretic factor; baroreceptor reflexes; Doppler cardiac output; endorphins; musclesympathetic nerve activity; opioids
endurance exercise is often followed by a persistent decrease in blood pressure (1, 2,16). The absence of reflex tachycardia and the altered vasoconstrictor responses to orthostatic stimuli seen in conjunction with the fall in blood pressure in these studies suggest that neuroregulatory mechanisms may be reset after submaximal or exhaustive exercise. Indeed, postexercise hypotension tends not to be accompanied by anticipated reflex increases in efferent muscle sympathetic nerve activity (MSNA) (14). The blood pressure and heart rate of spontaneously hypertensive rats (SHR) are also lower after exercise. This depressor response can be reversed by naloxone (lo-20 mg/kg iv) (37). Also, sustained and parallel reductions in blood pressure may be seen after exercise is simulated by electrical stimulation of the sciatic nerve or gastrocnemius muscle (37). Reductions in blood pressure after such stimulation is not accompanied, as might be expected, by baroreflex-mediated tachycardia and sympathoexcitation; indeed, splanchnic sympathetic nerve activity (SNA) is reduced in parallel with blood pressure. Both the fall in blood pressure and the reduction in efferent nerve activity in these preparations can be abolished by pretreatment with naloxone (lo-15 mg/kg iv) (37).
A SINGLE
2028
BOUT
OF PROLONGED
These observations, and supporting evidence from other experimental and human studies, led to the hypothesis that prolonged rhythmic exercise can reset the arterial baroreceptor centrally by activating sympathoinhibitory opioid systems (37). Evidence that central opioid pathways are involved in the autonomic adjustments to exercise in humans is sparse. Only some of the experimental evidence to date supports the concept that circulatory, metabolic, endocrine and thermoregulatory adjustments to submaximal and maximal exercise are influenced by pretreatment with high doses of naloxone (5,17,21,33,35), a selective and specific opioid antagonist with high affinity for p-receptors (30). The effects of naloxone on hemodynamics, MSNA, and plasma norepinephrine concentrations during the recovery period after exercise have not been defined. Our aims in this study were 1) to determine hemodynamic and neurohumoral sequelae to treadmill exercise in young untrained normal men and 2) to determine if hemodynamics and sympathetic activity after exercise can be altered by prior administration of high-dose naloxone. MATERIALS
AND
METHODS
Subjects
We studied nine young untrained normotensive subjects. Their mean age was 27 t 2 (SE) yr, their mean weight was 78 t 4 kg, and their mean body surface area was 1.93 t 0.05 m2. A medical history, physical examination, and laboratory investigations excluded hypertension, concurrent illness, and use of medication. All subjects gave an informed written consent. The study was approved by the Human Subjects Review Committee of the University of Toronto. Procedures Microneurogruphy. Multiunit recordings of postganglionic SNA were obtained with a unipolar tungsten electrode inserted selectively into muscle-nerve fascicles of
the peroneal nerve, posterior to the fibular head (39). The raw neurogram was amplified (by 20,000~50,000), filtered (at a bandwidth of 700-2,000 Hz), rectified, and integrated (time constant 0.1 s) to obtain a mean voltage display of MSNA. Bursts were differentiated from those arising from skin sympathetic nerves using previously accepted criteria (14,39). MSNA was quantitated both in
0161-7567/92 $2.00 Copyright 0 1992 the AmericanPhysiological Society
Downloaded from www.physiology.org/journal/jappl by ${individualUser.givenNames} ${individualUser.surname} (129.186.138.035) on January 16, 2019.
