alpha(1)-Adrenoceptor antagonist effect of (+/-)-dobutamine in rat isolated gastric artery preparation

(+/-)-Dobutamine at concentrations less than or equal to 10(-5) M did not e voke contractions of rat gastric artery segments. However, when the tissues were contracted with methoxamine, (+/-)-dobutamine evoked concentration-de pendent relaxation. The relaxant responses were not significantly affected by propranolol. In the same preparation, propranolol competitively antagoni zed isoprenaline-induced relaxation with a -log K-B value of 7.90 +/- 0.26. (+/-)-Dobutamine did not relax arterial ring segments precontracted with v asopressin (10(-7) M). (+/-)-Dobutamine antagonized noradrenaline-induced c ontractions of the gastric artery segments. The pA(2) value was 6.93 +/- 0. 20, and the slope of the Schild regression line was 1.22 +/- 0.14. This val ue (slope) was not significantly different from 1, indicating competitive a ntagonism, Pretreatment of gastric artery segments with dobutamine before p henoxybenzamine (PBZ) protected against inactivation of alpha(1)-adrenocept ors by PBZ. The dose ratio of prazosin (3 x 10(-9) M) and (+/-)-dobutamine (10(-5) M) in combination was close to the expected sum of their individual dose ratios minus 1, indicating interaction with a common site. It was the refore concluded that (+/-)-dobutamine evoked relaxation of rat gastric art ery segments by an action not involving beta-adrenoceptor activation but by blocking or, adrenoceptors.