Effects of norepinephrine on oxygenation of resting skeletal muscle R.F.COBURN Department Philadelphia,

ANDM.PENDLETON

of Physiology,

Pennsylvania

University 13104

of Pennsylvania

COBURN, R. F., AND M. PENDLETON. Effects of norepinephrine on oxygen&ion of resting skeletaZ muscle. Am. J. Physiol.

236(Z): H307-H313, 1979 or Am. J. Physiol.: Heart Circ. Physiol. 5(2): H307-H313, 1979.-Effects of arterial infusions of norepinephrine (NE) were studied in the canine skinned hindlimb. Myoglobin Paz changeswere estimated from shifts of carbon monoxide between blood and muscle. At an NEI concentration just below threshold for an effect on vascular resistance, myoglobin PO, and oxygen uptake rate (0%) did not change. At NE concentrations at which vascular resistance increased 117-216%and blood flow (Q) fell to 55438%of control, oxygen extraction per unit flow increasedbut myoglobin PO, and Vo, did not change significantly. At higher NF concentrations, with vascular resistance1974178%and blood flow 1745% of control, CO shiffsd out of blood into tissue, indicating a fall in myoglobin Po2, and oxygen up&&e &e fell as a function of &, despite increasesin oxygen extraction per unit flow. During these high-concentration NE infusions, the tissue became hypoxic under conditions in which femoral venous PO, was much greater than is seen during arterial hypoxemia experiments. We concludedthat NE at concentrations at which there were modesteffectson vascular resistance did not interfere with normal autoregulation of intracellular Poe, suggestingthat constriction of resistancevesselsdid not alter the capillary recruitment mechanism. The inability of skeletal muscleto maintain intracellular PO, during high NE concentrations under conditions in which maximal extraction of oxygen was not achieved is best explained by an effect of NE on the capillary recruitment mechanism. myoglobin PO,; microcirculation

OF OXYGEN to the skeletal muscle cell is critically dependent on the microcirculation. Previous studies have stressed the importance of local control of tissue oxygenation and also transcapillary exchange of other substrates by precapillary sphincters and resistance vessels. Precapillary sphincters, or an equivalent structure, appear to regulate the density of exchange vessels and, consequently, difiusion distances and capillary surface area (2,4,13,25,26,28); resistance vessels control total blood flow in the tissue. Experiments where- tissue PO, has been measured provided data which suggested oxygen tension in cells is finely regulated by local control mechanisms (oxygen autoregulation) under such extreme conditions as; hypoxic hypoxemia with arterial PO, as low as 40 mmHg (7, 8, 31) or under conditions where blood pressure was decreased by acute hemorrhage to about 60% of control (29).

DELIVERY

0363.6135/79/0000-0000$01.25

School

of Medicine,

The present study proposes to look at relationships between the microcirculation and tissue oxygenation in skeletal muscle by altering the circulation by constant infusion of norepinephrine (NE) into arterial blood and studying effects on intracellular Po2, oxygen uptake, and oxygen extraction as well as hemodynamic parameters. Thus, we looked at adaptation of the tissue to low flow states under conditions where resistance vessels could not participate (by dilatation) in oxygen autoregulation. We designed these experiments with the following questions in mind: a) Can precise autoregulation of intracellular PO, operate under conditions where resistance vessels are constricted? b) Does NE have an effect on microcirculatory mechanisms by which skeletal muscle is able to increase oxygen extraction &om blood in responses to falls in oxygen delivery? Norepinephrine effects on oxygenation of skeletal muscle in vivo have been studied previously with infusions into arterial blood or stimulation of sympathetic nerves in several different preparations with somewhat conflicting results. Studies on perfused canine hindlimb or gracilis muscle have found that oxygen uptake (VoJ falls during sympathetic stimulation resulting in increases in vascular resistance (21,221. No change in V% (13) or an increase in 00, (27) has been observed during NE infusions resulting in modest decreases in blood flow rate (13). Increases in V% were seen during sympathetic stimulation at low frequencies giving modest increases in vascular resistance; decreases in Vo, were seen during sympathetic stimulations at high frequenties resulting in large increases in vascular resistance (10). Norepinephrine infusions caused a fall in capillary filtration coefficient (CFC) (1) in the isolated perfused canine forelimb and an increase in CFC in the isolated dog hindlimb (13). Sympathetic stimulation caused a fall in 86Rb extraction in the isolated perfused canine gracilis muscle (26). In the isolated perfused cat hindlimb, constant NE infusions caused an initial fall in CFC followed by an increase (5) or an increase in CFC that was inhibited by propranolol(l7,18). The causes of conflicting findings in different studies in similar prep arations and species are not known but the existing data suggest: a) that NE may have a direct effect on the skeletal muscle cell causing an increase in aerobic metabolism which can be observed with NE concentrations low enough not to have marked effects on blood flow, b) decreases in Va seen during NE infusion result from falls in oxygen delivery and resultant falls in

