Diabetes-like action of intermittent fasting on sarcoplasmic reticulum Ca2+-pump ATPase and myosin isoenzymes can be prevented by sucrose

Tension cost correlates with mechanical and biochemical parameters in different myocardial contractility conditions

OBJECTIVES

Tension cost, the ratio of myosin ATPase activity to tension, reflects the economy of tension development in the myocardium. To evaluate the mechanical advantage represented by the tension cost, we studied papillary muscle contractility and the activity of myosin ATPase in the left ventricles in normal and pathophysiological conditions.

METHODS

Experimental protocols were performed using rat left ventricles from: (1) streptozotocin-induced diabetic and control Wistar rats; (2) N-nitro-L-arginine methyl ester (L-NAME) hypertensive and untreated Wistar rats; (3) deoxycorticosterone acetate (DOCA) salt-treated, nephrectomized and salt- and DOCA-treated rats; (4) spontaneous hypertensive rats (SHR) and Wistar Kyoto (WKY) rats; (5) rats with myocardial infarction and shamoperated rats. The isometric force, tetanic tension, and the activity of myosin ATPase were measured.

RESULTS

The results obtained from infarcted, diabetic, and deoxycorticosterone acetate-salt-treated rats showed reductions in twitch and tetanic tension compared to the control and sham-operated groups. Twitch and tetanic tension increased in the N-nitro-L-arginine methyl ester-treated rats compared with the Wistar rats. Myosin ATPase activity was depressed in the infarcted, diabetic, and deoxycorticosterone acetate salt-treated rats compared with control and sham-operated rats and was increased in N-nitro-L-arginine methyl ester-treated rats. These parameters did not differ between SHR and WKY rats. In the studied conditions (e.g., post-myocardial infarction, deoxycorticosterone acetate salt-induced hypertension, chronic N-nitro-L-arginine methyl ester treatment, and streptozotocin-induced diabetes), a positive correlation between force or plateau tetanic tension and myosin ATPase activity was observed.

CONCLUSION

Our results suggest that the myocardium adapts to force generation by increasing or reducing the tension cost to maintain myocardial contractility with a better mechanical advantage.

Renin-angiotensin blockade attenuates cardiac myofibrillar remodelling in chronic diabetes

Previous studies have shown that the renin-angiotensin system (RAS) is activated in diabetes and this may contribute to the subcellular remodelling and heart dysfunction in this disease. Therefore, we examined the effects of RAS blockade by enalapril, an angiotensin-converting enzyme inhibitor, and losartan, an angiotensin receptor AT1 antagonist, on cardiac function, myofibrillar and myosin ATPase activity as well as myosin heavy chain (MHC) isozyme expression in diabetic hearts. Diabetes was induced in rats by a single injection of streptozotocin (65 mg/kg; i.v.) and these animals were treated with and without enalapril (10 mg/kg/day; oral) or losartan (20 mg/kg/day; oral) for 8 weeks. Enalapril or losartan prevented the depressions in left ventricular rate of pressure development, rate of pressure decay and ventricular weight seen in diabetic animals. Both drugs also attenuated the decrease in myofibrillar Ca2+-ATPase, Mg2+-ATPase and myosin ATPase activity seen in diabetic rats. The diabetes-induced increase in beta-MHC content and gene expression as well as the decrease in alpha-MHC content and mRNA levels were also prevented by enalapril and losartan. These results suggest the occurrence of myofibrillar remodelling in diabetic cardiomyopathy and provide evidence that the beneficial effects of RAS blockade in diabetes may be associated with attenuation of myofibrillar remodelling in the heart.

Cardiac gene expression of natriuretic substances is altered in streptozotocin-induced diabetes during angiotensin II-induced pressure overload

OBJECTIVE

To gain insight into the cardiac adaptive mechanisms in diabetes, we studied whether angiotensin II (Ang II) alters expression of the atrial natriuretic peptide (ANP), B-type natriuretic peptide (BNP) and adrenomedullin (AM) genes in the left ventricle of the diabetic rat heart.

