Antagonism of ATP responses at P2X receptor subtypes by the pH indicator dye, Phenol red

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 P2X 1−4 receptors (P2X 1−4 Rs) expressed in Xenopus oocytes. 2 Phenol red Na + ‐salt antagonised adenosine 5′‐triphosphate (ATP) responses at P2X 1 R (IC 50 , 3  μ M ) and, at higher concentrations, also blocked P2X 2 R and P2X 3 R. Phenol red Na + ‐salt, purified of lipophilic contaminants, blocked P2X 1 R and P2X 3 R 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 P2X 1 R (IC 50 , 1.4  μ M ), whereas extract B was a potent antagonist at P2X 3 R (IC 50 , 4.1  μ M ). A bisphenolic compound (RS151030) found in these extracts was a potent antagonist at P2X 1 R (IC 50 , 0.3  μ M ) and at P2X 3 R (IC 50 , 2.4  μ M ). 4 Phenolphthalein base was a potent irreversible antagonist at P2X 1 R (IC 50 , 1  μ M ), whereas Phenolphthalein K + ‐salt was 25‐fold less potent here. 5 Phenolphthalein base was a reversible antagonist of ATP responses at rat P2X 4 R (IC 50 , 26  μ M ), whereas Phenolphthalein K + ‐salt was inactive. 6 Dimethyl sulphoxide (DMSO), used to dissolve lipophilic extracts, showed pharmacological activity by itself at rat P2X 1 R and P2X 4 R. 7 Thus, Phenol red and related compounds are antagonists at rat P2X 1 R, but are also active at other rat P2XRs. Phenolphthalein base is a newly identified, low potency antagonist of ATP responses at P2X 4 R. 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