Troponin subunits contribute to altered myosin ATPase activity in diabetic cardiomyopathy

Diabetes-induced chronic heart failure is due to defects in calcium transporting and regulatory contractile proteins: cellular and molecular evidence

Heart failure (HF) is a major deteriorating disease of the myocardium due to weak myocardial muscles. As such, the heart is unable to pump blood efficiently around the body to meet its constant demand. HF is a major global health problem with more than 7 million deaths annually worldwide, with some patients dying suddenly due to sudden cardiac death (SCD). There are several risk factors which are associated with HF and SCD which can negatively affect the heart synergistically. One major risk factor is diabetes mellitus (DM) which can cause an elevation in blood glucose level or hyperglycaemia (HG) which, in turn, has an insulting effect on the myocardium. This review attempted to explain the subcellular, cellular and molecular mechanisms and to a lesser extent, the genetic factors associated with the development of diabetes- induced cardiomyopathy due to the HG which can subsequently lead to chronic heart failure (CHF) and SCD. The study first explained the structure and function of the myocardium and then focussed mainly on the excitation-contraction coupling (ECC) processes highlighting the defects of calcium transporting (SERCA, NCX, RyR and connexin) and contractile regulatory (myosin, actin, titin and troponin) proteins. The study also highlighted new therapies and those under development, as well as preventative strategies to either treat or prevent diabetic cardiomyopathy (DCM). It is postulated that prevention is better than cure.

Ca2+ mishandling and mitochondrial dysfunction: a converging road to prediabetic and diabetic cardiomyopathy

Diabetic cardiomyopathy is defined as the myocardial dysfunction that suffers patients with diabetes mellitus (DM) in the absence of hypertension and structural heart diseases such as valvular or coronary artery dysfunctions. Since the impact of DM on cardiac function is rather silent and slow, early stages of diabetic cardiomyopathy, known as prediabetes, are poorly recognized, and, on many occasions, cardiac illness is diagnosed only after a severe degree of dysfunction was reached. Therefore, exploration and recognition of the initial pathophysiological mechanisms that lead to cardiac dysfunction in diabetic cardiomyopathy are of vital importance for an on-time diagnosis and treatment of the malady. Among the complex and intricate mechanisms involved in diabetic cardiomyopathy, Ca²⁺ mishandling and mitochondrial dysfunction have been described as pivotal early processes. In the present review, we will focus on these two processes and the molecular pathway that relates these two alterations to the earlier stages and the development of diabetic cardiomyopathy.

Impact of diabetes mellitus on the contractile properties of the left and right atrial myofilaments

OBJECTIVES

The incidence of diabetes mellitus in patients with ischaemic cardiomyopathy is increasing. To evaluate the impact of diabetes mellitus on contractility, we examined the calcium-induced force in left and right atrial myofilaments of patients with and without diabetes.

METHODS

We included 149 patients (106 without diabetes, 43 with diabetes), scheduled for elective coronary artery bypass grafting from August 2016 to June 2017. The left and right atria were excised and prepared for skinned fibre measurements (pCa-force curve). The unit for the force measurements is Millinewton (mN). Comprehensive demographic data as well as echocardiographic findings of the patients were collected.

RESULTS

We observed a significant decrease of left atrial force values in patients with diabetes, averaged over all calcium concentrations (patients with diabetes 0.50 ± 0.19 mN vs 0.68 ± 0.23 mN in patients without diabetes, P = 0.002) as well as in right atrial fibres (patients with diabetes 0.35 ± 0.17 mN vs 0.47 ± 0.21 mN in patients without diabetes, P = 0.005). There was a significant influence of repeated measurements (of the calcium concentrations) on force in left atrial myofilaments (P < 0.001). There was also a significant impact of diabetes on the force values of the different calcium concentrations in left atrial myofilaments (P 0.002). In right atrial myofilaments we also found a significant influence of repeated measurements (of the calcium concentrations) on force (P < 0.001). Additionally the impact of diabetes on the force values was significant (P = 0.005).

CONCLUSIONS

We demonstrated that diabetes mellitus has a significantly negative impact on calcium-induced force development in left and right atrial myofilaments.

