Inhibition of 5-carboxamidotryptamine-induced relaxation of guinea-pig ileum correlates with [125I]LSD binding.

5-Hydroxytryptamine (5-HT), 5-carboxamidotryptamine (5-CT), 8-OH-DPAT and RU 24969 relax the guinea-pig ileum precontracted with histamine. This relaxation was characterized using 5-CT as agonist with a series of 8 competitive antagonists. [125I]LSD binding was measured in a membrane preparation of the longitudinal muscle of the guinea-pig ileum in the presence of 3 X 10(-7) M cinanserin in order to suppress binding to 5-HT2 receptors. There was a significant correlation (P less than 0.01) between the antagonism of 5-CT-induced relaxation (pA2 values) and the affinity values of the antagonists for [125I]LSD binding (pKD values). It was also shown that 5-CT relaxed histamine-precontracted longitudinal muscle strips of the guinea-pig ileum. The results suggest that the 5-HT receptor-mediating relaxation in the guinea-pig ileum can be labelled with [125I]LSD and that this receptor does not belong to the 5-HT2, 5-HT1A, 5-HT1B or 5-HT1C receptor subtypes.