Abstract
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°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 (M 1, M 3 subtypes selective antagonist) > atropine (general) > pirenzepine (M 1). Methoctramine (M 2) didn’t antagonise the [ 3H]QNB binding efficiently. Acetylcholine and carbachol increased the intracellular Ca 2+ 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 M 1 and M 3 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.
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