Long-term exposure to high-altitude chronic hypoxia during gestation induces neonatal pulmonary hypertension at sea level

Maternal high-altitude hypoxia and suppression of ryanodine receptor-mediated Ca2+ sparks in fetal sheep pulmonary arterial myocytes

Ca(2+) sparks are fundamental Ca(2+) signaling events arising from ryanodine receptor (RyR) activation, events that relate to contractile and dilatory events in the pulmonary vasculature. Recent studies demonstrate that long-term hypoxia (LTH) can affect pulmonary arterial reactivity in fetal, newborn, and adult animals. Because RyRs are important to pulmonary vascular reactivity and reactivity changes with ontogeny and LTH we tested the hypothesis that RyR-generated Ca(2+) signals are more active before birth and that LTH suppresses these responses. We examined these hypotheses by performing confocal imaging of myocytes in living arteries and by performing wire myography studies. Pulmonary arteries (PA) were isolated from fetal, newborn, or adult sheep that lived at low altitude or from those that were acclimatized to 3,801 m for > 100 days. Confocal imaging demonstrated preservation of the distance between the sarcoplasmic reticulum, nucleus, and plasma membrane in PA myocytes. Maturation increased global Ca(2+) waves and Ca(2+) spark activity, with sparks becoming larger, wider, and slower. LTH preferentially depressed Ca(2+) spark activity in immature pulmonary arterial myocytes, and these sparks were smaller, wider, and slower. LTH also suppressed caffeine-elicited contraction in fetal PA but augmented contraction in the newborn and adult. The influence of both ontogeny and LTH on RyR-dependent cell excitability shed new light on the therapeutic potential of these channels for the treatment of pulmonary vascular disease in newborns as well as adults.

The heme oxygenase-carbon monoxide system in the regulation of cardiorespiratory function at high altitude

Pulmonary arterial hypertension is one of the most serious pathologies that can affect the 140 million people living at altitudes over 2500 m. The primary emphasis of this review is pulmonary artery hypertension in mammals (sheep and llamas) at high altitude, with specific focus on the heme oxygenase and carbon monoxide (HO-CO) system. We highlight the fact that the neonatal llama has neither pulmonary artery hypertension nor pulmonary vascular remodeling in the Andean altiplano. These neonates have an enhanced HO-CO system function, increasing the HO-1 protein expression and CO production by the pulmonary vessels, when compared to llamas raised at low altitude, or neonatal sheep raised at high altitude. The neonatal sheep has high altitude pulmonary artery hypertension in spite of enhancement of the NO system, with high eNOS protein expression and NO production by the lung. The gasotransmitters NO and CO are important in the regulation of the pulmonary vascular function at high altitudes in both high altitude acclimatized species, such as the sheep, and high altitude adapted species, such as the llama.

Preservation of serotonin-mediated contractility in adult sheep pulmonary arteries following long-term high-altitude hypoxia

Long-term hypoxia (LTH) can increase serotonin (5-HT) signaling as well as extracellular calcium entry in adult rodent pulmonary arteries (PA), and 5-HT is associated with pulmonary hypertension. Because LTH, 5-HT, and calcium entry are related, we tested the hypothesis that LTH increases 5-HT-mediated PA contractility and associated calcium influx through L-type Ca2+ channels, nonselective cation channels (NSCC), and reverse-mode sodium-Ca2+ exchange. We performed wire myography and confocal calcium imaging on pulmonary arteries from adult ewes that lived near sea level or were maintained at high-altitude (3801 m) for ∼110 days. LTH did not increase the arterial medial wall thickness, nor did it affect the potency or efficacy for 5-HT-induced PA contraction. Ketanserin (100 nM), a 5-HT2A antagonist, shifted the 5-HT potency to a far greater extent than 1 μM GR-55562, a 5-HT1B/D inhibitor. These influences were unaffected by LTH. The rank order for reducing 5-HT-induced PA contraction in normoxic animals was extracellular calcium removal≈10 mM Ni2+≈10 μM verapamil≈10 μM nifedipine with 50 μM SKF 96365>30 μM KB-R7943≈100 μM flufenamic acid≈10 μM nifedipine≈100 μM Gd3+> 100 μM La3+>500 μM Ni2+≈10 μM diltiazem≈50 μM 2-APB≈100 μM LOE 908. Contraction was not reduced by 100 μM spermine or 30 μM SN-6. LTH increased the effects of KB-R7943 and mitigated those of nifedipine but did not affect calcium responses in imaging studies. Overall, in adult sheep, arterial structure and 5-HT2A and 5HT1B/D functions are preserved following LTH while the role of NSCC-related calcium-dependent contraction is increased. These elements indicate preservation of PA contractility in LTH with minimal functional changes.

