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328
HORSTMAN,
GLESER,
AND
DELEHUNT
tion of 1, 2, 4, and 8 stimuli/s were examined. The data a. were similar in all five preparations; the pattern of Vo, 7. from one experiment is depicted in Fig. 1. The time ___ ------------_-----_-------f 6. _--4 Q response pattern of Q was identical to that of Vo,. In _--- _--_--; 5. _A-- e e this experiment, as in the other four, peak or steady/-9‘ _e-4 state Q and Vo,, at each stimulation frequency, was 0” ’ 3.0 ,$#/’ n.14 reached within 2 min. At each stimulation rate Q and 2. Vo, rose to 50% of the steady-state value within the first ,J” 1. --* 15 s of exercise. This extends the work of Kramer et al. (22) and Piiper et al. (25) by demonstrating that the 1 2 3 4 5 6 7 a 9 10 ii 12 STIMULATION RATE (per second) times to steady-state & and Vo, are independent of the FIG. 2. Muscle blood flow (0) and oxygen consumption (0) as a work intensity. rate. Stimulation applied in trains of 5 stimRate of stimulation. Vo, and Q, measured in 22 mus- function of stimulation uli of 1 ms duration at 25ms intervals. Means +SE for 14 preparacle preparations, increased with increasing stimulation tions are presented. rate until a plateau was reached (maximal values). Voz max and Q max were obtained at 5 stimuli/s in two 1.00 preparations, 8 stimuli/s in 14 preparations, and 12 : .80 stimuli/s in 6 preparations. Figure 2 shows mean t 2 standard error Vo, and Q, as a function of stimulation -2 -60 I rate, for the 14 preparations which plateaued at 8 stims uli/s. In these preparations Vo, max was 8.2 ml/min or 10.7 ml/l00 g min, while & max was 60 ml/min or 78 0" .4oa ml/100 g min. When these curves are plotted as the l y fraction of 30, max and & max versus stimulation rate, 0" .20, instead of absolute values, most of the differences be- + tween preparations are eliminated as the standard error 1 I rl I I at each stimulation rate was reduced by 75%. Therefore, 12 16 20 0 4 8 Vo, and & will be presented relative to the maximal STIMULI/TRAIN values of each preparation in subsequent graphs of the FIG. 3. Relative muscle oxygen consumption (iro,nio, max at 8 stimuli/s) as a function of number of stimuli per train at various resultant data. The curves for 30, and Q versus stimulastimulation rates. Train rate was adjusted to achieve 1 Cm>, 2 (+I, 4 tion rate in preparations which plateaued at other than (W, and 8 0) s t’lmuli/s. Means for 6 preparations are presented. 8 stimuli/s were similar to Fig. 2. Pattern of stimulation. The effects on Vo, and Q of varying the temporal pattern of stimulation were studing of 1, 2, 4, 5, 10, or 20 stimuli at an intratrain ied in six preparations to determine the pattern which frequency of 40 stimuli/s (i.e., stimuli were spaced at 25 would consistently result in the highest Vo, for any ms intervals). In these experiments, train rate was given overall stimulation rate. Figure 3 shows the effect adjusted so that the product of train rate and the numon Voz (the pattern of & was essentially the same as ber of stimuli per train equalled a given overall stimulathat of \io,> of varying the number of impulses per train tion rate. For example, at an overall rate of 2 stimuli/s, keeping the intratrain stimulation frequency constant. for trains consisting of 1, 2, 4, 5, 10, and 20 stimuli per Stimuli of 1 ms duration were delivered in trains consisttrain, respective train rates were 2, 1, .5, .4, .2, and .l trains/s. 7.0 At an overall rate of 1 stimuli/s, the number of stimuli RECOVERYSTIMULATION-~8 stim./sec I per train does not affect Vo, or Q except for trains of 20 6.0 stimuli where there are slight reductions of & and Vo2, At higher stimulation rates there are significant decreases in Q and vo, for trains of 10 and 20 stimuli. At rates of 8 stimuli/s, there were slightly lower Voz and Q for trains of 1 and 2 stimuli than for trains of 4 and 5 stimuli. At rates of 16 stimuli/s (not shown on the graph for reasons of visual clarity), there was a reduction in Q and 30, compared with 8 stimuli/s, regardless of the number of stimuli in each train. Since trains of 5 stimuli resulted in the highest Voz at every level of overall stimulation frequency, this pattern was used for all subsequent experiments. The effect of varying the intratrain stimulation rate 0 1 2 3 4 5 6 7 8 9 10 was also studied in five preparations. It was found that TIME (min 1 for intratrain frequencies varying between 25 and 50 FIG. 1. Muscle oxygen consumption as a function of time during 5 stimuli/s there were no differences in VOW. At lower min of stimulation at 1 (W, 2 (a), 4 (O), and 8 NIB) stimuli/s, each frequencies muscle contractions were tremorous, i.e., a followed by 5 min of recovery. Representative data from 1 preparation are presented. smooth summation contraction did not occur. l
0
1
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Vo,
RELATIVE
OF
0,
DELIVERY:
ISOLATED
