F. S. Fein

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

Depressed cardiac sarcoplasmic reticular function from diabetic rats.

Previous studies have demonstrated impaired contractile performance and delayed relaxation of hearts of diabetic rats. Male and female rats were made diabetic with intravenous streptozotocin and their hearts were studied 4 to 5 or 9 weeks later. Plasma glucose in female diabetics averaged 453 mg100 ml and in male diabetics 615 mg100 ml. Calcium uptake by isolated sarcoplasmic reticulum in the absence of oxalate were significantly lower in preparations from hearts of diabetic males and females than from controls. Rats pretreated with 3-0-methyl glucose before streptozotocin also had normal calcium binding by SR in the absence of oxalate. Sarcoplasmic reticulum Mg2+ ATPase and Ca2+Mg2+ (total) ATPase activities were significantly depressed in preparations from the diabetic animals. The results may partially explain the abnormalities in contraction and relaxation previously observed in hearts of diabetic animals.

The effect of streptozotocin-induced diabetes in rats on cardiac contractile proteins.

In order to determine whether diabetic cardiomyopathy in rats is associated with altered contractile proteins, male and female rats were made diabetic with intravenous streptozotocin (STZ). Calcium ATPase activity of cardiac actomyosin was significantly decreased after 1 week of diabetes and was depressed by 60% by 2 weeks. Rats pretreated with 3-O-methyl glucose to prevent the hyperglycemia caused by STZ had normal Ca2+-actomyosin ATPase activities, and non-diabetic rats whose food was restricted to keep their body and heart weights similar to those found in diabetic animals had only a slight fall in actomyosin ATPase activity. Ca2+-ATPase and actin-activated ATPase activities of pure myosin were similarly depressed in preparations from hearts of diabetic animals. Sodium dodecylsulfate gel electrophoresis and isoelectric focusing failed to reveal differences in the patterns of contractile proteins or light subunits between diabetics and controls, but pyrophosphate gels showed a shift in the myosin pattern. Because of depressed circulating thyroid hormone levels in diabetic animals, cardiac contractile proteins were also studied in preparations from thyroidectomized rats. Calcium activities of actomyosin and myosin ATPase were lower than values found in hearts of diabetic rats. When diabetic animals were kept euthyroid with thyroid replacement, actomyosin ATPase activity was still depressed. Thus STZ diabetes causes a significant decrease in cardiac contractile protein ATPase activity. This may be related to altered proportions of myosin isoenzymes.

Reversibility of diabetic cardiomyopathy with insulin in rats.

Diabetes mellitus causes a cardiomyopathy in human subjects, independent of atherosclerotic coronary artery disease. Ventricular papillary muscle function studies in chronically diabetic rats and rabbits have shown diminished contractility and a prolonged duration of contraction. In rats there was complete reversibility of these changes with insulin therapy. However, the effects of insulin on the myocardial mechanics of diabetic rabbits have not been studied. Therefore, rabbits diabetic for 3-4 mo (after alloxan injection) were treated with PZI insulin for 3-4 mo, and the mechanical performance of their right ventricular papillary muscles was compared with that of untreated diabetic animals and age-matched controls. Insulin therapy normalized serum glucose concentration. All abnormalities in papillary muscle function were completely reversed in insulin-treated animals. Norepinephrine (NE) dose responses were also evaluated in muscles from all groups. There were no differences in the positive inotropic effects of NE between groups. However, the data suggested, in diabetic animals a blunted response of peak relaxation rate to NE; this abnormality was reversed in muscles from treated animals. These findings indicate that previous work on diabetic rats can be extended to diabetic rabbits and suggest that chronic insulin therapy completely reverses the contractile alterations in hearts from these diabetic animals.

Hereditary and acquired cardiomyopathies in experimental animals: mechanical, biochemical, and structural features.

