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

Ketogenic Diet Ameliorates Cardiac Dysfunction via Balancing Mitochondrial Dynamics and Inhibiting Apoptosis in Type 2 Diabetic Mice

The ketogenic diet (KD) has been widely used in clinical studies and shown to hace an anti-diabetic effect, but the underlying mechanisms have not been fully elaborated. Our aim was to investigate the effects and the underling mechanisms of the KD on cardiac function in db/db mice. In the present study, db/db mice were subjected to a normal diet (ND) or KD. Fasting blood glucose, cardiac function and morphology, mitochondrial dynamics, oxidative stress, and apoptosis were measured 8 weeks post KD treatment. Compared with the ND, the KD improved glycemic control and protected against diabetic cardiomyopathy in db/db mice, and improved mitochondrial function, as well as reduced oxidative stress were observed in hearts. In addition, KD treatment exerted an anti-apoptotic effect in the heart of db/db mice. Further data showed that the PI3K/Akt pathway was involved in this protective effect. Our data demonstrated that KD treatment ameliorates cardiac dysfunction by inhibiting apoptosis via activating the PI3K-Akt pathway in type 2 diabetic mice, suggesting that the KD is a promising lifestyle intervention to protect against diabetic cardiomyopathy.

Enhanced inhibition of L-type calcium currents by troglitazone in streptozotocin-induced diabetic rat cardiac ventricular myocytes

1. Troglitazone, an insulin-sensitizing agent shown to improve cardiac function in both experimental animals and patients with diabetes, inhibits voltage-dependent L-type Ca(2+) currents (I(Ca,L)) in cardiac myocytes, which may underlie its cardioprotective effects. However, inhibition by troglitazone of I(Ca,L) in diabetic cardiac myocytes has not been characterized. 2. Using whole-cell voltage-clamp techniques, I(Ca,L) was measured in ventricular myocytes isolated from 4-6 weeks streptozotocin (STZ)-induced diabetic rats and age-matched control rats. 3. Under control conditions with CsCl internal solution, diabetic myocytes did not differ from control myocytes in membrane capacitance, current density or voltage-dependent properties of I(Ca,L). 4. Troglitazone decreased amplitude of I(Ca,L) in both control and diabetic myocytes in a concentration-dependent manner. This inhibition was more potent in diabetic than in control myocytes; half-maximum inhibitory concentrations of troglitazone measured at a holding potential of -50 mV were 4.3 and 9.5 micromol l(-1), respectively. 5. Troglitazone at 5 micromol l(-1) did not significantly influence the voltage dependency of steady-state inactivation or the inactivation time course of I(Ca,L) in either control or diabetic myocytes. 6. Since troglitazone inhibits I(Ca,L) more effectively in STZ-induced diabetic ventricular myocytes, this agent may prevent cardiac dysfunction in diabetes.

Troglitazone inhibits voltage-dependent calcium currents in guinea pig cardiac myocytes

BACKGROUND

It has been suggested that intracellular Ca2+ overload in cardiac myocytes leads to the development of diabetic cardiomyopathy. Troglitazone, an insulin-sensitizing agent, is a promising therapeutic agent for diabetes and has been shown to prevent diabetes-induced myocardial changes. To elucidate the underlying mechanism of troglitazone action on cardiac myocytes, the effects of troglitazone on voltage-dependent Ca2+ currents were examined and compared with classic Ca2+ antagonists (verapamil and nifedipine).

METHODS AND RESULTS

Whole-cell voltage-clamp techniques were applied in single guinea pig atrial myocytes. Under control conditions with CsCl internal solution, the voltage-dependent Ca2+ currents consisted of both T-type (ICa,T) and L-type (ICa,L) Ca2+ currents. Troglitazone effectively reduced the amplitude of ICa,L in a concentration-dependent manner. Troglitazone also suppressed ICa,T, but the effect of troglitazone on ICa,T was less potent than that on ICa,L. The current-voltage relationships for ICa,L and the reversal potential for ICa,L were not altered by troglitazone. The half-maximal inhibitory concentration of troglitazone on ICa,L measured at a holding potential of -40 mV was 6.3 micromol/L, and 30 micromol/L troglitazone almost completely inhibited ICa,L. Troglitazone 10 micromol/L did not affect the time courses for inactivation of ICa,L and inhibited ICa,L mainly in a use-independent fashion, without shifting the voltage-dependency of inactivation. This effect was different from those of verapamil and nifedipine. Troglitazone also reduced isoproterenol- or cAMP-enhanced ICa,L.

