A pharmacological comparison of the receptors mediating contractile responses to 5-hydroxytryptamine in the rat isolated caudal artery and fundic strip.

Contractile responses to 5-hydroxytryptamine (5-HT) and to a number of 5-HT-receptor agonists have been compared on the rat isolated caudal artery and stomach fundic strip. On the caudal artery, 5-HT was the most potent agonist tested. The 5-HT 1-like agonist, 5-carboxamidotryptamine (5-CT), was less potent than 5-HT and produced a lower maximum response. 8-Hydroxy-2-(di-n-propylamino)-tetralin (8-OH-DPAT) and RU24969 (5-methoxy-3(1,2,3,6-tetrahydropyridin-4-yl)1Hindole) were inactive as agonists and 8-OH-DPAT was not an antagonist. Ketanserin, ICI 169,369 (2-(2-dimethylaminoethylthio)-3-phenylquinoline hydrochloride) and ICI 170,809 (2-(2-dimethylamino-2-methylpropylthio)-3- phenylquinoline hydrochloride) were competitive antagonists of 5-HT on this preparation, indicating that 5-HT is acting via 5-HT2 receptors. In contrast, all the agonists produced contractions of the fundic strip (rank order of potency, 5-HT = 5-CT > RU24969 > 8-OH-DPAT). The maximum response to RU24969 was significantly lower than the maximum responses to the other agonists. Ketanserin was only a weak antagonist of 5-HT in the fundic strip, demonstrating that 5-HT2 receptors were not involved, but ICI 169,369 and ICI 170,809 were non-surmountable antagonists of 5-HT responses, as were methysergide and methiothepin. Since ICI 169,369 and ICI 170,809 are devoid of activity at 5-HT3 and 5-HT4 receptors, then these two subtypes would not appear to be implicated, a view that was confirmed in the case of 5-HT3 receptors by experiments using ondansetron.(ABSTRACT TRUNCATED AT 250 WORDS)