Evidence has been presented regarding alterations of contractile behavior muscle biochemistry, and ulstrastructure during the course of the hereditary hamster cardiomyopathy. Also, preliminary structural and mechanical data were presented on the acquired cardiomyopathy of diabetes mellitus in experimental animals. In the hamster model, contractile performance, measured as isometric tension and rate of tension development, was shown to be depressed throughout the course of the disease, whereas normalized force-velocity relationships returned to normal only during the compensated stages of hypertrophy. Force-frequency relationships were depressed in myopathic muscles, indicating the presence of alterations in the muscle activation system, namely, the biochemical and functional integrity of the sarcoplasmic reticulum. Analysis of the contractile proteins in myopathic muscle has revealed depressions of Ca2+ activity in purified myosin in addition to an independently increased neutral protease activity that results in the specific degradation of LC2 of myosin. Sympathetic time and norepinephrine turnover increase progressively during the course of the disease. These changes are accompanied by decreasing tissue levels of neorepinephrine and increasing levels of dopamine, indicating a shift in the rate-limiting step for norepinephrine synthesis. Alterations were also noted in nuclear protein composition and serotonin levels. Microscopically, the myolytic and calcification changes that characterize the hamster cardiomyopathy have been confirmed. In addition, contraction bands and lysosomal changes have been observed that may relate to cateholamine hypersensitivity. In the experimental model of diabetic cardiomyopathy, a significant alteration in relaxation process was demonstrated despite the fact that peak tension development and its rate of development were unaltered. Also, the length dependence of contractile behavior was altered when compared to that of age-matched controls, indicating a potential loss of contractility reserve. When animals with combined hypertension and diabetes were studied, bothe contraction and relaxation processes were affected to a greater degree.

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)

Altered myocardial response to ouabain in diabetic rats: Mechanics and electrophysiology

Diabetes induced by streptozotocin in rats is associated with changes in the mechanical function of isolated ventricular papillary muscle. Relaxation is slowed and shortening velocity is depressed. The effects of ouabain (10(-7) to 2 x 10(-4)M) and changes in extracellular calcium ([Ca2+]0 = 0.6 to 12 mM) on the mechanical and electrical properties of normal and diabetic papillary muscles were studied. High doses of ouabain caused a rise in resting tension and a fall in developed tension in both diabetic and control muscles. However, these changes were strikingly greater in diabetic muscles which developed partial contractures at 10(-4)M. The altered response to ouabain was observed in chronically (5 to 7 weeks or 3 months) but not acutely (less than 1 week) diabetic animals. Similarly, the response to ouabain was restored to normal after chronic (5 weeks) therapy with insulin but not after acute (4 days) therapy. In both normal and diabetic muscles, the mechanical effects of increasing [Ca2+]0 from 2.4 to 12.0 mM were qualitatively similar to those seen with ouabain at 10(-5) to 10(-4)M. Electrophysiologic studies showed that under control conditions action potentials of diabetic muscles were significantly longer than those of normal muscles. Treatment with progressively higher concentrations of ouabain (10(-7) to 10(-4)M) and [Ca2+]0 (2.4 to 12.0 mM) caused shortening of both normal and diabetic action potentials, but the effects of these interventions were much greater in the diabetics. These results suggest that the response of diabetic muscles to ouabain is markedly different from normal and that this altered response may be due to impaired regulation of intracellular Ca2+ levels in diabetic myocardium.

Myocardial alterations in diabetes and hypertension

Diabetes mellitus is a complex group of diseases that has hyperglycemia as a common metabolic abnormality. Although it is well-known that diabetic patients are susceptible to the effects of large vessel atherosclerosis with specific cardiac and cerebral complications, the association of diabetes mellitus with cardiac dysfunction caused by cardiomyopathy in the absence of significant coronary artery disease has been recognized for many years. However, the pathogenesis of diabetic cardiomyopathy remains unknown and has been somewhat controversial. Specifically, whether diabetes mellitus with its metabolic effects is sufficient to account for cardiomyopathy remains to be proven. This paper reviews the evidence for and against a metabolic etiology. In addition, we review the clinical and experimental evidence that supports the view that diabetes mellitus acts together with hypertension to produce structural damage in the heart that manifests as ventricular dysfunction and ultimately congestive heart failure. The concomitant effects of the metabolic derangements of diabetes and the vascular abnormalities associated with hypertension may lead to microvascular-induced tissue injury. Findings supporting this hypothesis are presented, along with observations suggesting that treatment with vasodilating calcium channel blockers or angiotensin converting enzyme inhibitors may be beneficial in regard to tissue pathology and mortality in experimental models. Recent clinical studies also support a role for the microcirculation in diabetics. Finally, it is suggested that if the microcirculation is pathogenetically involved in diabetic cardiomyopathy, then agents that improve microcirculatory flow along with tight control of hypertension may be as beneficial in the treatment or prevention of diabetic cardiomyopathy as strict metabolic control of hyperglycemia.

