Augmentation of the inotropic response to insulin in diabetic rat hearts

Overfeeding During Lactation in Rats is Associated with Cardiovascular Insulin Resistance in the Short-Term

Childhood obesity is associated with metabolic and cardiovascular comorbidities. The development of these alterations may have its origin in early life stages such as the lactation period through metabolic programming. Insulin resistance is a common complication in obese patients and may be responsible for the cardiovascular alterations associated with this condition. This study analyzed the development of cardiovascular insulin resistance in a rat model of childhood overweight induced by overfeeding during the lactation period. On birth day, litters were divided into twelve (L12) or three pups per mother (L3). Overfed rats showed a lower increase in myocardial contractility in response to insulin perfusion and a reduced insulin-induced vasodilation, suggesting a state of cardiovascular insulin resistance. Vascular insulin resistance was due to decreased activation of phosphoinositide 3-kinase (PI3K)/Akt pathway, whereas cardiac insulin resistance was associated with mitogen-activated protein kinase (MAPK) hyperactivity. Early overfeeding was also associated with a proinflammatory and pro-oxidant state; endothelial dysfunction; decreased release of nitrites and nitrates; and decreased gene expression of insulin receptor (IR), glucose transporter-4 (GLUT-4), and endothelial nitric oxide synthase (eNOS) in response to insulin. In conclusion, overweight induced by lactational overnutrition in rat pups is associated with cardiovascular insulin resistance that could be related to the cardiovascular alterations associated with this condition.

Caloric restriction attenuates aging-induced cardiac insulin resistance in male Wistar rats through activation of PI3K/Akt pathway

BACKGROUND AND AIM

Caloric restriction (CR) improves insulin sensitivity and is one of the dietetic strategies most commonly used to enlarge life and to prevent aging-induced cardiovascular alterations. The aim of this study was to analyze the possible beneficial effects of caloric restriction (CR) preventing the aging-induced insulin resistance in the heart of male Wistar rats.

METHODS AND RESULTS

Three experimental groups were used: 3 months old rats (3m), 24 months old rats (24m) and 24 months old rats subjected to 20% CR during their three last months of life (24m-CR). After sacrifice hearts were mounted in a perfusion system (Langendorff) and heart function in basal conditions and in response to accumulative doses of insulin (10^-9-10^-7 M), in the presence or absence of Wortmannin (10^-6 M), was recorded. CR did not attenuate the aging-induced decrease in coronary artery vasodilation in response to insulin administration, but it prevented the aging-induced downregulation of cardiac contractility (dp/dt) through activation of the PI3K/Akt intracellular pathway. Insulin stimulated in a greater extent the PI3K/Akt pathway vs the activation of the MAPK pathway and increased the protein expression of IR, GLUT-4 and eNOS in the hearts of 3m and 24m-CR rats, but not in the hearts of 24m rats. Furthermore, CR prevented the aging induced increase in endothelin-1 protein expression in myocardial tissue.

CONCLUSION

In conclusion CR partially improves cardiac insulin sensitivity and prevents the aging induced decrease in myocardial contractility in response to insulin administration through activation of PI3K/Akt pathway.

Morphology and Contractility of Cardiac Myocytes in Early Stages of Streptozotocin-Induced Diabetes Mellitus in Rats

Diabetic cardiomyopathy is the leading cause of mortality in type 1 diabetes. Thus study of cardiomyocyte morphology and function during early stages of diabetes using modern analytical methods is of critical importance. Therefore, using confocal microscopy, we determined metric parameters, volumes and contractility, with calcium transients in isolated left-ventricular myocytes at one week after induction of diabetes in rats. Myocyte volume analysis from 3D confocal scans was performed using an automated contour detection algorithm that took the actual shape of the myocytes into account. We showed a significant reduction in myocyte volume in diabetic animals. We also showed a significant reduction in length and width but not in thickness of the myocytes, which suggests disproportional reorganization of the structure of the heart tissue during short-term diabetes. From a functional point of view, we observed a significant decrease in cell shortening at a stimulation frequency of 0.5 Hz. This was accompanied by a decrease in calcium transient amplitude. Together, these data suggest that impaired calcium handling is one of the factors that contributes to the observed decrease in myocyte shortening during early stages of streptozotocin-induced diabetes in rats.

