Recombinant saphenous vein 5-HT(1B) receptors of the rabbit: Comparative pharmacology with human 5-HT(1B) receptors

Abstract

1. The rabbit recombinant saphenous vein 5-hydroxytryptamine(1B) (rb 5-HT(1B)) receptor stably transfected in rat C6-glial cells was characterized by measuring adenosine 3':5'-cyclic monophosphate (cyclic AMP) formation upon exposure to various 5-HT receptor ligands. The effects of agonists and antagonists were compared with their effects determined previously at the human cloned 5-HT(1B) (h 5-HT(1B)) receptor under similar experimental conditions. 2. Intact C6-glial cells expressing rb 5-HT(1B) receptors exhibited [3H]-5-carboxamidotryptamine (5-CT) binding sites with a K(d) of 0.80 ± 0.13 nM and a B(max) between 225 to 570 fmol mg-1 protein. The binding affinities of a series of 5-HT receptor ligands determined in a membrane preparation with [3H]-5-CT or [3H]-N-[4-methoxy-3-(4-methylpiperazin-1-yl)phenyl]-3-methyl-4-(4- pyridyl)benzamide (GR 125,743) were similar. With the exception of ketanserin, ligand affinities were comparable to those determined at the cloned h 5-HT(1B) receptor site. 3. rb 5-HT(1B) receptors were negatively coupled to cyclic AMP formation upon stimulation with 5-HT agonists. Of the several 5-HT agonists tested, 5-CT was the most potent, the potency rank order being: 5-CT > 5-HT > zolmitriptan > naratriptan > rizatriptan > sumatriptan > R(+)-8-(hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT). The maximal responses of these agonists were similar to those induced by 5-HT. The potency of these agonists showed a positive correlation (r2 = 0.87; P < 0.002) with their potency at the cloned h 5-HT(1B) receptor subtype. 4. 2'-Methyl-4-(5-methyl-[1,2,4]oxadiazol-3-yl)-biphenyl-4-carboxyl acid [4-methoxy-3-(4-methyl-piperazin-1-yl)-phenyl]-amide (GR 127,935), methiothepin and ketanserin each behaved as silent, competitive antagonists at rb 5-HT(1B) receptors; pK(B) values were 8.41, 8.32 and 7.05, respectively when naratriptan was used as an agonist. These estimates accorded with their binding affinities and the potencies found on 5-HT and/or sumatriptan-mediated contraction of isolated rabbit saphenous vein segments. 5. In conclusion, the recombinant saphenous vein 5-HT(1B) receptor of the rabbit shares important pharmacological similarities with the cloned h 5-HT(1B) receptor. However, ketanserin is a more potent antagonist of rb 5-HT(1B) receptors.