Comparative analysis of two types of 5-hydroxytryptamine receptor mediating vasorelaxation: differential classification using tryptamines

Abstract

Summary Two receptors mediating relaxant responses to 5-hydroxytryptamine (5-HT) were studied comparatively in rings of rabbit jugular vein contracted with U-46619 (10 nmol/l). At low concentrations of 5-HT (0.001–0.1 μmol/l) vascular relaxation was mediated indirectly by the endothelial 5-HT receptor previously described by Leff et al. (1987). In preparations denuded of endothelium, higher concentrations of the amine (0.03–30 μmol/l) caused relaxation responses directly, presumably via a receptor located on the smooth muscle cells. Similarity between the receptors was evident in that both were susceptible to antagonism by methysergide, but resistant to blockade by ketanserin and MDL 72222. In these respects, the receptors qualified for a ‘5-HT₁-like’ classification. Consistent with this, 5-carboxamidotryptamine demonstrated a higher agonist potency than 5-HT at the receptor mediating relaxation directly. However, in endothelium-intact jugular vein rings this potency order was reversed, showing that the endothelial 5-HT receptor did not satisfy completely the criteria for a ‘5-HT₁-like’ designation. When the activities of a single set of tryptamines were compared in endothelium-intact and -denuded jugular vein rings, different affinity and relative efficacy estimates were obtained, confirming that two distinct 5-HT receptors mediate relaxation responses in this tissue. The most striking difference between these two receptor types was demonstrated by α-methyl-5-HT since it expressed a high affinity comparable to 5-HT at the endothelial receptor, but was inactive at the receptor in endothelium-denuded preparations at concentrations up to 30 μmol/l. In antagonist studies, the butyrophenone spiperone (1 μol/l) likewise distinguished between these two receptor types, since it failed to block endothelium-dependent relaxations, but caused an 8-fold parallel, rightward displacement of the 5-HT concentration-effect curve obtained in endothelium-denuded preparations. The possibility that one or both of these receptors might mediate vasodepressor responses to tryptamines in vivo was investigated in pentobarbitone-anaesthetised rats treated with ketanserin. The relative hypotensive potencies of the tryptamines showed a close agreement with their relative affinities for the receptor type mediating directly vascular relaxation, but not with their relative affinities for the receptor type located on the vascular endothelium. In agreement with this, spiperone (1 mg/kg, i. v.) antagonised the hypotensive response to 5-carboxamidotryptamine, causing a 10-fold right-shift of the dose-effect curve. These results further support the utility of tryptamines as probes for 5-HT receptor classification and, in addition, suggest they can be used for the differential classification of receptors in vivo as well as in vitro.