Changes in collagenase and collagen gene expression after induction of aortocaval fistula in rats

Progressive ventricular dilatation commonly accompanies the transition to overt failure in chronically overloaded hearts; however, only recently have studies begun to elucidate underlying molecular alterations. In particular, the potential role of altered myocardial expression of the procollagenase gene in this process has not previously been examined. Biventricular hypertrophy and dilatation were produced in rats by creating an abdominal aortocaval fistula. The time courses of changes in expression of collagen I and III genes and of the procollagenase gene (matrix metalloproteinase-1, MMP-1) were assessed by Northern blot hybridization. Expression of all three genes increased promptly; however, collagenase gene expression peaked much earlier (8 h) than did expression of either of the collagen genes (7 days), and all returned to baseline levels by 45 days. These data corroborate earlier reports of increased collagen gene expression in this model, but more importantly, they provide the first evidence of concurrent activation of collagenase gene expression, suggesting that enhancement of collagen degradation may be a prerequisite for structural cardiac dilatation.

UL-FS 49 (zatebradine) does not affect arterial baroreflex in conscious normal or aortic-constricted rats

Heart-rate reduction is an important element of patient management during cardiac bypass surgery and in therapeutic measures for combating ischemia and relieving pain in patients with angina. UL-FS 49 is a novel bradycardic agent that purportedly acts solely on the sinoatrial node without potentially deleterious effects on arterial pressure and cardiac inotropism. However, little is known about influences of this agent on neuronal tissue and cardiovascular reflexes. Moreover, left ventricular hypertrophy, which often accompanies cardiovascular disease, is known to attenuate the arterial baroreflex and could have effects interactive with those of UL-FS 49. In this study, the effects of UL-FS 49 on the arterial baroreflex were tested in normal rats (N), rats with left ventricular hypertrophy 14 days after abdominal aortic constriction (AC), and sham-operated controls (SH). Arterial baroreflex sensitivity (BRS) was estimated as the slope of the relation between mean arterial pressure (independent variable) and the RR interval (dependent variable). At the time of study, the AC group had significantly greater mean arterial pressure than either SH or N (159 +/- 2, 122 +/- 3, and 124 +/- 3 mm Hg, respectively; mean +/- SEM, p < 0.01) and significantly greater left ventricular mass to body mass ratio than did SH (3.73 +/- 0.11, 2.33 +/- 0.11 mg/g; p < 0.01). As expected, BRS was significantly depressed in AC, compared with either SH or N (0.52 +/- 0.16, 1.48 +/- 0.12, 1.69 +/- 0.25 ms/mm Hg, respectively; p < 0.01). Despite its potent dose-dependent bradycardic effects in all three groups, UL-FS 49 did not affect BRS significantly in any group. These results show that the arterial baroreflex is largely unaffected by UL-FS 49 in both normal rats and rats with systemic hypertension and left ventricular hypertrophy.

Gated magnetic resonance imaging of normal and hypertrophied murine hearts

Transgenic murine models are being used increasingly to explore the molecular basis of heart disease. Until recently, there were no means for noninvasive assessment of changes in mass and function of the murine heart because of its very small size and high heart rate. Transthoracic echocardiography has now been utilized to obtain noninvasive estimates of murine left ventricular (LV) wall thicknesses, internal dimension, and mass. However, this approach is based on one-dimensional (M-mode) measurements of the LV at its midwall that take no account of variations in LV chamber and wall dimensions along other minor axes and at other anatomic levels. Thus asymmetries in LV geometry, which can affect LV mass estimates, may be undetected. In this study, gated (diastolic) magnetic resonance imaging (MRI) was utilized to obtain two-dimensional images of the LV at four anatomic levels in intact, anesthetized mice. In 17 normal CD-1 mice (body mass, 18-47 g; gravimetric LV mass, 51-135 mg), LV mass estimates produced from the MRI data correlated well (r = 0.87) with LV mass determined gravimetrically. In addition, this approach identified changes in LV mass and wall thickness-to-chamber diameter ratio in a group of seven aortic-constricted mice (body mass, 32-39 g; gravimetric LV mass, 119-198 mg) with compensated and decompensated LV hypertrophy. These findings suggest that utility of MRI for serial, noninvasive assessment of experimentally induced alterations in mass and geometry of the murine heart.

