The ouabain-induced [Ca]i increase in leech Retzius neurones is mediated by voltage-dependent Ca channels

In leech Retzius neurones the inhibition of the Na⁺–K⁺ pump by ouabain causes an increase in the cytosolic free calcium concentration (Ca²⁺]_i). To elucidate the mechanism of this increase we investigated the changes in Ca²⁺]_i (measured by Fura-2) and in membrane potential that were induced by inhibiting the Na⁺–K⁺ pump in bathing solutions of different ionic composition. The results show that Na⁺–K⁺ pump inhibition induced a Ca²⁺]_i increase only if the cells depolarized sufficiently in the presence of extracellular Ca²⁺. Specifically, the relationship between Ca²⁺]_i and the membrane potential upon Na⁺–K⁺ pump inhibition closely matched the corresponding relationship upon activation of the voltage-dependent Ca²⁺ channels by raising the extracellular K⁺ concentration. It is concluded that the Ca²⁺]_i increase caused by inhibiting the Na⁺–K⁺ pump in leech Retzius neurones is exclusively due to Ca²⁺ influx through voltage-dependent Ca²⁺ channels.