Kathleen G. Robinson

Myocardial Fibrosis and Stiffness With Hypertrophy and Heart Failure in the Spontaneously Hypertensive Rat

BACKGROUND Fibrosis is commonly found in association with cardiac hypertrophy and failure, but the relation of the connective tissue response to the development of impaired cardiac function remains unclear. We examined passive myocardial stiffness, active contractile function, and fibrosis in the spontaneously hypertensive rat (SHR), a model of chronic pressure overload in which impaired cardiac function follows a long period of stable hypertrophy. METHODS AND RESULTS We studied the passive and active mechanical properties of left ventricular (LV) papillary muscles isolated from normotensive Wistar-Kyoto (WKY) rats and spontaneously hypertensive rats (SHR) at the ages of 12 months and 20 to 23 months. Seven of 15 SHR between 20 and 23 months of age had findings consistent with heart failure (SHR-F). In comparison to preparations from WKY rats and nonfailing SHR (SHR-NF), papillary muscles from the SHR-F group demonstrated increased passive stiffness (central segment exponential stiffness constant, kcs: SHR-F 95.6 +/- 19.8, SHR-NF 42.1 +/- 9.7, WKY rats 39.5 +/- 9.5 (mean +/- SD); SHR-F P < .01 versus SHR-NF, WKY rats). The increase in stiffness was associated with an increase in LV collagen concentration (SHR-F 8.71 +/- 3.14, SHR-NF 5.83 +/- 1.20, WKY rats 4.78 +/- 0.70 mg hydroxyproline/g dry LV wt; SHR-F P < .01 versus SHR-NF, WKY rats); an increase in interstitial fibrosis, as determined histologically (SHR-F 13.5 +/- 8.0%, SHR-NF 4.9 +/- 2.1%, WKY rats 3.6 +/- 0.8%; SHR-F P < .01 versus SHR-NF, WKY rats); and impaired tension development (SHR-F 3.18 +/- 1.27, SHR-NF 4.41 +/- 1.04, WKY rats 4.64 +/- 0.85 kdyne/mm2; SHR-F P < .05 versus SHR-NF; P < .01 versus WKY rats). CONCLUSIONS The development of heart failure in the aging SHR is associated with marked myocardial fibrosis, increased passive stiffness, and impaired contractile function relative to age-matched nonfailing SHR and nonhypertensive control animals. These data suggest that fibrosis or events underlying the connective tissue response are important in the transition from compensated hypertrophy to failure in the SHR.

The spontaneously hypertensive rat as a model of the transition from compensated left ventricular hypertrophy to failure.

Studies of hemodynamics and intrinsic left ventricular myocardial function are carried out to investigate the transition from stable hypertrophy to cardiac decompensation in the aging (18-24 months) spontaneously hypertensive rat (SHR). Echocardiographic data in awake animals demonstrate increased end-diastolic and end-systolic volumes and depressed ejection fractions in left ventricles from SHR with failure (SHR-F) as compared to age matched hypertensive (SHR-NF) and non-hypertensive control animals (WKY). Cardiac catheterization data in anesthetized animals demonstrate depression of both systolic pressure and +dP/dt, and elevated end-diastolic pressure in the SHR-F relative to the two control groups. Since loading conditions and altered demand states may contribute to altered ventricular function, studies of isolated perfused hearts were carried out which demonstrate impaired systolic stress development in the SHR-F group under conditions in which loading conditions are controlled; in addition, it is observed that increasing perfusion pressure by 30 mm Hg has little effect on function. Depression of systolic function and increases in passive stiffness of isolated muscle preparations from the SHR-F indicate impairment of systolic and diastolic function at the tissue level. While all of the preparations studied have potential shortcomings, an integration of findings from these complementary approaches supports the conclusion that heart failure develops in the aging SHR. Furthermore, these data suggest that impaired function is due to changes in the intrinsic properties of the myocardium and that the connective tissue response may play an important role. These studies, in conjunction with the findings of others who have studied the aging SHR, provide support for the use of the aging SHR as a model of the transition from compensated hypertrophy to failure.

Effect of Angiotensin-Converting Enzyme Inhibition on Myocardial Fibrosis and Function in Hypertrophied and Failing Myocardium From the Spontaneously Hypertensive Rat

BACKGROUND After a period of stable hypertrophy, male spontaneously hypertensive rats (SHR) develop heart failure between 18 to 24 months of age, with depression of active myocardial function and increased passive stiffness. We tested the hypothesis that chronic ACE inhibition by captopril would prevent and possibly reverse impairment of myocardial function. METHODS AND RESULTS Male SHR and normotensive Wistar-Kyoto rats (WKY) were assigned to no treatment or captopril treatment (2 g/L in drinking water) begun at ages 12, 18, and 21 months; animals were studied at 24 months of age, or earlier when evidence of heart failure was found in SHR (mean age, 19+/-2 months). In an additional group, captopril treatment was begun when SHR developed heart failure; surviving animals were studied at 24 months of age. In untreated SHR, relative to WKY, isometric stress development at Lmax, maximum rate of stress development, and shortening velocity were depressed, whereas passive stiffness was increased, in association with the development of myocardial fibrosis. In the SHR treated before cardiac dysfunction, captopril administration attenuated hypertrophy and prevented contractile dysfunction, fibrosis, and increased passive stiffness. Captopril treatment begun after cardiac function was impaired reduced left ventricular hypertrophy but did not restore intrinsic contractile function or reduce fibrosis or passive stiffness. CONCLUSIONS In the male SHR, early treatment with captopril was associated with the most marked attenuation of dysfunction relative to the untreated SHR. Treatment initiated after the onset of heart failure improved clinical signs of heart failure and decreased left ventricular hypertrophy in surviving animals but did not reverse the fibrosis and contractile dysfunction associated with heart failure.

