External and internal effects of tetraethylammonium on voltage-dependent and Ca-dependent K+ currents components in molluscan pacemaker neurons.

Ca-dependent and voltage-dependent K+ currents in Aplysia pacemaker neurons were studied separately by electrophoretic injections of Ca2+ ions into the cells to induce the Ca-dependent component and by step depolarizations in the absence of external Ca2+ ions to induce the voltage-dependent component. Intracellular tetraethylammonium (TEA) blocks the voltage-dependent but not the Ca-dependent K+ current. Extracellular TEA blocks both components reversibly but is more effective in inhibiting the Ca-dependent K+ currents. Our results suggest that there are TEA receptor sites for the voltage-dependent K+ current on each side of the membrane, but only an external receptor site for the Ca-dependent K+ current.