Functional Characterization of Serotonin Receptors in Rat Isolated Aorta.

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Interactions of serotonin (5-HT) with different specific 5-HT receptors that can coexist in the same blood vessel sometimes generate opposite effects. The aim of this study was to characterize the functional role of previously described mRNAs for 5-HT receptors in the rat aorta (5-HT2A, 5-HT2B, 5-HT1B, 5-HT7) as well as to study the known 5-HT2A receptor-mediated constrictor response and investigate the influences of endothelium and preconstriction on the tissue in that response. A slight endothelium- and concentration-dependent relaxant effect was observed for 5-HT in aorta precontracted with either 5 μM phenylephrine (PE) or 1 μM prostaglandin F2α (PGF2α) in the presence of 0.3 μM ketanserin. EC50 values for 5-HT and α-methyl-5-HT relaxant responses after PE were 43.10±4.00 and 57.11±8.01 nM, respectively. pKB values for antagonists cyproheptadine and rauwolscine were 8.92±0.22 and 7.15±0.12, respectively. In nonprecontracted tissues, the contractile potency of 5-HT was higher in the absence of endothelium (EC50, μM): 2.60±0.28 and 4.12±0.21 in the absence and in the presence of endothelium, respectively. The differences were statistically significant (p<0.05). In precontracted tissues, the differences in EC50 values (2.22±0.40 and 4.65±0.60 μM without and with endothelium, respectively) were also statistically significant (p<0.05). pKB values for the 5-HT2A antagonist ketanserin were similar under all conditions tested. In conclusion, under our experimental conditions there are two functional 5-HT receptors in rat aorta: 5-HT2A contractile receptor in smooth muscle and a high-affinity relaxant receptor that mediates a very slight response and the pharmacology of which could be compatible with an endothelial 5-HT2B receptor.

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