Objectives     To investigate the effects of hypoxia-reoxygenation on the expressions of hypoxia-inducible factor-1 (HIF-1) and sodium/hydrogen exchanger-1 (NHE1) and the

activity of NHE1 in rat myocardial microvascular endothelial cells (RMMEVCs).     Methods     HIF-1 and NHE1 expression levels in RMMVECs cultured under anoxic condition and cobalt chloride-simulated anoxic condition were measured with Western blotting. Intracellular pH and pH recovery from acid loading and their responses to NHE1-specific blocker dimethyl amiloride (DMA) were measured by BCECF/AM. The recovery of NHE1 expression and activity was determined at 0

24

and 48 h after reoxygenation.      Results     HIF-1 and NHE1 expression levels increased under both anoxic condition and cobalt chloride-simulated anoxic condition. Hypoxia caused decrease of intracellular pH (0 h

7.42±0.06; 12 h

7.17±0.05; 24 h

7.06±0.04;

P

<

0.01) and intracellular pH recovery from acid loading [(U/min) 0 h

0.106±0.007; 12 h

0.059±0.005; 24 h

0.044±0.004;

P

<

0.01

]

. DMA lowered the intracellular pH recovery from acid loading at different hypoxia time points [(U/min) 0 h

-0.063; 12 h

-0.028; 24 h

-0.020

]

. NHE1 expression level reduced gradually after reoxygenation. Twenty four hours after reoxygenation

intracellular pH recovery from acid loading reached the highest point (0.119 U/min) under DMA-free condition

and DMA inhibition was the most effective (-0.076 U/min)

showing significant differences from those measured at 0 h and 48 h after reoxygenation (

P

<

0.01).     Conclusion    Under anoxic condition

HIF-1 increase may lead to NHE1 expression up-regulation. The activity of NHE1 decreases under anoxic condition but increases after reoxygenation. Changes of NHE1 expression and activity are possibly an important mechanism for pH regulation of RMMVECs in hypoxia.