Studies of Prevention, Treatment and Mechanisms of Heart Failure in the Aging Spontaneously Hypertensive Rat

The spontaneously hypertensive rat (SHR) is an animal model of genetic hypertension which develops heart failure with aging, similar to man. The consistent pattern of a long period of stable hypertrophy followed by a transition to failure provides a useful model to study mechanisms of heart failure with aging and test treatments at differing phases of the disease process. The transition from compensated hypertrophy to failure is accompanied by changes in cardiac function which are associated with altered active and passive mechanical properties of myocardial tissue; these events define the physiologic basis for cardiac decompensation. In examining the mechanism for myocardial tissue dysfunction, studies have demonstrated a central role for neurohormonal activation, and specifically the renin-angiotensin-aldosterone system. Pharmacologic attenuation of this system at differing points in the course of the process suggests that prevention but not reversal of myocardial tissue dysfunction is possible. The roles of the extracellular matrix, apoptosis, intracellular calcium, beta-adrenergic stimulation, microtubules, and oxygen supply-demand relationships in ultimately mediating myocardial tissue dysfunction are reviewed. Studies suggest that while considerable progress has been made in understanding and treating the transition to failure, our current state of knowledge is limited in scope and we are not yet able to define specific mechanisms responsible for tissue dysfunction. It will be necessary to integrate information on the roles of newly discovered, and as yet undiscovered, genes and pathways to provide a clearer understanding of maladaptive remodeling seen with heart failure. Understanding the mechanism for tissue dysfunction is likely to result in more effective treatments for the prevention and reversal of heart failure with aging. It is anticipated that the SHR model will assist us in reaching these important goals.

Direct Effects of Colchicine on Myocardial Function Studies in Hypertrophied and Failing Spontaneously Hypertensive Rats

-The aging spontaneously hypertensive rat (SHR) is a model in which the transition from chronic stable left ventricular hypertrophy to overt heart failure can be observed. Although the mechanisms for impaired function in hypertrophied and failing cardiac muscle from the SHR have been studied, none accounts fully for the myocardial contractile abnormalities. The cardiac cytoskeleton has been implicated as a possible cause for myocardial dysfunction. If an increase in microtubules contributes to dysfunction, then myocardial microtubule disruption by colchicine should promote an improvement in cardiac performance. We studied the active and passive properties of isolated left ventricular papillary muscles from 18- to 24-month-old SHR with evidence of heart failure (SHR-F, n=6), age-matched SHR without heart failure (SHR-NF, n=6), and age-matched normotensive Wistar-Kyoto rats (WKY, n=5). Mechanical parameters were analyzed before and up to 90 minutes after the addition of colchicine (10(-5), 10(-4), and 10(-3) mol/L). In the baseline state, active tension (AT) developed by papillary muscles from the WKY group was greater than for SHR-NF and SHR-F groups (WKY 5.69+/-1.47 g/mm2 [mean+/-SD], SHR-NF 3.41+/-1.05, SHR-F 2.87+/-0.26; SHR-NF and SHR-F P<0.05 versus WKY rats). The passive stiffness was greater in SHR-F than in the WKY and SHR-NF groups (central segment exponential stiffness constant, Kcs: SHR-F 70+/-25, SHR-NF 44+/-17, WKY 41+/-13 [mean+/-SD]; SHR-F P<0.05 versus SHR-NF and WKY rats). AT did not improve after 10, 20, and 30 minutes of exposure to colchicine (10(-5), 10(-4), and 10(-3) mol/L) in any group. In the SHR-F group, AT and passive stiffness did not change after 30 to 90 minutes of colchicine exposure (10(-4) mol/L). In summary, the data in this study fail to demonstrate improvement of intrinsic muscle function in SHR with heart failure after colchicine. Thus, in the SHR there is no evidence that colchicine-induced cardiac microtubular depolymerization affects the active or passive properties of hypertrophied or failing left ventricular myocardium.

Effect of chronic colchicine administration on the myocardium of the aging spontaneously hypertensive rat.

