Effects of ATP and vanadate on calcium efflux from barnacle muscle fibres

Abstract

Calcium ions carry the inward current during depolarization of barnacle muscle fibres and are involved in the contraction process¹. Intracellular ionized calcium ([Ca²⁺]_i) in barnacle muscle, as in other cells, is kept at a very low concentration, against a large electrochemical gradient^2,3. This large gradient is maintained by Ca²⁺ extrusion mechanisms. When [Ca²⁺]_i is below the contraction threshold⁴, Ca²⁺ efflux from giant barnacle muscle fibres is, largely, both ATP dependent and external Na⁺(Na₀⁺) dependent (see also refs 5,6). When [Ca²⁺]_i is raised to the level expected during muscle contraction (2–5 µM)⁷, most of the Ca²⁺ efflux from perfused fibres is Na₀ dependent; as in squid axons⁸, this Na₀⁺-dependent Ca²⁺ efflux is ATP independent. Orthovanadate is an inhibitor of (Na⁺ + K⁺) ATPase⁹ and the red cell Ca²⁺-ATPase¹⁰. We report here that vanadate inhibits ATP-promoted, Na₀⁺-dependent Ca²⁺ efflux from barnacle muscle fibres perfused with low [Ca²⁺]_i (0.2–0.5 µM), but has little effect on the Na₀⁺-dependent, ATP-independent Ca²⁺ efflux from fibres with a high [Ca²⁺]_i (2–5 µM). Nevertheless, ATP depletion or vanadate treatment of high [Ca²⁺]_i fibres causes an approximately 50-fold increase of Ca²⁺ efflux into Ca²⁺-containing lithium seawater. These results demonstrate that both vanadate and ATP affect Ca²⁺ extrusion, including the Na₀⁺-dependent Ca²⁺ efflux (Na-Ca exchange), in barnacle muscle.