Age-dependent variations of diastolic stiffness and collagen content in rat ventricular myocardium

The Effects of Healthy Aging on Right Ventricular Structure and Biomechanical Properties: A Pilot Study

Healthy aging has been associated with alterations in pulmonary vascular and right ventricular (RV) hemodynamics, potentially leading to RV remodeling. Despite the current evidence suggesting an association between aging and alterations in RV function and higher prevalence of pulmonary hypertension in the elderly, limited data exist on age-related differences in RV structure and biomechanics. In this work, we report our preliminary findings on the effects of healthy aging on RV structure, function, and biomechanical properties. Hemodynamic measurements, biaxial mechanical testing, constitutive modeling, and quantitative transmural histological analysis were employed to study two groups of male Sprague-Dawley rats: control (11 weeks) and aging (80 weeks). Aging was associated with increases in RV peak pressures (+17%, p = 0.017), RV contractility (+52%, p = 0.004), and RV wall thickness (+38%, p = 0.001). Longitudinal realignment of RV collagen (16.4°, p = 0.013) and myofibers (14.6°, p = 0.017) were observed with aging, accompanied by transmural cardiomyocyte loss and fibrosis. Aging led to increased RV myofiber stiffness (+141%, p = 0.003), in addition to a bimodal alteration in the biaxial biomechanical properties of the RV free wall, resulting in increased tissue-level stiffness in the low-strain region, while progressing into decreased stiffness at higher strains. Our results demonstrate that healthy aging may modulate RV remodeling via increased peak pressures, cardiomyocyte loss, fibrosis, fiber reorientation, and altered mechanical properties in male Sprague-Dawley rats. Similarities were observed between aging-induced remodeling patterns and those of RV remodeling in pressure overload. These findings may help our understanding of age-related changes in the cardiovascular fitness and response to disease.

The effects of asymmetric ventricular filling on left-right ventricular interaction in the normal rat heart

Heart failure is characterised by ventricular dysfunction and with the potential for changes to ventricular volumes constraining the mechanical performance of the heart. The contribution of this interaction from geometric changes rather than fibrosis or metabolic changes is unclear. Using the constant pressure Langendorff-perfused rat heart, the volume interaction between left ventricle (LV) and right ventricle (RV) was investigated. RV diastolic stiffness (P < 0.001) and developed pressure (P < 0.001) were significantly lower than LV. When the RV was fixed at the end-diastolic volume (EDV) or EDV + 50 %, both LV systolic and diastolic performance were unaffected with increasing LV balloon volume. However, at fixed LV volume, RV systolic performance was significantly decreased when LV volume increased to EDV + 50 % when RV volume was increased incrementally between 50 and 300 μl (P < 0.001). Systolic interaction in RV was noted as declining RV peak systolic load with increasing LV systolic pressure (P < 0.05) and diastolic interaction was noted for RV when LV volume was increased from EDV to EDV + 50 % (P < 0.05). RV diastolic wall stress was increased with increasing LV balloon volume (P < 0.05), but LV wall stress was unaltered at fixed RV balloon volume. Taken together, increasing LV volume above EDV decreased systolic performance and triggered ventricular constraint in the RV but the RV itself had no effect on the performance of the LV. These results are consistent with overload of the LV impairing pulmonary perfusion by direct ventricular interaction with potential alteration to ventilation-perfusion characteristics within the lung.

Age-related divergent remodeling of the cardiac extracellular matrix in heart failure: collagen accumulation in the young and loss in the aged

The incidence of heart failure (HF) increases with age. This study sought to determine whether aging exacerbates structural and functional remodeling of the myocardium in HF. HF was induced in young (~18 months) and aged sheep (>8 years) by right ventricular tachypacing. In non-paced animals, aging was associated with increased left ventricular (LV) end diastolic internal dimensions (EDID, P<0.001), reduced fractional shortening (P<0.01) and an increase in myocardial collagen content (P<0.01). HF increased EDID and reduced fractional shortening in both young and aged animals, although these changes were more pronounced in the aged (P<0.05). Age-associated differences in cardiac extracellular matrix (ECM) remodeling occurred in HF with collagen accumulation in young HF (P<0.001) and depletion in aged HF (P<0.05). MMP-2 activity increased in the aged control and young HF groups (P<0.05). Reduced tissue inhibitor of metalloproteinase (TIMP) expression (TIMPs 3 and 4, P<0.05) was present only in the aged HF group. Secreted protein acidic and rich in cysteine (SPARC) was increased in aged hearts compared to young controls (P<0.05) while serum procollagen type I C-pro peptide (PICP) was increased in both young failing (P<0.05) and aged failing (P<0.01) animals. In conclusion, collagen content of the cardiac ECM changes in both aging and HF although; whether collagen accumulation or depletion occurs depends on age. Changes in TIMP expression in aged failing hearts alongside augmented collagen synthesis in HF provide a potential mechanism for the age-dependent ECM remodeling. Aging should therefore be considered an important factor when elucidating cardiac disease mechanisms.

