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

Abstract

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.

Molecular Cloning of a Third Member of the Potassium-dependent Sodium-Calcium Exchanger Gene Family,NCKX3

“…That NCKX3 can be measured electrically, with charge moving in the same direction as Na ϩ , also places limits on the ionic stoichiometry of transport. Combined with the high degree of amino acid sequence similarity to NCKX1, which has been clearly established to transport 4 Na ϩ in exchange for 1 K ϩ and 1 Ca 2ϩ (23,24), it seems likely that NCKX3 has a similar stoichiometry.…”

“…Ca 2ϩ homeostasis must still be maintained under these conditions of membrane depolarization and reduced sodium gradient, and NCKX1 is the principal means by which Ca 2ϩ is extruded from rod outer segments. Such a function could not be achieved by a molecule operating with the 3:1 Na ϩ :Ca 2ϩ stoichiometry of NCX1, and indeed it has been demonstrated that NCKX1 couples the entry of four Na ϩ ions in exchange for the exit of one Ca 2ϩ and one K ϩ ion (23,24).…”

Why photoreceptors die (and why they don't)

“…23) that Ca 2þ enters the photoreceptor outer segment in darkness; it is then removed by a specialized transport protein that normally exchanges one Ca 2þ and one K þ on the cytosolic surface with four Na þ on the extracellular surface. (24) Light decreases the influx of Ca 2þ by closing the cGMP-gated channels, and continued extrusion by the Na þ / Ca 2þ -K þ transporter causes a decrease in the Ca 2þ concentration. This decrease plays an important role in light adaptation (see Ref.…”

“…Because this molecule exchanges four Na þ for one Ca 2þ and uses the free energy of both the Na þ and K þ gradients, it can in theory reduce the free Ca 2þ in the outer segment to below 1 nM. (24) It is well known, however (see Ref. 23), that the Ca 2þ concentration never reaches this theoretical limit.…”

K+-Dependent Na+/Ca2+Exchangers: Key Contributors to Ca2+Signaling

“…Early studies on isolated bovine rod outer segments were the first to functionally identify Na + /Ca 2+ exchange activity that was dependent on K + ions that were co-transported with Ca 2+ with an apparent stoichiometry of 4 Na + : 1 Ca 2+ + 1 K + (9,54). These functional properties were subsequently confirmed for recombinant NCKX1 and 2 using either 45 Ca 2+ uptake into insect cells or electrophysiological recordings in HEK293 cells (11,55,57).…”