Diabetic myocardial alterations in ultrastructure and function

Cell Culture Platforms with Controllable Stiffness for Chick Embryonic Cardiomyocytes

For several years, cell culture techniques have been physiologically relevant to understand living organisms both structurally and functionally, aiming at preserving as carefully as possible the in vivo integrity and function of the cells. However, when studying cardiac cells, glass or plastic Petri dishes and culture-coated plates lack important cues that do not allow to maintain the desired phenotype, especially for primary cell culture. In this work, we show that microscaffolds made with polydimethylsiloxane (PDMS) enable modulating the stiffness of the surface of the culture substrate and this originates different patterns of adhesion, self-organization, and synchronized or propagated activity in the culture of chick embryonic cardiomyocytes. Thanks to the calcium imaging technique, we found that the substrate stiffness affected cardiomyocyte adhesion, as well as the calcium signal propagation in the formed tissue. The patterns of activity shown by the calcium fluorescence variations are reliable clues of the functional organization achieved by the cell layers. We found that PDMS substrates with a stiffness of 25 kPa did not allow the formation of cell layers and therefore the optimal propagation of the intracellular calcium signals, while softer PDMS substrates with Young’s modulus within the physiological in vivo reported range did permit synchronized and coordinated contractility and intracellular calcium activity. This type of methodology allows us to study phenomena such as arrhythmias. For example, the occurrence of synchronized activity or rotors that can initiate or maintain cardiac arrhythmias can be reproduced on different substrates for study, so that replacement tissues or patches can be better designed.

The constant beat: cardiomyocytes adapt their forces by equal contraction upon environmental stiffening

Cardiomyocytes are responsible for the permanent blood flow by coordinated heart contractions. This vital function is accomplished over a long period of time with almost the same performance, although heart properties, as its elasticity, change drastically upon aging or as a result of diseases like myocardial infarction. In this paper we have analyzed late rat embryonic heart muscle cells' morphology, sarcomere/costamere formation and force generation patterns on substrates of various elasticities ranging from ∼1 to 500 kPa, which covers physiological and pathological heart stiffnesses. Furthermore, adhesion behaviour, as well as single myofibril/sarcomere contraction patterns, was characterized with high spatial resolution in the range of physiological stiffnesses (15 kPa to 90 kPa). Here, sarcomere units generate an almost stable contraction of ∼4%. On stiffened substrates the contraction amplitude remains stable, which in turn leads to increased force levels allowing cells to adapt almost instantaneously to changing environmental stiffness. Furthermore, our data strongly indicate specific adhesion to flat substrates via both costameric and focal adhesions. The general appearance of the contractile and adhesion apparatus remains almost unaffected by substrate stiffness.

Disruption of TRPV1-mediated coupling of coronary blood flow to cardiac metabolism in diabetic mice: role of nitric oxide and BK channels

We have previously shown transient receptor potential vanilloid subtype 1 (TRPV1) channel-dependent coronary function is compromised in pigs with metabolic syndrome (MetS). However, the mechanisms through which TRPV1 channels couple coronary blood flow to metabolism are not fully understood. We employed mice lacking TRPV1 [TRPV1((-/-))], db/db diabetic, and control C57BKS/J mice to determine the extent to which TRPV1 channels modulate coronary function and contribute to vascular dysfunction in diabetic cardiomyopathy. Animals were subjected to in vivo infusion of the TRPV1 agonist capsaicin to examine the hemodynamic actions of TRPV1 activation. Capsaicin (1-100 μg·kg(-1)·min(-1)) dose dependently increased coronary blood flow in control mice, which was inhibited by the TRPV1 antagonist capsazepine or the nitric oxide synthase (NOS) inhibitor N-nitro-l-arginine methyl ester (L-NAME). In addition, the capsaicin-mediated increase in blood flow was attenuated in db/db mice. TRPV1((-/-)) mice exhibited no changes in coronary blood flow in response to capsaicin. Vasoreactivity studies in isolated pressurized mouse coronary microvessels revealed a capsaicin-dependent relaxation that was inhibited by the TRPV1 inhibitor SB366791 l-NAME and to the large conductance calcium-sensitive potassium channel (BK) inhibitors iberiotoxin and Penetrim A. Similar to in vivo responses, capsaicin-mediated relaxation was impaired in db/db mice compared with controls. Changes in pH (pH 7.4-6.0) relaxed coronary vessels contracted to the thromboxane mimetic U46619 in all three groups of mice; however, pH-mediated relaxation was blunted in vessels obtained from TRPV1((-/-)) and db/db mice compared with controls. Western blot analysis revealed decreased myocardial TRPV1 protein expression in db/db mice compared with controls. Our data reveal TRPV1 channels mediate coupling of myocardial blood flow to cardiac metabolism via a nitric oxide-dependent, BK channel-dependent pathway that is corrupted in diabetes.

