Interactions of Physiological Ligands with the Ca Pump and Na / Ca Exchange in Squid Axons REINALDO DIPOLO and LUIS BEAUGE

We have studied the interaction of physiological ligands other than Nai and Ca; with the Ca pump and Na/Ca exchange in internally dialyzed squid axons. The results show the following. (a) Internal Mg` is an inhibitor of the Na,,-dependent Ca efflux . At physiological me, (4 mM), the inhibition amounts to ~50%. The inhibition is partial and noncompetitive with Ca;, and is not affected by Nai or ATP. The ATP-dependent uncoupled efflux is unaffected by Mg; up to 20 mM. Both components of the Ca efflux require me, for their activation by ATP. (b) At constant membrane potential, Ki is an important cofactor for the uncoupled Ca efflux . (c) Orthophosphate (P) activates the Na.-dependent Ca efflux without affecting the uncoupledcomponent. Activation by Pi occurs only in the presence of Mg-ATP or hydrolyzable ATP analogues. P; under physiological conditions has no effect on the uncoupled component; nevertheless, at alkaline pH, it inhibits the Ca pump, probably by product inhibition . (d) ADP is a potent inhibitor of the uncoupled Ca efflux . The Na.-dependent component is inhibited by ADP only at much higher ADP concentrations . These results indicate that (a) depending on the concentration of Ca?', Na± Mg$+, and Pi , the Na/Ca carrier can operate undera lowor highrate regime; (b) the interactions of Me', Pi , Na,:, and ATPwith the carrier are not interdependent; (c) the effect of Pi on the carrier-mediated Ca efflux resembles the stimulation of the Na.-dependent Ca efflux by internal vanadate; (d) the ligand effects on the uncoupled Ca efflux are of the type seen in the Ca pump in red cells and the sarcoplasmic reticulum.