Microtubule depolymerization normalizes in vivo myocardial contractile function in dogs with pressure-overload left ventricular hypertrophy.

BACKGROUND: Because initially compensatory myocardial hypertrophy in response to pressure overloading may eventually decompensate to myocardial failure, mechanisms responsible for this transition have long been sought. One such mechanism established in vitro is densification of the cellular microtubule network, which imposes a viscous load that inhibits cardiocyte contraction. METHODS AND RESULTS: In the present study, we extended this in vitro finding to the in vivo level and tested the hypothesis that this cytoskeletal abnormality is important in the in vivo contractile dysfunction that occurs in experimental aortic stenosis in the adult dog. In 8 dogs in which gradual stenosis of the ascending aorta had caused severe left ventricular (LV) pressure overloading (gradient, 152+/-16 mm Hg) with contractile dysfunction, LV function was measured at baseline and 1 hour after the intravenous administration of colchicine. Cardiocytes obtained by biopsy before and after in vivo colchicine administration were examined in tandem. Microtubule depolymerization restored LV contractile function both in vivo and in vitro. CONCLUSIONS: These and additional corroborative data show that increased cardiocyte microtubule network density is an important mechanism for the ventricular contractile dysfunction that develops in large mammals with adult-onset pressure-overload-induced cardiac hypertrophy.

Effects of gene deletion of the tissue inhibitor of the matrix metalloproteinase-type 1 (TIMP-1) on left ventricular geometry and function in mice.

Alterations in the expression and activity of the matrix metalloproteinases (MMPs) and the tissue inhibitors of the MMPs (TIMPs) have been implicated in tissue remodeling in a number of disease states. One of the better characterized TIMPs, TIMP-1, has been shown to bind to active MMPs and to regulate the MMP activational process. The goal of this study was to determine whether deletion of the TIMP-1 gene in mice, which in turn would remove TIMP-1 expression in LV myocardium, would produce time-dependent effects on LV geometry and function. Age-matched sibling mice (129Sv) deficient in the TIMP-1 gene (TIMP-1 knock-out (TIMP-1 KO), n=10) and wild-type mice (n=10) underwent comparative echocardiographic studies at 1 and 4 months of age. LV catheterization studies were performed at 4 months and the LV harvested for histomorphometric studies. LV end-diastolic volume and mass increased (18+/-4 and 38+/-3%, respectively, P<0.05) at 4 months in the TIMP-1 KO group; a significant increase compared to wild-type controls (P<0.05). At 4 months, LV and end-diastolic wall stress was increased by over two-fold in the TIMP-1 KO compared to wild type (P<0.05). However, LV systolic pressure and ejection performance were unchanged in the two groups of mice. LV myocyte cross-sectional area was unchanged in the TIMP-1 KO mice compared to controls, but myocardial fibrillar collagen content was reduced. Changes in LV geometry occurred in TIMP-1 deficient mice and these results suggest that constitutive TIMP-1 expression participates in the maintenance of normal LV myocardial structure.

Hypertrophic response to hemodynamic overload: role of load vs. renin-angiotensin system activation.

Myocardial hypertrophy is one of the basic mechanisms by which the heart compensates for hemodynamic overload. The mechanisms by which hemodynamic overload is transduced by the cardiac muscle cell and translated into cardiac hypertrophy are not completely understood. Candidates include activation of the renin-angiotensin system (RAS) and angiotensin II receptor (AT1) stimulation. In this study, we tested the hypothesis that load, independent of the RAS, is sufficient to stimulate cardiac growth. Four groups of cats were studied: 14 normal controls, 20 pulmonary artery-banded (PAB) cats, 7 PAB cats in whom the AT1 was concomitantly and continuously blocked with losartan, and 8 PAB cats in whom the angiotensin-converting enzyme (ACE) was concomitantly and continuously blocked with captopril. Losartan cats had at least a one-log order increase in the ED50 of the blood pressure response to angiotensin II infusion. Right ventricular (RV) hypertrophy was assessed using the RV mass-to-body weight ratio and ventricular cardiocyte size. RV hemodynamic overload was assessed by measuring RV systolic and diastolic pressures. Neither the extent of RV pressure overload nor RV hypertrophy that resulted from PAB was affected by AT1 blockade with losartan or ACE inhibition with captopril. RV systolic pressure was increased from 21 +/- 3 mmHg in normals to 68 +/- 4 mmHg in PAB, 65 +/- 5 mmHg in PAB plus losartan and 62 +/- 3 mmHg in PAB plus captopril. RV-to-body weight ratio increased from 0.52 +/- 0.04 g/kg in normals to 1.11 +/- 0.06 g/kg in PAB, 1.06 +/- 0.06 g/kg in PAB plus losartan and 1.06 +/- 0.06 g/kg in PAB plus captopril. Thus 1) pharmacological modulation of the RAS with losartan and captopril did not change the extent of the hemodynamic overload or the hypertrophic response induced by PAB; 2) neither RAS activation nor angiotensin II receptor stimulation is an obligatory and necessary component of the signaling pathway that acts as an intermediary coupling load to the hypertrophic response; and 3) load, independent of the RAS, is capable of stimulating cardiac growth.

