The basic aim of this grant proposal is to define and validate non-invasive techniques which allow the secretion and hepatic extraction of insulin to be accurately quantitated from peripheral C-peptide and insulin concentrations in human subjects. Insulin secretion rates will be derived from peripheral C-peptide concentrations by using a two compartment mathematical model to analyze the peripheral concentrations of C-peptide. Each subject will receive an intravenous bolus injection of biosynthetic human C-peptide and model parameters will be individually derived from the resulting decay curve. The secretion rate calculations will be based on individual model parameters. Alternative methods of deriving the post hepatic delivery rate of insulin will be compared in conscious mongrel dogs. Specific studies in dogs will compare hepatic extraction calculated as the difference between the secretion and the post hepatic delivery of the hormone with direct measurements based on fluxes of insulin across the liver. Experiments will be performed to determine if the conclusions of the dog studies can be applied to humans. New approaches to studying the effect of oral glucose and mixed meals on the peripheral clearance of insulin will be examined. These studies will involve the infusion of pork insulin as a non- iodinated tracer of human insulin and pork and human insulin will be separated by high pressure liquid chromatography. The accuracy with which urinary C-peptide reflects the secretion rate of insulin will be examined by determining the proportion of secreted C-peptide which is excreted unchanged in the urine. The reproducibility of this measure will be defined and the effects of changes in the plasma concentration of C- peptide and the ingestion of meals on urinary C-peptide clearance will be examined. Finally these techniques will be applied to a series of clinical studies in which the secretion and hepatic extraction of insulin will be studied in normal volunteers, obese subjects and patients with Type II diabetes. It is anticipated that the work performed in the next funding period will lead to improvements in the accuracy with which the secretion and hepatic extraction of insulin can be estimated in clinical studies. The use of well validated and quantitatively accurate approaches to studying these two processes should greatly enhance our understanding of the physiology of insulin secretion and its abnormalities which occur in obesity, diabetes and other disease states.
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