Capacity For Aerobic Metabolism Research Articles

Effects of fiber type and training on beta-adrenoceptor density in human skeletal muscle

The density and distribution of beta-adrenergic receptors in type I and II fibers of human gastrocnemius and quadriceps muscles were characterized in ten healthy sedentary subjects and in a subgroup of six subjects before and after 12 wk of endurance exercise training. Total tissue content of beta-receptors was measured in frozen sections of skeletal muscle biopsies incubated with 125I-labeled cyanopindolol in the presence and absence of 10(-5) M L-propranolol. The relative beta-receptor densities of type I and II fibers were delineated autoradiographically. Muscle fiber types were identified in adjacent serial sections by histochemical staining of myofibrillar adenosine-triphosphatase (ATPase) activity. Type I fibers had a threefold greater beta-receptor density than type II fibers of the same muscle [P less than 0.001; type I-to-type II fiber ratio of beta-receptor density was 3.06 +/- 0.43 (SD)]. Exercise training elicited a change in muscle fiber subtype composition (+34% type IIa and -42% type IIb; P less than 0.05 and P = 0.066, respectively), a 40% increase in citrate synthase activity of skeletal muscle (P = 0.01), and a 23% rise in peak oxygen uptake (P less than 0.001). However, no change in total tissue content of beta-receptors was observed after exercise training, even when receptor density was adjusted for preconditioning fiber type composition. Thus beta-receptor density of type I fibers of human skeletal muscle is threefold greater than that of type II fibers. Enhanced capacity for aerobic metabolism after endurance exercise training is not associated with upregulation of total beta-receptor density.

Lack of teratogenic effect of brief maternal insulin-induced hypoglycemia in rats during late neurulation.

We have previously shown that 1 h of maternal insulin-induced hypoglycemia is teratogenic to rat embryos during the initial stages of neurulation, when they are dependent on uninterrupted glycolysis (day 9.5-9.7 of development). To determine whether this vulnerability persists in later stages of neural tube and cardiac development, we infused insulin into 16 conscious pregnant rats for 1 h beginning after embryos had developed the capacity for aerobic glucose metabolism (day 10.6 of development). Half of the pregnant animals were allowed to become hypoglycemic (44 +/- 2 mg/dl) during the insulin infusions. The other half received glucose infusions to maintain normoglycemia (130 +/- 3 mg/dl). Normal plasma glucose levels were maintained in all animals after the insulin infusions. Embryos were examined on day 11.5 of development. At that time, 1 of 111 embryos from the normoglycemic group and 1 of 109 embryos from the hypoglycemic group were grossly malformed (P greater than .5). Means of embryo crown-rump length (4.15 +/- 0.03 vs. 4.14 +/- 0.03 mm), somite number (29.7 +/- 0.1 vs. 29.8 +/- 0.2), and total protein content (320 +/- 5 vs. 326 +/- 6 micrograms) were also similar in the two groups (P greater than .5). Thus, we could not detect an embryotoxic effect of 1 h of maternal insulin-induced hypoglycemia beginning at day 10.6 of development. This finding is in contrast to our prior demonstration that a similar period of hypoglycemia occurring earlier in neurulation (day 9.7) caused growth retardation and developmental anomalies in embryos.(ABSTRACT TRUNCATED AT 250 WORDS)

Thermal acclimation in fish: conservative and labile properties of swimming muscle

