The relationship between initial creatine phosphate breakdown and recovery oxygen consumption for a single isometric tetanus of the frog sartorius muscle at 20 degrees C

An Equivocal Final Link - Quantitative Determination of the Thermodynamic Efficiency of ATP Hydrolysis - Sullies the Chain of Electric, Ionic, Mechanical and Metabolic Steps Underlying Cardiac Contraction

Each beat of the heart completes the final step in a sequence of events commencing with electrical excitation-triggered release of Ca2+ from the sarcoplasmic reticulum which, in turn, triggers ATP-hydrolysis-dependent mechanical contraction. Given that Thermodynamics is inherently detail-independent, the heart can be thus be viewed as a mechanical pump - the generator of pressure that drives blood through the systemic and pulmonary circulations. The beat-to-beat pressure-volume work (W) of the heart is relatively straightforward to measure experimentally. Given an ability to measure, simultaneously, the accompanying heat production or oxygen consumption, it is trivial to calculate the mechanical efficiency: ε = W/ΔH where ΔH is the change of enthalpy: (W + Q), Q representing the accompanying production of heat. But it is much less straightforward to measure the thermodynamic efficiency: η = W/ΔG ATP , where ΔG ATP signifies the Gibbs Free Energy of ATP hydrolysis. The difficulty arises because of uncertain quantification of the substrate-dependent yield of ATP - conveniently expressed as the P/O2 ratio. P/O2 ratios, originally ("classically") inferred from thermal studies, have been considerably reduced over the past several decades by re-analysis of the stoichiometric coefficients separating sequential steps in the electron transport system - in particular, dropping the requirement that the coefficients have integer values. Since the early classical values are incompatible with the more recent estimates, we aim to probe this discrepancy with a view to its reconciliation. Our probe consists of a simple, thermodynamically constrained, algebraic model of cardiac mechano-energetics. Our analysis fails to reconcile recent and classical estimates of PO2 ratios; hence, we are left with a conundrum.

Oxygen uptake kinetics

Muscular exercise requires transitions to and from metabolic rates often exceeding an order of magnitude above resting and places prodigious demands on the oxidative machinery and O2-transport pathway. The science of kinetics seeks to characterize the dynamic profiles of the respiratory, cardiovascular, and muscular systems and their integration to resolve the essential control mechanisms of muscle energetics and oxidative function: a goal not feasible using the steady-state response. Essential features of the O2 uptake (VO2) kinetics response are highly conserved across the animal kingdom. For a given metabolic demand, fast VO2 kinetics mandates a smaller O2 deficit, less substrate-level phosphorylation and high exercise tolerance. By the same token, slow VO2 kinetics incurs a high O2 deficit, presents a greater challenge to homeostasis and presages poor exercise tolerance. Compelling evidence supports that, in healthy individuals walking, running, or cycling upright, VO2 kinetics control resides within the exercising muscle(s) and is therefore not dependent upon, or limited by, upstream O2-transport systems. However, disease, aging, and other imposed constraints may redistribute VO2 kinetics control more proximally within the O2-transport system. Greater understanding of VO2 kinetics control and, in particular, its relation to the plasticity of the O2-transport/utilization system is considered important for improving the human condition, not just in athletic populations, but crucially for patients suffering from pathologically slowed VO2 kinetics as well as the burgeoning elderly population.

Exercise: Kinetic considerations for gas exchange

The activities of daily living typically occur at metabolic rates below the maximum rate of aerobic energy production. Such activity is characteristic of the nonsteady state, where energy demands, and consequential physiological responses, are in constant flux. The dynamics of the integrated physiological processes during these activities determine the degree to which exercise can be supported through rates of O₂ utilization and CO₂ clearance appropriate for their demands and, as such, provide a physiological framework for the notion of exercise intensity. The rate at which O₂ exchange responds to meet the changing energy demands of exercise--its kinetics--is dependent on the ability of the pulmonary, circulatory, and muscle bioenergetic systems to respond appropriately. Slow response kinetics in pulmonary O₂ uptake predispose toward a greater necessity for substrate-level energy supply, processes that are limited in their capacity, challenge system homeostasis and hence contribute to exercise intolerance. This review provides a physiological systems perspective of pulmonary gas exchange kinetics: from an integrative view on the control of muscle oxygen consumption kinetics to the dissociation of cellular respiration from its pulmonary expression by the circulatory dynamics and the gas capacitance of the lungs, blood, and tissues. The intensity dependence of gas exchange kinetics is discussed in relation to constant, intermittent, and ramped work rate changes. The influence of heterogeneity in the kinetic matching of O₂ delivery to utilization is presented in reference to exercise tolerance in endurance-trained athletes, the elderly, and patients with chronic heart or lung disease.

