Aspartic acid 214 in Citrobacter freundii tyrosine phenol-lyase ensures sufficient C–H-acidity of the external aldimine intermediate and proper orientation of the cofactor at the active site

In the X-ray structure of tyrosine phenol-lyase (TPL) Asp214 is located at H-bonding distance from the N1 atom of the cofactor. This residue has been replaced with Ala and Asn and the properties of the mutant enzymes have been studied. The substitutions result in a decrease in the cofactor affinity of about four orders of magnitude. D214A and D214N TPLs do not catalyze the decomposition of l-Tyr and 3-fluoro-l-Tyr. They decompose substrates, containing better leaving groups with rates reduced by one or two orders of magnitude. Lognormal resolution of the spectra of the mutant enzymes revealed that the N1 atom of the cofactor is deprotonated. Spectral characteristics of internal and external aldimines of the mutant TPLs and the data on their interaction with quasisubstrates demonstrate that replacements of Asp214 lead to alteration of active site conformations. The mutant enzymes do not form noticeable amounts of a quinonoid upon interaction with inhibitors, but catalyze isotope exchange of C-alpha-proton of a number of amino acids for deuterium in (2)H(2)O. The k(ex) values for the isotope exchange of l-phenylalanine and 3-fluoro-l-tyrosine are close to the k(cat) values for reacting substrates. Thus, for the mutant TPLs the stage of C-alpha-proton abstraction may be considered as a rate-limiting for the whole reaction.

Role of Arginine 226 in the Mechanism of Tryptophan Indole-Lyase from Proteus vulgaris

In the spatial structure of tryptophanase from Proteus vulgaris the guanidinium group of arginine 226 forms a salt bridge with the 3;-oxygen atom of the coenzyme. The replacement of arginine 226 with alanine using site-directed mutagenesis reduced the affinity of the coenzyme for the protein by one order of magnitude compared to the wild-type enzyme. The catalytic activity of the mutant enzyme in the reaction with L-tryptophan was reduced 10(5)-fold compared to the wild-type enzyme. The rates of the reactions with some other substrates decreased 10(3)-10(4)-fold. The mutant enzyme catalyzed exchange of the C-alpha-proton in complexes with some inhibitors with rates reduced 10(2)-fold compared to the wild-type enzyme. Absorption and circular dichroism spectra of the mutant enzyme and the enzyme-inhibitor complexes demonstrate that the replacement of arginine 226 with alanine does not significantly affect the tautomeric equilibrium of the internal aldimine, but it leads to an alteration of the optimal conformation of the coenzyme-substrate intermediates.

Light-associated and processing-dependent protein binding to 5' regions of rbcL mRNA in the chloroplasts of a C4 plant

In amaranth, a C(4) dicotyledonous plant, the plastid rbcL gene (encoding the large subunit of ribulose-1,5-bisphosphate carboxylase) is regulated post-transcriptionally during many developmental processes, including light-mediated development. To identify post-transcriptional regulators of rbcL expression, three types of analyses (polysome heel printing, gel retardation, and UV cross-linking) were utilized. These approaches revealed that multiple proteins interact with 5' regions of rbcL mRNA in light-grown, but not etiolated, amaranth plants. Light-associated binding of a 47-kDa protein (p47), observed by UV cross-linking, was highly specific for the rbcL 5' RNA. Binding of p47 occurred only with RNAs corresponding to mature processed rbcL transcripts (5'-untranslated region (UTR) terminating at -66); transcripts with longer 5'-UTRs did not associate with p47 in vitro. Variations in the length of the rbcL 5'-UTR were found to occur in vivo, and these different 5' termini may prevent or enhance light-associated p47 binding, possibly affecting rbcL expression as well. p47 binding correlates with light-dependent rbcL polysome association of the fully processed transcripts in photosynthetic leaves and cotyledons but not with cell-specific rbcL mRNA accumulation in bundle sheath and mesophyll chloroplasts.

Citrobacter freundii tyrosine phenol-lyase: the role of asparagine 185 in modulating enzyme function through stabilization of a quinonoid intermediate

Asn185 is an invariant residue in all known sequences of TPL and of closely related tryptophanase and it may be aligned with the Asn194 in aspartate aminotransferase. According to X-ray data, in the holoenzyme and in the Michaelis complex Asn185 does not interact with the cofactor pyridoxal 5'-phosphate, but in the external aldimine a conformational change occurs which is accompanied by formation of a hydrogen bond between Asn185 and the oxygen atom in position 3 of the cofactor. The substitution of Asn185 in TPL by alanine results in a mutant N185A TPL of moderate residual activity (2%) with respect to adequate substrates, L-tyrosine and 3-fluoro-L-tyrosine. The affinities of the mutant enzyme for various amino acid substrates and inhibitors, studied by both steady-state and rapid kinetic techniques, were lower than for the wild-type TPL. This effect mainly results from destabilization of the quinonoid intermediate, and it is therefore concluded that the hydrogen bond between Asn185 and the oxygen at the C-3 position of the cofactor is maintained in the quinonoid intermediate. The relative destabilization of the quinonoid intermediate and external aldimine leads to the formation of large amounts of gem-diamine in reactions of N185A TPL with 3-fluoro-L-tyrosine and L-phenylalanine. For the reaction with 3-fluoro-L-tyrosine it was first possible to determine kinetic parameters of gem-diamine formation by the stopped-flow method. For the reactions of N185A TPL with substrates bearing good leaving groups the observed values of k(cat) could be accounted for by taking into consideration two effects: the decrease in the quinonoid content under steady-state conditions and the increase in the quinonoid reactivity in a beta-elimination reaction. Both effects are due to destabilization of the quinonoid and they counterbalance each other. Multiple kinetic isotope effect studies on the reactions of N185A TPL with suitable substrates, L-tyrosine and 3-fluoro-L-tyrosine, show that the principal mechanism of catalysis, suggested previously for the wild-type enzyme, does not change. In the framework of this mechanism the observed considerable decrease in k(cat) values for reactions of N185A TPL with L-tyrosine and 3-fluoro-L-tyrosine may be ascribed to participation of Asn185 in additional stabilization of the keto quinonoid intermediate.

