Effects of excitation-contraction uncoupling by stretch and hypertonicity on metabolism and tension in single frog muscle fibers

Metabolism and tension were examined in single fibers of the semitendinosus muscle of Rana pipiens at 15 degree C after excitation-contraction uncoupling by stretch and hypertonicity. Interrupted tetanic stimulation at 20 HZ for 150 s, of control fibers in isotonic Ringer at a rest sarcomere length (SL) of 2.3 micrometers, resulted in a steadily declining tension, stimulated glycolysis, and significantly reduced fiber phosphocreatine (PCr) and ATP concentrations. Stretching resting muscle fibers to an SL of 4.7 micrometers did not alter metabolite concentrations, but glucose-6-phosphate rose and PCr fell markedly when the stretched fibers were stimulated tetanically, although tension was absent. Immersion of untetanized fibers in 2.5 X isotonic Ringer produced a transient rise in resting tension, an increase in glucose-6-phosphate, and a significant reduction in PCr. During the transient rise in resting tension, PCr consumption per unit of tension-time integral was the same as that in fibers stimulated tetanically in isotonic Ringer. Tetanization of fibers in hypertonic solution did not further alter metabolite concentrations or produce tension. The results indicate that exposure to hypertonicity induces an increase in both tension and consumption of high-energy phosphate bonds (approximately P) in resting fibers, but stretch does not. during tetanic stimulation, stretch interferes with contraction but does not prevent activation, whereas hypertonicity inhibits activation as well as contraction.