Abnormal cardiac biochemistry in spontaneously diabetic Bio-Breeding/Worcester rat

Mechanisms of subcellular remodeling in heart failure due to diabetes

Diabetic cardiomyopathy is not only associated with heart failure but there also occurs a loss of the positive inotropic effect of different agents. It is now becoming clear that cardiac dysfunction in chronic diabetes is intimately involved with Ca(2+)-handling abnormalities, metabolic defects and impaired sensitivity of myofibrils to Ca(2+) in cardiomyocytes. On the other hand, loss of the inotropic effect in diabetic myocardium is elicited by changes in signal transduction mechanisms involving hormone receptors and depressions in phosphorylation of various membrane proteins. Ca(2+)-handling abnormalities in the diabetic heart occur mainly due to defects in sarcolemmal Na(+)-K(+) ATPase, Na(+)-Ca(2+) exchange, Na(+)-H(+) exchange, Ca(2+)-channels and Ca(2+)-pump activities as well as changes in sarcoplasmic reticular Ca(2+)-uptake and Ca(2+)-release processes; these alterations may lead to the occurrence of intracellular Ca(2+) overload. Metabolic defects due to insulin deficiency or ineffectiveness as well as hormone imbalance in diabetes are primarily associated with a shift in substrate utilization and changes in the oxidation of fatty acids in cardiomyocytes. Mitochondria initially seem to play an adaptive role in serving as a Ca(2+) sink, but the excessive utilization of long-chain fatty acids for a prolonged period results in the generation of oxidative stress and impairment of their function in the diabetic heart. In view of the activation of sympathetic nervous system and renin-angiotensin system as well as platelet aggregation, endothelial dysfunction and generation of oxidative stress in diabetes and blockade of their effects have been shown to attenuate subcellular remodeling, metabolic derangements and signal transduction abnormalities in the diabetic heart. On the basis of these observations, it is suggested that oxidative stress and subcellular remodeling due to hormonal imbalance and metabolic defects play a critical role in the genesis of heart failure during the development of diabetic cardiomyopathy.

RAGE and modulation of ischemic injury in the diabetic myocardium

OBJECTIVE

Subjects with diabetes experience an increased risk of myocardial infarction and cardiac failure compared with nondiabetic age-matched individuals. The receptor for advanced glycation end products (RAGE) is upregulated in diabetic tissues. In this study, we tested the hypothesis that RAGE affected ischemia/reperfusion (I/R) injury in the diabetic myocardium. In diabetic rat hearts, expression of RAGE and its ligands was enhanced and localized particularly to both endothelial cells and mononuclear phagocytes.

RESEARCH DESIGN AND METHODS

To specifically dissect the impact of RAGE, homozygous RAGE-null mice and transgenic (Tg) mice expressing cytoplasmic domain-deleted RAGE (DN RAGE), in which RAGE-dependent signal transduction was deficient in endothelial cells or mononuclear phagocytes, were rendered diabetic with streptozotocin. Isolated perfused hearts were subjected to I/R.

RESULTS

Diabetic RAGE-null mice were significantly protected from the adverse impact of I/R injury in the heart, as indicated by decreased release of LDH and lower glycoxidation products carboxymethyl-lysine (CML) and pentosidine, improved functional recovery, and increased ATP. In diabetic Tg mice expressing DN RAGE in endothelial cells or mononuclear phagocytes, markers of ischemic injury and CML were significantly reduced, and levels of ATP were increased in heart tissue compared with littermate diabetic controls. Furthermore, key markers of apoptosis, caspase-3 activity and cytochrome c release, were reduced in the hearts of diabetic RAGE-modified mice compared with wild-type diabetic littermates in I/R.

CONCLUSIONS

These findings demonstrate novel and key roles for RAGE in I/R injury in the diabetic heart.

Characterization of contractile function in diabetic hypertensive cardiomyopathy in adult rat ventricular myocytes

Diabetes and hypertension both produce myocardial dysfunction that accelerates cardiovascular morbidity and mortality. Coexistence of the two often results in a more severe cardiomyopathy than either process alone. The purpose of this study was to characterize the contractile function of diabetic hypertensive cardiomyopathy at the single myocyte level. Adult spontaneously hypertensive rats (SHR) and normotensive Wistar-Kyoto (WKY) rats were made diabetic with a single injection (55 mg/kg) of streptozotocin (STZ). Contractile properties of ventricular myocytes were evaluated, including peak shortening (PS), time-to-peak shortening (TPS), time-to-90% relengthening (TR90) and maximal velocities of shortening/relengthening (+/-dL/d t). The experimental animals exhibited enlarged heart size, elevated blood glucose and systolic blood pressure. PS was unchanged (SHR), enhanced (WKY-STZ) or depressed (SHR-STZ) compared to control (WKY). Myocytes from all experimental groups displayed prolonged TPS and TR90 compared to the WKY group, although only those from the hypertensive groups (SHR, SHR-STZ) were associated with reduced +/-dL/d t. Additionally, myocytes from the WKY-STZ but not the SHR or the SHR-STZ groups exhibited impaired responsiveness to increased extracellular Ca2+. Myocytes from the SHR-STZ group displayed a leftward shift of the stimulus frequency-peak shortening response curve compared to the WKY group. These results confirmed observations at the multicellular levels that combination of diabetes and hypertension results in a greater impairment of cardiac contractile function than is seen with either disease alone.

Altered cardiac excitation-contraction coupling in ventricular myocytes from spontaneously diabetic BB rats

Cardiac excitation-contraction (E-C) coupling abnormalities in chemically induced diabetes have been well defined. Heart dysfunction has also been reported in diabetes of genetic origin. The purpose of this study was to determine whether heart dysfunction in genetically predisposed diabetes is attributable to impaired E-C coupling at the cellular level. Myocytes were isolated from 1-yr-old BioBreed (BB) spontaneously diabetic-prone (BB/DP) rats and their diabetic-resistant littermates (BB/DR). Mechanical properties were evaluated by use of a video edge-detection system. Myocytes were electrically stimulated at 0.5 Hz. The contractile properties analyzed included peak shortening (PS), time-to-peak shortening (TPS), time-to-90% relengthening (TR(90)), and maximal velocities of shortening and relengthening (+/-dL/dt). Intracellular Ca(2+) handling was evaluated with fura 2 fluorescent dye. Myocytes from spontaneously diabetic hearts exhibited a depressed PS, prolonged TPS and TR(90), and reduced +/-dL/dt. Consistent with the mechanical response, myocytes from the BB/DP group displayed reduced resting and peak intracellular Ca(2+) concentration associated with a slowed Ca(2+)-transient decay. Furthermore, myocytes from BB/DP hearts were less responsive to increases in extracellular Ca(2+) and norepinephrine and equally responsive to increases in stimulation frequency and KCl compared with those from the BB/DR group. These results suggest that the genetic diabetic state produces altered cardiac E-C coupling, in part, because of abnormalities of the myocyte, similar to that demonstrable after chemically induced diabetes or during human diabetes.