Reproducibility. We were concerned with three types of reproducibility: 1) repeat measurements of Q and Vo, made during the same stimulation separated only by time; 2) repeat measurement of Q and Vo, under the same stimulation conditions, at different times during the experiment; and 3) measurements of Q and Vo, under the same stimulation conditions compared between the left and right gracilis muscles of the same animal. The first test of reproducibility estimates the reliability of the measuring techniques, the second estimates the reliability of the muscle’s preparation, and the third estimates the reliability of the total experimental technique. To examine type 1 reproducibility, 18 paired measures of Q and Vo, were compared. Each pair of measurements was made between 2 and 10 min after the onset of stimulation over a wide range of stimulation conditions in five .dogs. Coefficients of correlations between the first and second measurements were 0.99 for both Vo, and Q. There was no significant difference between the means of the first and second measurements. Type 2 reproducibility was estimated by comparing 30 pairs of measurements of Q and Vo, in 15 dogs over a wide range of stimulation parameters. Each pair of measurements was taken under identical stimulation conditions but at different times during the experiments. Coefficients of correlation between the first and second measurements were 0.98 for both 30, and Q. There was no significant difference between the means of the first and second measurements. In three dogs a total of 12 measurements of Q and Vo, were compared between the left and right gracilis muscles at various stimulation rates. Muscle weights were not significantly different. Coefficients of correlation between measurements from the left and right gracilis were 0.97 for both Vo, and Q. Measurements obtained from the right gracilis were not significantly different from those obtained from the left gracilis. Mood pressure. The relationship among blood pressure, Q, and Vo, was studied in five preparations. Blood pressure was adjusted by a screw clamp around the femoral artery proximal to the gracilis artery. In each preparation five levels of blood pressure were studied averaging 143, 112, 92, 80, and 50 mmHg. Stimulation rates were selected to produce a plateau in Vq, response for each blood pressure. In Figs. 4 and 5, Vo, and Q
(.64)
1
2 3 4 STIMULATION
5 6 7 0 RATE (per set )
329
MUSCLE
9
10
FIG. 4. Relative muscle blood flow as a function of stimulation rate at mean arterial blood pressures of 50 (N), 80 W, 92 (O), 112 (0), and 143 (0) mmHg; fraction of maximum (unclamped) BP, noted in parentheses. Means for 5 preparations are presented.
1.00 I .90. 80, % $ .7om E ? .60. '@ b .50. V X
; .40. 0" l a .3om .B
.20, .lO. 0
-II
1
g
g
[
2 3 4 STIMULATION
g
g g
g
5 6 7 0 RATE (per set )
g a
9
10
FIG. 5. Relative muscle oxygen consumption as a function of stimulation rate at mean arterial blood pressures of 50 cm), 80 W, 92 W, 112 (a), and 143 (0) mmHg; fraction of maximum (unclamped) BP;, noted in parentheses. Means for 5 preparations are presented.
measurements are presented relative to 30, max and Q max attained with unaltered blood pressure (average 143 mmHg). At low stimulation rates Vo, was unaffected when BP was lowered to as little as 60% of unaltered pressure, even though Q was progressively diminished by every reduction in pressure. At higher stimulation rates 30, -. was also diminished with lower BP. With decreased BP, both Vo, max and Q max were diminished and were attained-. at a lower stimulation rate. Under each condition of BP, Q max was reduced in exact proportion to the reduction in I?!? and the reduction of 30, max was nearly in proportion to the reduction in BP. Table 1 presents the measurements made at Vo, max for these and other experimental manipulations. Hemoglobin concentration (blood viscosity). The relationship among Hb concentration, Q, and Vo, was studied in five preparations exercised to maximum. Hb concentration was varied by bleeding and reinfusion of either dextrose or the dog’s own erythrocytes. Results at each stimulation rate were grouped for mean Hb concentrations (Hb) of 9.9, 12.5, 15.17,and 18.0 g/100 ml. Figures 6 and 7 present Q and Vo, relative to Q max and Vo, max obtained at normal Hb (Hb = 15.7 g/100 ml). Submaximal Vo, was relatively independent of Hb concentration, whereas submaximal Q increased with decreasing Hb concentration. Hb concentrations of 9.9, 12.5, and 15.7 g/l00 ml had no significant effect on Vo, max; higher Hb concentrations resulted in decreased Vo, max. Hypoxia. VO, and Q were studied in eight preparations while respiring the dog with hypoxic air mixtures. Figures 8 and 9 show the average Vo, and Q relative to the Vo, max and Q max of the normoxic preparations. Three curves are presented: 1) the mean of eight normoxie preparations with the average arterial oxyhemoglobin saturation (SaOJ equal to 96%; 2) the mean of three preparations with average SaO, = 73%; and3) the mean of five preparations with average SaO, = 62%. The average Q was higher for the hypoxic preparations than the normoxic at submaximal work; however, Q was not greatly different at maximal stimulation. Because of the greater blood flow with hypoxic work, 30, was not significantly different at any level of stimula-
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330
HORSTMAN,
GLESER,
AND
DELEHUNT
1. Maximal values for all measured and calculated parameters under various experimental conditions TABLE
sI 6 c8
112 92 80 50
94
*l 93 ?2 93 21 93 51 94 tl
15.1 -c.4 15.2 *.4 15.2 ?.3 15.4 2.4 15.3 2.4
19.4 -e.4 19.3 A.4 19.4 k.3 19.7 ?.3 19.8 +.4
12 +2 11 t2 9 tl 8 t2 9 t 2
11 +2 10 t2 9 -r_l 7 tl 8 Tl
15.7 2.3 15.8 4.4 15.4 2.4 15.9 *.4 15.6 2.4
2.4 2.3 2.2 2.3 1.9 2.2 1.5 L. 2 1.7 2.2
17.0 t.5 17.1 2.4 17.5 z.4 18.2 lr.3 18.1 -c.4
93
*5 72 54 59 +4 50 *3 33 23
18.0 15.8 2.9 2.5 13.9 12.3 2.8 2.6 11.4 10.3 2.8 2.5 9.9 9.1 2.7 A.4 6.5 6.0 +.7 2.5
143 24 112 +3 92 23 80 +2 50 -3
4
I
0
m
I
I
I
I
I1
I8
9’
12345678 STIMULATION
RATE
(per
10
ICC )
6. Relative muscle blood flow as a function of stimulation at mean hemoglobin concentrations of 9.9 (W), 12.5 (A), 15.7 CO), 18.0 0) g/100 ml. Means for 5 preparations are presented.
