Influence of exogenous fatty acids and ketone bodies on rates of lipolysis in isolated ventricular myocytes from normal and diabetic rats

Concurrent diabetes and heart failure: interplay and novel therapeutic approaches

Diabetes mellitus increases the risk of developing heart failure, and the co-existence of both diseases worsens cardiovascular outcomes, hospitalization and the progression of heart failure. Despite current advancements on therapeutic strategies to manage hyperglycemia, the likelihood of developing diabetes-induced heart failure is still significant, especially with the accelerating global prevalence of diabetes and an ageing population. This raises the likelihood of other contributing mechanisms beyond hyperglycemia in predisposing diabetic patients to cardiovascular disease risk. There has been considerable interest in understanding the alterations in cardiac structure and function in the diabetic patients, collectively termed as "diabetic cardiomyopathy". However, the factors that contribute to the development of diabetic cardiomyopathies is not fully understood. This review summarizes the main characteristics of diabetic cardiomyopathies, and the basic mechanisms that contribute to its occurrence. This includes perturbations in insulin resistance, fuel preference, reactive oxygen species generation, inflammation, cell death pathways, neurohormonal mechanisms, advanced glycated end-products accumulation, lipotoxicity, glucotoxicity, and posttranslational modifications in the heart of the diabetic. This review also discusses the impact of antihyperglycemic therapies on the development of heart failure, as well as how current heart failure therapies influence glycemic control in diabetic patients. We also highlight the current knowledge gaps in understanding how diabetes induces heart failure.

Myocardial triacylglycerol metabolism

Myocardial triacylglycerol (TAG) constitutes a highly dynamic fatty acid (FA) storage pool that can be used for an energy reserve in the cardiomyocyte. However, derangements in myocardial TAG metabolism and accumulation are commonly associated with cardiac disease, suggesting an important role of intramyocardial TAG turnover in the regulation of cardiac function. In cardiomyocytes, TAG is synthesized by acyltransferases and phosphatases at the sarcoplasmic reticulum and mitochondrial membrane and then packaged into cytosolic lipid droplets for temporary storage or into lipoproteins for secretion. A complex interplay among lipases, lipase regulatory proteins, and lipid droplet scaffold proteins leads to the controlled release of FAs from the cardiac TAG pool for subsequent mitochondrial β-oxidation and energy production. With the identification and characterization of proteins involved in myocardial TAG metabolism as well as the identification of the importance of cardiac TAG turnover, it is now evident that adequate regulation of myocardial TAG metabolism is critical for both cardiac energy metabolism and function. In this article, we review the current understanding of myocardial TAG metabolism and discuss the potential role of myocardial TAG turnover in cardiac health and disease. This article is part of a Special Issue entitled "Focus on Cardiac Metabolism".

Ketone bodies disturb fatty acid handling in isolated cardiomyocytes derived from control and diabetic rats

According to the current paradigm, fatty acid (FA) utilization is increased in the diabetic heart. Since plasma levels of competing substrates such as ketone bodies are increased during diabetes, the effect of those substrates on cardiac FA handling was explored. Cardiomyocytes were isolated from control and streptozotocin-treated diabetic rats and incubated with normal (80 microM) and elevated (160 microM) palmitate concentrations in the absence or presence of ketone bodies, including acetoacetate (AcAc). Comparing control cardiomyocytes under normal conditions (80 microM, no AcAc) with diabetic cardiomyocytes (160 microM, 3 mM AcAc) showed that palmitate uptake was increased from 35.2 +/- 4.8 to 60.2 +/- 14.0 nmol x 3 min(-1) x g wet weight(-1) respectively. Under these conditions, palmitate oxidation rates were comparable (58.9 +/- 23.6 versus 53.2 +/- 18.5 nmol x 30 min(-1) x g wet weight(-1)). However, in the absence of AcAc, palmitate oxidation was significantly enhanced in diabetic cardiomyocytes, indicating that ketone bodies are able to suppress cardiac FA oxidation in diabetes. The concomitantly increased FA uptake in diabetic cells, mainly due to the elevated extracellular FA levels, may be responsible for the accumulation of FA and triacylglycerol, as observed in the diabetic heart in situ.