METHODS

Diabetes was induced by streptozotocin (STZ; 60 mg/kg body weight intravenously). During the last 24 h of 2.5 or 7 weeks of treatment of male Wistar rats with STZ or vehicle, Ang II (33 microg/kg per h) was administered via osmotic minipumps.

RESULTS

Diabetes was associated with an increased left ventricular weight to body weight (LV/BW) ratio, an index of left ventricular hypertrophy, at week 7 but not at week 2.5, and with increased ANP mRNA content at 2.5 weeks, but not with altered expression of the AM and BNP genes. Mean arterial pressure and LV/BW ratio were increased by Ang II in all groups except in the 7-week diabetic group. Levels of ANP mRNA were increased fourfold (P < 0.001) and threefold (P < 0.05) by Ang II at 2.5 and 7 weeks in control animals, respectively, and 11-fold (P < 0.001) and sevenfold (P < 0.001) at 2.5 and 7 weeks in diabetic animals, respectively. Ang II increased ventricular concentrations of BNP mRNA in control and diabetic animals at 2.5 weeks (1.3-fold, P < 0.001; and 1.6-fold, P < 0.001) and at 7 weeks (1.3-fold, P < 0.05; and 1.8-fold, P < 0.001), respectively. Left ventricular levels of adrenomedullin mRNA were increased by treatment with Ang II for 24 h in 2.5-week diabetic animals.

CONCLUSION

Ang II markedly increased the levels of natriuretic peptide mRNAs in the left ventricle of normal and diabetic rat hearts, whereas it increased adrenomedullin mRNA levels only in 2.5-week diabetic rats and failed to cause hypertension in 7-week diabetic rats. Left ventricular levels of ANP and BNP mRNA were increased by Ang II in diabetic animals more than the additive effects of diabetes and Ang II alone, showing that Ang II induced an amplified response with respect to cardiac concentrations of ANP and BNP in diabetes.

Carnitine palmitoyltransferase-I, a new target for the treatment of heart failure: perspectives on a shift in myocardial metabolism as a therapeutic intervention

Although the heart is capable of extracting energy from different types of substrates such as fatty acids and carbohydrates, fatty acids are the preferred fuel under physiological conditions. In view of the presence of diverse defects in myocardial metabolism in the failing heart, changes in metabolism of glucose and fatty acids are considered as viable targets for therapeutic modification in the treatment of heart failure. One of these changes involves the carnitine palmitoyltransferase (CPT) enzymes, which are required for the transfer of long chain fatty acids into the mitochondrial matrix for oxidation. Since CPT inhibitors have been shown to prevent the undesirable effects induced by mechanical overload, e.g. cardiac hypertrophy and heart failure, it was considered of interest to examine whether the inhibition of CPT enzymes represents a novel approach for the treatment of heart disease. A shift from fatty acid metabolism to glucose metabolism due to CPT-I inhibition has been reported to exert beneficial effects in both cardiac hypertrophy and heart failure. Since the inhibition of fatty acid oxidation is effective in controlling abnormalities in diabetes mellitus, CPT-I inhibitors may also prove useful in the treatment of diabetic cardiomyopathy. Accordingly, it is suggested that CPT-I may be a potential target for drug development for the therapy of heart disease in general and heart failure in particular.

Therapeutic potential of CPT I inhibitors: cardiac gene transcription as a target

