Na+ - and Ca2+ -dependent components in action potentials of the ovulation hormone producing caudo-dorsal cells in Lymnaea stagnalis (Gastropoda)

Action potentials in the afterdischarge of the ovulation hormone producing caudo–dorsal cells (CDC) of Lymnaea stagnalis are strikingly different from electrically evoked spikes in the silent resting and inhibited states of these cells. Spikes evoked in the silent states consist of one fast peak (80–100 mV; 10–15 ms). The overshoot is Na⁺ - and Ca²⁺ -dependent. Spikes are blocked in Na⁺ -free saline and by TTX. Repolarization is retarded by TEA. Co²⁺ increases the overshoot. Active state action potentials (60–80 mV) last up to 125 ms, due to activation of a slow component following the TTX-sensitive spike. The slow component is Na⁺ - and Ca²⁺ -dependent. In normal saline it is blocked by Co²⁺ and La³⁺. In Ca²⁺ -free saline the remaining part of the slow component is blocked by La³⁺ only. The slow component is voltage-dependent in a graded fashion. Activation is bound to the active state in which the CDC are depolarized by 20 mV. TEA and Ca²⁺ -free saline greatly increase spike duration in the active state. This suggests that, in addition to the classical TEA-sensitive channel, a Ca²⁺ -dependent K⁺ channel is involved in repolarization of active state action potentials. The underlying membrane properties and the functional significance are discussed in relation to the pacemaking mechanism of the CDC.