Previous work has suggested that a Na-Ca exchanger may have a key role in visual transduction in retinal rods1-10. This exchanger is thought to maintain a low internal free Ca2+ concentration in darkness 4-10 and to contribute to the rod's recovery after light by removing any internally released Ca2+ (refs 1, 2, 6-8). Little else is known about this transport mechanism in rods. We describe here an inward membrane current recorded from single isolated rods which appears to be associated with such external Na+-dependent Ca2+ efflux activity. External Na+, but not Li+, could generate this current; high external K+ inhibited it while small amounts of La3+ (10 M) completely abolished it. The exchanger can also transport Sr2+, but not Ba2+ or other divalent cations. The exchange ratio was estimated to be 3Na+:1Ca2+. As well as demonstrating clearly the Na-Ca exchanger in the rod outer segment, our experiments also cast serious doubt on the commonly held view that light simply releases internal Ca2+ to bind to and block the light-sensitive conductance.
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