Muscarinic M1 and M3 receptors are present and increase intracellular calcium in adult rat anterior pituitary gland.

Physiological and biochemical evidence indicates the existence of functional muscarinic cholinergic receptors in the anterior pituitary. The selectivity of these receptors has been characterised by studying the binding of [3H]quinuclidinyl benzilate ([3H]QNB) and [3H]diphenyl-acetoxy-N-methyl-piperidine ([3H]4-DAMP) in membrane preparation of male rat anterior pituitary at 25 degrees C. Competition experiments with receptor selective muscarinic antagonists were used to characterise specific selective muscarinic receptor binding. Both [3H]QNB and [3H]4-DAMP bound to anterior pituitary membranes at low concentrations, binding was saturable and was potently displaced by 4-DAMP (M1, M3 subtypes selective antagonist) > atropine (general) > pirenzepine (M1). Methoctramine (M2) didn't antagonise the [3H]QNB binding efficiently. Acetylcholine and carbachol increased the intracellular Ca2+ level in 62% and 65% of cultured rat anterior pituitary cells in a dose-dependent manner, and this effect was prevented by pirenzepine. Based on these results we suggest that both M1 and M3 muscarinic receptors are present and active in the majority of cells in the rat anterior pituitary gland, but their physiological role in the adult rat remains to be examined.