Possible role of sodium-calcium pumps in tension development of vascular smooth muscle.

Observations were made on the tension development by helical strips of vascular smooth muscle from rabbit aorta and mesenteric artery. Based on the possibility that the pump which moves calcium into the cell coupled with sodium efflux, recently described for squid axon (l), exists also in vascular smooth muscle, studies were designed to evaluate changes in its activity in this muscle. Sodium competes with calcium for the carrier at the cell surface, hence the pump moves calcium into the cell more rapidly in a sodium-free medium. The strips developed more tension in a sodium-free solution, as if more ionized calcium were being delivered to the cell. Ouabain is known to block the sodium-potassium pump, thereby permitting an increase in sodium concentration inside the cell; this in turn will aceclerate the sodium-calcium pump. Ouabain potentiated the contractile responses of these muscles, suggesting that a higher concentration of ionized calcium is reaching the contractile protein.