Copyright 0 1979 the American Physiological Society

H307

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H308

R. F. COBURN

intracellular Paz, c) there may be a direct effect of NE on the precapillary sphincter (or equivalent structure if precapillary sphincters per se do not exist in, skeletal muscle (ll)), which in skeletal muscle appears to control opening or closing of capillaries, and d) it is possible that under some experimental conditions, compensatory (metabolic) dilatation occurs with resultant capillary recruitment that opposes direct effects of NE on microcirculatory vessels. Our data showed an increase in O2 extraction from blood during NE infusions which, at small NE concentrations giving modest increases in vascular resistance, completely compensated for decrease in oxygen delivery to the tissue and maintained mean myoglobin Paz near control level. During NE infusions at concentrations that caused marked increases in vascular resistance and falls in blood flow, the tissue became hypoxic. In these runs the efficiency of oxygen extraction from blood perfusing the tissue wis less than the blood seen during arterial hypoxemia. This is most likely due to a direct -antagonistic effect of NE on the mechanism of oxygen autoregulation. METHODS

Eight mongrel dogs (20-25 kg) were anesthetized with pentobarbital (30 mglkg iv). Small increments were given throughout the experiment. A tracheostomy was performed and a cannula inserted. This was attached to a respirator-rebreathing system, which has been described previously (19). This system prevented significant loss of carbon monoxide from the body stores during our experiments, which was important because of the method used to measure intracellular Paz. The respirator was adjusted to keep arterial pH and Pco2 as normal as possible. Small amounts of HCO,- were injected periodically during the experiment to counteract ‘metabolic acidosis. The skinned-hindleg preparation utilized in our experiments is shown in Fig. 1. The deep circumflex artery (and in some experiments the internal iliac artery) and veins were ligated. The skinned area was covered by cellophane during the experiment. A thermi&or or thermometer was placed under the cellophane and the temperature kept at 98’F with an infrared lamp placed over the preparation. We used this preparation because a primary goal was to disturb the preparation as little as possible. We know that the circulation of the hindlimb was not entirely isolated to inflow via the iliac artery and outflow via the femoral vein, because occlusion of the femoral vein reduced arterial flow only to approximately 10% of the preoccluded value eitherunder control conditions or during NE infusion. However, we accepted this source of error and think it does not have much effect on our data or conclusions. We did not disturb innervation to muscle or blood vessels and the results obtained reflect an intact tissue response including the possibility of neural reflex effects. The electromagnetic flow probe (Statham model M400) was calibrated with saline prior to the study and zeroed by complete occlusion of the vessel distal to the probe. We occasionally checked the calibration at the a