Effect of creatine phosphate on the contractile activity in acutely failing rat heart

The hypothesis was tested that infusion of a solution containing creatine phosphate (CP) into rats with acutely failing hearts would enhance recovery of cardiac function. The acutely failing heart was produced by constricting the ascending aorta. This overload produced failure in approximately 25 min. At the point of failure the constriction was removed and solutions containing sterile physiological saline (PSS), PSS and CP, PSS and creatine, or PSS and creatine plus phosphate were infused. Cardiac function was assessed from systolic and diastolic blood pressure, +/- dp/dt, heart rate, and cardiac work. Ca2+ uptake by isolated sarcoplasmic reticulum and the concentrations of selected blood and tissue metabolites were measured. Normal cardiac function was restored in the PSS-CP infused rats whereas all other treatments did not restore cardiac function. Adenosine triphosphate and CP had declined in the myocardium of the failing hearts while lactate was elevated. The concentrations of these metabolites were normal in the PSS-CP infused animals. The glycogen concentration in the myocardium was reduced following the constriction. Ca2+ uptake by isolated sarcoplasmic reticulum was depressed in the failed hearts but normal in the hearts of CP-infused animals. These results demonstrate that the infusion of CP into animals with failing hearts can be effective in restoring cardiac function.

Myosin heavy chain isoforms of human muscle after short-term spaceflight

The influence of microgravity on the myosin phenotype of skeletal muscle fibers in the vastus lateralis of eight crew members was studied before and after 5-day (n = 3) and 11-day (n = 5) spaceflights (space shuttle flights: STS-32, -33 and -34). Single-fiber electrophoresis analyses showed that the proportion of fibers expressing only slow (type I) myosin heavy chain (MHC) in the vastus lateralis was significantly lower after than before 11 days of spaceflight. Although the family of type II MHC isoforms was elevated post- compared with preflight, the distribution among the isoforms of type II MHC was not statistically different. Based on monoclonal and polyclonal antibodies specific for three adult MHC isoforms and single-fiber electrophoresis, approximately 3% of the fibers analyzed coexpressed all three adult MHC isoforms. The results from immunohistochemical staining with two different sets of antibodies indicate a reduction in the percentage of fibers expressing type I MHC as a result of spaceflight. The mean difference, however, was significant only when the fibers were categorized simply as type I or II. These changes appeared to be highly individualized among the astronauts. These results suggest that a rapid change in MHC isoform expression can occur in some muscle fibers after a relatively brief exposure to spaceflight.

Human fiber size and enzymatic properties after 5 and 11 days of spaceflight

Biopsies from the vastus lateralis muscle were obtained from three astronauts before and after two 5-day flights and from five astronauts before and after one 11-day flight (space shuttle flights: STS-32, -33, and -34). Muscle fibers from two separate samples from each biopsy were classified as type I and II or as type I, IIA, and IIB by using qualitative myofibrillar adenosinetriphosphatase (ATPase) staining. Cross-sectional area (CSA), number of capillaries per fiber, and the activities of succinate dehydrogenase (SDH), alpha-glycerophosphate dehydrogenase (GPD), and myofibrillar ATPase were determined from one sample of fibers of each myofibrillar ATPase type. Postflight biopsies had 6-8% fewer type I fibers than preflight. Mean fiber CSAs were 16-36% smaller after the 11-day flight with the relative effect being type IIB > IIA > I. Mean fiber CSAs were 11 and 24% smaller in type I and II fibers after 5 days of flight. Myofibrillar ATPase activities increased in type II but not in type I fibers after flight, whereas SDH activity was unaffected in either fast or slow fibers. GPD activity in type I fibers was approximately 80% higher (P > 0.05) postflight compared with preflight. Myofibrillar ATPase/SDH ratios in type II fibers were higher after than before flight, suggesting that some fast fibers were more susceptible to fatigue after flight. The GPD/SDH ratios were elevated in some type I fibers after spaceflight. The number of capillaries per fiber was 24% lower after than before flight, whereas the number of capillaries per unit CSA of muscle tissue was unchanged. These data suggest that adaptations in the size, metabolic properties, and vascularity of muscle fibers can occur rapidly in the space environment. These adaptations were qualitatively similar to those observed in animals after actual or simulated spaceflight conditions for short periods.

Effect of exercise on hexokinase distribution and mitochondrial respiration in skeletal muscle

Horses were subjected to treadmill running at 65% (submaximal) or 100% (maximal) VO2,max to examine the effects of exercise on subcellular distribution of hexokinase (HK) and on mitochondrial respiration. It is hypothesized that the fraction of HK bound to mitochondria will be reduced due to an elevation of glucose-6-phosphate (G-6-P) concentration in the exercising muscle and that such release of HK from mitochondria will depress oxidative phosphorylation. Changes in muscle G-6-P concentration, pH, subcellular HK distribution, mitochondrial respiration and other metabolites were determined in biopsy samples pre-exercise, immediately post-exercise and during the recovery phase. The fraction of HK associated with mitochondria decreased from 38% to 7% at the end of maximal exercise; exercise at VO2,max also reduced respiratory capacity of muscle homogenates by 20% and was associated with a fivefold increase in muscle [G-6-P], a potent agent known to dissociate HK from mitochondria. The HK distribution returned to normal within 60 min after exercise and the reassociation of the HK with mitochondria parallelled the removal of muscle G-6-P. No changes in muscle HK distribution and respiration were found following the submaximal exercise despite the fact that G-6-P was slightly elevated. Muscle concentrations of adenosine triphosphate, creatine phosphate and glycogen and pH dropped after exercise while lactate concentration increased. The amount of mitochondria-bound HK was also altered in vitro in a preparation of mitochondria isolated from rat skeletal muscle to examine the effect of the bound HK on mitochondrial respiration.(ABSTRACT TRUNCATED AT 250 WORDS)

Equine plasma and blood volumes decrease with dehydration but subsequently increase with exercise

The effects of dehydration and 40 min of exercise at approximately 40% of maximal O2 consumption on plasma volume (PV) and blood volume (BV) were studied in six horses. Horses were exercised while euhydrated (C); 4 h after administration of furosemide (1.0 mg/kg i.v.; FDH), which induced isotonic dehydration; and after 30 h without water (DDH), which caused hypertonic dehydration. Dehydration resulted in decreases of 6.3 and 9.9% for PV and BV, respectively, with FDH and 10.7 and 8.5%, respectively, with DDH. During exercise in C, PV and BV increased by 12.7 and 20.0%, respectively; during exercise with FDH, they increased by 11.7 and 26.1%, respectively; and during exercise with DDH, PV decreased by 1.3% from predehydration values, while BV increased by 18.7%. Hematocrit and total plasma protein concentration rose to higher values throughout exercise in FDH and DDH than in C; plasma [Na+] was higher in DDH than in FDH and C, [Cl-] was higher in DDH and lower in FDH than in C, and [K+] was lower in FDH and DDH than in C through exercise and recovery. From these results, we conclude that increases in PV and BV are normal features of low-intensity exercise in the horse. The increases in BV not only augment O2 carriage but also help maintain circulating volume. These increases can be modified by preexercise dehydration, the nature of which affects the extent of modification.

