Cardiac P2X ₄ Receptors

Abstract

Purinergic receptors have attracted growing interest as therapeutic targets. This perspective focuses on P2X 4 receptors as a new cardioprotective target in heart failure.P urinergic 1 receptors were initially conceived to describe the likely existence of membrane receptors responding to adenosine triphosphate (ATP) (P2 receptors) or its breakdown product adenosine (P1 receptors) in causing nonadrenergic, noncholinergic relaxation of gut smooth muscle. 1 Cloning of these receptors has yielded insights into their properties and signaling mechanisms, allowing classification into subfamilies of metabotropic (P2Y and P1) and ionotropic (P2X) receptors based on structure. 2 These receptors are now associated with a number of important roles in normal and pathophysiology. 3 Adenosine has been used extensively as a vasodilator and an antiarrhythmic, and efforts to target each adenosine receptor subtype as novel therapies are ongoing. 4 In this Perspective, we will focus our attention on P2 receptors in the heart (see Figure 1 and the Table for basic characteristics and known cardiac distribution) and more specifically on novel aspects of the P2X 4 receptor. Potential Role of P2X 4 Receptors in Response to Extracellular ATPIt has been long known that extracellular ATP can cause a modest increase in contractility and cytosolic calcium in cardiomyocytes. In adult ventricular myocytes from healthy wild-type mice, agonist 2-methylthioadenosine-5Ј-otriphosphate (2-mesATP) caused an increase in a nonselective cation current that was insensitive to antagonism by suramin or pyridoxalphosphate-6-azophenyl-2Ј,4Ј-disulphonic acid (PPADS). 5 The reversal potential of this current was similar to that of the cloned P2X 4 receptor. 6 The P2X 4 receptor, structurally similar to others in the P2X family, have EC50s for ATP similar to P2X 2,5,6 , higher than P2X 1,3, and approximately 1 magnitude lower than P2X 7 . Its unique properties include relative insensitivity suramin and PPADS and selective potentiation by ivermectin, permitting distinction from other P2X receptors. Moreover, 2-meSATP induced a greater increase of contractility in cardiomyocytes with cardiacspecific overexpression of the human P2X 4 receptor than in wild-type myocytes. 7 These results indicated that the P2X 4 receptor has a physiological effect in native murine myocytes, and the signal can be augmented with transgenic overexpression. Interestingly, these transgenic animals exhibit a modest increase in basal cardiac contraction without cardiac hypertrophy or failure. Enhanced in vivo contractility is not associated with an increased basal contractility of isolated myocytes, consistent with dissipation of endogenous extracellular ATP in isolated myocytes. This further supports the idea that extracellular ATP activates overexpressed P2X 4 receptor to achieve an enhanced in vivo contractile state. 7 Further characterization of 2-mesATP effect on P2X 4 -overexpressing cardiomyocytes showed that the enhanced myocyte contractile response was not accompanied by any chang...

Novel Protective Role of Endogenous Cardiac Myocyte P2X4 Receptors in Heart Failure

“…Possible candidates include cyclic GMP and protein kinase G or cellular protein nitrosylation. 5,30 Our study also has implications for the development of P2X4R antagonist as a new therapy to treat pain. 31 Because the endogenous cardiac myocyte P2X4R is cardioprotective, its antagonism during pain control may be deleterious to those individuals under cardiac stress, including HF.…”

“…For immunoprecipitation, we used mouse monoclonal eNOS (BD Bioscience; 1–3 μg) and rabbit polyclonal P2X4 (Alomone; 2 μg). L-N 5 -(1-iminoethyl) ornithine hydrochloride (L-NIO; Sigma-Aldrich) was administered via intraperitonel route at 40 mg/kg per day 20 beginning at 3 days before coronary ligation or TAC until cardiac function determinations at 7 days postsurgery.…”

“…Mice with cardiac-specific P2X4R overexpression had increased S-nitrosylation, cyclic GMP, NO formation, and were protected from postinfarct and pressure overload HF. Inhibitor of eNOS, L-N 5 -(1-iminoethyl)ornithine hydrochloride, blocked the salutary effect of cardiac P2X4R overexpression in postinfarct and pressure overload HF as did eNOS knockout. …”

“…Increasingly, these receptors are understood to communicate via purinergic signaling, 1,2 with evidence that extracellular ATP acting via its P2 purinergic receptors mediates a growing number of biological functions. 3–5 In adult ventricular myocytes from healthy wild-type (WT) mice, agonist 2-methylthioadenosine-5′-triphosphate (2-mesATP) causes an increase in a nonselective cation current partially insensitive to antagonism by suramin and pyridoxalphosphate-6-azophenyl-2′,4′-disulphonic acid, characteristic of P2X4 receptors (P2X4R). 6 This current’s reversal potential is similar to the cloned P2X4R.…”

Cardiac purinergic signalling in health and disease

“…Transgenic overexpression of P2X4R in mouse heart led to enhanced cardiac contractile performance after ischaemic infarction and increased survival at 1 and 2 months after infarction; this suggested that enhanced contractile function via P2X4R of the non-infarcted areas was likely to be a rescuing mechanism [606]. A recent review discusses P2X4R as targets for cardiac ischaemia [607].…”