Influence of β2- and β3-adrenoceptor agonists on contractile activity of the porcine myometrium in the luteal phase and the first days of pregnancy

This study analysed the relaxant properties of salbutamol (β₂-adrenoceptors agonist) and BRL 37344 (β₃-adrenoceptors agonist) regarding the contractility of porcine myometrium on days 10-14 of the oestrous cycle (cyclic group; n = 10) and on days 3-5 of pregnancy (early pregnant group; n = 6). The activity of myometrial strips (tension, frequency and amplitude) was recorded under isometric conditions using force transducers. The contractility was assessed further following the administration of increasing concentrations of the agonists (10⁻⁹-10⁻⁴ M), both with and without β-adrenoceptor antagonists (butaxamine - a selective β₂- adrenoceptor antagonist, propranolol- a non-selective β₁- and β₂-adrenoceptor antagonist and bupranolol - a non-selective β₁-, β₂- and β₃-adrenoceptor antagonist) at a concentration of 10⁻⁴ M. Although neither salbutamol nor BRL 37344 caused changes in the tension, at the highest concentrations they decreased the frequency and amplitude of contractions. These changes were more evident after salbutamol treatment and in the early pregnant group. Antagonists given alone did not cause changes in the parameters examined but changed some activity of the agonists. Butoxamine reduced the decrease in frequency and amplitude induced by salbutamol and produced a decrease in the tension after BRL 37344 treatment in the early pregnant group. Propranolol reduced the decrease in frequency and amplitude induced by salbutamol in both examined groups and did not cause significant changes in BRL 37344 activity. The administration of bupranolol before salbutamol treatment caused an increase in the tension and reduced the decrease in the frequency in the cyclic group. Moreover, bupranolol eliminated a decrease in frequency and induced an increase in amplitude caused by BRL 37344 in both groups and these changes were more evident in the early pregnant group. The data indicates that both β2- and β3-adenoreceptors are involved in the regulation of the contractility in both groups, but the changes after agonists and antagonists treatment are more evident in the early pregnant myometrium.