Prolonged ejection duration helps to maintain pump performance of the renal-hypertensive-diabetic rat heart: correlations between isolated papillary muscle function and ventricular performance in situ

OBJECTIVE

The purpose of this study was to examine the degree to which mechanical alterations in left ventricular papillary muscles of renal hypertensive-diabetic rat hearts correlate with functional measurements made on the same hearts in situ.

METHODS

Female Wistar rats weighing 170-200 g were made hypertensive by placing a 0.24 mm clip on the left renal artery, and made diabetic 1 week later by a single intravenous injection of streptozotocin (60 mg/kg). Approximately 3-5 months later hemodynamic measurements including left ventricular pressure and dP/dtmax, arterial pressure and aortic flow were made on control and hypertensive-diabetic hearts in situ and correlated with mechanical measurements in left ventricular papillary muscles isolated from the same hearts. Body and tissue weights and biochemical and histological measurements were made at the time of sacrifice.

RESULTS

Hypertensive-diabetic rats which survived to the time of study had decreased body weights, increased left ventricular weights and increased right ventricular weight to body weight and lung weight to body weight ratios. Those rats which died before the scheduled in-situ measurements had significantly more severe hypertension, greater left ventricular hypertrophy, increased right ventricular and lung weights, and more interstitial fibrosis than either surviving hypertensive-diabetics or controls. Rates of isometric tension development (normalized) and relaxation as well as shortening and relaxation velocities were significantly depressed in papillary muscles from hypertensive-diabetic rat hearts despite unchanged developed tension and peak shortening. Time to peak tension and time to peak shortening were markedly prolonged. Mean aortic flow was maintained in the hypertensive-diabetic group despite significant depression of left ventricular dP/dtmax (normalized), peak aortic flow, peak aortic flow acceleration and heart rate. There was also significant depression of left ventricular-dP/dtmax. Ejection duration was markedly prolonged and correlated with both time to peak shortening in vitro and with stroke volume in vivo.

CONCLUSIONS

Surviving hypertensive-diabetic rats were not in overt congestive heart failure; nevertheless, their hearts showed abnormal contractile performance which was qualitatively and quantitatively similar to that of left ventricular papillary muscles obtained from them. Depression of peak aortic flow, peak aortic flow acceleration and heart rate in the hypertensive-diabetic group was offset by increased ejection duration, resulting in normal mean aortic flow. The close correlation of ejection duration with time to peak shortening of the isolated papillary muscles suggests that it is a manifestation of an intrinsic change in the myocardium. To the extent that this prolongation is already maximized, further decreases in contractile speed would be expected eventually to cause depressed pump function and congestive heart failure. The possibility that this sequence of events occurred in the dying animals needs to be examined by evaluating in-vitro and in-vivo myocardial function at various stages of this disease model.

Effects of hypertrophy and heart failure on [Na+]i in pressure-overloaded guinea pig heart

Intracellular free sodium levels ([Na+]i) were assessed with 23Na nuclear magnetic resonance (NMR) spectroscopy in isolated, Langendorff-perfused normal, compensated hypertrophied, and hypertrophied failing guinea pig hearts under several conditions. Baseline [Na+]i measured with a shift reagent was significantly greater than normal in the compensated hypertrophied hearts (12.8 +/- 1.2 mmol/L v 6.4 +/- 0.7 mmol/L, means +/- SEM, P < .01), but not in the hypertrophied failing hearts (8.7 +/- 1.9 mmol/L, P = N.S.). Moreover, the highest levels of [Na+]i were seen just 3 to 4 weeks after aortic constriction. [Na+]i was inversely related to both time after aortic constriction (R = -0.71, P < .03) and to the degree of left ventricular hypertrophy (R = -0.79, P < .01), suggesting that the hypertrophied failing heart is capable of maintaining relatively normal [Na+]i. In addition, triple quantum filtered NMR measurements were made to assess changes in [Na+]i subsequent to altered perfusion or loading conditions. In hypertrophied failing hearts, but not normal hearts, low coronary perfusion pressure (60 cm H2O) was associated with relatively higher [Na+]i (ANOVA, P < .05), suggesting greater sensitivity of hypertrophied failing hearts to hypoperfusion. On the other hand, when all hearts were perfused at 90 cm H2O and intraventricular balloon volume was increased from 100 microL to 300 microL, [Na+]i increased significantly only in the normal guinea pig hearts (12.3 +/- 1.8%, P < .01). These findings suggest complex changes in the expression or modulation of proteins involved in Na+ regulation. Interpretation regarding the physiological significance of these changes depends on the specific mechanism(s) proposed. Previous work in this and other models of hypertrophy suggest that changes in the number or activity of both Na(+)-K(+)-ATPase and Na(+)-Ca2+ exchange proteins are involved.

