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Calcium is transported across the surface membrane of both nerve and muscle by a Na+-dependent mechanism, usually termed the Na:Ca exchange. It is well established from experiments on rod outer segments that one net positive charge enters the cell for every Ca2+ ion extruded by the exchange, which is generally interpreted to imply an exchange stoichiometry of 3 Na+:1 Ca2+. We have measured the currents associated with the operation of the exchange in both forward and reversed modes in isolated rod outer segments and we find that the reversed mode, in which Ca2+ enters the cell in exchange for Na+, depends strongly on the presence of external K+. The ability of changes in external K+ concentration ([K+]o) to perturb the equilibrium level of [Ca2+]i indicates that K+ is co-transported with calcium. From an examination of the relative changes of [Ca2+]o, [Na+]o, [K+]o and membrane potential required to maintain the exchange at equilibrium, we conclude that the exchange stoichiometry is 4 Na+:1 Ca2+, 1 K+ and we propose that the exchange should be renamed the Na:Ca, K exchange. Harnessing the outward K+ gradient should allow the exchange to maintain a Ca2+ efflux down to levels of internal [Ca2+] that are considerably lower than would be possible with a 3 Na+:1 Ca2+ exchange.

SUMMARY1. The processes regulating intracellular calcium in the outer segments of salamander rods have been investigated. The main preparation used was the isolated rod loaded with the Ca2+-sensitive photoprotein aequorin, from which outer segment membrane current and free [Ca2+]i could be recorded simultaneously. Two other preparations were also used: outer segment membrane current was recorded from intact, isolated rods using a suction pipette, and from detached outer segments using a whole-cell pipette. 11. High-and low-affinity components of internal Ca2+ buffering were also observed in intact rods, but the mean capacity of the high-affinity buffer, 241 4uM, was an order of magnitude larger than in aequorin-loaded rods. The difference is attributed to loss of this component of the buffer during the process of internal perfusion used to load aequorin.12. In detached outer segments internally perfused by a whole-cell pipette the capacity of the high-affinity calcium buffer declined with a time constant of around 15 min. It is concluded that this component of buffering is associated with a large, diffusible molecule.13. During internal perfusion the affinity for Ca2+ of the Na+-Ca2+, K+ exchange declined relative to that of the high-affinity buffer, probably because the affinity of the Na+-Ca2+. K+ exchange is controlled by a diffusible cytoplasmic modulator.

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