AND M. PENDLETON

NOREPI TO REBREATHING RESPIRATOR

FLOW PROBE FEMORAL VEIN

-&

BLOOD PRESSURE

FIG. 1. Canine skinned-hindlimb preparation. Left leg was skinned from ankle to body. A wire was tied tightly around the ankle to remove paw from preparation (not shown in this figure). NE injection catheter was inserted in right femoral artery and tip positioned just proximal to aortic bifurcation. The deep circumflex artery and vein and, in some experiments, the internal iliac artery and vein, were ligated. A flow probe was placed on the left iliac artery just superior to the inguinal ligament. A l-mm ID catheter was inserted into the left femoral vein without obstructing venous flow and the tip positioned just superior to the inguinal ligament. Blood pressure was measured via a catheter placed in a carotid artery.

end of the experiment by infusing saline at known rates through the in situ vessel. We did not correct for effects of change in the hematocrit on the flow probe signal. Norepinephrine solution was infused at a constant rate (0.1 to 0.5 ml/min) using an injection pump giving NE infusion rates of 1 x lo-* to 3 x 10m6 g/(min kg). We could not accurately estimate arterial blood NE concentrations since the blood flow rate at the tip of the catheter in the aorta was not known. Estimation of changes in muscle intracellular Paz was made from measurements of loss of carbon monoxide from blood perfusing the preparation. Previous studies have indicated that the partition of CO between that bound to hemoglobin in blood and that bound to myoglobin, is a function of myoglobin PO, and not sensitive to changes in capillary Paz over the range 3080 mmHg (7, 8). Thus loss of CO from blood perfusing the tissue indicates an increase in MbCO % saturation and a fall in intracellular PO,. Two to three runs were performed in each experiment. Prior to run 1, 10 ml of 100% CO were slowly added to the rebreathing system, which resulted in an increase in HbCO to 253.0% saturation, a range where it is technically easier to measure small shifts of CO out of blood from measurements of arterial and venous blood HbCO. Succinylcholine or n-tubocurarine was injected prior to each run. At the start of each run, several control arterial and femoral venous blood samples were drawn simultaneously for analysis of blood gases, 0, content, and HbCO % saturation. The NE infusion was then started and arterial and femoral venous samples were drawn every minute, at rates of approximately 0.2 ml/s, for 3-4 min. The drug infusion was stopped and the preparation allowed to recover for

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SKELETAL

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45 min prior to the second run. The first NE infusion was performed at a rate of injection which was shown to be just below the threshold (usually l-2 x lo-* g/ (min. kg)) for an effect on blood flow and vascular resistance. Succeeding runs used higher NE infusion rates that were randomly varied. Oxygen uptake was computed from measurements of oxygen content in arterial and femoral venous blood and blood flow rates. CO net flux between blood and tissue was determined from measured (a-v)HbCO (6) multiplied by mean flow rate during the time of sampling. Blood P%, Pco,, and pH were determined using appropriate electrodes. Oxygen content was either computed from Paz and pH using a standard canine oxygen dissociation curve or measured directly (Lex-O&on, Lexington Instruments). RESULTS

Control data from 21 runs in nine experiments are listed in Table 1. There was considerable scatter of blood flow, vascular resistance, and oxygen uptake data, some of which may be explained by markedly different muscle mass in dogs of different species and weights. We have not quantitated these data in terms of muscle mass because we are uncertain about the extent of perfusion of gluteal muscles in this preparation. Table 1 illustrates the stability of control flow and 00, measurements in consecutive runs. Our preparations all exhibited marked reactive hyperemia following a 10-s occlusion of the iliac artery. In parallel experiments (unpublished) it was shown that the preparation was capable of oxygen autoregulation during progressive hypoxic hypoxemia, in that intracelTABLE

1. Base-line

data Mean

Expt

1A 1B 2A 2B 2c 3A 3B 4A 4B

T;