The effects of telmisartan on mechanical responses of left ventricular papillary muscle in rats with streptozotocin-induced diabetes mellitus

The purpose of this study was to investigate the effects of telmisartan (5 × 10(-5) M) on the mechanical response of left ventricular papillary muscle in rats with streptozotocin-induced diabetes mellitus. We studied 32 rats; 16 were rendered diabetic by a single intravenous injection of streptozotocin (45 mg kg(-1) i.v.) and 16 formed a non-diabetic control group. Diabetic animals were divided into two groups: diabetic-telmisartan group and the diabetic-control group. Non-diabetic controls were further divided into the non-diabetic-telmisartan group and the non-diabetic-control group. We found: (1) Muscle twitch tension (P (0)) and contraction and relaxation rates were significantly lower in diabetic controls than in the other groups. (2) Telmisartan significantly increased P (0) in both diabetic and non-diabetic rats. (3) Times to peak tension and half-relaxation were significantly greater in groups DC and DT than in the non-diabetics. In conclusion, our data suggest that telmisartan attenuates diabetes-induced impairment of diabetic rat papillary muscles, and may thus be able to reduce cardiac complications in diabetes.

Dietary interaction of high fat and marginal copper deficiency on cardiac contractile function

OBJECTIVE

High-fat and marginally copper-deficient diets impair heart function, leading to cardiac hypertrophy, increased lipid droplet volume, and compromised contractile function, resembling lipotoxic cardiac dysfunction. However, the combined effect of the two on cardiac function is unknown. This study was designed to examine the interaction between high-fat and marginally copper-deficient diets on cardiomyocyte contractile function.

RESEARCH METHODS AND PROCEDURES

Weanling male rats were fed diets incorporating a low- or high-fat diet (10% or 45% of kcal from fat, respectively) with adequate (6 mg/kg) or marginally deficient (1.5 mg/kg) copper content for 12 weeks. Contractile function was determined with an IonOptix system including peak shortening (PS), time-to-PS, time-to-90% relengthening, maximal velocity of shortening/relengthening, and intracellular Ca(2+) ([Ca(2+)](I)) rise and decay.

RESULTS

Neither dietary treatment affected blood pressure or glucose levels, although the high-fat diet elicited obesity and glucose intolerance. Both diets depressed PS, maximal velocity of shortening/relengthening, and intracellular Ca(2+) ([Ca(2+)](I)) rise and prolonged time-to-90% relengthening and Ca(2+) decay without an additive effect between the two. Ca(2+) sensitivity, apoptosis, lipid peroxidation, nitrosative damage, tissue ceramide, and triglyceride levels were unaffected by either diet or in combination. Phospholamban (PLB) but not sarco(endo)plasmic reticulum Ca(2+)-ATPase was increased by both diets. Endothelial NO synthase was depressed with concurrent treatments. The electron transport chain was unaffected, although mitochondrial aconitase activity was inhibited by the high-fat diet.

DISCUSSION

These data suggest that high-fat and marginally copper deficient diets impaired cardiomyocyte contractile function and [Ca(2+)](i) homeostasis, possibly through a similar mechanism, without obvious lipotoxicity, nitrosative damage, and apoptosis.

Augmentation of insulin-stimulated ANP release through tyrosine kinase and PI 3-kinase in diabetic rats

The aim of present study was to define the effects of insulin on atrial dynamics and ANP release and its modification in diabetic rats. An isolated perfused beating atrial model was used from control and diabetic rats. Insulin was perfused with and without an inhibitor for tyrosine kinase or phosphatidylinositol 3-kinase (PI 3-kinase). Insulin increased the release of ANP and decreased atrial contractility in a dose-dependent manner. During the perfusion of 10(-10)M insulin, the release of ANP abruptly increased within 8min by approximately 40% and then decreased with time despite of continuous perfusion. In terms of increasing the dose of insulin, the time to reach the peak effect became faster and the slope to decrease became slower. In contrast, atrial contractility was gradually decreased with time. These effects were independent upon extracellular glucose. Genistein (10(-5)M) or lavendustin C (10(-5)M), a tyrosine kinase inhibitor, attenuated the release of ANP stimulated by insulin (10(-8)M). Wortmannin (10(-7)M) or LY294002 (10(-5)M), a PI 3-kinase inhibitor, also attenuated insulin-stimulated ANP release. However, both inhibitors for PI 3-kinase and tyrosine kinase did not cause any significant effects on negative inotropism by insulin. Insulin-stimulated ANP release was augmented in streptozotocin-treated rat atria. The density of insulin receptor markedly increased in diabetic hearts. These results suggest that insulin stimulates the release of ANP through PI 3-kinase and tyrosine kinase, and augmentation of insulin-stimulated ANP release in diabetic rat atria may be partly due to an upregulation of insulin receptor.