CONCLUSIONS

These results demonstrate that troglitazone inhibits voltage-dependent Ca2+ currents (T-type and L-type) and then antagonizes the effects of isoproterenol in cardiac myocytes, thus possibly playing a role in preventing diabetes-induced intracellular Ca2+ overload and subsequent myocardial changes.

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.

Ventricular repolarization is correlated with metabolic control in newly diagnosed type 2 diabetes

Good metabolic control may improve cardiac function in diabetic patients. It is not known, however, whether this functional improvement is associated with concomitant electrocardiographic changes. The aim of the present prospective study was to evaluate the quantitative electrocardiographic and vectorcardiographic correlates of metabolic control, left ventricular function and dimensions, and autonomic nervous function in patients with newly diagnosed Type 2 diabetes. We studied 35 patients (20 men, 15 women; age 52 +/- 6 years (mean +/- SD) with normal electrocardiograms at 1.5 and 15 months after the diagnosis of Type 2 (non-insulin-dependent) diabetes. During the follow-up, body weight decreased, and significant improvement was observed in metabolic control, cardiac function and autonomic nervous function. Concomitantly, maximal spatial vector of T wave increased from 238 +/- 122 to 284 +/- 141 microV (P < 0.01), and this increase was correlated with a decrease in glycosylated haemoglobin A1C (r = -0.45, P < 0.01) and plasma insulin (r = -0.46, P < 0.01). In addition, duration of QRS complex shortened from 94 +/- 9 to 92 +/- 8 ms (P < 0.05), and this shortening was correlated with an increase in heart rate variability (r = -0.34; P < 0.05) and a decrease in peak early to late left ventricular filling flow velocity (r = 0.35, P < 0.05). These changes were most prominent in patients with co-existing hypertension and coronary artery disease. In conclusion, improving metabolic control of diabetes is associated with changes in ventricular repolarization and shortening of QRS complex duration.

Influence of calcium channel blocker treatment on the mechanical properties of diabetic rat myocardium

The objective of this investigation was to determine whether calcium channel blocker (CCB) treatment effectively restores normal baseline mechanical function in diabetic myocardium and to evaluate its effect on the interval-strength relationship. Wistar rats were made diabetic with streptozotocin (55 mg/kg, IV). Left-ventricular papillary muscles from normal and diabetic (10 weeks) rats were superfused with Tyrode's solution at 30 degrees C. A subgroup of diabetic and normal animals received daily injections of verapamil or nifedipine (10 mg/kg, IP; 8 weeks) to compare the effectiveness of a phenylalkylamine to a dihydropyridine in reversing diabetes-induced contractile dysfunction in vitro. Muscles were electrically stimulated at 0.5 Hz with suprathreshold stimuli, and the following parameters were measured: peak tension developed, time to-peak tension, time-to-90% relaxation, and the maximum velocities of tension development and decay. Experimental diabetes was characterized by: severe hyperglycemia, hepatomegaly, reduced body weight gain, cardiomegaly, and increased plasma phospholipid levels. In addition, baseline values of peak tension developed, time to-peak tension, and time-to-90% relaxation were significantly greater in muscles from diabetic animals. Chronic nifedipine treatment reduced hyperglycemia and plasma phospholipid levels, normalized body weight gain, and reduced both heart and liver sizes in diabetic animals. Nifedipine treatment completely reversed diabetes-induced prolongation in both time-to-peak tension and time-to-90% relaxation. In diabetic myocardium, a slightly positive component was present in the interval-strength relationship between 0.01 and 1 Hz, resulting in a rightward shift in the entire curve across a wide range of stimulation frequencies (0.01-5 Hz). This positive component was absent in muscles from diabetic animals treated with both CCBs, and verapamil produced a leftward shift in the frequency response curve. The results of this study suggest that chronic nifedipine treatment may be more effective than verapamil in restoring normal baseline myocardial mechanical function, reducing hyperglycemia and hyperlipidemia, as well as attenuating both cardiac and liver enlargement in experimental diabetes. In contrast, verapamil treatment tended to normalize more effectively the inotropic response to changes in stimulation frequency in diabetic myocardium.