Maternal obesity impairs fetal cardiomyocyte contractile function in sheep

Obesity is a major public health problem worldwide. In the United States, one-third of women of reproductive age are obese. Human studies show that maternal obesity (MO) predisposes offspring to cardiovascular disease. However, the underlying mechanisms remain unclear. Given the similarities between pregnancy in sheep and humans, we studied sheep to examine the impact of MO on fetal cardiomyocyte contractility at term. We observed that MO impaired cardiomyocyte contractility by reducing peak shortening and shortening/relengthening velocity, prolonging time to relengthening. MO disrupted Ca²⁺ homeostasis in fetal cardiomyocytes, increasing intracellular Ca²⁺ and inducing cellular Ca²⁺ insensitivity. The Ca²⁺-release channel was impaired, but Ca²⁺ uptake was unaffected by MO. The upstream kinases that phosphorylate the Ca²⁺-release channel-ryanodine receptor-2, PKA, and calmodulin-dependent protein kinase II-were activated in MO fetuses. Contractile dysfunction was associated with an increased ratio of myosin heavy chain (MHC)-β to MHC-α and upregulated cardiac troponin (cTn)-T and tropomyosin, as well as cTn-I phosphorylation. In summary, this is the first characterization of the effects of MO on fetal cardiomyocyte contractility. Our findings indicate that MO impairs fetal cardiomyocyte contractility through altered intracellular Ca²⁺ handling, overloading fetal cardiomyocyte intracellular Ca²⁺ and aberrant myofilament protein composition. These mechanisms may contribute to developmental programming by MO of offspring cardiac function and predisposition to later life cardiovascular disease in the offspring.-Wang, Q., Zhu, C., Sun, M., Maimaiti, R., Ford, S. P., Nathanielsz, P. W., Ren, J., Guo, W. Maternal obesity impairs fetal cardiomyocyte contractile function in sheep.

Complement Activation in Patients With Diabetic Nephropathy

Introduction

Complement activation plays a role in various organs in patients with diabetes. However, in diabetic nephropathy (DN), the role of complement activation is poorly understood. We examined the prevalence and clinical significance of complement deposits in the renal tissue of cases with type 1 and type 2 diabetes with and without DN.

Methods

We measured the prevalence of glomerular C4d, C1q, mannose-binding lectin (MBL), and C5b-9 deposits in 101 autopsied diabetic cases with DN, 59 autopsied diabetic cases without DN, and 41 autopsied cases without diabetes or kidney disease. The presence of complement deposits was scored by researchers who were blinded with respect to the clinical and histological data.

Results

C4d deposits were more prevalent in cases with DN than in cases without DN in both the glomeruli (46% vs. 26%) and the arterioles (28% vs. 12%). C1q deposits were also increased in the glomerular hili (77% vs. 55%) and arterioles (33% vs.14%), and were correlated with DN (P < 0.01). MBL deposits were only rarely observed. C5b-9 deposits were more prevalent in the cases with diabetes mellitus (DM) than in the cases without DM (69% vs. 32%; P < 0.001). Finally, glomerular C4d and C5b-9 deposits were correlated with the severity of DN (ρ = 0.341 and 0.259, respectively; P < 0.001).

Conclusion

Complement activation is correlated with both the presence and severity of DN, suggesting that the complement system is involved in the development of renal pathology in patients with diabetes and is a promising target for inhibiting and/or preventing DN in these patients.

Removal of Abnormal Myofilament O-GlcNAcylation Restores Ca2+ Sensitivity in Diabetic Cardiac Muscle

