Chelation of Intracellular Calcium Prevents Stimulation of Glucose Transport by Insulin and Insulinomimetic Agents in the Adipocyte. Evidence for a Common Mechanism*

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Abstract
Vanadate, concanavalin A (Con A), H2O2, and the phorbol ester, 12-O-tetradecanoyl phorbol 13-acetate (TPA), seemingly unrelated compounds, have effects on cellular metabolism similar to those of insulin. The mechanism(s) by which these diverse compounds act and their relevance to insulin action remain unclear. The hypothesis that increased intracellular calcium ([Ca2+]i) may provide the common basis for the stimulatory effects of these agents on glucose uptake in adipocytes was tested. This was accomplished by preincubating cells with the membrane-permeant, ester, quin2-AM (2-[(2-bis[carboxymethyl] amino-5-methylphenoxy]-6-methoxy-8-bis[carboxymethyl]-aminoquinolinetetrakis [acetoxymethyl] ester), which is rapidly accumulated and hydrolyzed by esterases to form the impermeant tetracarboxylate chelator form, quin2. Vanadate, Con A, H2O2, and TPA stimulate D-glucose uptake in adipocytes approximately 3-, 3-, 2-, and 0.6-fold, respectively, compared to the 5- to 10-fold stimulation of D-glucose uptake obtained with insulin. Preincubation with quin2-AM produced a dose-dependent inhibitory effect only on the stimulated portion of glucose transport without affecting basal (unstimulated) transport. The concentrations for half-maximal inhibition (IC50) of stimulated glucose transport by quin2-AM were 26, 35, 25, 14, and 34 .mu.M for insulin, vanadate, Con A, H2O2, and TPA, respectively. Quin2-AM maximally inhibited stimulated glucose uptake by greater than 85% for all of the insulin mimetic agents. In contrast the maximal inhibition of insulin-stimulated glucose transport by quin2-AM was 55 .+-. 4%. Therefore, stimulation of glucose transport by insulin and other diverse compounds appears to involve at least one common calcium-dependent intermediate step.

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