The ionic basis of the resting potential of molluscan unstriated muscle

Abstract

The membrane potential (Vm) of unstriated, non-spiking fibres from the buccal retractor muscle of the opisthobranch molluscPhiline aperta is primarily determined by the distribution of the potassium ion across the membrane. In salines where potassium is varied and chloride remains constant or nearly so, the membrane potential varied with log external K+ with a slope of 50.6 (±2.3) mV per decade. In chloride-free salines the slope was 48.5 mV per decade. The experiments were conducted at temperatures of 18–20° C. A ten-fold reduction in external chloride concentration depolarised the fibres by around 10 mV, indicating that chloride permeability makes some contribution to Vm. In salines where [K]0·[Cl]0 is constant the Nernst slope was 55.8 mV per decade compared with the theoretical value of 58 mV. The experimental data suggest that the internal potassium concentration of the fibres is 247±31 mM and pNa/pK is 0.01, giving a predicted value of Vm in sea water of −72 mV. The membrane potential of 90 fibres measured in sea water was −74.2±1.3 mV. The membrane contains an electrogenic sodium pump which contributes 4–5 mV to the membrane potential.