Role of regulatory proteins (troponin-tropomyosin) in pathologic states

In vertebrate striated muscle, troponin-tropomyosin is responsible, in part, not only for transducing the effect of calcium on contractile protein activation, but also for inhibiting actin and myosin interaction when calcium is absent. The regulatory troponin (Tn) complex displays several molecular and calcium binding variations in cardiac muscles of different species and undergoes genetic changes with development and in various pathologic states. Extensive reviews on the role of tropomyosin (Tm) and Tn in the regulation of striated muscle contraction have been published describing the molecular mechanisms involved in contractile protein regulation. In our studies, we have found an increase in Mg2+ ATPase activity in cardiac myofibrils from dystrophic hamsters and in rats with chronic coronary artery narrowing. The abnormalities in myofibrillar ATPase activity from cardiomyopathic hamsters were largely corrected by recombining the preparations with a TnTm complex isolated from normal hamsters indicating that the TnTm may play a major role in altered myocardial function. We have also observed down regulation of Ca2+ Mg2+ ATPase of myofibrils from hypertrophic guinea pig hearts, myocardial infarcted rats and diabetic-hypertensive rat hearts. In myosin from diabetic rats, this abnormality was substantially corrected by adding troponin-tropomyosin complex from control hearts. All of these disease models are associated with decreased ATPase activities of pure myosin and in the case of rat and hamster models, shifts of myosin heavy chain from alpha to beta predominate. In summary, there are three main troponin subunit components which might alter myofibrillar function however, very few direct links of molecular alterations in the regulatory proteins to physiologic and pathologic function have been demonstrated so far.

Diabetic cardiomyopathy

Diabetic cardiomyopathy as a distinct entity was first recognized by Rubler et al. in diabetics with congestive heart failure (CHF), who had no evidence of coronary atherosclerosis. The Framingham study showed a 2.4-fold increased incidence of CHF in diabetic men and a 5.1-fold increase in diabetic women over 18 years. Pathological studies show left ventricular hypertrophy and fibrosis with varying degrees of small vessel disease, the functional significance of which is uncertain. Hypertension was recognized as an important cofactor in the development of fatal congestive heart failure in diabetics. On cardiac catheterization, in patients symptomatic of heart failure, either congestive or restrictive patterns have been observed. In contrast, asymptomatic diabetics had decreased left ventricular compliance but normal systolic function on hemodynamic study. Noninvasive studies show alterations in systolic and especially diastolic function, particularly in diabetics with microvascular complications and/or coexistent hypertension. Using load-independent measures of contractility, however, systolic function was generally found to be normal in asymptomatic normotensive diabetics. Experimental studies have focused on the mildly diabetic dog and the severely diabetic rat. Decreased left ventricular compliance and increased interstitial connective tissue were observed in chronically diabetic dogs. In contrast, ventricular myocardium from diabetic rats exhibits a reversible decrease in the speed of contraction, prolongation of contraction, and a delay in relaxation. These mechanical changes are associated with a decreased myosin ATPase, a shift in myosin isoenzyme distribution, alterations in a variety of Ca2+ fluxes, and changes in responses to alpha- and beta-adrenergic and cholinergic stimulation. These biochemical changes may be secondary to alterations in carbohydrate, lipid, and adenine nucleotide metabolism in the diabetic heart.(ABSTRACT TRUNCATED AT 250 WORDS)

Abnormal changes in transmitral flow after acute exposure to nitroglycerin and nifedipine in uncomplicated insulin-dependent diabetes mellitus: a Doppler echocardiographic study

To study the left ventricular (LV) diastolic function in uncomplicated insulin-dependent diabetic patients without hypertension (n = 25) and comparable controls (n = 15), the effect of acute administration of nitroglycerin and nifedipine on cavity dimensions and transmitral flow pattern was investigated by Doppler echocardiography. At baseline no significant differences in any of the LV Doppler echocardiographic parameters were seen in the two groups. Only in diabetics did nitroglycerin diminish LV diastolic and systolic diameter significantly (p < 0.0002 and 0.004, respectively), reducing the stroke volume from 70 +/- 16 to 64 +/- 20 ml, p < 0.01. The decrease in stroke volume correlated significantly to hemoglobin (Hb) A1c level (r = 0.42, p = 0.036). An excessive preload reducing effect on venous capacitance vessels is assumed in diabetes, and this preload reducing effect was also reflected in transmitral flow pattern where E-wave/A-wave ratio decreased significantly only in the diabetic subjects (p < 0.0005). Nifedipine induced the same degree of sympathetic activation in the two groups, but an increase in LV early peak filling rate (E wave) was seen in diabetes only (63.3 +/- 13.5 to 66.8 +/- 13.5 mm, p < 0.01). The atrial filling of LV (A wave) was increased significantly in both groups. Thus an abnormal LV diastolic function can be disclosed by these pharmacologic challenges. Regulation of preload and its influence on transmitral flow pattern in diabetes deserves further investigation, as does the influence of calcium antagonists on early relaxation in the diabetic myocardium.