Ability of insulin to increase calcium binding by adipocyte plasma membranes.

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Abstract
Ca specifically binds to [rat] adipocyte plasma membranes, demonstrating 2 classes of binding sites having affinity constants of 4.5 .times. 10-4 M-1 and 2.0 .times. 103 M-1. Insulin (100 microunits/ml) added directly to the isolated plasma membranes caused no alteration in Ca binding, whereas incubation of the adipocytes with 100 microunits/ml of insulin resulted in a 25.0 .+-. 1.6% increase in Ca binding to the subsequently isolated plasma membranes. The increase in Ca 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. A 2nd pool of Ca in adipocyte plasma membranes was identified by atomic absorption analysis; it was more than 2.times. larger than the maximum binding capacity of the Ca binding system. This pool of Ca was stable, did not participate in the 45Ca2+ exchange, and was unaltered by insulin treatment. A similar stable pool of Mg exists in plasma membranes and was also unaffected by insulin treatment. The increased capacity of the isolated plasma membranes to bind Ca after insulin treatment of the cells may represent an important bioregulating mechanism and supports the concept that Ca may play an important role in the effector system for insulin.

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