Effects of verapamil and manganese ion on potassium-induced transmitter release in rat diaphragm muscles

Effects of verapamil and manganese ion (Mn++) were examined on the rise in the frequency of miniature end-plate potential (m.e.p.p.) induced by hypertonic 10 mM potassium chloride (H10K) in rat diaphragm muscles. H10K consistently caused a 10-fold increase of the m.e.p.p. frequency and depolarized the membrane potential of muscle fiber by approximately 10 mV. The effect of H10K on the m.e.p.p. frequency disappeared in Ca++-free solutions. Verapamil (5 x 10(-5) M) and Mn++ (1 mM) consistently increased the m.e.p.p. frequency by factors of 3 and 5, respectively. The stimulatory effect of H10K on the m.e.p.p. frequency was abolished by Mn++ but was not affected by verapamil. These data suggest that the effect of verapamil on the motor nerve terminal is different from the effect on the cardiac muscle and on the vascular and intestinal smooth muscle.