Activation of the Na+/H+ exchanger is required for reperfusion-induced Ins(1,4,5)P-3 generation

Post-ischemic reperfusion causes a change in inositol phosphate responses t o norepinephrine from primary generation of inositol(1,4)bisphosphate (Ins( 1,4)P-2) to generation of inositol(1,4,5)trisphosphate (Ins(1,4,5)P-3) that is required for the initiation of reperfusion arrhythmias. The current stu dy was undertaken to investigate the role of Na+/H+ exchange in facilitatin g this transient change in inositol phosphate response. Rat hearts were sub jected to 20 min ischemia followed by 2 min reperfusion and Ins(1,4,5)P-3 c ontent was measured by mass analysis or by anion-exchange HPLC following [H -3]inositol labeling. Reperfusion caused generation of [H-3]Ins(1,4,5)P-3 ( 1732 +/- 398 to 3103 +/- 214, cpm/g tissue, mean +/- S.E.M., n = 5, P<0.01) and the development of arrhythmias. Inhibition of Na+/H+ exchange, by repe rfusing at pH 6.3 or by pretreating with HOE-694 (10 nM-3 <mu>M) or HOE-642 (3 muM) prevented the [H-3]Ins(1,4,5)P-3 generation, without causing any s uppression of norepinephrine release. Increases in Ins(1,4,5)P-3 mass were similarly reduced by inhibition of Na+/H+ exchange. Thus, activation of Na/H+ exchange is required for the enhanced Ins(1,4,5)P-3 response observed u nder reperfusion conditions, and prevention of Ins(1,4,5)P-3 generation may be an important contributor to the anti-arrhythmic actions of inhibitors o f Na+/H+ exchange. (C) 2000 Academic Press.