The Effects of Vitamin A on Insulin Release and Glucose Oxidation in Isolated Rat Islets*

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Abstract
The effects of vitamin A, a membrane surface-active agent, were tested on glucose (16.7 mM)-induced biphasic insulin release from collagenase-isolated rat islets. Efforts were made to correlate the effects of vitamin A with glucose oxidation. Vitamin A (10-4 M) inhibited 1st- and 2nd phase insulin release; 10-5 M vitamin A inhibited 2nd phase release only and to a lesser extent than that observed with 10-4 M vitamin A; and 10-6 M vitamin A had no effect. Vitamin A (10-7 M) stimulated biphasic insulin release. Exposure to high glucose (27.8 mM) overcame the effects of 10-4 M vitamin A on 1st phase release, but not on 2nd phase release of insulin. Exposure to 10-5 M hydrocortisone opposed the effects of 10-4 M vitamin A on both phases of insulin release. Vitamin A (10-4 and 10-5 M) inhibited glucose oxidation by islets, as measured by the production of 14CO2 from [14C]glucose. The effects of vitamin A on insulin release were dissociated in part from those effects on glucose oxidation, in that hydrocortisone opposed the effect of vitamin A on insulin release but not on glucose oxidation. The effects of vitamin A on insulin secretion can best be explained by the interaction of vitamin A at multiple sites affecting the membrane and intracellular glucose oxidation.