Characterization and expression of ATP P2X(4) receptor from embryonic chick skeletal muscle

Previous pharmacological experiments have indicated the existence of ATP P2 X receptors in chick embryonic skeletal muscles. In this study we cloned a P2X(4)-like cDNA encoding a protein of 385 amino acids, which shares 75% an d 76% identity with rat and human P2X(4) receptors, respectively. Functiona l studies of this cP2X(4) receptor expressed in Xenopus oocytes showed that ATP induced a fast inward current, which was partially desensitized upon p rolonged application of ATP The ATP-induced currents were concentration-dep endent, with an EC50 of 9.5 muM. Adenosine 5 ' -O-(thio)triphosphate and 2- methylthioATP very weak agonists. alpha,beta -methyleneATP was almost inact ive. In contrast to their potentiating effects on recombinant rat P2X(4) re ceptors, both suramin and pyridoxalphosphate-6-azophenyl-2 ' ,4 ' -disulfon ic acid partially blocked ATP-induced currents. TrinitrophenylATP was able to block ATP-induced response completely, with an IC50 of 4.7 muM. Northern blot and RT-PCR analysis showed that cP2X(4) mRNAs were mainly expressed i n skeletal muscle, brain, and gizzard of day 10 chick embryos. Lower levels of expression were also detected in liver, heart, and retina. Whole-mount in situ hybridization showed that cP2X(4) mRNAs were expressed in the brain , spinal cord, notochord, gizzard, and skeletal muscle. The physiological f unctions of cP2X(4) receptors in embryonic skeletal muscle remain unclear a t present. (C) 2001 Wiley-Liss, Inc.