Progression from hypertrophic to dilated cardiomyopathy in mice that express a mutant myosin transgene.

A mouse model of hypertrophic cardiomyopathy (HCM) was created by expression of a cardiac alpha-myosin transgene including the R(403)Q mutation and a deletion of a segment of the actin-binding domain. HCM mice show early histopathology and hypertrophy, with progressive hypertrophy in females and ventricular dilation in older males. To test the hypothesis that dilated cardiomyopathy (DCM) is part of the pathological spectrum of HCM, we studied chamber morphology, exercise tolerance, hemodynamics, isolated heart function, adrenergic sensitivity, and embryonic gene expression in 8- to 11-mo-old male transgenic animals. Significantly impaired exercise tolerance and both systolic and diastolic dysfunction were seen in vivo. Contraction and relaxation parameters of isolated hearts were also decreased, and lusitropic responsiveness to the beta-adrenergic agonist isoproterenol was modestly reduced. Myocardial levels of the G protein-coupled beta-adrenergic receptor kinase 1 (beta-ARK1) were increased by more than twofold over controls, and total beta-ARK1 activity was also significantly elevated. Induction of fetal gene expression was also observed in transgenic hearts. We conclude that transgenic male animals have undergone cardiac decompensation resulting in a DCM phenotype. This supports the idea that HCM and DCM may be part of a pathological continuum rather than independent diseases.

Pathophysiology of sodium and water retention in heart failure.

Heart failure is a leading cause of morbidity and mortality. In the United States, there are more than 5 million patients with heart failure and over 500,000 newly diagnosed cases each year. Numerous advances have been made in our understanding of the pathophysiologic mechanisms contributing to sodium and water retention in this condition. Important alterations in the sympathetic nervous system and the renin-angiotensin-aldosterone system have been described in heart failure, allowing the use of mechanism-specific treatments such as beta-adrenergic receptor antagonism and angiotensin-converting enzyme inhibition. As our understanding of the roles of the natriuretic peptides and the arginine vasopressin-aquaporin-2 system in the pathophysiology of heart failure evolves, treatments directed toward the alterations in these systems in heart failure can be further developed.

Myocardial-directed overexpression of the human beta(1)-adrenergic receptor in transgenic mice.

The beta(1)-adrenergic receptor (AR) is the dominant subtype in non-failing and failing myocardium. beta(1)-AR signaling, by the endogenous neurotransmitter norepinephrine, is central to the regulation of myocardial contractility. In heart failure, the beta(1)-AR undergoes subtype-selective downregulation which may protect against the increased cardiac adrenergic drive associated with this pathophysiological state. To examine the hypothesis that chronically increased beta(1)-AR mediated signaling has adverse myocardial effects, transgenic mice overexpressing the human beta(1)-AR in a cardiac-selective context were produced, utilizing an alpha-myosin heavy chain (MHC) promoter. In these mice, beta(1)-AR protein abundance was approximately 24-46-fold (1-2 pmol/mg protein) that of wild-type mice. Histopathological examination of young (4 months old) and old (approximately 9 months old) transgenic mouse hearts consistently demonstrated large areas of interstitial replacement fibrosis, marked myocyte hypertrophy and myofibrilar disarray. In addition, increased expression of the pre-apoptotic marker, Bax, was observed coincident with regions of fibrosis accompanied by an increased apoptotic index, as measured by TUNEL assay. Older non-transgenic mice exhibited a slight tendency towards a decreased fractional shortening, whereas older beta(1)-AR transgenic mice had a marked reduction in fractional shortening (%FS approximately 30) as determined by echocardiography. Additionally, older beta(1)-AR transgenic mice had an increased left ventricular chamber size. In summary, cardiac-directed overexpression of the human beta(1)-AR in transgenic mice leads to a significant histopathological phenotype with no apparent functional consequence in younger mice and a variable degree of cardiac dysfunction in older animals. This model system may ultimately prove useful for investigating the biological basis of adrenergically-mediated myocardial damage in humans.

Cardiac myosin heavy chains lacking the light chain binding domain cause hypertrophic cardiomyopathy in mice.

