J. M. Capasso

Effects of gonadectomy and hormonal replacement on rat hearts.

To evaluate the effects of sex hormones on heart function and biochemistry, gonadectomy (GX) was performed in postpubertal male (M) and female (F) rats and compared with sham-operated controls (SH). The groups were MSH; MGX; MGX replaced with testosterone 3 mg/day s.c. (MGX + T), FSH, and FGX replaced with estrogen 2 mg/day (FGX + E), progesterone 0.4 mg/day (FGX + P), estrogen and progesterone (FGX + EP), or testosterone 2 mg/day (FGX + T). Body weight was decreased in MGX and was decreased further in MGX + T. Heart weight was decreased in both MGX and MGX + T. Body weights were increased in FGX and FTX + P and were increased further in FGX + T but were normal in FGX + E and FGX + EP. Heart weights were unchanged in F groups except in FGX + T, where it was increased. Cardiac performance in perfused hearts, as measured by stroke work, ejection fraction, fractional shortening and mean velocity of circumferential fiber shortening, was decreased in MGX but was slightly increased in MGX + T. Papillary muscle studies showed increases in time to peak tension and one-half relaxation in MGX, but these were decreased in MGX + T. Isotonic shortening studies showed decreased velocity of shortening in MGX and increased velocity in MGX + T. Heart function was significantly decreased in FGX and FGX + P compared with FSH but was similar to FSH in FGX + E and FGX + EP. FGX + T had greater stroke work and ejection fraction than FSH and FGX.(ABSTRACT TRUNCATED AT 250 WORDS)

Regulation of angiotensin II receptors on ventricular myocytes after myocardial infarction in rats.

To determine the effects of acute myocardial infarction on the regulation of angiotensin II (Ang II) receptors and contractile performance of left and right ventricular myocytes, coronary artery ligation was surgically induced in rats, and Ang II receptor density and affinity and the mechanical properties of surviving muscle cells were examined 1 week later. Physiological determinations of cardiac pump function revealed the presence of ventricular failure, which was associated at the cellular level with a depression in the velocity of myocyte shortening and relengthening, a prolongation of time to peak shortening, and a reduction in the extent of cell shortening. These abnormalities in single-cell function were more prominent in left than in right ventricular myocytes. Cellular hypertrophy was documented by increases in cell length and width, which were also greater in the spared myocytes of the infarcted left ventricle. Reactive hypertrophy was accompanied by a 1.84- and 1.85-fold increase in the density of Ang II receptors on left and right myocytes, respectively. On the other hand, the affinity of Ang II receptors for the radiolabeled antagonist was not altered. However, Ang II-stimulated phosphoinositol turnover was enhanced by 3.7- and 2.5-fold in left and right myocytes, respectively, after infarction. Ventricular myocytes were found to possess the AT1 receptor subtype exclusively. In conclusion, myocardial infarction leads to impairment in the contractile behavior of the remaining cells and to the activation of Ang II receptors and effector pathway associated with these receptors, which may be involved in the reactive growth adaptation of the viable myocytes.

Cellular basis of ventricular remodeling after myocardial infarction

To determine whether acute left ventricular failure associated with myocardial infarction leads to architectural changes in the spared nonischemic portion of the ventricular wall, large infarcts were produced in rats, and the animals were sacrificed 2 days after surgery. Left ventricular end-diastolic pressure was increased, whereas left ventricular dP/dt and systolic pressure were decreased, indicating the presence of severe ventricular dysfunction. Absolute infarct size, determined by measuring the fraction of myocyte nuclei lost from the left ventricular free wall, averaged 63%. Transverse midchamber diameter increased by 20%, and wall thickness diminished by 33%. The number of mural myocytes in this spared region of the left ventricular free wall decreased by 36% and the capillary profiles by 40%. Thus, side-to-side slippage of myocytes in the myocardium occurs acutely in association with ventricular dilation after a large myocardial infarction. In order to analyze the chronic consequences of myocardial infarction on ventricular remodeling, a second group of experiments was performed in which the left coronary artery was ligated and the functional and structural properties of the heart were examined 1 month later. In infarcts affecting an average 38% of the free wall of the left ventricle (small infarcts), reactive hypertrophy in the spared myocardium resulted in a complete reconstitution of functioning tissue. However, left ventricular end-diastolic pressure was increased, left ventricular dP/dt was decreased, and diastolic wall stress was increased 2.4-fold. After infarctions resulting in a 60% loss of mass (large infarcts), a 10% deficit was present in the recovery of viable myocardium. Functionally, ventricular performance was markedly depressed, and diastolic wall stress was increased 9-fold. The alterations in loading of the spared myocardium were due to an increase in chamber volume and a decrease in the myocardial mass/chamber volume ratio that affected both infarct groups. Thus, decompensated eccentric ventricular hypertrophy develops chronically after infarction and growth processes in myocytes are inadequate for normalization of wall stress when myocyte loss involves nearly 40% or more of the cells of the left ventricular free wall. The persistence of elevated myocardial and cellular loads may sustain the progression of the disease state toward end-stage congestive heart failure.

