Influence of β-adrenoceptor-mediated relaxation on α-adrenoceptor-mediated contraction of rat pulmonary artery to adrenaline or noradrenaline

Abstract

Abstract Concentration-response (contraction) curves to either adrenaline or noradrenaline were obtained on isolated ring preparations of pulmonary artery from rats. In preparations from young rats the curve for adrenaline was bell-shaped, unless β-adrenoceptors were blocked with propranolol (1 × 10−6 M). The maximum contraction to adrenaline was less in the absence than in the presence of propranolol. In preparations from aged rats the adrenaline curve was no longer bell-shaped, even in the absence of propranolol. This reflected a decrease in the β-adrenoceptor-mediated relaxant responses of preparations from aged rats, seen as a separation between the concentration-response (relaxation) curves to adrenaline on preparations (phenoxybenzamine-treated, KCl-contracted) from young and aged rats. In preparations from young rats the noradrenaline curve was not bell-shaped, but if the preparations were from young rats treated with thyroxine (T4), then a bell-shaped curve for noradrenaline was obtained, unless β-adrenoceptors were blocked by propranolol. These data could be explained by an increase in β-adrenoceptor-mediated relaxant responses of preparations from T4-treated rats, seen as a separation in the concentration-response (relaxation) curves to noradrenaline on preparations from control and T4-treated rats, respectively. Thus (i) α-adrenoceptor-mediated contractile responses of rat pulmonary artery preparations, to adrenaline or noradrenaline, can be attenuated by activation of β-adrenoceptors, mediating relaxation, and (ii) the extent of this attentuation changes under the influence of factors, such as ageing or T4-treatment, which modify β-adrenoceptor-mediated relaxation in this blood vessel type.