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The voltage-gated Ca2+ current in cone photoreceptors operates over only a small part of the physiological voltage range produced by light and, consequently, appears insufficient for controlling transmitter release. We have used a whole-cell voltage clamp to measure membrane current and the capacitance change produced by exocytosis in solitary cone and rod photoreceptors isolated from the salamander retina. In both types of photorecep tor, Ca*+ influx through voltagegated Ca*+ channels initiated exocytosis. In addition, Caz+ influx through a cCMP-gated channel in the inner segment and synaptic processes of cones also initiated exocytosis. The cCMP-gated current sustained exocytosis over the entire physiological voltage range. tntroduction

Rod and cone photoreceptors have different functional tasks. Rods are optimized for detecting the absorption of single photons. Cones detect high rates of photon …