Analysis of sodium and potassium conductances in the procaine end‐plate potential

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Abstract
1. The effect of procaine on neuromuscular transmission was studied on the sartorius muscle of the frog, Rana pipiens, with intracellular microelectrodes.2. Mean quantal content of the e.p.p. in high Mg Ringer was little affected by procaine.3. The short circuit effect of the e.p.p. on the action potential, which was an indication of change in potassium conductance (ΔgK) during the e.p.p., was investigated on the muscle fibre immobilized in hypertonic Ringer. It was found that procaine had little effect on ΔgK.4. The equilibrium potential of the e.p.p. (Ee.p.p.) was measured in hypertonic Ringer. Ee.p.p. was 12 mV in control experiment and was decreased to 3·1 mV in 10−4 procaine. The ratio ΔgNa/ΔgK was calculated to be 2·75 in control and 2·11 in 10−4 procaine.5. E.p.p.s were recorded extracellularly to study changes in the time course of ΔgNa in procaine. Procaine reduced the peak magnitude of ΔgNa; following the peak, ΔgNa fell quickly to a lower level, which decayed much more slowly.6. An end‐plate model with separate sodium and potassium pathways is proposed to explain the procaine e.p.p. Procaine is assumed to affect the time course of ΔgNa.7. E.p.p.s simulated from the proposed end‐plate model with theoretical ΔgNa and ΔgK agreed satisfactorily with actual procaine e.p.p.