FIG.
rate and
gm % gm% gm %
1.00 .90 3
Ht) Concn, g/100 ml 9.9
2 * .ao, ‘ij 95 52 93 22 92 23 89 22
12.5 15.7 18.0
Hypoxiasao,, 96
92
k3 92 22 96 -+3 95 23 95 23
97 $2 95 +1 94 t2 93 tl
9.9 z.3 12.5 t .3 15.7 T.5 18.0 T.2
13.2 t.3 16.3 k.4 20.2 -c.5 22.9 +.3
7 *2 9 +2 10 -+ 2 12 +2
8 21 10 +l 10 ?l 11 T2
10.4 t.3 12.7 t.4 15.9 T.4 18.6 T-.3
1.2 z.2 1.9 e.3 2.2 k.3 2.6 2.3
12.0 2.4 14.4 *.4 18.0 2.4 20.3 2.3
129 25 104 -+4 85 +5 60 t7
17.0 2.9 17.0 2.8 17.2 k.6 13.7 t 1.0
15.5 k.7 15.0 *.5 15.3 2.5 12.2 It.9
136 t4 143 ~6 139 *6 135 t5
E .70, s x .60, 6
.50,
l> 2
.40 I
‘>
.20 4 r/
73 62
97 A3 40 *4 34 *2
96 -2 73 z3 62 t3
15.4 2.4 15.6 k.4 15.7 *.3
20.3 *.4 15.7 ?.4 13.3 2.3
10 t2 7 -+2 8 22
9 21 8 -t2 8 +l
15.5 5.3 15.5 +.4 15.7 -c.3
1.9 *.3 1.7 2.4 1.7 2.3
18.4 -t.3 14.0 2.3 11.6 2.3
91 c4 98 ~8 102 -c6
18.5 T.7 15.4 21.0 13.6 5.6
16.5 2.5 13.8 t.8 12.0 2.5
140 +4 150 26 155 24
91 51 635 *lo
94 +l 100 CO
15.9 2.5 16.3 2.5
20.4 t .4 24.0 +.3 --
10 -t2 12 22
10 tl 12 t2
16.2 f .4 16.7 2.4
2.2 2.4 2.8 2.4
18.2 2.4 21.2 2.3
86 25 79 *5
17.5 2.9 18.9 +.7
15.7 2.6 16.7 t.7
141 t , the metabolic and hemodynamic responses to adjustments in hemoglobin concentration and hypoxia corresponded exactly to responses in our experiments with isolated contracting muscle. From these experiments, it can be concluded that oxygen delivery and not utilization, is the critical component in limiting the oxygen consumption of working muscle. At maximal exercise, oxygen consumption varies in exact proportion to oxygen delivery and is a function of oxygen delivery. During submaximal exercise, the muscle vasculature possesses the capacity to maintain oxygen delivery in the face of decreased perfusion pressure and hypoxia by vasodilatation. However, during maximal exercise, the vasculature is maximally dilated and behaves as a set of rigid tubes. Factors such as arterial blood pressure and viscosity determine maximal blood flow, and these combined with arterial oxygen content determine the maximal oxygen delivery and oxygen consumption that can be achieved. Findings in experiments with isolated contracting muscle are substantially identical to those in the intact, exercising animal and demonstrate the appropriateness of the gracilis preparation for the study of working muscle. The authors are indebted to Thomas Tryon and Richard Oswald for their valuable technical assistance. This work was presented, in part, at Annual Meetings of the American College of Sports Medicine, Seattle, Wash., Spring 1973, and Knoxville, Tenn., Spring 19’74. Preliminary reports have appeared in abstract form (Med. Sci. Sports 5: 64, 1973; Med. Sci. Sports 6: 77, 1974). A preliminary report was also submitted to the American Heart Association in competition for the 1974 Louis N. Katz Basic Science Research Prize for Young Investigators. The opinions or assertions contained herein are the private views of the author(s) and are not to be construed as official or as reflecting the views of the Department of the Army or the Department of Defense. In conducting the research described in this report, the investigators adhered to the “Guide for Laboratory Animal Facilities and Care,” prepared by the National Academy of Sciences-National Research Council. Present address of M. Gleser: Yale University, School of Medicine, New Haven, Conn. Present address of J. Delehunt: Institute of Environmental Stress, Univ. of California, Santa Barbara, Calif. Received
for publication
31 March
1975.
