The subtype of serotonin (5-HT) receptors in the rat pulmonary artery

Serotonin (5-HT) and some alpha-agonists induced a marked contraction of ring segments of the rat pulmonary artery. Phenylephrine and 5-HT caused maximum contractions in the artery amounting to 91% and 66%, respectively, of the maximum effect of noradrenaline. Clonidine failed to induce contractions in the artery. The alpha 1-adrenoceptor blocker prazosin (1 X 10(-6)M) and alpha 2-adrenoceptor blocker yohimbine (5 X 10(-7)M) did not antagonize 5-HT-induced contractions. However 5-HT2 receptor antagonists ketanserin (3 X 10(-9)-1 X 10(-7)M), mianserin (3 X 10(-9)-1 X 10(-7)M) and pizotifen (3 X 10(-9)-1 X 10(-7)M) competitively antagonized 5-HT-induced contractions in the tissue studied, whereas the antagonist methysergide (5 X 10(-10)-1 X 10(-7)M) caused a noncompetitive antagonism. The pA2 values for ketanserin (8.98), mianserin (9.1) and pizotifen (9.14) against 5-HT were not different from the values obtained in other tissues which were shown to contain 5-HT2 receptors. The results suggest that 5-HT receptors in rat main pulmonary artery are of the 5-HT2 subtype.