NHE and ICAM-1 expression in hypoxic/reoxygenated coronary microvascular endothelial cells

Although Na+/H+ exchange (NHE) has been implicated in myocardial reperfusio n injury, participation of coronary microvascular endothelial cells (CMECs) in this pathogenesis has been poorly understood. NHE-induced intracellular Ca2+ concentration ([Ca2+](i)) overload in CMECs may increase the synthesi s of intercellular adhesion molecules (ICAM), which is potentially involved in myocardial reperfusion injury. The present study tested the hypothesis that NHE plays a crucial role in [Ca2+](i) overload and ICAM-1 synthesis in CMECs. Primary cultures of CMECs isolated from adult rat hearts were subje cted to acidic hypoxia for 30 min followed by reoxygenation. Two structural ly distinct NHE inhibitors, cariporide and 5-(N-N-dimethyl)-amiloride (DMA) , had no significant effect on the acidic hypoxia-induced decrease in intra cellular pH (pH(i)) of CMECs but significantly retarded pH(i) recovery afte r reoxygenation. These NHE inhibitors abolished the hypoxia- and reoxygenat ion-induced increase in [Ca2+](i). Expression of ICAM-1 mRNA was markedly i ncreased in the vehicle-treated CMECs 3 h after reoxygenation, and this was significantly inhibited by treatment with cariporide, DMA, or Ca2+-free bu ffer. In addition, enhanced ICAM-I protein expression on the cell surface o f CMECs 8 h after reoxygenation was attenuated by treatment with cariporide , DMA, or Ca2+-free buffer. These results suggest that NHE plays a crucial role in the rise of [Ca2+](i) and ICAM-1 expression during acidic hypoxia/r eoxygenation in CMECs. We propose that inhibition of ICAM-1 expression in C MECs may represent a novel mechanism of action of NHE inhibitors against is chemia-reperfusion injury.