Joseph M. Capasso

Altered myocardial mechanics in diabetic rats.

Diabetes mellitus is associated frequently with congestive heart failure in humans, even in the absence of associated coronary disease or hypertension. Nevertheless, the effects of the diabetic state on myocardial mechanics have not been studied. Accordingly, diabetes was induced in female Wistar rats by injection of streptozotocin (60 mg/kg). Left ventricular papillary muscles were studied 5,10, and 30 weeks later and compared with controls. Relaxation was delayed significantly and velocity of shortening was depressed at all loads. However, the passive and active force-length curves, as well as the series elastic properties, were not altered. The changes in cardiac performance were found over a range of muscle lengths, stimulus frequencies, and bath concentrations of calcium, glucose, and norepinephrine. The duration of diabetes had no major effect on the mechanical changes observed. The possible influences of drug-induced cardiac toxicity, malnutrition, and altered thyroid hormone levels have been considered; the latter two factors could not be excluded completely from having some influence on the mechanical properties of diabetic cardiac muscle. Evidence is cited showing abnor-malities in calcium uptake by sarcoplasmic reticulum and depressed actomyosin ATPase activity. Thus, a cardiomyopathic state has been produced in the rat consequent to the induction of experimental diabetes mellitus. Various mechanisms for this entity have been suggested. Circ Res 47: 922-933, 1980

Physiologic cardiac hypertrophy corrects contractile protein abnormalities associated with pathologic hypertrophy in rats.

To evaluate the combined effects of cardiac overload imposed by hypertension and by chronic exercise, male and female rats were made hypertensive by unilateral renal artery stenoses and made to exercise in an 8-10-wk swimming program. Sedentary normotensive animals, sedentary hypertensive animals and normotensive animals exposed to the swimming program were also studied. Hypertension was associated with the development of cardiac hypertrophy, and this was exaggerated in hypertensive swimmers. Actomyosin, Ca2+-myosin, and actin-activated Mg2+-myosin ATPase activities were enhanced in normotensive swimmers, depressed in hypertensives and were normal or increased in hypertensive swimmers. Myosin isoenzyme analysis showed a predominant V1 pattern in normals; an increase in percent V1 isoenzyme is swimmers; a predominant V3 pattern in hypertensives; and a return to the predominant V1 pattern in hypertensive swimmers. These findings suggest that the hypertrophy imposed by hypertension and hypertrophy imposed by physical training using a chronic swimming program are distinctly different biological phenomena. Physical training by swimming prevents the changes in cardiac myosin induced by hypertension despite the exaggeration of hypertrophy.

Morphology, composition, and function of struts between cardiac myocytes of rat and hamster

SummaryThe morphology, composition, and function of struts that interconnect the lateral surfaces of cardiomyocytes were examined in the hearts of rats and hamsters. Methods included brightfield and fluorescent light microscopy, secondary and backscatter scanning electron microscopy, and transmission electron microscopy in conjunction with silver stain, cationic dye, and antibody to type-I collagen. These studies reveal a twisted, beaded appearance and a complex substructure of collagen fibrils embedded in a ground substance that has a positive reaction with cationic dye. A hierarchy of patterns of branching and attachment was seen among intercellular struts ranging in diameter from 0.1 μm to several urn. The hypothesis that struts tether not only the surfaces but the contractile lattices of laterally adjacent myocytes is supported by the following: (a) the attachments of struts to the collagen weave of the sarcolemma, often lateral to the level of Z bands, (b) the presence of collagen type I in a composite material arrangement, (c) the relative dispositions and configurational changes of struts and myocyte surfaces in various physiological states and induced, non-physiological perturbations of cardiac muscle, (d) the corrugated sarcolemmas with infoldings near Z bands, and (e) the continuity of intracellular filaments from Z bands to the inner aspect of the sarcolemma in relaxed and contracted myocytes. Implications of struts acting as tethers and sites for storage of energy in the motions of myocytes during the cardiac cycle are discussed.

Myocardial biochemical, contractile, and electrical performance after imposition of hypertension in young and old rats.

