Metabolic determinants of the onset of acidosis in exercising human muscle: a 31P-MRS study

Onset of intracellular acidosis during muscular exercise has been generally attributed to activation or hyperactivation of nonoxidative ATP production but has not been analyzed quantitatively in terms of H+ balance, i.e., production and removal mechanisms. To address this issue, we have analyzed the relation of intracellular acidosis to H+balance during exercise bouts in seven healthy subjects. Each subject performed a 6-min ramp rhythmic exercise (finger flexions) at low frequency (LF, 0.47 Hz), leading to slight acidosis, and at high frequency (HF, 0.85 Hz), inducing a larger acidosis. Metabolic changes were recorded using 31P-magnetic resonance spectroscopy. Onset of intracellular acidosis was statistically identified after 3 and 4 min of exercise for HF and LF protocols, respectively. A detailed investigation of H+ balance indicated that, for both protocols, nonoxidative ATP production preceded a change in pH. For HF and LF protocols, H+ consumption through the creatine kinase equilibrium was constant in the face of increasing H+ generation and efflux. For both protocols, changes in pH were not recorded as long as sources and sinks for H+approximately balanced. In contrast, a significant acidosis occurred after 4 min of LF exercise and 3 min of HF exercise, whereas the rise in H+ generation exceeded the rise in H+ efflux at a nearly constant H+ uptake associated with phosphocreatine breakdown. We have clearly demonstrated that intracellular acidosis in exercising muscle does not occur exclusively as a result of nonoxidative ATP production but, rather, reflects changes in overall H+ balance.

Fish muscle energy metabolism measured during hypoxia and recovery: an in vivo 31P-NMR study

Three fish species were exposed to graded hypoxia levels and allowed to recover. Levels of high-energy phosphate compounds in epaxial white muscle were monitored by in vivo 31P nuclear magnetic resonance (NMR) spectroscopy. Furthermore, O2 consumption of the animals was measured. With increasing hypoxia load, metabolic parameters started to change in the following order: phosphocreatine (PCr)-to-Pi ratio (decrease), O2 consumption (decrease), [PCr] (decrease), intracellular pH (pHi; decrease), Pi (increase), free ADP concentration ([ADP]free; increase), [ATP] (decrease). PCr levels fell with the PO2. After each increment, the [PCr] reached a stable plateau value while, in some cases, a recovery was observed. This recovery could be explained because the balance between anaerobic and aerobic metabolism is continuously fluctuating during hypoxia as a consequence of changes in the activity of the fish. Consequently the [ADP]free are fluctuating, resulting in an activation of the creatine kinase reaction and the anaerobic glycolysis. In all three species, anaerobic glycolysis was activated, but in contrast to anoxia exposure, metabolic suppression was absent. The changes of [ADP]free and [H+] (which influences the position of the creatine kinase equilibrium) are species dependent. Species differences observed in the other parameters were small. It is concluded that the pattern of the activation of anaerobic metabolism under deep hypoxia is different from that under anoxia.

31P-NMR of high-energy phosphates in perfused rat heart during metabolic acidosis

Intracellular pH (pHi), intracellular free magnesium concentration ([Mg2+]i), and high-energy phosphates in Langendorff perfused rat hearts were evaluated by 31P-nuclear magnetic resonance (NMR) during metabolic acidosis. During acidosis, cardiac pHi approached that of the perfusing solution (pH approximately 6.7) and [Mg2+]i increased. In hearts perfused with glucose as the sole carbon source, the ratio of [phosphocreatine] to [ATP] decreased during acidosis. In contrast, in hearts supplemented with pyruvate (either 2.8 or 10 mM) this ratio increased during acidosis. Oxygen consumption decreased in hearts perfused with glucose only and with pyruvate-glucose. Using the creatine kinase equilibrium constant, we find that [MgADP] is significantly decreased in pyruvate-perfused hearts but is not significantly altered in glucose-perfused hearts during metabolic acidosis. These data indicate that [MgADP] may be the regulator of cardiac oxidative phosphorylation in the presence of excess pyruvate; however, during metabolic acidosis in hearts perfused with glucose only, ATP synthesis appears limited by the availability of pyruvate via glycolysis.