Vanadate inhibits uncoupled Ca efflux but not Na--Ca exchange in squid axons

Nerve cells can maintain a very low intracellular calcium concentration ([Ca2+]i) against large Ca2+ electrochemical gradients (see ref. 1 for review). The properties of the calcium efflux from these cells depend on [Ca2+]i (ref. 2), and within the physiological range, most Ca efflux depends on ATP (which stimulates with high affinity) and is insensitive to Na1, Na0 and Ca0 (uncoupled Ca efflux). When the [Ca2+]i is well above the physiological range, Ca efflux becomes only partially dependent on ATP (acting now with low affinity), is inhibited by Nai and is stimulated by Na0 and Ca0 (Na--Ca exchange). Orthovanadate, a powerful inhibitor of the (Na+ + K+)ATPase and the Na pump, also inhibits the Ca-stimulated ATPase activity, which is the enzymatic basis for the uncoupled Ca pump, in human red cells. The experiments reported here show that in squid axons the ATP-dependent uncoupled Ca efflux can be fully and reversibly inhibited by vanadate, whereas concentrations of vanadate 10 times higher have no effect on the Na--Ca exchange. This is another indication that the uncoupled Ca efflux represents an ATP-driven Ca pump, and supports the suggestion that the uncoupled Ca efflux and Na--Ca exchange are mediated by different mechanisms.