How does glucose insulin potassium improve hemodynamic performance? Evidence for altered expression of beta-adrenoreceptor and calcium handling genes

BACKGROUND

Glucose insulin potassium (GIK) improves hemodynamic performance after coronary artery surgery (CABG). We investigated whether this is associated with changes in gene expression of beta1-adrenergic receptor (ADRB1) or other calcium handling proteins.

METHODS AND RESULTS

During a randomized double-blind placebo-controlled trial, 48 patients undergoing on-pump CABG, allocated to receive pre-ischemic placebo (5% dextrose) or GIK (40% dextrose, K+ 100 mmol.L(-1), insulin 70 u.L(-1); 0.75 mL.kg(-1).h(-1)) continued for 6 hours after the removal of the aortic cross-clamp (AXC), underwent left ventricular biopsy for analysis of specific mRNAs immediately before AXC, before release of AXC, and 10 minutes after reperfusion (placebo n=24, GIK n=24). GIK or placebo was infused for a mean of 79+/-21 minutes or 79+/-18 minutes pre-ischemia respectively. Serial hemodynamic measurements were performed. Biopsy samples were snap-frozen and stored at -80 degrees C, mRNA was extracted and TaqMan real-time polymerase chain reaction was performed to investigate expression of ADRB1, sarcoplasmic reticulum Ca-ATPase (SERCA2a), and phospholamban (PLB). GIK significantly increased cardiac index versus placebo (P=0.037). TaqMan reverse-transcriptase polymerase chain reaction showed significantly greater ADRB1 mRNA expression at all time points (4.9-fold, 7.4-fold, and 15.6-fold increase, respectively; P<0.001), significantly greater SERCA2a mRNA expression after reperfusion (13.2-fold; P<0.001), and increased PLB mRNA expression at pre-ischemia and reperfusion (P<0.001 for both time-points) in GIK groups versus placebo.

CONCLUSIONS

The beneficial hemodynamic effects of GIK therapy are associated with increased ADRB1 and SERCA2a mRNA expression. Further work is therefore warranted to investigate these mRNA effects at the protein level.

High-fat diet-induced juvenile obesity leads to cardiomyocyte dysfunction and upregulation of Foxo3a transcription factor independent of lipotoxicity and apoptosis

BACKGROUND

Obesity is associated with dyslipidemia, which leads to elevated triglyceride and ceramide levels, apoptosis and compromised cardiac function.

METHODS

To determine the role of high-fat diet-induced obesity on cardiomyocyte function, weanling male Sprague-Dawley rats were fed diets incorporating 10% of kcal or 45% of kcal from fat. Mechanical function of ventricular myocytes was evaluated including peak shortening (PS), time-to-PS (TPS), time-to-90% relengthening (TR90) and maximal velocity of shortening and relengthening (+/- dl/dt). Intracellular Ca properties were assessed using fluorescent microscopy.

RESULTS

High-fat diet induced hyperinsulinemic insulin-resistant obesity with depressed PS, +/- dl/dt, prolonged TPS/TR90 reduced intracellular Ca release and Ca clearing rate in the absence of hypertension, diabetes, lipotoxicity and apoptosis. Myocyte responsiveness to increased stimulus frequency and extracellular Ca was compromised. SERCA2a and phospholamban levels were increased, whereas phosphorylated phospholamban and potassium channel (Kv1,2) were reduced in high-fat diet group. High-fat diet upregulated the forkhead transcription factor Foxo3a, and suppressed mitochondrial aconitase activity without affecting expression of the caloric sensitive gene silent information regulator 2 (Sir2), protein nitrotyrosine formation, lipid peroxidation and apoptosis. Levels of endothelial nitric oxide synthase (NOS), inducible NOS, triglycerides and ceramide were similar between the two groups.