Hypertensive-diabetic cardiomyopathy in rats

Left ventricular papillary muscle function, transmembrane action potentials, myosin adenosinetriphosphatase (ATPase) and isoenzyme distribution, and myocardial pathology were studied in hypertensive (H), diabetic (D), hypertensive-diabetic (HD), and control (C) rats. There was approximately 50% relative left ventricular hypertrophy in H and HD rats. Relative lung and liver weights were greater in HD rats. Peak velocity of shortening tended to decrease progressively in H, D, and HD rats. The duration of contraction and relaxation was markedly prolonged in Ds and HDs. The length-developed tension relation was blunted in HDs. The negative inotropic effect of verapamil was similar in all groups. Resting membrane potential and amplitude were decreased in D and HD rats. Action potential duration was increased in H, D, and especially HD rats. The shortening of action potential duration with increased stimulus frequency was greater in H, D, and especially HD rats than in Cs. Left ventricular myosin ATPase and V1 isoenzyme content decreased progressively in H, D, and HD rats. Right ventricular V1 isoenzyme content was not affected in H rats but was markedly decreased in D and HD rats. Left (and right) ventricular pathology was unchanged in rats with diabetes but was increased in rats with hypertension. These data suggest that the combination of myocardial pathology (due to hypertension) and cellular dysfunction (caused mainly by diabetes) may result in cardiomyopathy and congestive heart failure in the HD rat.

Beneficial effects of diltiazem on the natural history of hypertensive diabetic cardiomyopathy in rats

Hypertensive diabetic rats develop a cardiomyopathy characterized by systolic and diastolic ventricular dysfunction, myocardial hypertrophy and fibrosis, pulmonary congestion and a very high mortality rate. Alterations in contractile proteins and sarcoplasmic reticular calcium (Ca2+) transport in diabetic myocardium and their partial reversal with verapamil suggest that calcium channel blockade may prevent death from congestive heart failure in hypertensive diabetic rats. A large group of rats with renovascular hypertension and streptozotocin diabetes were divided into four groups: untreated animals (Group 1) and animals treated with 100 (Group 2), 300 (Group 3) or 600 (Group 4) mg/kg per day of sustained release diltiazem mixed in their food. Treatment was begun shortly after the onset of hypertension and diabetes. Mortality rates after 4 months were 59% (19 of 32), 53% (17 of 32), 27% (7 of 26) and 35% (12 of 34) in Groups 1, 2, 3 and 4, respectively; the mortality rate in age-matched control rats was 5% (1 of 19). The reductions in mortality rates in Groups 3 and 4 were statistically significant. Diltiazem did not change systolic blood pressure, serum glucose concentration, heart rate or left ventricular mass. There was a trend to decreased left ventricular interstitial fibrosis and perivascular fibrosis in diltiazem-treated animals. Ventricular collagen concentration was similar in untreated hypertensive diabetic and control rats; levels were higher in hypertensive diabetic rats that died than in those that survived. There was a trend to decreased collagen concentration as diltiazem dose increased. Myosin isoenzyme distribution was not changed in Groups 3 and 4 (in comparison with Group 1). In all hypertensive diabetic groups, rats that died had a higher blood pressure, heart rate, relative left ventricular mass, lung weight and lung water than did survivors. The mortality rate was two to three times higher among rats with an initial blood pressure greater than or equal to 180 mm Hg. The beneficial effects of diltiazem on survival were most significant among rats with severe hypertension.