ENDORPHINS terms of burst frequency (burstsfmin) and (to adjust for the pulse synchronous nature of MSNA) burst incidence (bursts/l00 cardiac cycles). Plethysmography. Calf blood flow (ml min-’ 100 ml of calf volume-‘) was estimated by venous occlusion plethysmography. A Whitney strain gauge was applied to the midportion of the calf. Circulation to the foot was interrupted by an ankle cuff inflated to 180 mmHg. A second cuff was applied above the popliteal crease. Sequential 50-mmHg inflations and subsequent deflations of the proximal cuff were timed to give four estimates of calf blood flow per minute. Echocardiography. With subjects in the left lateral position, the parasternal long axis view was used to measure the aortic ring diameter (II) as well as left ventricular and left atria1 dimensions (~-MHZ mechanical scan transducer, Ultramark 8, Advanced Technology Laboratories, Bothell, WA). Cardiac output was calculated from the product of stroke volume and heart rate. Stroke volume was calculated from the product of the mean timevelocity integral and the cross-sectional area of the aortic orifice (A) calculated as A = r X (D/2)2 (18). With subjects supine we measured instantaneous flow velocity in the ascending aorta using continuous wave Doppler directed through the suprasternal window (2.25MHz CW Doppler transducer, Advanced Technology Laboratories). The continuous Doppler beam was initially directed to the aortic valve and then adjusted to obtain the maximum aortic flow velocity. Doppler signals were recorded onto super VHS videotape simultaneously with the electrocardiogram and the phonocardiogram. Timevelocity integrals recorded during held expiration were planimetered (Nova Microsonics, Mahwah, NJ). At least 10 consecutive cardiac cycles were averaged to derive a mean value for time-velocity integrals for these calculations. l
l
Protocol Study design. High-dose naloxone (0.4 mg/kg iv) or vehicle was administered 30 min before exercise in a random double-blind placebo controlled study design. Experiments were performed on two occasions with an interval of 1 mo, in the same laboratory at constant room temperature and humidity, and at the same time of day. Subjects were instructed to maintain their usual diet before the study, to avoid exercise for 48 h before the study, and to avoid alcohol, caffeine-containing beverages, and tobacco during the 12 h preceding each study day. Subjects were studied supine. An indwelling salinelock catheter (20 gauge, Deseret Medical, Sandy, UT) was introduced into a right arm antecubital vein for blood sampling. Blood pressure was measured by an automatic cuff recorder (Physio-Control Lifestat 200, Redmond, WA). Respiratory fluctuations were measured continuously using a respiratory belt connected to a Statham P23 ID pressure transducer (Gould, Cleveland, OH). Preexercise. After ZO-min supine rest, blood pressure was measured every minute, and heart rate, the electrocardiogram, respiratory excursions, and the mean voltage neurogram (left peroneal nerve) of muscle sympathetic nerve activity were recorded continuously (Gould
AND EXERCISE
2029
model 2800s ink recorder) over a 15-min baseline period. The last 5 min were used to represent preexercise baseline values. Calf blood flow (right leg) was measured over the last 2 min of this period. The microelectrode was removed from the peroneal nerve; venous blood was withdrawn for the determination of plasma norepinephrine by high-performance liquid chromatography with electrochemical detection (l3), atria1 natriuretic factor (ANF) by radioimmunoassay of extracted plasma samples (13), and luteinizing hormone concentrations by radioimmunoassay; and Doppler cardiac output was determined. We then gave either naloxone (0.4 mg/kg in a concentration of 10 mg/dl; Narcan, DuPont Pharmaceuticals) or vehicle (DuPont Pharmaceuticals) by intravenous bolus over 10 min. Blood pressure and heart rate were monitored for a further 20 min. Subjects then exercised at their assigned target heart rate for 45 min. Exercise. Exercise was performed on a commercially available treadmill (T2000, Complex Engineering, Markham, Ontario, Canada) with a speed and grade that could be adjusted to maintain subjects’ heart rates at 70% of their resting heart rate (RHR) reserve (11). The latter was calculated as 0.7 [ (220 - age) - RHR] + RHR. Cooled by a nearby fan, subjects exercised for 45 min. Small amounts of water were provided ad libitum. Korotkoff phase V was used to determine diastolic blood pressure during exercise. Postexercise. Subjects lay supine after exercise. Blood pressure, heart rate, the electrocardiogram, respirations, and MSNA (right peroneal nerve) were recorded continuously until 60 min after exercise. As in our previous protocol (14), data obtained over the last 5 min of this period were used to represent postexercise values. Calf blood flow (left leg) was measured over the last 2 min of this hour, after which the microelectrode was removed, a second series of blood samples was obtained, the left leg was lowered, and cardiac output was calculated. The latter measurements were acquired -90 min after exercise. Derived
Values
Mean arterial pressure (MAP) was calculated as the diastolic pressure plus one-third of the pulse pressure. Total peripheral resistance (TPR) was estimated by the formula TPR (dyn se cmm5) = 80 X MAP (mmHg)/CO l
where CO is cardiac output. Calf vascular resistance units) was calculated as the pressure divided by the mean calf blood flow. Plasma volume was derived tration and hematocrit (12).