Effects of dehydration on thermoregulatory responses of horses during low-intensity exercise

Effects of dehydration on thermoregulatory and metabolic responses were studied in six horses during 40 min of exercise eliciting approximately 40% of maximal O2 consumption and for 30 min after exercise. Horses were exercised while euhydrated (C), 4 h after administration of furosemide (FDH; 1.0 mg/kg i.v.) to induce isotonic dehydration, and after 30 h without water (DDH) to induce hypertonic dehydration. Cardiac output was significantly lower in FDH (144.1 +/- 8.0 l/min) and in DDH (156.6 +/- 6.9 l/min) than in C (173.1 +/- 6.2 l/min) after 30 min of exercise. When DDH, FDH, and C values were compared, dehydration resulted in higher temperatures in the middle gluteal muscle (41.9 +/- 0.3, 41.1 +/- 0.2, and 40.6 +/- 0.2 degrees C, respectively) and pulmonary artery (40.8 +/- 0.3, 40.1 +/- 0.2, and 39.7 +/- 0.2 degrees C, respectively). Temperatures in the superficial thoracic vein and subcutaneous sites on the neck and back and peak sweating rates on the neck and back were not significantly different in DDH and C. In view of higher core temperatures during exercise after dehydration and decrease in cardiac output without concomitant increases in peripheral temperatures or reduced sweating rates, we conclude that the impairment of thermoregulation was primarily due to decreased transfer of heat from core to periphery.

Dissipation of metabolic heat in the horse during exercise

Horses were exercised at 40, 65, and 90% of their maximum O2 uptake (VO2max) until moderately fatigued (approximately 38, 15, and 9 min, respectively) to assess heat loss through different routes. Approximately 4,232, 3,195, and 2,333 kcal of heat were generated in response to exercise at these intensities. Of this, approximately 7, 16, and 20% remained as stored heat 30 min postexercise. Respiratory heat loss, estimated from the temperature difference between blood in the pulmonary and carotid arteries and the cardiac output, was estimated to be 30, 19, and 23% of the heat produced during exercise at the three intensities. The kinetics of the increases in muscle and blood temperature were similar, with the greatest change in temperature occurring in muscle (+3.8, 5.2, and 6.1 degrees C after exercise at 40, 65, and 90% of VO2max, respectively). The temperature of blood in the superficial thoracic vein was approximately 2 degrees C below that of arterial blood at rest. This difference had increased to approximately 3 degrees C during the last minute of exercise. The rate of sweating at sites on the back and neck increased with exercise intensity to a common peak of approximately 40 ml.m-2.min-1. If complete evaporation had occurred, water loss in response to exercise (estimated to be 12, 10, and 7.7 liters for the different intensities of exercise) greatly surpassed that required for dissipation of the metabolic heat load.

Substrates for muscle glycogen synthesis in recovery from intense exercise in man

1. Intramuscular glyconeogenesis from lactate after intense exercise was examined by using the one-legged knee extension model which enables evaluation of metabolism in a well-defined muscle group. 2. In seven subjects measurements of leg blood flow and arterial-venous differences of various substrates were performed in individuals after intense, exhaustive knee extensor exercise lasting 3.0 min. Muscle glycogen and lactate concentrations were determined in the quadriceps muscle immediately after exercise and three times during 1 h of recovery. 3. Muscle glycogen increased from 93.7 +/- 6.7 (+/- S.E.M.) to 108.8 +/- 8.1 mmol (kg wet wt)-1 during the recovery period. Muscle lactate was 27.1 +/- 2.1 mmol (kg wet wt)-1 at the end of exercise and decreased to 14.5 +/- 2.1, 6.7 +/- 1.1, and 3.0 +/- 0.5 mmol (kg wet wt)-1 after 3, 10 and 60 min of recovery, respectively. 4. More than two-thirds of the lactate that accumulated in the muscle during the intense exercise was released into the blood. It was estimated that between 13 and 27% of the lactate could have been converted to glycogen. This corresponded to a glycogen resynthesis rate from lactate of 0.17-0.34 and 0.002 mmol glucosyl units min-1 (kg wet wt)-1 for the first 10 and last 50 min of recovery, respectively. 5. The O2 debt of the leg was 1.5 l of which the resynthesis of ATP, creatine phosphate (CP) and glycogen and reloading of haemoglobin (Hb) and myoglobin (Mb) only could account for one-third. It is proposed that the elevated oxygen uptake during recovery is linked to the metabolic use of intramuscular triacylglycerol.

Respiratory and metabolic responses in the horse during moderate and heavy exercise

Thoroughbred horses were exercised to fatigue on a treadmill at 62% and 100% of their VO2max. Hypoxemia occurred at the onset of exercise under both exercise conditions. This hypoxemia persisted to fatigue during the heavy exercise but progressively diminished as the exercise continued and had disappeared by the end of exercise at the lighter load. As a result of the hypoxemia the oxygen content of arterial blood during exercise at VO2max was 17% below its carrying capacity. However, under both experimental conditions the CaO2 still exceeded that of rest owing to an elevation in hemoglobin concentration. The temperature of blood at the point of fatigue was similar, 41.0 +/- 0.2 degrees C and 41.1 +/- 0.2 degrees C, for exercise at 62% and 100% VO2max, respectively. Muscle samples collected at rest and at the termination of exercise did not demonstrate major differences between the exercise conditions except for a higher [lactate] and lower pH following the heavy exercise. From these results it can be suggested that the combined effects of an elevated body temperature, changes in muscle pH, and oxygen delivery may all be factors contributing to limit exercise capacity in the horse.

Ammonia metabolism during intense dynamic exercise and recovery in humans

This study examined the dynamics for ammonia (NH3) metabolism in human skeletal muscle during and after intense one-legged exercise. Subjects (n = 8) performed dynamic leg extensor exercise to exhaustion (3.2 min). Muscle NH3 release increased rapidly to a maximum of 314 +/- 42 mumol/min and declined immediately on cessation of exercise. Recovery was complete in approximately 20 min. Arterial [NH3] increased less rapidly and reached its maximum 2-3 min into recovery. These data demonstrate that NH3 clearance is more sensitive to the cessation of exercise than is NH3 release from skeletal muscle. Muscle [NH3] increased three to fourfold during exercise and represented 74 +/- 8% of the total net NH3 formation. Thus the change in muscle [NH3] alone underestimates the NH3 production. There was no evidence that the muscle-to-venous blood NH3 ratio shifts in accordance with the H+ data. Thus other factors must contribute to the NH3 release from active muscle. The total net NH3 formed corresponded with the intramuscular inosine 5'-monophosphate accumulation, suggesting that the NH3 was derived from AMP deamination. Changes in the known modulators of AMP deaminase (ATP, ADP, H+) were moderate, so the mechanisms initiating the deamination remain obscure.