CONCLUSIONS

Collectively, our data show that high-fat diet-induced obesity resulted in impaired cardiomyocyte function, upregulated Foxo3a transcription factor and mitochondrial damage without overt lipotoxicity or apoptosis.

Cardiac dysfunction induced by high-fat diet is associated with altered myocardial insulin signalling in rats

AIMS/HYPOTHESIS

Diabetic cardiomyopathy (DCM) is common in type 2 diabetes. In DCM, insulin resistance may alter cardiac substrate supply and utilisation leading to changes in myocardial metabolism and cardiac function. In rats, exposure to excessive alimentary fat, inducing a type 2 diabetic phenotype, may result in myocardial insulin resistance and cardiac functional changes resembling DCM.

MATERIALS AND METHODS

Rats received high-fat (HFD) or low-fat (LFD) diets for 7 weeks. Prior to killing, insulin or saline was injected i.p. Contractile function and insulin signalling were assessed in papillary muscles and ventricular lysates, respectively.

RESULTS

Fasting and post-load blood glucose levels were increased in HFD- vs LFD-rats (all p < 0.02). Mean heart weight, but not body weight, was increased in HFD-rats (p < 0.01). HFD-hearts showed structural changes and triglyceride accumulation. HFD-muscles developed higher baseline and maximum forces, but showed impaired recovery from higher workloads. Insulin-associated modulation of Ca2+-induced force augmentation was abolished in HFD-muscles. HFD reduced insulin-stimulated IRS1-associated phosphatidylinositol 3'-kinase activity and phosphorylation of protein kinase B, glycogen synthase kinase-3beta, endothelial nitric oxide synthase, and forkhead transcription factors by 40-60% (all p < 0.05). Insulin-mediated phosphorylation of phospholamban, a critical regulator of myocardial contractility, was decreased in HFD-hearts (p < 0.05).

CONCLUSIONS/INTERPRETATION

HFD induced a hypertrophy-like cardiac phenotype, characterised by a higher basal contractile force, an impaired recovery from increased workloads and decreased insulin-mediated protection against Ca2+ overload. Cardiac dysfunction was associated with myocardial insulin resistance and phospholamban hypophosphorylation. Our data suggest that myocardial insulin resistance, resulting from exposure to excessive alimentary fat, may contribute to the pathogenesis of diabetes-related heart disease.

Insulin resistance induces hyperleptinemia, cardiac contractile dysfunction but not cardiac leptin resistance in ventricular myocytes

Insulin resistance is a metabolic syndrome commonly seen in obesity. Leptin, the obese gene product, plays a role in the regulation of cardiac function. Elevated leptin levels have been demonstrated under insulin-resistant states such as obesity and hypertension, although their role in cardiac dysfunction is unknown. This study was designed to determine the impact of prediabetic insulin resistance on leptin levels and leptin-induced cardiac contractile response. Whole-body insulin resistance was generated with a 10-week dietary sucrose feeding. Contractile and intracellular Ca(2+) properties were evaluated in ventricular myocytes using an IonOptix system. The contractile indices analyzed included peak shortening (PS), time-to-PS (TPS), time-to-90% relengthening (TR(90)), maximal velocity of shortening/relengthening (+/-dL/dt), fura-fluorescence intensity change (deltaFFI) and decay rate (tau). Sucrose-fed rats displayed significantly elevated body weight and plasma leptin levels, depressed PS, +/-dL/dt, shortened TPS, prolonged TR(90) and tau, as well as reduced deltaFFI compared to the starch-fed control group. Leptin (1-1000 nM) elicited a concentration-dependent depression of PS and deltaFFI in myocytes from both starch and sucrose groups. Leptin-induced contractile depression was abolished by the nitric oxide synthase inhibitor Nomega-nitro-L-arginine methyle ester, elevation of the extracellular Ca(2+) concentration, the Janus activated kinase 2 inhibitor AG-490 or the mitogen activated protein kinase inhibitor SB203580 in myocytes from both sucrose and starch groups. Moreover, AG-490 and SB203580 unmasked a positive response of PS in myocytes from both groups. These data indicate that insulin resistance directly induces hyperleptinemia and cardiac contractile dysfunction, without affecting leptin-mediated cardiac contractile function at the myocyte level.