Calcium-activated conductance in skate electroreceptors: voltage clamp experiments.

Abstract

Voltage clamp experiments allowed further characterization of the Ca2+-dependent repolarizing process in skate [Raja oscellata and R. erinacea] electroreceptor epithelium. Four current components were described; a prolonged capacity current, a leakage current, an early active current which flows inward across the lumenal membranes of the receptor cells and a late current which flowed outward. The leakage and capacity currents were linear and may be subtracted from the total current, giving net active currents. The early active current was carried by Ca2+ and did not undergo inactivation for at least several seconds. When large stimuli exceeded the reversal potential for the early Ca2+ current, the late current was suppressed. Reduction of the ionized Ca2+ concentration in the lumen lowered the reversal potential for the early current and the suppression potential for the late current by the same amount. The late current is apparently initiated by a Ca2+ influx into the cytoplasm. During pulses of moderate duration, activation of the late current did not begin until a fixed amount of Ca2+ entered the receptor cells. The required amount of Ca2+ was reduced if a recent Ca2+ influx had occurred. The Ca2+ activated outward current may be mediated by a distinct macromolecule that is insensitive to voltage. Such macromolecules are likely to have an important role in the regulation of electrical activity in excitable cells.