Pressure-overload hypertrophy is unabated in mice devoid of AT1A receptors.

Mechanisms controlling cardiac growth are under intense investigation. Among these, the renin-angiotensin system has received great interest. In the current study, we tested the hypothesis that the renin-angiotensin system was not an obligate factor in cardiac hypertrophy. We examined the left ventricular hypertrophic response to a pressure overload in mice devoid of the AT1A receptor, the putative major effector of the growth response of the renin-angiotensin system. Aortic banding produced similar transband gradients in wild-type and AT1A knockout mice. The left ventricular mass-to-body weight ratio increased from 3.44 +/- 0.08 to 5.62 +/- 0.25 in wild-type ascending aortic-banded mice. The response in the knockout mice was not different (from 2.97 +/- 0.13 to 5.24 +/- 0.37). We conclude that the magnitude of cardiac hypertrophy is not affected by the absence of the AT1A receptor and its signaling pathway and that this component of the renin-angiotensin system is not necessary in cardiac hypertrophy.

An adult canine model of progressive left ventricular pressure overload.

This article details the development of a model of progressive left ventricular pressure overload (LVPO) in the adult dog. LVPO was induced by banding the proximal ascending aorta in 69 adult conditioned dogs. The base of the aorta was exposed through a right thoracotomy. A tunnel was created by blunt dissection between the aorta and pulmonary arteries. An aortic band was constructed by passing umbilical tapes through the lumen of gortex tubing. This band was placed through the tunnel, then tied around a balloon dilatation catheter. The distal end of the balloon catheter was closed with an injection cap and positioned in a subcutaneous pocket. Aortic stenosis was induced by filling the balloon catheter with saline. A predetermined amount of LVPO was created by adjusting the amount of aortic stenosis. At 2, 4, and 6 weeks after aortic banding the LVPO was increased by transcutaneous injection of saline into the balloon catheter. At 8 weeks the dogs were evaluated for sufficiently decreased cardiac contractility and used acutely in one of several studies. The article also discusses perioperative management, postoperative care, and complications that were encountered during the development of the model. Postoperative pain was managed by the combined use of preemptive and postoperative opioids, local nerve blocks, and nonsteroidal anti-inflammatory drugs. Notable intraoperative complications included atrial and ventricular arrhythmias and pulmonary artery laceration during the banding procedure. The most significant postoperative complications were aortic ruptures and congestive heart failure. The success rate of this model has increased from 20% (year 1) to 65% (year 3). This success has been attributed to improvements in band design, surgical technique, and postoperative management.

Premorbid determinants of left ventricular dysfunction in a novel model of gradually induced pressure overload in the adult canine.

BACKGROUND: When a pressure overload is placed on the left ventricle, some patients develop relatively modest hypertrophy whereas others develop extensive hypertrophy. Likewise, the occurrence of contractile dysfunction also is variable. The cause of this heterogeneity is not well understood. METHODS AND RESULTS: We recently developed a model of gradual proximal aortic constriction in the adult canine that mimicked the heterogeneity of the hypertrophic response seen in humans. We hypothesized that differences in outcome were related to differences present before banding. Fifteen animals were studied initially. Ten developed left ventricular dysfunction (dys group). Five dogs maintained normal function (nl group). At baseline, the nl group had a lower mean systolic wall stress (96 +/- 9 kdyne/cm2; dys group, 156 +/- 7 kdyne/cm2; P < .0002) and greater relative left ventricular mass (left ventricular weight [g]/body wt [kg], 5.1 +/- 0.36; dys group, 3.9 +/- 0.26; P < .02). On the basis of differences in mean systolic wall stress at baseline, we predicted outcome in the next 28 dogs by using a cutoff of 115 kdyne/cm2. Eighteen of 20 dogs with baseline mean systolic stress > 115 kdyne/cm2 developed dysfunction whereas 6 of 8 dogs with resting stress < or = 115 kdyne/cm2 maintained normal function. CONCLUSIONS: We conclude that this canine model mimicked the heterogeneous hypertrophic response seen in humans. In the group that eventually developed dysfunction there was less cardiac mass despite 60% higher wall stress at baseline, suggesting a different set point for regulating myocardial growth in the two groups.

