ABSTRACT. Studies were performed on isolated pulmonary arterial segments to investigate dopamine receptormediated relaxant effects at different times during development. Dopamine receptor-mediated relaxant effects can only be observed when vessels are precontracted with prostaglandin Fz, and in the presence of al, ag P, and serotonergic blockade. Helical strips of pulmonary arteries from rabbits of different ages (2, 7, 14, 30, and 90 days), partially precontracted by prostaglandin Fz, were tested for their responses to dopamine in the presence of prazosin M), yohimbin M), propranolol M), and methysergide M). Strips from 2-and 7-day-old rabbits were not induced to relax by dopamine, whereas those from 14-, 30-, and 90-day-old animals, after cumulative application of dopamine, underwent concentration-dependent relaxation. Dopamine (half the maximum response) concentration decreased during the development of rabbits from 14 to 90 days old. Mean values for apparent dopamine ED50 (half the maximum response) concentrations in the arteries of 14-, 30-, and 90-day old animals were 4.94 + 0.40, 2.02 + 0.30, and 0.113 + 0.028 wM, respectively.The effects of various dopamine antagonists on dopamineinduced relaxation were not markedly different at different ages. These findings indicate that dopamine receptor function is not fully developed in the pulmonary arteries of newborn rabbits, but matures as the age of the rabbit increases. (Pediatr Res 24: 160-165, 1988) Abbreviations ED50 concentration, concentration that produced half the maximum response pAz value, the negative logarithm of the molar concentration of the antagonist that produces a 2-fold shift to the right of the dose-response curve for the agonist Dopamine stimulates PI-adrenergic receptors causing positive inotropy and has been used to treat shock and congestive heart failure. In high doses, it also stimulates al-adrenergic receptors causing vasoconstriction. In low doses, however, specific dopaminergic receptors mediate vasodilation.In the cardiovascular system specific dopaminergic receptors are found in renal, mesenteric, splenic, coronary, cerebral, and femoral arteries. A vascular dopaminergic receptor classification has been proposed by Goldberg and Kohli (1, 2), in which the DA, receptor subserves smooth muscle relaxation, and the DA2 Received December 29, 1987; accepted March 18, 1988. Correspondence Dr. Miyako Mori, Department of Anesthesiology, Kobe University School of Medicine, Kusunoki-cho-7, Chuo-ku, Kobe, 650 Japan.Supported by grants from the Ministry of Education, Japan.receptor subserves the inhibition of noradrenaline release from postganglionic sympathetic nerves. Previously, we confirmed the existence of DAI dopamine receptors in the adult rabbit pulmonary artery (3). Dopamine produced a concentration-dependent relaxation of arterial strips contracted with prostaglandin F2, in the presence of prazosin (al-blocker), yohimbin (a2-blocker), propranolol (P-blocker), and methysergide (serotonin-blocker). The pulmonary circulatory system un...