REFERENCES 1. AGARWAL, J. B., R. PALTOO, AND W. H. PALMER. Relative viscosity of blood at varying hematocrits in pulmonary circulation. J. AppZ. Physiol. 29: 866-871, 1970. 2. ASTRAND, P., AND K. RODAHL. Textbook of Work Physiology. New York: McGraw, 1970. 3. BASKIN, R. J. The variation of muscle oxygen consumption with velocity of shortening. J. Gen. PhysioZ. 49: 9-15, 1965. 4. BASKIN, R. J. The variation of muscle oxygen consumption with load. J. Physiol., London 181: 270-281, 1965. 5. BASKIN, R. J., AND V. N. GALLUZZI. Oxygen consumption in frog sartorius muscle: variation following an applied stretch. Am. J. PhysioZ. 211: 525-528, 1966. 6. CHANCE, B., G. MAURIELLO, AND X. M. AUBERT. ADP arrival at muscle mitochondria following a twitch. In: MuscZe as a Tissue, edited by K. Rodahl and S. M. Horvath. New York: McGraw, 1962, p. 128-145. 7. CLAUSEN, J. P., AND N. A. LASSEN. Muscle blood flow during exercise in normal man studied by the l”“Xe clearance method.
Cardiovascular Res. 5: 245-254, 1971. 8. FALES, J. T., S. R. HEISEY, AND K. L. ZIERLER. Dependency of oxygen consumption of skeletal muscle on number of stimuli during work in the dog. Am. J. Physiol. 198: 1333-1342, 1960. 9. FALES, J. T., AND K. L. ZIERLER. Relation between length, tension, and heat: frog sartorius muscle, brief tetani. Am. J. Physiol. 216: 70-75, 1969. 10. FOLKOW, B., B. GASKELL, AND B. A. WASSLER. Blood flow through limb muscles during heavy rhythmic exercise. Acta Physiol. Stand. 80: 61-72, 1970. 11. FOLKOW, B., AND H. D. HALICKA. A comparison between red and white muscle with respect to blood supply, capillary surface area and oxygen uptake during rest and exercise. Microvascular Res. 1: 1-14, 1968. 12. GLESER, M. A. Effects of hypoxia and physical training on hemodynamic adjustment to one-legged exercise. J. Appl. Physiol. 34: 655-659, 1973. 13. GLESER, M. A., D. H. HORSTMAN, AND R. P. MELL~. The effects
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334
14.
15.
16.
17.
18.
19. 20. 21
22
23
24
25
26.
27.
HORSTMAN, on Vo, max of adding arm work to maximal leg work. Med. Sci. Sports 6: 104-107, 1974. HARTLEY, L. H., J. A. VOGEL, AND M. LANDOWNE. Central, femoral, and brachial circulation during exercise in hypoxia. J. AppZ. Physiol. 34: 87-90, 1973. HIRVONEN, L., AND R. R. SONNENSCHEIN. Relation between blood flow and contraction force in active skeletal muscle. Circulation Res. 10: 94-104, 1962. HORSTMAN, D. H., M. A. GLESER, D. WOLFE, T. TRYON, AND J. DELEHUNT. Effects of hemoglobin reduction on Vo, max and related hemodynamics in exercising dogs. J. AppZ. Physiol. 37: 97-102, 1974. J~BSIS, F. F., AND J. C. DUFFIELD. Force, shortening, and work in muscular contraction: relative contributions to overall energy utilization. Science 156: 1388-1392, 1967. J~BSIS, F. F., AND W. N. STAINSBY. Oxidation of NADH during contractions of circulated mammalian skeletal muscle. Resp. PhysioZ. 4: 292-300, 1968. KJELLMER, I. Effect of exercise on the vascular bed of skeletal muscle. Acta Physiol. Stand. 62: 18-30, 1964. KJELLMER, I. Studies on exercise hyperemia. Acta Physiol. Stand. 64: Suppl. 