Myosin is a chemomechanical motor that converts chemical energy into the mechanical work of muscle contraction. More than 40 missense mutations in the cardiac myosin heavy chain (MHC) gene and several mutations in the two myosin light chains cause a dominantly inherited heart disease called familial hypertrophic cardiomyopathy. Very little is known about the biochemical defects in these alleles and how the mutations lead to disease. Because removal of the light chain binding domain in the lever arm of MHC should alter myosin's force transmission but not its catalytic function, we tested the hypothesis that such a mutant MHC would act as a dominant mutation in cardiac muscle. Hearts from transgenic mice expressing this mutant myosin are asymmetrically hypertrophied, with increases in mass primarily restricted to the cardiac anterior wall. Histological examination demonstrates marked cellular hypertrophy, myocyte disorganization, small vessel coronary disease, and severe valvular pathology that included thickening and plaque formation. Skinned myocytes and multicellular preparations from transgenic hearts exhibited decreased Ca2+ sensitivity of tension and decreased relaxation rates after flash photolysis of diazo 2. These experiments demonstrate that alterations in myosin force transmission are sufficient to trigger the development of hypertrophic cardiomyopathy.

Diagnosis and treatment of diastolic heart failure.

Up to 40% of patients with heart failure have isolated diastolic dysfunction. With proper management, the prognosis is generally more favorable than in systolic dysfunction. Distinguishing diastolic from systolic dysfunction is essential since the optimal therapy for one condition may aggravate the other. New echocardiographic methods enable accurate diagnoses.

The pulmonary circulation of homozygous or heterozygous eNOS-null mice is hyperresponsive to mild hypoxia.

Acute hypoxic vasoconstriction and development of hypoxic pulmonary hypertension (PHTN) are unique properties of the pulmonary circulation. The pulmonary endothelium produces vasoactive factors, including nitric oxide (NO), that modify these phenomena. We tested the hypothesis that NO produced by endothelial nitric oxide synthase (eNOS) modulates pulmonary vascular responses to hypoxia using mice with targeted disruption of the eNOS gene (eNOS-/-). Marked PHTN was found in eNOS-/- mice raised in mild hypoxia when compared with either controls or eNOS-/- mice raised in conditions simulating sea level. We found an approximate twofold increase in partially and fully muscularized distal pulmonary arteries in eNOS-/- mice compared with controls. Consistent with vasoconstriction being the primary mechanism of PHTN, however, acute inhalation of 25 ppm NO resulted in normalization of RV pressure in eNOS-/- mice. In addition to studies of eNOS-/- mice, the dose-effect of eNOS was tested using heterozygous eNOS+/- mice. Although the lungs of eNOS+/- mice had 50% of normal eNOS protein, the response to hypoxia was indistinguishable from that of eNOS-/- mice. We conclude that eNOS-derived NO is an important modulator of the pulmonary vascular response to chronic hypoxia and that more than 50% of eNOS expression is required to maintain normal pulmonary vascular tone.

Microtubules modulate cardiomyocyte beta-adrenergic response in cardiac hypertrophy.

The role of microtubules in modulating cardiomyocyte beta-adrenergic response was investigated in rats with cardiac hypertrophy. Male Sprague-Dawley rats underwent stenosis of the abdominal aorta (hypertensive, HT) or sham operation (normotensive, NT). Echocardiography and isolated left ventricular cardiomyocyte dimensions demonstrated cardiac hypertrophy in the HT rats after 30 wk. Cardiomyocyte microtubule fraction was assayed by high-speed centrifugation and Western blot. In contrast to previous reports of increased microtubules after acute pressure overload, microtubule fraction for HT was significantly lower than that for NT. Cardiomyocytes were exposed to either 1 microM colchicine, 10 microM taxol, or equivalent volume of vehicle. Colchicine decreased microtubules, and taxol increased microtubules in both groups. Cardiomyocyte cytosolic calcium ([Ca2+]c) and shortening/relaxation dynamics were assessed during exposure to increasing isoproterenol concentrations. The beta-adrenergic response for these variables in the HT group was blunted compared with NT. However, increased microtubule assembly by taxol partially recovered the normal beta-adrenergic response for time to peak [Ca2+]c, time to peak shortening, and mechanical relaxation variables. Microtubule assembly may play a significant role in determining cardiomyocyte beta-adrenergic response in chronic cardiac hypertrophy.

