Studies on segmental differences in sensitivity to adrenergic agonists in pulmonary arteries isolated from the...

This study was designed to examine the existence and possible mechanisms of segmental differences in sensitivity to adrenergic substances in the pulmonary vasculature of the guinea pig. The main pulmonary artery was found to be less sensitive to epinephrine, norepinephrine and phenylephrine than its left and right main branches. Because there were no differences in sensitivities between the left and right branches, most comparisons were made between the main and left branches. The difference in sensitivity was larger for adrenergic agonists which are known to be taken up by adrenergic neurons (epinephrine, norepinephrine, phenylephrine) than for an agonist which is not taken up by these neurons (methoxamine). The left artery from reserpine-treated animals was only slightly more sensitive (1.5-fold) to barium chloride than the main pulmonary artery. Cocaine potentiated to a greater extent in the main than in the left pulmonary artery the amines which are taken up by adrenergic neurons. The left arterial branch was less responsive to transmural electrical stimulation than the main artery in the absence and presence of cocaine. [3H]Norepinephrine uptake was larger in main arterial segments than in the branches and histochemical fluorescence studies demonstrated a greater degree of adrenergic innervation in the proximal segments. The results indicate that a major explanation for the differences in sensitivity between the vascular segments is a smaller degree of adrenergic neuronal uptake of agonist in the arterial branches.