(l/min)
(expressed as resistance quotient of mean arterial of four to six measures of from hemoglobin
concen-
Statistics
Data were expressed as means +- SE. We used both paired t test and two-way analysis of variance (ANOVA) for comparisons between pre- and postexercise values. P values of ~0~05 were considered significant. To quantitate the effect of naloxone on the relationship between blood pressure and (direct and indirect)
Downloaded from www.physiology.org/journal/jappl by ${individualUser.givenNames} ${individualUser.surname} (129.186.138.035) on January 16, 2019.
2030
ENDORPHINS
AND EXERCISE
FIG. 1. Systolic and diastolic blood pressures and heart rates (means t SE; n = 9) at baseline, during and after injection of either vehicle or intravenous naloxone (IV), during exercise, and in recovery period. Open circles, on vehicle day; solid circles, on naloxone day. (Diastolic blood pressure is deleted from exercise component.)
200 n E CL
150
P CD z
a
+& L
100
z x
.-
Treadmill
0
Exercise
Supine
.
rIl’~~~“~‘*‘5 10 15
5 10 15 M
25 30 5 10 15 20 25 30 35 40 45 5 10 15 20 25 30 35 10 45 50 55 60
indexes of sympathetic activity, we employed a permutation test based on a randomized reference distribution (4). Under each drug condition, we obtained the Pearson correlation between the percent change in the two variables of interest and calculated an absolute difference between these two correlation coefficients obtained on the naloxone and placebo days. Assuming the null hypothesis that naloxone has no effect on the correlation between blood pressure and that particular measure of sympathetic outflow, and assuming random allocation of vehicle or naloxone, this difference should be independent of the order that each drug was allocated to the subjects. We calculated the 2n possible correlations between these variables based on the 212potential placebodrug allocations for the n subjects and counted the number of correlations in which such absolute differences were at least as large as we observed in our study. The P value was derived from the ratio of this number to the total number of possibilities. P values of ~0.05 were considered significant. RESULTS
Preexercise. There were no symptoms such as nausea, drowsiness, or dysphoria as clues to whether subjects had received intravenous naloxone or vehicle, and neither blood pressure nor heart rate increased over the 30-min observation period after this injection (Fig. 1). Exercise data. All nine subjects completed the exercise protocol. Mean work loads during exercise were comparable on the 2 study days (treadmill grade 13.9 & 1.2 vs.
14.7 t 0.9%; speed 7.4 & 0.3 vs. 7.4 t 0.4 km/h). Mean work distance was also similar on the 2 study days (5.5 t 0.2 km after vehicle; 5.5 t 0.2 km after naloxone). Although average achieved heart rates at this work load were higher on the vehicle day (146 t 5 beats/min) than on the naloxone day (139 t 6 beats/min; P < 0.05), the ratios of achieved heart rate to target heart rate were similar (92 t 2% after vehicle; 87 t 3% after naloxone) (Fig. 1). Naloxone had no effect on blood pressure during exercise (2-way ANOVA). Neurohumoral and hemodynamic variables after exer-
cise and naloxone. The effects of exercise and naloxone on these variables appear in Tables 1 and 2. Mean and diastolic blood pressures were lower and heart rate higher 60 min after exercise on both study days. This postexercise hypotension was caused by a reduction in peripheral resistance rather than cardiac output. Although this modest postexercise hypotension had dissipated by 90 min after exercise, cardiac output remained above resting values (P < 0.002 after vehicle; P < 0.05 after naloxone), because of significantly higher heart rates during the recovery period (P < 0.005 on both days). Total peripheral resistance was lower after exercise on the vehicle (placebo) day (P < 0.004), but not after naloxone, because of a trend to higher mean arterial pressure and lower cardiac output after naloxone compared with vehicle (Table 2). Calf vascular resistance was lower 60 min after exercise on the vehicle (placebo) day (P < 0.02). Calf vasodilation was absent after naloxone (Table 1, Fig. 2). Exercise had minor nonsignificant effects on plasma volume on both study days (vehicle -2.3 t 3.6%, n = 7;
Downloaded from www.physiology.org/journal/jappl by ${individualUser.givenNames} ${individualUser.surname} (129.186.138.035) on January 16, 2019.
