Abstract
Adult obese (fa/fa) Zucker rats hypersecrete insulin in response to glucose and other secretagogues. Functional changes in islet alpha 2-adrenoceptors (8) and glycolytic regulation (9) have been reported. In this study, the development of these biochemical lesions in islets isolated from suckling (3 week old) and weanling (5 week old) lean and fa/fa rats was investigated and compared to results in adult animals. Glucose (15 mM)-induced insulin secretion was inhibited by mannoheptulose (MH) in lean (n = 8) but not fa/fa (n = 10) adult rats, indicating loss of sensitivity of glucokinase to competitive inhibition. Sensitivity to MH was somewhat reduced in the islets of 3- and 5-week-old fa/fa (n = 7 and 12) compared to lean (n = 15 and 9) rats, requiring 30-100 fold higher concentrations to achieve significant inhibition. At 3 weeks of age fa/fa rats did not differ from lean controls in either islet insulin content or body weight, but both parameters were increased in fa/fa rats by 5 weeks. The presence of altered alpha 2-adrenoceptor function in fa/fa rats could not be confirmed in this study. Unlike the previous report, prazosin did not antagonize alpha 2-agonist mediated inhibition of insulin secretion. The presence of defective regulation of the glycolytic pathway by mannoheptulose in suckling and weanling rats may contribute to development of hyperinsulinemia in fa/fa rats.
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