Nature 337, 740 - 743 (23 February 1989); doi:10.1038/337740a0

Extrusion of calcium from rod outer segments is driven by both sodium and potassium gradients

L. Cervetto^*, L. Lagnado, R. J. Perry, D. W. Robinson & P. A. McNaughton

Physiological Laboratory, Downing Street, Cambridge CB2 3EG, UK
^* Istituto di Neurofisiologia del CNR, via San Zeno 51, 56100 Pisa, Italy

Calcium is transported across the surface membrane of both nerve^{1, 2} and muscle³ 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^4–7, which is generally interpreted to imply an exchange stoichiometry of 3 Na⁺:l 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⁺:l 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⁺¹:1 Ca2+ exchange.

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