Guinea-pig sympathetic neurons express varying proportions of two distinctP2X receptors

Characterization of P2X receptors on neurons of guinea-pig superior cervica l ganglion (SCG) has been carried out using a whole-cell voltage-clamp tech nique. Application of ATP and alpha,beta-methylene ATP (alpha beta-MeATP) produced fast activating inward currents, which desensitized slowly. The maximum re sponse to alpha beta-MeATP was 36 +/- 23 % (range 0.1-100 %) of that evoked by ATP in the same cell. Co-application of alpha beta-MeATP (300 mu M) with ATP (300 mu M) produced a response that was 97 +/- 1% of that given by ATP alone. Following desensi tization with alpha beta-MeATP, the decrease in response to ATP was equal t o the absolute reduction in response to alpha beta-MeATP in the same cell. The concentration-response curve for alpha beta-MeATP had an EC50 of 42 mu M and a Hill coefficient of 1.17. For cells where the ratio of alpha beta-M eATP/ATP currents at 100 mu M was < 0.1, the ATP concentration-response cur ve had an EC50 of 56 mu M and a Hill coefficient of 1.95. However, in cells where the ratio was > 0.7, the curve had an EC50 of 60 mu M and a Hill coe fficient of 0.97. The response to 100 mu M alpha beta-MeATP was inhibited by 2' (or 3')-O-tri nitrophenyl-ATP (TNP-ATP) with an IC50 of 70 nM. However, on cells where th e ratio of alpha beta-MeATP/ATP currents was < 0.1, ATP was inhibited by TN P-ATP with an IC50 of 522 nM. Immunohistochemical staining with antibodies raised against, rat P2X(2) and P2X(3) epitopes suggested that both subunits were expressed by guinea-pig SCG neurons. We conclude that varying proportions of two distinct P2X receptors coexist on the cell bodies of individual guinea-pig SCG neurons, which may correspo nd to homomeric P2X(2) and heteromeric P2X(2/3) receptors.