The complex of cGMP-gated channel and Na+/Ca2+, K+ exchanger in rod photoreceptors

Ca2+ is an important signal ion in photoreceptors for recovery after excitation and light adaptation. It enters the outer segment as a minor fraction of the dark current through cGMP-gated channels and is extruded in the same cell compartment by Na+/Ca2+, K+ exchange. Channel and exchanger are located exclusively in the plasma membrane, but not in the cytoplasmic membrane stack, the discs, which contain the visual pigment rhodopsin. The channel consists presumably of two alpha-subunits and two beta-subunits, whereas the exchanger is a monomeric protein. Recently, considerable evidence has been accumulated indicating that both proteins form a complex which is bound to peripherin/rds, an integral protein of the disc rim. This review focuses on the complex of cGMP-gated channel and Na+/Ca2+, K+ exchanger. The possibility of direct functional interaction between channel and exchanger is discussed. Furthermore, the consequences of different subunit arrangements of the channel for the channel-exchanger complex are considered. Finally, a Ca2+ diffusion model is presented which examines the possibility that Ca2+ currents are locally restricted to the close vicinity of the channel.