Milrinone and theophylline act as lower oesophageal sphincter relaxing agents: a comparative pharmacodynamic study in the rabbit

This study demonstrates that the inotropic agent milrinone and the bronchodilator drug theophylline exert a relaxing effect on the rabbit lower oesophageal sphincter in vitro. The relaxing effect of milrinone and theophylline, which is concentration-dependent, involves a second messenger 3',5'-cyclic adenosine monophosphate pathway and most probably it is accomplished through inhibition of phosphodiesterase (PDE) type III, as according to the obtained results it is not significantly modified either by nicotinic acid, an inhibitor of adenylate cyclase, or by the inhibitor of nitric oxide-synthetase Nω-nitro- l-arginine methylester and the purinergic antagonist suramin; moreover, it persists under non-adrenergic non-cholinergic conditions and it is both hexamethonium- and tetrodotoxin-insensitive. Both milrinone and theophylline display equal efficacy, comparable to that of the calcium blocker verapamil and the non-selective PDE inhibitor papaverine, but milrinone appears 50 times more potent than theophylline and three times less potent than verapamil, as, according to the pIC₅₀ values the potency rank of order is found to be verapamil (5.56) > milrinone (5.12) > theophylline (3.42). The here obtained pharmacodynamic profiles of the drugs suggest that both milrinone and theophylline may be considered as potent relaxing agents of the lower oesophageal sphincter.