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 (I_p) and NCX current (I_NCX) 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 I_pfrom a basal level of 0.52 ± 0.02 to 0.58 ± 0.03 pA/pF. 2-meSATP also increased the Ca²⁺ entry mode of I_NCX (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 I_NCX 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. KB-R7943 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 I_NCXparticipates in P2X agonist-stimulated contractions. In ventricular myocytes from wild-type mice, the P2X agonist could increase I_NCX, 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.