Neuromuscular block by verapamil and diltiazem and inhibition of acetylcholine release

Abstract

The effects of Ca 2+-channel antagonists, verapamil, diltiazem and nifedipine, on the neuromuscular transmission were studied in the isolated mouse phrenic nerve-diaphragm preparations. All 3 drugs increased the twitch response evoked by direct single stimulation at 10–100 μM. The neuromuscular transmission at 0.1 Hz was blocked by verapamil and diltiazem, but not by nifedipine, only at very high concentrations (⩾100 μM). In the time course of block, no endplate potential (e.p.p.) could be recorded, whenever the junction failed to elicit an action potential, suggesting that the block is due to an axonal conduction failure. Conduction block became apparent in both axon and muscle at low concentrations (>10 μM) of verapamil and diltiazem at 100 Hz. When the safety margin of neuromuscular transmission was reduced by tubocurarine or low Ca 2+ plus high Mg 2+, verapamil and diltiazem, but not nifedipine, reduced the single twitch response to nerve stimulation at concentrations that did not cause axon conduction block. The inhibition was dependent on the frequency of nerve stimulation, enhanced by low-Ca 2+ and antagonized by high-Ca 2+. Verapamil (50 μM) inhibited the mean amplitude of the median size miniature e.p.p. by only8 %, whereas it increased the frequency by 4–5 fold and the proportion of both small and giant miniature e.p.p.s. The e.p.p. amplitude was inhibited by verapamil by about 67% in low-Ca 2+ media and by about 38% in normal Tyrode. Similar but somewhat lesser effect was obtained with diltiazem. It is concluded that verapamil and diltiazem, but not nifedipine, inhibit the transmitter release. This effect in normal Tyrode, however, is largely due to an inhibition of Na +-channel. An inhibition of Ca 2+-channel may be involved in low-Ca 2+ media.