The effect of myoglobin-facilitated oxygen transport on the basal metabolism of papillary muscle.

A mathematical model of oxygen diffusion into cylindrical papillary muscles is presented. The model partitions total oxygen flux into its simple and myoglobin-facilitated components. The model includes variable sigmoidal, exponential, or hyperbolic functions relating oxygen partial pressure to both fractional myoglobin saturation and rate of oxygen consumption. The behavior of the model was explored for a variety of saturation- and consumption-concentration relations. Facilitation of oxygen transport by myoglobin was considerable as indexed both by the elevation of oxygen partial pressure on the longitudinal axis of the muscle and by the fraction of total oxygen flux at the muscle center contributed by oxymyoglobin. Despite its facilitation of oxygen flux at the muscle center, myoglobin made only a negligible contribution to the total oxygen consumption averaged over the muscle cross-section. Hence the presence of myoglobin fails to explain either the experimentally determined basal metabolism-muscle radius relation or the stretch effect observed in isolated papillary muscle.     

Oxygen transport during hemodilution with a perfluorocarbon-based oxygen carrier: effect of altitude and hyperoxia.

Oxygen delivery and consumption after hemodilution with a perfluorocarbon-based oxygen carrier (PFCOC) was evaluated at sea level and at 2,600 m above sea level. Fifteen anesthetized rats were subjected to a two-exchange normovolemic hemodilution of 40% of the circulating blood volume each. First exchange was performed with a colloid solution. Second exchange was with 80% PFCOC and 20% colloid. Animals were then ventilated with 100% oxygen. Experiments were performed at barometric pressure of 1.0 atm (sea-level group, n=9) or 0.74 atm (2,600-m group, n=6). Blood gases, hematocrit, fluorocrit, and hemoglobin content were measured at baseline and 15 min after each exchange. After hemodilution, total arterial content was not modified by the PFCOC in either group. In contrast, arteriovenous oxygen difference increased significantly in both groups, as did the oxygen extraction ratio. In the second exchange, although total arterial content was similar between the two groups, the perfluorocarbon and plasma phases contributed significantly more at sea level. Arteriovenous oxygen difference was significantly less at sea level with a higher contribution from the perfluorocarbon and plasma phases. In conclusion, hemodilution with a PFCOC induced changes in oxygen delivery and consumption that differ with altitude. The 2,600-m group exhibited a higher oxygen extraction ratio and arteriovenous oxygen difference, with reduced oxygen delivery and unloading from both the fluorocarbon and plasma phase. Therefore, the efficacy of PFCOCs at 2,600 m above sea level is reduced, and altitude must be taken into account when PFCOCs are used.     

The effects of hyperoxia on oxygen uptake during acute anemia

The effects of normobaric hyperoxia on the oxygen uptake (VO2) and cardiovascular responses of the whole body and hindlimb during anemia were investigated. Anesthetized, paralyzed dogs were ventilated for 20-min periods with room air (normoxia), 100% O2 (hyperoxia), and returned to room air. Anemia (hematocrit = 15%) was then induced by isovolemic dextran-for-blood exchange and the normoxia, hyperoxia, normoxia sequence was repeated. Whole body VO2 and cardiac output rose following anemia, and then fell (p less than 0.05) with hyperoxia during anemia. These responses were not abolished by beta-blockade with propranolol (1 mg/kg, iv) or bilateral vagotomy. The hindlimb data for blood flow and VO2 were similar in direction to those of the whole body but were more variable. Section of the sciatic and femoral nerves did not appear to have significant effect on the limb responses to hyperoxia. The decrease in whole body and hindlimb VO2 with hyperoxia during anemia may have resulted from a redistribution of capillary blood flow away from exchange vessels in response to the elevated PO2.     

Effects of hyperoxia on skeletal muscle carbohydrate metabolism during transient and steady-state exercise.

