Ability of insulin to increase calcium binding by adipocyte plasma membranes

Calcium specifically binds to adipocyte plasma membranes, demonstrating two classes of binding sites having affinity constants of 4.5 x 10(4) M-1 and 2.0 x 10(3) M-1. Insulin (100 microunits/ml) added directly to the isolated plasma membranes caused no alteration in calcium binding, whereas incubation of the adipocytes with 100 microunits/ml of insulin resulted in a 2.50 +/- 1.6% increase in calcium binding to the subsequently isolated plasma membranes. The increase in calcium binding produced by insulin resulted from an increase in the maximum binding capacities of both classes of binding sites without alteration in their affinity constants. Additionally, a second pool of calcium in adipocyte plasma membranes has been identified by atomic absorption analysis; it was more than two times larger than the maximum binding capacity of the calcium binding system. This pool of calcium was stable, did not participate in the 45Ca2+ exchange, and was unaltered by insulin treatment. A simialr stable pool of magnesium exists in plasma membranes and was also unaffected by insulin treatment. The increased capacity of the isolated plasma membranes to bind calcium after insulin treatment of the cells may represent an important bioregulating mechanism and supports the concept that calcium may play an important role in the effector system for insulin.