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Objective and Methods: Left ventricular hypertrophy in human and experimental hypertension is not always associated with pressure overload but seems to precede an increase in blood pressure. In this study, performed in male 5-week-old prehypertensive spontaneously hypertensive rats (SHR; n = 65) and age-matched Wistar-Kyoto rats (n = 56), the relationship between myocardial structure and activation of the adrenergic and nitric oxide systems was evaluated., Results: Body weight, blood pressure and heart rate were similar in both groups. A higher left ventricle/body weight ratio was found in SHR, as a result of greater mononuclear (+47%) and binuclear (+43%) myocyte volumes, without changes in interstitial collagen. Both adrenergic and nitric oxide pathways were activated in SHR, as expressed by higher myocardial norepinephrine content, tyrosine hydroxylase activity, myocardial nitric oxide synthase 3 expression and protein nitration, indicating greater peroxynitrite (ONOO) generation from nitric oxide and superoxide. No difference was measured in nitric oxide synthase 1 expression, whereas nitric oxide synthase 2 was undetectable. A positive correlation between myocardial tyrosine hydroxylase activity and protein nitration was observed in SHR (r = 0.328; P < 0.01). Early treatment with a superoxide dismutase mimetic, 4-hydroxy-2,2,6,6-tetramethyl piperidinoxyl, from the third to the fifth week of age, reduced ONOO generation, protein nitration and sympathetic activation in SHR without changes in myocardial structure., Conclusion: In prehypertensive SHR, left ventricular hypertrophy is associated with adrenergic and nitrosative imbalance. Early superoxide dismutase mimetic treatment in SHR effectively reduces higher myocardial ONOO generation, sympathetic activation, and heart rate without affecting the development of myocardial hypertrophy.

Experimental and clinical evidence suggests kinin involvement in adaptive myocardial growth. Kinins are growth-inhibitory to cardiomyocytes. Knockout of kinin B2 receptor (B2R) signaling causes dilated and failing cardiomyopathy in 129/J mice, and a 9-bp deletion polymorphism of human B2R is associated with reduced receptor expression and exaggerated left ventricular growth response to physical stress. We reasoned that genetic background and aging may significantly influence the impact of B2R mutation on cardiac phenotype. The theory was challenged in C57BL/6 mice, a strain that naturally differs from the 129/J strain, carrying 1 instead of 2 renin genes. C57BL/6 B2R knockouts (B2R-KO) showed higher blood pressure and heart rate levels (P<0.05) compared with wild-type controls (WT) at all ages examined. At 12 months, left ventricular contractility and diastolic function were mildly altered (P<0.05) and histological and morphological analyses revealed ventricular hypertrophy and cardiomyocyte enlargement in B2R-KO (P<0.01). Reparative fibrosis was enhanced by 208% and capillary density reduced by 38% (P<0.01). Functional and structural alterations induced by B2R deletion in C57BL/6 mice were less severe than those reported previously in the 129/J strain. We conclude that interaction of B2R signaling with other genetic determinants influences aging-related changes in myocardial structure and function. These findings may help us understand the role of kinins in the development of cardiac failure.

In 47 male adult Wistar rats with 4-wk aortic coarctation (AC) and 39 age-matched sham-operated rats (SO) chronically instrumented for telemetry electrocardiogram recording, we investigated the mechanisms of arrhythmogenesis in moderate cardiac hypertrophy, with an approach from "in vivo" toward the cellular level, analyzing 1) stress-induced cardiac arrhythmias in all rats and 2) myocardial fibrosis in 35 animals and action potential duration and density of hyperpolarization-activated current in 19 others at the ventricular level. Aortic banding increased arterial blood pressure, cardiac weight, and ventricular myocyte volume by 11, 25, and 14%, respectively (P < 0.001-0.05). Ventricular arrhythmias occurred at similar rates in AC and SO rats throughout the stress procedure. Action potential duration and hyperpolarization-activated current were about twice as great and myocardial fibrosis about four times greater in AC animals (P < 0.005-0.05). Electrocardiogram data also revealed more supraventricular arrhythmias in AC rats during the baseline period and after stress and fewer atrioventricular block episodes after stress (P < 0.05). Thus stress-induced supraventricular and atrioventricular nodal, but not ventricular, arrhythmias were affected in moderate cardiac hypertrophy when ventricular morphofunctional alterations were evident.