Contractile dysfunction and increased deposition of O-linked β-N-acetyl-d-glucosamine (O-GlcNAc) in cardiac proteins are a hallmark of the diabetic heart. However, whether and how this posttranslational alteration contributes to lower cardiac function remains unclear. Using a refined β-elimination/Michael addition with tandem mass tags (TMT)-labeling proteomic technique, we show that CpOGA, a bacterial analog of O-GlcNAcase (OGA) that cleaves O-GlcNAc in vivo, removes site-specific O-GlcNAcylation from myofilaments, restoring Ca(2+) sensitivity in streptozotocin (STZ) diabetic cardiac muscles. We report that in control rat hearts, O-GlcNAc and O-GlcNAc transferase (OGT) are mainly localized at the Z-line, whereas OGA is at the A-band. Conversely, in diabetic hearts O-GlcNAc levels are increased and OGT and OGA delocalized. Consistent changes were found in human diabetic hearts. STZ diabetic hearts display increased physical interactions of OGA with α-actin, tropomyosin, and myosin light chain 1, along with reduced OGT and increased OGA activities. Our study is the first to reveal that specific removal of O-GlcNAcylation restores myofilament response to Ca(2+) in diabetic hearts and that altered O-GlcNAcylation is due to the subcellular redistribution of OGT and OGA rather than to changes in their overall activities. Thus, preventing sarcomeric OGT and OGA displacement represents a new possible strategy for treating diabetic cardiomyopathy.

Streptozotocin-induced diabetes prolongs twitch duration without affecting the energetics of isolated ventricular trabeculae

BACKGROUND

Diabetes induces numerous electrical, ionic and biochemical defects in the heart. A general feature of diabetic myocardium is its low rate of activity, commonly characterised by prolonged twitch duration. This diabetes-induced mechanical change, however, seems to have no effect on contractile performance (i.e., force production) at the tissue level. Hence, we hypothesise that diabetes has no effect on either myocardial work output or heat production and, consequently, the dependence of myocardial efficiency on afterload of diabetic tissue is the same as that of healthy tissue.

METHODS

We used isolated left ventricular trabeculae (streptozotocin-induced diabetes versus control) as our experimental tissue preparations. We measured a number of indices of mechanical (stress production, twitch duration, extent of shortening, shortening velocity, shortening power, stiffness, and work output) and energetic (heat production, change of enthalpy, and efficiency) performance. We calculated efficiency as the ratio of work output to change of enthalpy (the sum of work and heat).

RESULTS

Consistent with literature results, we showed that peak twitch stress of diabetic tissue was normal despite suffering prolonged duration. We report, for the first time, the effect of diabetes on mechanoenergetic performance. We found that the indices of performance listed above were unaffected by diabetes. Hence, since neither work output nor change of enthalpy was affected, the efficiency-afterload relation of diabetic tissue was unaffected, as hypothesised.

CONCLUSIONS

Diabetes prolongs twitch duration without having an effect on work output or heat production, and hence efficiency, of isolated ventricular trabeculae. Collectively, our results, arising from isolated trabeculae, reconcile the discrepancy between the mechanical performance of the whole heart and its tissues.

Protection of the heart by treatment with a divalent-copper-selective chelator reveals a novel mechanism underlying cardiomyopathy in diabetic rats

Background

Intracellular calcium (Ca²⁺) coordinates the cardiac contraction cycle and is dysregulated in diabetic cardiomyopathy. Treatment with triethylenetetramine (TETA), a divalent-copper-selective chelator, improves cardiac structure and function in patients and rats with diabetic cardiomyopathy, but the molecular basis of this action is uncertain. Here, we used TETA to probe potential linkages between left-ventricular (LV) copper and Ca²⁺ homeostasis, and cardiac function and structure in diabetic cardiomyopathy.

Methods

We treated streptozotocin-diabetic rats with a TETA-dosage known to ameliorate LV hypertrophy in patients with diabetic cardiomyopathy. Drug treatment was begun either one (preventative protocol) or eight (restorative protocol) weeks after diabetes induction and continued thereafter for seven or eight weeks, respectively. Total copper content of the LV wall was determined, and simultaneous measurements of intracellular calcium concentrations and isometric contraction were made in LV trabeculae isolated from control, diabetic and TETA-treated diabetic rats.

Results

Total myocardial copper levels became deficient in untreated diabetes but were normalized by TETA-treatment. Cardiac contractility was markedly depressed by diabetes but TETA prevented this effect. Neither diabetes nor TETA exerted significant effects on peak or resting [Ca²⁺]_i. However, diabetic rats showed extensive cardiac remodelling and decreased myofibrillar calcium sensitivity, consistent with observed increases in phosphorylation of troponin I, whereas these changes were all prevented by TETA.

