A Contribution of an Electrogenic Na⁺ Pump to Membrane Potential in Aplysia Neurons

The resting membrane potential (RMP) of Aplysia neurons is very temperature-dependent, and in some cells increases with increasing temperature by as much as 2 mv/°C. RMP at room temperature may significantly exceed the potassium equilibrium potential, which can be determined by measurement of the equilibrium point of the spike after potential. The hyperpolarization on warming is completely abolished by ouabain, replacement of external Na⁺ by Li⁺, removal of external K⁺, and by prolonged exposure to high Ca⁺⁺, while it is independent of external chloride but is increased by cocaine (3 x 10^-3 M). In an identified cell that shows a marked temperature dependence of RMP, both the potassium equilibrium potential and the membrane resistance were found to be relatively independent of temperature. The hyperpolarization on warming, which may increase RMP by as much as 50%, can most reasonably be ascribed to the activity of an electrogenic Na⁺ pump.