5-HYDROXYTRYPTAMINE-INDUCED CONTRACTION OF THE MARMOSET AORTA IS MEDIATED BY A 5-HT1-LIKE RECEPTOR

1. 5‐Hydroxytryptamine (5‐HT) exerts both contractile and relaxant effects in the marmoset isolated aorta, actions that are unaffected by the 5‐HT2 antagonist ketanserin. The aim of the present study was to define the receptors mediating the contractile activity of 5‐HT in the marmoset aorta. 2. Contractile responses were elicited in aortic rings that were either: (i) precontracted submaximally with the thromboxane A2 agonist U44069 in order to amplify the responses; or (ii) exposed to Nω‐nitro‐L‐arginine (100 μmol/L) plus LY 53857 (0.1 μmol/L; a 5‐HT2 receptor antagonist shown previously to inhibit relaxation). The effect of 5‐HT on adenosine 3′,5′‐cyclic monophosphate (cAMP) formation was also investigated. 3. The effects of agonists and antagonists comprised: (i) agonist potencies in the order 5‐carboxamidotryptamine > 5‐HT > sumatriptan > 8‐hydroxy‐2‐(di‐n‐propylamino)tetralin; (ii) inhibition of contractile action of 5‐HT by the 5‐HT1D antagonist GR 127935; (iii) a contractile response to methysergide; (iv) a lack of effect of tropisetron, an antagonist of 5‐HT3 and 5‐HT4 receptors; and (v) inhibition of forskolin‐stimulated cAMP formation by 5‐HT (in the presence of LY 53857), indicative of negative coupling to adenylate cyclase. 4. The above effects fulfil the criteria for a 5‐HT1‐like receptor. In view of the previous finding that this contractile response is insensitive to ketanserin, it is concluded that the contractile effects of 5‐HT in the marmoset aorta are mediated exclusively by a 5‐HT1‐like receptor.