The current investigation was designed to evaluate whether the proportion of mononucleated binucleated, trinucleated and tetranucleated myocytes varies in the left ventricle, interventricular septum and right ventricular free wall with aging, cardiac hypertrophy and ischemic cardiomyopathy. In addition, the number and dimensional properties of myocytes were measured to determine whether a relationship existed between myocyte size and number, and organ hypertrophy. For this purpose, 72 normal hearts were obtained from individuals who died from causes other than cardiovascular disease and compared with 81 hypertrophied hearts and 95 with ischemic cardiomyopathy. The age interval examined varied from 26 to 93 years. The analysis of enzymatically dissociated myocytes in control left ventricles demonstrated that mononucleated, binucleated, trinucleated, trinucleated and tetranucleated myocytes comprised 74%, 25.5%, 0.4% and 0.1% of the entire myocyte population. Similarly, mononucleated myocytes constituted the prevailing cell population of the interventricular septum and right ventricular free wall. Aging, myocardial hypertrophy and ischemic cardiomyopathy did not change the percentage of mononucleated and multinucleated myocyte in the ventricular myocardium. Cardiac hypertrophy and ischemic cardiomyopathy were characterized by comparable increase in myocyte size in spite of a significant difference in the magnitude of myocardial hypertrophy. Myocyte number was increased in hypertrophied hearts, whereas myocyte cell loss occurred in ischemic cardiomyopathy. In conclusion, aging, cardiac hypertrophy and ischemic cardiomyopathy do not alter the fractions of mononucleated and multinucleated myocytes in the myocardium.

Objectives: This study investigated the changes in myocyte size and number in the left and right ventricles that occur with aging in the female and male heart., Background: Differences in life span between women and men may be related to a better preservation of myocardial structure in the female heart with aging. On this basis, the hypothesis was advanced that the aging process has a different impact on the integrity of the myocardium in the two genders., Methods: Morphometric methodologies were applied to analyze the changes in number and size of ventricular myocytes in the hearts of 53 women and 53 men. The changes in mononucleated and binucleated myocytes with age were determined in enzymatically dissociated cells. The age interval examined varied from 17 to 95 years., Results: Aging was associated with a preservation of ventricular myocardial mass, aggregate number of mononucleated and binucleated myocytes, average cell diameter and volume in the female heart. In contrast, nearly 1 g/year of myocardium was lost in the male heart, and this phenomenon accounted for the loss of approximately 64 million cells. This detrimental effect involved the left and right sides of the heart. In the remaining cells, myocyte cell volume increased at a rate of 158 microns3/year in the left and 167 microns3/year in the right ventricle., Conclusions: Aging does not lead to myocyte cell loss and myocyte cellular reactive hypertrophy in women, indicating that gender differences may play a significant role in the detrimental effects of the aging process on the heart.

To determine the effects of chronic anemia on the functional and structural characteristics of the heart, 1-month-old male rats were fed a diet deficient in iron and copper, which led to a hemoglobin concentration of 4.63 g/dl, for 8 weeks. At sacrifice, under fentanyl citrate and droperidol anesthesia, systolic, diastolic, and mean arterial blood pressures were decreased, whereas differential pressure was increased. Left ventricular systolic pressure and the ventricular rate of pressure rise (mmHg/s) were reduced by 9% and 14%, respectively. Moreover, developed peak systolic ventricular pressure and maximal dP/dt diminished 14% and 12%. After perfusion fixation of the coronary vasculature and the myocardium, at a left ventricular intracavitary pressure equal to the in vivo measured end diastolic pressure, a 10% thickening of the left ventricular wall was measured in association with a 13% increase in the equatorial cavitary diameter and a 44% augmentation in ventricular mass. The 52% hypertrophy of the right ventricle was characterized by an 11% thicker wall and a 37% larger ventricular area. The 33% expansion in the aggregate myocyte volume of the left ventricle was found to be due to a 14% myocyte cellular hypertrophy and a 17% myocyte cellular hyperplasia. These cellular parameters were calculated from the estimation of the number of myocyte nuclei per unit volume of myocardium in situ and the evaluation of the distribution of nuclei per cell in enzymatically dissociated myocytes. Myocyte cellular hyperplasia provoked a 9% increase in the absolute number of cells across the left ventricular wall. In contrast, myocyte cellular hypertrophy (42%) was responsible for the increase in myocyte volume of the right ventricle. The proliferative response of left ventricular myocytes was not capable of restoring diastolic cell stress, which was enhanced by the changes in ventricular anatomy with anemia. In conclusion, chronic anemia induced an unbalanced load on the left ventricle, which evoked a hyperplastic reaction of preexisting myocytes, in an attempt to normalize diastolic wall and myocyte stress.

