Showing papers in "American Journal of Physiology in 1973"

Function of the thick ascending limb of Henle's loop.

Abstract: BURG, MAURICE, AND NORDICA GREEN. Function of the thick ascending limb of Henle’s loop. Am. J. Physiol. 224(3) : 659-668. 1973.-The thick ascending limb of Henle’s loop was dissected from rabbit kidneys and perfused in vitro. There was net NaCl absorption from the tubule lumen with resultant lowering of lumen NaCl concentration below that of the bath. Since the electrical PD was positive in the lumen, Cl is absorbed actively. Na absorption is largely passive as indicated by comparison of 1) Na flux ratio and PD (at fast perfusion rates); and 2) Na concentration ratio and PD (at slow perfusion rates). However, the possibility of some active Na transport is not ruled out. The PD results from Cl transport. It is eliminated reversibly by removal of Cl from bath and lumen (SO4 substitution) and by ouabain. The PD is increased by removal of Na from bath and lumen (choline substitution). Na permeability is greater than Cl permeability, as indicated by I) the NaCl diffusion potential that is lumen positive when lumen NaCl concentration is low, and 2) radioisotope measurements of ion fluxes. Water permeability (Lp) is low, which combined with the NaCl transport accounts for the dilution of the urine in this segment.

Furosemide effect on isolated perfused tubules.

Abstract: BURG, MI., L. STONER, J. CARDINAL, AND N. GREEN. Furosemide effect on isolated perfused tubules. Am. J. Physiol. 225(l): 119-124. 1973 .-Proximal convoluted tubules, thick ascending limbs of Henle’s loop, and cortical collecting tubules were dissected from rabbit kidneys, and perfused in vitro. The effect of the diuretic agent furosemide on electrolyte transport and electrical potential difference (PD) in these segments was determined. Furosemide in the lumen (lo-” to lOA M) inhibited active Cl transport in the thick ascending limbs of Henle’s loop with resultant decrease in net NaCl absorption and electrical PD. Concentrations of furosemide as high as 10 -4 M in the bath had little effect. Furosemide at low4 M also had no definite effect on proximal convoluted tubules and cortical collecting tubules whether placed in the bath and/or the lumen. We conclude that the principal renal action of this drug is to inhibit active Cl transport in the thick ascending limb of Henle’s loop.

Carrier-mediated blood-brain barrier transport of short-chain monocarboxylic organic acids

Abstract: OLDENDORF, WILLIAM H. Carrier-mediated blood-brain barrier transport of short-chain monocarboxylic organic acids. Am. J. Physiol. 224(6) : 1450-l 453. 1973.-The permeability of the blood-brain barrier (BBB) to 15 14C-labeled organic acids of metabolic interest was studied by injecting the test acids rapidly into the common carotid artery and decapitating the rat 15 set later. Brain uptake during the first brain circulatory passage was measured relative to tritiated water injected simultaneously as a highly diffusible internal standard. Competitive inhibition of uptake of the radiolabeled test substance was measured by including unlabeled competitive substances in the injected solution. Uptake of straight-chain saturated monocarboxylic acids increased with chain length and was virtually complete at lengths greater than that of hexanoate. Smaller acids exhibited uptakes which were saturable and, in the case of lactate, stereospecific. No measurable uptake of dior tricarboxylic acids was observed. Cross-inhibition was demonstrated between pyruvate and acetate, propionate, L-lactate, and butyrate but was not shown with octanoate and decanoate. Cross-inhibition was not demonstrated between pyruvate and L-phenylalanine, L-arginine, or D-glucose, indicating that the monocarboxylic acid carrier system demonstrated here is independent of blood-brain barrier amino acid and glucose transport systems.

Effects of ischemia on function and metabolism of the isolated working rat heart.