Given the rapid thermal equilibration of most fish with their environment, thermal compensation of metabolic and contractile properties is essential for the maintenance of locomotory capacities over a wide range of temperatures. The response of fish swimming performance, contractile properties of isolated fibers, myosin ATPase activity, and metabolic systems for ATP generation to short- and long-term changes in temperature have received sufficient study to allow one to identify certain constrained and labile properties. Sustained swimming performance and its components generally have their optimal performance and lowest thermal sensitivity within the range of temperatures frequently encountered by the organism. These principles are particularly well established for isolated enzyme systems. Furthermore, swimming performance and most of its components demonstrate thermal compensation on the evolutionary time scale. Temperature acclimation also leads to compensatory responses which, while quite species-specific, consistently increase the capacity for sustained swimming at low temperatures. The position of the thermal optimum for locomotion in relation to the width of a species' tolerance limits aids in predicting the species' capacity for thermal compensation during acclimation. Goldfish (Carassius auratus) and common carp (Cyprinus carpio), which tolerate temperatures 25–30 °C below their optimum for locomotion, show thermal compensation in terms of contractile properties, myosin ATPase activity, the proportion of red fibers in their axial musculature, and the levels of aerobic enzymes in their musculature. By contrast, striped bass (Morone saxatilis) and chain pickerel (Esox niger), which have lower optimal temperatures for locomotion, only increase the proportion of red fibers and (or) the levels of aerobic enzymes with cold acclimation. Finally, lake whitefish (Coregonus clupeaformis), which have their optimal temperature for locomotion at 12 °C, show none of these responses. Given that when thermal compensation occurs, aerobic enzymes in red muscle generally increase, the capacity of red muscle to generate ATP seems more temperature sensitive than other metabolic or contractile properties. Whether this compensatory response serves to counteract the effect of temperature on diffusive exchange between mitochondria and the cytoplasm or its effect on the catalytic capacity of aerobic metabolism remains to be established.

Adaptive variation in the mammalian respiratory system in relation to energetic demand: IV. Capillaries and their relationship to oxidative capacity

We analyzed the capillarity of the heart, diaphragm, M. vastus medialis and M. semitendinosus of dogs, goats, ponies and calves ( n = 3 each). Blocks of tissue were preserved, processed and photographed bye electron microscopy. Using morphometric techniques we estimated capillary density, capillary lenght density and total capillary length in these muscles. The highly aerobic dogs and ponies had greater total capillary lenghts and larger muscles than the less aerobic goats and calves. A significant correlation was found between capillary length density (J V(c,f)) and mitochondrial volume density (V V(mt,f)) which was: J V(c,f) = 258 + 1.25·10 4 V V(mt,f). From this correlation we calculated an average of 14 km or 0.22 ml of capillaries per milliliter of mitochondria. With these values and data from blood gas analysis (Karas et al., 1987), we calculated a mean minimum transit time for blood in capillaries of approximately 0.5 sec for all four species. At the tissue level, the greater aerobic metabolic capacity of dogs and ponies was supported in equal parts by the larger capillary supply of the muscle tissue and by the higher oxygen carrying capacity of the blood.

Health Benefits of Exercise in an Aging Society

The effects of regular aerobic exercise are important to an aging society increasingly preoccupied with exercise. Traditionally, most attention has been directed to the relationship between a physically active life-style and cardiovascular mortality. In an aging society, however, active life expectancy and maintenance of independence may be as important as effects of regular exercise on longevity. Regular exercise results in increased maximum aerobic capacity due to peripheral changes in muscle (increased capacity for aerobic metabolism and improved substrate and oxygen extraction with a widened arteriovenous oxygen difference) and also due to cardiovascular changes with increased stroke volume and cardiac output in normal persons. "Therapeutic benefits" of conditioning probably occur at submaximal work loads common to everyday activity, when cardiac work and myocardial oxygen consumption are less for any given work load, muscles are more efficient, and relative oxygen requirements are less. Aging is associated with a linear decline in maximum aerobic capacity. The rate of decline is twofold greater when comparing sedentary with physically active middle-aged men. Thus, regular exercise could conceivably lower functional aerobic age by slowing this functional decline. Exercise, particularly excessive exercise, is also associated with serious hazards, including sudden death, nonfatal myocardial infarction, excessive fatigue, hyperthermia, and significant musculoskeletal problems. Accounts of the health effects of exercise should consider a wide range of risks and benefits, especially those related to improving function, minimizing disability, and prolonging independent living.

