The action of verapamil on the rate of spontaneous release of transmitter at the frog neuromuscular junction

Abstract

Verapamil is known to reduce Ca 2+ entry in a variety of cells. At 10 −5 M it produces a small reduction in MEPP frequency at the frog neuromuscular junction, whereas the rate of spontaneous release rises following treatment at a concentration of 10 −4 M. This latter effect is augmented by raising [Ca 2+] 0 to 9 mM or, more especially, by raising the temperature from 17 to 23°C. It is argued that both these opposing effects are related to the action of verapamil in modiying [Ca 2+] i at the presynaptic terminals and its is suggested that the drug can affect both inward Ca 2+ flux (so reducing the steady-state position of [Ca 2+] i) and also, at higher concentration, either inhibit the membrane Ca 2+ pump or cause the release of Ca 2+ from intracellular Ca 2+ stores (so raising [Ca 2+] i).