In the squid axon Na+/Ca2+ exchanger the state of the Cai-regulatory site influences the affinities of the intra- and extracellular transport sites for Na+ and Ca2+

In squid axons, intracellular Mg(2+) reduces the activity of the Na(+)/Ca(2+) exchanger by competing with Ca(i)(2+) for its regulatory site. The state of the Ca(i)-regulatory site (active-inactive) also alters the apparent affinity of intra- and extracellular transport sites. Conditions that hinder the binding of (low pH(i), low [Ca(2+)](i), high [Mg(2+)](i)) diminish the apparent affinity of intracellular transport sites, in particular for Na(i) due to its synergism with H(+) inhibition, but less noticeably for Ca(i)(2+) because of its antagonism towards (H(i)(+) + Na(i)(+)) and Mg(i)(2+) inhibitions. These are kinetic effects unrelated to the true affinity of the sites. With the Ca(i)-regulatory site saturated, the intracellular transporting sites are insensitive to [H(+)](i) and to ATP. Likewise, the state of the Ca(i)-regulatory site (activated or inactivated) influences the affinity of the extracellular Ca(o) and Na(o)-transport sites (trans effects). In this case, the effects are opposite to those predicted by any of the transport schemes proposed for the Na(+)/Ca(2+)exchanger; i.e. its mechanism remains unexplained. In addition to their intrinsic importance for a full understanding of the properties of the Na(+)/Ca(2+) exchanger, these findings show a new way by which the state of the Ca(i)-regulatory site may determine net movements of Ca(2+) through this system.