INTERACTIONS OF PHYSIOLOGICAL LIGANDS WITH THE CA PUMP AND NA/CA EXCHANGE IN SQUID AXONS

The interaction of physiological ligands other than Nai and Cai with the Ca pump and Na/Ca exchange was studied in internally dialyzed squid axons. Internal Mg2+ is an inhibitor of the Nao-dependent Ca efflux. At physiological Mgi2+ (4 mM), the inhibition amounts to .apprx. 50%. The inhibition is partial and noncompetitive with Cai, and is not affected by Nai or ATP. The ATP-dependent uncoupled efflux is unaffected by Mgi up to 20 mM. Both components of the Ca efflux require Mgi2+ for their activation by ATP. At constant membrane potential, Ki is an important cofactor for the uncoupled Ca efflux. Orthophosphate (Pi) activates the Nao-dependent Ca efflux without affecting the uncoupled component. Activation by Pi occurs only in the presence of Mg-ATP or hydrolyzable ATP analogs. Pi under physiological conditions has no effect on the uncoupled component; nevertheless, at alkaline pH, it inhibits the Ca pump, probably by production inhibition. ADP is a potent inhibitor of the uncoupled Ca efflux. The Nao-dependent component is inhibited by ADP only at much higher ADP concentrations. These results indicate that depending on the concentration of Cai2+, Nai+ and Pi, the Na/Ca carrier can operate under a low- or high-rate regime; the interactions of Mgi2+, Pi, Nai+ and ATP with the carrier are not interdependent; the effect of Pi on the carrier-mediated Ca efflux resembles the stimulation of the Nao-dependent Ca efflux by internal vanadate; the ligand effects on the uncoupled Ca efflux are of the type seen in the Ca pump in red cells and the sarcoplasmic reticulum.