Abstract: NEELY, J. R., M. J. ROVETTO, J. T. WHITMER, AND H. IL MORGAN. Effects of ischemia on function and metabolism of the isolated wcrking rut heart. Am. J. Physiol. 225(3): 651-658. 1973.-Two techniques have been developed for inducing whole-heart ischemia in the isulated working rat heart, These procedures allowed total coronary Aow to be reduced in hearts that were performing external mechanical work. Reducing the rate of coronary flow resulted in myocardial ischemia as indicated by reduced oxygen delivery and consumption, increased production, and tissue accumulation of lactate, and ventricular failure. A 50y0 reduction in coronary flow resulted in ventricular failure within 8 min. Metabolism of the ischemic hearts was characterized by depletion of tissue highenergy phosphates and accumulation of ADP and AMP. Within 2 min, creatine phosphate and ATP had decreased by 70 and 2597,, respectively, and the level of ADP had increased by 100 %. After 20 min of ischemia, tissue levels of ATP had decreased by 70%, ADP had returned to control levels, and AMP had increased about fivefold. Irreversible ventricular failure occurred in 50 and 100% of the hearts after exposure to about 20 and 30 min of ischemia, respectively,

Effects of parathyroid hormone on renal tubular reabsorption of calcium, sodium, and phosphate.

Abstract: AGUS, ZALMAN S., LAURENCE B. GARDNER, LAURENCE H. BECK, AND MARTIN GOLDBERG. Efects of parathyroid hormone on renal tubular reabsorption of calcium, sodium, and phosphate. Am. J. Physiol. 224(5) : 1143-l 148. 1973.-In order to study the relationships between sodium, calcium, and phosphate reabsorption in the proximal tubule and the effects of parathyroid hormone (PTH) on these relationships, recollection micropuncture and clearance studies were performed in dogs. Proximal tubular fluid/ plasma ultrafiltrate ratios for calcium ((TP/UF)cJ and phosphate ((TF/UF) ) p were 1.1 and 0.7 during the nondiuretic state, and reabsorption of both ions was directly proportional to sodium and fluid reabsorption. PTH resulted in a parallel inhibition of proximal tubular sodium and calcium reabsorption but caused a disproportionate inhibition of phosphate reabsorption ((TF/UF), rose to 0.9). The bulk of rejected phosphate was excreted in the urine. Sodium clearance rose slightly, whereas calcium clearance fell despite the proximal inhibition. Dibutyryl cyclic adenosine monophosphate (cyclic AMP) produced identical effects in the proximal tubule and similar urinary changes. Thus, the reabsorption of calcium and phosphate is closely related to sodium transport in the proximal tubule. Parathyroid hormone, probably via cyclic AMP, inhibits the proximal reabsorption of all three ions, but in a nonparallel manner. Phosphaturia results because the proximally rejected phosphate is not reabsorbed distally to any great extent. On the other hand, PTH reduces calcium excretion by stimulating distal calcium reabsorption out of proportion to that of sodium.

Evidence for nonadrenergic inhibitory nerves in the guinea pig trachealis muscle.

Abstract: COBURN, R. F., AND T. TOMITA. Evidence for nonadrenergic inhibitory nerves in the guinea pig trachealis muscle. Am. J. Physiol. 224(5) : 1072-1080. 1973-The inhibitory innervation of the guinea pig trachealis muscle was studied by placing muscle strips in an organ bath containing Krebs solution equilibrated with 97 y. 02 and 3 To CO2 and measuring the effects of field stimulation on isometric tension in the presence of hyoscine or atropine. In strips obtained from the thoracic trachea: a) relaxations elicited by field stimulation were not sigificantly influenced by propranolol (l-6 X 10B6 g/ml) or practalol (l-5 X 10B4 g/ml). b) Large fieldstimulation relaxations were seen after pretreatment of the animals with approximately 4 mg/kg reserpine. Incubation of these strips with norepinephrine did not influence the magnitude of the relaxations. Catecholamine fluorescence could not be seen after reserpine. c) The muscle was not sensitive to the adrenergic neuron blocking effects of bretylium ( l-6 X 1 Om6 g/ml). d) Maximal relaxations were elicited at lower stimulus frequencies than found with sympathetic nerve stimulation in other tissues. These findings suggest that all or most of the inhibitory innervation is nonadrenergic. Cervical trachealis muscle contained some adrenergic nerves as evidenced by partial suppression of field-stimulation relaxations with beta-blocking drugs and sensitivity to the adrenergic neuron-blocking effects of bretylium.

Central and peripheral circulatory changes after training of the arms or legs

Abstract: cardiac output (Q,), regional arteriovenous oxygen differences, changes to the bradycardia of training. The second is to (axillary and femoral (a-v)02 diff), hepatic clearance of indoevaluate the relative importance of central and peripheral cyanine green (ICG clearance), and aortic blood pressure during factors for other circulatory, respiratory, and metabolic moderate and heavy submaximal exercise. Arm training caused a effects of training. This was done by comparing the response pronounced reduction in HR during arm exercise, whereas only to exercise performed with trained and nontrained muscle a small reduction was seen during exercise performed with nongroups after training of either the arms or the legs. The trained leg muscles. Leg training, however, reduced HR almost equally during leg exercise and arm exercise. After both types present paper deals with data on circulatory variables.