244, 1965. KRAMER, K., F. OBAL, AND W. QUENSEL. Untersuchungen uber den Muskelstoffwechsel des Warmblutters. III. Mitteilung. Die Sauerstoffaufnahme des Muskels wahrend rythmischer Tatigkeit. Pfluegers Arch. 241: 717-729, 1939. KRAMER, K., W. QUENSEL, AND K. E. SCHAFER. Untersuchungen uber den Muskelstoffwechsel des Warmbluters. IV. Mitteilung. Beziehungen zwischen Sauerstoffaufnahme und Milchsaureabgabe des Muskels wahrend der Tatigkeit. Pfluegers Arch. 241: 730-740, 1939. LEVY, M. N., AND L. SHARE. The influence of erythrocyte concentration upon the pressure-flow relationships in the dog’s hind limb. Circulation Res. 1: 247-255, 1953. MERCKER, H., B. OCHWADT, AND W. SCHOEDEL. Der Einfluss der Erregungsfrequenz und der Belastung auf Durchbluting und Sauerstoffaufnahme des Skeietmuskels. PfZuegers Arch. 251: 73-82, 1949. PIIPER, J., P. E. DIPRAMPERO, AND P. CERRETELLI. Oxygen debt and high-energy phosphates in gastrocnemius muscle of the dog. Am. J. Physiol. 215: 523-531, 1968. PIIPER, J., P. CERRETELLI, F. CUTTICA, AND F. MANGILI. Energy metabolism and circulation in dogs exercising in hypoxia. J. AppZ. Physiol. 21: 1143-1149, 1966. QUENSEL, W., AND K. KRAMER. Untersuchungen uber den Mus-
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kelstoffwechsel des Warmbluters. II. Mitteilung. Die Sauerstoffaufnahme des Muskels wahrend der tetanischen Kontraktion. Pfluegers Arch. 241: 698-716, 1939. Ross, J. M., H. M. FAIRCHILD, J. WELDY, AND A. C. GUYTON. Autoregulation of blood flow by oxygen lack. Am. J. Physiol. 202: 21-25, 1962. Ross, J., G. A. KAISER, AND F. J. KLOCKE. Observations on the role of diminished oxygen tension in the functional hyperemia of skeletal muscle. CircuZation Res. 15: 473-484, 1964. SHEPERD, A. P., H. J. GRANGER, E. E. SMITH, AND A. C. GUYTON. Local control of tissue oxygen delivery and its contribution to the regulation of cardiac output. Am. J. PhysioZ. 225: 747-755, 1973. STAINSBY, W. N. Autoregulation of blood flow in skeletal muscle during increased metabolic activity. Am. J. Physiol. 202: 273276, 1962. STAINSBY, W. N. Autoregulation in skeletal muscle. Circulation Res. 15, Suppl. 1: 39-47, 1964. STAINSBY, W. N. Oxygen uptake for isotonic and isometric twitch contractions of dog skeletal muscle in situ. Am. J. Physiol. 219: 435-439, 1970. STAINSBY, W. N., AND J. K. BARCLAY. Relation of load, rest length, work and shortening to oxygen uptake by in situ dog semitendinosis. Am. J. PhysioZ. 22: 1238-1242, 1971. STAINSBY, W. N., AND J. T. FALES. Oxygen consumption for isometric tetanic contractions of dog skeletal muscle in situ. Am. J. Physiol. 224: 687-691, 1973. STAINSBY, W. N., AND A. B. OTIS. Blood flow, blood oxygen tension, oxygen uptake and oxygen transport in skeletal muscle. Am. J. Physiol. 206: 858-866, 1964. STAINSBY, W. N., AND H. G. WELCH. Lactate metabolism of contracting dog skeletal muscle in situ. Am. J. Physiol. 211: 177-183, 1966. STENBERG, J., B. EKBLOM, AND R. MESSEN. Hemodynamic response to work at simulated altitude, 4000 m. J. AppZ. Physiol. 21: 1589-1594, 1966. VOGEL, J. A., AND M. A. GLESER. Effect of carbon monoxide in oxygen transport during exercise. J. Appl. PhysioZ. 32: 234-239, 1972. WHITTAKER, R. R. F., AND F. R. WINTON. The apparent viscosity of blood flowing in the isolated hindlimb of the dog and its variation with corpuscular concentration. J. PhysioZ., London 78: 339-369, 1933. WRIGHT, D. L., AND R. R. SONNENSCHEIN. Relations among activity, blood flow, and vascular state in skeletal muscle. Am. J. Physiol. 208: 782-789, 1965.