Muscle responses of thoroughbreds to conventional race training and detraining

Ten healthy sedentary Thoroughbreds with previous race training experience were trained conventionally for 9 weeks. Muscle biopsy samples were obtained before and after training and after 6 weeks of detraining pasture rest. Biopsy samples were obtained from the right deltoid, triceps, vastus lateralis, middle gluteal, biceps femoris, and semitendinosus muscles. The deep-frozen biopsy samples were analyzed for activities of succinate dehydrogenase (SDH), 3-hydroxy-acylcoenzyme A dehydrogenase (HAD), and phosphorylase (PHOS) and for glycogen concentration. The triceps and gluteal muscle samples were also serially sectioned and stained for myofibrillar actomyosin adenosine triphosphatase (ATPase) activity after alkaline (pH 10.3) and sequential acidic (pH 4.34) ATPase inactivation. Fiber types I (alkaline preincubation), IIA1, IIA2, and IIA3 (sequential acidic preincubation over 5 minutes) were identified and were evaluated for fiber-type distribution and fiber areas. Increases in response to training were observed in deltoid and vastus muscle SDH and gluteal muscle HAD activities, and deltoid muscle glycogen concentration (P less than 0.05 to P less than 0.01). Changes in PHOS activity were not observed. Type-IIA1, -IIA2, and -IIA3 fiber areas in triceps muscle were increased in response to training (P less than 0.05 to P less than 0.01). Changes in fiber-type distribution did not occur in response to training. Changes in muscle enzyme activities, glycogen concentration, fiber types, and fiber areas were not seen from posttraining to detraining. Further increases were observed when detraining values were compared with pretraining values in deltoid, triceps, vastus, gluteal, and biceps femoris muscle SDH activities and in gluteal muscle glycogen concentration (P less than 0.05 to P less than 0.01).(ABSTRACT TRUNCATED AT 250 WORDS)

Kinetics of VO2 and VCO2 in the horse and comparison of five methods for determination of maximum oxygen uptake

To determine whether maximum oxygen uptake (VO2max) in the horse in influenced by type of exercise test, five different protocols were evaluated in eight untrained Thoroughbreds exercised on a treadmill. With all protocols, horses were given a 5 min warm-up on a 10 per cent treadmill slope. Three protocols were at a 10 per cent slope and included: 1) increasing the running speed by 1 to 2 m/sec every 60 secs from 4 m/sec to a maximum of 12 m/sec; 2) running at 12 m/sec until fatigue; and 3) running for 3 to 4 mins at speeds ranging from 6 to 12 m/sec with rest pauses between exercise bouts. The fourth protocol employed a 24 per cent slope where animals exercised for 5 mins at 2 m/sec followed by 3 mins at 5 m/sec, and 2 mins at 6 m/sec. The fifth test involved running at 11 m/sec while the slope of the surface was increased every 2 mins until the horses could not maintain the pace. A plateau in VO2 occurred in all protocols except when exercise was performed on a 24 per cent slope. Slow speed exercise at a steep grade (24 per cent) produced the highest VO2 (P < 0.05) even though there was no plateau in the VO2. A steady state for VO2 and VCO2 existed 90 secs after the onset of exercise with all protocols which involved 2 mins or more at each speed. The VO2 and VCO2 values at all speeds of the rapid incremental exercise test were not different to those found at steady state in the third exercise protocol.

Standardized exercise test and daily heart rate responses of thoroughbreds undergoing conventional race training and detraining

Ten healthy sedentary male Thoroughbreds with previous race training experience were studied for 14 weeks. Horses were trained for 9 weeks, using a program designed after those used commonly in the United States. Horses were trained conventionally by slow trotting (250 m/min) for 2 weeks and galloping (390 to 450 m/min) for 4 weeks, followed by 3 weeks of galloping (440 to 480 m/min) and intermittent sprinting exercises (breezes) at distances between 600 and 1,000 m (900 to 950 m/min). The horses were then pasture rested for 5 weeks. A standardized exercise test (SET) involving an 800-m gallop at 800 m/min was administered before and after the 9-week training period and after the 5-week detraining period. Heart rate (HR) was monitored during exercise and at standardized intervals after exercise for 60 minutes. Venous blood for determination of plasma lactate concentration was obtained at 5 minutes after exercise. Heart rate was monitored daily at rest, during exercise, and through the first 60 minutes of recovery. Venous plasma samples (for lactate determination) were obtained 5 minutes after the sprinting exercises. Horses were observed daily before exercise for signs of lameness and were not allowed to train if lame. Differences after 9 weeks' training were seen in the SET recovery HR at 0.5 through 5 minutes after exercise (P less than 0.05 to P less than 0.01). Differences after detraining were seen in the SET recovery HR at 40 and 60 minutes after exercise (P less than 0.05 to P less than 0.01). Neither training nor detraining resulted in differences in plasma lactate concentration after the SET gallop.(ABSTRACT TRUNCATED AT 250 WORDS)

Anaerobic energy production and O2 deficit-debt relationship during exhaustive exercise in humans

