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

We investigated the cytosolic free calcium concentration ([Ca2+]i) of leech Retzius neurons in situ while varying the extracellular Ca2+ concentration via the bathing solution ([Ca2+]B). Changing [Ca2+]B had only an effect 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 at higher [Ca2+]B. The changes in the relationship between [Ca2+]i and membrane potential upon varying [Ca2+]B could be reversed by changing extracellular pH. We conclude that [Ca2+]B affects [Ca2+]i by modulating Ca2+ influx through voltage-dependent Ca2+ channels via the electrochemical Ca2+ gradient and the surface potential at the extracellular side of the plasma membrane. These two parameters are affected in a counteracting way: Raising 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, and vice versa.