Membrane potential produced directly by the Na⁺ pump in Aplysia neurons

1. 1. A sudden cooling of Aplysia neurons preconditioned at room temperature results in a reversible depolarization of up to 35 mV which is dependent upon Na⁺ and K⁺ and is abolished by ouabain. 2. 2. In K⁺-free sea water or ouabain a sudden cooling results in a hyperpolarization of up to 20 mV which is dependent upon external Na⁺. 3. 3. The depolarization on cooling in the control presumably results from the temperature dependence of an electrogenic Na⁺ pump which contributes directly to membrane potential. 4. 4. The hyperpolarization on cooling in absence of pump activity reflects a high P_Na+ P_K+ ratio and a higher Q₁₀ for P_Na+ than for P₊.