5-HT receptors mediating contraction in the rat-tail artery

We studied the effects of 5-HT in resistance vessels. Compounds acting on 5-HT2A and 5-HT1B receptors were tested on rat-tail arterial rings in varying experimental conditions. We tested 5-HT from 50 nM to 100 microM: the pD2 was 6.4 +/- 0.1. We evaluated vasoconstriction by 5-HT in tissues slightly depolarized with 30 mM KCl. In this condition, the 5-HT concentration-related contraction started at lower concentration in comparison to control tissues. Preincubation with 50 nM ketanserin, a 5-HT2A-antagonist, and 1 microM prazosin, an alpha1-antagonist, strongly inhibited concentration-related contraction by 5-HT: the pD2 was 3.2 +/- 0.2. Moreover, we experimented alpha-methyl-5-HT (alpha-me-5-HT) and 5-carboxamidotryptamine (5-CT), selective agonists at 5-HT2A and 5-HT1B receptors, respectively. Both agonists induced concentration-related contraction; the potency order observed was 5-HT > alpha-me-5-HT > 5-CT. Finally, we studied SB 224289, a selective 5-HT1B-antagonist, on contraction by 5-HT in control and in depolarized conditions. 0.2 microM SB 224289 significantly inhibited vasoconstriction induced by 5-HT in depolarized vascular tissues. The data indicate that vasoconstriction induced by 5-HT is mainly dependent on 5-HT2A receptors; however, 5-HT1B receptors are also present in rat-tail artery.