Metabolic and functional characteristics of erythrocytes from Glycera and Noetia

1. 1. Annelid and molluscan red blood cells (RBC) may de differentiated metabolically from vertebrate RBC by their increased permeability to substrate, their magnitude of amino acid catabolism and their higher aerobic metabolism. 2. 2. At 22°C, Glycera and Noetia RBC oxidize glucose and glutamate to CO 2 without accumulation of either d- or l-lactate. By comparison, the oxidation of glutamate by rat and chicken RBC is negligible at this temperature despite its incorporation into the cells. 3. 3. At 37°C, chicken RBC oxidize glutamate at a rate 4 times greater than at 22°C, with oxygen uptake still lower than that in Noetia RBC at 32°C. At 37°C, rat RBC do not increase their oxidation of glutamate above that at 22°C, but oxygen uptake increases to slightly more than half that of chicken RBC. 4. 4. Our finclings indicate that RBC of these two invertebrate species have both a higher aerobic metabolism and lower anerobic capacity than vertebrate RBC. 5. 5. Moreover, the annelid and molluscan RBC have a relatively lower activity of the pentose phosphate (PPO 4) pathway than vertebrate RBC, as evidenced by their higher thermal sensitivity of oxygen uptake and their higher *C 1O 2/*C 6O 2 isotope ratio.

Metamorphosis of the American eel, Anguilla rostrata LeSeur: I. Changes in metabolism of skeletal muscle.

Unequivocal demarcation between immature, nonmigratory yellow eels and migratory silver eels of greater sexual maturity is possible by measuring eye diameter and retinal capillary length, which undergo a 1.5- and 2.3-fold increase during metamorphosis, respectively. Anatomical arrangement of trunk musculature is similar in the two groups except for an increased depth of slow muscle in silver eel. Histochemical analysis reveals a progressive increase in numbers of "displaced" fast fibres within slow muscle of the lateral line triangle in maturing eels, although these are unlikely to affect recruitment pattern of muscle fibre types. Previous studies have suggested greater involvement of fast muscle in locomotion of migratory eels. In contrast, estimates of enzyme activity in fast muscle suggest an inadequate aerobic capacity to fuel sustained activity. Myoglobin content is extremely low, around 0.4 nM g wet wt-1. Prolonged anaerobic metabolism is also discounted as a migratory strategy. Increased energy provision for migration is apparently derived from increased capacity for both aerobic carbohydrate metabolism and mitochondrial fatty acid oxidation within slow muscle of silver eels. Activity of hexokinase (HK) shows a 1.6-fold increase (to 0.51 microM g wet wt-1) and carnitine palmitoyltransferase (CPT) a 3.1-fold increase (to 0.22 microM g wet wt-1 min-1), suggesting a maximal flux through these pathways of 18 and 14 ATP equivalents, respectively. However, the fatty acyl transferase system of skeletal muscle mitochondria displays up to threefold greater activity with palmitoleoyl CoA (C16:1) as substrate than with the usual palmitoyl CoA (C16:0). Slow muscle of silver eel is therefore capable of deriving aerobic energy from free fatty acids and carbohydrate in the ratio 2.3:1. Differences in aerobic enzyme activities are not paralleled by myoglobin content of slow muscle, being 15 and 16 nM g wet wt-1 for yellow and silver eel, respectively. Structural reorganization of muscle fibres during metamorphosis, however, results in a twofold elevation of cytoplasmic myoglobin concentration in silver eel. It would appear that dramatic differences in metabolic capacity between life history stages of eel is required to overcome locomotory inefficiency of yellow eels and to "preadapt" silver eels for migratory activity. This increased locomotory capacity may be amplified by a subsequent training response.

Effects of methamphetamine on arterioles of rat caudate nucleus and midbrain.

