Temperature and pH dependence of energy balance by 31P- 1H-MRS in anaerobic frog muscle

The temperature (T)-dependence of energy consumption of resting anaerobic frog gastrocnemii exposed to different, changing electrochemical gradients was assessed. To this aim, the rate of ATP resynthesis (DfP/Dt) was determined by 31P- and 1H-MRS as the sum of the rates of PCr hydrolysis (D[PCr]/Dt) and of anaerobic glycolysis (D[La]/ Dt, based on a fP/La ratio of 1.5). The investigated T levels were 15, 20 and 25 jC, whereas initial extracellular pH (pHe) values were 7.9, 7.3 and 7.0, i.e. higher, equal or lower, respectively, than intracellular pH (pHi). The latter was changing with T according to the neutrality point (dpH/dT = 0.0165pH units/jC). Both rates of PCr hydrolysis and of lactate accumulation and that of their sum, expressed as DfP/Dt, were highly T-dependent. By contrast, the pHe-dependence of the muscle energy balance was nil or extremely limited at 15 and 20 jC, respectively, but remarkable at 25 jC (with a depression of the ATP resynthesis rate up to 25% with a decrease of pHe from 7.9 to 7.0). The pHedependent reduction of metabolic rate was associated with a down-regulation of anaerobic glycolysis due to reduced activity of ion transporters controlling acid–base balance and/or to a shift from Na+/H+ to a more efficient Na+-dependent Cl /HCO3 exchanger. Uncoupling of glycogenolysis from P-metabolite concentrations, both as function of T (z20 jC) and of pHe (V7.3), was also shown, attributable to a T-dependence of glycolytic enzyme activity and/or H+ ion transport systems. The described metabolic slowdown observed in isolated muscle preparations subjected to the combined regimes of anoxia/acidosis implies that the mechanism determining survival time at the cellular level is mediated by exchange transport systems. A similar mechanism might affect muscle metabolism of homeotherms during chronic hypoxia and/or ischemia.