Na-Ca or Na-Ca-K exchange in rod photoreceptors.

Remarkably dynamic Ca z ÷ fluxes control Ca 2 ÷ homeostasis in the outer segments of vertebrate rod photoreceptors (ROS). Ca 2 ÷ fluxes are believed to be carried by only two different membrane proteins embedded in the ROS plasma membrane; these two proteins as well as some of the other main molecular components of ROS are schematically depicted in Fig. 1. ROS are the sites of the process of visual transduction; both visual excitation to light stimuli and adaptation to ambient background light originate in this organelle and are mediated by modulating a current that flows through the outer segment plasma membrane in darkness and is carried by Na ÷ and Ca 2 ÷ ions. Absorption of a photon by the visual pigment rhodopsin initiates the c G M P cascade reducing the aqueous concentration of the excitatory messenger c G M P and reducing the influx o fNa ÷ and Ca 2 ÷ via the cGMP-gated channel; for recent reviews on the c G M P cascade see Stryer (1986) and Chabre and Deterre (1989), for a review on the cGMP-dependent channel see Yau and Baylor (1989). Light causes a decrease