Synergistic interactions between human transfected adenosine A(1) receptors and endogenous cholecystokinin receptors in CHO cells

The effect of G(i) coupled receptor activation (adenosine A(1) and 5-HT1B receptors) on cholecystokinin receptor-stimulated inositol phosphate accumulation has been investigated in Chinese hamster ovary cells transfected with the human adenosine A(1) receptor cDNA (CHO-A1). CHO cells constitutively express the 5-HT1B receptor [Berg, Clarke, Sailstad, Saltzman and Maayani (1994) Mol. Pharmacol. 46, 477-484]. Our previous studies using CHO-A1 cells have revealed that both the adenosine A(1) and 5-HT1B receptor are negatively coupled to adenylyl cyclase activity and stimulate increases in [Ca2+](i) through a pertussis toxin-sensitive pathway. In the present study the selective adenosine A(1) receptor agonist N-6-cyclopentyladenosine stimulated a pertussis toxin-sensitive increase in total [H-3]inositol phosphate accumulation. The sulphated C-terminal octapeptide of cholecystokinin (CCK-8) stimulated a robust and pertussis toxin-insensitive increase in [H-3]inositol phosphate accumulation through the activation of CCKA receptors. Co-stimulation of CHO-A1 cells with N-6-cyclopentyladenosine and CCK-8 produced a synergistic increase in [H-3]inositol phosphate accumulation. The synergistic interaction between N-6-cyclopentyladenosine and CCK-8 was abolished in pertussis toxin-treated cells. Synergy between N-6-cyclopentyladenosine and CCK-8 still occurred in the absence of extracellular calcium. The 5-HT1B receptor agonist 5-carboxyamidotryptamine did not stimulate a measurable increase in [H-3]inositol phosphate accumulation. Furthermore, 5-carboxyamidotryptamine had no significant effect on CCK-8 mediated [H-3]inositol phosphate production. Activation of endogenous P-2U receptors (G(q)/G(11) coupled) with ATP gamma S produced a significant increase in [H-3]inositol phosphate accumulation. Co-stimulation of CHO-A1 cells with ATP gamma S and CCK-8 produced additive increases in [H-3]inositol phosphate accumulation. These data indicate that CHO-A1 cells may prove a useful model system in which to investigate further the mechanisms underlying the intracellular 'cross-talk' between phospholipase C coupled receptors (G(q)/G(11) linked) and G(i)/G(o) coupled receptors.