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Proliferative growth of the ventricular myocyte (cardiomyocyte) is primarily limited to embryonic, fetal and very early neonatal periods of heart development. In contrast, cardiomyocyte maturation, as evidenced by cellular hypertrophy, is a long-term process that can occupy the bulk of the life-span of the mature organism. As the newborn myocyte undergoes a 'transition' from proliferative to hypertrophic growth, ventricular remodeling of the non-myocyte compartment is characterized by increased extracellular matrix (ECM) formation and coronary capillary angiogenesis. A role for ventricular-derived growth factors (GFs) in these inter-related processes are examined in an animal model of altered heart development produced by neonatal aortic banding. The suprarenal abdominal aorta of five day old rat pups were banded (B), sham operated (S), or untreated (C) and ventricular tissue (left ventricular free wall and septum) obtained at 7-, 14-, and 21-days post-intervention. Using Northern blot RNA hybridizations, expression of growth factors (GFs) and/or GF-receptors (GFR's) temporally associated with heart development were evaluated. Transcript levels for TGF-beta 1, IGF-II, and their associated cell surface receptors were increased in B animals. Concomitant changes in extracellular matrix (ECM) genes (as evaluated by Collagens Type I, III, and IV) were also increased in B animals. In addition, transcript levels for the vascular morphogenesis and remodeling-related protein SPARC (Secreted Protein, Acidic and Rich in Cysteine) was also elevated in the B animals. In several instances, S animals demonstrated changes in steady state transcript levels for genes which may influence myocyte maturation during the postnatal period. This suggests that normal autocrine/paracrine growth regulatory stimuli and responses can be modified (by surgical intervention and/or abdominal aortic banding) and these perturbations in gene expression may be related to previously documented changes in myocyte cell number, vascular composition, and ventricular architecture of the banded, neonatal heart. Future studies using this model will provide an opportunity to evaluate and possibly identify the stimuli and signal transduction machinery that regulate the final phases of myocyte proliferation, stimulate capillary formation and ECM deposition, and orchestrate the transition to hypertrophic growth during heart development.

Chronic activation of the circulating renin-angiotensin-aldosterone system (RAAS), as can occur with unilateral renal ischemia (URI), is associated with an adverse structural remodeling of the right and left ventricles characterized by reparative (i.e., microscopic scars) and reactive (i.e., perivascular/interstitial) fibrosis. The time course and cells involved in fibroplastic and fibrogenic phases of these events are unclear. Hearts were examined over the course of 8 weeks in rats infused with either angiotensin II or aldosterone, and compared to rats with URI. Tissue sections from the same heart were stained with hematoxylin and eosin, collagen specific picrosirius red, or immunolabeled with PCNA or alpha smooth muscle actin antibody. With angiotensin II or renal ischemia, fibroblast proliferation, presenting as focal accumulations at both sites of myocyte necrosis and widespread perivascular locations, was present in each ventricle on days 2 and 4, but not thereafter, alpha-Smooth muscle actin containing cells (myofibroblasts) appeared at day 2 and persisted through week 2 with renal ischemia and week 6 with angiotensin II. Macrophages, neutrophils and lymphocytes were transiently found at sites of necrosis between day 2-4 of renal ischemia. AngII-induced necrotic sites were characterized by macrophages and lymphocytes from day 2 through week 6, and neutrophils at day 2-4. Increased collagen volume fraction, presenting as immature scars associated with fibroblast clusters and interstitial/perivascular fibrosis, was evident on day 14 in both ventricles. In contrast, fibroblast proliferation during aldosterone infusion did not appear in both ventricles until week 3 and was associated with a subsequent reparative and reactive fibrosis as early as 4 weeks. Myofibroblasts became evident between 3-6 weeks; macrophages and lymphocytes were seen between 3-8 weeks. Neutrophils were not seen at any time point with aldosterone. Thus, the temporal cellular response and appearance of myocardial fibrosis associated with chronic elevations in angiotensin II and/or aldosterone differ. We conclude that separate pathogenic mechanisms are operative with these effector hormones of the RAAS.

Objective: Cardiac myocyte necrosis is associated with an endogenous increase in angiotensin II or a subpressor dose of angiotensin II. In each case, raised plasma angiotensin II, together with aldosterone, results in cardiac myocyte necrosis followed by scarring. Arterial hypertension is non-contributory. The aim of this study was to determine whether myocyte necrosis is induced directly by angiotensin II, or indirectly by catecholamines or aldosterone released from adrenal glands in response to angiotensin II., Methods: Right and left ventricular myocardium was examined over a two week period in rats receiving a subpressor yet cardiotoxic dose of angiotensin II (150 ng.min-1.kg-1 subcutaneously) alone or together with total adrenalectomy, bilateral medullectomy, or the aldosterone receptor antagonist spironolactone (150-200 mg.d-1.kg-1). Findings were compared to unoperated age/sex matched controls. Coronal sections obtained from the base, equator and apex of the heart were stained with antifibronectin antibody to detect myocyte injury, picrosirius red for fibrillar collagen, or haematoxylin and eosin., Results: In both right and left ventricles (a) fibronectin labelling was distributed within injured cardiac myocytes and adjacent interstitium of both ventricles in rats receiving angiotensin II or angiotensin II plus spironolactone; (b) fibronectin labelling of myocytes was markedly diminished or absent in the ventricles of rats with total adrenalectomy or bilateral medullectomy; and (c) there was microscopic scarring of both ventricles after two weeks of either angiotensin II or angiotensin II plus spironolactone, but not in medullectomised or adrenalectomised animals., Conclusions: In the presence of increased circulating levels of angiotensin II, bilateral medullectomy and total adrenalectomy (in contrast to spironolactone) reduced myocyte injury and scarring of the right and left ventricular myocardium, suggesting that the cytotoxic effect of angiotensin II is largely indirect and related to circulating catecholamines released by the adrenal medulla.