To determine the effects of long-term pressure overload on the structural mechanisms implicated in wall remodeling of the right ventricle, a mild pulmonary artery banding was applied to rats approximately 2 months old, and the animals were killed 150 days later. The surgical procedure resulted in a 60% reduction in the cross-sectional area of the constricted vessel and a 52% increase in the weight of the right ventricle. The hypertrophic myocardial response was associated with an elevation in right ventricular systolic pressure (from 33 +/- 11 mm Hg to 71 +/- 12 mm Hg), right ventricular end-diastolic pressure (from 3 +/- 1 mm Hg to 10 +/- 3 mm Hg), and central venous pressure (from 2 +/- 0.2 mm Hg to 10 +/- 3 mm Hg). The 76% increase in wall thickness after pulmonary artery stenosis was the result of a 24% lateral expansion of cardiac muscle cells and a 44% increase in the number of myocytes across the ventricular wall. The intermyocyte distance was also increased by 22%. These cellular adaptations occurred with no alterations in total myocyte length, average sarcomere length, and volume composition of the myocardium. Ventricular wall area was decreased by 14%, which suggests a small reduction in chamber volume. Myocyte growth was accompanied by proportional expansions of mitochondrial and myofibrillar components, so that the ratio of mitochondria to myofibrils in the cytoplasm remained essentially constant. In conclusion, ventricular remodeling in this model of chronic pressure hypertrophy is characterized by increases in cellular diameter and number that would both tend to decrease the magnitude of systolic and diastolic stresses on a per cell basis and thus improve the myocardial response to a prolonged and sustained mechanical load.

To determine whether a prolonged duration of mechanical load on the ventricular myocardium leads to capillary proliferation, constriction of the pulmonary artery was produced in rats at 3 mo of age, and the hearts were examined 200 days later. Functionally, elevations of right ventricular systolic pressure, from 30 +/- 12 to 67 +/- 11 mmHg, and right ventricular end-diastolic pressure, from 2.4 +/- 1.4 to 8.6 +/- 2.4 mmHg, were found. Anatomically, a 69% enlargement in right ventricular mass was observed, and this hypertrophic growth was characterized by a 78% thickening of the wall with no change in ventricular wall area, the latter calculated from the quotient of ventricular volume and wall thickness. Morphometric data showed a 44% increase in the capillary-to-myocyte ratio, a 66% augmentation of the total length of capillaries in the whole ventricle, and a 77% greater number of capillaries across the ventricular wall. Furthermore, these indexes of neogenesis of capillary units were associated with a more than three fold increase in the numerical density of mast cell profiles in the myocardium. In conclusion, capillary proliferation occurs in the adult rat heart, and this process appears to be coupled with mast cell hyperplasia.

To determine whether anemia-induced cardiac hypertrophy affects ventricular size and shape and the component structures of the capillary network of the left and right ventricles, young male rats were fed an iron- and copper-deficient diet for 7 weeks. By that time, blood hemoglobin content fell to 5 +/- 1 g/dl, and packed cell volume fell to 18 +/- 3%. To further characterize the implications of anemia, red blood cell number, hemoglobin corpuscular content, systemic arterial pressure, heart rate, and blood viscosity were measured. Moreover, the changes in ventricular weights were analyzed in terms of the alterations in ventricular wall area and ventricular wall thickness to establish the impact of the elevation in load associated with a high cardiac output state on ventricular remodeling. The quantitative properties of the capillary circulation were also examined biventricularly by low power electron microscopic morphometry to evaluate the adaptive growth potential of the coronary microcirculation in this form of cardiac hypertrophy. Anemia was found to interfere with the production of red blood cells and their mean corpuscular hemoglobin content and resulted in a 40% reduction in blood viscosity and a 12% and 27% decrease in systolic and diastolic blood pressure, respectively. The changes in heart rate were not statistically significant. In comparison with control animals, heart weight increased by 50%, but the enlargement in right ventricular mass (65%) was greater than that of the left ventricle (47%). Ventricular hypertrophy occurred with increases in wall area and wall thickness although the former increased consistently more than the latter in either ventricle. Tissue growth was accompanied by a 60% lengthening of the capillary network, which in combination with an increase in capillary diameter resulted in a 65% and 34% expansion in capillary luminal volume and 56% and 20% larger luminal surface density in the left and right sides of the heart, respectively. In conclusion, hypochromic microcytic anemia leads to eccentric ventricular hypertrophy with a significant amount of capillary proliferation that may tend to protect the myocardium from the increased potential for ischemic injury.