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328
HORSTMAN,
GLESER,
AND
DELEHUNT
tion of 1, 2, 4, and 8 stimuli/s were examined. The data a. were similar in all five preparations; the pattern of Vo, 7. from one experiment is depicted in Fig. 1. The time ___ ------------_-----_-------f 6. _--4 Q response pattern of Q was identical to that of Vo,. In _--- _--_--; 5. _A-- e e this experiment, as in the other four, peak or steady/-9‘ _e-4 state Q and Vo,, at each stimulation frequency, was 0” ’ 3.0 ,$#/’ n.14 reached within 2 min. At each stimulation rate Q and 2. Vo, rose to 50% of the steady-state value within the first ,J” 1. --* 15 s of exercise. This extends the work of Kramer et al. (22) and Piiper et al. (25) by demonstrating that the 1 2 3 4 5 6 7 a 9 10 ii 12 STIMULATION RATE (per second) times to steady-state & and Vo, are independent of the FIG. 2. Muscle blood flow (0) and oxygen consumption (0) as a work intensity. rate. Stimulation applied in trains of 5 stimRate of stimulation. Vo, and Q, measured in 22 mus- function of stimulation uli of 1 ms duration at 25ms intervals. Means +SE for 14 preparacle preparations, increased with increasing stimulation tions are presented. rate until a plateau was reached (maximal values). Voz max and Q max were obtained at 5 stimuli/s in two 1.00 preparations, 8 stimuli/s in 14 preparations, and 12 : .80 stimuli/s in 6 preparations. Figure 2 shows mean t 2 standard error Vo, and Q, as a function of stimulation -2 -60 I rate, for the 14 preparations which plateaued at 8 stims uli/s. In these preparations Vo, max was 8.2 ml/min or 10.7 ml/l00 g min, while & max was 60 ml/min or 78 0" .4oa ml/100 g min. When these curves are plotted as the l y fraction of 30, max and & max versus stimulation rate, 0" .20, instead of absolute values, most of the differences be- + tween preparations are eliminated as the standard error 1 I rl I I at each stimulation rate was reduced by 75%. Therefore, 12 16 20 0 4 8 Vo, and & will be presented relative to the maximal STIMULI/TRAIN values of each preparation in subsequent graphs of the FIG. 3. Relative muscle oxygen consumption (iro,nio, max at 8 stimuli/s) as a function of number of stimuli per train at various resultant data. The curves for 30, and Q versus stimulastimulation rates. Train rate was adjusted to achieve 1 Cm>, 2 (+I, 4 tion rate in preparations which plateaued at other than (W, and 8 0) s t’lmuli/s. Means for 6 preparations are presented. 8 stimuli/s were similar to Fig. 2. Pattern of stimulation. The effects on Vo, and Q of varying the temporal pattern of stimulation were studing of 1, 2, 4, 5, 10, or 20 stimuli at an intratrain ied in six preparations to determine the pattern which frequency of 40 stimuli/s (i.e., stimuli were spaced at 25 would consistently result in the highest Vo, for any ms intervals). In these experiments, train rate was given overall stimulation rate. Figure 3 shows the effect adjusted so that the product of train rate and the numon Voz (the pattern of & was essentially the same as ber of stimuli per train equalled a given overall stimulathat of \io,> of varying the number of impulses per train tion rate. For example, at an overall rate of 2 stimuli/s, keeping the intratrain stimulation frequency constant. for trains consisting of 1, 2, 4, 5, 10, and 20 stimuli per Stimuli of 1 ms duration were delivered in trains consisttrain, respective train rates were 2, 1, .5, .4, .2, and .l trains/s. 7.0 At an overall rate of 1 stimuli/s, the number of stimuli RECOVERYSTIMULATION-~8 stim./sec I per train does not affect Vo, or Q except for trains of 20 6.0 stimuli where there are slight reductions of & and Vo2, At higher stimulation rates there are significant decreases in Q and vo, for trains of 10 and 20 stimuli. At rates of 8 stimuli/s, there were slightly lower Voz and Q for trains of 1 and 2 stimuli than for trains of 4 and 5 stimuli. At rates of 16 stimuli/s (not shown on the graph for reasons of visual clarity), there was a reduction in Q and 30, compared with 8 stimuli/s, regardless of the number of stimuli in each train. Since trains of 5 stimuli resulted in the highest Voz at every level of overall stimulation frequency, this pattern was used for all subsequent experiments. The effect of varying the intratrain stimulation rate 0 1 2 3 4 5 6 7 8 9 10 was also studied in five preparations. It was found that TIME (min 1 for intratrain frequencies varying between 25 and 50 FIG. 1. Muscle oxygen consumption as a function of time during 5 stimuli/s there were no differences in VOW. At lower min of stimulation at 1 (W, 2 (a), 4 (O), and 8 NIB) stimuli/s, each frequencies muscle contractions were tremorous, i.e., a followed by 5 min of recovery. Representative data from 1 preparation are presented. smooth summation contraction did not occur. l
0
1
Downloaded from www.physiology.org/journal/ajplegacy by ${individualUser.givenNames} ${individualUser.surname} (129.081.226.078) on January 22, 2019.
