Characterization and expression of ATP P2X4 receptor from embryonic chick skeletal muscle

Previous pharmacological experiments have indicated the existence of ATP P2X receptors in chick embryonic skeletal muscles. In this study we cloned a P2X₄‐like cDNA encoding a protein of 385 amino acids, which shares 75% and 76% identity with rat and human P2X₄ receptors, respectively. Functional studies of this cP2X₄ receptor expressed in Xenopus oocytes showed that ATP induced a fast inward current, which was partially desensitized upon prolonged application of ATP. The ATP‐induced currents were concentration‐dependent, with an EC₅₀ of 9.5 μM. Adenosine 5′‐O‐(thio)triphosphate and 2‐methylthioATP very weak agonists. α,β‐methyleneATP was almost inactive. In contrast to their potentiating effects on recombinant rat P2X₄ receptors, both suramin and pyridoxalphosphate‐6‐azophenyl‐2′,4′‐disulfonic acid partially blocked ATP‐induced currents. TrinitrophenylATP was able to block ATP‐induced response completely, with an IC₅₀ of 4.7 μM. Northern blot and RT‐PCR analysis showed that cP2X₄ mRNAs were mainly expressed in 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 cP2X₄ mRNAs were expressed in the brain, spinal cord, notochord, gizzard, and skeletal muscle. The physiological functions of cP2X₄ receptors in embryonic skeletal muscle remain unclear at present. Drug Dev. Res. 53:22–28, 2001. © 2001 Wiley‐Liss, Inc.