This study compared the effects of inspiring either a hyperoxic (60% O(2)) or normoxic gas (21% O(2)) while cycling at 70% peak O(2) uptake on 1) the ATP derived from substrate phosphorylation during the initial minute of exercise, as estimated from phosphocreatine degradation and lactate accumulation, and 2) the reliance on carbohydrate utilization and oxidation during steady-state cycling, as estimated from net muscle glycogen use and the activity of pyruvate dehydrogenase (PDH) in the active form (PDH(a)), respectively. We hypothesized that 60% O(2) would decrease substrate phosphorylation at the onset of exercise and that it would not affect steady-state exercise PDH activity, and therefore muscle carbohydrate oxidation would be unaltered. Ten active male subjects cycled for 15 min on two occasions while inspiring 21% or 60% O(2), balance N(2). Blood was obtained throughout and skeletal muscle biopsies were sampled at rest and 1 and 15 min of exercise in each trial. The ATP derived from substrate-level phosphorylation during the initial minute of exercise was unaffected by hyperoxia (21%: 52.2 +/- 11.1; 60%: 54.0 +/- 9.5 mmol ATP/kg dry wt). Net glycogen breakdown during 15 min of cycling was reduced during the 60% O(2) trial vs. 21% O(2) (192.7 +/- 25.3 vs. 138.6 +/- 16.8 mmol glycosyl units/kg dry wt). Hyperoxia had no effect on PDH(a), because it was similar to the 21% O(2) trial at rest and during exercise (21%: 2.20 +/- 0.26; 60%: 2.25 +/- 0.30 mmol.kg wet wt(-1).min(-1)). Blood lactate was lower (6.4 +/- 1.0 vs. 8.9 +/- 1.0 mM) at 15 min of exercise and net muscle lactate accumulation was reduced from 1 to 15 min of exercise in the 60% O(2) trial compared with 21% (8.6 +/- 5.1 vs. 27.3 +/- 5.8 mmol/kg dry wt). We concluded that O(2) availability did not limit oxidative phosphorylation in the initial minute of the normoxic trial, because substrate phosphorylation was unaffected by hyperoxia. Muscle glycogenolysis was reduced by hyperoxia during steady-state exercise, but carbohydrate oxidation (PDH(a)) was unaffected. This closer match between pyruvate production and oxidation during hyperoxia resulted in decreased muscle and blood lactate accumulation. The mechanism responsible for the decreased muscle glycogenolysis during hyperoxia in the present study is not clear.     

Hydrogen ion concentration and oxygen uptake in an isolated canine hindlimb.

Oxygen utilization (VO2) and lactate production by an isolated perfused canine hindlimb was evaluated at various hydrogen ion concentrations. A membrane lung perfusion system was established such that blood flow and temperature could be fixed at normal levels. Oxygen, nitrogen, and carbon dioxide (CO2) gas flows to the membrane lung were independently regulated to provide a fixed arterial oxygen content (CaO2). By changing CO2 flow, the pH of the arterial blood was varied between 6.9 and 7.6 at 10-min intervals. The mean O2 delivery (CaO2 X blood flow) was between 16.3 ML O2/min and 20.5 ml O2/min. Standard error of the mean in each dog, however, was less than 0.4 ml O2/min. VO2 was linearly related to the pH of the perfusing blood: VO2% = 100.1 pH - 643 (r = 0.866). Oxygen consumption was inversely related to PCO2: VO2% = -0.62 PCO2 + 124, but the correlation was less good (r = 0.729). Lactate production was linearly related to the pH of the perfusing blood (above a pH of 7.4): lactate produced = 22.5 pH - 162.5 (r = 0.75). At a pH below 7.4, lactate was not produced. Oxygen consumption of skeletal muscle appears critically dependent on extracellular fluid pH. A change in pH of 0.1 alters VO2 almost exactly 10%. Alkalosis is a potent stimulus to lactic acid production by skeletal muscle.     

The effect of temperature on canine papillary muscle

Measurements of the total duration,t _Aof the action potential for canine papillary muscle in the temperature range 25–45t_A = texp(Q/k_B T),t_A = \tau \exp (Q/k_B T),     

Effect of temperature on the myoglobin-facilitated transport of oxygen in skeletal muscle.

