Sequential expression of three receptor subtypes for extracellular ATP in developing rat skeletal muscle

In this study, we investigated the expression of the P2X receptor subtypes (P2X(1-7)) during the development of skeletal muscle and in relation to acetylcholine receptors in the rat embryo and pup. By using immunohistochemistry, we showed that three receptor subtypes, P2X(2), P2X(5), and P2X(6), were expressed in developing skeletal muscle. The timing and pattern of receptor expression seemed to be precisely regulated. P2X(2), P2X(5), and P2X(6) were expressed in a sequential manner, which was consistent for all regional muscles tested (intercostal, paravertebral, and lower limb): P2X(5) expression appeared first (E15-E18) followed by P2X(6) (E16-E18), and finally P2X(2) (E18-adult). At no developmental stage did we observe colocalization of P2X(2) and acetylcholine receptors. In the case of P2X(2) and P2X(6), immunoreactivity was found to be widespread, immunopositive cells being apparent throughout the muscle. However, staining for P2X(5), both at the beginning and end of expression, was restricted to regions of muscle close to the myotendinous junctions. Because the timing of receptor expression is closely related to key events in skeletal muscle development, notably the generation of secondary myotubes and the redistribution of acetylcholine receptors, it is possible that ATP-signaling by means of P2X receptors could be involved in these processes.