1. Eight subjects performed one-legged, dynamic, knee-extensor exercise, first at 10 W followed by 10 min rest, then at an intense, exhaustive exercise load (65 W) lasting 3.2 min. After 60 min recovery, exercise was performed for 8-10 min each at 20, 30, 40 and 50 W. Measurements of pulmonary oxygen uptake, heart rate, blood pressure, leg blood flow, and femoral arterial-venous differences of oxygen content and lactate were performed as well as determination of ATP, creatine phosphate (CP) inosine monophosphate (IMP) and lactate concentrations on biopsy material from the quadriceps muscle before and immediately after the intense exercise, and at 3, 10 and 60 min into recovery. 2. Individual linear relations (r = 0.95-1.00) between the power outputs for submaximal exercise and oxygen uptakes (leg and pulmonary) were used to estimate the energy demand during intense exercise. Pulmonary and leg oxygen deficits determined as the difference between energy demand and oxygen uptake were 0.46 and 0.48 l (kg active muscle)-1, respectively. Limb and pulmonary oxygen debts (oxygen uptake during 60 min of recovery - pre-exercise oxygen uptake) were 0.55 and 1.65 l (kg active muscle)-1, respectively. 3. During the intense exercise, muscle [ATP] decreased by 30% and [CP] by 60% from resting concentrations of 6.2 and 22.4 mmol (kg wet wt)-1, respectively, and [IMP] increased to 1.1 mmol (kg wet wt)-1. Muscle [lactate] increased from 2 to 28.1 mmol (kg wet wt)-1, and the concomitant net lactate release was 14.8 mmol (kg wet wt)-1 or about 1/3 of the total net lactate production. During recovery 70% of the accumulated lactate was released to the blood, and the nucleotides and CP returned to about 40 and 85% of pre-exercise values at 3 and 10 min of recovery, respectively. 4. Total reduction in ATP and CP (and elevation of IMP) during the intense exercise amounted to 16.4 mmol ATP (kg wet wt)-1, which together with the lactate production accounted for 83.1 mmol ATP (kg wet wt)-1. In addition 6-8 mmol ATP (kg wet wt)-1 are made available related to accumulation of glycolytic intermediates including pyruvate (and alanine). Estimated leg oxygen deficit corresponded to an ATP production of 94.7 mmol ATP kg-1; this value included 3.1 mmol kg-1 related to unloading of HbO2 and MbO2.(ABSTRACT TRUNCATED AT 400 WORDS)

Effects of acetazolamide on metabolic and respiratory responses to exercise at maximal O2 uptake

Changes in blood gases, ions, lactate, pH, hemoglobin, blood temperature, total body metabolism, and muscle metabolites were measured before and during exercise (except muscle), at fatigue, and during recovery in normal and acetazolamide-treated horses to test the hypothesis that an acetazolamide-induced acidosis would compromise the metabolism of the horse exercising at maximal O2 uptake. Acetazolamide-treated horses had a 13-mmol/l base deficit at rest, higher arterial Po2 at rest and during exercise, higher arterial and mixed venous Pco2 during exercise, and a 48-s reduction in run time. Arterial pH was lower during exercise but not in recovery after acetazolamide. Blood temperature responses were unaffected by acetazolamide administration. O2 uptake was similar during exercise and recovery after acetazolamide treatment, whereas CO2 production was lower during exercise. Muscle [glycogen] and pH were lower at rest, whereas heart rate, muscle pH and [lactate], and plasma [lactate] and [K+] were lower and plasma [Cl-] higher following exercise after acetazolamide treatment. These data demonstrate that acetazolamide treatment aggravates the CO2 retention and acidosis occurring in the horse during heavy exercise. This could negatively affect muscle metabolism and exercise capacity.

The effect of high-intensity exercise on the respiratory capacity of skeletal muscle

The effect of high-intensity exercise on the respiratory capacity of skeletal muscle was studied in horses which ran five 600-m bouts on a track with 2 min of rest between exercise bouts, or once to fatigue on a treadmill at an intensity that elicited the maximal oxygen uptake. Venous blood and biopsy samples of the middle gluteal muscle were collected at rest, after each exercise bout, and 30 and 60 min post-exercise. Blood samples were analyzed for lactate concentration and pH and muscle samples for metabolites, pH, and respiratory capacity. Venous blood and muscle pH declined to 6.91 +/- 0.02 and 6.57 +/- 0.02, respectively, after the fifth track run and to 6.98 +/- 0.02 and 6.71 +/- 0.07, respectively, after treadmill running. Muscle metabolite changes were consistent with the metabolic response to high-intensity exercise. Muscle respiratory capacity declined greater than 20% (P less than 0.05) after a single exercise bout and was 45% of the control value after the fifth track run. Tissue respiration was depressed 60 min post-exercise but was normal 24 h later. These observations suggest that high-intensity exercise impairs the respiratory capacity of the working muscle. Although this occurred in parallel with reductions in pH, other factors could be responsible for this response.

Altered sarcoplasmic reticulum function after high-intensity exercise

This study examined the effects of acute high-intensity exercise on the rate and capacity of Ca2+ uptake and Ca2+-stimulated adenosinetriphosphatase (ATPase) activity of the sarcoplasmic reticulum and the reversibility of these effects. Thoroughbred horses were run at maximal O2 uptake on a high-speed treadmill until fatigued. Muscle temperatures and biopsy samples were collected at rest, immediately after exercise, and 30 and 60 min after exercise. Blood samples were collected at rest and 5 min after exercise. Muscle and blood (lactate concentration) were three- and fivefold greater than pre-exercise values. Muscle temperature and pH immediately after post-exercise were 43 degrees C and 6.55, respectively, but approached rest values by 60 min after exercise. The initial rate and maximal capacity of Ca2+ uptake of muscle homogenates and isolated sarcoplasmic reticulum were significantly depressed immediately after exercise. This depression was paralleled by decreased activity of the Ca2+-stimulated ATPase. However, both Ca2+ uptake (rate and capacity) and Ca2+4-ATPase activity had returned to normal by 60 min after exercise. These findings demonstrate that changes in sarcoplasmic reticulum function after high-intensity exercise may be induced but not sustained by local changes in muscle pH and/or temperature.

Exercise-induced hypercapnia in the horse

The effects of exercise intensity and duration on blood gases in thoroughbred horses were studied to characterize the apparent exercise-induced failure in pulmonary gas exchange that occurs in these animals. In response to 2 min of exercise, arterial CO2 tension (PaCO2) decreased in mild and moderate exercise, returned to normocapnic levels in moderate to heavy exercise, and rose 5-10 Torr above resting values during very heavy exercise when CO2 production (VCO2) exceeded 20 times the resting value, and mixed venous CO2 tension approximated 140 Torr. Exercise-induced hypoxemia occurred at the onset of heavy exercise and was associated with the absence of a hyperventilatory response and an alveolar-arterial PO2 difference that increased four to six times above rest with very heavy exercise. PaCO2 was related to VCO2 but not fb, as changes in breathing frequency (fb) of 8-20 breaths/min at comparable VCO2 did not affect PaCO2. Prolonging very heavy exercise from 2 to 4 min caused a severe metabolic acidosis (arterial pH less than 7.15) and hypoxemia was maintained; however, CO2 was no longer retained, as PaCO2 gradually fell to below resting levels, due to an increased tidal volume at constant fb. We conclude that a truly compensatory hyperventilation to very heavy exercise in the horse is not achieved because of the excessive volumes and flow rates required by their extraordinarily high VCO2 and VO2. On the other hand, the frank CO2 retention during short-term high-intensity exercise occurs even though the horse is not apparently mechanically obligated to tolerate it.

