Muscarinic cholinergic modulation of insulin response to an intravenous glucose tolerance test in normal man.

The effect of pirenzepine, a specific muscarinic cholinergic receptor antagonist, on insulin and glucagon responses to an intravenous injection of glucose was investigated in eight normal adult subjects. These volunteers received two iv glucose tolerance tests (0.33 g/kg) before and after the oral administration of 125 mg of pirenzepine (three doses of 25 mg during the day before the experiment and a fourth dose of 50 mg 2 h before glucose injection). Treatment with pirenzepine neither altered basal blood glucose levels nor affected glucose tolerance after the injection of the glucose load. In addition, it did not modify basal plasma insulin and glucagon levels and the decrement of glucagon in response to glucose injection. In contrast, pirenzepine significantly decreased insulin release induced by glucose administration. In man, during the present experimental conditions, the muscarinic cholinergic system modulates insulin, but not glucagon response to an iv glucose injection.