Diabetic cardiomyopathy

Diabetic cardiomyopathy is a distinct primary disease process, independent of coronary artery disease, which leads to heart failure in diabetic patients. Epidemiological and clinical trial data have confirmed the greater incidence and prevalence of heart failure in diabetes. Novel echocardiographic and MR (magnetic resonance) techniques have enabled a more accurate means of phenotyping diabetic cardiomyopathy. Experimental models of diabetes have provided a range of novel molecular targets for this condition, but none have been substantiated in humans. Similarly, although ultrastructural pathology of the microvessels and cardiomyocytes is well described in animal models, studies in humans are small and limited to light microscopy. With regard to treatment, recent data with thiazoledinediones has generated much controversy in terms of the cardiac safety of both these and other drugs currently in use and under development. Clinical trials are urgently required to establish the efficacy of currently available agents for heart failure, as well as novel therapies in patients specifically with diabetic cardiomyopathy.

Substrate stiffness affects the functional maturation of neonatal rat ventricular myocytes

Cardiac cells mature in the first postnatal week, concurrent with altered extracellular mechanical properties. To investigate the effects of extracellular stiffness on cardiomyocyte maturation, we plated neonatal rat ventricular myocytes for 7 days on collagen-coated polyacrylamide gels with varying elastic moduli. Cells on 10 kPa substrates developed aligned sarcomeres, whereas cells on stiffer substrates had unaligned sarcomeres and stress fibers, which are not observed in vivo. We found that cells generated greater mechanical force on gels with stiffness similar to the native myocardium, 10 kPa, than on stiffer or softer substrates. Cardiomyocytes on 10 kPa gels also had the largest calcium transients, sarcoplasmic calcium stores, and sarcoplasmic/endoplasmic reticular calcium ATPase2a expression, but no difference in contractile protein. We hypothesized that inhibition of stress fiber formation might allow myocyte maturation on stiffer substrates. Treatment of maturing cardiomyocytes with hydroxyfasudil, an inhibitor of RhoA kinase and stress fiber-formation, resulted in enhanced force generation on the stiffest gels. We conclude that extracellular stiffness near that of native myocardium significantly enhances neonatal rat ventricular myocytes maturation. Deviations from ideal stiffness result in lower expression of sarcoplasmic/endoplasmic reticular calcium ATPase, less stored calcium, smaller calcium transients, and lower force. On very stiff substrates, this adaptation seems to involve RhoA kinase.

Ultrastructure and histochemistry of rat myocardial capillary endothelial cells in response to diabetes and hypertension

Insufficient growth and rarefaction of capillaries, followed by endothelial dysfunction may represent one of the most critical mechanisms involved in heart damage. In this study we examined histochemical and ultrastructural changes in myocardial capillary endothelium in two models of heart failure streptozotocin-induced diabetes mellitus (STZ) and NO-deficient hypertension in male Wistar rats. Diabetes was induced by a single i.v. dose of STZ (45 mg/kg) and chronic 9-week stage was analysed. To induce NO-deficient hypertension, animals were treated with inhibitor of NO synthase L-nitroarginine methylester (L-NAME) (40 mg/kg) for 4 weeks. Left ventricular tissue was processed for enzyme catalytic histochemistry of capillary alkaline phosphatase (AlPh), dipeptidyl peptidase IV (DPP IV), and endothelial NO synthase/NADPH-diaphorase (NOS) and for ultrastructural analysis. In diabetic and hypertensive rats, lower/absent AlPh and DPP IV activities were found in focal micro-areas. NOS activity was significantly reduced and persisted only locally. Quantitative evaluation demonstrated reduction of reaction product intensity of AlPh, DPP and NOS by 49.50%, 74.36%, 20.05% in diabetic and 62.93%, 82.71%, 37.65% in hypertensive rats. Subcellular alterations of endothelial cells were found in heart of both groups suggesting injury of capillary function as well as compensatory processes. Endothelial injury was more significant in diabetic animals, in contrast the adaptation was more evident in hypertensive ones. CONCLUDING: both STZ-induced diabetes- and NO-deficient hypertension-related cardiomyopathy were accompanied by similar features of structural remodelling of cardiac capillary network manifested as angiogenesis and angiopathy. The latter was however, predominant and may accelerate disappearance of capillary endothelium contributing to myocardial dysfunction.

