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

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Abstract
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/ min. from muscle into rinsing solution) decreased slightly with time. On the basis of the temperature-sensitivity of Ca efflux from auricles the activation energy was calculated to have a value of 5900 cal/mole, corresponding to a Q10 of 1.35. 2,4-Dinitrophenol (5. 5x10-5-5. 5xlO-4M) had either no effect on Ca efflux or increased it slightly. Compared to control efflux in 1. 8mM [Ca] 0 [external Ca concentration] 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 depended to a large extent on the ratio [Ca2+]0/[Na+]2. The affinity for Na of the activation site for Ca efflux (carrier) is much less than for Ca. 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. 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. The results are interpreted in terms of a modified exchange diffusion mechanism (Ussing, 1947) which is responsible for Ca extrusion from mammalian cardiac muscle.