Receptor mechanisms for 5-hydroxytryptamine in isolated human umbilical artery and vein

In order to characterize the 5-hydroxytryptamine (5-HT) receptors, the contractile effects of both 5-HT and alpha-adrenoceptor agonists were studied in helically-cut human umbilical artery and vein in the presence of different antagonists. Noradrenaline, phenylephrine and clonidine caused maximum contractions in vein amounting to only 9%, 14% and 11% of the maximum effect of 5-HT, respectively. The maximum effects of noradrenaline and phenylephrine in artery were only 18% and 29% of that of 5-HT. Clonidine failed to induce contractions in the artery. The alpha 1-adrenoceptor blocker, prazosin (1 X 10(-6) and 1 X 10(-5)M) did not antagonize 5-HT-induced contractions in the tissues studied. However, yohimbine, an alpha 2-adrenoceptor blocker, shifted the concentration-effect curves of the 5-HT to the right without any reduction in the maximum responses in both tissues. The pA2 values for yohimbine against 5-HT (6.46 and 6.72 in artery and vein, respectively) were different from the values for yohimbine against alpha 2-adrenoceptor agonists (8.2 and 8.6 against clonidine and M-7, respectively) in dog saphenous vein which contains postsynaptic alpha 2-adrenoceptors. 5-HT2 receptor antagonists, ketanserin (1 X 10(-7)-1 X 10(-6)M) and mianserin (1 X 10(-7)-1 X 10(-6)M) competitively antagonized 5-HT-induced contractions in both tissues. The pA2 values for ketanserin (7.89 and 7.86 in artery and vein, respectively) and mianserin (8.06 and 7.73 in artery and vein, respectively) were significantly lower than those obtained in other tissues which were shown to contain 5-HT2 receptors by the other authors. The results suggest that alpha-adrenoceptors play a minor role in 5-HT-induced contractions in both tissues. Yohimbine antagonized 5-HT action competitively, by blocking probably 5-HT receptors. The results also indicate that 5-HT receptors in human umbilical vessels are less sensitive to certain 5-HT2 antagonists than those of other tissues. A third subtype of 5-HT receptors different from the 5-HT1 and 5-HT2 receptors may be present in the human umbilical artery and vein.