Active adaptation of the tethered mitral valve: insights into a compensatory mechanism for functional mitral regurgitation.

BACKGROUND: In patients with left ventricular infarction or dilatation, leaflet tethering by displaced papillary muscles frequently induces mitral regurgitation, which doubles mortality. Little is known about the biological potential of the mitral valve (MV) to compensate for ventricular remodeling. We tested the hypothesis that MV leaflet surface area increases over time with mechanical stretch created by papillary muscle displacement through cell activation, not passive stretching. METHODS AND RESULTS: Under cardiopulmonary bypass, the papillary muscle tips in 6 adult sheep were retracted apically short of producing mitral regurgitation to replicate tethering without confounding myocardial infarction or turbulence. Diastolic leaflet area was quantified by 3-dimensional echocardiography over 61+/-6 days compared with 6 unstretched sheep MVs. Total diastolic leaflet area increased by 2.4+/-1.3 cm(2) (17+/-10%) from 14.3+/-1.9 to 16.7+/-1.9 cm(2) (P=0.006) with stretch with no change in the unstretched valves despite sham open heart surgery. Stretched MVs were 2.8 times thicker than normal (1.18+/-0.14 versus 0.42+/-0.14 mm; P<0.0001) at 60 days with an increased spongiosa layer. Endothelial cells (CD31(+)) coexpressing alpha-smooth muscle actin were significantly more common by fluorescent cell sorting in tethered versus normal leaflets (41+/-19% versus 9+/-5%; P=0.02), indicating endothelial-mesenchymal transdifferentiation. alpha-Smooth muscle actin-positive cells appeared in the atrial endothelium, penetrating into the interstitium, with increased collagen deposition. Thickened chordae showed endothelial and subendothelial alpha-smooth muscle actin. Endothelial-mesenchymal transdifferentiation capacity also was demonstrated in cultured MV endothelial cells. CONCLUSIONS: Mechanical stresses imposed by papillary muscle tethering increase MV leaflet area and thickness, with cellular changes suggesting reactivated embryonic developmental pathways. Understanding such actively adaptive mechanisms can potentially provide therapeutic opportunities to augment MV area and reduce ischemic mitral regurgitation.

Effect of Losartan on Mitral Valve Changes After Myocardial Infarction.

BACKGROUND: After myocardial infarction (MI), mitral valve (MV) tethering stimulates adaptive leaflet growth, but counterproductive leaflet thickening and fibrosis augment mitral regurgitation (MR), doubling heart failure and mortality. MV fibrosis post-MI is associated with excessive endothelial-to-mesenchymal transition (EMT), driven by transforming growth factor (TGF)-ß overexpression. In vitro, losartan-mediated TGF-ß inhibition reduces EMT of MV endothelial cells. OBJECTIVES: This study tested the hypothesis that profibrotic MV changes post-MI are therapeutically accessible, specifically by losartan-mediated TGF-ß inhibition. METHODS: The study assessed 17 sheep, including 6 sham-operated control animals and 11 with apical MI and papillary muscle retraction short of producing MR; 6 of the 11 were treated with daily losartan, and 5 were untreated, with flexible epicardial mesh comparably limiting left ventricular (LV) remodeling. LV volumes, tethering, and MV area were quantified by using three-dimensional echocardiography at baseline and at 60 ± 6 days, and excised leaflets were analyzed by histopathology and flow cytometry. RESULTS: Post-MI LV dilation and tethering were comparable in the losartan-treated and untreated LV constraint sheep. Telemetered sensors (n = 6) showed no significant losartan-induced changes in arterial pressure. Losartan strongly reduced leaflet thickness (0.9 ± 0.2 mm vs. 1.6 ± 0.2 mm; p < 0.05; 0.4 ± 0.1 mm sham animals), TGF-ß, and downstream phosphorylated extracellular-signal-regulated kinase and EMT (27.2 ± 12.0% vs. 51.6 ± 11.7% α-smooth muscle actin-positive endothelial cells, p < 0.05; 7.2 ± 3.5% sham animals), cellular proliferation, collagen deposition, endothelial cell activation (vascular cell adhesion molecule-1 expression), neovascularization, and cells positive for cluster of differentiation (CD) 45, a hematopoietic marker associated with post-MI valve fibrosis. Leaflet area increased comparably (17%) in constrained and losartan-treated sheep. CONCLUSIONS: Profibrotic changes of tethered MV leaflets post-MI can be modulated by losartan without eliminating adaptive growth. Understanding the cellular and molecular mechanisms could provide new opportunities to reduce ischemic MR.

Myocardial Infarction Alters Adaptation of the Tethered Mitral Valve.

