Cardiac P2X purinergic receptors as a new pathway for increasing Na+ entry in cardiac myocytes.

P2X4 receptors (P2X4Rs) are ligand-gated ion channels capable of conducting cations such as Na+. Endogenous cardiac P2X4R can mediate ATP-activated current in adult murine cardiomyocytes. In the present study, we tested the hypothesis that cardiac P2X receptors can induce Na+ entry and modulate Na+ handling. We further determined whether P2X receptor-induced stimulation of the Na+/Ca2+ exchanger (NCX) has a role in modulating the cardiac contractile state. Changes in Na+-K+-ATPase current (Ip) and NCX current (INCX) after agonist stimulation were measured in ventricular myocytes of P2X4 transgenic mice using whole cell patch-clamp techniques. The agonist 2-methylthio-ATP (2-meSATP) increased peak Ip from a basal level of 0.52 ± 0.02 to 0.58 ± 0.03 pA/pF. 2-meSATP also increased the Ca2+ entry mode of INCX (0.55 ± 0.09 pA/pF under control conditions vs. 0.82 ± 0.14 pA/pF with 2-meSATP) at a membrane potential of +50 mV. 2-meSATP shifted the reversal potential of INCX from -14 ± 2.3 to -25 ± 4.1 mV, causing an estimated intracellular Na+ concentration increase of 1.28 ± 0.42 mM. These experimental results were closely mimicked by mathematical simulations based on previously established models. KBR7943 or a structurally different agent preferentially opposing the Ca2+ entry mode of NCX, YM-244769, could inhibit the 2-meSATP-induced increase in cell shortening in transgenic myocytes. Thus, the Ca2+ entry mode of INCX participates in P2X agonist-stimulated contractions. In ventricular myocytes from wild-type mice, the P2X agonist could increase INCX, and KB-R7943 was able to inhibit the contractile effect of endogenous P2X4Rs, indicating a physiological role of these receptors in wild-type cells. The data demonstrate a novel Na+ entry pathway through ligand-gated P2X4Rs in cardiomyocytes. [ABSTRACT FROM AUTHOR]