Human 5-HT1B receptor stimulated inositol phospholipid hydrolysis in CHO cells:: synergy with Gq-coupled receptors

We have previously reported that the transfected G(i)/G(o) protein-coupled human adenosine A(1) receptor(expressed at 200 fmol/mg of protein) and the endogenous 5-HT1B receptor (not detectable using radioligand binding) suppress forskolin-stimulated cyclic AMP accumulation and stimulate increases in [Ca2+](i) in Chinese hamster ovary cells (CHO). In addition, co-activation of the adenosine A(1) receptor (but not the 5-HT1B receptor) potentiates the hydrolysis of inositol phospholipids elicited by receptors coupled to G(q)-proteins (Dickenson and Hill, 1996. fur. J. Pharmacol. 320, 141-151). In order to establish whether this difference in ability to modulate G(q)-coupled receptor responses is a consequence of low 5-HT1B receptor density, we have stably transfected CHO-KI cells with the human 5-HT1D beta cDNA (the human homologue of the rodent 5-HT1B receptor). We initially isolated a clonal cell line (designated CHO5-HT1B cells) displaying moderate specific [H-3]5-HT binding (pK(d) of 8.17 +/- 0.07 and a B-max of 140 fmol/mg protein). In CHO5-HT1B cells, the selective human 5-HT1B/1D receptor agonist sumatriptan produced a concentration-dependent inhibition of forskolin-stimulated cyclic AMP accumulation (pEC(50) = 7.92 +/- 0.04). Sumatriptan also elicited a moderate and pertussis toxin-sensitive increase in [H-3]inositol phosphate formation in CHO-5HT(1B) cells (pEC(50) = 6.51 +/- 0.05). Finally, sumatriptan synergistically enhanced P-2U purinoceptor stimulated [H-3]inositol phosphate accumulation through a pertussis toxin-sensitive mechanism. These findings clearly show the significance of 5-HT1B receptor expression level in determining whether 5-HT1B receptor activation can modulate the accumulation of [H-3]inositol phosphates elicited by a G(q)-protein coupled receptor. The observation that 5-HT1B receptor activation can potentiate G(q)-coupled receptor stimulated second messenger responses may have an important physiological role in the regulation of vascular smooth muscle contraction. (C) 1998 Elsevier Science B.V. All rights reserved.