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. Received: 23 January 2001/Revised: 23 June 2001