Effect of Urea on Excitation-Contraction Coupling in Frog Sartorius Muscle.

Mechanical and ultrastructural studies were carried out on the frog sartorius muscle immersed in hypertonic Ringer's solution containing 400mM urea and thereafter was returned to normal Ringer's solution. 2. Twitch tension began to fall immediately after immersion of muscle in the hypertonic urea solution. Five minutes after the immersion twitch tension reached its minimal values and then recovered. The recovered twitch in the urea-treated muscle was markedly potentiated and the duration of plateau and the phase of relaxation were prolonged. 3. Twitch and potassium contracture were abolished in the muscle which was returned to normal Ringer's solution following a one hour treatment with hypertonic urea solution, but the muscle still retained its ability to respond to caffeine. 4. Resting potentials in the urea-treated and the urea-removed muscles decreased by about 10 mV. In the urea-treated fiber, a long series of discharges was observed following a single electrical stimulation, and voltage of the negative afterpotential was increased. In the urea-removed fiber, the action potential was still evoked, but the voltage of the negative afterpotential was considerably decreased. 5. The continuity between sarcolemma and T-tubular membrane was clearly demonstrated in electron micrographs of the urea-removed muscle. However, the distance between the membrane of T-tubule and the membrane of terminal cisterns in the urea-removed muscle was longer than that in the normal and urea-treated muscles. On the basis of these results, the effect of urea on the excitation-contraction coupling in frog sartorius muscle was discussed.