Echocardiographic assessment of left ventricular mass and systolic function in mice

The increasing use of transgenic mouse models for investigating the mechanisms of cardiac growth and function has made it important to develop noninvasive methods for assessing murine cardiac anatomy, size, and function. At present, murine cardiac mass can be determined only at necropsy. Left ventricular (LV) function can be assessed by use of various catheterization techniques, but these approaches are usually terminal procedures and provide no information about chamber anatomy and dimensions. Although transthoracic echocardiography has been used to study the LVs of rats and larger animals, the considerably smaller LV masses and somewhat faster heart rates of mice pose significant challenges to obtaining good-quality echocardiograms. In this study we tested the hypothesis that transthoracic echocardiography can image the murine LV as well as provide assessments of LV mass and function. Our results in a series of 33 mice, including normal, transgenic, and aortic-banded subgroups, demonstrate the capability of transthoracic two-dimensionally directed M-mode echocardiography in mice to (1) obtain good-quality images, (2) produce estimates of LV mass having good correlations with directly determined LV mass in normal mice, (3) detect LV hypertrophy noninvasively in different experimental models, and (4) identify impaired LV systolic function. Thus, echocardiography appears to be a promising approach for noninvasively assessing LV mass and function in mice.

Cardiac contractile proteins in hypertrophied and failing guinea pig heart

OBJECTIVE

The aim was to study changes in contractile proteins which accompany marked hypertrophy and heart failure in mammalian hearts initially containing predominantly V3 isomyosin.

METHODS

Left ventricular myosin and myofibrillar ATPase activity and right ventricular actomyosin ATPase activity were measured in normal guinea pig hearts, in hearts which were hypertrophied as a result of progressive left ventricular systolic overload following ascending aortic banding, and in hypertrophied hearts from animals which showed signs of overt congestive heart failure. Male guinea pigs weighing 225-275 g at the time of aortic banding were used for the studies.

RESULTS

Left ventricular myosin and myofibrillar ATPase activity and right ventricular actomyosin ATPase activity were correlated with body weight, left and right ventricular weight, and left ventricular peak systolic pressure during aortic occlusion. Left ventricular myosin ATPase activity and right ventricular actomyosin ATPase activity were markedly depressed in hypertrophied ventricles compared to control ventricles. Cardiac myofibrillar ATPase activity was lower in hypertrophied failing hearts than in control hearts over a wide range of calcium concentrations. In control animals and in those without heart failure, there was a nearly identical inverse relationship between left ventricular mass up to 1600 mg and myosin ATPase activity. Hypertrophied failing hearts were larger but showed little further reduction in cardiac myosin ATPase activity. Representative gel scans of non-dissociating pyrophosphate gels of left ventricular myosin from an 8 d postoperative aortic constricted animal and from its age and weight matched control showed predominantly V3 isomyosin with small amounts of V1 isoenzyme. However, preparations taken from guinea pigs 16 d after aortic constriction showed only the V3 isoform, whereas the V1 isoform was still apparent in control. Hypertrophied failing left ventricles developed less pressure per unit mass during brief aortic occlusion than non-failing left ventricles with comparable myosin ATPase activities.

CONCLUSIONS

These observations raise important questions as to the distribution of myosin isoforms in the normal adult guinea pig, and the possibility that myosin ATPase activity might be altered by post-translational modification. Although cardiac myosin ATPase activity correlates with left ventricular performance, it cannot fully explain the depressed performance of failing hearts in this model. Additional immunological studies of cardiac contractile proteins are required as well as studies designed to explore the implications of altered myosin ATPase activity for both contractile function and overall cellular homeostasis.