The effects of renovascular hypertension on the biochemical, contractile, and electrical performance of myocardial tissue from rats of various ages has been examined. Male Fischer rats, 2, 7, 12, and 17 months old, were made hypertensive by constriction of the left renal artery. Ten weeks after the onset of hypertension, left ventricular papillary muscles were isolated from those four groups when 5, 10, 15, and 20 months old, respectively. Mechanical performance and transmembrane electrical events were recorded simultaneously. Contractile protein enzyme activity was determined in the same hearts from which papillary muscles were used for acquisition of mechanical and electrical information. There was a slight increase in blood pressure in control groups as a function of age while blood pressure maintained a range of approximately 179–188 mm Hg for all hypertensive groups. Heart weight of control animals increased significantly from 5 months to 20 months of age from 539 ± 26 to 1088 ± 56 mg, representing an increase of 101%. In hypertensive animals, heart weight increased 50% in 5-month-, 15% in 10-month-, 50% in 15-month-, and 11.7% in 20-month-old animals. Although control groups revealed alterations in mechanical, electrical, and biochemical parameters that increased as a function of age, the magnitude of the biochemical, contractile, and electrical response to hypertension varied monotonically with the extent of myocardial hypertrophy, rather than age per se. Adaptation to the stress of hypertension was observed in each age group, and was revealed as prolongation of mechanical and electrical timing parameters, depression of the load-velocity relation, and contractile protein enzyme activity. Thus, the stress of hypertension, which was tolerated by the 10-and 20-month-old animals with lesser relative hypertrophy and lesser changes in measured parameters, may represent a differential adaptation to the stress of hypertension.

Sex differences in myocardial contractility in the rat.

SummaryWe examined the intrinsic contractile performance of papillary muscles removed from the left ventricle of male and female Wistar rats. Muscles were studied isometrically and isotonically, stimulated at 0.1 Hz, perfused with Tyrodes solution having an external calcium concentration=2.4 and maintained at 30°C. In addition, we examined muscle response to changes in external calcium, added norepinephrine or verapamil and alterations in contraction frequency. No significant change in peak isometric development tension was observed between male and female preparations. However, muscles from male rats showed a significantly greater isometric time-to-peak tension and time to 1/2 relaxation with a depression of both the maximum rate of tension rise and maximum rate of tension decay. Isotonically, although peak shortening showed no difference between male and female preparations, the maximum velocities of shortening and relaxation were significantly depressed in muscles from male rats. Muscles from male animals also displayed significant prolongation of the time-to-peak shortening and time-to-peak velocity of shortening. These differences in papillary muscle performance were found over a wide range of muscle lengths, stimulus frequencies and bath concentrations of calcium, norepinephrine and verapamil. Thus differences in intrinsic contractile performance between papillary muscle from male and female rats have been characterized.ZusammenfassungWir untersuchten die kontraktilen Eigenschaften linksventrikulärer Papillarmuskeln von männlichen und weiblichen Wistar-Ratten unter isometrischen und isotonische Bedingungen bei einer Reizfrequenz von 0,1 Hz und Perfusion mit Tyrodelösung (Ca2+ 2,4 mmol/l, 30°C). Weiterhin untersuchten wir die Antwort der Muskeln auf Änderungen der Ca2+-Außenkonzentration, Verabfolgung von Noradrenalin oder Verapamil sowie Änderungen der Kontraktionsfrequenz. Die Präparate von männlichen und weiblichen Tieren unterschieden sich nicht bezüglich der entwickelten isometrischen Spitzenspannung. Jedoch zeigten die Muskeln von männlichen Ratten eine signifikant längere isometrische Anstiegszeit; auch die Halbwertszeit der Erschlaffung war verlängert, die maximale Anstiegsgeschwindigkeit und Erschlaffungsgeschwindigkeit herabgesetzt. Obwohl unter isotonischen Bedingungen keine Unterschiede bezüglich der maximalen Verkürzung vorlagen, war die Maximalgeschwindigkeit der Verkürzung und Relaxation bei Muskeln von männlichen Ratten signifikant geringer. Auch unter isotonischen Bedingungen wurden die maximale Verkürzung sowie die maximale Verkürzungsgeschwindigkeit später erreicht. Diese Unterschiede in der Mechanik der Papillarmuskeln wurden über einen weiten Bereich der Vordehnung, der Reizfrequenz und Ca-Außenkonzentration sowie auch unter Noradrenalin und Verapamil verzeichnet. Es konnten also geschlechtsspezifische Unterschiede in den kontraktilen Grundeigenschaften der Papillarmuskeln nachgewiesen werden.