Conclusions

Diabetes causes cardiomyopathy through a copper-mediated mechanism that incorporates myocardial copper deficiency, whereas TETA treatment prevents this response and maintains the integrity of cardiac structure and myofibrillar calcium sensitivity. Altered calcium homeostasis may not be the primary defect in diabetic cardiomyopathy. Rather, a newly-described copper-mediated mechanism may cause this disease.

Molecular role of GATA binding protein 4 (GATA-4) in hyperglycemia-induced reduction of cardiac contractility

Background

Diabetic cardiomyopathy, a diabetes-specific complication, refers to a disorder that eventually leads to left ventricular hypertrophy in addition to diastolic and systolic dysfunction. In recent studies, hyperglycemia-induced reactive oxygen species (ROS) in cardiomyocytes have been linked to diabetic cardiomyopathy. GATA binding protein 4 (GATA-4) regulates the expression of many cardio-structural genes including cardiac troponin-I (cTnI).

Methods

Streptozotocin-induced diabetic rats and H9c2 embryonic rat cardiomyocytes treated with a high concentration of glucose (a D-glucose concentration of 30 mM was used and cells were cultured for 24 hr) were used to examine the effect of hyperglycemia on GATA-4 accumulation in the nucleus. cTnI expression was found to be linked to cardiac tonic dysfunction, and we evaluated the expression levels of cTnI and GATA-4 by Western blot analysis.

Results

Cardiac output was lowered in STZ-induced diabetic rats. In addition, higher expressions of cardiac troponin I (cTnI) and phosphorylated GATA-4 were identified in these rats by Western blotting. The changes were reversed by treatment with insulin or phlorizin after correction of the blood sugar level. In H9c2 cells, ROS production owing to the high glucose concentration increased the expression of cTnI and GATA-4 phosphorylation. However, hyperglycemia failed to increase the expression of cTnI when GATA-4 was silenced by small interfering RNA (siRNA) in H9c2 cells. Otherwise, activation of ERK is known to be a signal for phosphorylation of serine105 in GATA-4 to increase the DNA binding ability of this transcription factor. Moreover, GSK3β could directly interact with GATA-4 to cause GATA-4 to be exported from the nucleus. GATA-4 nuclear translocation and GSK3β ser9 phosphorylation were both elevated by a high glucose concentration in H9c2 cells. These changes were reversed by tiron (ROS scavenger), PD98059 (MEK/ERK inhibitor), or siRNA of GATA-4. Cell contractility measurement also indicated that the high glucose concentration decreased the contractility of H9c2 cells, and this was reduced by siRNA of GATA-4.

Conclusions

Hyperglycemia can cause systolic dysfunction and a higher expression of cTnI in cardiomyocytes through ROS, enhancing MEK/ERK-induced GATA-4 phosphorylation and accumulation in the cell nucleus.

Interplay between impaired calcium regulation and insulin signaling abnormalities in diabetic cardiomyopathy

According to the International Diabetes Federation the number of people between the ages of 20 and 79 years diagnosed with diabetes mellitus is projected to reach 380 million worldwide by 2025. Cardiovascular disease, including heart failure, is the major cause of death in patients with diabetes. A contributing factor to heart failure in such patients is the development of diabetic cardiomyopathy--a clinical myocardial condition distinguished by ventricular dysfunction that can present independently of other risk factors such as hypertension or coronary artery disease. This disorder has been associated with both type 1 and type 2 diabetes, and is characterized by early-onset diastolic dysfunction and late-onset systolic dysfunction. The development of diabetic cardiomyopathy and the cellular and molecular perturbations associated with the pathology are complex and multifactorial. Hallmark mechanisms include abnormalities in regulation of calcium homeostasis, and associated abnormal ventricular excitation-contraction coupling, metabolic disturbances, and alterations in insulin signaling. An emerging concept is that disruptions in calcium homeostasis might be linked to diminished insulin responsiveness. An understanding of the cellular effect of these abnormalities on cardiomyocytes should be useful in predicting the maladaptive cardiac structural and functional consequences of diabetes.

