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1 Many types of culture media contain a pH-sensitive dye. One commonly occurring dye, Phenol red sodium (Na+) salt, was tested for blocking activity at rat P2X1−4 receptors (P2X1−4Rs) expressed in Xenopus oocytes. 2 Phenol red Na+-salt antagonised adenosine 5′-triphosphate (ATP) responses at P2X1R (IC50, 3 μM) and, at higher concentrations, also blocked P2X2R and P2X3R. Phenol red Na+-salt, purified of lipophilic contaminants, blocked P2X1R and P2X3R by acting as an insurmountable antagonist. 3 Two lipophilic extracts of Phenol red antagonised ATP responses at P2XRs. Extract A was a potent antagonist at P2X1R (IC50, 1.4 μM), whereas extract B was a potent antagonist at P2X3R (IC50, 4.1 μM). A bisphenolic compound (RS151030) found in these extracts was a potent antagonist at P2X1R (IC50, 0.3 μM) and at P2X3R (IC50, 2.4 μM). 4 Phenolphthalein base was a potent irreversible antagonist at P2X1R (IC50, 1 μM), whereas Phenolphthalein K+-salt was 25-fold less potent here. 5 Phenolphthalein base was a reversible antagonist of ATP responses at rat P2X4R (IC50, 26 μM), whereas Phenolphthalein K+-salt was inactive. 6 Dimethyl sulphoxide (DMSO), used to dissolve lipophilic extracts, showed pharmacological activity by itself at rat P2X1R and P2X4R. 7 Thus, Phenol red and related compounds are antagonists at rat P2X1R, but are also active at other rat P2XRs. Phenolphthalein base is a newly identified, low potency antagonist of ATP responses at P2X4R. Culture media containing these red dyes should be used cautiously in future pharmacological studies of P2XRs. Also, wherever possible, the solvent DMSO should be used with caution. British Journal of Pharmacology (2005) 145, 313–322. doi:10.1038/sj.bjp.0706187

1. The effect of adenosine A1-receptor and P2-purinoceptor agonists on [3H]-inositol phosphate accumulation has been investigated in CHO-K1 cells transfected with the human adenosine A1-receptor. 2. Adenosine receptor agonists stimulated [3H]-inositol phosphate accumulation in CHO-K1 cells with a rank potency order of N6-cyclopentyladenosine (CPA) > 5'-N-ethylcarboxamidoadenosine (NECA) > 2-chloroadenosine > N6-2-(4-aminophenyl) ethyladenosine (APNEA). The responses to both CPA and APNEA were antagonized by the A1 selective antagonist, 1,3-dipropylcyclopentylxanthine (DPCPX) yielding KD values of 1.2 nM and 4.3 nM respectively. 3. ATP, UTP and ATP gamma S were also able to stimulate [3H]-inositol phosphate accumulation in these cells with EC50 values of 1.9 microM, 1.3 microM and 5.0 microM respectively. 2-Methyl-thio-ATP was a weak agonist of this response (EC50 > 100 microM). 4. The [3H]-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 [3H]-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 [3H]-inositol phosphate responses to histamine and ATP in CHO-K1 cells transfected with the bovine histamine H1-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.