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The localization of neuropeptide Y (NPY) and atrial natriuretic peptide (ANP) in the endothelial cells of human umbilical blood vessels was studied using the pre-embedding peroxidase-antiperoxidase (PAP) technique for electron microscopy and avidin-biotin-complex (ABC) immunostaining for endothelial cells cultured from umbilical vein. Subpopulations of NPY- and ANP-immunoreactive endothelial cells were present in term umbilical vein and artery. The umbilical vein contained more positive cells than the artery. The percentage of NPY- and ANP-immunoreactive umbilical vein cells in culture was 32% and 44%, respectively, out of a total of 3013 cells examined. The possibility that these potent vasoactive substances located in the endothelial cells of the non-innervated umbilical vessels are involved in the local regulation of blood flow is discussed.

Human umbilical vessels are devoid of nerves and therefore endothelial cells may play an important role in the control of feto-placental blood flow. The pharmacological effects of 5-hydroxytryptamine, histamine and endothelin were examined in umbilical arteries and veins from legal terminations (gestational age 8-17 weeks, n=12) and normal term vaginal deliveries (gestational age 38-41, n=12). Immunocytochemistry of human unbilical vessels indicated that 5-hydroxytryptamine, histamine and endothelin were localised in subpopulations of endothelial cells of both artery and vein in late, but not early, pregnancy. 5-Hydroxytryptamine (10 nM-30 microM) caused sustained concentration-dependent contractions in all vessels from early and late pregnancy. Histamine (0.1 microM-30 mM) also caused sustained contractions in all vessels from late pregnancy but only 27% of arteries and 41% of veins from early pregnancy responded. Endothelin (10 pM-30 nM) caused slow long-lasting contractions in all vessels from early and late pregnancy. Atrial natriuretic peptide and neuropeptide Y did not alter vascular tone. The endothelium may thus play an autocrine/paracrine role, by synthesizing and releasing the above reactive substances in late pregnancy to influence feto-placental blood flow.

Endothelial cells of human umbilical vein and artery, both in situ and in culture, were examined ultrastructurally and at the light-microscope level using either the pre-embedding peroxidase-antiperoxidase or avidin-biotin peroxidase complex immunostaining techniques. Vasoactive intestinal polypeptide (VIP), substance P (SP), calcitonin gene-related peptide (CGRP) and arginine-vasopressin (AVP) immunoreactivity were localized in subpopulations of endothelial cells of the term umbilical vein and artery. The percentage of VIP-, SP-, CGRP- and AVP-immunoreactive cells in the umbilical vein was 12, 10, 11, and 7.5%, respectively, out of a total of 5,364 cells (from 15 umbilical cords) examined. The artery contained fewer VIP-, SP-, and CGRP-immunoreactive cells, but more AVP-immunoreactive cells, than the vein. In conclusion, subpopulations of endothelial cells in the human umbilical vein and artery contain the neuropeptides VIP, SP, CGRP and AVP, although their physiological roles are not yet known.