Chemical energy balance in amphibian and mammalian muscles

A quantitative comparison of the utilization of creatine phosphate and ATP during a muscle contraction with the extent of recovery oxygen consumption afterward is used to establish a biochemical energy balance. In prolonged contractions of sartorii from Rana temporaria and in tetani of mouse hindlimb muscles, the predicted stoichiometric relationship, delta approximately P/delta O2 = 6.2, is observed. Thus a simple energetic paradigm of a separable contractile and recovery phase is quantitatively valid. In contrast, in brief contractions of sartorii of R. pipiens and R. temporaria, the ratio of delta approximately P/delta O2 is lower than the predicted value of 6.2. This biochemical energy imbalance observed in brief tetani of frog muscles implies: 1) a significant and continued utilization of high-energy phosphates after mechanical relaxation and 2) that measurement of high-energy phosphate utilization during contraction does not measure the total energy cost for the contraction. In the fast, pale, mouse extensor digitorum longus muscle, phosphorylation in an 18,000-dalton myosin light chain occurred during contraction and was correlated with twofold decrease in the isometric maintenance energy cost. Thus the energy cost for maintaining isometric tension in fast and slow twitch mammalian muscles depends on the physiological conditions of the contraction and appears to be variable according to myosin light chain phosphorylation.