Vo,
RELATIVE
OF
0,
DELIVERY:
ISOLATED
Reproducibility. We were concerned with three types of reproducibility: 1) repeat measurements of Q and Vo, made during the same stimulation separated only by time; 2) repeat measurement of Q and Vo, under the same stimulation conditions, at different times during the experiment; and 3) measurements of Q and Vo, under the same stimulation conditions compared between the left and right gracilis muscles of the same animal. The first test of reproducibility estimates the reliability of the measuring techniques, the second estimates the reliability of the muscle’s preparation, and the third estimates the reliability of the total experimental technique. To examine type 1 reproducibility, 18 paired measures of Q and Vo, were compared. Each pair of measurements was made between 2 and 10 min after the onset of stimulation over a wide range of stimulation conditions in five .dogs. Coefficients of correlations between the first and second measurements were 0.99 for both Vo, and Q. There was no significant difference between the means of the first and second measurements. Type 2 reproducibility was estimated by comparing 30 pairs of measurements of Q and Vo, in 15 dogs over a wide range of stimulation parameters. Each pair of measurements was taken under identical stimulation conditions but at different times during the experiments. Coefficients of correlation between the first and second measurements were 0.98 for both 30, and Q. There was no significant difference between the means of the first and second measurements. In three dogs a total of 12 measurements of Q and Vo, were compared between the left and right gracilis muscles at various stimulation rates. Muscle weights were not significantly different. Coefficients of correlation between measurements from the left and right gracilis were 0.97 for both Vo, and Q. Measurements obtained from the right gracilis were not significantly different from those obtained from the left gracilis. Mood pressure. The relationship among blood pressure, Q, and Vo, was studied in five preparations. Blood pressure was adjusted by a screw clamp around the femoral artery proximal to the gracilis artery. In each preparation five levels of blood pressure were studied averaging 143, 112, 92, 80, and 50 mmHg. Stimulation rates were selected to produce a plateau in Vq, response for each blood pressure. In Figs. 4 and 5, Vo, and Q
(.64)
1
2 3 4 STIMULATION
5 6 7 0 RATE (per set )
329
MUSCLE
9
10
FIG. 4. Relative muscle blood flow as a function of stimulation rate at mean arterial blood pressures of 50 (N), 80 W, 92 (O), 112 (0), and 143 (0) mmHg; fraction of maximum (unclamped) BP, noted in parentheses. Means for 5 preparations are presented.
1.00 I .90. 80, % $ .7om E ? .60. '@ b .50. V X
; .40. 0" l a .3om .B
.20, .lO. 0
-II
1
g
g
[
2 3 4 STIMULATION
g
g g
g
5 6 7 0 RATE (per set )
g a
9
10
FIG. 5. Relative muscle oxygen consumption as a function of stimulation rate at mean arterial blood pressures of 50 cm), 80 W, 92 W, 112 (a), and 143 (0) mmHg; fraction of maximum (unclamped) BP;, noted in parentheses. Means for 5 preparations are presented.
measurements are presented relative to 30, max and Q max attained with unaltered blood pressure (average 143 mmHg). At low stimulation rates Vo, was unaffected when BP was lowered to as little as 60% of unaltered pressure, even though Q was progressively diminished by every reduction in pressure. At higher stimulation rates 30, -. was also diminished with lower BP. With decreased BP, both Vo, max and Q max were diminished and were attained-. at a lower stimulation rate. Under each condition of BP, Q max was reduced in exact proportion to the reduction in I?!? and the reduction of 30, max was nearly in proportion to the reduction in BP. Table 1 presents the measurements made at Vo, max for these and other experimental manipulations. Hemoglobin concentration (blood viscosity). The relationship among Hb concentration, Q, and Vo, was studied in five preparations exercised to maximum. Hb concentration was varied by bleeding and reinfusion of either dextrose or the dog’s own erythrocytes. Results at each stimulation rate were grouped for mean Hb concentrations (Hb) of 9.9, 12.5, 15.17,and 18.0 g/100 ml. Figures 6 and 7 present Q and Vo, relative to Q max and Vo, max obtained at normal Hb (Hb = 15.7 g/100 ml). Submaximal Vo, was relatively independent of Hb concentration, whereas submaximal Q increased with decreasing Hb concentration. Hb concentrations of 9.9, 12.5, and 15.7 g/l00 ml had no significant effect on Vo, max; higher Hb concentrations resulted in decreased Vo, max. Hypoxia. VO, and Q were studied in eight preparations while respiring the dog with hypoxic air mixtures. Figures 8 and 9 show the average Vo, and Q relative to the Vo, max and Q max of the normoxic preparations. Three curves are presented: 1) the mean of eight normoxie preparations with the average arterial oxyhemoglobin saturation (SaOJ equal to 96%; 2) the mean of three preparations with average SaO, = 73%; and3) the mean of five preparations with average SaO, = 62%. The average Q was higher for the hypoxic preparations than the normoxic at submaximal work; however, Q was not greatly different at maximal stimulation. Because of the greater blood flow with hypoxic work, 30, was not significantly different at any level of stimula-
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330
HORSTMAN,
GLESER,
AND
DELEHUNT
1. Maximal values for all measured and calculated parameters under various experimental conditions TABLE
sI 6 c8
112 92 80 50
94
*l 93 ?2 93 21 93 51 94 tl
15.1 -c.4 15.2 *.4 15.2 ?.3 15.4 2.4 15.3 2.4
19.4 -e.4 19.3 A.4 19.4 k.3 19.7 ?.3 19.8 +.4
12 +2 11 t2 9 tl 8 t2 9 t 2
11 +2 10 t2 9 -r_l 7 tl 8 Tl
15.7 2.3 15.8 4.4 15.4 2.4 15.9 *.4 15.6 2.4
2.4 2.3 2.2 2.3 1.9 2.2 1.5 L. 2 1.7 2.2
17.0 t.5 17.1 2.4 17.5 z.4 18.2 lr.3 18.1 -c.4
93
*5 72 54 59 +4 50 *3 33 23
18.0 15.8 2.9 2.5 13.9 12.3 2.8 2.6 11.4 10.3 2.8 2.5 9.9 9.1 2.7 A.4 6.5 6.0 +.7 2.5
143 24 112 +3 92 23 80 +2 50 -3
4
I
0
m
I
I
I
I
I1
I8
9’
12345678 STIMULATION
RATE
(per
10
ICC )
6. Relative muscle blood flow as a function of stimulation at mean hemoglobin concentrations of 9.9 (W), 12.5 (A), 15.7 CO), 18.0 0) g/100 ml. Means for 5 preparations are presented.
