Preservation of Serotonin-Mediated Contractility in Adult Sheep Pulmonary Arteries Following Long-Term High-Altitude Hypoxia

Abstract

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.