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In this work, we compared the effect of K+ on vesicles derived from the longitudinal (LSR) and terminal cisternae (HSR) of rabbit white muscle. In HSR, K+ was found to inhibit both the Ca2+ accumulation and the heat released during ATP hydrolysis by the Ca2+-ATPase (SERCA1). This was not observed in LSR. Valinomycin abolished the HSR Ca2+-uptake inhibition promoted by physiological K+ concentrations, but it did not modify the thermogenic activity of the Ca2+ pump. The results with HSR are difficult to interpret, assuming that a single K+ is binding to either the ryanodine channel or to the Ca2+-ATPase. It is suggested that an increase of K+ in the assay medium alters the interactions among the various proteins found in HSR, thus modifying the properties of both the ryanodine channel and SERCA1.

(1) Vanadate (pentavalent vanadium) inhibits with high affinity ( K 0.5 = 3 μ M ) the ATP-dependent Ca2+ efflux in reconstituted ghosts from human red cells. (2) To inhibit Ca2+ efflux vanadate has to have access to the inner surface of the cell membrane. (3) The inhibitory effect of vanadate is potentiated by intracellular Mg2+ and by intracellular K+. (4) Ca2+ in the external medium antagonizes the inhibitory effect of vanadate.

Ca2+ transport was studied in membrane vesicles of alkalophilic Bacillus. When Na+-loaded membrane vesicles were suspended in KHCO3/KOH buffer (pH 10) containing Ca2+, rapid uptake of Ca2+ was observed. The apparent Km value for Ca2+ measured at pH 10 was about 7 microM, and the Km value shifted to 24 microM when measured at pH 7.4. The efflux of Ca2+ was studied with Ca2+-loaded vesicles. Ca2+ was released when Ca2+-loaded vesicles were suspended in medium containing 0.4 M Na+. Ca2+ was also transported in membrane vesicles driven by an artificial pH gradient and by a membrane potential generated by K+-valinomycin in the presence of Na+. These results indicate the presence of Ca2+/Na+ and H+/Na+ antiporters in the alkalophilic Bacillus A-007