Characteristics and preliminary observations of the influence of electromyostimulation on the size and function of human skeletal muscle during 30 days of simulated microgravity

During 30 days (d) of bedrest, the practicality of using electromyostimulation (EMS) as a deterrent to atrophy and strength loss of lower limb musculature was examined. An EMS system was developed that provided variable but quantifiable levels of EMS, and measured torque. The dominant leg of three male subjects was stimulated twice daily in a 3-d on/1-d off cycle during bedrest. The non-dominant leg of each subject acted as a control. A stimulator, using a 0.3 ms monophasic 60 Hz pulse waveform, activated muscle tissue for 4 s. The output waveform from the stimulator was sequenced to the knee extensors (KE), knee flexors (KF), ankle extensors (AE), and ankle flexors (AF), and caused three isometric contractions of each muscle group per minute. Subject tolerance determined EMS intensity. Each muscle group received four 5-min bouts of EMS each session with a 10-min rest between bouts. EMS and torque levels for each muscle action were recorded directly on a computer. Overall average EMS intensity was 197, 197, 195, and 188 mA for the KE, KF, AF, and AE, respectively. Overall average torque development for these muscle groups was 70, 16, 12, and 27 Nm, respectively. EMS intensity doubled during the study, and average torque increased 2.5 times. Average maximum torque throughout a session reached 54% of maximal voluntary for the KE and 29% for the KF. Reductions in leg volume, muscle compartment size, cross-sectional area of slow and fast-twitch fibers, strength, and aerobic enzyme activities, and increased leg compliance were attenuated in the legs which received EMS during bedrest.(ABSTRACT TRUNCATED AT 250 WORDS)

Structural and metabolic characteristics of human skeletal muscle following 30 days of simulated microgravity

Percutaneous needle biopsy samples were obtained from the vastus lateralis and soleus muscles before and after 30 d of 6 degree head-down bedrest to determine the influence of this model of simulated microgravity on human skeletal muscle. Fiber atrophy was evident in both muscles with both fast-twitch and slow-twitch fiber cross-sectional areas decreasing. Predominant atrophy of slow-twitch fibers was not evident. The soleus had a greater proportion of slow-twitch fibers than the vastus lateralis before bedrest. Neither muscle showed a change in fiber type percentage with bedrest. Phosphofructokinase and lactate dehydrogenase activities in the soleus and vastus lateralis muscles were similar before and after bedrest. The activities of beta-hydroxyacyl-CoA dehydrogenase and citrate synthase, however, were reduced during bedrest with these responses being somewhat greater in the soleus. While the ultrastructure of most of the fibers of the soleus and vastus lateralis appeared normal after bedrest, evidence of remodeling was present in both muscles. The proliferation of core/targetoid lesions, honeycomb networks, regenerating satellite cells, necrotic foci and myofibrillar disorganization after bedrest indicates that force development is an important factor in determining the organization of the fine structure of muscle. The results indicate that short-duration exposure to simulated microgravity decreases fiber size and the capacity for aerobic energy supply of human skeletal muscle. Moreover, disorganization of the contractile machinery occurs. Thus, it appears that bedrest alters the "normal" load-time constraints imposed on skeletal muscle sufficiently to change its inherent structural and metabolic characteristics.

Evidence that cysteine 298 is in the active site of tryptophan indole-lyase

Escherichia coli tryptophan indole-lyase (tryptophanase) mutants, with cysteine residues 294 and 298 selectively replaced by serines, have been prepared by site-directed mutagenesis. Both mutant enzymes are highly active for beta-elimination reactions measured with both L-tryptophan and S-(o-nitrophenyl)-L-cysteine. The Cys-294----Ser mutant enzyme is virtually identical to the wild type with respect to pyridoxal phosphate binding (KCO = 2 microM), cofactor absorption spectrum (lambda max = 420 and 337 nm) and pH dependence (pK alpha = 7.3), pH profile for catalysis, and rate of bromopyruvic acid inactivation. In contrast, the Cys-298----Ser mutant enzyme exhibits a reduced affinity for pyridoxal phosphate (KCO = 6 microM), a shift in the cofactor absorption spectrum to 414 nm and an altered pK alpha = 8.5, an alkaline shift in the pH profile for catalysis, and resistance to inactivation of the apoenzyme by bromopyruvic acid. The C298S mutant enzyme (wherein cysteine 298 is altered to serine) also undergoes an isomerization to an unreactive state upon storage at 4 degrees C. These results demonstrate that the sulfhydryl groups of Cys-294 and Cys-298 are catalytically nonessential. However, these data suggest that Cys-298 is located within or very near the active site of the enzyme and is the reactive cysteine residue previously observed by others.

Maximum O2 uptake, O2 debt and deficit, and muscle metabolites in Thoroughbred horses

This study determined maximal O2 uptake (VO2max), maximal O2 deficit, and O2 debt in the Thoroughbred racehorse exercising on an inclined treadmill. In eight horses the O2 uptake (VO2) vs. speed relationship was linear until 10 m/s and VO2max values ranged from 131 to 153 ml.kg-1.min-1. Six of these horses then exercised at 120% of their VO2max until exhaustion. VO2, CO2 production (VCO2), and plasma lactate (La) were measured before and during exercise and through 60 min of recovery. Muscle biopsies were collected before and at 0.25, 0.5, 1, 1.5, 2, 5, 10, 15, 20, 40, and 60 min after exercise. Muscle concentrations of adenosine 5'-triphosphate (ATP), phosphocreatine (PC), La, glucose 6-phosphate (G-6-P), and creatine were determined, and pH was measured. The O2 deficit was 128 +/- 32 (SD) ml/kg (64 +/- 13 liters). The O2 debt was 324 +/- 62 ml/kg (159 +/- 37 liters), approximately two to three times comparative values for human beings. Muscle [ATP] was unchanged, but [PC] was lower (P less than 0.01) than preexercise values at less than or equal to 10 min of recovery. [PC] and VO2 were negatively correlated during both the fast and slow phases of VO2 during recovery. Muscle [La] and [G-6-P] were elevated for 10 min postexercise. Mean muscle pH decreased from 7.05 (preexercise) to 6.75 at 1.5 min recovery, and the mean peak plasma La value was 34.5 mmol/l.(ABSTRACT TRUNCATED AT 250 WORDS)

