Decrease in the size of tetanic responses produced by nitrendipine by extracellular calcium ion removal without blocking twitches or action potentials in skeletal muscle

The effects of removing extracellular Ca ions or of adding the organic calcium channel antagonist, nitrendipine, were tested on twitches and tetani (1 00 Hz for 2 sec) in frog toe muscles. Under conditions that did not reduce or that potentiated twitches, both procedures reduced the size of the tetanic responses. This depression was seen as an inability to maintain the maximum tetanic tension for more than 0.5 sec. Intracellular microelectrode recordings showed that the muscle fibers were depolarized (mean about 23 mV) during the stimulus train and the fiber only slowly repolarized after the train. The latter effect is the “late negative afterpotential ” and it is produced by the accumulation of K ions in the t-tubules during the action potential train. Neither the depolarization nor the late negative afterpotentials were decreased in amplitude by nitrendipine. These results indicat