Cardiac dysfunction in aging conscious rats: altered cardiac cytoskeletal proteins as a potential mechanism

The objective of this study was to test the hypothesis that the mechanism mediating left ventricular (LV) dysfunction in the aging rat heart involves, in part, changes in cardiac cytoskeletal components. Our results show that there were no significant differences in heart rate, LV pressure, or LV diameter between conscious, instrumented young [5.9 +/- 0.3 mo (n = 9)] and old rats [30.6 +/- 0.1 mo (n = 10)]. However, the first derivative of LV pressure (LV dP/dt) was reduced (8,309 +/- 790 vs. 11,106 +/- 555 mmHg/s, P < 0.05) and isovolumic relaxation time (tau) was increased (8.7 +/- 0.7 vs. 6.3 +/- 0.6 ms, P < 0.05) in old vs. young rats, respectively. The differences in baseline LV function in young and old rats, which were modest, were accentuated after beta-adrenergic receptor stimulation with dobutamine (20 mug/kg), which increased LV dP/dt by 170 +/- 9% in young rats, significantly more (P < 0.05) than observed in old rats (115 +/- 5%). Volume loading in anesthetized rats demonstrated significantly impaired LV compliance in old rats, as measured by the LV end-diastolic pressure and dimension relationship. In old rat hearts, there was a significant (P < 0.05) increase in the percentage of LV collagen (2.4 +/- 0.2 vs. 1.3 +/- 0.2%), alpha-tubulin (92%), and beta-tubulin (2.3-fold), whereas intact desmin decreased by 51%. Thus the cardiomyopathy of aging in old, conscious rats may be due not only to increases in collagen but also to alterations in cytoskeletal proteins.

Quantitative analysis of tenascin in chordae tendineae of human left ventricular papillary muscle with aging

A computer image analysis was done to investigate the distribution of tenascin and collagenous matrix components (collagen types I and III) in the chordae tendineae of the anterior cusp of the left ventricular papillary muscle, using confocal laser scanning microscopy in both a 20s age group and an 80s-90s age group. Tenascin and the collagenous matrix components showed marked accumulation in the 80s-90s age group with elastic fiber concentration being increased in the same region as that in the 20s age group. The highest content of intersurface layer staining with an anti-tenascin antibody and with anti-collagen type III in the inner layer was found in the 80s-90s age group. These content levels were low in each region in the 20s age group. The distribution and content of these extracellular matrix changes are related to the changes in mechanical performance of the cardiac muscle with age.

Left ventricular geometric remodeling and residual stress in the rat heart

Theoretical considerations and observations of residual stress suggest that geometric remodeling in the heart may also alter residual stress and strain. We investigated whether changes in left ventricular geometry during physiologic growth were associated with corresponding changes in myocardial residual strain. In anesthetized rats from eight age groups ranging from 2-25+ weeks, the heart was arrested and isolated, and equatorial slices were obtained. The geometry of the intact, unloaded state was recorded, as well as the "opening angle" of the stress-free configuration after radial resection of the tissue slice. The tissue was fixed and embedded for histological examination of collagen area fraction. Heart weight increased 10-fold with age and unloaded internal radius increased almost 4-fold. However, wall thickness increased only 66 percent, so that the ratio of wall thickness to internal radius decreased significantly from 2.22 +/- 0.29 (mean +/- SD) at 2 weeks to 0.81 +/- 0.47 at 25 weeks. Opening angle of the stress-free slice decreased significantly from 87 +/- 16 deg at 2 weeks to 51 +/- 16 deg, and correlated linearly with wall thickness/radius ratio. Collagen area fraction increased with age. Hence physiologic ventricular remodeling in rats decreases myocardial residual strain in proportion to the relative reduction in wall thickness-radius ratio.

Development of the collagen network of the human fetal myocardium: an immunohistochemical study

We have studied the development of the collagen network within the ventricular myocardium of sixteen human fetuses of gestational age 12 weeks to 22 weeks. Using the technique of indirect immunohistochemistry with antibodies raised against collagen Types I and III, we have demonstrated that the 12 week old heart has a rudimentary endomysial collagen structure which coexpresses both collagen types. Perimysial structures evolve with the onset of the second trimester and are collagen III positive before expressing collagen Type I. No differences in collagen deposition were detected between the right and left ventricular free walls, but collagen content of the interventricular septum appeared relatively high and expressed in thick highly organised fibrils. Transventricular gradients of collagen distribution were seen for both collagen isoforms which persisted with increased age. Intraventricular differences in collagen deposition were marked due to the insertion of the atrioventricular valves and tension apparatus. These findings suggest that the collagen network of the fetal myocardium is composed of copolymer fibrils, possibly rich in collagen type III which evolve at a time when the mechanical efficiency of the fetal heart must improve to keep pace with the escalating demands of a rapidly growing body.