BACKGROUND: In patients with myocardial infarction (MI), leaflet tethering by displaced papillary muscles induces mitral regurgitation (MR), which doubles mortality. Mitral valves (MVs) are larger in such patients but fibrosis sets in counterproductively. The investigators previously reported that experimental tethering alone increases mitral valve area in association with endothelial-to-mesenchymal transition. OBJECTIVES: The aim of this study was to explore the clinically relevant situation of tethering and MI, testing the hypothesis that ischemic milieu modifies mitral valve adaptation. METHODS: Twenty-three adult sheep were examined. Under cardiopulmonary bypass, the papillary muscle tips in 6 sheep were retracted apically to replicate tethering, short of producing MR (tethered alone). Papillary muscle retraction was combined with apical MI created by coronary ligation in another 6 sheep (tethered plus MI), and left ventricular remodeling was limited by external constraint in 5 additional sheep (left ventricular constraint). Six sham-operated sheep were control subjects. Diastolic mitral valve surface area was quantified by 3-dimensional echocardiography at baseline and after 58 ± 5 days, followed by histopathology and flow cytometry of excised leaflets. RESULTS: Tethered plus MI leaflets were markedly thicker than tethered-alone valves and sham control subjects. Leaflet area also increased significantly. Endothelial-to-mesenchymal transition, detected as α-smooth muscle actin-positive endothelial cells, significantly exceeded that in tethered-alone and control valves. Transforming growth factor-ß, matrix metalloproteinase expression, and cellular proliferation were markedly increased. Uniquely, tethering plus MI showed endothelial activation with vascular adhesion molecule expression, neovascularization, and cells positive for CD45, considered a hematopoietic cell marker. Tethered plus MI findings were comparable with external ventricular constraint. CONCLUSIONS: MI altered leaflet adaptation, including a profibrotic increase in valvular cell activation, CD45-positive cells, and matrix turnover. Understanding cellular and molecular mechanisms underlying leaflet adaptation and fibrosis could yield new therapeutic opportunities for reducing ischemic MR.

Twelve-Hour Hypothermic Machine Perfusion for Donor Heart Preservation Leads to Improved Ultrastructural Characteristics Compared to Conventional Cold Storage.

BACKGROUND Hypothermic machine perfusion of donor hearts has the theoretical advantage of continuous aerobic metabolism and washes out toxic metabolic byproducts. Here, we studied the effect of hypothermic machine perfusion on cardiac myocyte integrity when hearts are preserved for longer ischemic times (12 hours). MATERIAL AND METHODS Pig hearts were harvested and stored in Celsior® solution for 12 hours using either conventional cold storage on ice (12 h CS, n=3) or pulsatile perfusion with the Paragonix Sherpa Perfusion™ Cardiac Transport System at different flow rates (12 h PP, n=3 or 12 h PP low flow, n=2). After cold preservation, hearts were reperfused using an LV isovolumic Langendorff system. Controls (n=3) were reperfused immediately after organ harvest. Biopsies were taken from the apex of the left ventricle before storage, after storage and after reperfusion to measure ATP and endothelin-1 content in the tissue. TUNEL staining for signs of apoptosis and electron microscopy of the donor hearts were performed. RESULTS 12 h PP hearts showed significantly more weight gain than 12 h CS and controls after preservation. Pulsatile perfused hearts showed less ATP depletion, lower endothelin-1 levels and less apoptosis after preservation compared to CS. Electron microscopy showed damaged muscle fibers, endothelial cell rupture, and injury of mitochondria in the 12 h CS group, while machine perfusion could preserve the cell structures. CONCLUSIONS Hypothermic machine perfusion of donor hearts can preserve the cell structures better than conventional cold storage in prolonged ischemic times. Hypothermic pulsatile perfusion may therefore enable longer preservation times of donor hearts. Whether this method is able to avoid primary graft failure after orthotopic heart transplantation remains to be evaluated in further studies.

Preservation of donor hearts using hypothermic oxygenated perfusion.

BACKGROUND: Hypothermic machine perfusion of donor hearts enables continuous aerobic metabolism and washout of toxic metabolic byproducts. We evaluated the effect of machine perfusion on cardiac myocyte integrity in hearts preserved for 4 h in a novel device that provides pulsatile oxygenated hypothermic perfusion (Paragonix Sherpa Perfusion™ Cardiac Transport System). MATERIAL AND METHODS: Pig hearts were harvested and stored in Celsior® solution for 4 h using either conventional cold storage on ice (4-h CS, n=6) or the Sherpa device (4-h pulsatile perfusion (PP), n=6). After cold preservation, hearts were evaluated using a non-working heart Langendorff system. Controls (n=3) were reperfused immediately after organ harvest. Biopsies were taken from the apex of the left ventricle before storage, after storage, and after reperfusion to measure ATP content and endothelin-1 in the tissue. Ultrastructural analysis using electron microscopy was performed. RESULTS: Four-hour CS, 4-h PP, and control group did not show any significant differences in systolic or diastolic function (+dP/dt, -dP/dt, EDP). Four-hour PP hearts showed significantly more weight gain than 4-h CS after preservation, which shows that machine perfusion led to myocardial edema. Four-hour CS led to higher endothelin-1 levels after preservation, suggesting more endothelial dysfunction compared to 4-h PP. Electron microscopy revealed endothelial cell rupture and damaged muscle fibers in the 4-h CS group after reperfusion, but the cell structures were preserved in the 4-h PP group. CONCLUSIONS: Hypothermic pulsatile perfusion of donor hearts leads to a better-preserved cell structure compared to the conventional cold storage method. This may lead to less risk of primary graft failure after orthotopic heart transplantation.

A stentless trileaflet valve from a sheet of decellularized porcine small intestinal submucosa.

PURPOSE: The purpose of this study was to investigate the function of a trileaflet pulmonary valve constructed from a sheet of porcine small intestinal submucosa. DESCRIPTION: In four sheep, the native pulmonary valve and a segment of the pulmonary trunk was excised and replaced with a trileaflet valve constructed from decellularized porcine small intestinal submucosa. The valve construct was created from a sheet of the xenograft material by a method of involuting flaps of tissue inside a cylinder of itself. The function of the valve was assessed by echocardiography, catheter pullback across the valve, and observation of an excised valve in a flow simulator. EVALUATION: The valve constructs exhibited low gradients and symmetrical leaflet movement with good mobility when tested under physiologic conditions in an acute sheep model. CONCLUSIONS: This method offers a means to create a functional trileaflet valve replacement from a sheet of tissue.