Chronic effects of type 2 diabetes mellitus on cardiac muscle contraction in the Goto-Kakizaki rat

Type 2 diabetes mellitus accounts for more than 90% of all cases of diabetes mellitus, and cardiovascular complications are the major cause of mortality and death in diabetic patients. The chronic effects of type 2 diabetes mellitus on heart function have been investigated in the Goto-Kakizaki (GK) rat. Experiments were performed in GK rats and age-matched Wistar control rats at 18 months of age. The progressive effects of diabetes on glucose metabolism were monitored periodically by application of the glucose tolerance test. Ventricular action potentials were measured in isolated, perfused heart. Shortening and intracellular Ca(2+) were measured in electrically stimulated ventricular myocytes. The GK rats displayed mild fasting hyperglycaemia and progressively worsening glucose tolerance. At 18 months of age and 180 min after intraperitoneal injection of glucose (2 g (kg body weight)(-1)), blood glucose was 436 +/- 47 mg dl(-1) in GK rats compared with 153 +/- 18 mg dl(-1) in control animals. Heart weight to body weight ratio was significantly increased in GK rats (4.10 +/- 0.09 mg g(-1), n = 5) compared with control animals (3.36 +/- 0.22 mg g(-1), n = 4). Spontaneous heart rate was slightly reduced in GK rats compared with control rats. Although the amplitude of shortening was not altered, the amplitude of the Ca(2+) transient was significantly increased in myocytes from GK rats (0.78 +/- 0.11 ratio units) compared with control rats (0.50 +/- 0.06 ratio units). Despite progressively worsening glucose metabolism, at 18 months of age the contractile function of the heart appears to be well preserved.

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.

Depressed cardiac tension cost in experimental diabetes is due to altered myosin heavy chain isoform expression

Cardiac disease in diabetes presents as impaired left ventricular contraction and relaxation; however, the mechanisms underlying contractile protein dysfunction during the progression of disease are unknown. Accordingly, we assessed Ca(2+)-dependent tension development and tension-dependent ATP consumption (tension cost) in a rat model early (6 wk) and late (12 wk) after the onset of diabetes (50 mg/kg iv streptozotocin) using mechanical force- and enzyme-coupled UV absorbance measurements. Myofilament Ca(2+) sensitivity and maximal tension were unchanged between groups at either time point. Cross-bridge cycling rate was significantly decreased in diabetes, as indexed by tension cost (early control 5.4 +/- 0.4 and early diabetes 4.2 +/- 0.3; and late control 6.0 +/- 0.2 and late diabetes 4.2 +/- 0.2; P < 0.05). Because rodent models of cardiac disease are confounded by altered myosin isoform distribution, myosin content was determined by SDS-PAGE and densitometry. The cardiac content of alpha-myosin in diabetes was decreased to 41% +/- 4.1 at 6 wk and 32.5% +/- 2.9 at 12 wk of diabetes (early control 77.8% +/- 3.3 and late control 73.6% +/- 2.5). Separate control experiments demonstrated a linear decrease in tension cost with decreased alpha-myosin content. Given this, the depression of tension cost in this rodent model of diabetes could be fully explained by the altered myosin isoform distribution.

Myocardial function in rat genetic models of low and high aerobic running capacity

We recently evaluated treadmill aerobic running capacity in 11 inbred strains of rats and found that isolated working left ventricular function correlated (r = 0.86) with aerobic running capacity. Among these 11 strains the Buffalo (BUF) hearts produced the lowest and the DA hearts the highest isolated cardiac output. The goal of this study was to investigate the components of cardiac function (i.e., coronary flow, heart rates, stroke volume, contractile dynamics, and cross-bridge cycling) to characterize further the BUF and DA inbred strains as potential models of contrasting myocardial performance. Cardiac performance was assessed using the Langendorff-Neely working heart preparation. Isolated DA hearts were superior (P < 0.05) to the BUF hearts for cardiac output (63%), stroke volume (60%), aortic +dP/dt (47%), and aortic -dP/dt (46%). The mean alpha/beta-myosin heavy chain (MHC) isoform ratio for DA hearts was 21-fold higher relative to BUF hearts. At the steady-state mRNA level, DA hearts had a fivefold higher alpha/beta-ratio than the BUF hearts. The mean rate of ATP hydrolysis by MHCs was 64% greater in DA compared with BUF ventricles. These data demonstrate that the BUF and DA strains can serve as genetic models of contrasting low and high cardiac function.

