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Abstract:
The properties of the 45Ca efflux systems in Saccharomyces cerevisiae were investigated in yeast cells grown overnight in medium containing 45Ca. Efflux was measured in medium containing glucose and Tris-Hepes buffer adjusted to the required pH. In the absence of permeable cations in the medium, at pH 5.2, 20% of the cellular Ca was extruded from the cells during the first 2 h. There was no further decrease in the amount of cellular Ca during an additional 24 h of incubation. The initial rate of Ca efflux was markedly reduced with the increase in the pH of the medium. On the other hand the efflux during the second phase (2-24 h) increased with the increase in medium-pH up to pH 7.5. It is suggested that the initial rapid phase of Ca efflux, in the absence of permeable cations, represents transport across the plasma membrane and is mediated via a Ca2+/H+ antiport. The second phase represents the release of Ca sequestered in some cellular organelles, probably the vacuoles, and is mediated via a different mechanism. Addition of Ca or Mg to the medium markedly stimulated the rate of Ca efflux from both cellular compartments. At the same time a predominant influx of divalent cations was observed. This exchange between intracellular Ca and extracellular divalent cations was not affected by the pH of the medium between pH 5.2 and 7.0. Both processes, Ca efflux and Ca-Mg exchange, required cellular energy; they were almost completely inhibited in the absence of glucose and the presence of antimycin A, a respiratory inhibitor.