Depressed intracellular calcium transients and contraction in myocytes from hypertrophied and failing guinea pig hearts

We investigated the basis for impaired left ventricular function of hearts in which hypertrophy was produced by gradual pressure overload. We measured myoplasmic free calcium concentration ([Ca2+]i) with fura-2 and sarcomere shortening in single myocytes isolated from control hearts and hypertrophied failing hearts. Diastolic [Ca2+]i was normal, but [Ca2+]i at the peak of contraction was depressed in myocytes from failing hypertrophied hearts. Increasing drive rate from 0.20 Hz to 5.00 Hz increased both diastolic and peak [Ca2+]i. Norepinephrine (3 x 10(-6) M) increased diastolic [Ca2+]i in all cells and tended to normalize peak [Ca2+]i in myocytes from hypertrophied failing hearts during 5.00 Hz drive. Depressed peak [Ca2+]i in the hypertrophied cells was paralleled by significant decreases in both the velocity and percent of sarcomere shortening, which were measured in cells not loaded with fura-2. Sarcomere length was correlated with estimates of [Ca2+]i in intact cells and with controlled levels of [Ca2+] in chemically "skinned" myocytes. A plot of sarcomere length against [Ca2+] gave a single continuous relationship that spanned resting and peak values at all drive rates in both the control and hypertrophied myocytes. Thus heart failure in this model is reflected in impaired myocyte contraction, which is closely related to reduced levels of [Ca2+]i during systole rather than to depressed myofilament sensitivity to Ca2+.

Compensatory hypertrophy and failure in gradual pressure-overloaded guinea pig heart

Left ventricular hypertrophy has been produced in the guinea pig by a procedure that gradually increases left ventricular afterload. A mildly constricting band was placed around the ascending aortas of very young guinea pigs (225-275 g). With growth to 500-1,000 g, left ventricular systolic pressure increased and left ventricular hypertrophy developed. In approximately 50% of these animals, the hypertrophy was associated with normal left ventricular function and with no unusual symptoms or evidence of heart failure. The other animals developed dyspnea, which appeared an average of 41 days after banding. Dyspneic animals had normal body weight, markedly increased right ventricular and lung weights, decreased left ventricular norepinephrine content, diminished maximum left ventricular pressure generating capacity, and a significantly higher incidence of left ventricular interstitial and perivascular fibrosis. These findings demonstrate that even when left ventricular overload is imposed gradually by banding the aortas of young animals, myocardial decompensation ultimately ensues in a significant proportion of such animals. The slow imposition of loading, the slow rate of decompensation, and the ability to identify animals in heart failure by clinical dyspnea make this model uniquely valuable for studies on the mechanisms of heart failure.

Sympathetic changes during development of cardiac hypertrophy in aortic-constricted rats

Cardiac hypertrophy is frequently associated with sympathetic changes that include increased myocardial norepinephrine turnover, depleted myocardial norepinephrine stores, decreased myocardial responses to sympathetic nerve stimulation, and elevated plasma catecholamines. To better understand these events mechanistically, the time course of each was assessed in rat hearts subjected to aortic constriction-induced pressure overload. There was no evidence of increased left ventricular norepinephrine turnover in abdominal aortic-constricted rats, when compared with sham-constricted animals, during the first 3 days postoperatively. Moreover, their turnover rate constants tended to be lower during this period, then increased significantly by day 7. Plasma catecholamines were increased and left ventricular norepinephrine stores were decreased only on day 7. Heart rate responses to maximal sympathetic nerve stimulation were significantly reduced on the third postoperative day. Thus the decrease in norepinephrine stores coincided with changes in left ventricular norepinephrine turnover and plasma catecholamines, whereas the reduction in heart rate responses did not. This pattern suggests an initial reflex decrease in myocardial sympathetic tone, followed by baroreceptor resetting, with an eventual increase in general sympathetic outflow. The period of increasing catecholamine stimulation occurred after a relative left ventricular hypertrophy had developed but before a significant increase in absolute left ventricular mass. Thus catecholamines may still importantly contribute to the hypertrophy seen in this model.

