Light- and voltage-dependent increases of calcium ion concentration in molluscan photoreceptors

Changes in cytoplasmic calcium levels have been measured in photoreceptors from Hermissenda crassicornis using the indicator dye Arsenazo III. Following stimulation with light, the dye absorbance, measured at the Ca-sensitive wavelength pair 660-690 nm increased and remained elevated for 10s of seconds. The absorbance change began during the initial part of the photodepolarization, but the early time course was obscured by the stimulus flash. Absorbance at a second wavelength pair, 630-690 nm, changed only a small amount during the photoresponse. This wavelength dependence of the dye absorbance was the same as that observed for direct Ca injections into larger Hermissenda neurons. Pairing the light stimulus with depolarizing current (0.5 nA) increased the dye absorbance, while sufficient hyperpolarizing current (0.5-1 nA) markedly reduced the dye response to a paired light flash. Depolarizing current alone gave a small, slowly rising dye absorbance change. Light- and current-induced dye absorbance change was greatly reduced by external Cd. We conclude that light stimulus causes an increase in cytoplasmic Ca, which is accentuated by extrinsic depolarizing input, and that most of the increase results from transmembrane influx.