Characterization of the reverse Na/Ca exchange in squid axons and its modulation by Cai and ATP. Cai-dependent Nai/Cao and Nai/Nao exchange modes

We have used dialyzed squid axons to characterize the ouabain- and bumetanide-insensitive Na efflux components and their relation to the operation of the Na/Ca exchange mechanism. In axons dialyzed with solutions containing nearly physiological concentrations of K, Na, and Mg, three components of the Na efflux can be distinguished: Cai-activated, Cao-dependent Na efflux ("reverse" Na/Ca exchange); Cai-activated, Nao-dependent Na efflux; and Cai-independent, ATP-activated, Nao-dependent Na efflux. We have studied the effects of internal alkalinization, Mgi, Cao, and the ATP analogue [gamma-thio]ATP (ATP gamma S) on the different components of the Na efflux. The results show the following: (a) internal alkalinization activates both Cao- and Nao-dependent Na efflux components provided that Cai is present; (b) Mgi inhibits both the Cai-activated, Cao- and Nao-dependent Na efflux components; (c) Cao inhibits the Nao-dependent component by competition for a common site; (d) ATP gamma S activates both Nao- and Cao-dependent Na efflux components only in the presence of Cai; and (e) ATP activates the Nai/Nao and Nai/Cao exchanges, causing a 10-fold increase in the affinity of the reverse Na/Ca exchange toward Cai. In the absence of Cai, ATP stimulates an Nao-dependent Na efflux that is not affected either by internal alkalinization or high Cao. The ATP analogue does not activate the Cai-independent Na/Na exchange system. These experiments demonstrate that the Cai-activated Na/Na exchange is a mode of operation of the Na/Ca exchange mechanism that substantially contributes to Na movement during the activation of the Na/Ca antiporter. The experimental evidence obtained on the Cai-independent Na/Na exchange component shows that this system is not part of the Na/Ca exchange.