Effects of internal calcium and sodium on photocurrent kinetics in toad rod photoreceptors.

Reducing external calcium from 1.0 to 0.1 mM increased the flash sensitivity and the time-to-peak (tp) of linear responses of rod photoreceptors. The quantitative relation between flash sensitivity and tp was the same as during light adaptation. However, the calcium ionophore A23187 reduced sensitivity without affecting response kinetics. Thus, increasing intracellular calcium did not mimic the effect of steady light on kinetics. An increase in internal sodium might play some role in the increase in tp observed in low external calcium.