Phosphocreatine and Inorganic Phosphate in Working and Resting Muscles of Rats, Studied with Radioactive Phosphorus

The rapid hydrolysis of large quantities of phosphocreatine is one of the characteristic initial processes which occur during muscular contraction (l-3). Minimal concentrations are found after 1 or 2 minutes of work and little further change is apparent as work continues. The isotope technique would seem to offer a new approach to the question of whether the small quantity of phosphocreatine remaining after the beginning of work is involved actively in the continuation of this process. Sacks (4) measured the relative radioactivity of the phosphocreatine of cat muscle in situ after a 15 second period of isometric tetanus and found that the contraction did not modify the rate of turnover of the remaining phosphocreatine. We have now used radioactive phosphorus to study the effect of continuous isotonic contraction at the rate of three twitches per second on the phosphocreatine and inorganic phosphate of muscle, the effect of one period of work followed by rest, and the effect of repeated alternating periods of work and rest. The changes found in the concentrations of the phosphate compounds of the muscles studied under the conditions of work and rest in these experiments were similar to those previously reported (2). In muscle which had contracted 180 times in 1 minute, as compared with the resting control muscle of the unstimulated leg, these changes were found: approximately a 3-fold increase of inorganic phosphate, a decrease to a fifth of phosphocreatine, and a decrease to a half of adenosine triphosphate. As long as the muscle continued to work, no major changes in the content of these phosphates occurred. Subsequent rest of 5 minutes restored the resting values of inorganic phosphate and phosphocreatine, but the resynthesis of adenosine triphosphate was not as rapidly completed.