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Abstract

Vertebrate rod photoreceptors hyperpolarize in response to light by means of the closure of cGMP-gated channels, present in the plasma membrane of the outer segment, which mediate an inwardly directed cation flow referred to as the dark current. Although these channels are most permeable to monovalent cations, they also exhibit an appreciable permeability to the divalent cations Ca²⁺ and Mg²⁺. Indeed, calcium ions entering the cell via the cGMP-gated channel constitute 10-15% of the dark current. The sodium-calcium exchanger of rod photoreceptors is responsible for the extrusion of calcium ions entering the outer segment cytosol through the cGMP-gated channel under conditions of darkness. The driving force for this calcium extrusion process is an inwardly directed electrochemical Na⁺ gradient together with an out wardly directed electrochemical K⁺ gradient. The stoichiometry of the exchange process under physiological conditions is now accepted to be 4 Na_o⁺: 1 Ca_i²⁺ + 1 K_i and is therefore electrogenic.