Ventilatory responses of the horse to exercise: effect of gas collection systems

Experiments were undertaken to determine whether respiratory masks worn by horses exercising strenuously on a treadmill may interfere with normal gas exchange. Four collection systems, two flow-through systems and two incorporating one-way valve systems with subject-generated airflow were studied. Six horses performed standard treadmill exercise tests consisting of a 2-min warm up followed by galloping 1 min each at 8,9, and 10 m/s. Each horse exercised six times while wearing each of the four respiratory masks. Each flow-through system was used twice with flow rates of 2,360 and 3,840 l/min for one system, and 3,840 and 6,300 l/min for the other. Arterial blood gas tensions were measured during exercise at each speed for each system and were compared with values measured when the horses performed the same test without wearing a mask. Hypercapnia developed during exercise with each of the respiratory masks except with the 6,300-l/min flow-through system. All horses became hypoxemic during every exercise test, but it was most severe when systems incorporating one-way valves were used. This, plus the degree of hypercapnia observed and a suboptimal heart rate-O2 uptake relationship, indicated that such systems severely impede ventilation and suggest that experiments performed while utilizing them do not represent the normal exercise condition.

Some properties of different skeletal muscle fiber types: comparison of reference bases

Several biochemical components of the white portion of the gastrocnemius (WGM), plantaris (PM), and soleus (SM) muscles of the rat and middle gluteal (MGM) muscle of the horse were compared based on wet and dry weight, protein, and total creatine concentrations ([TCr]). The water content was similar for the rat hindlimb muscles, however, the concentrations of protein, ATP, phosphocreatine (PCr), creatine, and glycogen ranked as SM less than PM less than WGM for all reference bases except total creatine. In contrast, concentrations of ATP, creatine, and PCr were similar in all muscles studied when expressed as ratios of [TCr]. Horse MGM had the lowest percent of water and protein per gram wet or dry weight but highest glycogen concentration of the muscles studied, irrespective of the reference base used to express concentrations. Coefficients of variation were lowest when muscle constituents were related to [TCr]. It is concluded that expressing muscle constituents relative to total creatine results in the smallest variation and is a good method for making comparisons between muscles of similar fiber composition. However, essential information concerning different types of muscle may be lost when this reference base is used.

Exercise intensity, training, diet, and lactate concentration in muscle and blood

With some, but not all, types and intensities of exercise, lactate accumulates in the blood and in the muscles engaged in the exercise. A great deal of attention has been directed towards attempting to understand the dynamics of lactate production and removal at the onset of exercise, during exercise, and during the recovery process following exercise. It has been hoped that an unravelling of these events would provide a key to understanding cellular metabolism and its regulation during exercise. The purpose of this introductory paper to a symposium on lactate is to present a brief overview of some of the conditions that influence the rate and magnitude of lactate accumulation during exercise. It is pointed out that many conditions influence the rate and magnitude of the accumulation of lactate in blood and muscles. Included are diet, state of physical fitness, and the type and duration of the exercise. We have cautioned against trying to evaluate the state of oxygen delivery to muscle and the state of tissue oxygenation from the appearance of lactate in blood. We have pointed out the positive aspects of lactate production based on how it augments the cellular supply of ATP, thereby allowing for high intensity exercise, and also the negative aspects that develop as a result the reduction in pH which adversely influences many cellular processes essential for muscular activity.

Adaptive response of hypertrophied skeletal muscle to endurance training

The response of hypertrophied soleus and plantaris muscle of rats to endurance training was studied. Hypertrophy was produced by bilateral extirpation of the gastrocnemius muscle. A 13-wk training program of treadmill running initiated 30 days after removal of the gastrocnemius muscle accentuated (P less than 0.01) the hypertrophy. Succinate dehydrogenase activities of the enlarged muscles of sedentary rats were similar to those of normal animals, as were the increases associated with training. Phosphorylase and hexokinase activities were unaltered as a result of the experimental perturbations. Rates of glycogen depletion during exercise were lower (P less than 0.01) in the liver and soleus and plantaris muscles of endurance-trained animals. No difference existed in the rate of glycogen depletion of normal and hypertrophied muscle within the sedentary or trained groups. These data demonstrate that extensively hypertrophied muscle responds to training and exercise in a manner similar to that of normal muscle.

Sympathoadrenal system and activation of glycogenolysis during muscular activity

Comparisons were made of the appearance of phosphorylase (PHOS) a and lactate (LA) during electrical stimulation of the gastrocnemius (GM) and soleus (SM) muscles of normal and sympathectomized (SYMPX) rats. Ten-second stimulation at 3 Hz increased PHOS a approximately fourfold in the GM of normal rats, whereafter it declined during stimulation until at 60 s it was similar to rest. The increase in PHOS a of GM from SYMPX rats after 10 s of stimulation was approximately 50% that of normal rats. Stimulation of the SM produced smaller and slower increases in PHOS a with the peak occurring after 60 s, which remained constant to 90 s. SYMPX did not alter this effect in the SM. LA production and creatine phosphate depletion in the GM were continuous throughout stimulation and uninfluenced by SYMPX. This was true for the SM with the exception of LA production being greater after SYMPX. [ATP] was unchanged by electrical stimulation. The rate and magnitude of the PHOS a appearance was a function of stimulation frequency. Reversion of PHOS to the b form after stimulation was rapid, with approximately 50% of the peak value being attained in 2.5 s, and at 5 s the values were those of rest. These data demonstrate that an intact sympathoadrenal system is not obligatory for the initiation of glycogenolysis in skeletal muscle.

Differences in metabolic potential of skeletal muscle fibres and their significance for metabolic control

The role of an increase in oxidative potential of skeletal muscle in the enhanced work capacity and greater use of fat as a fuel after endurance training is discussed. Evidence is presented to illustrate that this adaptive response is probably expressed at the cellular level by a more rapid translocation into the mitochondria of the ADP generated during contractile activity. The consequence of this is a tighter control over the glycolytic process thereby creating more favourable conditions for the entry of acetyl units derived from beta-oxidation of fatty acids into the citric acid cycle.

Conversion of rat muscle fiber types. A time course study

Rats were used in this study to determine the time course of conversion of muscle fiber types. The right or left gastrocnemius muscle was removed thereby causing an overload on the ipsilateral soleus and plantaris muscles. The contralateral limb served as a control. The type II to type I fiber conversion was followed histochemically in the soleus and plantaris muscles for one to six weeks following surgery. Muscle sections were stained for myofibrillar actomyosin ATPase and NADH tetrazolium reductase. The type I population in the soleus muscle was 99.3% six weeks after synergist removal. The plantaris muscle underwent a two fold increase in the percentage of type I fibers after six weeks. Transitional fibers were prominent in the plantaris muscle and reached their peak at 4% (P less than 0.05) of the total population, four weeks after surgery.

