Partial Characterization of K+ and Ca2+ Uptake Systems in the Halotolerant Alga Dunaliella salina

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Abstract
The uptake of K+ and Ca2+ in Dunaliella salina is mediated by two distinct carriers: a K+ carrier with a high selectivity against Na+, Li+, and choline+ but not towards Rb+, K+, Cs+, or NH4+, and a Ca2+ carrier with a high selectivity against Mg2+. The latter is specifically blocked by La3+ and by Cd2+. Apparent Km values for K+ and Ca2+ uptake are 2.5 and 0.8 millimolar, respectively, and their maximal calculated fluxes are 22 and 0.8 nanomoles per square meter per second, respectively. Effects of permeable ions and ionophores on K+ and Ca2+ uptake suggest that the driving force for their uptake is the transmembrane electrical potential. Inhibitors of ATP production, typical inhibitors of plasma membrane H+-ATPases and protonionophores inhibit K+ and Ca2+ uptake and accelerate K+ efflux. The results suggest that an H+-ATPase in the cell membrane provides the driving force for K+ and Ca2+ uptake. Efflux measurements from 86Rb+ and 45Ca2+ loaded cells suggest that part of the intracellular K+ and most of the intracellular Ca2+ is nonexchangeable with the extracellular pool. Correlations between phosphate and K+ contents and the effect of phosphate on K+ efflux suggest intracellular associations between K+ and polyphosphates. On the basis of these results, it is suggested that: (a) K+ and Ca2+ uptake in D. salina is driven by the transmembrane electrical potential which is generated by the action of an H+-ATPase of the plasma membrane. (b) Part of the intracellular K+ is associated with polyphosphate bodies, while most of the intracellular Ca2+ is accumulated in intracellular organelles in the algal cells.