20 28

22 24

4c

5A 5B 5C 6A 6B 7A 7B 8A 8B 9A 9B

21

20 21 23 21

Q, mllmin

73 73 264 280 214 37 36 60 70 70 120 110 120 35 27 26 31 39 41 73 55

h, mllmin

1.6 2.3 14.2 14.0 12.1 2.0 2.1 2.5 2.3 2.4 2.0 2.9 2.3 1.3 0.95 1.3

1.1 1.6 1.3 2.3 2.8

3.59 Mean 88.3 *SE k16.8 zko.93 &, flow; oo,, 0, uptake.

AI% lickPressure,

98 100 105 110 110 104 116 92 96 96 116 120 116 140 140 100

116 112 104 104

100 109.3 22.9

Arterial

Femoral

‘@* -Hg

PH

101

7.27 7.32 7.25 7.25 7.23 7.27 7.32 7.37 7.35 7.39 7.33 7.37 7.42 7.34 7.34 7.32 7.34 7.36 7.33 7.25 7.42

92 71 90 82

101 118 111

100 98 125 124 108 107 114 100 128 94 90 73 75

100.1 23.7

7.33

to.01

P% mmHg

53 47 38 43 44 45 45 43 45 44 65 57 57 52 52 47 54 47 50 53 44 48.8 Al.4

Venous

PH

7.29 7.27 7.26 7.24 7.23 7.29 7.29 7.35 7.34 7.32 7.33 7.34 7.38 7.31 7.28 7.32 7.32 7.33 7.32 7.33 7.37 7.31 20.01

lular PO, was maintained until Pao, fell to 35-40 mmHg and femoral venous PO, to about 20 mmHg, as demonstrated previously in our laboratory using a slightly different preparation (7). The pattern of iliac arterial flow during NE infusion varied in different experiments. About half of the runs showed an initial increase in iliac flow followed by reduction in flow. In most runs the fall in flow was progressive during the 3- to 4-min infusion period; however, in some experiments flow decreased and then “escaped.” In all experiments there was a marked hyperemia after discontinuation of NE infusion. Figure 2 illustrates the most typical hemodynamic pattern seen in these experiments. Tachyphylaxis was usually observed in that second and third runs required higher rates of NE infusion to cause equivalent falls in iliac arterial flow. Systemic arterial blood pressure was unchanged in runs using NE infusion rates that had no effect or moderate effects on iliac blood flow. In runs using the highest NE infusion rate, systemic mean pressure increased 2-10 mmHg. Femoral venous pressure did not change during infusion. In the five runs where NE infusion rates were below threshold for an effect on resistance vessels (group A, Table 2), there was no consistent effect on oxygen uptake, CO partition between blood and tissue, oxygen extraction, or femoral venous. PO&. Figure 3 shows a plot of Vo, measurements during runs where NE concentrations were sufficient to cause a decrease in iliac arterial flow. No consistent change in Voz was detectable with NE infusions where flow fell from 88% to 55% of control; however, in runs where larger falls in flow were seen, marked decreases in Voz were observed. Figure 4 shows the difference between arterial and femoral venous HbCO ((a-v)HbCO) determined in the same experiments as Vo, data plotted in Fig. 3. (a-v)HbCO was not significantly different from zero under conditions where flow fell only modestly, 88% to 55% of control, and, as shown in Fig. 3, Vo, did not fall. When flow fell below approximately 55% of control, (a-v)HbCO became positive indicating shifts of CO into tissue. Thus, CO shifts occurred under conditions where Vo, was less than control value (Fig. 3). We divided the data, on the basis of findings in Figs. 3 and 4, into group B where flows fell to 55-88% of control and group C where flows fell to values below 55% of control. In the group B runs where Voz did not change and CO did not shiff, femoral venous Pop always fell compared to the control prior to NE, and oxygen extraction always increased (Table 2). In group C runs where CO shifted into the tissue and 00, fell, femoral venous PO, decreased to lower levels than found in group B runs and oxygen extractions were greater than found ingroup B runs. CO shiRs (positive (a-v)HbCO) were seen during the first 30 s after fall in flow below 50% of control, and CO continued to shift out of perfusing blood for at least 4 min after fall in flow. After cessation of NE infusion, CO shifted out of muscle into blood over at least a 4-min period.