Microvascular spasm as a cause of cardiomyopathies and the calcium-blocking agent verapamil as potential primary therapy

The origin of cardiomyopathies, a major cause of cardiac disability and death, has been largely unexplained. Pathologic features, common to all cardiomyopathies independent of origin, include ventricular hypertrophy and diffuse scarring with variable amounts of ventricular dilatation. This problem was studied experimentally in 2 models of congestive cardiomyopathy: the hereditary cardiomyopathic Syrian hamster and the hypertensive-diabetic rat. In both the genetic and the acquired disease models, there is focal myocytolytic necrosis followed by healing with focal scars, ventricular wall hypertrophy, ventricular dilatation with congestive heart failure and, ultimately, death. In view of the heterogeneous pathologic features of both diseases, silicone rubber perfusions have been used to study the microcirculation of the heart in these animals; microvascular spasm has been demonstrated early in the disease associated with small areas of myocytolytic necrosis that undergo subsequent fibrosis. Reactive hypertrophy then ensues as a compensatory response to this myocellular necrosis; it is the combination of cell loss and slowly decreasing contractility resulting from the reactive hypertrophy, which culminates in a cardiomyopathy. Administration of verapamil or prazosin to the cardiomyopathic Syrian hamster prevents microvascular spasm and development of cardiomyopathic changes in the myocardium. In view of these and other findings related to the anatomy and hyperreactivity of microcirculation, it is concluded that hypertrophic congestive cardiomyopathies may be caused by focal cell loss due to microvascular spasm and reperfusion injury, with the subsequent development of focal fibrosis and reactive hypertrophy in response to the myocardial necrosis.(ABSTRACT TRUNCATED AT 250 WORDS)

Chronic coronary artery constriction leads to moderate myocyte loss and left ventricular dysfunction and failure in rats

Coronary artery narrowing, ranging from 19% to 61%, was induced in rats and ventricular performance, myocardial damage, and myocyte hypertrophy were examined 1 mo later. Animals were separated into two groups, exhibiting ventricular dysfunction and failure, respectively. Dysfunction consisted of a 2.4-fold increase in left ventricular end diastolic pressure (LVEDP), 15% decrease in left ventricular peak systolic pressure (LVPSP), 24% reduction in developed pressure (DP), and a 16% depression in-dP/dt. Failure was defined on the basis of a 4.7-fold elevation in LVEDP, and a 26%, 47%, 45%, and 41% decrease in LVPSP, DP, +dP/dt, and -dP/dt. Moreover, in this group, right ventricular end diastolic and systolic pressures increased 5.5- and 1.2-fold. Left and right ventricular weights expanded 23% and 51% with dysfunction and 30% and 56% with failure. Left ventricular hypertrophy was characterized by ventricular dilation and wall thinning which were more severe in the failing animals. Foci of damage were found in both groups but tissue injury was more prominent in the endomyocardium and in failing rats. Finally, myocyte loss in the ventricle was 10% and 20% with dysfunction and failure whereas the corresponding enlargements of the unaffected myocytes were 34% and 53%. Thus, coronary narrowing led to abnormalities in cardiac dynamics with an increase in diastolic wall stress and extensive ventricular remodeling in spite of a moderate loss of myocytes and compensatory reactive hypertrophy of the viable cells.

Hypertensive cardiomyopathy. Myocyte nuclei hyperplasia in the mammalian rat heart.

