The effect of pH on Ca 2+ extrusion mechanisms in dialyzed squid axons

Abstract

The effect of internal and external pH, on the components of the Ca 2+ efflux have been investigated in internally dialyzed squid axons. (1) Internal pH: a fall in intracellular pH (below 7.3) inhibited both the ATP-dependent uncoupled (Ca 2+ pump) (50% at pH i 6.3) and the Na 0 +-dependent Ca 2+ efflux (forward Na +/Ca 2+ exchange) (50% at pH i 6.8). Internal alkalinization to pH 8.8 had no effect on the uncoupled component but markedly increases (4-fold) the Na 0 +-dependent Ca 2+ efflux. (2) External pH: altering the external pH from 7.3 to 9.0 had no effect on the Na 0 +-dependent Ca 2+ efflux mechanism. In the absence of Ca 0 2+, alkalinization to pH 0 8.8 caused a reduction in the magnitude of the uncoupled Ca 2+ pump. This inhibition is markedly enhanced by the presence of Ca 2+ in the external medium. As for the case of the sarcoplasmic reticulum Ca 2+-ATPase, this combined inhibitory effect of high pH 0 and Ca 2+ 0 is most probably related to a reversal of the cycle of the ATP driven Ca 2+ pump. The marked differences in the pH dependence of the components of the Ca 2+ efflux support the model of two separate mechanisms of Ca 2+ extrusions in squid axons: Ca 2+ pump and Na +/Ca 2+ exchange.