Evidence that 5-HT2 receptors predominantly mediate the contraction of the rat basilar artery to 5-hydroxytryptamine.

We characterised the 5-hydroxytryptamine (5-HT) receptor subtype which mediates the contraction of the rat isolated basilar artery, mounted in a myograph, by using agonists and antagonists for 5-HT1-like, 5-HT2, 5-HT3 and 5-HT4 receptors. The rank order of potency for a range of selective agonists was: 5-HT > alpha-methyl-5-HT > 5-carboxamidotryptamine > 2-methyl-5-HT > sumatriptan. The maximum contractions for these agonists (Emax) was less than for 5-HT while 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT) was devoid of contractile activity. Ketanserin antagonised the contractile effect of 5-HT in the rat basilar artery with a provisional pA2 estimate of 9.3 +/- 0.2. A similar antagonism was observed against alpha-methyl-5-HT. Ergometrine (0.01-1 microM) was devoid of any agonist activity but antagonised the contractile effect of 5-HT on the rat basilar artery with a provisional pKB of 8.7 (7.8-9.8, 95% confidence limits). The 5-HT3 and 5-HT4 receptor antagonist ICS 205930 (10 microM) did not alter the response to 5-HT. The high potency and efficacy of alpha-methyl-5-HT, poor effect of sumatriptan and antagonism of 5-HT by ergometrine and ketanserin, support the conclusion that the 5-HT2 receptors are primarily responsible for the 5-HT-induced contraction of rat basilar artery.