MgATP counteracts intracellular proton inhibition of the sodium-calcium exchanger in dialysed squid axons.

Intracellular Na(+) and H(+) inhibit Na(+)-Ca(2+) exchange. ATP regulates exchange activity by altering kinetic parameters for Ca(2+)(i), Na(+)(i) and Na(+)(o). The role of the Ca(2+)(i)regulatory site on Na(+)(i)-H(+)(i)-ATP interactions was explored by measuring the Na(+)(o)-dependent (45)Ca(2+) efflux (Na(+)(o)-Ca(2+)(i) exchange) and Ca(2+)(i)-dependent (22)Na(+) efflux (Na(+)(o)-Na(+)(i) exchange) in intracellular-dialysed squid axons. Our results show that: (1) without ATP, inhibition by Na(+)(i) is strongly dependent on H(+)(i). Lowering the pH(i) by 0.4 units from its physiological value of 7.3 causes 80 % inhibition of Na(+)(o)-Ca(2+)(i) exchange; (2) in the presence of MgATP, H(+)(i) and Na(+)(i) inhibition is markedly diminished; and (3) experiments on Na(+)(o)-Na(+)(i) exchange indicate that the drastic changes in the Na(+)(i)-H(+)(i)-ATP interactions take place at the Ca(2+)(i) regulatory site. The increase in Ca(2+)(i) affinity induced by ATP at acid pH (6.9) can be mimicked by a rise in pH(i) from 6.9 to 7.3 in the absence of the nucleotide. We conclude that ATP modulation of the Na(+)-Ca(2+) exchange occurs by protection from intracellular proton and sodium inhibition. These findings are predicted by a model where: (i) the binding of Ca(2+) to the regulatory site is essential for translocation but not for the binding of Na(+)(i) or Ca(2+)(i) to the transporting site; (ii) H(+)(i) competes with Ca(2+)(i) for the same form of the exchanger without an effect on the Ca(2+)(i) transporting site; (iii) protonation of the carrier increases the apparent affinity and changes the cooperativity for Na(+)(i) binding; and (iv) ATP prevents both H(+)(i) and Na(+)(i)-effects. The relief of H(+) and Na(+) inhibition induced by ATP could be important in cardiac ischaemia, in which a combination of acidosis and rise in Na(+) occurs.