On the composition of the cytosol of relaxed skeletal muscle of the frog

This review summarizes a variety of estimates for the concentrations of the principal cytosolic constituents in frog skeletal muscle. From these estimates (listed in the APPENDIX), we chose representative values and used electroneutrality and osmotic considerations to ensure that all major constituents have been considered. Given total cytosolic concentrations of these constituents from the literature, we employed a computer program to calculate the concentrations of all the major ionic species in the cytosol. In relaxed muscle, electroneutrality and osmotic constraints are fulfilled if, in addition to diffusible species, the charge contribution of the myofilaments is considered. Mean buffer power of the diffusible cytosolic species is calculated to be less than one-third of that experimentally determined for whole muscle. Computations indicate that recent estimates of intracellular free magnesium concentration approximately 1 mM are likely to be correct.

Oxygen consumption of single muscle fibres of Rana temporaria and Xenopus laevis at 20 degrees C

1. Oxygen consumption of contracting single muscle fibres of Rana temporaria and Xenopus laevis was investigated at 20 degrees C. 2. Single fibres of the tibialis anterior muscle of Rana and the iliofibularis muscle of Xenopus were mounted in a chamber containing Ringer solution. The solution was stirred and its partial pressure of oxygen (PO2) was continuously measured polarographically. 3. Steady-state rates of oxygen consumption (VO2) of single fibres were determined as a function of twitch frequency (0.2-12 stimuli s-1, depending on the type of fibre). VO2 increased with twitch frequency until a plateau value (VO2,max) was reached. VO2,max of different fibres ranged from 0.042 to 0.169 nmol O2 s-1 mg-1 dry weight in Rana and from 0.045 to 0.412 nmol O2 s-1 mg-1 dry weight in Xenopus. Under VO2,max conditions oxygen availability was not the limiting factor. 4. VO2 after injection of the uncoupler carbonyl cyanide m-chlorophenylhydrazone (CCCP) into the chamber correlated with VO2,max, suggesting that VO2,max is determined by mitochondrial density. This suggestion was confirmed by the observation that a close relationship exists between VO2,max and succinate dehydrogenase activity in three different fibre types of Xenopus. 5. At VO2,max a considerable amount of oxygen was taken up after the twitch train by most fibres, indicating that the oxidative ATP synthesis cannot match ATP hydrolysis. Xenopus muscle fibres with high oxidative capacity did not show this phenomenon. 6. The results are discussed in relation to the occurrence of anoxic cores in muscle fibres and the maximum steady-state contractile activity attainable by the fibres.

First-order kinetics of muscle oxygen consumption, and an equivalent proportionality between QO2 and phosphorylcreatine level. Implications for the control of respiration

In frog sartorius muscle, after a tetanus at 20 degrees C, during which an impulse-like increase occurs in the rate of ATP hydrolysis, the rate of O2 consumption (QO2) reaches a peak relatively quickly and then declines monoexponentially, with a time constant not dependent on the tetanus duration (tau = 2.6 min in Rana pipiens and 2.1 min in Rana temporaria). To a good approximation, these kinetics are those of a first-order impulse response, and the scheme of reactions that couple O2 consumption to extramitochondrial ATP hydrolysis thus behaves as a first-order system. It is first deduced and then demonstrated directly that while QO2(t) is monoexponential, it changes in parallel with the levels of creatine and phosphorylcreatine, with proportionality constants +/- 1/tau p, where p is the P/O2 ratio in vivo. From this, it is further deduced that the mitochondrial creatine kinase (CK) reaction is pseudo-first order in vivo. The relationship between [creatine] and QO2 predicted by published models of the control of respiration is markedly different from that actually observed. As shown here, the first-order kinetics of QO2 are consistent with the hypothesis that respiration is rate-limited by the mitochondrial CK reaction; this has as a corollary the "creatine shuttle" hypothesis.

Reappraisal of diffusion, solubility, and consumption of oxygen in frog skeletal muscle, with applications to muscle energy balance

Previously we tested the validity of the one-dimensional diffusion equation for O2 in the excised frog sartorius muscle and used it to measure the diffusion coefficient (D) for O2 in this muscle and the time course of its rate of O2 consumption (Qo2) after a tetanus (Mahler, 1978, 1979, J. Gen. Physiol., 71:533-557, 559-580, 73:159-174). A transverse section of the frog sartorius is in fact well fit by a hemi-ellipse with width divided by maximum thickness averaging 5.1 +/- 0.2. Using the previous techniques with the two-dimensional diffusion equation and this hemi-elliptical boundary yields a value for D that is 30% smaller than reported previously; the revised values at 0, 10, and 22.8 degrees C are 6.2, 7.9, and 10.8 X 10(-6) cm2/s, respectively. After a tetanus at 20 degrees C, Qo2 rose quickly to a peak and then declined exponentially, with a time constant (tau) approximately 15% faster than that reported previously; tau averaged 2.1 min in Rana temporaria and 2.6 min in Rana pipiens. A technique was devised to measure the solubility (alpha) of O2 in intact, respiring muscles, and yielded alpha (muscle)/alpha (H2O) = 1.26 +/- 0.04. With these modifications, the values for O2 consumption obtained with the diffusion method were in agreement with those measured by the direct method of Kushmerick and Paul (1976, J. Physiol. [Lond.]., 254:693-709). Using results from both methods, at 20 degrees C the ratio of phosphorylcreatine split during a tetanus to O2 consumption during recovery ranged from 5.2 to 6.2 mumol/mumol, and postcontractile ATP hydrolysis was estimated to be 13.6 +/- 4.1 (n = 3) nmol/mumol total creatine.