To determine whether long-term hypertension leads to hyperplasia of myocyte nuclei in the heart, a phenomenon suspected to occur in humans, renal hypertension was produced in rats and the animals were killed 8 mo later. Arterial blood pressure remained elevated for approximately 5 mo, but decreased progressively in the last 3 mo so that at 8 mo this parameter was practically identical to that found in controls. Moreover, left ventricular end diastolic pressure was markedly increased in experimental animals in association with a substantial decrease in left ventricular dP/dt. The alteration of these physiological measurements was indicative of severe ventricular dysfunction. Quantitative analysis of the transmural distribution of myocyte nuclei in the left ventricle showed 36 and 23% increases in myocyte nuclei concentration in the epimyocardium and endomyocardium, respectively. These changes in nuclei were accompanied by 25 and 16% reductions in myocyte cell volume per nucleus in the outer and inner layers of the wall. In conclusion, long-term hypertension leads to impairment of ventricular function and proliferation of nuclei in myocytes.

Efficacy of angiotensin-converting enzyme inhibition and AT1 receptor blockade on cardiac pump performance after myocardial infarction in rats

Summary: To determine whether cardiac unloading by inhibition of angiotensin I (AI) to AII conversion by captopril or blockade of the AII receptor (AT1) by losartan was more effective in prevention of the detrimental hemodynamic consequences of myocardial infarction (MI), inhibition of metabolic production of AII by captopril was compared with blockade of AT1 with losartan in Sprague-Dawley rats with large MI. Infarcts were created by surgical occlusion of the left main coronary artery and oral drug therapy initiated immediately and continued until hemodynamic evaluation seven days later. Heart weight was unchanged in untreated infarcted animals, whereas captopril reduced heart weight in control animals and losartan increased heart weight in infarcted animals. Left ventricular (LV) peak systolic blood pressure (SBP) was lower in treated and untreated infarcted animals. Although captopril reduced end-diastolic pressure (EDP) to a greater degree than losartan, all infarcted group showed an increase in this parameter with respect to similarly treated controls. LV peak rates of pressure increase and decay in infarcted hearts were decreased significantly more by captopril than by losartan administration. Captopril also impaired right side cardiac function more than losartan when peak rate of pressure increase was evaluated. Thus, inhibition of the effects of AII during cardiac failure improved but did not normalize cardiac pump performance. Although inhibition of AII production by captopril produced its beneficial effect by reducing diastolic BP (DBP) and SBP, preventing AII binding by administration of losartan appeared to elicit its therapeutic effect by an improvement in myocardial contractility.

Cardiac conditioning ameliorates cardiac dysfunction associated with renal hypertension in rats.

To explore the effect of physiologic hypertrophy superimposed on pathologic hypertrophy, hearts from female control rats (C), renal hypertensive rats (H), rats conditioned with a 10-12 wk swimming program (Sw), and hypertensive rats trained by the swimming program (H-Sw) were perfused in an isolated working rat-heart apparatus. Systolic blood pressure was approximately 100 mmHg in C and Sw and was 160 mmHg in H and H-Sw. The swimming program had no effect on blood pressure. Compared with C, heart weight was increased by 30% in Sw, 47% in H, and 77% in H-Sw. At high preload and afterload, cardiac output (milliliters per gram dry LV weight) was decreased in H, increased in Sw, and partially restored towards normal in H-Sw. Ejection fraction, percent fractional shortening, and mean velocity of circumferential fiber shortening were enhanced in Sw, depressed in H, and normalized in H-Sw when compared with C. Coronary flow and myocardial oxygen consumption in this series of hearts were depressed in H, with no restoration in H-Sw, but coronary effluent lactate/pyruvate ratios were only elevated in the hearts of H-Sw. Coronary vascular responses were examined in a second series of experiments which used microspheres. In this series, the depressed coronary flow observed in H was partially restored towards normal in H-Sw and the inner/outer myocardial flow ratio was normal when hearts were perfused at 140 cm aortic pressure but was somewhat depressed in both H and H-Sw when the hearts were perfused at 80 cm aortic pressure. These studies demonstrate that hypertrophic hearts from renal hypertensive rats have diminished coronary flow and depressed cardiac function when they are studied in the isolated working heart apparatus, yet there is no evidence of myocardial ischemia. Superimposition of a chronic swimming program results in increased hypertrophy but restoration of cardiac function partially or completely to normal. Thus, pathologic and physiologic hypertrophy are biologically distinct entities. Physiologic hypertrophy may partially ameliorate the defects associated with pathologic hypertrophy.