No differences in arteriolar metabolic profiles from rat caudate nucleus, ventral tegmental area or substantia nigra were observed between saline-control and methamphetamine HCl-treated (20 mg/kg, i.p. twice daily for 10 consecutive days) animals. Arterioles from the above regions of forebrain and midbrain are metabolically active vessels with a capacity for aerobic and anaerobic metabolism. These results suggest that a high dose of methamphetamine does not alter cerebral arteriolar metabolism in the areas examined.

Strain variation in Hymenolepis diminuta: enzyme profiles

In comparative study of respiratory metabolism, it was established that the relative proportions of respiratory end-products (succinic, acetic and lactic acids) differed consistently in two strains of Hymenolepis diminuta (Toronto and ANU). The ANU strain produced more lactic acid and less succinic acid under aerobic and anaerobic conditions. In the shift from aerobic to anaerobic conditions both strains compensated by increasing their outputs of succinic acid. The ANU strain possessed significantly higher activities of hexokinase, pyruvate kinase, lactate dehydrogenase, cytosolic and mitochondrial malic enzyme and cytosolic α-glycerophosphate dehy drogenase. The Toronto strain had significantly higher activities of fumarase, succinate dehydrogenase, and fumarate reductase. There were no significant differences in the activities of phosphoenolpyruvate carboxykinase and malic dehydrogenase between strains. The fumarase activity in the Toronto strain was 16 times that of the ANU strain, its K m (malate) was 0.8mM, as opposed to 2.5 mM, and it was less sensitive to inhibition by NAD or ATP. These observations are consistent with the patterns of end-product formation in the two strains. Ratios of end-products and calculations of approximate redox balance suggest that the Toronto strain may have a greater capacity for aerobic metabolism.

Activities of key enzymes of aerobic and anaerobic metabolism in middle gluteal muscle from trained and untrained horses.

The effect of physical training on the in vitro activities of key enzymes that provide quantitative information on the maximum capacities of anaerobic and aerobic metabolism has been investigated in the gluteal muscle of the horse. Training had no effect on the activities of 6-phosphofructokinase or creatine kinase, suggesting that there was no effect on the capacity of anaerobic metabolism in this muscle. However, the activities of hexokinase and citrate synthase were increased, indicating that training increased the capacity of aerobic metabolism. For comparative purposes, muscle fibre composition and enzyme activities were also determined in a group of foals and a group of broodmares.

A histochemical study of cerebral cortical vessels and ganglionic vessels of the caudatoputamen in aging normotensive rats.

The goal was to describe the metabolic profile of ganglionic and cortical arteries and arterioles in aging normotensive male rats. Five enzymes indicative of key metabolic pathways in the vessel walls were semiquantitatively evaluated using bright-field histochemical microscopy. Lactate dehydrogenase showed significant reactivity which increased with vessel diameter in cortical and ganglionic vessels in all age groups tested. Succinate dehydrogenase and cytochrome oxidase showed little reactivity in both cortical and ganglionic vessels, suggesting a reduced role for aerobic metabolic pathways. Myosin ATPase reactivity was high in cortical and ganglionic vessels. Only this enzyme showed an increased reactivity that was correlated with the age and diameter of the vessel. Glucose-6-phosphate dehydrogenase reactivity was more pronounced in cortical than ganglionic vessels, suggesting that the hexose-monophosphate-shunt may be more active in the cortical vessels. There were no regional differences in enzyme reactivity throughout the caudatoputamen. In conclusion, both the cortical and ganglionic vessels are metabolically active, with significant anaerobic glycolysis, and reduced, but observable capacity for aerobic metabolism. The decreased myosin ATPase reactivity and the low level of glucose-6-phosphate dehydrogenase reactivity in the ganglionic arterioles of senescent rats may contribute to the susceptibility of these vessels to cerebrovascular accidents.

Influence of aerobic metabolism on IMP accumulation in fast-twitch muscle.

