The effect of pH on Ca2+ extrusion mechanisms in dialyzed squid axons.
The effect of internal and external pH, on the components of the Ca2+ efflux have been investigated in internally displayed squid axons. (1) Internal pH: a fall in intracellular pH (below 7.3) inhibited both the ATP-dependent uncoupled (Ca2+ pump) (50% at pHi 6.3) and the Na+o-dependent Ca2+ efflux (forward Na+/Ca2+ exchange) (50% at pHi 6.8). Internal alkalinization of pH 8.8 had no effect on the uncoupled component but markedly increased (4-fold) the Na+o-dependent Ca2+ efflux. (2) External pH: altering the external pH from 7.3 to 9.0 had no effect on the Na+o-dependent Ca2+ efflux mechanism. In the absence of Ca2+o, alkalinization to pHo 8.8 caused a reduction in the magnitude of the uncoupled Ca2+ pump. This inhibition is markedly enhanced by the presence of Ca2+ in the external medium. As for the case of the sarcoplasmic reticulum Ca2+-ATPase, this combined inhibitory effect of high pHo and Ca2+o is most probably related to a reversal of the cycle of the ATP driven Ca2+ pump. The marked differences in the pH dependence of the components of the Ca2+ efflux support the model of two separate mechanisms of Ca2+ extrusion in squid axons: Ca2+ pump and Na+/Ca2+ exchange.
D000255:Adenosine Triphosphate / Q000378:metabolism; D000818:Animals; D001369:Axons / Q000378:metabolism*; D001693:Biological Transport, Active; D002118:Calcium / Q000378:metabolism*; D049832:Decapodiformes; D003956:Dialysis; D006863:Hydrogen-Ion Concentration; D012964:Sodium / Q000378:metabolism
D000255:Adenosine Triphosphate; D012964:Sodium; D002118:Calcium