Intracellular ionic and metabolic regulation of squid nerve Na+/Ca2+ exchanger

Intracellular Na(+) and H(+) synergistically inhibit the squid Na(+)/Ca(2+) exchanger by reducing the affinity for Ca(2+) of its regulatory site. MgATP antagonizes H(+)(i) and Na(+)(i) inhibition; this effect must occur through a phosphorylation-dephosphorylation process, because exogenous protein phosphatases prevent MgATP activation of the exchanger. Protection by ATP against H(+)(i) and Na(+)(i) inhibition happens by decreasing the apparent affinity for the synergistic binding of these cations to the carrier. In this way ATP modifies the apparent affinity for Ca(2+) of its regulatory site. Mg(2+) ions play an important role in the process because they are essential for ATP activation of Na(+)/Ca(2+) exchange but can also promote deactivation of the ATP upregulated exchanger. At constant [ATP], activation at low [Mg(2+)](i) is followed by deactivation as [Mg(2+)](i) is increased. The most likely explanation for deactivation is stimulation of endogenous phosphatases. We developed a kinetic model that predicts all H(+)(i), Na(+)(i), and MgATP described above. This scheme includes the following conditions: (i) The binding of Ca(2+) to the regulatory site is essential for the binding of Na(+)(i) or Ca(2+)(i) to the transporting sites. (ii) The binding of a first H(+)(i) to the carrier displaces Ca(2+)(i) from its regulatory site and allows binding of one Na(+) forming a H.E(1).Na complex. The H.E(1).Na complex can bind a second H(+)(i) forming a dead-end inhibitory H(2).E(1).Na complex. (iii) MgATP, through an unspecified phosphorylation process, decreases the apparent affinity for the synergistic H(+)(i) and Na(+)(i) binding to the carrier.