Voltage-dependent Ca2+ influx into identified leech neurones

We determined the relationships between the intracellular free Ca2+ concentration ([Ca2+](i)) and the membrane potential (E(m)) of six different neurones in the leech central nervous system: Retzius, 50 (Leydig), AP, AE, P, and N neurones. The [Ca2+](i) was monitored by using iontophoretically injected fura-2. The membrane depolarization evoked by raising the extracellular K+ concentration ([K+](o)) up to 89 mM caused a persistent increase in [Ca2+](i), which was abolished in Ca2+-free solution indicating that it was due to Ca2+ influx. The threshold membrane potential that must be reached in the different types of neurones to induce a [Ca2+](i) increase ranged between -40 and -25 mV. The different threshold potentials as well as differences in the relationships between [Ca2+](i) and E(m) were partly due to the cell-specific generation of action potentials. In Na+-free solution, the action potentials were suppressed and the [Ca2+](i)/E(m) relationships were similar. The K+-induced [Ca2+](i) increase was inhibited by the polyvalent cations Co2+, Ni2+, Mn2+, Cd2+ and La3+, as well as by the cyclic alcohol menthol. Neither the polyvalent cations nor menthol had a significant effect on the K+-induced membrane depolarization. Our results suggest that different leech neurones possess voltage-dependent Ca2+ channels with similar properties.