Changes of enzyme activities in the myocardium and skeletal muscle fibres of cardiomyopathic hamsters. A cytophotometrical study

Myocardial Steatosis and Necrosis in Atria and Ventricles of Rats Given Pyruvate Dehydrogenase Kinase Inhibitors

Pharmaceutical therapies for non-insulin-dependent diabetes mellitus (NIDDM) include plasma glucose lowering by enhancing glucose utilization. The mitochondrial pyruvate dehydrogenase (PDH) complex is important in controlling the balance between glucose and fatty acid substrate oxidation. Administration of pyruvate dehydrogenase kinase inhibitors (PDHKIs) to rats effectively lowers plasma glucose but results in myocardial steatosis that in some instances is associated primarily with atrial and to a lesser degree with ventricular pathology. Induction of myocardial steatosis is not dose-dependent, varies from minimal to moderate severity, and is either of multifocal or diffuse distribution. Ventricular histopathology was restricted to few myocardial degenerative fibers, while that in the atrium/atria was of either acute or chronic appearance with the former showing myocardial degeneration/necrosis, acute myocarditis, edema, endothelial activation (rounding up), endocarditis, and thrombosis associated with moderate myocardial steatosis and the latter with myocardial loss, replacement fibrosis, and no apparent or minimal association with steatosis. The evidence from these evaluations indicate that excessive intramyocardial accumulation of lipid may be either primarily adverse or represents an indicator of other adversely affected cellular processes.

Changes in metabolic profile and population of skeletal muscle fibers of mice overexpressing calsequestrin: influence of losartan

In heart failure, exertional fatigue of skeletal muscles can occur. A transgenic mouse overexpressing calsequestrin can be regarded as an animal model of heart failure. The aims of the present study were to investigate, whether at the time of cardiac failure the composition of fiber types of skeletal muscles was altered, what kind of alterations in glycolytic and oxidative enzyme activities occurred in different muscle fiber types and whether these were affected by the administration of the angiotensin II receptor blocker, losartan. Hemodynamic parameters were determined using a working heart preparation. Four groups of mice were investigated: wild-type (WT) mice and transgenic (TG) mice overexpressing calsequestrin, with and without losartan treatment. Enzyme activities were measured in homogenates of Rectus femoris muscle and in muscle fibers, which were typed by their metabolic profile. Calcineurin expression was measured by Western blotting. Succinate dehydrogenase activity was increased by 275% in R. femoris muscle homogenates of TG compared to WT mice. This was due to a 57% increase in slow oxidative fibers, which was accompanied by an increased calcineurin expression in TG muscles. This increase was attenuated by losartan treatment. With respect to glycerol-3-phosphate-dehydrogenase (GPDH), no difference was evident comparing WT and TG. Treatment with losartan resulted in a down-regulation of GPDH in WT and TG. In conclusion, changes in skeletal muscles occur in this mouse model of heart failure and these changes were antagonized by losartan. In contrast to heart failure patients, in the mouse model a shift to the oxidative phenotype of skeletal muscle was noted, possibly due to enhanced calcineurin expression.

Losartan reduces mortality in a genetic model of heart failure

Altered Ca(2+) homoeostasis accompanies heart failure. As a model of heart failure, transgenic mice (TG) with selective overexpression of calsequestrin (CSQ) in the heart were used. CSQ is the main Ca(2+) binding protein in the lumen of the junctional sarcoplasmic reticulum. Overexpression of CSQ leads to hypertrophy, fibrosis, heart failure, cardiac arrhythmias, and ultimately premature death compared to littermate controls (WT). In the present study, cardiac hypertrophy was noted at 2 months of age (relative heart weight 6.4 +/- 0.2 mg/g in WT and 11.2 +/- 0.3 mg/g in TG, n = 7, p < 0.05) which progressed at 5 months of age (relative heart weight 15.5 +/- 1.1 mg/g in TG, n = 11). Furthermore, an increased degree of fibrosis (from 0.29 +/- 0.04 in WT to 0.77 +/- 0.06 in TG, n = 8, p < 0.05) was quantified by sirius red staining. Cardiac function was greatly impaired in TG as exemplified by reduced pressure development and cardiac arrhythmias. It is hypothesized that losartan, an inhibitor of angiotensin II receptors, might be able to attenuate these detrimental effects. Hence, TG and WT were treated for 1 or 4 months perorally with losartan (5 mg/kg/day) or solvent alone (control conditions) starting at 4 weeks of age. Under control conditions, none of the WT died within the observation period whereas all TG died within 9 months. Losartan treatment reduced the mortality of TG: Mean life span was raised from 116 to 193 days (n = 18 end, p < 0.05). Likewise, losartan reduced relative heart weight and the degree of fibrosis. In addition, losartan improved hemodynamic parameters, like left ventricular pressure and its first derivative. However, losartan treatment did not modify overexpression of CSQ in the heart of TG. These results imply that the angiotensin II receptor (type 1) contributes to heart failure due to CSQ overexpression, as its blockade improved survival.

