Influence of the endothelium and nitric oxide on the contractile responses evoked by 5‐HT 1D receptor agonists in the rabbit isolated saphenous vein

1 We investigated whether contractile responses evoked by 5‐HT 1D receptor agonists were influenced by the endothelium (E) and nitric oxide (NO) in the rabbit isolated saphenous vein. 2 Saphenous vein rings were set up for isometric tension recording in oxygenated (5% CO 2 in O 2 ) Krebs solution (pH 7.4) containing (10 −6 m ): idazoxan (1), indomethacin (10), ketanserin (0.1), prazosin (10), and N ω nitro‐L‐arginine methyl ester (L‐NAME; 0 or 10), a NO synthase inhibitor. In some experiments, the E was removed mechanically. 3 5‐Hydroxytryptamine (5‐HT), 5‐carboxamidotryptamine (5‐CT) and sumatriptan (Sum) contracted rabbit saphenous vein rings in the potency order (pD 2 range) of 5‐CT(7.2–7.6) > 5‐HT(6.2–7.1) > Sum(5.0–5.8), irrespective of the presence or absence of the E or L‐NAME ( n = 9–37 per group) indicating that the potencies of the 3 agonists were not significantly affected by either the E or L‐NAME. 4 Efficacy, as assessed by the maximal contractile response (E max ), was significantly greater for Sum compared to 5‐HT and 5‐CT with intact E irrespective of the presence (77 ± 3, 62 ± 3, and 50 ± 3 mN respectively; P < 0.05 Sum versus 5‐HT and 5‐CT) or absence (26 ± 3, 14 ± 4, and 13 ± 2 mN respectively; P < 0.05 Sum versus 5‐HT and 5‐CT) of L‐NAME. In E‐denuded rings, the E max values were all higher than in E‐intact rings and did not differ between the 3 agonists (36 ± 4, 37 ± 4, and 36 ± 5 mN for Sum, 5‐HT and 5‐CT, respectively; P > 0.5 between the 3 agonists) indicating that an endothelium‐derived relaxing factor (EDRF) counteracted the constrictor activities of the 5‐HT 1D receptor agonists and raising the possibility that a component of the Sum‐induced contractile responses was E‐dependent. Without E, the presence of L‐NAME did not significantly affect the E max values of the 3 agonists (41 ± 4, 41 ± 5, and 41 ± 4 mN for Sum, 5‐HT, and 5‐CT respectively; P > 0.5 between the 3 agonists) indicating that the NO synthase inhibited was of endothelial origin. 5 Potentiation of the E max of the 3 agonists by L‐NAME was significantly albeit partially reversed by L‐arginine (10 −2 m ) indicating that NO synthase was indeed inhibited by L‐NAME. Furthermore, in the presence of E, potentiation of E max of the 3 agonists by L‐NAME was mimicked by methylene blue (10 −5 m ) providing further evidence that NO was involved in the attenuation by the E of the contractile responses induced by the 5‐HT 1D receptor agonists. 6 In the presence of an intact E and L‐NAME, contractile responses elicited by 5‐HT and Sum were competitively antagonized by the non‐selective 5‐HT 1D receptor antagonist, methiothepin (pA 2 : 9.4 and 8.8; slopes: 0.66 and 0.81, respectively) and the highly selective 5‐HT 1D receptor antagonist, GR 127935 (pA 2 : 9.0 in each case; slopes: 1.04 and 0.93, respectively) indicating that contractions were mediated through activation of a single population of 5‐HT 1D receptors. Contractile responses elicited by 5‐CT were also competitively antagonized by methiothepin and GR 127935, but non parallel rightward shifts of the concentration‐response curves were observed suggestive of the involvement of additional but as yet unidentified receptors in mediating the 5‐CT‐induced responses. 7 In conclusion, the efficacy, but not the potency, of 5‐HT, 5‐CT and Sum in evoking 5‐HT 1D receptor‐mediated contractile responses are subject to a substantial inhibitory influence of the E and of an EDRF (probably NO).