FURTHER CHARACTERIZATION OF 5-HYDROXYTRYPTAMINE RECEPTORS (PUTATIVE 5-HT2B) IN RAT STOMACH FUNDUS LONGITUDINAL MUSCLE

1 The present study was undertaken to isolate and characterize pharmac ologically homogeneous populations of 5-hydroxytryptamine (5-HT) recep tors from a possible mixed receptor population mediating contraction o f 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. 2 To minimize spontaneou s activity and any influence of circular muscle on the contractile res ponse, narrow (1-1.5 x 20 mm) segments of mucosa-denuded longitudinal muscle were used. Under these conditions, blockade of monoamine oxidas e with pargyline (100 mu M 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 af fected responses to 5-HT. 3 In pargyline pretreated preparations, the order of potency of a number of tryptamine analogues was as follows: 5 -MeO-T greater than or equal to alpha-Me-5-HT greater than or equal to 5-HT>5-carboxamidotryptamine (5-CT)>tryptamine>2-Me-5HT. In addition several ligands known to act as agonists at either 5-HT2A or 5-HT2C re ceptors including 1-m-chlorophenylpiperazine (m-CPP), Ru 24969, MK 212 and SCH 23390 were also agonists in rat fundus whilst sumatriptan, re nzapride 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 w ith an interaction at a single site. High concentrations of 2-Me-5-HT evoked relaxations which were blocked by phentolamine (1 mu M) suggest ing an interaction with alpha-adrenoceptors. m-CPP often evoked biphas ic concentration-effect curves with a second contractile phase which w as insensitive to yohimbine at concentrations higher than required for antagonism of responses to 5-HT. 4 LY 53857, methiothepin, methysergi de, ritanserin and ICI 170809 were potent but non-surmountable antagon ists of 5-HT in rat fundus. In contrast, several ligands behaved as su rmountable antagonists with the following order of potency: rauwolscin e>yohimbine=mesulergine greater than or equal to>mianserin=SB 204070 g reater than or equal to WY 26703 greater than or equal to SB 200646>pi renpirone greater than or equal to renzapride. DAU 6285, granisetron, spiperone, ketanserin, phentolamine and GR 127935 did not affect respo nses to 5-KT at concentrations up to 1 mu M. The agonist and concentra tion independent profile of antagonism supported a single site interac tion for both agonists and antagonists. 5 We conclude that despite sma ll differences concerning the enantiomeric selectivity and affinity of rauwolscine and yohimbine, the close pharmacological identity of 5-HT receptors in rat stomach fundus and I:he recently cloned 5-HT2B recep tor is maintained. SB 200646, which demonstrates some selectivity for 5-HT receptors in rat stomach fundus, should provide a useful ligand f or confirmation of this view and allow discrimination of 5-HT2B functi on both in vitro and in vivo.