Myopathy-dependent changes in activity of ATPase, SDH and GPDH and NOS expression in the different fibre types of hamster muscles

Proximal (vastus lateralis) and distal (gastrocnemius) muscles of 100-day-old normal and myopathic BIO TO-2 hamsters were analysed to study the effects of myopathy on the different muscle fibre types: SO (slow oxidative), FOG (fast oxidative glycolytic) and FG (fast glycolytic). Cytophotometric measurements of enzyme activities (myofibrillic adenosine triphosphatase, succinate dehydrogenase and glycerol-3-phosphate dehydrogenase), Western blot analysis of nitric oxide synthase (NOS) I, II, III isoforms and NOS II immunohistochemistry were performed. The following alterations were found in myopathic muscle fibres: all fibre types of both proximal and distal myopathic muscles showed decreased myofibrillic adenosine triphosphatase activity indicating depressed contractility. This was associated with depressed oxidative activity of the muscle fibres. A shift to more glycolytic metabolism was observed, mainly in FG fibres of proximal muscle. We found an increased NOS II expression in both myopathic muscle types investigated. It means that increased NO production inhibits force generation in myopathic muscle. NOS II immunoreactivity was found mainly in the cytoplasm of FG fibres. NOS I and NOS III expression was not significantly effected by this form of myopathy. Our findings demonstrate that muscle fibres of proximal and distal skeletal muscles of 100-day-old cardiomyopathic BIO TO-2 hamsters are altered with respect to contractility, metabolism and NOS II expression. FG fibres of the proximal muscle were effected most strongly.

Capillarization in skeletal muscle of rats with cardiac hypertrophy

PURPOSE

Exercise intolerance during chronic heart failure (CHF) is localized mainly in skeletal muscle. A decreased capillarization may impair exchange of oxygen between capillaries and muscle tissue and in this way contribute to exercise intolerance. We assessed changes in capillary supply in plantaris and diaphragm muscles of a rat aorta-caval fistula (ACF) preparation, a volume overload model for CHF.

METHODS

An ACF was created under equithesin anesthesia. Plantaris and diaphragm muscles were removed 6 wk postsurgery and examined for myosin heavy chain (MyHC) content and capillary supply.

RESULTS

Cardiac hypertrophy was 96% (P < 0.002) after ACF. The Type IIb MyHC content of the plantaris muscles increased (33.9 +/- 3.3 vs 49.8 +/- 3.8%; mean +/- SEM) at the expense of Type IIa MyHC (17.6 +/- 1.8 vs 11.2 +/- 1.7%) in ACF rats (P < 0.05). In the diaphragm, the number of Type I (32.1 +/- 2.3 vs 40.6 +/- 2.7%) and IIb fibers (40.6 +/- 1.9 vs 49.6 +/- 3.6%) increased at the expense of Type IIa fibers (26.8 +/- 2.5 vs 9.4 +/- 0.9%) (P < 0.05). The capillary number per fiber did not change, and this indicated that no capillary loss occurred with ACF. Also, the capillary density was maintained in the diaphragm and plantaris muscles of ACF rats. Furthermore, the coupling between fiber type, size, and metabolic type of surrounding fibers, with the capillary supply to a fiber, was maintained in rats with an ACF.

CONCLUSION

The cardiac hypertrophy induced by volume overload seems adequate to prevent atrophy and changes in the microcirculation of limb and diaphragm muscles.