Cholinergic muscarinic effects on insulin release in mice

Insulin secretion and blood glucose homeostasis were studied in mice following administration of cholinergic agonists, antagonists and other possible modifiers of cholinergic insulin secretory mechanisms. It was observed that administration to mice of the cholinergic agonists acetylcholine, carbachol and pilocarpine resulted in an increase in plasma immuno-reactive insulin levels accompanied by a significant fall in blood glucose levels. Nicotine had no effect. Carbachol was found to enhance insulin release in a dose-dependent manner. Muscarinic blockade by atropine or methylatropine totally suppressed carbachol-induced insulin secretion. No blocking effect was accomplished by beta-adrenoceptor blockade. Glucose-induced insulin secretion was not affected by atropine in normal non-fasted mice. In mice fasted for 24 h, however, the insulin response to glucose was impaired by atropine suggesting that the nutritional state is important for cholinergic modulation of glucose-induced insulin response. Pretreatment of animals with the glycogenolytic hydrolase, acid amyloglucosidase, enhanced tolbutamide-induced insulin release but did not influence insulin secretion stimulated by carbachol. Pretreatment with the monoamine oxidase inhibitor, pargyline, plus L-dopa, leading to an intracellular accumulation of dopamine in the insulin cells, totally suppressed carbachol-induced insulin release. It is suggested that, in mice, cholinergic stimulation promotes insulin secretion through activation of muscarinic receptors on the insulin cell. Blockade of these receptors does not influence glucose-stimulated insulin release in the non-fasting state, but may impair the insulin response to glucose after fasting. Cholinergic stimulation of insulin release is inhibited after L-dopa-induced accumulation of dopamine in the insulin cells. In contrast to tolbutamide-induced insulin release cholinergic insulin release is not dependent on acid amyloglucosidase activity.