Inhibitors of carnitine palmitoyl-transferase I (CPT I), the key enzyme for the transport of long-chain acyl-coenzyme A (acyl-CoA) compounds into mitochondria, have been developed as agents for treating diabetes mellitus Type 2. Findings that the CPT I inhibitor, etomoxir, has effects on overloaded heart muscle, which are associated with an improved function, were unexpected and can be attributed to selective changes in the dysregulated gene expression of hypertrophied cardiomyocytes. Also, the first clinical trial with etomoxir in patients with heart failure showed that etomoxir improved the clinical status and several parameters of heart function. In view of the action of etomoxir on gene expression, putative molecular mechanisms involved in an increased expression of SERCA2, the Ca(2+) pump of sarcoplasmic reticulum (SR) and alpha-myosin heavy chain (MHC) of failing overloaded heart muscle are described. The first 225 bp of human, rabbit, rat and mouse SERCA2 promoter sequence have high identity. Various cis-regularory elements are also given for the promoter of the rat cardiac alpha-MHC gene. It is hypothesised that etomoxir increases glucose-phosphate intermediates resulting in activation of signalling pathway(s) mediated by phosphatases. Regarding the possible direct action of etomoxir on peroxisome proliferator activated receptor alpha (PPAR-alpha) activation, it could upregulate the expression of various enzymes that participate in beta-oxidation, thereby modulating some effects of CPT 1 inhibition. Any development of alternative drugs requires a better understanding of the signal pathways involved in the altered gene expression. In particular, signals need to be identified which are altered in overloaded hearts and can selectively be re-activated by etomoxir.

Biphasic changes in heart performance with food restriction in rats

To examine effects of food restriction resembling very-low-calorie dieting on heart performance, normal rats were fed 25% of ad libitum food intake for 14 days. Although heart weight decreased (P < 0.05) after 5 days, left ventricular systolic pressure as well as rates of pressure development and fall were increased (P < 0.05) at 7 days and decreased (P < 0.05) after 14 days. Systolic and diastolic blood pressures were also increased from 5 to 7 days and decreased after 14 days. The increased hemodynamic performance of heart was associated with a raised plasma norepinephrine concentration, which peaked at day 7 of food restriction; epinephrine concentration was increased (P < 0.05) also at day 7. An increased catecholamine synthesis was indicated by the raised (P < 0.05) plasma dopamine beta-hydroxylase activity at 3 days, but this was decreased (P < 0. 05) at 14 days. The concentration of dopamine in the heart was increased (P < 0.05) at 2-14 days, of norepinephrine at 7-14 days, and of epinephrine at 10 and 14 days. Food restriction thus appears initially to be associated with an enhanced catecholamine influence on the heart and is followed by a depressed cardiac performance.

Modification of catecholamine-induced changes in heart function by food restriction in rats

In view of the common practice of dieting for weight reduction, the influence of severe food restriction (about 25% of ad libitum intake) on adrenergic mechanisms was studied. Cardiac norepinephrine and epinephrine concentrations as well as plasma norepinephrine levels, were increased upon feeding a restricted diet to rats for 14 days in comparison with control rats that ingested about 30 g food/ day. Bradycardia as well as characteristic electrocardiographic abnormalities, including prolongation of the QRS and QT intervals, were observed in food-restricted rats. Diet-restricted rats did not develop ventricular arrhythmias in response to epinephrine injections as readily as control rats. Depression in both + dP/dt and -dP/dt of the heart in situ as well as reductions in the inotropic responses to epinephrine were evident in diet-restricted rats. Beta-adrenergic binding studies revealed a significant decrease in receptor density, but the dissociation constant for binding was also depressed in the food-restricted rat heart. Downregulation of the beta-adrenergic receptors in the heart may explain the lack of an epinephrine-induced increase in contractile force development as well as arrhythmias in food-restricted rats. These data demonstrate that severe food restriction has marked effects on adrenergic mechanisms and heart function, and thus some caution should be exercised at early periods of this therapy for weight reduction.

Effect of alpha adrenergic stimulation and carnitine palmitoyl transferase I inhibition on hypertrophying adult rat cardiomyocytes in culture