Effects of sympathectomy on heart size and function in aortic-constricted rats

Effects of chemical sympathectomy with 6-hydroxydopamine on left ventricular mass, norepinephrine content, and performance in aortic-constricted and sham-constricted rats were examined. Sympathectomy did not affect mean arterial pressure but it led to lower final body weights in aortic-constricted rats. Of the hypertensive aortic-constricted rats, those that had also been sympathectomized showed greater increases in left ventricular weight-to-body weight, right ventricular weight-to-body weight, and lung weight-to-body weight ratios. Left ventricular norepinephrine content was depressed by either sympathectomy or aortic constriction and was severely depleted by their combination. Sympathectomy had no effect on maximal cardiac output or left ventricular stroke work during rapid saline infusion. In aortic-constricted rats, however, it lowered heart rate during the infusion and increased left ventricular end-diastolic pressure at peak stroke work. This evidence suggests that elimination of the adrenergic nervous system's chronotropic influence promotes greater left ventricular filling, and the resultant increase in preload may importantly contribute to the stimulation of cardiac hypertrophy.

Effects of plasma norepinephrine elevation on the heart's adaptation to chronic aortic constriction in rats

Chronically elevated plasma norepinephrine has the potential for supporting function of diseased hearts, yet may also initiate harmful biochemical and (or) structural changes in the myocardium. The present study investigated the dosage-related effects of chronic norepinephrine infusion on markers of myocardial damage and then tested the influence of a relatively low norepinephrine infusion rate (0.05 microgram X kg-1 X min-1) on the heart's adaptation to pressure overload in aortic constricted rats. Norepinephrine infusion at 0.50 microgram X kg-1 X min-1 led to significantly increased myocardial hydroxyproline concentration and significant mortality. A rate of 0.25 microgram X kg-1 X min-1 increased myocardial hydroxyproline concentration and mortality in aortic constricted rats but had no such effects on sham-operated rats. The lowest rate tested (0.05 microgram X kg-1 X min-1) significantly increased mean arterial pressure and lung weight of aortic constricted rats, without affecting the degree of left ventricular hypertrophy. This infusion rate and aortic constriction each increased plasma norepinephrine and impaired cardiac performance during rapid preloading, although their combination did not cause further impairment. Thus, it appears that even modest plasma norepinephrine elevation has a negative effect on the heart's adaptation to sustained pressure overload.

Plasma catecholamine measurements in resting and stressed conscious rats, using high performance liquid chromatography with electrochemical detection

Plasma epinephrine (EPI) and norepinephrine (NE) were measured in conscious, unrestrained rats at rest, and following exercise, cold stress and hemorrhage. Heart rate and mean arterial pressure were monitored at all points. Minor modifications of a standard, commercially available methodology achieved good chromatographic separation of both EPI and NE. Sensitivity was sufficient for all NE measurements and for EPI measurements in the stress conditions. Estimates of EPI by this procedure were somewhat higher than those in other reports. Absolute values of NE, as well as the selective, qualitative changes found in both EPI and NE under each stress condition, were consistent with those found in other studies which used the radioenzymatic method. Blood withdrawal of two 1.5 ml samples, 3 hours apart, had no detectable effect on EPI, NE, mean arterial pressure or heart rate in otherwise non-stressed rats.

Chronic norepinephrine infusion and adrenergic function of hypertrophied hearts

The ability of chronic subpressor norepinephrine infusion to prevent depletion of myocardial norepinephrine stores and deterioration of myocardial adrenergic nerve function in congestive heart failure was examined. Four groups of adult male Wistar rats were subjected to aortic constriction or sham aortic constriction, along with either norepinephrine infusion or infusion of vehicle alone. After 7 days of treatment, these animals were evaluated for myocardial norepinephrine content, ventricular mass, and resting hemodynamics. Chronotropic and inotropic responses to electrical stimulation of the left and right cervical sympathetic trunks and to intravenous injections of tyramine and norepinephrine were measured. Chronic norepinephrine infusion normalized myocardial norepinephrine content in aortic-constricted rats but did not significantly affect heart size or mortality. Norepinephrine infusion diminished responses to sympathetic nerve stimulation, despite apparently normal myocardial norepinephrine sensitivity. The results suggest that chronic norepinephrine infusion suppresses neurotransmission in the heart by some means unrelated to its norepinephrine store. This suppression may be compensatory and reversible.