Description: Small-conductance Ca 2+ -activated K + (SK Ca ) channels are crucial in regulating vascular tone and blood pressure. The present study tested the hypothesis that SK Ca channels play an important role in uterine vascular adaptation in pregnancy, which is inhibited by chronic hypoxia during gestation. Uterine arteries were isolated from nonpregnant and near-term pregnant sheep maintained at sea level (300 m) or exposed to high-altitude (3801 m) hypoxia for 110 days. Immunohistochemistry revealed the presence of SK Ca channels type 2 (SK2) and type 3 (SK3) in both smooth muscles and endothelium of uterine arteries. The expression of SK2 and SK3 channels was significantly increased during pregnancy, which was inhibited by chronic hypoxia. In normoxic animals, both SK Ca channel opener NS309 and a large-conductance (BK Ca ) channel opener NS1619 relaxed norepinephrine-contracted uterine arteries in pregnant but not nonpregnant sheep. These relaxations were inhibited by selective SK Ca and BK Ca channel blockers, respectively. NS309-induced relaxation was largely endothelium-independent. In high-altitude hypoxic animals, neither NS1691 nor NS309 produced significant relaxation of uterine arteries in either nonpregnant or pregnant sheep. Similarly, the role of SK Ca channels in regulating the myogenic reactivity of uterine arteries in pregnant animals was abrogated by chronic hypoxia. Accordingly, the enhanced SK Ca channel activity in uterine arterial myocytes of pregnant animals was ablated by chronic hypoxia. The findings suggest a novel mechanism of SK Ca channels in regulating myogenic adaptation of uterine arteries in pregnancy and in the maladaptation of uteroplacental circulation caused by chronic hypoxia during gestation.

Description: Estrogen receptor-α (ERα) plays a key role in the adaptation of increased uterine blood flow in pregnancy. Chronic hypoxia is a common stress to maternal cardiovascular homeostasis and causes increased risk of preeclampsia. Studies in pregnant sheep demonstrated that hypoxia during gestation downregulated ERα gene expression in uterine arteries. The present study tested the hypothesis that hypoxia causes epigenetic repression of the ERα gene in uterine arteries via heightened promoter methylation. Ovine ERα promoter of 2035 bp spanning from –2000 to +35 of the transcription start site was cloned. No estrogen or hypoxia-inducible factor response elements were found at the promoter. Two transcription factor binding sites, USF –15 and Sp1 –520 , containing CpG dinucleotides were identified, which had significant effects on the promoter activity. The USF element binds transcription factors USF1 and USF2, and the Sp1 element binds Sp1, as well as ERα through Sp1. Deletion of the Sp1 site abrogated 17β-estradiol–induced increase in the promoter activity. In normoxic control sheep, CpG methylation at the Sp1 but not the USF site was significantly decreased in uterine arteries of pregnant as compared with nonpregnant animals. In pregnant sheep exposed to long-term high-altitude hypoxia, CpG methylation at both Sp1 and USF sites in uterine arteries was significantly increased. Methylation inhibited transcription factor binding and the promoter activity. The results provide evidence of hypoxia causing heightened promoter methylation and resultant ERα gene repression in uterine arteries and suggest new insights of molecular mechanisms linking gestational hypoxia to aberrant uteroplacental circulation and increased risk of preeclampsia.

Description: Our previous study demonstrated that increased Ca 2+ -activated K + (BK Ca ) channel activity played a key role in the normal adaptation of reduced myogenic tone of uterine arteries in pregnancy. The present study tested the hypothesis that chronic hypoxia during gestation inhibits pregnancy-induced upregulation of BK Ca channel function in uterine arteries. Resistance-sized uterine arteries were isolated from nonpregnant and near-term pregnant sheep maintained at sea level (300 m) or exposed to high-altitude (3801 m) hypoxia for 110 days. Hypoxia during gestation significantly inhibited pregnancy-induced upregulation of BK Ca channel activity and suppressed BK Ca channel current density in pregnant uterine arteries. This was mediated by a selective downregulation of BK Ca channel β1 subunit in the uterine arteries. In accordance, hypoxia abrogated the role of the BK Ca channel in regulating pressure-induced myogenic tone of uterine arteries that was significantly elevated in pregnant animals acclimatized to chronic hypoxia. In addition, hypoxia abolished the steroid hormone-mediated increase in the β1 subunit and BK Ca channel current density observed in nonpregnant uterine arteries. Although the activation of protein kinase C inhibited BK Ca channel current density in pregnant uterine arteries of normoxic sheep, this effect was ablated in the hypoxic animals. The results demonstrate that selectively targeting BK Ca channel β1 subunit plays a critical role in the maladaption of uteroplacental circulation caused by chronic hypoxia, which contributes to the increased incidence of preeclampsia and fetal intrauterine growth restriction associated with gestational hypoxia.