Post-contractile phosphocreatine splitting in muscle as revealed by time-resolved 31P nuclear magnetic resonance.

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Abstract
A 25 mm diameter chamber was designed and constructed to study the 31P NMR spectra from a considerable mass of toad and frog muscles (16 sartorii weighing 5-10 g) which were maintained in a well-oxygenated condition at 4.degree. C. The biochemical changes were measured that accompany contraction and recovery with improved time-resolution. Using this apparatus, splitting of phosphocreatine (PCr) continues for a few min after relaxation. Subsequently the PCr is rebuilt by oxidative processes in the familiar way, with a time-constant .simeq. 10 min. By studying tetanic contractions of various durations, the time-course of the post-contractile PCr splitting is similar to that of the heat production that cannot yet be accounted for by known chemical changes. Myosin and actomyosin ATPase reactions most likely underlie the post-contractile ATP utilization. Evidently, the post-contractile ATP utilization is responsible for the unexplained enthalpy mentioned above.