HEPATIC PORTAL INSULIN ANTIBODY INFUSION INCREASES, BUT INSULIN DOES NOT ALTER, SPONTANEOUS MEAL SIZE IN RATS

To investigate the acute effects of pancreatic insulin on spontaneous feeding in rats fed adlibitum, insulin or insulin antibodies were infused into the hepatic portal vein during the first meal of either the light or dark phase. Infusions (3 min, 0.033 ml/min) were remotely controlled, and a computerized system recorded meal patterns. In separate crossover tests, 1, 2, 4, 8, and 16 mU insulin/meal did not affect meal size or subsequent intermeal interval (P > 0.10). In one test, nocturnal meal duration was decreased by 2 mU insulin/meal (19%, P < 0.05). Infusions of polyclonal antibodies to human insulin with in vitro rat insulin binding capacity of 20 or 50 mU increased the size of the first nocturnal meal by 24 and 29% (P < 0.05), respectively. Meal duration was reliably increased only by the smaller antibody dose. Subsequent intermeal interval was unaffected by either antibody dose. The stimulatory effect of insulin antibody infusion on meal size indicates that antagonism of circulating insulin during meals interferes with the control of meal termination. Thus insulin appears to play a role in the physiological control of nocturnal spontaneous feeding in rats. Exogenous insulin may have failed to decrease meal size because of a ceiling effect.