Effects of adrenaline on triacylglycerol synthesis and turnover in ventricular myocytes from adult rats

Ca2+-tolerant myocytes were isolated with endogenous triacylglycerol (TAG) stores prelabelled with [3H]palmitate and subsequently incubated for a 1h chase period with [14C]palmitate, 2% albumin and 5mM glucose. Measurements were then made of [14C]palmitate conversion into TAG and phospholipids, of loss of [3H]TAG, of glycerol release and of change in the total TAG content. Rates of de novo synthesis of TAG were calculated by a balance method. With 0. 5mM palmitate present, 5 microM adrenaline increased de novo synthesis of TAG by 81% and incorporation of [14C]palmitate into phospholipids by 59%. Significant increases in these processes with adrenaline were also seen with 0.08, 0.14 and 0.26 mM palmitate. The beta-agonist isoprenaline had little effect on de novo synthesis of TAG and had no effect on [14C]palmitate conversion into phospholipids. The alpha1-agonist phenylephrine mimicked adrenaline in increasing [14C]palmitate conversion into phospholipids but had no effect on de novo synthesis of TAG. Adrenaline did not significantly alter the myocyte glycerol 3-phosphate content but caused a persistent 40% increase in the activity of the form of glycerolphosphate acyltransferase found predominantly in the sarcoplasmic reticulum. With 0.5 mM palmitate present, the value [14C]TAG formed -decrease in [3H]TAG consistently exceeded the enzymically measured change in cell TAG content. From this it was suggested that the specific radioactivity of [3H]TAG pool(s) mobilized during the chase period was lower than that of the overall cell TAG. In the basal state, complete mobilization of TAG measured as glycerol release was low, but cycling of TAG to diacylglycerol or monoacylglycerol and back to TAG appeared to be high. With adrenaline present, glycerol release was increased 5-6-fold but recycling of lower acylglycerols to TAG was abolished. Glycerol release was inhibited by increasing extracellular palmitate from 0.08 to 0.5 mM. Adrenaline partially over-rode this effect.

Improved isolation of cardiomyocytes by trypsination in addition to collagenase treatment

The present study was undertaken to develop an improved and stabilized method for isolating cardiomyocytes from perfused rat heart. Different lots of the commercial collagenases used for isolating cardiomyocytes give variable results both with respect to the total cell yield and the percentage of elongated cells obtained. When trypsin was present both before and during collagenase treatment of the tissue, the performance of the collagenases was improved and stabilized, and a high and stable cell yield (7.5 x 10(6) cells per heart), and a high percentage of elongated cells (about 70%) was regularly obtained. The cells possessed alpha 1-adrenergic binding sites with binding properties (Bmax = 43.5 fmol/mg protein and Kd = 125.5 pmol/l) in agreement with values previously reported. The cells were able to respond functionally, as the cellular uptake of 86Rb+ increased by 18% after alpha 1-adrenoceptor stimulation with phenylephrine. These criteria indicate that the cells were well preserved during the isolation procedure.

Myocardial cell vulnerability to exogenous phospholipase attack

Myocardial cell vulnerability to phospholipase C (PC-PLC) attack was investigated in three different preparations of rat myocardial cells: triacylglycerol (TG)-loaded, hypothermic/rewarmed and energy depleted myocytes. The attack by PC-PLC was evaluated as PC-PLC induced glycerol output due to the combined action of phospholipase C and intracellular lipases. PC-PLC induced glycerol output was significantly higher (p < 0.05) in all three myocyte preparations, compared to their respective controls. Cell morphology (% rod shaped myocytes) of TG-loaded or hypothermic/rewarmed myocytes was not different from their controls, whereas energy depleted myocytes almost exclusively were rounded up, due to hypercontraction of the myofilaments. Hypothermic/rewarmed and energy depleted myocytes showed a significantly higher release of lactate dehydrogenase (LDH), compared to their controls although the difference was much more pronounced in the latter. Finally, the cellular contents of ATP were maintained both in TG-loaded and hypothermic rewarmed myocytes, while energy depleted myocytes contained only about 25% of the normal ATP level. These results demonstrate that attack from exogenously added phospholipases can occur, not only in seriously damaged cardiac myocytes, but in myocytes with a more subtle damage as well.