Calcium efflux from internally dialyzed squid axons: The influence of external and internal cations

Abstract

Abstract Internal dialysis techniques have been used to examine the influence of external and internal cations on Ca efflux from ATP‐depleted squid axons. The main observation is that Ca efflux is promoted by external Na and inhibited by internal Na. The Na 0 ‐dependent Ca efflux appears to be a function of [Na] , and is also affected by the membrane potential; a 25 mV depolarization may cause as much as an e‐fold decrease in Ca efflux. These data are consistent with a counter‐transport exchange of 3Na + –for–1Ca 2+ . A Ca 0 ‐dependent Ca efflux has also been observed; it is prominent in Na sea water or Le sea water, and is markedly diminished in choline sea water. This flux is consistent with the idea of a Ca‐Ca exchange diffusion process. Taken together, the Na 0 ‐ and the Ca 0 ‐dependent Ca effluxes fit a two‐site model for carrier‐mediated Ca transport; one site binds two Na + or one Ca 2+ , while the second site can bind either one Na + or one Li + . The data reported here suggest that both sites must be filled on the inward journey, but that only the Ca‐binding site need be occupied on the outward journey of the carrier. A mechanism of this type could derive sufficient energy from the Na and voltage gradients to maintain a [Ca 2+ ] 0 /[Ca 2+ ] i concentration ratio of about 10 4 in the absence of ATP. The present experiments do not, however, rule out the possible participation of a metabolically driven Ca transport mechanism in vivo.