ENDORPHINS TABLE
1. Hemodynumic
und neurohumorul
2031
AND EXERCISE
vuriubles before and 60 min after treadmill
exercise
Vehicle Variable
Preexercise
Systolic blood pressure, mmHg Mean arterial pressure, mmHg Diastolic blood pressure, mmHg Heart rate, beatsfmin Calf blood flow, ml. min-’ 100 ml-’ Calf vascular resistance, units Muscle sympathetic nerve activity, burstslmin Muscle sympathetic nerve activity, bursts/IO0 cardiac cycles Plasma norepinephrine, nmol/l Atria1 natriuretic factor, pg/ml Luteinizing hormone, W/l Plasma volume, ml/100 ml blood l
Naloxone Postexercise
12lt3
Preexercise
Postexercise
117t3
121k3
119+2
7813-f
80+2*
67+4"f 4.5t0.6 2Ok3*
85t3 67t3 57k4 3.0t0.4 31t4
3.OkO.4 (8) 30t4 (8)
19+3
21t5
23~13
27k5
34s 1.2~0*2 24t3 3.1+0.5 60tl
32t6 1.6t0.3
38t3
38+6 (8) 1.2kO.2 18+2* 4.o-to.4* 59+2 (7)
83t3 64t3 57-t4 3.2t0.3 28+3
59+3t
(8)
60t3t 65t4t
0.9t0.2
19t2*
22+3 2.4kO.5 61+1
2.4tU.5 58t2 (7)
(8)
Values are means t SE in 9 subjects except where indicated in parentheses. * P < 0.05; “f P < 0.005.
naloxone -2.4 t 2.7%, II = 7). Plasma ANF was lower after exercise on the both study days; this decrease occurred in the absence of changes in either left atria1 diameter index (e.g., 14.4 t 0.5 vs. 14.4 t 0.7 mm/m2 before exercise; n = 8 on the naloxone day) or left ventricular end-diastolic diameter index (22.7 t 3.7 vs. 25.6 t 1.1 mm/m2 before exercise, n = 6). No naloxone-specific effects were detected, but twoway ANOVA documented significant naloxone-exercise interactions for plasma luteinizing hormone concentration (P = 0.023) and a trend toward similar interactions for calf blood flow (P = 0.078) and calf vascular resistance (P = 0.068). High-quality mean voltage neurograms were obtained on all four sessions (i.e., both study sessions on both study days) in eight of these nine subjects, and paired data for plasma norepinephrine on both study days were obtained in eight of the nine subjects. These data also appear in Table 1. Despite the fall in mean arterial blood pressure 60 min after exercise, average mean values for MSNA and plasma norepinephrine concentration at this time were not increased reflexively above baseline on either day. Analysis of individual relationships between changes in blood pressure and changes in indexes of sympathetic outflow on the vehicle day revealed an inverse correlation between changes in plasma norepinephrine concentration and changes in mean arterial pressure (r = -0.76; n = 8; P < 0.005) and an inverse correlation between changes in MSNA and changes in mean arterial TABLE
2. Hemodynumics
before and 90 min after treadmill
pressure (r = -0.66; rz = 9; P = 0.05) after exercise (Fig. 3). The inverse relationship between plasma norepinephrine concentration and changes in mean arterial pressure was transformed into a positive relationship by naloxone (from -0.76 to +O.Sl; n = 8; P < 0.05 by the permutation test) (Fig. 3). In contrast to the inverse relationship documented on the vehicle day, there was a significant positive correlation between changes in sympathetic nerve activity and changes in mean blood pressure (r = 0.77; rz = 8; P < 0.05) (Fig. 3) after exercise and naloxone. Naloxone abolished calf vasodilation after exercise without affecting mean values for peroneal nerve muscle sympathetic activity (Fig. 2, Table 1). Naloxone appeared to transform an inverse correlation between changes in plasma norepinephrine and changes in calf vascular resistance on the vehicle day (r = -0.46; n = 8; P = 0.25) into a positive relationship (r = +0.71; n = 8; P < 0.05) (Fig 4). DISCUSSfON In this study of healthy untrained young normotensive subjects, 45 min of treadmill exercise at ~60% maximal oxygen consumption (11) were followed by a reduction in blood pressure. Postexercise hypotension was caused by a fall in peripheral and calf vascular resistance; cardiac output was increased. Mean values for MSNA and plasma norepinephrine concentration were similar beexercise
