Calcitonin: antagonism at intestinal muscarinic receptors.

The action of calcitonin was studied on the motility of isolated innervated segments of rabbit and guinea-pig intestines as well as longitudinal muscle with adherent myenteric plexus dissected from the guinea-pig ileum. Calcitonin (0.25 muM) antagonized contractile responses to acetylcholine and the cholinergic response to electrical field stimulation. This hormonal effect was relatively specific since it was not observed at nicotinic receptors or adrenoceptors, nor did calcitonin act as a local anaesthetic or directly on the contractile machinery of smooth muscle. Perivascular adrenergic and intrinsic non-adrenergic inhibitory responses also were unaffected by calcitonin. However, calcitonin did have antihistaminic properties directed against H1-receptors. The concentration of calcitonin required to achieve muscarinic antagonism in our experiments is not reached at the resting level of circulating hormone.