Long-term maternal hypoxia: the role of extracellular Ca2+ entry during serotonin-mediated contractility in fetal ovine pulmonary arteries

Antenatal maternal long-term hypoxia (LTH) can alter serotonin (5-HT) and calcium (Ca(2+)) signaling in fetal pulmonary arteries (PAs) and is associated with persistent pulmonary hypertension of the newborn (PPHN). In humans, the antenatal maternal hypoxia can be secondary to smoking, anemia, and chronic obstructive pulmonary disorders. However, the mechanisms of antenatal maternal hypoxia-related PPHN are unresolved. Because both LTH and 5-HT are associated with PPHN, we tested the hypothesis that antenatal maternal LTH can increase 5-HT-mediated PA contraction and associated extracellular Ca(2+) influx through L-type Ca(2+) channels (Ca(L)), nonselective cation channels (NSCCs), and reverse-mode sodium-calcium exchanger (NCX) in the near-term fetus. We performed wire myography and confocal-Ca(2+) imaging approaches on fetal lamb PA (∼ 140 days of gestation) from normoxic ewes or those acclimatized to high-altitude LTH (3801 m) for ∼110 days. Long-term hypoxia reduced the potency but not the efficacy of 5-HT-induced PA contraction. Ketanserin (100 nmol/L), a 5-HT(2A) antagonist, shifted 5-HT potency irrespective of LTH, while GR-55562 (1 µmol/L), a 5-HT(1B/D) inhibitor, antagonized 5-HT-induced contraction in normoxic fetuses only. Various inhibitors for Ca(L), NSCC, and reverse-mode NCX were used in contraction studies. Contraction was reliant on extracellular Ca(2+) regardless of maternal hypoxia, NSCC was more important to contraction than Ca(L), and reverse-mode NCX had little or no role in contraction. Long-term hypoxia also attenuated the effects of 2-APB and flufenamic acid and reduced Ca(2+) responses observed by imaging studies. Overall, LTH reduced 5HT(1B/D) function and increased NSCC-related Ca(2+)-dependent contraction in ovine fetuses, which may compromise pulmonary vascular function in the newborn.

Role of the α-adrenergic system in femoral vascular reactivity in neonatal llamas and sheep: a comparative study between highland and lowland species

Using an integrative approach at the whole animal, isolated vessels, and molecular levels, we tested the hypothesis that the llama, a species that undergoes pregnancy under the influence of the chronic hypoxia of high altitude, delivers offspring with an increased α-adrenergic peripheral vascular reactivity compared with neonates from lowland species. We studied the femoral vascular response to acute hypoxia in vivo, the reactivity of femoral vessels ex vivo, and the expression of femoral α₁-adrenergic receptor subtypes using RT-PCR in vitro. The increase in femoral resistance during hypoxia was 3.6 times greater in newborn llamas than newborn sheep (P < 0.05). The sensitivity of the contractile response to noradrenaline (pD₂ = 5.18 ± 0.06 vs. 4.84 ± 0.05, P < 0.05) and the maximal response (R_max = 101.3 ± 1.4 vs. 52.4 ± 1.4% K⁺_max, P < 0.05) and sensitivity (pD₂ = 5.47 ± 0.03 vs. 4.57 ± 0.05, P < 0.05) to phenylephrine were higher in femoral vessels from newborn llamas than newborn sheep. Competitive inhibition with prazosin of noradrenaline-induced contraction followed by Schild analysis showed higher affinity in the llama than the sheep (pA₂ = 10.08 ± 0.093 vs. 8.98 ± 0.263, respectively, P < 0.05), consistent with greater α_1B-adrenergic receptor transcript expression observed in small femoral arteries from neonatal llama. The llama newborn demonstrates significantly greater α-adrenergic peripheral vascular reactivity compared with neonates from lowland species that could be partially explained by preferential expression of α_1B-adrenergic receptor subtype.