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 K_m (concentration giving half-maximal activity) for Ca²⁺ was 60 nM. In the presence of 0.4 μM free Ca²⁺, the K_m, for ATP was 15 μM, and the K_m, for Mg²⁺ was 4 μM. 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.