Left ventricular failure induced by long-term hypertension in rats.

To determine whether the duration of hypertension is an essential component in the evolution of myocardial dysfunction, renal artery constriction was performed in male Fischer 344 rats at 4 months of age, and in vivo global cardiac performance of sham-operated and experimental animals was evaluated 8 months later. Systemic arterial blood pressure increased to 173 +/- 5 mm Hg 2 weeks after the arteries were clipped and remained elevated for the following 5 months. Blood pressure decreased over the remaining 3 months to a value not significantly different from control rats that were killed, 132 +/- 4 mm Hg. After 8 months of renovascular hypertension, we observed that the elevated level of systolic arterial pressure was accompanied by a distinct absence of left ventricular hypertrophy when measured at the ventricular weight level. Moreover, left ventricular end-diastolic pressure increased in hypertensive animals from 6.0 to 24.0 mm Hg while peak left ventricular pressure was identical to controls. In addition, peak +dP/dt and -dP/dt were depressed in hypertensive animals. Although stroke volume was unaltered, cardiac output in renal artery clipped animals was depressed by 34% while total peripheral resistance was elevated by 50%. Ventricular chamber remodeling in the hearts of hypertensive animals was evidenced as a 19% increase in the transverse and a 16% increase in the longitudinal axes of the left ventricle with a 27% diminution of wall thickness. Myocardial damage, in the form of myocyte loss and replacement fibrosis, increased in the hearts of hypertensive animals resulting in a ninefold augmentation in the volume fraction of collagen within the ventricular wall. These alterations in the architectural properties of chamber geometry coupled with the abnormalities in contractile performance resulted in a severe reduction in ejection fraction from 82% to 47% and a marked elevation in transmural diastolic and systolic stress in hypertensive animals. The gradient in stress across the ventricular wall, from epicardium to endocardium, revealed a direct correlation with the regional distribution of myocardial damage. In conclusion, the loading state of the myocardium, tissue injury, and myocardial fibrosis all appear to be critical determinants in the genesis of left ventricular failure in long-term pressure overload.

Myocyte mitotic division in the aging mammalian rat heart.

To determine whether myocyte mitotic division occurs in the adult mammalian heart and whether this cellular process is affected by aging, we measured the percentage of myocyte nuclei showing metaphase chromosomes in myocytes isolated from the left and right ventricles of rats at 8-12, 19-24, and 28-32 months after birth. Metaphase chromosomes were found at all ages in both ventricles. However, from 8-12 to 28-32 months, the fraction of nuclei exhibiting metaphase chromosomes increased 6.3-fold and 2.3-fold in the left and right ventricles, respectively. Thus, myocyte cellular hyperplasia is present in the adult and aging myocardium as a compensatory mechanism to regenerate tissue mass and recover function, which are lost with the progression of life and senescence.

Myocardial mechanical, biochemical, and structural alterations induced by chronic ethanol ingestion in rats.

