Benzamide action at alpha 2-adrenoceptors modifies catecholamine-induced contraction and relaxation of circular smooth muscle from guinea-pig stomach.

Dopamine was shown to act on the circular smooth muscle of the stomach body to cause contraction at a yohimbine-sensitive site (alpha 2) and a relaxation at a prazosin-sensitive site (alpha 1). Metoclopramide and tiapride failed to modify either response, failed to antagonise a relaxation to phenylephrine at alpha 1 sites in the same tissue, and failed to modify the contractions caused by dopamine and phenylephrine at an alpha 1-adrenoceptor site in the pyloric sphincter. However, (+)- and (-)-sultopride and (+)-sulpiride antagonised the dopamine-induced contractions of the stomach body indicating an alpha 2-antagonist action. An ability to attenuate the relaxation of this tissue may reflect a displacement of the contraction curve to the right rather than an alpha 1-antagonist action since the response to phenylephrine was not antagonised either in this tissue or in the pyloric sphincter. Within the central nervous system the (-)-enantiomers of sultopride and sulpiride have a highly selective dopamine receptor blocking action. This contrasts with the present findings in the stomach musculature of a non-stereospecific antagonism at alpha 2-type adrenoceptors.