Comparison of fiber types in skeletal muscles from ten animal species based on sensitivity of the myofibrillar actomyosin ATPase to acid or copper

Comparisons were made of the histochemical characteristics of skeletal muscle from 10 animal species. The basic comparison was made from the staining patterns for the myofibrillar actomyosin ATPase produced by preincubation of fresh frozen cross-sections of muscle at alkaline pH (10.30) or acid pH (4.60) with those produced by preincubation in media containing Cu2+ at alkaline pH (10.30), near neutral pH (7.40), or acid pH (4.60). Muscle sections were also stained for reduced nicotinamide adenine dinucleotide tetrazolium reductase and alpha-glycerophosphate dehydrogenase to provide an indication of the relative oxidative and glycolytic capacity of the different fiber types. Type II fibers in mixed fibered muscles were either very sensitive, moderately sensitive, or relatively insensitive to inactivation of the myofibrillar actomyosin ATPase after acid preincubation. These fibers were identified as type IIA1, IIA2, and IIA3, respectively. The myofibrillar actomyosin ATPase of the type I fibers of these muscles, with the exception of those in mouse muscle, was activated by pretreatment with acid. A separation of animal species was possible based on the stability of the IIA1 fibers to inclusion of Cu2+ in the preincubation medium. For one group of animals (rat, mouse, monkey, man, dog, rabbit, and cow), a reciprocal relationship existed between lability to acid and stability to Cu2+ for type IIA1 and IIA3 fibers, respectively. For the second group of animals (horse, ass, and cat) there was a parallel relationship between lability or stability of the type IIA1 and IIA3 fibers to pretreatment with either acid or Cu2+.

Cardiac contractility, cAMP concentration, cAMP-dependent protein kinase, and phosphorylase activation during acute pressure overload

The relationship between increases in myocardial contractility and cAMP and protein kinase activity were studied for hearts of normal rats and those with altered sympathectic capacity produced by the combined treatments of adrenalectomy, and 6-hydroxydopamine and propranolol injections. Increases in myocardial contractility, evaluated from intra-ventricular pressure changes, were produced by occlusion of the ascending aorta for 15, 20, or 25 s. Resting peak left ventricular pressure and the rate of rise of left ventricular pressure were lower (P less than 0.05) in sympathectomized animals, however, aortic occlusion abolished these differences. Time to peak tension and the relationship between end-diastolic pressure and developed pressure were unchanged by sympathectomy. ATP and CP concentrations in freeze clamped samples of the myocardium were lower (P less than 0.05) in both groups after aortic occlusion whereas lactate was elevated (P less than 0.05). Sympathectomy delayed and reduced the magnitude of the increase in the phosphorylase a/a + b ratio produced by aortic occlusion. Myocardial cAMP concentration was increased in the normal rats but decreased in sympathectomized animals after aortic occlusion. cAMP-dependent protein kinase activity followed the pattern of cAMP. The results demonstrate that heart possesses the capacity to increase its contractility to an acute, short-term overload even when devoid of sympathetic control.

Role of carbohydrate in exercise

Carbohydrate is an important energy source during exercise. During short, heavy exercise it may be the only energy source for the working muscle and may be derived exclusively from the glycogen stores within the muscle fibers themselves. During prolonged, submaximal exercise the magnitude of the contribution that carbohydrate makes to the total fuel consumed depends upon a number of factors, including (1) the intensity of the exercise, (2) the duration of the exercise, (3) the state or type of training that the individual has engaged in, or both, and (4) the diet previously consumed. Due to their limited storage in the body and the relative ease with which these stores can be manipulated by combinations of diet and/or training, a knowledge of the dynamics of carbohydrate metabolism is an important component in the arsenal of knowledge for coaches and athletes.

The muscle fiber composition of skeletal muscle as a predictor of athletic success. An overview

Human skeletal muscle is composed of varying percentages of fiber types. This percent composition varies widely between muscles and among individuals. The fiber composition of some skeletal muscle could be construed as being advantageous to successful performance in selected athletic event. However, this relationship is not sufficiently close to warrant the conclusion that the fiber composition of the muscle per se is the determinant of the superior performance of elite athletes. Reasonably good evidence exists to support the position that the fiber composition of a muscle is the result of a genetic endowment. Although muscle fibers are mutable, present evidence is equivocal as to whether habitual participation in given type of physical activity is responsible for high percentages of a given fiber type being present in the muscles of some athletes. Although considerable knowledge has come from the study of muscle samples obtained from sedentary individuals, athletes of a wide range of performance capacity, and individuals before and after training, a considerable gap remains for a full understanding of how the characteristics of muscle are related to performance capacity. The observation that considerable variation exists in the percent distribution of the fibers within a muscle and that athletes with a wide range of fiber populations in their muscles can be successful in the same athletic event cautions against the routine application of the biopsy technique to estimate the fiber distribution of muscles and also to use such data as a routine screening procedure for predicting athletic success. The point, as was made in an earlier paper, that the biopsy technique for studying muscle is a research tool, will probably continue to be true for the near future.

Response of skeletal muscle to training

Physical training induces adaptive changes in skeletal muscle. These changes are localised to the active muscle with their magnitude depending upon the nature, i.e. time and intensity, of the training regimen. The most notable changes are increased concentrations of mitochondria and glycogen. With endurance training there are major changes in metabolism in that there is a greater contribution of fat to the total metabolism during submaximal exercise. This results in a conservation of the stores of glycogen with the net result of increasing total exercise capacity. This increased use of fat during submaximal exercise appears to be more closely related to the elevations in the concentration of mitochondria in muscle than to changes in total body maximal oxygen uptake. The combination of a greater contribution of fat to the metabolism and the elevated concentration of stored glycogen are prime factors contributing to the enhanced endurance capacity after endurance training. The mechanism for the greater use of fat after endurance training is discussed. Evidence now supports the hypothesis that this is due to a tighter control over the Embden-Meyerhof pathway as a result of the greater concentration of mitochondria. The effect of heavy resistance exercise on the size and strength of skeletal muscle is discussed. Some attention is focused on the recently revived controversy concerning whether muscle enlargement is the result of a hypertrophy of pre-existing fibres or of hyperplasia. It is concluded that although there is considerable evidence to support the development of hypertrophy in response to heavy resistance exercise, the contention that a splitting of fibres occurs to produce a greater fibre number is presently poorly supported.