Effect of calcium on [125I]insulin binding to rat adipocytes

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Abstract
The specific binding of [125I]insulin to rat adipocytes was reduced in the absence of extracellular C. The addition of C to the C-free medium during incubation restored [125I]insulin binding towards normal. The specific binding of insulin was significantly increased with C concentrations as low as 0.5 mM and maximal binding occurred with 5 mM. C. Scatchard analysis of the data suggests 2 major binding sites, one a high-affinity low-capacity site (Kd, 1.5 .times. 1010 M-1) and the other a lower-affinity high-capacity site (Kd, 4.7 .times. 109 M-1). There was a 50% decrease in the number of high-affinity sites in absence of extracellular C. The dissociation curve of receptor-bound insulin was nonlinear both in the absence and presence of extracellular C suggesting receptor heterogeneity. The dissociation rate of receptor-bound insulin was greater when insulin was bound in the absence of extracellular C than in its presence. Extracellular C, by increasing the number of high-affinity receptor sties, may alter the ratio of high-affinity to low-affinity receptors for insulin in adipocytes.