Metabolism of bears before, during, and after winter sleep

Abstract: NELSON, RALPH A., HEINZ W. WAHNER, JAMES D. JONES, RALPH D. ELLEFSON, AND PAUL E. ZOLLMAN. Metabolism of bears before, during, and after winter sleep. Am. J. Physiol. 224(Z) : 491-496. 1973.-Three bears were studied before, during, and after winter sleep. The study lasted 97 days in the 1st year and 102 days in the 2nd year. None of the bears received food or water during winter sleep. In winter sleep the respiratory quotient decreased, blood lipids and creatinine increased, but no changes were noted in blood proteins, amino acids, ammonia, uric acid, urea, sodium, potassium, water content of plasma and red cells, and thyroid hormones. Blood glucose decreased slightly and the hematocrit increased slightly but changes were not significant statistically. In winter sleep, volume of urine appearing in the bladder and urinary nitrogenous compounds derived from protein catabolism were reduced by 95%. When urea was injected intravenously during winter sleep, diuresis occurred and 60-90% of the 10-g dose appeared in the urine. It is proposed that bears achieve their independence from the need of intake of food and water for 100 days or more at near normal body temperatures, without urinating or defecating, by not producing catabolic products of protein metabolism requiring urinary excretion.

Mitochondria and sarcoplasmic reticulum function in cardiac hypertrophy and failure

Abstract: WOOD, AND ARNOLD SCHWARTZ. Mitochondria and sarcoplasmic reticulum function in cardA hypertrophy and failure. Am. J. Physiol. 224(3) : 497-502. 1973.-Mitochondria and “sarcoplasmic reticulum” cardiac relaxing system (CRS) were isolated from rabbit hearts that were in various stages of hypertrophy and failure induced by progressive stenosis of the ascending aorta. The latter was produced by Ameroid clips that gradually swell causing left ventricular overload. Left ventricular and peak isovolumic pressures were measured prior to removal of the hearts for biochemical studies. Mitochondria exhibited marked increases in respiratory activity during early stages of hypertrophy, followed by decreases during later stages. Density gradient analysis revealed alterations in mitochondrial populations during hypertrophy and failure. Mitochondrial calcium uptake was not significantly different in hypertrophied hearts compared to controls, but was markedly reduced in failing hearts. In addition, previously bound calcium was more easily “released” from mitochondria isolated from failing hearts than from control hearts. CRS from hypertrophied and failing hearts revealed significantly decreased rates and total binding of calcium. The results suggest compensatory increases in mitochondrial respiratory function during early hypertrophy. Alteration in intracellular calcium metabolism is suggested with a depression of calcium-binding activity of CRS preceding that of mitochondria.

Length-dependent changes in myocardial contractile state

Abstract: PARMLEY, WILLIAM W., AND LEONARD CHUCK. Length-dependent changes in myocardial contractile state. Am. J. Physiol. 224(5) : 11951199. 1973.-The present study describes an alteration of myocardial contractile state which is intimately associated with changes in muscle length. Seventy-one right ventricular cat papillary muscles were studied isometrically or isotonically in vitro. When muscle length was increased from 10 y0 less than L TnaX up to L mIlx, there was an’ immediate rise in isometric force due to the Starling mechanism. This was followed by a gradual secondary increase in force (average 25 %), which occurred over the subsequent 10 min. T’his effect was reversible, since a reduction in muscle length to lOY, less than L mBX produced a fall in force due to the Starling mechanism, followed by a gradual secondary reduction in force over the next 10 min to the original base line. This slow secondary change in contractile state was unrelated to the cross-sectional area of the muscles or catecholamine depletion by reserpine and persisted during hypoxia. The same effect was seen during isotonic contractions with a preload only, suggesting that it was primarily a length-dependent, rather than force-dependent, phenomenon. This study indicates that one cannot simply relate muscle performance to length without knowledge of the previous history.

