Abstract 13146: Cardiac P2X Purinergic Receptors as a New Pathway for Increasing Intracellular Na+ in Ventricular Myocytes: P2X Agonist Stimulates Na+ Pump, Na+/Ca2+ Exchange and Contraction
The P2X4 receptor (P2X4R), a purinergic receptor subtype, is a non-selective cation channel activated by extracellular ATP. We previously demonstrated that cardiac-specific transgenic overexpression of this channel (Tg) can enhance contractility and protect against calsequestrin overexpression and post-infarct heart failure. Using Tg myocytes as a model, we tested the hypothesis that P2X4R-induced contractility increase is due to enhanced Na+ entry via the receptor, which subsequently modulates the currents of Na+/Ca2+ exchanger (NCX) and Na+-K+ ATPase. Similar to previous results, activation of cardiac P2X4R by 3 μM 2me-SATP induced an inward current at -80mV in 21 of 32 Tg myocytes (1.34±0.25pA/pF). In these myocytes, K+-activated Na+ pump current (Ip) or Ni+-sensitive NCX current (INCX) was tested further with whole cell patch clamp. Ip peak increased from 0.52 ±0.02 pA/pF at control to 0.58±0.03 pA/pF after 3 minutes of 2me-SATP superperfusion (P<0.05, N=11), suggesting that P2X4R-induced increase of intracellular Na+ exited the myocytes via the Na+ pump. 2me-SATP also caused a significant increase of reverse mode INCX (0.55±0.09 pA/pF vs 0.82±0.14pA/pF at 50mV, P<0.05, N=10), while showing minimal effect on forward mode INCX. The reversal potential of INCX shifted from -14±2.3mV under control to -25±4.1mV in response to 2me-SATP; this shift is estimated to require a net increase of 1.28±0.42 mM Na+ that is in turn sensed by the NCX. KB-R7943, a selective inhibitor of reverse mode INCX, partially inhibited 2me-SATP-induced increase of cell shortening. 3 μM 2me-SATP alone induced a 29.3±3.7% increase of cell shortening in 16 out of 23 Tg myocytes. Subsequent co-treatment with 5 μM KB-R7943 in these cells led to a smaller increase of contraction (13.8±3.9% increase of cell shortening above basal level, P<0.05 paired t-test), suggesting a significant contribution from reverse mode INCX to P2X4R-induced contractility increase. Our results confirm that P2X4R can increase intracellular Na+, which subsequently stimulates Ip and INCX. The increase of reverse mode INCX contributes to enhanced contractions in Tg myocytes. Exit of the P2X4R-mediated intracellular Na+ increase via the Na+-K+ ATPase can help maintain intracellular Na+ homoeostasis.
- © 2013 by American Heart Association, Inc.