Significant activation of AMP deaminase in fast-twitch muscle leads to a loss of ATP and accumulation of NH4 and IMP. Although this occurs during severe metabolic stress caused by intense contraction conditions, the process is probably influenced by the muscle's capacity for aerobic metabolism. We evaluated this possibility during moderately intense (5 Hz) contraction conditions in situ by following the time course of NH4 and IMP accumulation in fast-twitch, low-oxidative white (FTW) and fast-twitch, high-oxidative red (FTR) muscle of the rat. A high rate of IMP formation, resulting in a 50% loss of ATP content, occurred in normal FTW, but not FTR muscle, during contractions when blood flow was intact. Eliminating blood flow prior to contractions, however, removed the distinction between the FTR and FTW muscle. The FTR fiber section now produced a high IMP content and a stoichiometric loss of ATP. Thus the ability of the FTR fiber to sustain this contraction effort without an ATP loss is due to its greater functional capacity for aerobic metabolism. The FTW muscle section of trained animals exhibited a reduced accumulation of IMP and a smaller loss of ATP during the same 5-Hz stimulation. The mitochondrial content and peak blood flow of this FTW fiber section is increased by training. Thus it is probable that the cellular conditions leading to a significant accumulation of IMP in fast-twitch muscle are determined by the metabolic stress established by the contraction effort, relative to the muscle fiber's functional capacity for aerobic metabolism.

Selected enzyme activities in mouse cardiac muscle during training and terminated training.

We studied the effects of running-training, heavy exercise and termination of training on the heart weight, the ratio heart to body weight and the cardiac muscle activities of actomyosin ATPase, citrate synthase, succinate dehydrogenase, cytochrome c oxidase, malate dehydrogenase, adenylate kinase and beta-glucuronidase with adult male NMRI-mice. Stable hypertrophy (6-7%), estimated by the ratio heart or ventricle weight to body weight, was achieved by 28 exercises and it was dependent on the running speed (20 vs. 25 m X min-1). The withdrawal of training for 5-61 days did not permanently decrease the heart weight or the heart to body weight ratio to the level of sedentary controls. The activity of enzymes of energy metabolism or actomyosin ATPase were not affected by training, heavy exercise or terminated training. beta-glucuronidase activity slightly (20-25%) increased in the trained animals and remained at a higher level during the period of terminated training. The results suggest that the capacity for aerobic metabolism of normal mice heart is sufficient to meet the enhanced demand for ATP imposed by running-training and that the heart enlargement occurs in equal proportions with the enzymatic potential of the cardiac tissue.

Spinal cord vasculature of the rat: a histochemical study of the metabolism of arteries and arterioles.

Spinal cord arteries and arterioles of adult female rats were examined histochemically to determine their metabolic profiles. The metabolic pathways evaluated included those related to aerobic (oxidative phosphorylation, Kreb's cycle and respiratory chain) and anaerobic (glycolysis) capacity, hexosemonophosphate-shunt activity, beta-oxidation of fat and adenosine triphosphate utilization. The amounts of deoxyribonucleic and ribonucleic acids were determined as an indication of protein synthesis. The present findings indicate that arteries of the rat spinal cord are metabolically active with high capacities for both aerobic and anaerobic metabolism, and possess a significant potential for nucleic acid and protein synthesis. Lipid catabolism, via beta-oxidation of fat, may serve as one source of energy. The arteries also demonstrate a high capacity for utilization of adenosine triphosphate. In contrast, the spinal cord arterioles show a lower capacity for aerobic metabolism and lipid utilization, while anaerobic glycolysis may be a main source of energy. The arterioles also demonstrate a significant potential for nucleic acid and protein synthesis, in addition to a high capacity for adenosine triphosphate utilization.

