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Induced Metabolic Acidosis is Protective in a Rat Model of Hypoxic Pulmonary Hypertension: Direct Effect on Vascular Smooth Muscle Cell (VSMC) Proliferation and Migration

H. Christou

E. Arons

S. A. Mitsialis

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Conference: 2008 International PHA Conference and Scientific Sessions

Release Date: 06.20.2008

Presentation Type: Abstracts

Christou H., Arons E., Mitsialis S.A.

Brigham and Women’s Hospital and Children’s Hospital, Boston, MA, USA

BACKGROUNDPulmonary Artery Hypertension (PAH) is a devastating disease with high mortality. Current vasodilator therapies are variably successful in ameliorating PAH, but it is the combination of these with anti-proliferative and/or pro-apoptotic approaches that is likely to decrease mortality by reversing the underlying pulmonary arteriolar remodeling. The prevailing mechanistic hypothesis is that pulmonary vasoconstriction and pulmonary vascular smooth muscle cell (VSMC) proliferation use overlapping signaling processes, therefore vasodilator therapies also exert anti-proliferative effects.  Despite the general validity of this hypothesis, emerging experimental evidence supports that acidosis, a known pulmonary vasoconstrictor, may be paradoxically protective in the setting of PAH through mechanisms that are independent of its effects on vascular tone. Our hypothesis is that induced metabolic acidosis will inhibit VSMC proliferation and migration and that this will translate to a protective effect in experimental PAH.

METHODSPrimary rat VSMCs were stimulated to grow in the setting of extracellular acidosis (EA) or physiologic pH. VSMC proliferation was assessed by cell number, BrdU incorporation, and cell cycle analysis by flow cytometry, and VSMC migration in response to Platelet Derived Growth Factor-BB (PDGF-BB) was assessed in Transwell plates. Adult male rats were treated with 1.5% NH4Cl in the drinking water for 5 days prior to and during one or two weeks of hypoxic exposure. Arterial blood gases were obtained to confirm metabolic acidosis, right ventricular hypertrophy (RVH) was assessed by the Fulton's index, and pulmonary vascular remodeling was quantified by morphometric analysis at the end of the experimental period. Statistical analysis was done by Analysis of Variance (ANOVA).

RESULTSVSMC number in response to growth factor stimulation was lower in the setting of EA and this corresponded to decreased BrdU incorporation and lower percentage of cells in the S+G2/M phases of the cell cycle. VSMC migration in response to PDGF-BB was also inhibited by EA. Hypoxic animals treated with NH4Cl had significantly lower systemic pH compared to hypoxic controls (7.22 vs. 7.37, p=0.002). These animals also had significantly lower Fulton's indices compared to untreated hypoxic animals (0.32 vs. 0.44, p=0.006). The pulmonary vascular remodeling seen in hypoxic animals after two weeks was significantly ameliorated in hypoxic animals with induced metabolic acidosis (percent medial thickness 0.33 vs. 0.20, p<0.001).

COMMENTS AND CONCLUSIONS: Induced metabolic acidosis has a direct inhibitory effect on VSMC proliferation and migration in culture and can protect from RVH and pulmonary vascular remodeling associated with hypoxic PAH in vivo. Ongoing studies aim to identify the molecular mechanisms and signaling pathways through which acidosis impacts on vascular cell function.