The presence of maxi-K channels has been reported in the mitochondria of the cardiac myocytes where it plays a cytoprotective role. Using real-time PCR we have observed that severe chronic hypoxia induces the down-regulation of the b1-subunit in neonatal rat cardiomyocytes in culture, and that this effect is persistent and dose-dependent. We have studied the molecular mechanisms underlying this effect and the physiological significance of this regulation in the heart.

The transcription inhibitor DRB and the translational inhibitor cycloheximide abolish the down-regulation of the b1-subunit induced by hypoxia in this preparation, indicating that regulation occurs at the transcriptional level and that it depends on the novo synthesis of proteins. Small-interference RNA (si-RNA) for HIF-1a and HIF-2a reveals that HIF-1a is not responsible for the effect. However si-RNA for HIF-2a induces a partial reversion of the effect. This is consistent with the stabilization of HIF-2a protein in hypoxia observed by western-blot analysis even in the presence of si-RNA. To study the functional significance of the down-regulation of b1-subunit during chronic hypoxia we performed simulated ischemia assays on cardiomyocytes previously cultured under normoxic and hypoxic conditions. We observed that the hypoxic treatment increased cell viability during the simulated ischemia, and that the b1-subunit si-RNA treatment (in normoxia) also increased cell viability. This increase in cell viability was smaller than the one elicited by the hypoxic treatment, indicating that the down-regulation of the b1-subunit during hypoxia could contribute to cardioprotection induced by chronic hypoxia.