Long-term, serum supplemented cultures of rat adult ventriculocytes were utilized to study the tropic effects of the alpha-agonist phenylephrine and of the carnitine palmitoyltransferase I inhibitor etomoxir. Cell protein and the rate of incorporation of phenylalanine were measured, corrected for cellular DNA content and utilized as an index for hypertrophy and of anabolic activity of the cells, respectively. The mRNA level of ANF was utilized as an index for the pathological phenotypic change (i.e., switch to fetal gene program), and that of the Na-channel--a constantly expressed gene in normal and hypertrophic cardiomyocytes--served as an internal control. Both mRNAs were quantified at various stages in culture by competitive reverse transcriptase PCR. The size of control myocytes steadily increased for over 3 weeks. The cells were completely redifferentiated and reached a maximum of anabolic activity 2 weeks after plating. Secretion and mRNA levels of ANF were increased severalfold after 7-8 days. Addition of 10 microM phenylephrine considerably speeded up cell growth. Maximum anabolic activity and complete redifferentiation were reached already after 1 week. Levels of mRNA and of ANF release increased 30-40 fold. Interestingly, induction of ANF gene transcription lagged behind the redifferentiation of the cells. Ten microM etomoxir inhibited the oxidation of palmitic acid and stimulated that of exogenous glucose by adult cardiomyocytes. In spite of its clear effect on fuel utilization, etomoxir had no direct hypertrophic effect on the myocytes in culture and did not inhibit the stimulatory action of alpha-agonists. Reactivation of the fetal gene program, as visualized by ANF production, was not reversed by etomoxir.

Modification of myosin isozymes and SR Ca(2+)-pump ATPase of the diabetic rat heart by lipid-lowering interventions

To define metabolic influences on cardiac myosin expression and sarcoplasmic reticulum (SR) Ca(2+)-stimulated ATPase streptozotocin-diabetic rats were treated for 9-10 wk with etomoxir, an inhibitor of carnitine palmitoyl transferase I (CPT-1) and fatty acid synthesis, or an antilipolytic drug, acipimox. Etomoxir reduced myosin V3 of diabetic rats but did not normalize it. However, the high serum triglyceride, free-fatty acid and cholesterol concentrations in diabetic animals were greatly reduced. After bypassing the CPT-1 inhibition with a medium-chain fatty acid (miglyol) diet, the V3 contents and serum lipids were still reduced in the etomoxir-treated diabetic rats; V3 was also reduced in diabetic rats fed miglyol or treated with acipimox. Since low serum insulin or triiodothyronine concentrations in diabetic rats were not improved by these interventions but changes in V3 were correlated with those in triglyceride, free-fatty acid and cholesterol concentrations, it is likely that myosin may be influenced by some metabolic factors. To assess the role of adrenergic influences, diabetic rats (7-8 wk) were treated with an antisympathotonic drug, moxonidine, a beta-adrenoceptor blocking drug, propranolol, and a bradycardic drug, tedisamil. Myosin V3 was not reduced significantly in moxonidine-treated or propranolol-treated rats in comparison to untreated diabetic rats. Serum thyroid hormones and insulin were not altered, whereas triglycerides were reduced but not significantly by these antiadrenergic agents. Lowering serum lipids in diabetic rats by treatment with etomoxir, miglyol and acipimox increased the depressed SR Ca(2+)-stimulated ATPase activity. On the other hand, in diabetic rats treated with moxonidine, propranolol or tedisamil, the ATPase activity was not increased significantly. These results suggest that normalization of blood lipids is important for improving subcellular organelle function in diabetic hearts with impaired glucose utilization.

Insulin resistance, hyperinsulinemia, and cardiovascular disease. The need for novel dietary prevention strategies