Objective: The aim was to determine if long term treatment with nifedipine or nisoldipine affects structural remodelling of cardiac myocytes and is effective in attenuating or preventing reparative and reactive myocardial fibrosis in essential hypertension., Methods: Five and a half month old male spontaneously hypertensive rats and Wistar Kyoto (WKY) rats received either 1000 ppm nifedipine or nisoldipine or no treatment (controls) for 22 weeks. Haemodynamic variables were measured and hearts recovered from animals of each group. Cardiac myocytes were isolated by retrograde coronary perfusion with collagenase. Cell volume was determined by Coulter analysis, cell length by direct measurement, and cross sectional area by volume/length. Cardiac myocyte number was calculated for both ventricles. Tissue sections from perfusion fixed hearts were stained with picrosirius red and myocardial collagen was analysed. Reparative fibrosis was assessed by the presence of microscopic scarring and reactive (interstitial and perivascular) fibrosis was quantified., Results: Nifedipine and nisoldipine significantly decreased systolic and diastolic blood pressure in spontaneously hypertensive rats, although normotensive pressures did not result. Left ventricular weight relative to body weight was significantly decreased in nifedipine and nisoldipine treated compared with untreated spontaneously hypertensive rats, although values remained significantly greater than WKY controls. Cardiac myocyte volume was slightly decreased with attenuation of blood pressure in spontaneously hypertensive rats, but no significant differences were found. Cardiac myocyte number for each ventricle was similar between groups. Microscopic scarring was significantly decreased in nifedipine and nisoldipine treated compared with untreated spontaneously hypertensive rats and interstitial and perivascular fibrosis were substantially reduced., Conclusions: Nifedipine and nisoldipine: (a) attenuate hypertension and decrease left ventricular mass in spontaneously hypertensive rats, but the associated decrease in myocyte size was not significant; (b) are cardioprotective as indicated by a significant decrease in the incidence of microscopic scarring; and (c) decrease the extent of reactive, both interstitial and perivascular, fibrosis normally found in untreated spontaneously hypertensive rats. Nifedipine and nisoldipine have the potential to positively alter myocardial pathology in essential hypertension.

In this report we review the replacement (i.e., scarring) and reactive (i.e., perivascular and interstitial fibrosis) fibrous tissue responses found in the myocardium in response to effector hormones of the renin-angiotensin-aldosterone system. Experimental data are presented to indicate: a) endogenous or exogenous elevations in plasma angiotensin II are associated with acute cardiac myocyte necrosis and subsequent microscopic scarring; b) chronic elevations in plasma aldosterone (ALDO), relative to Na+ intake, are associated with a perivascular and interstitial fibrosis of the coronary and systemic circulations and are also seen in response to chronic administration of the mineralocorticoid hormone deoxycorticosterone (DOC); and c) chronic mineralocorticoid excess, due to ALDO or DOC, is associated with enhanced urinary K+ excretion, cardiac myocyte necrosis and scarring. Pharmacologic agents which interfere with these effector hormones (e.g., ACE inhibition and ALDO receptor antagonism) protect the myocardium against this pathologic structural remodeling created by the reactive and replacement (reparative) fibrosis. Evidence is also presented to indicate that chronic ACE inhibition is associated with a regression in reactive myocardial fibrosis. Based on these experimental findings we would suggest that clinical trials are indicated to address the prevention and regression of myocardial fibrosis--an important determinant of pathologic structural remodeling and abnormal myocardial stiffness.

Changes in hemodynamic load cause alterations in cardiac myocyte size, with regional variations in myocyte size distribution possible within the ventricular wall. We studied regional changes in cellular dimensions and their distribution in two models of cardiac hypertrophy and in cardiac atrophy in the rat. Combined volume-pressure overload was produced by 3,3',5-triiodo-L-thyronine (T3) treatment; atrophy was produced by heterotopic isotransplantation. Our previous data from long-term pressure overload after aortic constriction were used for comparison. Isolated ventricular myocytes were obtained after in vitro coronary perfusion with collagenase. Cell volume and its distribution were determined; cell length was directly measured by image analysis, and cross-sectional area was estimated from the cell volume/cell length ratio, assuming a cylindrical model. Myocyte hypertrophy resulting from hyperthyroidism and aortic constriction was primarily due to increased cross-sectional area. In both cases, the relative response was greater in the right ventricle than in the left ventricle. Within the left ventricle, epimyocardial myocytes enlarged the most. Aortic constriction and T3 treatment predominantly increased the size of smaller myocytes. Heterogeneity in myocyte size increased after constriction but remained relatively unaffected after T3 treatment. Atrophy of left ventricular myocytes was due to a proportional decrease in cell length and cross-sectional area, with the greatest decrease in the left ventricular endomyocardium. Atrophy predominantly affected larger myocytes, resulting in a more homogeneous overall population of smaller myocytes. We conclude that various alterations in load lead to diverse remodeling in the myocyte population throughout the ventricular wall. In general, smaller myocytes show the highest growth potential, whereas larger myocytes exhibit the highest potential to atrophy.