Vehicle
Naloxone
Variable
Preexercise
Postexercise
Preexercise
Postexercise
Systolic blood pressure, mmHg Mean arterial pressure, mmHg Diastolic blood pressure, mmHg Heart rate, beatslmin Stroke volume, ml Cardiac output, l/min Total peripheral resistance, dyn s cmm5
118t3 81t3
116t2
116t2 79t3
118t2
62t3 56t3 7Ok-5
59t3
l
l
3.9kO.3 1,739+164
78t3 66+4$ 67t5
4.4+0.3t 1,516*178$
6Ozk3 57t4
69k7 3.8-+0.3 1,764,168
80-r-3 61t4 63-t4$ 67+6 4.2t0.4* 1,659*234
Values are means + SE for 9 subjects. * P < 0.05; t P < 0.01; $ P < 0.005. Downloaded from www.physiology.org/journal/jappl by ${individualUser.givenNames} ${individualUser.surname} (129.186.138.035) on January 16, 2019.
2032
ENDORPHINS
). 40 .r: > .zi a 30 Q1 LF S-E .-*g z;20ra dV cu e 57 lo-
ia
AND EXERCISE
lation would therefore appear to be a peripheral effect of naloxone. Reductions in blood pressure after running exercise have been observed in humans and SHR (1, 14, 16, 37). Withdrawal of sympathetic vasoconstrictor tone, as a result of activation of central opioidergic systems during exercise, may mediate this response in SHR (37), but mechanisms of postexercise hypotension in humans appear to be more complex. In our subjects the vasodilation during recovery from exercise could not be attributed to withdrawal of vasoconstrictor tone, as significant reductions in calf vascular resistance were not paralleled by decreases in peroneal MSNA, Several indirect observations suggest that neural mechanisms might contribute to postexercise hypotension in humans (1, 2, 7, 8, 10, 14, 16). In normotensive subjects, baroreflex control of heart rate is enhanced for up to 24 h after exercise to exhaustion (lo), whereas the effects of prior exercise on the arterial baroreflex control of forearm vascular resistance have not been clearly established. Bennett et al. (1) reported a fivefold increase in reflex forearm vasoconstriction (in response to -50 mmHg of lower body negative pressure) 30 min after exercise, Cleroux et al. (7) found forearm vascular responses to arterial baroreceptor unloading to be unaffected by prior exercise, and Bjurstedt et al. (2) observed marked impairment of orthostatic tolerance during the first 0.5 h of recovery from exhaustive exercise of short duration. The microneurographic technique used in our study allows direct quantitation of central sympathetic outflow
----I---
Post-exercise
Pre-exercise
2. Calf vasodilation after exercise and vehicle (open circles) (* P < 0.05) was abolished by naloxone (solid circles), but mean values for sympathetic nerve activity were unchanged. (Data are from 7 subjects in whom complete data on calf vascular resistance and muscle sympathetic nerve activity were available.) FIG.
fore and after exercise, but in individual subjects changes in resting mean arterial pressure 60 min after exercise were inversely related to changes in sympathetic activity. Naloxone, given before exercise, did not prevent postexercise hypotension but transformed these inverse correlations into positive relationships and attenuated both calf and peripheral vasodilation after exercise without altering mean values for MSNA. Attenuation of vasodiVehicle
(n=9; I= -0.66;
Naloxone
pdI.05)
(nd;
I
Vehicle
(n=8; I= -0.76; p