Glycolytic enzymes in different types of skeletal muscle: adaptation to exercise

Abstract: BALDWN, K. M., W. W. WINDER, R. L. TERJUNG, AND J. 0. HOLLOSZY. Glycoi’ytic enzymes in di$erent types of skeletal muscle: adaptation to exercise. Am. J. Physiol. 225(4): 962-966. 1973.-The level of activity of hexokinase increased 1700/;, in red skeletal muscle (red portion of quadriceps), 50y0 in intermediate (soleus), and 30% in white skeletal muscle (white portion of quadriceps) in rats subjected to a program of treadmill running. The only other change found in the levels of the glycolytic enzymes in white muscle was a 15 y0 decrease in lactate dehydrogenase activity. In red muscle a small but consistent decrease (approximately 20(%) occurred in the levels of activity of glycogen phosphorylase, phosphofructokinase, glyceraldehyde-3-phosphate dehydrogenase, pyruvate kinase, lactate dehydrogenase, and cytoplasmic aglycerolphosphate dehydrogenase in response to the exercise program. In contrast to the decrease in the levels of these enzymes in red muscle, the exercise induced a 50% increase in cytoplasmic a-glycerolphosphate dehydrogenase activity, and 18-35 y. increases in the levels of phosphorylase, phosphofructokinase, glyceraldehyde-3-phosphate dehydrogenase, and pyruvate kinase in intermediate muscle. As a result of the adaptions induced by exercise, the red and intermediate types of skeletal muscle become more like heart muscle in their enzyme patterns.

Kinetics of unidirectional glucose transport into the isolated dog brain

Abstract: BETZ, A. LORRIS, DAVID D. GILBOE, DAVID L. YUDILEVICH, AND LESTER R. DREWES. Kinefics of unidz’rechzal glucose fransport into the isoluted dog brain. Am. J. Physiol. 225(3): 586592* 1973.An indicator-dilution technique with 22Na as the intravascular marker was used to measure unidirectional transport of D-glucose6-3H from blood into the isolated dog brain. The rate of unidirectional glucose transport, v, was calculated from the equation v = (E 0.036) A Fp/W, where E is the fractional extraction of glucose from the blood, A is the arterial plasma glucose concentration, Fp/W is the plasma flow rate per unit weight of brain, and 0.036 is a correction for glucose diffusion. The diffusion correction was based on the fractional extraction of D-fructose-6-“H, a hexose that is not transported into brain. The results of experiments in which Fp/W was varied while A was held constant indicate that an increased flow rate results in an increased rate of glucose uptake. Because of the Aow effect, the kinetics of unidirectional glucose transport were studied in six brains in which Fp/W was held constant while A was varied between 4.3 and 60 mM. An apparent Kna for glucose transport of 8.26 =t 1.67 mM and a Vmax of 1.75 + .I 1 pmoles/g of brain per minute were calculated. The kinetics were not significantly altered in the presence of pentobarbital (25 mg/liter) or insulin (4 pg/lOO ml), This technique is generally applicable to any isolated, perfused tissue.

Electrolyte transport across isolated large intestinal mucosa

Abstract: BINDER, HENRY J., AND CLAUDIA L. RAWLINS. Electrolyte transport across isolated large intestinal mucosa. Am. J. Physiol. 225(5) : 1232-l 239. 1973.-Transmural electrolyte fluxes, JfA and JzJt, were determined across rat colonic mucosa stripped of its serosal and part of its muscular layer to determine the mechanism of ion transport in the large intestine. In a glucose-free, IX&-Ringer solution short-circuit current (ISC) was 1.2 =k 0.1 pEq/hr-cm2; J $$, 2.5 =k 0.2 pEq/hr*cm2; Jzi,, 1.9 & 0.9 pEq/hr*cm? However, when 10 mM glucose was present in both mucosal and serosal solutions, Jf$ and JfJt increased to 8.8 =k 0.3 and 9.1 k 0.9 pEq/hr cm2, respectively, but I,, was not changed. Selective removal of Cl or HCQ in the presence of glucose significantly decreased Jr$. JE& was also significantly inhibited by the selective removal of HCQ or Na in the presence of glucose. JFz and Jz& but not I sc were inhibited by acetazolamide. These results are best explained by the presence of two Na-transport systems: an “electrogenic” and a “neutral” or coupled Na-Cl transport. It is proposed that the coupled NaCl absorption may represent two related ion exchanges: Na-H and Cl-HC03. This model of ion transport is consistent with electrolyte movement in vivo.

