Sodium‐calcium exchange in the outer segments of bovine rod photoreceptors.

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Abstract
Intact rod outer segments (r.o.s.) isolated from bovine retinas were used to measure net Ca2+ fluxes using the optical Ca2+ indicator Arsenazo III. Ca2+ fluxes were observed, which could change the internal Ca2+ content of isolated r.o.s. by as much as 0.5 mM s‐1. The Ca2+ content of isolated intact r.o.s. was strongly dependent on the Na/Ca ratio in the isolation medium, and could be made less than 0.1 mol Ca2+ mol‐1 rhodopsin (zero Ca2+ in isolation medium) or up to 7 mol Ca2+ mol‐1 rhodopsin (zero Na+ in isolation medium). Ca2+ efflux from r.o.s. rich in Ca2+ was observed only when Na+ was added to the external medium (as opposed to any other alkali cation); in Ca2+‐depleted r.o.s. Ca2+ uptake required the presence of internal Na+ and was inhibited selectively by external Na+. These results suggest that Na‐Ca exchange across the plasma membrane operated freely in both directions and controlled the internal Ca2+ concentration in r.o.s. Na+‐stimulated Ca2+ efflux depended on the external Na+ concentration in a sigmoidal way. This suggests that the simultaneous binding of two Na ions is rate limiting for transport. In Ca2+‐depleted r.o.s. and in the absence of external Na+, 1 mol Ca2+ mol‐1 rhodopsin (or 3 mM‐total Ca2+) could be taken up within 1 min by intact r.o.s. at a free external Ca2+ concentration of about 1 microM. Only part of the internal Ca2+ was available for Na‐Ca exchange. The external Na+ and K+ concentration as well as the temperature were factors controlling the accessibility of internal Ca2+ to participate in Na‐Ca exchange. Ca2+ fluxes in r.o.s. with a permeabilized plasma membrane but intact disk membranes were very similar to those observed in intact r.o.s.; Na‐Ca exchange could operate in both directions across the disk membrane. In addition to Na‐Ca exchange, leaky r.o.s. also showed a guanosine 3', 5'‐cyclic monophosphate (cyclic GMP)‐induced Ca2+ release that was about 1/20 of the rate of Na‐Ca exchange. Na‐Ca exchange could release 1.5 mol Ca2+ mol‐1 rhodopsin from disks as compared with a cyclic‐GMP‐induced release of 0.15 mol Ca2+ mol‐1 rhodopsin.

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