To determine the effects of moderate ethanol consumption on the mechanical, biochemical, and structural characteristics of the heart, myocardial mechanical performance, contractile protein enzyme activity, and the number and size of myocytes were measured in male Fischer 344 rats after the ingestion of 30% oral ethanol. Papillary muscles removed from the left ventricle were greater in length, weight, and cross-sectional area than the corresponding muscles from the right side. However, no differences were found between control and ethanol-treated myocardium when either the left or right side was compared separately. Chronic ethanol ingestion resulted in an increase in resting tension in left ventricular muscles, with no alteration in peak developed tension. Moreover, time to peak tension was significantly prolonged, whereas a depression was observed in the peak rate of isometric tension development. Isotonically, left muscles from ethanol-treated rats revealed a prolongation of time to peak shortening and a marked depression in the velocity of shortening at physiological loads. No changes were noted in muscles from the right ventricle. Contractile protein enzyme activity revealed no differences in myofibrillar Mg(2+)-ATPase activity in right and left ventricular myocardium between control and ethanol-treated rats in the presence of EGTA. However, at physiological activating levels of calcium, an upward shift of the myofibrillar Mg(2+)-ATPase activity-calcium curve occurred in left myocardium, whereas a depression in this relation was seen in the right ventricle. As a result of chronic ethanol intake, a decrease was noted in the volume percent of myocardium occupied by myocytes, and that myocyte cell volume per nucleus was found to remain essentially constant throughout the various layers of the ventricular wall. Importantly, a 14% significant decrease in the total number of myocyte nuclei was demonstrated in the left ventricular myocardium of rats on chronic ethanol consumption. Thus, chronic but moderate alcohol ingestion resulted in depressed contractile performance, alterations in myofibrillar Mg(2+)-ATPase activity, and myocyte loss. These events may serve to function as preliminary indicators of the onset of heart failure of alcoholic origin in this animal model.

Ventricular loading is coupled with DNA synthesis in adult cardiac myocytes after acute and chronic myocardial infarction in rats

To determine whether the overload associated with myocardial infarction and ventricular failure in rats is coupled with activation of DNA synthesis in the remaining left and right ventricular myocytes, flow cytometric analysis was performed on myocyte nuclei prepared from both ventricles 7 and 30 days after coronary occlusion. In addition, oral captopril was administered in separate groups of control and experimental rats to establish whether a relation existed between attenuation of ventricular loading and magnitude of DNA synthesis in myocytes. Results demonstrated that left ventricular failure and right ventricular dysfunction at 7 days after infarction were biventricularly associated with marked increases in the number of myocyte nuclei in the G2M phase of the cell cycle. In contrast, the fraction of nuclei in the G0+G1 phase decreased. In comparison with the earlier time point, the 30-day interval was characterized by a significant magnitude of cardiac hypertrophy, a moderate amelioration of ventricular pump function, and a decrease in the percentage of myocyte nuclei in the G2M phase in both ventricles. However, 30 days after infarction, the number of right ventricular myocyte nuclei in the S and G2M phases remained elevated with respect to control animals. Captopril therapy reduced the extent of ventricular loading and the population of myocyte nuclei in the cell cycle at 7 days. In conclusion, DNA synthesis in myocyte nuclei may represent an important adaptive component of the myocardial response to infarction.

Alterations in collagen cross-linking impair myocardial contractility in the mouse heart.

A number of genetic disorders in humans are associated with defects in the synthesis and metabolism of collagen, which are accompanied by multiple cardiovascular disease processes. To determine whether genetically determined cross-linking abnormalities of collagen may alter cardiac function, left ventricular papillary muscles of mice with a genetic defect in the cross-linking of collagen (Movbr) were studied in vitro. With respect to controls, increases in time to peak tension, from 102 ± 1.4 to 125 ± 5.4 msec (p<0.001), and time to one-half relaxation, from 76 ± 3.0 to 98 ± 6.1 msec (/J<0.05), were measured. Moreover, resting tension at the length associated with maximum developed isometric force (L) was elevated, from 11.1 ± 1.7 to 19.3 ± l.l mN/mm2 (p<0.001), and a similar difference was also seen throughout the physiological range of muscle lengths. In contrast, developed tension was depressed at 93-97% of L. Peak rate of tension rise and decay were diminished whereas time to peak rate of tension rise was prolonged. Isotonically, a decrease in the magnitude of peak shortening at L, from 4.0 ± 0.5 to 2.0 ± 0.2% (p<0.04), and an increase in time to peak shortening, from 100 ± 2.3 to 129 ± 2.8 msec (p< 0.001), were seen. In addition, peak velocities of shortening and relengthening were diminished in the Movbr mouse heart. In conclusion, the impairment in collagen cross-linking alters cardiac mechanics by a reduction in force-generating ability and a prolongation of the timing parameters of the systolic and diastolic phases of contraction in vitro.