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H310

R. F. COBURN

AND M. PENDLETON

200 Arterial Blood Pressure mm Hg

I loo FIG. 2. Systemic arterial blood pressure and iliac arterial mean blood flow, during constant NE infusion into distal aorta. Blood pressure recording was not obtained during sampling of arterial blood. Oscillations in blood pressure and flow were synchronous with respiration.

0

100 Arterial r /A “Mean” 50 Blood Flow ml/min 0 IT”-----Noiepi 6.1 x 10’7 g/(min x Kg) TABLE

T

50

Infusion Off

2. Effects of NE infusions Vascular Resistance, -HE! l/min

(a-v)02 c-g, ml

Venoue POS -43

h, ml/min

Group A (n = 5), NE below 2008 3.70 3.10 21970 kO.80 a.20 2090 3.30 3.18 t1850 k2.10 t1.30

Control NE

hK2, ml O,/ml flow

Mbco

resistance threshold 49.2 0.031 ~0.008 k3.0 49.7 0.033 No effect 24.2 eO.008

Group B (n = 13), flow fell to 5595% of control 49.1 0.031 1972 3.40 2.90 k2.2 to.003 t1517 to.80 20.27 45.0 0.048 No effect 3411 4.65 2.96 t907* t1.2* to.35 &2.4* t0.002*

Control NE

Group C (n = 22), flow fell to 1745% of control 2012 3.90 3.59 48.2 0.026 21870 tl.40 to.65 t2.0 kO.002 0.065 Increased* 12725 6.00 2.19 41.3 *2410* It1.7* 20.89" +1.7* to.o03*

Control NE

* Significantly

Data are given as means t, SD. paired analysis.

different by

E+o.3\ l e

0 P

0 -e-

----e-----e-

-0. It,

le 0

0

0

I

1

25

a

50

L

75

0

b

loo

Ar teriol Blood Flow (% Control) 4. Arterial-femoral venous differences in %HbCO saturation ((a-v)HbCO) as a function of iliac arterial blood flow. Plotted (av)HbCO are mean data from three sequential arterial and venous pairs drawn during the first 3 min of low flow. Plotted flow is mean flow during time period during which these samples were drawn. Line through data was drawn by hand. FIG.

0

120 IO!,

0

0

0

0

0

0

f

t0

20

0/

0

DISCUSSION

l 0

0

30 40 50 60 70 80 90 100

Arterial Blood Flow % Control 3. Comparison of iliac arterial flow and oxygen uptake (fro,). Data were obtained during constant NE infusion. Each point corresponds to data obtained from simultaneous arterial and femoral venous blood samples taken over a 30. to 40-s period. Each plotted flow value is the mean flow during the sampling period. Open square indicates control data just prior to start of NE infusion. Drawn line shows the least-squares slope of the relationship between arterial blood flow and 00, over the range 18-55% of control blood flow, assuming the relationship is linear. FIG.

Krogh theory (16) has been a basis for analysis of oxygenation of tissues, and although it is known to be an oversimplification, the concept that capillary PO, and the diffusion distance between capillary and mitochondria are important parameters is widely accepted. Experiments performed under conditions where oxygen delivery to skeletal muscle has been varied have indicated that recruitment of oxygen exchange vessels with resultant decreased diffusion distances is an important factor in maintaining oxygenation of muscle (2-4, 12, 13,15,20,25,26). As indicated above, in skeletal muscle there is evidence that capillary PO, and density of exchange vessels are finely regulated, under conditions of decreased oxygen delivery, to maintain intracellular PO, above hypoxic levels (7,8, 13,31). It is possible that O2 autoregulation mechanisms in skeletal muscle are due entirely to alterations in the microcirculation. The finding that there is a nearly linear relationship between CFC and O2 extraction per unit flow, during low

