The ouabain-induced [Ca2+] increase in leech Retzius neurones is mediated by voltage-dependent Ca2+ 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. (C) 2001 Elsevier Science B.V. All rights reserved.