An analysis of thermal effects on the facilitative transport of oxygen in skeletal muscle fibers is presented. Steady-state mass and energy transport balances are written and solved analytically or numerically using a finite-difference procedure. It is shown that no significant spatial thermal gradients exist due to internal reactions or bulk conduction effects across a muscle fiber. At typical muscle conditions, it is predicted that increased global temperature reduces the fraction of oxygenated myoglobin, increases local oxygen concentrations, and increases the percentage of oxygen flux attributed to oxy-myoglobin. The maximum supportable oxygen consumption rate, mO2max, is defined as the highest consumption rate sustainable without developing anoxic regions at the center of the fiber. By considering only temperature sensitive effects within fibers, mO2max is found to increase slightly with temperature at low temperatures. This increase is due to thermal effects on the diffusion coefficients as opposed to effects associated with the kinetics of the myoglobin-oxygen reaction. If the simulations include the temperature effect associated with oxygen solubility in blood plasma, mO2max decreases with temperature. A sensitivity analysis was performed by varying the values of relevant parameters. The maximum consumption rate was least affected by parameters associated with the kinetic and equilibrium constants and most affected by the diffusion coefficients and the concentration of myoglobin.     

Glucose metabolism in perfused skeletal muscle. Interaction of insulin and exercise on glucose uptake.

1. The interaction of insulin and isometric exercise on glucose uptake by skeletal muscle was studied in the isolated perfused rat hindquarter. 2. Insulin, 10 m-i.u./ml, added to the perfusate, increased glucose uptake more than 10-fold, from 0.3-0.5 to 5.2-5.4 mumol/min per 30g of muscle in hindquarters of fed and 48h-starved rats respectively. In contrast, it did not stimulate glucose uptake in hindquarters from rats in diabetic ketoacidosis. 3. In the absence of added insulin, isometric exercise, induced by sciatic-nerve stimulation, increased glucose uptake to 4 and 3.4 mumol/min per 30g of muscle in fed and starved rats respectively. It had a similar effect in rats with moderately severe diabetes, but it did not increase glucose uptake in rats with diabetic ketoacidosis or in hindquarters of fed rats that had been "washed out" with an insulin-free perfusate. Insulin, at concentrations which did not stimulate glucose uptake in resting muscle, restored the stimulatory effect of exercise in these situations. 4. The stimulation of glucose uptake by exercise was independent of blood flow and the degree of tissue hypoxia; also it could not be reproduced by perfusing resting muscle with a medium previously used in an exercise experiment. 5. At rest glucose was not detectable in muscle cell water of fed and starved rats even when perfused with insulin. In the presence of insulin, a small accumulation of glucose, 0.25 mM, was noted in the muscle of ketoacidotic diabetic rats, suggesting inhibition of glucose phosphorylation, as well as of transport. 6. During exercise, the calculated intracellular concentration of glucose in the contracting muscle increased to 1.1-1.6mM in the fed, starved and moderately diabetic groups. Insulin significantly increased the already high rates of glucose uptake by the hindquarters of these animals but it did not alter the elevated intracellular concentration of glucose. 7. In severely diabetic rats, exercise did not cause glucose to accumulate in the cell in the absence of insulin. In the presence of insulin, it increased glucose uptake to 6.1 mumol/min per 30g of muscle and intracellular glucose to 0.72 mM. 8. The data indicate that the stimulatory effect of exercise on glucose uptake requires the presence of insulin. They suggest that in the absence of insulin, glucose uptake is not enhanced by exercise owing to inhibition of glucose transport into the cell.     

Substrate stimulation of para-aminohippuric acid transport: effect on uptake and runout.

The ability of penicillin pretreatment to increase PAH accumulation by slices of newborn rabbit renal cortex was dissected into two components, uptake and runout. The oxygen-requiring component of the uptake process was significantly enhanced by penicillin treatment, whereas runout was unaffected. Kinetically, the data suggest that penicillin alters the affinity of the transport system for PAH. Due to the limitations of such a kinetic analysis, no conclusions may be drawn from such a suggestion. However, it may be concluded that penicillin pretreatment increases renal accumulation of PAH solely by stimulating the uptake process. Elucidation of the molecular changes involved will require techniques more sophisticated than uptake into renal cortical slices.     

