Regional contractile responses in pulmonary artery to alpha- and beta-adrenoceptor agonists

Contractile sensitivity and reactivity to alpha- and beta-adrenoceptor stimulation was studied in incubated rabbit pulmonary artery cylindrical segments of differing diameters. Distinct differences were noted between the responses of extra- and intra-pulmonary pulmonary arteries to norepinephrine and isoproterenol. The sensitivity to norepinephrine was largest in the intrapulmonary pulmonary arteries. Arterial reactivity to norepinephrine was greatest in the larger of the intrapulmonary vessel segments, diminishing considerably as the vessels became smaller. Cocaine did not cause substantial alterations in the response of any of the arterial segments to the alpha-agonist. Phentolamine, however, exerted its influence primarily in the smaller arterial segments. Vascular sensitivity to isoproterenol was least in the intrapulmonary pulmonary arteries. These smaller vessel segments, however, were more reactive to isoproterenol than were the extrapulmonary pulmonary arterial segments. Propranolol, at a concentration of 10(-8) M, was an effective antagonist of the beta-agonist; at a concentration of 10(-7) M, however, this antagonist was related to isoproterenol-induced arterial contraction, apparently by stimulation of alpha-receptor sites. The results of this study demonstrated a regional heterogeneity in the contractile response of the pulmonary artery to alpha- and beta-stimulation. The extrapulmonary arterial segments were found to be more sensitive to beta-stimulation than were the smaller, intrapulmonary, segments. The intrapulmonary arterial segments, on the other hand, were found to be more sensitive to alpha-stimulation than were the extrapulmonary segments.