Energetics studies of muscles of different types.

Abstract

31P-NMR studies were performed in isolated perfused striated and smooth muscles. Important qualitative and quantitative differences were found in resting muscles. In resting fast-twitch skeletal muscle the chemical potential of ATP obtained from the measured intracellular pH, ATP and inorganic phosphate concentrations and from the ADP concentrations calculated from the position of the creatine kinase equilibrium was -72 kJ/mol ATP. This high value was the result of a very low free ADP and inorganic phosphate content. In resting slow-twitch skeletal muscle, in smooth muscle, and in cardiac muscle at low work rates (literature data), the chemical potential of ATP was lower (approximately -50 to -60 kJ/mol), the difference being primarily due to a much higher inorganic phosphate content (especially in slow-twitch and smooth muscle) and/or a higher ADP concentration (especially in cardiac muscle). Upon stimulation or, for the heart, working at higher work rates, the pattern of chemical changes of phosphocreatine, creatine and inorganic phosphate was the same for all types of muscle. The phosphocreatine levels decreased and the inorganic phosphate concentration increased stoichiometrically without a change in the ATP content so long as the phosphocreatine pool was not totally depleted (greater than or equal to 10%). The rate and extent of these chemical changes was dependent on the inherent ATPase and ATP synthesis rates. The exception was in the intracellular pH changes. In fast-twitch and smooth muscle, pH decreased with contractile activity, as expected from the large glycolytic capacity. However, an alkalinization was observed in slow-twitch skeletal muscle and this difference was attributed to the uptake of H+ during the net hydrolysis of phosphocreatine to creatine plus inorganic phosphate, and to the absence of significant lactate production. The pH of cardiac muscle does not appear to change with work load. The common bioenergetic pattern in all types of muscles is consistent with a graded increase in ADP concentration (from below to well above the apparent Km for nucleotide translocase ANT) with increasing work as a regulator of mitochondrial respiration. In fast-twitch muscle these changes are also accompanied by large changes in inorganic phosphate concentration (3-30 mM) which may also play a role in metabolic regulation.(ABSTRACT TRUNCATED AT 400 WORDS)