Chronic hypoxia suppresses pregnancy-induced upregulation of large-conductance Ca2+-activated K+ channel activity in uterine arteries

Abstract

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.

Figure 1

Chronic hypoxia decreases whole-cell K⁺ currents in uterine arteries of pregnant sheep. Arterial myocytes were freshly isolated from uterine arteries of normoxic and hypoxic sheep. Whole-cell K⁺ currents were recorded in the absence or presence of tetraethylammonium (TEA; 1.0 mmol/L) or iberoitoxin (IBTX; 100.0 nmol/L). A, Normoxic nonpregnant animals. B, Normoxic pregnant animals. C, Hypoxic nonpregnant animals. D, Hypoxic pregnant animals. Data are mean±SEM of 7 to 10 cells from 5 to 8 animals of each group. *P<0.05 vs control (Ctr). ○, ctr; ●, TEA; ■, IBTX.

Figure 2

Chronic hypoxia suppresses Ca²⁺-activated K⁺ (BK_Ca) current density in uterine arteries of pregnant sheep. Arterial myocytes were freshly isolated from uterine arteries of normoxic and hypoxic sheep. BK_Ca current density was determined in the presence of tetraethylammonium (TEA; 1.0 mmol/L). A, Nonpregnant animals. B, Pregnant animals. Data are mean±SEM of 7 to 10 cells from 5 to 8 animals of each group. *P<0.05 vs normoxia. ○, normoxia; ●, hypoxia.

Figure 3

Tetraethylammonium (TEA) has no effects on myogenic tone in uterine arteries of hypoxic animals. Uterine arteries were isolated from nonpregnant (NPUA) and pregnant (PUA) hypoxic sheep. Pressure-dependent myogenic tone was determined in the absence or presence of TEA (1.0 mmol/L). Data are mean±SEM of tissues from 5 to 6 animals of each group. ○, PUA-control; ●, PUA-TEA; □, NPUA-control; ■, NPUA-TEA.

Figure 4

Chronic hypoxia inhibits steroid hormone-induced upregulation of Ca²⁺-activated K⁺ (BK_Ca) channel activity in uterine arteries. Uterine arteries were isolated from nonpregnant sheep and were treated ex vivo with 17β-estradiol (E2β; 0.3 nmol/L) plus progesterone (P4; 100.0 nmol/L) under 20.5% O₂ and 10.5% O₂, respectively, for 48 hours. Arterial myocytes were then isolated and whole-cell K⁺ currents were recorded in the absence or presence of tetraethylammonium (TEA; 1.0 mmol/L). A, Whole-cell K⁺ currents in myocytes of normoxic animals without hormonal treatment. *P<0.05 vs control (Ctr). B, Whole-cell K⁺ currents in myocytes of normoxic animals with hormonal treatment. *P<0.05 vs control (Ctr). C, BK_Ca current density in myocytes of normoxic animals without and with hormonal treatment. *P<0.05, +E2β/P4 vs −E2β/P4. D, Whole-cell K⁺ currents in myocytes of hypoxic animals without hormonal treatment. *P<0.05 vs control (Ctr). E, Whole-cell K⁺ currents in myocytes of hypoxic animals with hormonal treatment. *P<0.05 vs control (Ctr). F, BK_Ca current density in myocytes of hypoxic animals without and with hormonal treatment. Data are mean±SEM of 8 to 10 cells from 6 animals of each group. A, B, D, and E, ○, ctr; ●, TEA. C and F, ○, −E2β-P4;●, +E2β-P4.

Figure 5

Chronic hypoxia inhibits pregnancy- and steroid hormone-induced upregulation of Ca²⁺-activated K⁺ (BK_Ca) channel β1 subunit in uterine arteries. Protein abundance of BK_Ca channel α (αBK_Ca) and β1 (α1BK_Ca) subunits was determined by Western blot analyses. A, Freshly isolated uterine arteries of pregnant sheep in normoxic and hypoxic animals. *P<0.05 vs normoxia. B, Freshly isolated uterine arteries from nonpregnant (NPUA) and pregnant (PUA) sheep of hypoxic animals. C, Uterine arteries from nonpregnant sheep of hypoxic animals were treated ex vivo with 17β-estradiol (E2β; 0.3 nmol/L) plus progesterone (P4; 100.0 nmol/L) under 10.5% O₂ for 48 hours. Data are mean±SEM of tissues from 5 to 6 animals of each group. A, □, normoxia; ■, hypoxia. B, □, PUA; ■, NPUA. C, □, −E₂β&P₄; ■, +E₂β&P₄.

Figure 6

Chronic hypoxia has no effects on voltage-gated K⁺ (K_V) channel activity in uterine arteries. Arterial myocytes were freshly isolated from uterine arteries of pregnant sheep in normoxic and hypoxic animals. Whole-cell K⁺ currents were recorded in the absence or presence of 4-aminopyridine (4-AP; 5.0 mmol/L). K_V current density was determined as the fraction of the whole-cell K⁺ currents blocked by 4-AP. Data are mean±SEM of 5 cells from 5 animals of each group. ○, normoxia; ●, hypoxia.

Figure 7

Chronic hypoxia diminishes protein kinase C (PKC)-mediated modulation of Ca²⁺-activated K⁺ (BK_Ca) channel in uterine arteries. Arterial myocytes were freshly isolated from uterine arteries of pregnant sheep in normoxic (A) and hypoxic (B) animals. Whole-cell K⁺ currents were recorded in the absence or presence of 4-aminopyridine (4-AP; 5.0 mmol/L) and 4-AP plus phorbol 12,13-dibutyrate (PDBu; 1.0 µmol/L), respectively. Data are mean±SEM of 5 cells from 5 animals of each group. a, P<0.05 vs control (Ctr); b, P<0.05 vs 4-AP. ○, ctr; ●, 4-AP; ■, 4-AP+PDBu.