Changes in plasma glucose, insulin and glucagon levels, glucose tolerance tests and insulin sensitivity with age in the rat.

Glucose and insulin relationships with aging were studied in fed rats. Levels of basal circulating glucose did not change while those of RIA-insulin increased and RIA-glucagon decreased lineary with animal weight. The oral glucose tolerance test revealed a greater increase in blood glucoe in adult and old rats than in prepuberals, while the rise in plasma insulin was faster and greater in the oldest group. After intravenous glucose load, plasma insulin increase was greater in adult than in prepuberal and old rats, and in the latter grup values remained elevated for a longer period. The hypoglycemic response to i.v. insulin was greatest in the prepuberals with no difference between adult and old rats. In prepuberals, the augmented insulin sensitivity was counteracted by retarded insulinotropic glucose action and an enhanced basal glucagon level, while in the old animals normoglycemia was maintained due to an augmented secretory response of B cells, counteracted by reduced sensitivity to endogenous insulin.