Regional variation of lamb blood vessel responsiveness to vasoactive agents during fetal development.

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Abstract
Types (8) of blood vessels were isolated from the fetal lamb and compared for their responsiveness to norepinephrine (NE), serotonin (5-hydroxytryptamine, 5HT) and acetylcholine (ACh). Fetuses ranged from 53 days to term in gestational age and from 0.05-4 kg in weight. The maximal contractile responses to the 3 agents were unequal among the vessels in the immature, premature or mature periods of gestation. Vessels in the mature period were of 3 classes, in increasing order of maximal response to NE per unit cross-sectional area of vascular wall: thoracic aorta, ductus arteriosus and pulmonary artery; common carotid, ulnar and mesenteric arteries; and renal artery and saphenous vein. 5HT and ACh were less consistent than NE in magnitude of their effect and often elicited dilation or failed to cause a response in immature and premature vessels. Vessels were categorized according to gestation-related changes in their maximal responses. Responses to NE and 5HT of all vessels, corrected for the cross-sectional area, either remained unchanged or increased at different rates. The ductal contractile response to ACh decreased toward term. The aorta, pulmonary artery and ductus arteriosus were different from one another in their affinity to NE, although no change in affinity occurred with progress in fetal maturation. Quantitative and qualitative diversity in the response to NE, 5HT and ACh apparently exists among blood vessels from early prenatal life.