Calcium efflux from barnacle muscle fibers. Dependence on external cations

Calcium-45 was injected into single giant barnacle muscle fibers, and the rate of efflux was measured under a variety of conditions. The rate constant (k) for (45)Ca efflux into standard seawater averaged 17 x 10(-4) min(-1) which corresponds to an efflux of about 1-2 pmol/cm(2).s. Removal of external Ca (Ca(o)) reduced the efflux by 50%. In most fibers about 40% of the (45)Ca efflux into Ca-free seawater was dependent on external Na (Na(o)); treatment with 3.5 mM caffeine increased the magnitude of the Na(o)-dependent efflux. In a few fibers removal of Na(o), in the absence of Ca(o), either had no effect or increased k; caffeine (2-3.5 mM) unmasked an Na(o)-dependent efflux in these fibers. The Na(o)-dependent Ca efflux had a Q(10) of about 3.7. The data are consistent with the idea that a large fraction of the Ca efflux may be carrier-mediated, and may involve both Ca-Ca and Na-Ca counterflow. The relation between the Na(o)-dependent Ca efflux and the external Na concentration is sigmoid, and suggests that two, or more likely three, external Na(+) ions may activate the efflux of one Ca(+2). With a three-for-one Na-Ca exchange, the Na electrochemical gradient may be able to supply sufficient energy to maintain the Ca gradient in these fibers. Other, more complex models are not excluded, however, and may be required to explain some puzzling features of the Ca efflux such as the variable Na(o)-dependence.