Calcium transport by plasma membranes from a glucose-responsive rat insulinoma.

Inside-out plasma membrane vesicles from a glucose-responsive rat insulinoma showed an ATP- and Mg2(+)-dependent uptake of Ca2+. The Km (concentration giving half-maximal activity) for Ca2+ was 60 nM. In the presence of 0.4 microM free Ca2+, the Km for ATP was 15 microM, and the Km for Mg2+ was 4 microM. Glucose (30 mM) decreased Ca2+ uptake by 50%, while other insulin secretagogues had no effect, except for glyceraldehyde, 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.