FIG.
rate and
gm % gm% gm %
1.00 .90 3
Ht) Concn, g/100 ml 9.9
2 * .ao, ‘ij 95 52 93 22 92 23 89 22
12.5 15.7 18.0
Hypoxiasao,, 96
92
k3 92 22 96 -+3 95 23 95 23
97 $2 95 +1 94 t2 93 tl
9.9 z.3 12.5 t .3 15.7 T.5 18.0 T.2
13.2 t.3 16.3 k.4 20.2 -c.5 22.9 +.3
7 *2 9 +2 10 -+ 2 12 +2
8 21 10 +l 10 ?l 11 T2
10.4 t.3 12.7 t.4 15.9 T.4 18.6 T-.3
1.2 z.2 1.9 e.3 2.2 k.3 2.6 2.3
12.0 2.4 14.4 *.4 18.0 2.4 20.3 2.3
129 25 104 -+4 85 +5 60 t7
17.0 2.9 17.0 2.8 17.2 k.6 13.7 t 1.0
15.5 k.7 15.0 *.5 15.3 2.5 12.2 It.9
136 t4 143 ~6 139 *6 135 t5
E .70, s x .60, 6
.50,
l> 2
.40 I
‘>
.20 4 r/
73 62
97 A3 40 *4 34 *2
96 -2 73 z3 62 t3
15.4 2.4 15.6 k.4 15.7 *.3
20.3 *.4 15.7 ?.4 13.3 2.3
10 t2 7 -+2 8 22
9 21 8 -t2 8 +l
15.5 5.3 15.5 +.4 15.7 -c.3
1.9 *.3 1.7 2.4 1.7 2.3
18.4 -t.3 14.0 2.3 11.6 2.3
91 c4 98 ~8 102 -c6
18.5 T.7 15.4 21.0 13.6 5.6
16.5 2.5 13.8 t.8 12.0 2.5
140 +4 150 26 155 24
91 51 635 *lo
94 +l 100 CO
15.9 2.5 16.3 2.5
20.4 t .4 24.0 +.3 --
10 -t2 12 22
10 tl 12 t2
16.2 f .4 16.7 2.4
2.2 2.4 2.8 2.4
18.2 2.4 21.2 2.3
86 25 79 *5
17.5 2.9 18.9 +.7
15.7 2.6 16.7 t.7
141 t , the metabolic and hemodynamic responses to adjustments in hemoglobin concentration and hypoxia corresponded exactly to responses in our experiments with isolated contracting muscle. From these experiments, it can be concluded that oxygen delivery and not utilization, is the critical component in limiting the oxygen consumption of working muscle. At maximal exercise, oxygen consumption varies in exact proportion to oxygen delivery and is a function of oxygen delivery. During submaximal exercise, the muscle vasculature possesses the capacity to maintain oxygen delivery in the face of decreased perfusion pressure and hypoxia by vasodilatation. However, during maximal exercise, the vasculature is maximally dilated and behaves as a set of rigid tubes. Factors such as arterial blood pressure and viscosity determine maximal blood flow, and these combined with arterial oxygen content determine the maximal oxygen delivery and oxygen consumption that can be achieved. Findings in experiments with isolated contracting muscle are substantially identical to those in the intact, exercising animal and demonstrate the appropriateness of the gracilis preparation for the study of working muscle. The authors are indebted to Thomas Tryon and Richard Oswald for their valuable technical assistance. This work was presented, in part, at Annual Meetings of the American College of Sports Medicine, Seattle, Wash., Spring 1973, and Knoxville, Tenn., Spring 19’74. Preliminary reports have appeared in abstract form (Med. Sci. Sports 5: 64, 1973; Med. Sci. Sports 6: 77, 1974). A preliminary report was also submitted to the American Heart Association in competition for the 1974 Louis N. Katz Basic Science Research Prize for Young Investigators. The opinions or assertions contained herein are the private views of the author(s) and are not to be construed as official or as reflecting the views of the Department of the Army or the Department of Defense. In conducting the research described in this report, the investigators adhered to the “Guide for Laboratory Animal Facilities and Care,” prepared by the National Academy of Sciences-National Research Council. Present address of M. Gleser: Yale University, School of Medicine, New Haven, Conn. Present address of J. Delehunt: Institute of Environmental Stress, Univ. of California, Santa Barbara, Calif. Received
for publication
31 March
1975.
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