Modulation of Ca2+ influx in leech Retzius neurons II. Effect of extracellular Ca2+

We investigated the cytosolic free calcium concentration ([Ca2+](i)) of lee ch Retzius neurons in situ while varying the extracellular Ca2+ concentrati on via the bathing solution ([Ca2+](B))- Changing [Ca2+](B) had only an eff ect on [Ca2+](i) if the cells were depolarized by raising the extracellular K+ concentration. Surprisingly raising [Ca2+](B) from 2 to 10 mm caused a decrease in [Ca2+](i), and an increase was evoked by reducing [Ca2+](B) to 0.1 mM. These changes were not due to shifts in membrane potential. At low [Ca2+](B) moderate membrane depolarizations were sufficient to evoke a [Ca2 +](i) increase, while progressively larger depolarizations were necessary a t higher [Ca2+](B). The changes in the relationship between [Ca2+](i) and m embrane potential upon varying [Ca2+](B) could be reversed by changing extr acellular pH. We conclude that [Ca2+](B) affects [Ca2+](i) by modulating Ca 2+ influx through voltage-dependent Ca2+ channels via the electrochemical C a2+ gradient and the surface potential at the extracellular side of the pla sma membrane. These two parameters are affected in a counteracting way: Rai sing the extracellular Ca2+ concentration enhances the electrochemical Ca2 gradient and hence Ca2+ influx, but it attenuates Ca2+ channel activity by shifting the extracellular surface potential to the positive direction, an d vice versa.