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SKELETAL

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flow rates in skeletal muscle (2, 13), supports this concept. The possible importance of changes in the 0, diffision coefficient in tissue or alteration in geometry of the capillary-tissue respiratory unit is not known. At the present time, it is reasonable to conclude that the limiting factor determining efficiency of oxygen extraction at a given capillary PO, is the diffision distance between capillary and mitochondria. It can be shown using the Krogh equation that prolongation of transit times without recruitment of capillaries can explain only a fraction of the increased 0, extraction per unit flow and cannot explain autoregulation of intracellular PO,. Granger et al. (13) provided evidence that both resistance vessels and exchange vessels were involved in oxygen autoregulation in skeletal muscle during low blood flow states. Recruitment of oxygen exchange vessels appeared to be most important at flow rates where venous PO, was not markedly decreased, but as venous PO, fell, dilatation of resistance vessels became important. Previous studies of.relative roles of these two known mechanisms have not included measurements of intracellular Po2, and it is possible that tissue Po2 varied widely. The advantages and limitations of the CO method of estimating mean myoglobin Paz have been discussed in detail previously (7,8). This method seemed appropriate for the present study because it gives a mean value for myoglobin PO, in myoglobin-containing muscle fibers. CO shifts out. of blood into muscle appear to reflect increases in carboxymyoglobin (MbCO) since the finity of myoglobin and CO at a given PO, is greater than the affinity of CO for reaction with other intracellular hemoproteins, and the concentration of myoglobin in red muscle is approximately 100 times greater than that of cytochrome a3 (9). Normal “mean myoglobin Po2” computed from muscle biopsy measurements of MbCO in canine hamstring muscle or canine myocardium is 46 mmHg. This value depends on the validity of the oxymyoglobin dissociation curve and equilibrium constant for the reaction of CO and oxymyoglobin used in the calculation, and this range of values could be only qualitative. We compute that in the present experiments changes in mean myoglobin PO, of 2 mmHg can be detected from measurements of shifts of CO out of blood. In our previous experiments (7, 8) it was found during progressive arterial hypoxemia that the partition of CO between blood and skeletal muscle remained constant as Pa, fell from 250 to 35-40 mmHg, but as Pa,, fell below this value there was a large shift of CO out of blood into skeletal muscle. A similar finding occurred in the canine heart (8), and shiffs into tissue occurred at the same Pa4 during progressive hypoxemia as reversal of a-v lactate concentrations. Other evidence that the CO shift in myocardium or skeletal muscle is due to fall in intracellular PO, to levels below the critical PO, for oxidative phosphorylation has been discussed previously (8). Calculated mean myoglobin Peg values, under conditions where CO has shifted into muscle, give computed values less than 1 mmHg. The lack of a CO shiftwith falls in Pa9 to levels as low as 35-40 mmHg or falls in mean arterial blood pressure to levels of 60-70 mmHg (29) (anesthetized dogs) is inter-

preted as indicating that relatively small falls in intracellular Paz occurred (computed falls of l-l .5 mm Hg) and that intracellular PO, is precisely autoregulated (7, 8)