Metabolic meaning of elevated levels of oxidative enzymes in high altitude adapted animals: An interpretive hypothesis

It is commonly observed that during acclimatization to altitude oxidative enzyme activities increase per g wet weight of tissue. To examine this problem in long-term adapted animals we measured citrate synthase (CS), hydroxyacylCoA dehydrogenase (HOAD), pyruvate kinase (PK), and lactate dehydrogenase (LDH) activities/g of myocardium in two domestic species (llama and alpaca) and a high altitude deer, the taruca. In all these species, we found an upward scaling of oxidative capacity (indicated by absolute activities of CS and HOAD) but a downward scaling of anaerobic/aerobic metabolic potentials of the heart (indicated by low ratios of LDH/CS, and LDH/HOAD, but high ratios of PK/LDH). As the direction and magnitude of these long-term adaptations are the same as in shorter-term acclimizations, we wondered why a similar pattern at the enzyme level correlates with the right shift of the O 2 dissociation curve (ODC) in the latter case, but with a left shifted ODC in the former. We hypothesize that in the long term, increased oxidative enzyme activities allow increased maximum flux capacity of aerobic metabolism. This in turn call for physiological adjustments in O 2 transfer systems; flux limits of the former must be matched by flux limits of the latter. Only then can an acceptably high scope for aerobic activity be achieved despite reduced O 2 availability in inspired air. Such long-term match-up invariably calls for a left-shifted ODC plus other well known adjustments in O 2 transport. In the short term, right shifting the ODC may increase the total amount of aerobic work possible (by favoring O 2 unloading and thus raising tissue O 2 concentration), yet maximum flux capacity cannot be changed much because mitochondrial metabolism is designed for maintaining stable rates of ATP synthesis even at widely varying O 2 tensions. That is why even in short-term acclimization, in order to increase flux capacity, the activities of oxidative enzymes also must be increased.

Maintenance of the adaptation of skeletal muscle mitochondria to exercise in old rats

The purpose of this study was to determine whether the adaptive increase in mitochondrial enzymes in skeletal muscle induced by a constant exercise program is altered by aging. Male pathogen-free Long-Evans rats were exercised by means of swimming for 3h/d, 5d/wk beginning at age 6 months and studied at ages 9 and 24 months. The levels of activity of the four mitochondrial enzymes used as indicators of the capacity for aerobic metabolism (citrate synthase, succinate DH, 3-hydroxyacyl-CoA-DH, and 3-ketoacid-CoA-transferase) were significantly increased in epitrochlearis muscle in both the 9-month and 24-month-old swimmers. The increases ranged from approximately 20% for 3-hydroxyacyl-CoA-DH to 50-60% for 3-ketoacid-CoA-transferase. Neither the 24-month-old sedentary rats nor the 24-month-old swimmers had significantly lower muscle enzyme levels than the comparable 9-month-old rats. Thus, it appears that aging does not result in a progressive decline in the capacity of muscle for aerobic metabolism or a progressive impairment in the ability to maintain an increase in muscle mitochondrial enzymes in response to chronic exercise in healthy rats.

Relationship between muscle QO2 and fatigue during repeated isokinetic contractions.

The percent slow-twitch fibers (%ST) of the vastus lateralis muscle and selected indicators of maximum aerobic power (QO2, VO2 max, and leg extensor VO2 max) were compared with the rate of fatigue development during 45 s of repeated maximum isokinetic leg extension-flexion contractions (180 degrees/s) on 13 males. Subjects with muscle respiratory capacities (QO2) above the mean (2,472 microliter O2 X h-1 X g-1) displayed a significantly slower rate of fatigue development than those subjects with QO2 values below the mean. In addition, indicators of maximum aerobic power were correlated with the percent of maximum power and power/ml fat-free thigh volume at 5, 15, 30, and 45 s during the fatigue test. The correlations increased progressively during the fatigue test and became significant at 45 s. This trend was not observed when the %ST fibers were compared with the percent of maximum power and power/ml fat-free thigh volume at these time intervals. These data suggest that 1) the muscle tissue's capacity for aerobic metabolism influences the rate of fatigue development even during a strenuous short-term isokinetic exercise task, and 2) factors other than fiber type may interact to confound the prediction of fiber type by noninvasive means.

