The dependence of calcium efflux from cardiac muscle on temperature and external ion composition

Abstract

SUMMARY1. Exchangeable Ca in guinea-pig auricles and ventricular trabeculae of sheep and calf hearts was labelled with 45Ca and the loss of radioactivity into inactive rinsing solutions of different ion composition was measured for periods up to 6 hr. At no time did the decrease of radioactivity in the muscle follow a single exponential course, while the rate coefficient k (= fraction of 45Ca lost per minute from muscle into rinsing solution) decreased slightly with time.2. On the basis of the temperature-sensitivity of Ca efflux from auricles the activation energy has been calculated to have a value of 5900 cal/mole, corresponding to a Qlo of 135. 2,4-Dinitrophenol (5.5 x 10-5-55 x 104 M) had either no effect on Ca efflux or increased it slightly.3. Compared to control efflux in 1-8 mm [Ca]. Ca efflux decreased to 70 % in Ca-free solution, to 20 % in Ca-free, Na-free solution and to 65 % in Ca-containing, Na-free solution, NaCl being replaced by either sucrose or LiCl. Quantitatively, Ca efflux from auricles has been shown to depend to a large extent on the ratio [Ca2+]0/[Na+]'. The affinity for Na of the activation site for Ca efflux (carrier) is much less than for Ca.4. The efflux from muscles soaked for about 2 hr in Ca-free solution was not linearly related to Ca-concentration in the tissue but followed a square law.5. While Ca content in auricles increased in Ca-containing, Na-poor solution it decreased again when Tyrode solution was readmitted indicating a Na-sensitive Ca net transport in cardiac muscle.6. The results are interpreted in terms of a modified exchange diffusion mechanism (Ussing, 1947) which is responsible for Ca extrusion from mammalian cardiac muscle.

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.

“…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.…”