Relaxation of Guinea Pig Aorta Induced by 5-HT_2B Receptor. Exploration of Potential Mechanisms

Vascular relaxation induced by 5-HT_2B receptor is thought to be mediated by stimulation of endothelial nitric oxide synthase (eNOS), but this assumption still requires investigation. To investigate potential mechanisms, BW723C86 (5-HT_2B receptor agonist) was added to guinea pig aorta rings precontracted by KCl, phenylephrine (Phe) or 5-hydroxytryptamine (5-HT). The effect of N^ω-nitro-L-arginine methyl ester (L-NAME) (NOS inhibitor), charybdotoxin (potassium channels blocker) or endothelium removal on the BW723C86 response was also explored. 5-HT_2B receptor expression was evaluated through immunofluorescence and NOS expression by means of qPCR and ELISA. Stimulation of the 5-HT_2B receptor relaxed all precontracted aorta rings, and L-NAME fully abolished such relaxations, while charybdotoxin prevented the relaxation only in aortas precontracted by Phe or 5-HT. Immunolabelling confirmed a strong expression of the 5-HT_2B receptor in endothelium and much less in smooth muscle. Interestingly, BW723C86 was still able to produce relaxation on endothelium-denuded aortas precontracted by KCl or Phe but caused a potent contraction on 5-HT precontracted aortas. Because it is known that NOS activity is upregulated by KCl and Phe, but downregulated by 5-HT, we explored whether smooth muscle NOS could be responsible for these effects, but L-NAME did not prevent it. These results pointed out that mechanisms other than NOS activation are also involved in the relaxing effect of 5-HT_2B receptor stimulation.