Insulin resistance and hyperglycemia are associated with suppressed endothelial mediated dilation in humans. The mechanisms potentially involve oxidant injury and elevation of protein kinase C activity. While a glucose concentration of 250-300 mg/dl induces endothelial injury in normal rats, insulin resistance may lower the concentration to less than 200 mg/dl. In vivo mechanisms by which insulin resistance and acute hyperglycemia impair endothelial microvascular regulation will be studied in normal and insulin resistant Zucker obese rats. Data obtained during normoglycemia in normal and insulin resistant rats will be used to determine whether chronic insulin resistance is associated with dysfunction of nitric oxide production in the intestinal vasculature. Hyperglycemic solution will then be topically applied to the intestine. In normal rats topical 300 mg/dl hyperglycemia causes a rapid and prolonged impairment of endothelial dependent dilation. It is proposed that oxidant injury associated with increased eicosanoid metabolism initially destroy nitric oxide during hyperglycemia. In addition, a mechanism independent of radical formation leads to sustained loss of nitric oxide function. This mechanism may be caused by activation of beta II protein kinase C, and is expressed primarily as increased resistance of small arteries and large arterioles. To confirm this hypothesis, vessel diameter, blood flow and nitric oxide concentration will be measured during flow and agonist mediated stimulation of nitric oxide release. If oxidants initially destroy nitric oxide during hyperglycemia, pretreatment with oxygen free radical scavengers will maintain the nitric oxide concentration during the early phase of hyperglycemia. Further inhibitors of beta II PKC will improve the nitric oxide dependent mechanisms over longer term hyperglycemia. Positive results will confirm that chronic insulin resistance predisposes the microvasculature to abnormalities of endothelial function. However, antioxidants and inhibition of beta II isoenzymes of PKC may protect endothelial function.
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