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

Abstract

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 (Δ∼P/Δ t) was determined by 31P- and 1H-MRS as the sum of the rates of PCr hydrolysis (Δ[PCr]/Δ t) and of anaerobic glycolysis (Δ[La]/ Δ t, based on a ∼P/La ratio of 1.5). The investigated T levels were 15, 20 and 25 °C, 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/d T=−0.0165 pH units/°C). Both rates of PCr hydrolysis and of lactate accumulation and that of their sum, expressed as Δ∼P/Δ t, were highly T-dependent. By contrast, the pHe-dependence of the muscle energy balance was nil or extremely limited at 15 and 20 °C, respectively, but remarkable at 25 °C (with a depression of the ATP resynthesis rate up to 25% with a decrease of pHe from 7.9 to 7.0). The pHe-dependent 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 −/HCO 3 − exchanger. Uncoupling of glycogenolysis from P-metabolite concentrations, both as function of T (≥20 °C) and of pHe (≤7.3), 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.