The stoichiometry of Na-Ca+K exchange in rod outer segments isolated from bovine retinas

Ca2+ extrusion in the outer segments of retinal rods (ROS) is mediated by a protein that couples both the inward Na+ gradient and the outward K+ gradient to Ca2+ extrusion. Na(+)-stimulated Ca2+ release from ROS requires internal K+ and is accompanied by release of internal K+, whereas a slow component of Na(+)-stimulated Ca2+ release does not require K+. In this paper we discuss our observations on the K+ transport via Na-Ca+ K exchange in bovine ROS, on the electrogenicity and stoichiometry of the ROS Na-Ca+ K exchanger, and on the mechanism on coupling Ca2+ to K+ via this protein. Finally, we discuss briefly the physiological implications of Na-Ca+ K exchange.