Studies on the mechanism of action of acetylcholine antagonists on rat parasympathetic ganglion cells.

Abstract

The mode of action of ACh [acetylcholine] antagonists on the parasympathetic neurons of the submandibular ganglion of the rat was studied using a 2-microelectrode voltage-clamp technique. The currents produced by various agonists (carbachol, ACh, suberylcholine) were studied in steady state and after voltage steps, before and after perfusion of various antagonists. For 3 antagonists (tubocurarine, hexamethonium and decamethonium) the blocking action increases with hyperpolarization. For 3 other antagonists (surugatoxin, trimetaphan and mecamylamine) the effects observed at low concentrations appear to be independent of membrane potential, but in some cases voltage dependence of the block was seen for mecamylamine. The block produced by tubocurarine, hexamethonium and decamethonium increases with the agonist concentration, an observation which supports a sequential scheme in which the antagonist blocks the open channel-receptor complex. The block produced by trimetaphan and mecamylamine decreases slightly with increased agonist concentration, which in turn suggests that the 2 compounds are competitive antagonists, preventing binding of the agonists to the closed channel-receptor complex. In the cases where the block is voltage dependent, voltage jumps trigger slow relaxations which are not present in control conditions. In the case of tubocurarine and hexamethonium, the relaxation after a hyperpolarizing voltage jump corresponds to a decrease in conductance. In the case of decamethonium, the slow relaxation is in the opposite direction. The slow relaxations seen with tubocurarine and hexamethonium are speeded by an increase of the antagonist concentration; the slow relaxations seen with decamethonium are slowed by an increase of the decamethonium concentration. The steady-state observations and the relaxations can be interpreted in terms of a scheme in which tubocurarine, hexamethonium and decamethonium act by blocking the channels opened by the cholinergic agonists. The 2 types of slow relaxation are those predicted if tubocurarine and hexamethonium dissociate slowly from the channel and decamethonium dissociates rapidly. An additional effect of tubocurarine is described, which consists of the rising phase of the response to an ionophoretic pulse. Possible mechanisms of the effect are discussed.