Insulin resistance associated with hyperinsulinemia (metabolic syndrome) emerged in recent years as an important health risk which is present in approximately 25% of the normal population in western industrialized societies. Insulin resistance as assessed for the whole body arises from a reduced glucose utilization of skeletal muscle. If the metabolic syndrome persists over a prolonged period of time, detrimental influences on the cardiovascular system become apparent involving diabetes mellitus, hypertension, and arteriosclerosis. Of particular pathogenic relevance is an unbalanced influence of insulin arising either from a diminished or enhanced insulin action depending on whether the various tissues of the body exhibit a reduced or unchanged insulin sensitivity. Since insulin resistance and hyperinsulinemia appear to be affected by various lifestyle factors, the unique opportunity exists of reducing cardiovascular mortality by correcting this syndrome at a time when degenerative changes have not occurred in the cardiovascular system. Of great importance is the finding that dietary factors can have a modulatory action on insulin sensitivity. In animal experiments, an increased intake of (saturated) fat and refined carbohydrates increased insulin resistance. Since psychosocial distress is expected to be associated with a sustained activation of the sympathoadrenal axis, it is likely also to aggravate the metabolic syndrome. A factor with a beneficial action appears to be physical exercise. In view of the high incidence of cardiovascular diseases, further research on lifestyle factors with an insulin-sensitizing or insulin-desensitizing action is required. Of prime importance is the reevaluation of established dietary recommendations and diets should be designed which take into account the individual cardiovascular risk factor profile.

Effect of positive inotropic agents on myosin isozyme population and mechanical activity of cultured rat heart myocytes

To examine whether catecholamines have a direct effect on myosin heavy chain expression of heart myocytes or whether they act via an altered work load, myocytes from neonatal rat hearts were cultured in thyroid hormone-free media containing various positive inotropic and chronotropic agents. The velocity and frequency of contraction of the myocytes were monitored using an optoelectronic system. After 3-5 days of culture, myosin isozyme populations, cellular cAMP content, and 2-deoxy-D-glucose uptake of the myocytes were determined. Compared with myocytes cultured in the absence of inotropic agents (32.6 +/- 3.5% V1), the proportion of myosin V1 was significantly (p less than 0.05) increased in the case of 1 microM isoproterenol (48.2 +/- 5.9% V1), 1 microM forskolin (57.1 +/- 11.7% V1), and 1 mM dibutyryl cAMP (79.1 +/- 2.0% V1). Dibutyryl cAMP increased V1 to a similar level as 30 nM triiodothyronine did (70.2 +/- 13.0% V1). Only a small increase was observed in myocytes cultured in the presence of 10 microM phenylephrine (40.4 +/- 8.4% V1), 10 microM ouabain (40.6 +/- 11.9% V1), or 10 microM Bay K 8644 (40.7 +/- 11.7% V1). The agents with a marked effect on myosin heavy chain expression resulted in a higher cAMP content; isoproterenol and forskolin also stimulated 2-deoxy-D-glucose uptake. All agents resulted in a higher velocity of contraction; with the exception of ouabain, frequency of contraction was also increased. A change in Ca2+ concentration in the medium from 1.3 to 2.4 mM resulted in a small increase in V1 (40.7 +/- 5.2% V1) but had the same effect on contraction velocity as dibutyryl cAMP did. Furthermore, 10 nM isoproterenol also increased V1 in myocytes that were arrested with 10 microM verapamil. The increase in V1 in the case of dibutyryl cAMP, isoproterenol, and forskolin is thus most probably not a correlate of the increased mechanical activity but of the high cellular cAMP content.

The metabolic syndrome and signal transduction of gene expression

Epidemiological studies have clearly shown that the so-called metabolic syndrome which is linked to insulin resistance and a reduced glucose utilization of muscle represents an important risk factor for cardiovascular disease. However, only little is known of the intracellular consequences of insulin resistance. An important feature of an altered substrate utilization is related to signal transduction of gene expression. For the example of myosin heavy chain expression, it is shown that metabolic signals exist which reflect the fuel flux and substrate utilization of the heart muscle cell. The signals were characterized in functional states of the heart associated with altered metabolic influences (fasting, diabetes, sucrose feeding, increased calorie intake, carnitine palmitoyltransferase inhibition). In the pressure-overloaded heart, metabolic interventions which are expected to increase glucose utilization (sucrose feeding, captopril treatment) have a pronounced effect. Although a link with gene expression remains to be established, it should be noted that the sarcoplasmic reticulum Ca(2+)-pump activity seems to be affected in a functionally comparable manner. It is concluded that metabolic signals alter the protein phenotype of heart muscle and it is expected that a deranged signal transduction affects, not only the heart, but also vascular muscle.