Cation Selectivity of the cGMP-Gated Channel of Mammalian Rod Photoreceptors

Abstract

Biochemical and electrophysiological attempts to elucidate the mechanism of phototransduction in vertebrate rod cells have identified a cation permeant channel in the plasma membrane of rod outer segments being responsible for the inward current in the dark which is directly and reversibly activated by cGMP. The light-induced rapid decrease in inward current and concomitant hyperpolarisation are caused by the closure of these channels due to hydrolysis of the agonist cGMP and subsequent desorption from the channel protein (for reviews see Stryer, 1986; Kaupp and Koch, 1986; Lamb, 1986; Pugh, 1987). To date, most biochemical investigations of the cGMP-gated channel were carried out on bovine rod outer segments, whereas electrical characteristics of the channel were almost exclusively determined on amphibian rod cells (Yau and Baylor, 1989). To fill this gap, we examined the electrical properties of the cGMP-gated channel of bovine rod photoreceptors by employing the patch clamp method. The selectivity of the cation channel for monovalent ions was obtained by exposing excised patches to solutions containing different cation species and determining reversal voltages (V _rev ) and macroscopic conductances (g). The dependence of macroscopic current upon cGMP concentration yielded EC₅₀ values.