INHIBITORY EFFECTS OF HGCL2 ON EXCITATION-SECRETION COUPLING AT THE MOTOR-NERVE TERMINAL AND EXCITATION-CONTRACTION COUPLING IN THE MUSCLE-CELL

1. Indirect and direct twitch (0.1-Hz) stimulation of the rat phrenic nerve-diaphragm disclosed that the inhibitory effect of HgCl2, 3.7 x 1 0(-5) M, on the neuromuscular transmission and in the muscle cell, was accelerated by 10-sec periods of 50-Hz tetanic stimulation every 10 m in. This activity-dependent enhancement suggested an inhibitory mechan ism of HgCl2 related to the development of fatigue, like membrane depo larization or decreased excitability, decreased availability of transm itter, or interference with the factors controlling excitation-secreti on coupling of the nerve terminal, i.e. (Ca2+)(0) or (Ca2+)(i), and ex citation-contraction coupling in the muscle cell, i.e., (Ca2+)(i). 2. During both indirect and direct stimulation, HgCl2-induced inhibition was enhanced markedly by pretreatment with caffeine, which releases Ca 2+ from endoplasmic and sarcoplasmic reticulum in the nerve terminal a nd muscle cell, respectively. This caffeine-induced enhancement was co mpletely antagonized by dantrolene, which inhibits the caffeine-induce d release. However, dantrolene alone did not antagonize the HgCl2-indu ced inhibition. 3. Since caffeine depletes the intracellular Ca2+ stor es of the smooth endoplasmic reticulum, HgCl2 probably inhibits by bin ding to SH groups of transport proteins conveying the messenger functi on of (Ca2+)(i). In the muscle cell this leads to inhibition of contra ction. In the nerve terminal, an additional enhancement of the HgCl2-i nduced inhibition, by inhibiting reuptake of choline by TEA and tetani c stimulation, suggested that HgCl2 inhibited a (Ca2+)(i) signal neces sary for this limiting factor in resynthesis of acetylcholine. 4. The (Ca2+)(0) signal necessary for stimulus-induced release of acetylcholi ne was not affected by HgCl2. Hyperpolarization in K+-free solution an tagonized the inhibitory effect of HgCl2 at indirect stimulation, and Ca2+-free solution enhanced the inhibitory effect at direct stimulatio n. K+ depolarization, membrane electric field increase with high Ca2+, membrane stabilization with lidocaine, and half-threshold stimulation , did not change the inhibitory effect of HgCl CH3HgCl, 1.85 x 10(-5) M, disclosed a synergistic interaction with caffeine during direct, bu t not during indirect, stimulation.