‘We have made rough calculations showing that the shift of CO out of blood that occurred in the present experiments over a 3-min period in low flow rate runs was about that expected if mean myoglobin PO, dropped to levels similar to those seen with severe hypoxic hypoxemia where MbCO % saturation increased to levels as great, as 200% of control (7, 8). Assuming muscle mass of the hindlimb to equal 3% of total weight and an average myoglobin concentration of 3 mg/g (7), a doubling of MbCO corresponds to a shift of CO of about 0.1 ml. CO efYlux from blood in our experiments, where flow was less than 55% of control, was computed to average 0.14 ml in this 3-min period. The slow shift of CO is not explained, but probably is due. to slow decrease in mean myoglobin PO, resulting, at least in part, from the progressive decrease in flow that occurred over this time period. A limitation of our experiments is that the intracellular PO, probe, CO binding to myoglobin, gives information only about myoglobincontaining muscle fibers, whereas flow and Vo, data are influenced by the total cells in the preparation. However, the high myoglobin concentration in several muscle groups of the dog hindlimb suggests a high fraction of red fibers (7), and it is known that oxygen uptake and blood flow to red muscle is 3-10 times that of white muscle (12, 14, 23, 24). Furthermore, the critical flow below which CO shifted into tissue in our experiments was the same for fall in oxygen uptake of the tissue. It seems justified to assume that in our preparation oxygen uptake and flow rates reflect mostly myoglobin-containing muscle fibers. Many of our blood samples were collected at a time when flow rate was changing and there was not a steady state. We cannot compute effects on fro, and mean myoglobin PO, resulting from this lack of steady state. We were able to confirm the findings of others (21,22, 26) that NE in hindlimb arterial blood can decrease flow to the extent that oxygen uptake is impaired. The evidence obtained in the present study that intracellular PO, fell to low levels complements this finding and suggests the fall in Vo, was a result of fall in intracellular PO,. During NE infusions at concentrations which constricted resistance vessels, the efficiency of extraction of oxygen from blood perfusing the tissue always increased. In runs where increases in resistance and decreases in flow were modest, autoregulatory mechanisms were able to completely compensate for fall in oxygen delivery and to preserve intracellular PO, near control levels. But these mechanisms failed to keep myoglobin PO, normoxic when flows were reduced to less than 55% of control. In runs where falls in flow and 0, delivery were modest and myoglobin PO, was maintained near control, autoregulation of intracellular PO, was similar to that observed with falls in 0, delivery due to arterial hypoxemia (7,8) as discussed above or hypotension (29). Since autoregulation of intracellular PO, was appropri-

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R. F. COBURN

H312

AND M. PENDLETON

N E infusion A,rteriol Hypoxemia

exchange vessels or a decrease in uniformity of tissue OoJQ. There is support in the literature for a fall in capillary density during NE infusion (513, 26). Renkin and Rose11 (26) have cited arguments against the presence of significant true vascular shunts, in support of the rubidium method of estimating the capillary diffusing capacity. However, Baker (1) has published evidence that NE infusion in perfused canine forelimbs resulted in redistribution of flow to shunt vessels. It is evident that if, during NE infusion, venous blood leaving gas exchange capillaries had a PO, of 20-25 mmHg Femoral Venous PO, mmHg (the critical femoral venous PO, found during arterial hypoxemia), whereas femoral venous Poe was 40-45 FIG. 5. Comparison of critical femoral venous PO, observed durmmHg (respective oxygen contents of approximately 4 ing NE infusions and during arterial hypoxemia. “Critical Po2” is the PoZ below which CO shifted out of blood into muscle. CO shifts and 13 ml/l00 ml), that blood exiting from gas exchange are given in terms of ratio MbCO/HbCO where both are in percent capillaries would have to be diluted about threefold saturation. Control MbCO/HbCO is approximately unity (7, 8). NE giving a true shunt of about 70% of total blood flow in data: critical PO, (plot 1) was obtained from three experiments the tissue. Since it is unlikely that this large percent of where flows were 55-65% of control and CO shifts did not occur. We also plotted mean values from experiments where CO shifts occurred blood perfusing the hindlimb was shunted from gas (pZot 2). CO shifts were converted into MbCO/HbCO as described in exchange vessels during NE infusion, because there is DISCUSSION. Arterial hypoxemia data: ‘critical venous Peg (plot 3) evidence that NE has qualitatively similar effects on was determined in a previous study from shifts of 14C0 or 12C0 blood flow to red and to white skeletal muscle (12, 14, during progressive arterial hypoxemia. Plot 4 is mean femoral 24) and because of previous studies (5, 13, 26) cited venous PO, and CO shifts in three muscle biopsy experiments (7) during severe arterial hypoxemia. Interrupted lines are meant to above, we conclude that shunting can not entirely connect data, rather than to indicate a physiological relationship. explain the less efficient 0, extraction. Therefore, the Shaded area indicates mean control MbCO/HbCO t 2 SE (7, 8). more likely explanation is a direct effect of NE on the precapillary sphincter or equivalent structure which ate in these NE runs it is suggested that NE did not opposed metabolic vasodilatation or that maximal reantagonize or alter this mechanism. This indicates that cruitment of capillaries was not possible at low flow autoregulation of intracellular Po2 can operate despite rates resulting from large increases in vascular resistconstriction of resistance vessels. It is still possible that ance in these experiments. NE had a constrictive effect on the precapillary sphincIn runs where maximal falls in flow were achieved ter or equivalent structure which was completely counthere was an increase in systemic blood pressure; thereteracted by metabolic vasodilatation. The finding that fore we cannot exclude the possibility of a baroreceptorVo2 was not flow dependent until flow was depressed to mediated dilator reflex having an effect on our data. low values is consistent with previous data (28, 30) This reflex should not explain the inability of our obtained by changing perfusion rates of isolated dog preparation to maximally extract oxygen from blood, hindlimb muscles without pharmacologic manipulation. and it is unlikely that decreased endogenous NE release In runs where high NE concentrations were used, would have much effect during infusion of a high NE myoglobin-containing muscle became hypoxic at a relaconcentration in arterial blood. tively high femoral venous PO, and relatively low O2 It is possible that oxygen uptake was increased due to extraction per unit flow, compared to that found in the metabolic effects of NE on skeletal muscle since 00, same preparation during arterial hypoxemia (7,S) (Fig. increased in four of five runs at flow rates 65.88% of 5) or found by others in contracting skeletal muscle (13, control. However, we do not have sufficient data on this 15, 28). There are at least two possible explanations for NE concentration to allow us to be convinced that this this finding: a) NE may have a direct constricting effect was a real finding on the precapillary sphincter (or equivalent structure), opposing metabolic -dilatation and resulting in less This study was supported by Grant HL-19737. capillary recruitment, or b) there could be a redistribution of blood flow with increased a-v shunting past gas Received 15 June 1978; accepted in final form 19 September 1978. l 0-0