Congenital duplication of the tricuspid valve presenting as a circumscribed obstructive mass in the right ventricular outflow tract.

Congenital duplication of the tricuspid valve is a rare condition. We present a case of a 32-year-old man with an increase in intensity of a long-standing heart murmur and intermittent palpitations. The murmur was evaluated by transthoracic and transesophageal echocardiography that identified a mass in the right ventricle and right ventricular outflow tract resulting in partial obstruction of the right ventricular outflow tract. This mass was determined to be congenital duplication of the tricuspid valve by histologic examination after surgical excision.

Comparison of amlodipine and long-acting diltiazem in the treatment of mild or moderate hypertension.

The comparative effects of the once a day calcium channel antagonists amlodipine and long-acting diltiazem were assessed in a parallel design, investigator-blinded, multicenter trial in 123 patients with diastolic blood pressures ranging from 95 to 114 mm Hg before treatment. Patients were randomized to one of the two drugs and titrated at 2-week intervals to 5 or 10 mg of amlodipine or 180, 240, or 360 mg of long-acting diltiazem during a 10-week treatment period. Both drugs significantly reduced resting, sitting, standing, and 24-h ambulatory systolic and diastolic pressures. Amlodipine caused significantly greater reductions in sitting and standing systolic pressures, standing diastolic pressures, and 24-h ambulatory systolic and diastolic pressures versus diltiazem. Sitting systolic pressures were reduced from 151.9 +/- 2.0 (SE) at baseline to 137.9 +/- 1.8 mm Hg with amlodipine treatment and from 149.0 +/- 2.1 to 145.1 +/- 2.5 mm Hg with diltiazem. Sitting diastolic pressures were reduced from 100.2 +/- 0.6 to 87.8 +/- 1.0 mm Hg with amlodipine and from 101.1 +/- 1.0 to 91.9 +/- 1.1 mm Hg with diltiazem. Reductions in standing systolic pressures after treatment were -12.1 +/- 1.5 mm Hg amlodipine v -4.6 +/- 1.5 mm Hg diltiazem (P < .01), and reductions in standing diastolic pressures were -11.8 +/- 0.9 mm Hg amlodipine v -8.6 +/- 0.9 mm Hg diltiazem (P < .02). Heart rates did not change significantly with either drug during the study. Two subjects in each group dropped out because of adverse experiences. Although both agents were well tolerated and reduced blood pressures consistently over the 10-week test period, amlodipine was more effective than diltiazem in reducing systolic and diastolic blood pressures to the target pressures of < 140 mm Hg systolic and < 90 mm Hg diastolic over a range of doses widely used in clinical practice.

The effect of angiotensin II on glomerular function and morphology in the rat.

We have observed alterations of glomerular and afferent arteriolar structure in nephrotoxic and ischemic models of acute renal failure. These alterations may have been caused by angiotensin II. To test this notion, we infused the peptide directly into the left renal artery of anesthetized rats. At sub-pressor doses of 1.9 and 6.5 ng/100g body weight/min, the peptide caused dose-related decreases in glomerular filtration rate similar to those that occur in models of acute renal failure. In some animals, methacrylate casts of the infused and contralateral kidneys were made for later examination by scanning electron microscopy. In other animals, the kidneys were fixed with glutaraldehyde for examination of the glomerular epithelium and endothelium by scanning electron microscopy. Cast analysis showed no detectable alteration of afferent arteriolar diameter, and only moderate bilateral changes in the tapering of the arterioles. In contrast, the diameter of the casted glomerular tuft was significantly reduced by the highest dose of angiotensin II. No alterations of the glomerular epithelium or endothelial fenestrae were observed. We conclude that angiotensin II probably did not mediate the changes in glomerular structure in our earlier studies of acute renal failure, even though similar reductions of glomerular filtration rate occurred. Reduced diameter of the glomerular tuft is consistent with a possible role for angiotensin II as a modulator of intrinsic contractile elements in glomeruli.