Resistance of diabetic rat hearts to Ca overload-related injury. Histochemical and ultrastructural study

The enzymatic histochemical and ultrastructural alterations of the rat heart during development of streptozotocin (STZ) induced diabetic cardiomyopathy were studied. Moreover, the response of the isolated diabetic hearts to Ca overload-Ca paradox-was investigated. In the early stage of diabetes (1 week of diabetes), no apparent histochemical changes were observed but gentle alterations of the ultrastructure of the myocytes and particularly capillaries were found. Structural changes of the myocytes and microangiopathy accompanied by decreased activities of some enzymes (phosphorylase, various dehydrogenases, ATPase) progressed with time and were more pronounced late in diabetes (9 weeks). Ca paradox induced severe structural damage of the majority of cardiomyocytes and loss of the cellular integrity, and marked decrease in activities of all enzymes. However, in acute diabetic heart only partial Ca paradox was observed. It was manifested by transmural heterogeneity of structural and enzymatic histochemical changes. Evident preservation of the ultrastructure and enzyme activities of the myocardium was revealed in late stage (9 weeks) of diabetes. It can be concluded that diabetes results in prevention of the Ca overload in rat myocardium in vitro. Disturbances in coronary perfusion associated with microangiopathy as well as altered Ca handling and depressed heart function may account for delayed development of Ca paradox in diabetic heart.

Coronary blood flow in chronic insulin-dependent diabetic dogs

Diabetic patients appear to be at an increased risk for perioperative morbidity and mortality following coronary artery bypass grafting. Many have suggested that microangiopathy is a primary cause. Using radionuclide labelled microspheres, we measured the perfusion of the subendocardium, midmyocardium, subepicardium, and the subendocardium/subepicardium ratio in alloxan-induced diabetic and normal dogs. We found no statistical difference in the myocardial perfusion of dogs made diabetic for five months when compared to normal dogs. By using repeated measures two-factor analysis of variance-regression model, changing blood glucose levels had no effect on coronary blood flow in either the diabetic or normal dogs.

Structural manifestations of diabetic cardiomyopathy in the rat and its reversal by insulin treatment

The structural effects of diabetes and subsequent insulin treatment upon the contractile and supporting elements of the rat myocardium were examined at progressive stages of both untreated and treated disease. Diabetes was induced by intravenous injection of alloxan, and tissue was examined after 6, 12, and 26 weeks. Insulin treatment began after 12 weeks of diabetes and tissue from these animals was examined after the same intervals. Within the cardiocytes, diabetes produced a focal yet progressive loss of myofibrils, transverse tubules, and sarcoplasmic reticulum, and separation of the fasciae adherens was evident at the intercalated disk. Mitochondrial damage was not evident. These cytoplasmic alterations were accompanied by intercellular and perivascular deposition of connective tissue, thickening of the endothelial cytoplasm with pinocytotic hyperactivity, and characteristic basal laminar changes. When insulin treatment began after 12 weeks of diabetes, most, but not all, of these changes were reversed, and this reversal was essentially complete within 6-12 weeks. Even with longer periods of insulin treatment, cardiocytes still exhibited scattered areas of myofibril loss and extracellular matrix was retained. In contrast, diabetic changes in the intercalated disk and capillaries, including their basal laminae, were completely and rapidly reversed. It is hypothesized that the structural manifestations of diabetic cardiomyopathy consist of two major components; the first is a short-term, physiologic adaptation to metabolic alterations, while the other represents degenerative changes for which the myocardium has only a limited capacity for repair.

Effect of acute hypoxia on cardiac function in alloxan-diabetic dogs

The aim of the present study was to examine the left ventricular and haemodynamic parameters during acute hypoxia induced by carbon monoxide intoxication and/or after norepinephrine administration in metabolically healthy and alloxan-diabetic dogs. In metabolically healthy animals after carbon monoxide intoxication followed by norepinephrine administration myocardial oedema occurred with a concomitant elevation of diastolic stiffness in the left ventricular wall, resulting in impaired cardiac performance. In the alloxan-diabetic animals no myocardial oedema developed and therefore no further increase of the originally elevated left ventricular diastolic stiffness could be observed. Close correlations were demonstrable between the enhanced water content and diastolic stiffness of the left ventricular wall in metabolically healthy animals as well as between the myocardial contractile force and cardiac output index in both control and diabetic dogs. In contrast to the controls, following norepinephrine administration per se decrease of cardiac performance could be observed in the alloxan-diabetic animals. Based on these results, it can be concluded that an increase of left ventricular diastolic stiffness impairs the cardiac performance in acute hypoxia following norepinephrine administration both in control and in diabetic animals, even if different reasons are responsible for it.