Further characterization of 5‐hydroxytryptamine receptors (putative 5‐HT2B) in rat stomach fundus longitudinal muscle

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Abstract
The present study was undertaken to isolate and characterize pharmacologically homogeneous populations of 5‐hydroxytryptamine (5‐HT) receptors from a possible mixed receptor population mediating contraction of the longitudinal muscle of rat stomach fundus. Our aim was to extend the pharmacological characterization of the 5‐HT2B receptor which is reported to be expressed in this preparation. To minimize spontaneous activity and any influence of circular muscle on the contractile response, narrow (1–1.5 × 20 mm) segments of mucosa‐denuded longitudinal muscle were used. Under these conditions, blockade of monoamine oxidase with pargyline (100 μm for 15 min) caused a leftward displacement of concentration‐effect curves for both 5‐methoxytryptamine (5‐MeO‐T) and tryptamine. Neither pargyline nor a number of uptake inhibitors affected responses to 5‐HT. In pargyline pretreated preparations, the order of potency of a number of tryptamine analogues was as follows: 5‐MeO‐T≥α‐Me‐5‐HT≥ 5‐HT> 5‐carboxamidotryptamine (5‐CT)> tryptamine>2‐Me‐5‐HT. In addition several ligands known to act as agonists at either 5‐HT2A or 5‐HT2C receptors including 1‐m‐chlorophenylpiperazine (m‐CPP), Ru 24969, MK 212 and SCH 23390 were also agonists in rat fundus whilst sumatriptan, renzapride and 8‐hydroxy‐2‐(di‐n‐propylamino) tetralin (8‐OH‐DPAT) were very weak or inactive. With the exception of 2‐Me‐5‐HT and m‐CPP, most agonists produced monophasic concentration‐effect curves consistent with an interaction at a single site. High concentrations of 2‐Me‐5‐HT evoked relaxations which were blocked by phentolamine (1 μm) suggesting an interaction with α‐adrenoceptors. m‐CPP often evoked biphasic concentration‐effect curves with a second contractile phase which was insensitive to yohimbine at concentrations higher than required for antagonism of responses to 5‐HT. LY 53857, methiothepin, methysergide, ritanserin and ICI 170809 were potent but non‐surmountable antagonists of 5‐HT in rat fundus. In contrast, several ligands behaved as surmountable antagonists with the following order of potency: rauwolscine > yohimbine = mesulergine ≥ mianserin = SB 204070 ≥ WY 26703 ≥ SB 200646 > pirenpirone ≥ renzapride. DAU 6285, granisetron, spiperone, ketanserin, phentolamine and GR 127935 did not affect responses to 5‐HT at concentrations up to 1 μm. The agonist and concentration independent profile of antagonism supported a single site interaction for both agonists and antagonists. We conclude that despite small differences concerning the enantiomeric selectivity and affinity of rauwolscine and yohimbine, the close pharmacological identity of 5‐HT receptors in rat stomach fundus and the recently cloned 5‐HT2B receptor is maintained. SB 200646, which demonstrates some selectivity for 5‐HT receptors in rat stomach fundus, should provide a useful ligand for confirmation of this view and allow discrimination of 5‐HT2B function both in vitro and in vivo.