Platelet Activating Factor Stimulates Sodium-Proton Exchange

Abstract

Sodium-hydrogen exchanger (NHE), the principal sarcolemmal acid extruder in ventricular myocytes is stimulated by a variety of autocrine/paracrine factors and contributes to myocardial injury and arrhythmias during ischemia/reperfusion (IR). Platelet-activating factor (PAF, 1-O-alkyl-2-acetyl-sn-glycero-3-phosphocholine) is a potent proinflammatory phospholipid that is released in the heart in response to oxidative stress and promotes myocardial IR injury. PAF stimulates NHE in neutrophils and platelets, but its effect on cardiac NHE (NHE1) is resolved. We utilized quiescent guinea pig ventricular myocytes bathed in icarbonate-free solutions and used epifluorescence to measure intracellular pH (pHi). Methylcarbamyl-PAF (C-PAF, 200 nM), a metabolically-stable analog of PAF, significantly increased steady-state pHi. The alkalosis was completely blocked by the NHE inhibitor, cariporide, and by sodium-free bathing solutions, indicating it was mediated by NHE activation. C-PAF also significantly increased the rate of acid xtrusion induced by intracellular acidosis. The ability of C-PAF to increase steady-state pHi was completely blocked by the PAF receptor inhibitor WEB 2086 (10 μM), indicating the PAF receptor is required. A mitogen-activated protein (MAP) kinase kinase (MEK) inhibitor (PD98059, 25μM), also completely blocked the rise in pHi induced by C-PAF, suggesting participation of the MAP kinase signaling cascade downstream of the PAF receptor. Inhibition of protein kinase C (PKC) with GF109203X (1 μM) and chelerythrine (2 μM) did not significantly affect the alkalosis induced by C-PAF. In summary, these results provide evidence that: a) PAF stimulates cardiac NHE1, b) the effect occurs via the PAF receptor, and c) signal relay requires participation of the MAP kinase cascade.