The influence of calcium on sodium efflux in squid axons

Abstract

SUMMARY1. Previous work has.shown that the sodium efflux from the axons of Loligo forbesi increa 2. The increase in efflux in lithium was unaffected by ouabain but was abolished by removal of external calcium; in these respects it differed from the potassium-dependent sodium efflux which was abolished by ouabain but not reduced by removal of external calcium.3. Strontium but not magnesium could replace calcium in activating the ouabain-insensitive sodium efflux; lanthanum had an inhibitory effect.4. Replacing all the holine chloride or dextrose gave a rise in Na efflux which was abolishey ouabain but not by removal of external calcium.5. The rise in Na efflux resulting from partial replacement of NaCl by dextrose or choline chloride consisted of two components one of which was ouabain-insensitive and calcium-dependent and the other was inhibited by ouabain but calcium-insensitive.6. The ouabain-insensitive component of the Na efflux was activated by low concentrations of Na, Li or K but inhibited by high concentrations of Na and to a lesser extent Li. The inhibiting effect of high Na was of the kind expected if these ions displace calcium from an external site.7. The ouabain-insensitive component of the Na efflux was abolished by cyanide, had a Qlo of 2-7; and was roughly proportional to [Na]?. It was much more variable in magnitude than the ouabain-sensitive, potassiumdependent component of the sodium efflux. 8. The calcium influx increased five to fortyfold when external NaCl

The receptor function of the Na+, K+-activated adenosine triphosphatase system

“…But what mechanism links the sodium pump to calcium fluxes? Reuter & Seitz (1968) discovered that the heart muscle contains a calcium-sodium exchange system, which was also found in other muscles (Blaustein, 1976) and in nerve (Baker et al, 1969). This system reversibly exchanges Na+ and Ca2+ ions across the membrane at a 2-4 Na+: 1 Ca+ stoichiometry (Mullins, 1979).…”

Alterations in [Ca2+]i mediated by sodium‐calcium exchange in smooth muscle cells isolated from the guinea‐pig ureter.

“…With regard to P. I. AARONSON AND C. D. BENHAM Ca2+ extrusion, however, similar (although wide) ranges of temperature dependence have been reported for the Na+-Ca2+ exchanger, and for the ATP-driven Ca2+ pump (e.g. Baker et al 1969;Larsen, Hinds & Vincenzi, 1978;Kimura et al 1987), so that the relative contributions of these two processes may not have been markedly perturbed. In spite of these potential drawbacks, however, the technique has proved useful for characterizing the properties of Na+{Ca2+ exchange in cardiac cells both when [Ca2+], was buffered (Kimura et al 1987) and when it was allowed to vary (Barcenas-Ruiz et al 1988).…”

“…Sodium-calcium (Na+-Ca2+) exchange is a process by which the electrochemical Na+ and Ca2+ gradients across cellular membranes are coupled, and thus interact (Reuter & Seitz, 1968;Baker, Blaustein, Hodgkin & Steinhardt, 1969). Although the resulting equilibrium thermodynamic relationship between these gradients has long been appreciated , the physiological significance of the exchange process in many of the various types of cell where it has been observed has remained unclear (van Breemen, Aaronson & Loutzenhiser, 1979;Eisner & Lederer, 1985).…”

Effects of Na removal and readmission on the mechanical response in the guinea-pig tracheal smooth muscle.

“…One of the mechanisms which affects the tension development when the Na concentration is modified may be a Na-Ca exchange process, originally proposed for the Ca flux in cardiac muscle (REUTER and SEITZ, 1968) and squid axon (BAKER et al, 1969). In this mechanism, the transport of Ca is coupled with that of Na in an opposite direction, and it is driven by a transmembrane gradient of Na concentration.…”