Block of-quantal end-plate currents of mouse muscle by physostigmine and procaine

Quantal endplate currents (qEPCs) were recorded from hemidiaphragms of mice by means of a macro-patch-clamp electrode. Excitation was blocked with tetrodotoxin, and quantal release was elicited by depolarizing pulses through the electrode. Physostigmine (Phys) or procaine (Proc) was applied to the recording site by perfusion of the electrode tip. Low concentrations of Phys increased the amplitude and prolonged the decay time constants of qEPCs from similar to 3 to similar to 10 ms, due to block of acetylcholine-esterase. With 20 mu M to 2 mM Phys or Proc, the decay of qEPCs became biphasic, an initial short time constant tau(s) decreasing to <1 ms with 1 mM Phys and to similar to 0.3 ms with 1 mM Proc. The long second time constant of the decay, tau(1), reached values of less than or equal to 100 ms with these blocker concentrations. The blocking effects of Phys and Proc on the qEPC are due to binding to the open channel conformation. A method is described to extract the rate constants of binding (b(p)) from the sums 1/tau(s) + 1/tau(1), and the rates of unbinding (b(-p)) from tau(0).tau(s)(-1).tau(1)(-1) (tau(0) is the decay time constant of the control EPC). For Phys and Proc b(p) of 1.3 and 5.10(6) M-1 s(-1) and b(-p) of 176 and 350 s(-1), respectively, were found. Using these rate constants and a reaction scheme for the nicotinic receptor together with the respective rate constants determined before, we could model the experimental results satisfactorily.