Chronic nonocclusive coronary artery constriction impairs ventricular function, myocardial structure, and cardiac contractile protein enzyme activity in rats.

To determine the effects of chronic nonocclusive coronary constriction on cardiac hemodynamics, structural integrity, and contractile protein enzyme activity, the left coronary artery was narrowed in rats, and measurements of ventricular performance, magnitude, and distribution of tissue damage and myofibrillar Mg2+ and Ca2+ myosin ATPase activities were evaluated 1 month later. In the presence of coronary artery stenosis averaging 58%, three levels of involvement of global cardiac performance were identified, and the rats were divided accordingly. In the first group, only left ventricular end-diastolic pressure (LVEDP) was increased; in the second group, LVEDP and left ventricular +dP/dt and/or -dP/dt were affected; and in the third group, LVEDP, left ventricular +dP/dt and -dP/dt, and right ventricular end-diastolic pressure were impaired. Thus, left ventricular moderate dysfunction, severe dysfunction, and failure occurred with coronary narrowing. On a structural basis, coronary constriction resulted in an ongoing process characterized by acute myocytolytic necrosis and foci of replacement fibrosis in different stages of healing. The number of these lesion profiles in the left ventricular myocardium increased 4.7-, 4.4-, and 8.3-fold in rats with moderate dysfunction, severe dysfunction, and failure, respectively. Biochemically, Mg(2+)-ATPase activity of myofibrils increased biventricularly when moderate dysfunction was present. However, this parameter decreased with the appearance of severe dysfunction, reaching control values in ventricular failure. Ca2+ myosin ATPase activity was reduced in the left ventricle of rats with severe dysfunction and failure, whereas it was elevated in the right ventricle of rats with severe dysfunction. In conclusion, a fixed lesion of the left main coronary artery with a modest reduction in vessel luminal diameter generates a conditioned state of the heart characterized by a continuous loss of myocytes and replacement scarring, which, in combination with alterations in contractile protein enzyme activity, may be responsible for a number of abnormalities in cardiac dynamics ranging from moderate dysfunction to pump failure.

Differences in load dependence of relaxation between the left and right ventricular myocardium as a function of age in rats.

To determine whether the variation in the magnitude of work load sustained by the left and right ventricles during adulthood and senescence affects the load-dependent aspect of relaxation, posterior papillary muscles from the left and right ventricles of rats at 4, 10, and 20 months of age were studied under variably loaded conditions in vitro. Because of differences between the life spans of Fischer and Sprague-Dawicy rats, the functional characteristics of relaxation were investigated to evaluate the possibility of a differential age-associated response in these two strains of animals. The kinetic performance of the diastolic phase of myocardial contraction was measured by assessing the relative time during which load bearing occurred in a series of afterloaded isotonic twitches. This measurement was expressed as the ratio of the duration of afteiioaded isotonic shortening and relengthening to the time required for isometric force to decline to the same level daring isometric relaxation. A ratio of less than unity identified a load-dependent state whereas a value greater than one reflected a load-independent condition. Results snowed that the right myocardium was completely load independent whereas the left myocardium was fully load dependent at all physiological afterloads. Aging reduced the load independence of the right ventricle and the load dependence of the left ventricle in Fischer rats. In contrast, no aging effect on the properties of afteiioaded isotonic relaxation was seen in Sprague-Dawiey rats. In conclusion, distinct differences exist in the mechanical dynamics of inactivation between the left and right ventricular myocardium. Aging reduced these variations in Fischer rats but had no apparent influence in Sprague-Dawiey animals up to 20 months after birth.