Calcium transport affinity, ion competition and cholera toxin effects on cytosolic Ca concentration

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Abstract
The physiological relevance of an apparent ionophore activity of cholera toxin towards Ca2+ has been examined in several different systems designed to measure affinity, specificity, rates of ion transfer, and effects on intracellular ion concentrations. Half-maximal transfer rates across porcine jejunal brush-border vesicles were obtained at a concentration of 0.20 μM Ca2+. When examined in the presence of competing ions the transfer process was blocked by very low concentrations of La3+ or Cd2+. Sr2+, Ba2+ and Mg2+ were relatively inefficient competitors for Ca2+ transport mediated by cholera toxin. The relative affinities observed would be compatible with a selectivity for Ca2+ transfer at physiological ion concentrations, as well as an inhibition of this ionophore activity by recognized antagonists of cholera toxin such as lanthanum ions. Entry rates of Ca2+ into brush-border vesicles exposed to cholera toxin were large enough to accelerate the collapse of a Ca2+ gradient generated by endogenous Ca, Mg-ATPase activity. The treatment of isolated jejunal enterocytes with cholera toxin caused a significant elevation in cytosolic Ca2+ concentrations as measured by Quin-2 fluorescence. This effect was specifically prevented by prior exposure of the cholera toxin to excess ganglioside GM1. We conclude that cholera toxin has many of the properties required for promoting transmembranes Ca2+ movement in membrane vesicles and appears to be an effective Ca2+ ionophore in isolated mammalian cells.