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

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.