Effects of chronic beta-adrenergic blockade on the left ventricular and cardiocyte abnormalities of chronic canine mitral regurgitation.

The mechanism by which beta blockade improves left ventricular dysfunction in various cardiomyopathies has been ascribed to improved contractile function of the myocardium or to improved beta-adrenergic responsiveness. In this study we tested two hypotheses: (a) that chronic beta blockade would improve the left ventricular dysfunction which develops in mitral regurgitation, and (b) that an important mechanism of this effect would be improved innate contractile function of the myocardium. Two groups of six dogs with chronic severe mitral regurgitation were studied. After 3 mo both groups had developed similar and significant left ventricular dysfunction. One group was then gradually beta-blocked while the second group continued to be observed without further intervention. In the group that remained unblocked, contractile function remained depressed. However, in the group that received chronic beta blockade, contractile function improved substantially. The contractility of cardiocytes isolated from the unblocked hearts and then studied in the absence of beta receptor stimulation was extremely depressed. However, contractility of cardiocytes isolated from the beta-blocked ventricles was virtually normal. Consistent with these data, myofibrillar density was much higher, 55 +/- 4% in the beta-blocked group vs. 39 +/- 2% (P < 0.01) in the unblocked group; thus, there were more contractile elements to generate force in the beta-blocked group. We conclude that chronic beta blockade improves left ventricular function in chronic experimental mitral regurgitation. This improvement was associated with an improvement in the innate contractile function of isolated cardiocytes, which in turn is associated with an increase in the number of contractile elements.

The effects of complete versus incomplete mitral valve repair in experimental mitral regurgitation.

Severe mitral regurgitation (regurgitant fraction 0.75 +/- 0.02) was created in eight dogs by our closed-chest chordal rupture technique. After 3 months of chronic mitral regurgitation all indices of contractile function were depressed. Mitral valve repair was then attempted. Postoperative regurgitant fraction was reduced compared with the preoperative value in all eight dogs. Concomitantly, forward cardiac output increased in all dogs and pulmonary capillary wedge pressure fell in all dogs. However, in some dogs, significant regurgitation persisted despite repair. Postoperative regurgitant fraction ranged from 0% to 60%. Postoperative residual regurgitant fraction was related significantly to postoperative cardiac output (r = 0.99), pulmonary capillary wedge pressure (r = 0.77), ejection fraction (r = 0.75), and two indices of contractile function--the mass-corrected end-systolic stress volume relationship (r = 0.87) and end-systolic stiffness (r = 0.93). In general, these parameters returned to their normal values before mitral regurgitation when postoperative regurgitant fraction was less than 30%. Myocytes isolated from the ventricles at the end of study also demonstrated normal contractile function when regurgitant fraction was less than 30%.

Native beta-adrenergic support for left ventricular dysfunction in experimental mitral regurgitation normalizes indexes of pump and contractile function.

BACKGROUND: It is generally accepted that the adrenergic nervous system provides inotropic support for the failing heart. However, the magnitude of this support has never been studied extensively. The present study was performed to test the hypothesis that the adrenergic nervous system is capable of maintaining indexes of pump and contractile function in the normal range despite significant innate myocardial depression. METHODS AND RESULTS: We used our model of experimental canine mitral regurgitation, which produces left ventricular dysfunction after 3 months of volume overload. We studied indexes of contractile function on and off beta-blockade at baseline and again on and off beta-blockade 3 months after chronic mitral regurgitation had induced significant contractile dysfunction. At baseline, acute beta-blockade caused insignificant reductions in the mass-corrected slope of the end-ejection stress-volume relation (EESVR), the end-systolic stiffness constant, and the ejection fraction-end-systolic stress and the mean velocity of circumferential fiber shortening (VCF)-end-systolic stress relations. After 3 months of chronic mitral regurgitation, all indexes of contractile function were normal in the unblocked state except for the VCF-stress relation, which was mildly reduced. However, after acute beta-blockade after 3 months of chronic mitral regurgitation, the EESVR fell to 303 +/- 27 versus 443 +/- 24 during acute beta-blockade before mitral regurgitation was created (P < .05), and the end-systolic stiffness constant was reduced to 2.54 +/- 0.15 versus 3.27 +/- 0.11 (P < .05). Only after beta-blockade was the ejection fraction-stress relation significantly reduced for dogs with chronic mitral regurgitation. The VCF-stress relation became markedly more abnormal. The viscosity-velocity relation of myocytes isolated from the ventricles of the dogs with mitral regurgitation confirmed that substantial innate contractile depression was present. CONCLUSIONS: After 3 months of chronic mitral regurgitation, the adrenergic nervous system was able to maintain most indexes of contractile function in the normal range despite significant depression in innate contractile function. Thus, in the absence of beta-blockade, significant innate contractile depression may be obscured by adrenergic support.