Relative importance of kidney and liver in synthesis of arginine by the rat

Abstract: FEATHERSTON, W. R.,Q. R. ROGERS, AND R. A. FREEDLAND. Relative importance of kidney and liver in synthesis of arginine by the rat. Am. J. Physiol. 224(l) : Yl27-129. 1973.-The relative contributions of the kidneys and liver to the total synthesis of arginine in the rat were studied by following the conversion of intravenously injected L-citrulline-ureido-14C to L-arginine guanido-14C in intact controls or in rats with either the kidneys, or liver, or both completely ligated. The incorporation of the labeled arginine into the trichloroacetic acid-precipitable protein from liver, kidney, skeletal muscle, brain, and lung was determined. A greater decrease in the specific activity of skeletal muscle, lung, and liver protein was observed when the kidneys were ligated than occurred in the skeletal muscle, lung, and kidney protein when the liver was ligated. The specific activity of skeletal muscle protein was approximately 3.3 times higher when the liver was ligated than when the kidneys were ligated. Ligation of either the kidney, liver, or both had less of an effect on lowering the specific activity of brain protein than the ligation had on the specific activity of skeletal muscle protein indicating the brain may be synthesizing a portion of its own arginine from citrulline. The results provide evidence that the kidneys synthesize a considerably larger portion of the endogenous arginine used for body protein synthesis than does the liver.

Influences of hepatic portal receptors on hypothalamic feeding and satiety centers.

Abstract: &IMITT, n/TARU. hy%mms Uf hepi& purtd raCe@rS on @POthalamic feeding and satiety centers. Am. J. Physiol. 225(5) : IO89IO95 1973.-Evidence for a neural pathway from the hepatic portal vein to the lateral hypothalamus was found. Injections of hypertofiic saline and/or glucose into the portal vein of the rat produced changes in firing rate of certain lateral hypothalamic neurons. No responses to such injections were found in the supraoptic, paraventricular, or ventromedial nuclei. The response could be either one of increase or decrease in firing rate. Section of splanchnic nerves bilaterally or cord transection at the level of T5 eliminated these responses. Severing both vagi did not abolish but exaggerated the response. Ammonium chloride injection into the portal vein did affect hypothalamic units, but this action was not abolished by section of the spinal cord or severing afferent nerves.

Micropuncture study of electrolyte transport across papillary collecting duct of the rat.

Abstract: DIEZI, JACQUES, PIERRETTE MICHOUD, JORGE ACEVES, AND GERHARD GIEBISCH. Micropuncture study of electrolyte transport across papillary collecting duct of the rat. Am. J. Physiol. 224(3): 623-634. 1973.-Samples of collecting duct (CD) fluid were obtained by micropuncture from different sites along the exposed renal papilla of young rats. Fluid was analyzed for inulin-14C, K, Na, and/or Cl. Experiments were carried out in rats on I) normal electrolyte intake or after 2) Na depletion, 3) K depletion, 4) Cl depletion, 5) K loading, and 6) saline loading. Net water and Na reabsorption was observed in all experimental conditions. Na reabsorption was stimulated by Na depletion. The highest rates of Na reabsorption approaching saturation obtained in saline-loaded rats. Net movement of K was frequently absent in control and saline-loaded animals; Na and K depletion resulted in consistent K reabsorption, whereas K loading induced K secretion in some animals. Significant net reabsorption of Cl occurred in control and Cl-depleted animals. From consideration of the electrochemical potential gradient, it is thought that Na, K, and Cl reabsorption across the collecting duct epithelium is active in nature.

Substrate depletion in different types of muscle and in liver during prolonged running.

Abstract: BALDWIN, K. M., J. S. REITMAN, R. L. TERJUNG, W. W, WINDER, AND J. 0. HOLLOSZY. Substrate depktion in dz’fmnt types of muscle and in liver during prolonged running. Am. J. Physiol. 2 2 5(5) : 1045-l 050. 1973,KDepletion of glycogen and triglyceride stores in red, white, and intermediate types of skeletal muscle and of glycogen in liver was studied in rats subjected to prolonged exercise. Groups of rats were killed after 15, 60, and 120 min of running up an 8’ incline at a) 1 mph, b) I mph with I min at 1.5 mph every 9 min, and c) alternate 30-set intervals at 0.5 and 1,5 mph. White muscle underwent little or no change in glycogen and triglyceride concentrations in response to the three exercise tests. In red muscle and intermediate muscle, the decrease in glycogen concentration ranged from 42 to 727, after 2 hr; glycogen decreased most sharply during the first 15 min of all three exercise tests and then stabilized or decreased more slowly. Triglyceride concentration in red muscle decreased approximately 4Oaj’, after 2 hr of all three exercise tests. Little or no change in triglyceride concentration occurred in white and intermediate muscle. Liver glycogen decreased progressively during all three exercise tests and was depleted approximately 85y0 after 2 hr. These results suggest that in the rat performing prolonged moderate exercise, a) white muscle contracts infrequently and b) liver glycogen can be a more important source of energy than muscle glycogen.

