The overall goal of these investigations is to determine the mechanism(s) responsible for insulin resistance among newborn dogs. We and others have demonstrated that the neonatal period is characterized by imprecise regulation of glucose metabolism as manifested by fasting hypoglycemia and alimented induced hyperglycemia. Using the euglycemic hyperinsulinemic clamp, we have demonstrated that neonatal insulin resistance in vivo is due to a postreceptor defect as demonstrated by a reduced maximum rate of insulin-mediated glucose uptake and by attenuated suppression of endogenous glucose production. These preliminary projects have led to further testable hypotheses related to mechanisms of insulin action due to immaturity, counterregulatory hormones, and FFA which may cause defects at the level of the insulin receptor tyrosine kinase activity, and the cellular uptake of glucose. Presently there is a paucity of studies which have simultaneously investigated the kinetic and postreceptor factors which may be responsible for neonatal insulin resistance. In addition to reduced peripheral tissue glucose uptake the newborn demonstrated the attenuated suppression of hepatic glucose production. We will investigate the relationship of this phenomenon with techniques to quantitate insulin's effect on the suppression of gluconeogenesis. This will be determined at the substrate flux level in addition to the enzyme activity and mRNA expression level. We hypothesize that ongoing gluconeogenesis is responsible for the poor suppression of hepatic glucose output in the newborn mammal. We also propose to employ the euglycemic hyperinsulinemic clamp in newborn dogs, combined with specific perturbations which are present during the neonatal period (e.g., epinephrine, FFA) and analyze alterations of insulin-mediated glucose uptake. In addition we will determine the suppression of endogenous glucose production, receptor binding characteristics, autophosphorylation of the insulin receptor, and phosphorylation of receptor-associated proteins. The insulin sensitive adult dog will serve as the appropriate control. It is anticipated that the results of the proposed investigations will increase our understanding of neonatal insulin resistance and further our knowledge about insulin physiology in general. The understanding of the relationship between insulin and glucose in animal models of human metabolic problems will enable more rational care for high-risk infants.
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