Coronary blood flow after the regression of pressure-overload left ventricular hypertrophy.

Abnormal coronary blood flow (CBF) in long-standing left ventricular (LV) pressure-overload hypertrophy has been associated with ischemia and LV dysfunction. Thus, goals of therapy in pressure overload are not only the relief of the overload itself but also regression in hypertrophy and subsequent improvement in CBF. However, little is known about CBF in humans or in large mammals after the relief of pressure overload, when the hypertrophy has regressed. This study was performed to test the hypothesis that, even 6 months after the relief of pressure overload in the dog, CBF would still be abnormal. Three groups of dogs were studied: 1) normal control dogs (NL group), 2) dogs with LV pressure-overload hypertrophy (LVH group), and 3) dogs that had developed LV pressure-overload hypertrophy but in whom the pressure overload was relieved 6 months before the final study (LVH Reg group). CBF was studied in conscious dogs by use of the radiolabeled microsphere technique at rest, during rapid atrial pacing, and during maximum coronary vasodilation produced by adenosine infusion. The ratio of LV weight (g) to body weight (kg) (LVBW) was 4.2 +/- 0.3 in the NL group, 7.1 +/- 0.6 in the LVH group, and 7.7 +/- 0.5 in the LVH Reg group before pressure-overload relief (p = NS, LVH versus LVH Reg). Six months after removal of the pressure overload, the LVBW in the LVH Reg group had fallen to 5.5 +/- 0.3 (p < 0.05), but this LVBW was still greater than that in the NL group (p < 0.05). During rapid atrial pacing, endocardial and epicardial CBF rose significantly in NL dogs. However, during rapid atrial pacing, endocardial CBF fell from 1.18 +/- 0.22 to 0.7 +/- 0.20 ml/min per gram in the LVH group (p < 0.05) and did not rise in the LVH Reg group. During adenosine infusion, endocardial blood flow increased in NL dogs from 1.63 +/- 0.13 to 4.0 +/- 0.3 ml/min per gram and increased to a similar level in the LVH Reg group. Although CBF increased during adenosine infusion in the LVH group, the increase was less than that in the NL or LVH Reg group (p < 0.05). Minimum coronary vascular resistance was similar in NL dogs (14 +/- 2 units) and LVH Reg dogs (18 +/- 3 units, p = NS) but was significantly elevated (32 +/- 10 units) in LVH dogs (p < 0.05).(ABSTRACT TRUNCATED AT 400 WORDS)

Left ventricular mechanics and myocyte function after correction of experimental chronic mitral regurgitation by combined mitral valve replacement and preservation of the native mitral valve apparatus.

BACKGROUND: Contractile function improves after correction of experimental mitral regurgitation, but ejection performance becomes depressed when mitral valve replacement involves chordal transection. A role for chordal transection in producing the depressed ejection performance was suspected but uncertain. Therefore, in this study, we tested two specific hypotheses: 1) that contractile function would improve and, in conjunction with chordal preservation, would allow for preserved ejection performance and 2) that improved left ventricular contractile function after surgery would be reflected in the function of myocytes isolated from the affected left ventricles. METHODS AND RESULTS: We examined ventricular contractile function and ejection performance and isolated myocyte function after correction of experimental mitral regurgitation (chordal rupture) with mitral valve replacement that involved chordal preservation. After 3 months of chronic mitral regurgitation, the average regurgitant fraction of seven dogs was 0.77 +/- 0.04. End-diastolic volume had increased from 79 +/- 5 to 132 +/- 10 cm3 (p < 0.05). At that time, all indexes of left ventricular contractile function were depressed. Three months after mitral valve replacement with chordal preservation, end-diastolic volume fell to 100 +/- 4 cm3 (p < 0.05). At this time, all indexes of contractile function had returned to normal. End-systolic stress and ejection fraction after mitral valve replacement were similar to their baseline levels. Viscosity-velocity curves (analogous to force-velocity curves) of myocytes isolated from the affected left ventricles were similar to those of myocytes isolated from normal left ventricles. CONCLUSIONS: We conclude that mitral valve replacement with chordal preservation allows ventricular contractile function to return to normal. Normal global ventricular function, in turn, is associated with normal function of the individual myocytes that compose the left ventricular chamber. Further, chordal preservation allowed for loading and ejection performance to return to premorbid levels.