Characterization of alpha-adrenoceptors mediating contraction in isolated ovine umbilical vein.

Responses to alpha-adrenoceptor agonists were studied in isolated umbilical vessels obtained from fetal lambs within 2 weeks of term (term = 145 days). Norepinephrine produced concentration-dependent contractions of the umbilical vein, but not in the umbilical artery. The contraction elicited by norepinephrine in the umbilical vein was not potentiated by cocaine (10(-5) M). Epinephrine also produced contractions of the umbilical vein with a potency similar to that of norepinephrine. Phenylephrine was less potent than norepinephrine while clonidine and xylazine were inactive in producing contractions of the umbilical vein. The dissociation constant (KA) of norepinephrine in the umbilical vein was 172 +/- 41 nM. There was little, if any, alpha-adrenoceptor reserve in the umbilical vein. Contraction produced to norepinephrine in the umbilical vein was effectively blocked by prazosin and phentolamine. Yohimbine was 500 times less potent than prazosin in blocking norepinephrine-induced contraction in the umbilical vein. Idazoxan only minimally antagonized the contractions elicited to norepinephrine. We conclude that alpha 1-adrenoceptors are present and mediate vasoconstriction in the ovine umbilical vein and that alpha 2-adrenoceptors are not present in this tissue.