A Histochemical Study of the Coronary Vasculature in Euthyroid and Hyperthyroid Rats

Coronary arteries and arterioles in the left and right ventricles from normal and hyperthyroid rats were examined histochemically to determine and to compare their metabolic activities. The test animals were made hyperthyroid by administration of desiccated thyroid for 8-10 weeks. Using histochemical techniques, selected enzymes and components of key metabolic pathways were examined. These pathways included an evaluation of aerobic (oxidative phosphorylation, Kreb's cycle and respiratory chain) and anaerobic metabolic capacity, hexose-monophosphate shunt activity, amounts of deoxyribonucleic and ribonucleic acids present and activity of beta-oxidation of fatty acids. Our results indicate that normal coronary arteriolar metabolism is predominantly aerobic. The findings also suggest a reduction in aerobic metabolism with an accompanying increase in anaerobic potential in the hyperthyroid coronary arterioles. Thus, during thyrotoxicosis, the coronary arterioles may partially shift from aerobic to anaerobic metabolism. Moreover, in both the normal and thyrotoxic rat heart, the coronary microvasculature appears quite stable with little cell proliferation. In contrast, both the control and hyperthyroid rat coronary arteries appear to utilize primarily anaerobic pathways, while the control and hyperthyroid myocardium seem highly dependent upon aerobic metabolism. The tremendous reduction in glucose-6-phosphate dehydrogenase activity in hyperthyroid, when compared to normal coronary arteries and some larger arterioles, implies a reduced capacity for nucleic acid and protein synthesis in the test animals.

A Histochemical Evaluation of Metabolism in the Coronary Vasculature of the Primate

Coronary arteries and arterioles in the left ventricle from the primate Macaca fascicularis were histochemically examined to evaluate their metabolic profiles. Succinate dehydrogenase and cytochrome oxidase activities were assessed to evaluate aerobic metabolic capacity, while myosin ATPase activity was determined as an index of ATP utilization for contraction. Anaerobic capacity was evaluated from lactate dehydrogenase and glycogen reactivity. Glucose-6-phosphate dehydrogenase was examined to determine capacity of the hexose-monophosphate-shunt, while the amounts of deoxyribonucleicc and ribonuclei acids were assessed as possible indicators of protein synthesis. Succinate dehydrogenase and cytochrome oxidase demonstrated slight reactivity in both coronary arteries and arterioles indicating a low capacity for aerobic metabolism. Myosin ATPase showed strong activity in arteries and even stronger reactivity in arterioles, suggesting that arteriolar smooth muscle is more capable of utilizing ATP. Glucose-6-phosphate dehydrogenase activity was extremely low in both arteries and arterioles, while deoxyribonucleic and ribonucleic acids demonstrated only slight to moderate reactivity in both arteries and arterioles, indicating that under normal conditions the coronary vasculature appears quite stable with little cell proliferation.

Physiological and biochemical effects of iron deficiency on rat skeletal muscle.

Young rats were made iron deficient by feeding them a low-iron diet for 8 wk. Iron deficiency resulted in a 50% decrease in cytochrome c and cytochrome oxidase and a 26% decrease in mitochondrial glycerol-3-phosphate dehydrogenase activity in skeletal muscle. Respiratory capacity of muscle homogenates was reduced 55%. After 8 days of iron treatment, respiratory capacity, cytochrome c, cytochrome oxidase, and glycerol-3-phosphate dehydrogenase had returned 50% toward normal. Maximum O2 uptake of contracting hindlimb muscles averaged 8.5 mumol O2.min-1.g-1 in control, 4.3 mumol O2.min-1.g-1 in iron-deficient, and 6.2 mumol O2.min-1.g-1 in the 8-day-iron-repleted rats. Muscle fatigue during 10 min of stimulation was greater in the iron-deficient group. Lactate concentration in red muscle was higher in iron-deficient than in control rats after stimulation. The muscle fatigue and lactate responses returned 50% toward normal during 8 days of iron treatment. We conclude that iron deficiency results in a decrease in skeletal muscle capacity for aerobic metabolism and, by this mechanism, increases susceptibility to fatigue.