In squid axons, ATP modulates Na+-Ca2+ exchange by a Ca2+i-dependent phosphorylation.

In squid axons ATP stimulates both the forward and reverse modes of the Na+-Ca2+ exchange by changing the affinity of the carrier towards Na+ and Ca2+ ions. Whether ATP activates the Na+-Ca2+ antiporter allosterically or is hydrolyzed during activation is still debated. The hypothesis that ATP modulates the Na+-Ca2+ exchange through phosphorylation has been tested by means of [gamma-S]ATP, an ATP analog that can act as a substrate for kinases but not for ATPases. Steady-state Ca2+ efflux was measured in squid axons dialyzed without ATP and containing either 0.7 or 100 microM Ca2+i. Addition of 1 mM [gamma-S]ATP markedly increases the Na+o-dependent component of the Ca2+ efflux. The activation by [gamma-S]ATP: requires the presence of Mg2+i, is partially reversible upon removing the analog, is greater than that caused by ATP and only operates on the exchange system since no activation of the ATP-dependent uncoupled Ca2+ efflux (Ca2+ pump) can be detected. 22Na+ experiments were used to monitor the Cao-dependent Na+ efflux (reverse Na+-Ca2+ exchange). Without Ca2+i and ATP, Na+ efflux is very small ('leak'). [gamma-S]ATP does not activate the efflux of Na+ in the absence of Ca2+i. In the presence of Ca2+i the ATP analog stimulates both the Cao- and Nao-dependent Na+ efflux components. Interestingly, neither the Na+ pump, Ca2+i-independent Na+-Na+ exchange, Nai+-Mg2+i exchange or Na+/K+/Cl- cotransport are affected by [gamma-S]ATP. The experiments indicate that a Ca2+i-dependent phosphorylation occurs during the activation of the Na+-Ca2+ exchange by ATP.