Direct plasma radioimmunoassay for rat amylin-(1-37): concentrations with acquired and genetic obesity

Amylin (islet-associated polypeptide) is a 37-amino acid peptide that is cosecreted with insulin from the pancreatic beta-cell. Accurate measurement of its plasma levels is important for delineating the physiological range over which amylin acts. We describe a reproducible, highly specific, and sensitive radioimmunoassay for direct measurement of plasma amylin-(1-37). We measured changes in portal and systemic plasma amylin and insulin in three groups of anesthetized rats: lean young adult and old adult Wistar rats with acquired obesity, and Wistar fatty [WDF/TaFa (fa/fa)] rats, a model of genetic obesity and insulin resistance derived from the Wistar strain. Changes in response to fasting, feeding, and intravenous stimulation with glucose plus arginine were assessed. We find that the amylin-to-insulin ratio is constant in fasted or fed young and old rats because of proportionate increases in both entities with aging. In genetically obese Wistar rats, amylin and insulin levels are three- to tenfold higher than in lean young or obese old normal controls. Islet stimulation by feeding or intravenous glucose plus arginine resulted in a decreased amylin-to-insulin molar ratio in all groups. When normalized for the degree of islet stimulation, amylin-to-insulin ratios were significantly elevated in genetically obese vs. normal rats, both in the portal and systemic circulation. These results demonstrate that aging-related weight gain in normal rats is associated with moderate and proportional increases in amylin and insulin, whereas genetic obesity is characterized by elevated amylin and an increased amylin-to-insulin ratio. Implications for the pathogenesis of insulin resistance and obesity are discussed.