Characterization of end-plate conductance in transected frog muscle: modification by drugs

Cutaneous pectoris muscles of Rana pipiens were transected distal to the innervated region. Within 10 min, membrane potentials (Em's) of -33 +/- 2.5 mV and end-plate potentials (3-15 mV) were recorded unaccompanied by muscle action potentials or twitch. The fall in Em was associated with a net loss of [K+]i and a net gain of [Na+]i. Although input resistance fell by 50% and the space constant was slightly reduced in the transected muscle fibers, end-plates could be adequately voltage-clamped with two microelectrodes. End-plate currents (e.p.c.s) with rise times of 350 to 700 musec were recorded as a function of holding potential (Vm). The current-voltage relationship of peak e.p.c.s over the range of -70 to +20 mV was linear and the reversal potential (-6.6 +/- 2.2 mV) was the same as that found for intact muscle fibers. The decay phase of e.p.c.s could be described as a single exponential at all Vm's and had a voltage and temperature dependence similar to that described for e.p.c.s of glycerol-treated muscles. Tubocurarine (0.3 microM) caused a significant decrease in the time constant (tau) of e.p.c. decay and e.p.c. amplitude. The depression of e.p.c. amplitude by tubocurarine was reversed by 4-aminopyridine while the decrease of tau was not. Atropine (10(-4) M) caused a monotonic shortening of e.p.c.s at a Vm of -90 mV but e.p.c.s recorded at +50 mV were biphasic. Lidocaine, a quaternary nitrogen analog of lidocaine (QX314), lobeline and hexafluorenium were studied also in transected muscle and their effects on the parameters of e.p.c. are described. Both lobeline (50 microM) and hexafluorenium caused a decrease of tau and eliminated the voltage dependence of tau at negative Vm's. The transected muscle can be used for the study of conductance kinetics of end-plate and for the study of drug action uncomplicated by the presence of other drugs of Mg++ to eliminate contraction.