Effect of amiloride on sodium transport across body surfaces of freshwater animals.

Abstract: KIRSCHNER, LEONARD B., LEWIS GREEN~ALD, AND THEODORE H. KERSTETTER. Effect of amiloride on sodium transport across body surfaces of freshwater animals. Am. J. Physiol. 224(4): 832-837. 1973.-Amiloride, added to an external bathing solution at a concentration of 1 X 10m4 M, inhibited sodium influx in intact trout (79 %), frogs (460/,), and crayfish (90 yO). It had no effect on efflux across the body surface. Chloride fluxes, measured simultaneously, were also unaffected. Ammonia efflux was inhibited by amiloride to a lesser extent in trout (300/,) and crayfish (54’7,). It was not excreted at measurable rates by the frog in the presence or absence of the inhibitor. Hydrogen ion excretion, calculated on the assumption that only NH3 (not NHJ+) was excreted, was inhibited by amiloride in all three animals: trout 56 %, frogs 407,, and crayfish >90%. The data suggest that the initial step in active sodium transport across the body surface is similar in freshwater animals from at least two phyla. Under natural conditions (a dilute external medium), this step appears to require a coupled Na+/H+ exchange to maintain electrical neutrality. The amiloride data also admit the possibility of NHJ+ excretion balancing a fraction of the Na+ influx in some animals.

Respiratory control of arterial pH as temperature changes in rainbow trout Salmo gairdneri.

Abstract: RANDALL, D. J., AND J. N. CAMERON. Respiratory control of arterial pH as temperature changes in rainbow trout Salmo gairdneri. Am. J. Physiol. 225(4) : 997-1002. 1973.-Arterial blood pH decreased as temperature increased, dpH/dT was 0.017 unit/“C and was independent of the acclimation history of the fish. Ventilation volume increased in proportion to oxygen uptake with temperature. Changes in water flow over the gills with temperature were associated with large changes in breathing rate and small changes in the volume of water pumped each breathing cycle. Percent oxygen extraction from water flowing over the gills and arterial oxygen tension did not change with temperature. The partial pressure of CO2 in arterial blood and the content of molecular CO2 in plasma did not change with temperature. Decreases in blood pH were associated with a fall in plasma total COP and bicarbonate.

Energy production of rat extensor digitorum longus muscle.

Abstract: WENDT, I. R., AND C. L. GIBBS. Energy production of rat extensor digitorum longus muscle. Am. J. Physiol. 224(5) : 1081-1086 1973.The myothermic technique was employed to investigate the energy production of the fast-twitch extensor digitorum longus (EDL) muscle of the rat. Both isotonic and isometric tetani were studied. The total energy liberated during an isotonic tetanus was found to be maximal with loads approximately 0.6 PO (where P, is the peak isometric tension) and was always minimal for the lightest loads. The mechanical efficiency of these muscles appeared to be quite low (maximally 7.3-14.2y0 with the stimulus parameters employed). In isometric experiments, the maintenance heat rate varied with muscle length. At 1, length, the heat rate was 32.6 meal/g muscle per second, and it fell with tension when the muscles were either stretched or shortened. In stretched muscles the tension-independent heat rate was 11.3 meal/g muscle per second and the activation heat, in response to a single stimulus, was estimated to be 0.57 meal/g muscle over both the initial and recovery phases. These values are considerably higher than comparable values reported for the slow-twitch soleus muscle, and it is suggested that the greater energy expenditure of the fast-twitch EDL muscle reflects higher myosin ATPase activity and a more extensive and active sarcoplasmic reticulum. It is concluded that the fasttwitch EDL muscle sacrifices energetic economy in return for speed of response.