Physical and biochemical energy balance during an isometric tetanus and steady state recovery in frog sartorius at 0 degree C

Frog sartorius muscle stimulated isometrically for 3 s every 256 s to attain a steady state in which initial heat (QI), recovery heat (QR), rate of O2 consumption (JO2), and isometric force (PO) generated are constant for each cycle. For a 3-s tetanus given every 256 s, JO2 was 0.106 mumol/(min . g blotted weight), approximately 71% of the maximum rate observed, whereas lactate production was negligible under these conditions. QI, QT(= QI + QR), and QT/QI were 88.2, 181.5, 2.06 mJ/g blotted weight, respectively. The high-energy phosphate breakdown (delta approximately P) breakdown during the first 3-s tetanus was not different from that during a contraction in the steady state and averaged 1.1 mumol/g blotted weight. Less than half of the initial heat could be accounted for in terms of the extent of the known chemical reactions occurring during contraction. From the stoichiometry of the theoretical biochemical pathways, the amount of ATP synthesized in the steady state exceeds delta approximately P during contraction by more than twofold, corresponding to an apparent ADP:O ratio of 1.5. If it is assumed that carbohydrate oxidation is the only net chemical reaction in the steady state, the total heat production can be explained on the basis of the measured JO2. Under this assumption, heat production during recovery was less than that expected on the basis of the oxygen consumption and delta approximately P during contraction. These observations support the hypothesis that the unexplained enthalpy production and low apparent ADP:O ratio are causally related, i.e., that the reaction(s) producing the unexplained heat during contraction is reversed during the recovery period.

Kinetics of oxygen consumption after a single flash of light in photoreceptors of the drone (Apis mellifera)

The time course of the rate of oxygen consumption (QO2) after a single flash of light has been measured in 300-micrometers slices of drone retina at 22 degrees C. To measure delta QO2(t), the change in QO2 from its level in darkness, the transients of the partial pressure of O2 (PO2) were recorded with O2 microelectrodes simultaneously in two sites in the slice and delta QO2 was calculated by a computer using Fourier transforms. After a 40-ms flash of intense light, delta QO2, reached a peak of 40 microliters O2/g.min and then declined exponentially to the baseline with a time constant tau 1 = 4.96 +/- 0.49 s (SD, n = 10). The rising phase was characterized by a time constant tau 2 = 1.90 +/- 0.35 s (SD, n = 10). The peak amplitude of delta QO2 increased linearly with the log of the light intensity. Replacement of Na+ by choline, known to decrease greatly the light-induced transmembrane current, caused a 63% decrease of delta QO2. With these changes, however, the kinetics of delta QO2 (t) were unchanged. This suggest that the recovery phase is rate-limited by a single reaction with apparent first-order kinetics. Evidence is provided that suggests that this reaction may be the working of the sodium pump. Exposure of the retina to high concentrations of ouabain or strophanthidin (inhibitors of the sodium pump) reduced the peak amplitude of delta QO2 by approximately 80% and increased tau 1. The increase of tau 1 was an exponential function of the time of exposure to the cardioactive steroids. Hence, it seems likely that the greatest part of delta QO2 is used for the working of the pump, whose activity is the mechanism underlying the rate constant of the descending limb of delta QO2 (t).

31P NMR reveals increased intracellular pH after fertilization in Xenopus eggs

31P NMR spectra of mature eggs of the frog (Xenopus laevis) were taken prior to and after both fertilization and activation by a Ca2+/H+ ionophore (A23187). The eggs were constantly perfused with fresh well-buffered solution during the experiments, and the intracellular pH (pHi) was determined from the pH-dependent chemical shift of the internal Pi peak. The detection of this Pi peak in the presence of overlapping yolk phosphoprotein signals was accomplished by a T2 experiment which discriminated against the broader yolk phosphoprotein peak. The average pHi of the unfertilized, fertilized, and activated eggs was 7.42, 7.66, and 7.64, respectively. Thus, a cytoplasmic alkalinization of 0.24 pH unit occurs within 90 min 90 min after fertilization. These values are practically identical to pHi measurements made in this laboratory on Xenopus eggs by using pH-sensitive glass microelectrodes. These 31P NMR studies also indicate that extracellular pH changes as large as 3 pH units had no effect on pHi. We also found that phosphocreatine levels are very sensitive to metabolic perturbations such as oxygen depletion or metabolic inhibitor application. These treatments resulted in a rapid decrease in the phosphocreatine concentration; the ATP concentration declined only slowly after the phosphocreatine peak had disappeared.