Abstract

The effects of benzoquinonium (Bz) at the skeletal neuromuscular junction have been studied using intracellular microelectrodes in the isolated tenuissimus muscle of the cat. End-plate potentials (e.p.p.s) recorded in a muscle fiber in which transmission was blocked by Bz had a longer time course than e.p.p.s of a similar amplitude recorded in muscles blocked by tubocurarine (Tc). The facilitatory drugs neostigmine, edrophonium, ambenonium and methoxyambenonium had no effect on the e.p.p. in muscles blocked by Bz. When the Bz was removed and replaced by Tc, the effects of the drugs were diminished when compared to the effects on a control e.p.p. recorded in a muscle blocked by Tc. Bz, up to a concentration of 10^-7 M, had no effect on miniature end-plate potential (m.e.p.p.) amplitude and time course. At this concentration, the amplitude was decreased without any change in the end-plate membrane resting potential. In a muscle blocked by Tc, a weak facilitatory action was observed when Bz was added at concentrations of 10^-7 M and 10^-6 M. Bz possibly decreased the quantal content of the e.p.p. at neuromuscular blocking doses. It is concluded that Bz has both pre- and postjunctional actions.