3.0 r o--o

REFERENCES C. H. Norepinephrine and phenoxybenzamine redistribution of dog blood flow and, volume. Proc. Sot. Exp. Biol. Med. 138: 6880692,197l. 2. BEER, G., AND L. R. YONCE. Blood flow, oxygen uptake, and capillary filtration in resting skeletal muscle. Am, J. Physiol. 1. BAKER,

223: 492-498,1972. 3. BOURDEAU-MARTINI,

J., AND C. R. HONIG. Control of coronary intercapillary distance: effect of arterial PC& and pH. Microvast. Res. 6: 286-296,1973. 4. BOURDEAU-MARTINI, J., C. L. ODOROFF, AND C. R. HONIG. Dual effect of oxygen on magnitude and uniformity of coronary intercapillary distance. Am. J. PhysioZ. 226: 800-810, 1974. 5. COBBOLD, A., B. FOLKOW, I. KJELLMER, AND S. MELLANDER.

Nervous and local chemical control of pre-capillary sphincters in skeletal muscle as measured by changes in filtration coefficient. Actu Physiol. &and. 57: 180-192, 1963. 6. COBURN, R. F., G. K. DANIELSON, W. S. BLAKEMORE, AND R. E. FORSTER. Carbon monoxide in blood: analytical method and sources of error. J. AppZ. Physiol. 19: 510-515, 1964. 7. COBURN, R. F., AND L. B. MAYEREL Myoglobin 0, tension determined from measurements of carboxymyoglobin in skeletal muscle. Am. J. PhysioZ. 220: 66-74, 1971. 8. COBURN, R. F., F. PLOEGMAKERS, P. G~NDRIE, AND R. ABBOUD. Myocardial myoglobin oxygen tension. Am. J. PhysioZ. 224: 870876,1973. 9. DRABKIN,

D. L. The distribution

of the chromoproteins,

hemo-

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H313

OXYGENATION

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