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

We have designed and constructed a 25 mm diameter chamber in order to study the phosphorus nuclear magnetic resonance (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 degrees C. We have thus been able to measure the biochemical changes that accompany contraction and recovery with improved time-resolution. Using this apparatus it is shown that splitting of phosphocreatine (PCr) continues for a few minutes after relaxation. Subsequently the PCr is rebuilt by oxidative processes in the familiar way, with a time-constant congruent to 10 min. By studying tetanic contractions of various durations we have shown that 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. The results suggest that the post-contractile ATP utilization is responsible for the unexplained enthalpy mentioned above.