| Hypoxia, a state of inadequate supply of oxygen, can be experienced under many physiological and pathophysiological situations. For instance, high altitude sojourns result in sustained hypoxia (SH); whereas intermittent hypoxia (IH) is experienced in the pathological setting of recurrent apneas characterized by periodic cessations of breathing. Previous studies have shown that hypoxia modulates K+ channel activity and thereby alters cellular excitability. The human-ether-a-go-go K+ channel regulates the resting membrane potential and excitability in a wide variety of cells. In the present study we examined the impact of SH and IH on hERG K+ channel protein expression in cell culture models. In HEK293 cells stably transfected with the hERG potassium channel (HEK-hERG cells), SH causes reduction both in hERG protein and in hERG current, and these effects are reversible upon re-oxygenation. In contrast, IH exposure causes a smaller but significant reduction in mature protein and hERG current with a concomitant increase in immature hERG protein in the endoplasmic reticulum (ER) in HEK-hERG cells. These changes were not reversed even after 48 hours of re-oxygenation. In a neuroblastoma cell line that endogenously expresses hERG channels, IH reduces mature hERG protein as in the HEK-hERG cells, without accumulation of immature protein in the ER. IH had no effect on hERG mRNA levels in either cell line, suggesting that transcriptional regulation does not account for IH-evoked changes in hERG protein. Bafilomycin A, a blocker of lysosomal degradation, does not prevent the effects of IH, suggesting that IH does not act by increasing endocytosis and degradation of hERG. Rather, the effects of IH are temperature sensitive, and are therefore likely due to impaired protein folding in the ER. Two days exposure to IH led to a small increase in current density in HEK293 cells stably transfected with the cardiac sodium channel, Nav1.5; whereas SH had no effect, suggesting that the effects of hypoxia are not uniform among membrane ion channel proteins. The cytosolic chaperone Hsp90 is required for hERG trafficking and has been implicated in regulation of hERG channel protein by hypoxia. We found that Nav1.5, in contrast to hERG, does not rely on Hsp90 for trafficking. This may in part underlie the differences in the effects of IH and SH on these two ion channels. |
