Inside-out plasma membrane vesicles from a glucose-responsive rat insulinoma showed an ATP- and Mg2+-dependent uptake of Ca2+. The Km(concentration giving halfmaximal activity) for Ca2+was 60 nM. In the presence of 0.4 µ free Ca2+, the Kmfor ATP was 15 µM, and the Kmfor Mg2+was 4 µM. Glucose (30 mM) decreased Ca2+uptake by 50%, while other insulin secretagogues had no effect, except for glyceralde- hyde, which stimulated Ca2+uptake. Calmodulin increased the uptake of Ca2+, while trifluoperazine and vanadate inhibited the uptake. The Ca2+- and Mg2+-dependent ATPase from this tumor has a 10- to 20-fold higher requirement for Ca2+, which suggests that this enzyme is not responsible for Ca2+transport, rather, Ca2+transport activity represents only a small fraction of the total Ca2+-ATPase activity. The physiological importance of Ca2+transport in insulin secretion is evident from the inhibition of Ca2+uptake by glucose, which leads to a decrease in Ca2+efflux from the cell. This inhibition would lead to an increase in intracellular free Ca2+and insulin release.
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