Dual effect of Na sub(i) super(+) on Ca super(2+) influx through the Na super(+)/Ca super(2+) exchanger in dialyzed squid axons. Experimental data confirming the validity of the squid axon kinetic model

We propose a steady-state kinetic model for the squid Na super(+)/Ca super(2+) exchanger that differs from other current models of regulation in that it takes into account, within a single kinetic scheme, all ionic [intracellular Ca super(2+) (Ca sub(i) super(2+))-intracellular Na super(+) (Na sub(i) super(+))-intracellular H sub(i) super(+)] and metabolic (ATP) regulations of the exchanger in which the Ca sub(i) super(2+)-regulatory pathway plays the central role in regulation. Although the integrated ionic-metabolic model predicts all squid steady-state experimental data on exchange regulation, a critical test for the validity of it is the predicted dual effect of Na sub(i) super(+) on steady-state Ca super(2+) influx through the exchanger. To test this prediction, an improved technique for the estimation of isotope fluxes in squid axons was developed, which allows sequential measurements of ion influx and effluxes. With this method, we report here two novel observations of the squid axon Na super(+)/Ca super(2+) exchanger. First, at intracellular pH (7.0) and in the absence of MgATP, Na sub(i) super(+) has a dual effect on Ca super(2+) influx: inhibition at low concentrations followed by stimulation at high Na sub(i) super(+) concentrations, reaching levels higher than those seen without Na sub(i) super(+). Second, in the presence of MgATP, the biphasic response to Na sub(i) super(+) disappears and is replaced by a sigmoid activation. Furthermore, the model predicts that Ca super(2+) efflux is monotonically inhibited by Na sub(i) super(+), more pronouncedly without than with MgATP. These results are predicted by the proposed kinetic model. Although not fully applicable to all exchangers, this scheme might provide some insights on expected net Ca super(2+) movements in other tissues under a variety of intracellular ionic and metabolic conditions.