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Kalkman HO. 
“Characterization of serotonin (5-HT) receptors in the isolated guinea-pig ileum”. 
Naunyn-Schmiedeberg's Arch. Pharmacol.. 1985;329(Suppl R82).
2-{125I}LSD labels a binding site in membranes of guinea-pig ileum that is different from the 5-HT2 binding site (labelled with the same ligand) in rat brain membranes. Under appropriate conditions 5-HT can either relax or contract the isolated guinea-pig ileum. Contraction is best measured on strips of longitudinal muscle. Relaxation can be quantified using the ileum with intact mucosa,, precontracted with 1 mcM histamine. The antagnoistic potentcy (pA2-values) of 8 5-HT antagnoists with respect to inhibition of relaxation, was assessed from Schild-plots. Contraction induced via activation of 5-HT-D receptors, was obtained after desensitization of 5-HT-M receptors. Inhibition of responses to 1 mcM 5-HT was determined for 13 antagonists (pIC50). The antagonistic potencies obtained in both pharmacological tests were compared with pKD-values derived from binding assays in ileum and brain. pD2-values of 5-HT and 11 analogues were determined for contraction of the rat aorta in the presence of 1 mcM prazosin (a 5-HT2 model) and guinea-pig ileum relaxation. No correlation was found (r=0.20). These results suggest that the 5-HT-D receptor mediating contraction is a 5-HT2 receptor. The receptor mediating relaxation is not a 5-HT2 receptor and may be identified in a binding assay with 2-[125I]LSD on membranes of guinea-pig ileum.
Notes # : Abstract 327
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