Colchicine has been demonstrated to suppress the release of fibroblast growth factors, retard collagen formation and augment collagenase activity. Trials with colchicine in patients with hepatic fibrosis have suggested clinical benefit. The development of impaired myocardial function in the spontaneously hypertensive rat (SHR) is associated with a marked increase in myocardial fibrosis. The present study was carried out to test the hypothesis that chronic colchicine administration to the SHR would prevent the development of fibrosis and impaired myocardial performance.Colchicine (1 mg/l drinking water) was administered to male SHR and WKY rats from at age 13 months until 24 months or until evidence of heart failure was observed. Age-matched untreated SHR and colchicine treated and untreated WKY served as controls. At study, active and passive properties of isolated left ventricular muscle preparations were determined. Myocardial fibrosis was assessed by measuring hydroxyproline and histologic determination of interstitial cross-sectional area. Increases in LV hydroxyproline and interstitial area were found in untreated SHR relative to WKY; passive myocardial stiffness was increased and active muscle properties were depressed. In comparing colchicine treated vs untreated SHR, no differences in hydroxyproline, interstitial area or intrinsic myocardial function were found. In the WKY, colchicine increased myocardial interstitium and passive stiffness without changing hydroxyproline. Active myocardial function was not depressed.Thus, chronic colchicine administration neither attenuated the development of interstitial fibrosis nor prevented impaired myocardial function in the SHR. Colchicine treatment was associated with increased interstitium in WKY with increased passive myocardial stiffness. (Mol Cell Biochem 166: 45-54, 1997)

L-arginine fails to prevent ventricular remodeling and heart failure in the spontaneously hypertensive rat.

BACKGROUND The effects of long-term oral administration of L-arginine, a substrate for nitric oxide (NO) production, on left ventricular (LV) remodeling, myocardial function and the prevention of heart failure (HF) was compared to the angiotensin-converting enzyme (ACE) inhibitor captopril in a rat model of hypertensive HF (aged spontaneously hypertensive rat (SHR)). METHODS SHRs and age-matched normotensive Wistar-Kyoto (WKY) rats were assigned to either no treatment, treatment with L-arginine (7.5 g/l in drinking water) or captopril (1 g/l in drinking water) beginning at 14 months of age, a time when SHRs exhibit stable compensated hypertrophy with no hemodynamic impairment; animals were studied at 23 months of age or at the time of HF. RESULTS In untreated SHR, relative to WKY, there was significant LV hypertrophy, myocardial fibrosis, and isolated LV muscle performance and response to isoproterenol (ISO) were depressed; and, 7 of 10 SHRs developed HF. Captopril administration to six SHRs attenuated hypertrophy and prevented impaired inotropic responsiveness to ISO, contractile dysfunction, fibrosis, increased passive stiffness, and HF. In contrast, L-arginine administration to SHR increased LV hypertrophy and myocardial fibrosis while cardiac performance was depressed; and 7 of 9 SHRs developed HF. In WKY, L-arginine treatment but not captopril resulted in increased LV weight and the contractile response to ISO was blunted. Neither L-arginine nor captopril treatment of WKY changed fibrosis and HF did not occur. CONCLUSION These data demonstrate that in contrast to captopril, long-term treatment with L-arginine exacerbates age-related cardiac hypertrophy, fibrosis, and did not prevent contractile dysfunction or the development of HF in aging SHR.

Participação do estado contrátil e do relaxamento miocárdico na disfunção ventricular durante a transição hipertrofia-falência cardíaca

PURPOSE: To investigate the participation of contractile state and relaxation in cardiac muscle dysfunction during the transition from stable hypertrophy to cardiac decompensation in aging spontaneously hypertensive rats (SHR). METHODS: Isolated left ventricular papillary muscle function was studied in SHR with heart failure (SHR-F), in age-matched SHR without evidence of heart failure (SHR-NF), and in nonhypertensive controls Wistar-Kyoto rats (WKY). Muscles were analised in isometric and isotonic contractions in Krebs-Henseleit solution with calcium concentration of 1.25mM at 28oC. RESULTS: Papillary muscles from SHR-F and SHR-NF demonstrated decreased active tension development and shortening velocity relative to normotensive WKY (p 0.05). CONCLUSION: These data suggest that the progression from stable hypertrophy to heart failure is associated with changes in the passive stiffness and is not related to depression of myocardial contractile function.PURPOSE To investigate the participation of contractile state and relaxation in cardiac muscle dysfunction during the transition from stable hypertrophy to cardiac decompensation in aging spontaneously hypertensive rats (SHR). METHODS Isolated left ventricular papillary muscle function was studied in SHR with heart failure (SHR-F), in age-matched SHR without evidence of heart failure (SHR-NF), and in nonhypertensive controls Wistar-Kyoto rats (WKY). Muscles were analysed in isometric and isotonic contractions in Krebs-Henseleit solution with calcium concentration of 1.25 mM at 28 degrees C. RESULTS Papillary muscles from SHR-F and SHR-NF demonstrated decreased active tension development and shortening velocity relative to normotensive WKY (p < 0.05). SHR-F and SHR-NF did not differ. Compared with SHR-NF and WKY, muscle passive stiffness was increased in the failing SHR (p < 0.05 versus WKY and SHR-NF). This parameter did not differ between SHR-NF and WKY (p > 0.05). CONCLUSION These data suggest that the progression from stable hypertrophy to heart failure is associated with changes in the passive stiffness and is not related to depression of myocardial contractile function.