Interactions of physiological ligands with the Ca pump and Na/Ca exchange in squid axons

Abstract

We have studied the interaction of physiological ligands other than Nai and Cai with the Ca pump and Na/Ca exchange in internally dialyzed squid axons. The results show the following. (a) Internal Mg2+ is an inhibitor of the Nao-dependent Ca efflux. At physiological Mgi2+ (4 mM), the inhibition amounts to ~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. (b) At constant membrane potential, Ki is an important cofactor for the uncoupled Ca efflux. (c) 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 analogues. Pi under physiological conditions has no effect on the uncoupled component; nevertheless, at alkaline pH, it inhibits the Ca pump, probably byproduct inhibition. (d) 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 (a) depending on the concentration of Cai2+, Nai+ Mgi2+, and Pi, the Na/Ca carrier can operate under a low- or high rate regime; (b) the interactions of Mgi2+, Pi, Nai+, and ATP with the carrier are not interdependent; (c) the effect of Pi on the carrier-mediated Ca efflux resembles the stimulation of the Nao-dependent Ca efflux by internal vanadate;(d) the lig and effects on the uncoupled Ca efflux are of the type seen in the Ca pump in red cells and the sarcoplasmic reticulum. © 1984, Rockefeller University Press., All rights reserved.