SYNERGY BETWEEN THE INOSITOL PHOSPHATE RESPONSES TO TRANSFECTED HUMAN ADENOSINE A(1)-RECEPTORS AND CONSTITUTIVE P-2-PURINOCEPTORS IN CHO-K1 CELLS

1 The effect of adenosine A(1)-receptor and P-2-purinoceptor agonists on [H-3]-inositol phosphate accumulation has been investigated in CHO-K1 cells transfected with the human adenosine A(1)-receptor. 2 Adenosine receptor agonists stimulated [H-3]-inositol phosphate accumulation in CHO-K1 cells with a rank potency order of N-6-cyclopentyladenosine (CPA) > 5'-N-ethylcarboxamidoadenosine (NECA) > 2-chloroadenosine > N-6-2-(4-aminophenyl) ethyladenosine (APNEA). The responses to both CPA and APNEA were antagonized by the A(1) selective antagonist, 1,3-dipropylcyclopentylxanthine (DPCPX) yielding K-D values of 1.2 nM and 4.3 nM respectively. 3 ATP, UTP and ATP gamma S were also able to stimulate [H-3]-inositol phosphate accumulation in these cells with EC(50) values of 1.9 mu M, 1.3 mu M and 5.0 mu M respectively. 2-Methyl-thio-ATP was a weak agonist of this response (EC(50) > 100 mu M). 4 The [H-3]-inositol phosphate response to CPA was completely attenuated by pertussis toxin treatment (24 h; 100 ng ml(-1)). In contrast, the responses to ATP, UTP and ATP gamma S were only reduced by circa 30% in pertussis toxin-treated cells. 5 The simultaneous addition of CPA and either ATP, UTP or ATP gamma S produced a large augmentation of [H-3]-inositol phospholipid hydrolysis. This was due to an increase in the maximal response and was significantly greater than the predicted additive response for activation of these two receptor systems. The synergy was not observed in pertussis toxin-treated cells. 6 No synergy was observed between the [H-3]-inositol phosphate responses to histamine and ATP in CHO-K1 cells transfected with the bovine histamine H-1-receptor. In these cells the response to histamine was completely resistant to inhibition by pertussis toxin treatment. 7 This study provides a clear demonstration of a synergy between pertussis toxin-sensitive and insensitive receptor systems in a model cell system which is an ideal host for transfected cDNA sequences. This model system should provide a unique opportunity to unravel the mechanisms underlying this example of receptor cross-talk involving phospholipase C.