Preservation of glucose metabolism in hypertrophic GLUT4-null hearts

GLUT4-null mice lacking the insulin-sensitive glucose transporter are not diabetic but do exhibit abnormalities in glucose and lipid metabolism. The most striking morphological consequence of ablating GLUT4 is cardiac hypertrophy. GLUT4-null hearts display characteristics of hypertrophy caused by hypertension. However, GLUT4-null mice have normal blood pressure and maintain a normal cardiac contractile protein profile. Unexpectedly, although they lack the predominant glucose transporter in the heart, GLUT4-null hearts transport glucose and synthesize glycogen at normal levels, but gene expression of rate-limiting enzymes involved in fatty acid oxidation is decreased. The GLUT4-null heart represents a unique model of hypertrophy that may be used to study the consequences of altered substrate utilization in normal and pathophysiological conditions.

Experimental diabetes is associated with functional activation of protein kinase C epsilon and phosphorylation of troponin I in the heart, which are prevented by angiotensin II receptor blockade

A cardiomyopathy that is characterized by an impairment in diastolic relaxation and a loss of calcium sensitivity of the isolated myofibril has been described in chronic diabetic animals and humans. To explore a possible role for protein kinase C (PKC)-mediated phosphorylation of myofibrillar proteins in this process, we characterized the subcellular distribution of the major PKC isoforms seen in the adult heart in cardiocytes isolated from diabetic rats and determined patterns of phosphorylation of the major regulatory proteins, including troponin I (TnI). Rats were made diabetic with a single injection of streptozotocin, and myocardiocytes were isolated and studied 3 to 4 weeks later. In nondiabetic animals, 76% of the PKC epsilon isoform was located in the cytosol and 24% was particulate, whereas in diabetic animals, 55% was cytosolic and 45% was particulate (P < .05). PKC delta, the other major PKC isoform seen in adult cardiocytes, did not show a change in subcellular localization. In parallel, TnI phosphorylation was increased 5-fold in cardiocytes isolated from the hearts of diabetic animals relative to control animals (P < .01). The change in PKC epsilon distribution and in TnI phosphorylation in diabetic animals was completely prevented by rendering the animals euglycemic with insulin or by concomitant treatment with a specific angiotensin II type-1 receptor (AT1) antagonist. Since PKC phosphorylation of TnI has been associated with a loss of calcium sensitivity of intact myofibrils, these data suggest that angiotensin II receptor-mediated activation of PKC may play a role in the contractile dysfunction seen in chronic diabetes.

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Diabetes mellitus is associated with a distinct cardiomyopathy. Whether cardiac myofilament function is altered in human diabetes mellitus is unknown. Myocardial biopsies were obtained from seven diabetic patients and five control, nondiabetic patients undergoing coronary artery bypass surgery. Myofilament function was assessed by determination of the developed force-Ca2+ concentration relation in skinned cardiac cells from flash-frozen human biopsies. Separate control experiments revealed that flash freezing of biopsy specimens did not affect myofilament function. All patients in the diabetes mellitus cohort were classified as Type 2 diabetes mellitus patients, and most showed signs of diastolic dysfunction. Diabetes mellitus was associated with depressed myofilament function, that is, decreased Ca2+ sensitivity (29%, P < 0.05 vs. control) and a trend toward reduction of maximum Ca2+-saturated force (29%, P = 0.08 vs. control). The slope of the force-Ca2+ concentration relation (Hill coefficient) was not affected by diabetes, however. We conclude that human diabetes mellitus is associated with decreased cardiac myofilament function. Depressed cardiac myofilament Ca2+ responsiveness may underlie the decreased ventricular function characteristic of human diabetic cardiomyopathy.