Activation of the Na+/H+Exchanger is Required for Reperfusion-induced Ins(1,4,5)P3Generation

Post-ischemic reperfusion causes a change in inositol phosphate responses to norepinephrine from primary generation of inositol(1,4) bis phosphate (Ins(1,4)P2) to generation of inositol(1,4,5) tris phosphate (Ins(1,4,5)P3) that is required for the initiation of reperfusion arrhythmias. The current study was undertaken to investigate the role of Na+/H+exchange in facilitating this transient change in inositol phosphate response. Rat hearts were subjected to 20 min ischemia followed by 2 min reperfusion and Ins(1,4,5)P3content was measured by mass analysis or by anion-exchange HPLC following [3H]inositol labeling. Reperfusion caused generation of [3H]Ins(1,4,5)P3(1732±398 to 3103±214, cpm/g tissue, mean± , n=5, P<0.01) and the development of arrhythmias. Inhibition of Na+/H+exchange, by reperfusing at pH 6.3 or by pretreating with HOE-694 (10 n –3 μ ) or HOE-642 (3 μ ) prevented the [3H]Ins(1,4,5)P3generation, without causing any suppression of norepinephrine release. Increases in Ins(1,4,5)P3mass were similarly reduced by inhibition of Na+/H+exchange. Thus, activation of Na+/H+exchange is required for the enhanced Ins(1,4,5)P3response observed under reperfusion conditions, and prevention of Ins(1,4,5)P3generation may be an important contributor to the anti-arrhythmic actions of inhibitors of Na+/H+exchange.