Older type 2 diabetic males do not exhibit abnormal pulmonary oxygen uptake and muscle oxygen utilization dynamics during submaximal cycling exercise

There are reports of abnormal pulmonary oxygen uptake (Vo(2)) and deoxygenated hemoglobin ([HHb]) kinetics in individuals with Type 2 diabetes (T2D) below 50 yr of age with disease durations of <5 yr. We examined the Vo(2) and muscle [HHb] kinetics in 12 older T2D patients with extended disease durations (age: 65 ± 5 years; disease duration 9.3 ± 3.8 years) and 12 healthy age-matched control participants (CON; age: 62 ± 6 years). Maximal oxygen uptake (Vo(2max)) was determined via a ramp incremental cycle test and Vo(2) and [HHb] kinetics were determined during subsequent submaximal step exercise. The Vo(2max) was significantly reduced (P < 0.05) in individuals with T2D compared with CON (1.98 ± 0.43 vs. 2.72 ± 0.40 l/min, respectively) but, surprisingly, Vo(2) kinetics was not different in T2D compared with CON (phase II time constant: 43 ± 17 vs. 41 ± 12 s, respectively). The Δ[HHb]/ΔVo(2) was significantly higher in T2D compared with CON (235 ± 99 vs. 135 ± 33 AU·l(-1)·min(-1); P < 0.05). Despite a lower Vo(2max), Vo(2) kinetics is not different in older T2D compared with healthy age-matched control participants. The elevated Δ[HHb]/ΔVo(2) in T2D individuals possibly indicates a compromised muscle blood flow that mandates a greater O(2) extraction during exercise. Longer disease duration may result in adaptations in the O(2) extraction capabilities of individuals with T2D, thereby mitigating the expected age-related slowing of Vo(2) kinetics.     

Peak muscle perfusion and oxygen uptake in humans: importance of precise estimates of muscle mass.

The knee extensor exercise model was specifically developed to enable in vivo estimates of peak muscle blood flow and O(2) uptake in humans. The original finding, using thermodilution measurements to measure blood flow in relation to muscle mass [P. Andersen and B. Saltin. J. Physiol. (Lond.) 366: 233-249, 1985], was questioned, however, as the measurements were two- to threefold higher than those previously obtained with the (133)Xe clearance and the plethysmography technique. As thermodilution measurements have now been confirmed by other methods and independent research groups, we aimed to address the impact of muscle mass estimates on the peak values of muscle perfusion and O(2) uptake. In the present study, knee extensor volume was determined from multiple measurements with computer tomography along the full length of the muscle. In nine healthy humans, quadriceps muscle volume was 2.36 +/- 0.17 (range 1. 31-3.27) liters, corresponding to 2.48 +/- 0.18 (range 1.37-3.43) kg. Anthropometry overestimated the muscle volume by approximately 21-46%, depending on whether quadriceps muscle length was estimated from the patella to either the pubic bone, inguinal ligament, or spina iliaca anterior superior. One-legged, dynamic knee extensor exercise up to peak effort of 67 +/- 7 (range 55-100) W rendered peak values for leg blood flow (thermodilution) of 5.99 +/- 0.66 (range 4.15-9.52) l/min and leg O(2) uptake of 856 +/- 109 (range 590-1,521) ml/min. Muscle perfusion and O(2) uptake reached peak values of 246 +/- 24 (range 149-373) and 35.2 +/- 3.7 (range 22.6-59. 6) ml. min(-1). 100 g muscle(-1), respectively. These peak values are approximately 19-33% larger than those attained by applying anthropometric muscle mass estimates. In conclusion, the present findings emphasize that peak perfusion and O(2) uptake in human skeletal muscle may be up to approximately 30% higher than previous anthropometric-based estimates that use equivalent techniques for blood flow measurements.     

Methotrexate: studies on the cellular metabolism. I. Effect on mitochondrial oxygen uptake and oxidative phosphorylation

Effect of methotrexate (MTX) on mitochondrial oxygen uptake, oxidative phosphorylation and on the activity of several enzymes linked to respiratory chain was studied. MTX was able to inhibit state III respiration activated by ADP and to decrease the respiratory coefficient with the substrates alpha-ketoglutarate and glutamate; these effects became pronounced when mitochondria were pre-incubated with MTX for 10 min. No effect was observed on ATPase activity of undamaged or broken mitochondria; the same was true for NADH-oxidase, NADH-dehydrogenase, NADH-cytochrome c reductase, succinate oxidase, and cytochrome c oxidase activity. The effect on the steady-state of cytochrome b, as well as, the inhibitory effect on state III of respiration with NAD+-linked substrates, offers a reasonable possibility to suggesting that the inhibition site of MTX could be in a place anterior to cytochrome b region, and not linked to respiratory chain.