Calcium-Dependent Action Potentials in Giant Salivary Gland Cells of the LeechHaementeria Ghilianii: Calcium Removal Induces Dependence on Sodium

Abstract

ABSTRACTAn electrophysiological study was made of the giant, non-coupled salivary gland cells of the leech Haementeria ghilianii (de Filippi, 1849). Resting membrane potential (−40 mV to −80 mV) was primarily dependent on K+, with a small contribution from a Na+ conductance and an electrogenic Na+ pump. Resting Cl− permeability was low. The cells generated overshooting action potentials (70-110 mV, 100-400 ms) which appeared to be mediated exclusively by Ca2+ because they were unaffected by removal of external Na+ and were blocked by 5 mmol 1−1 Co2+. Removal of external Ca2+ and addition of 1 mmol 1−1 EGTA produced spontaneous action potentials of reduced amplitude (peaking at about OmV) and greatly increased duration [typically tens of seconds but sometimes resulting in sustained depolarizations (plateau potentials) extending up to 30min or more]. Action potential amplitude was then dependent on external Na+ concentration, and action potentials were abolished by removal of Na+. The responses were blocked by 5 mmol 1−1 Co2+, indicating that they were produced by Na+ flowing through Ca2+ channels. Addition of micromolar concentrations of Ca2+ to Ca2+-free saline de-creased spike duration and amplitude, suggesting a competition between Na+ and Ca2+. An electrogenic Na+ pump was activated by removal of Ca2+, presumably as a result of the influx of Na+ during spiking; this produced large increases in membrane potential which occurred spontaneously or when Ca2+ was reintro-duced. In normal saline, spike overshoot and duration were increased when the temperature was lowered by 10°C, whereas in Ca2+-free solution, they were reduced by this change. This suggests that the Ca2+ channel may be differentially affected by cooling, depending on the presence or absence of Ca2+