5-Hydroxytryptamine (5-HT) mediates potent relaxation in the sheep isolated pulmonary vein via activation of 5-HT4 receptors.

1. We investigated the potent 5-hydroxytryptamine (5-HT)-mediated vasorelaxation of the sheep pulmonary vein. Here we present evidence that this response is due to activation of 5-HT4 receptors. 2. 5-HT (1-1000 nM) caused concentration-dependent, maintained relaxations (pEC50 = 8.4 +/- 0.1) of isolated rings of sheep pulmonary vein pre-contracted with endothelin-1 (3 nM). 3. The relaxation response to 5-HT was unaffected by either removal of the endothelium or by inhibition of NO-synthase by NG-nitro-L-arginine (100 microM). 4. Ketanserin, methiothepin, methysergide and MDL 72222 at concentrations that selectively block 5-HT2, 5-HT1-like and 5-HT3 receptors respectively, had no effect on the concentration-relaxation curve to 5-HT. 5. ICS 205-930 (1-10 microM) competitively antagonized the concentration-relaxation curve to 5-HT with a pA2 of approximately 6.7. 6. Increasing the concentration of ICS 205-930 from 10 to 30 microM did not cause a further rightward shift of the 5-HT concentration-relaxation curve. The pEC50 of 6.50 for 5-HT in the presence of ICS 205-930 (30 microM) was taken as an estimate of the affinity of 5-HT for 5-HT1-like receptors since methiothepin (10 nM) unmasked further competitive inhibition of 5-HT in the presence of this concentration of ICS 205-930. 7. Other 5-HT agonists including 5-carboxamidotryptamine (5-CT), alpha-methyl-5-HT and BIMU 8 (but not 2-methyl-5-HT) also relaxed the pulmonary vein.The response to 5-CT was inhibited by methiothepin (10 nM) and methysergide (100 nM) but unaffected by ICS 205-930 (30 microM), whilst that toa-methyl-5-HT and BIMU 8 was unaffected by methiothepin (10 nM) but blocked by ICS 205-930(estimated pKB values of 6.4 and 6.9 respectively). Relaxation curves to both 5-HT and BIMU 8 were unaffected by cocaine (6 microM).8. In conclusion, these results indicate that the sheep pulmonary vein possesses 5-HT4 receptors that mediate potent endothelium-independent relaxation. 5-HT,-like relaxant receptors are also present in this tissue but 5-HT has a lower affinity at these receptors. This preparation may thus provide a robust and sensitive bioassay for future development of selective 5-HT4 receptor agonists and antagonists.