The endosomal compartment is an insulin-sensitive recruitment site for GLUT4 and GLUT1 glucose transporters in cardiac myocytes.

In nonstimulated cardiomyocytes, the glucose transporter GLUT4 is confined to intracellular vesicles forming at least two populations: a storage pool enriched in GLUT4 (pool 1) and an endosomal pool containing both GLUT4 and GLUT1 (pool 2). We have now studied the dynamics of these pools in response to insulin or the mitochondrial inhibitor rotenone in rat cardiomyocytes. Rotenone recruited GLUT4 and GLUT1 to the cell surface from endosomal pool 2 without affecting pool 1. Kinetic experiments were consistent with rotenone acting on an intracellular compartment that is in close connection with the plasma membrane. In contrast, insulin caused rapid, complete depletion of GLUT4 from pool 1 and reduced the GLUT1 content of pool 2 by approximately 50%, whereas, surprisingly, no net decrease in GLUT4 occurred in this pool. Subsequent insulin withdrawal resulted in slow replenishment of pool 2 with GLUT1 and of pool 1 with GLUT4. When pool 1 was still largely depleted of GLUT4, a second insulin challenge did reduce GLUT4 in pool 2 and stimulated glucose transport to the same extent as the first insulin treatment. In conclusion, the storage pool is the primary source of GLUT4 in response to insulin, but not to rotenone. In addition, the endosomal compartment is an important recruitment site of both GLUT1 and GLUT4 when the storage pool is either unaffected (rotenone) or depleted (by a previous insulin challenge). GLUT4 mobilized by insulin from the storage pool may pass through an intermediary (possibly endosomal) compartment on its way to the cell surface.     

Elevated glucose activates protein synthesis in cultured cardiac myocytes

Diabetes mellitus results in chronic hyperglycemia, a serious metabolic disorder associated with a markedly increased risk of cardiovascular disease. However, the effects of high glucose (HG) on cardiac myocyte growth have not been fully clarified. In this study, the effect of glucose on cardiac myocyte growth was examined using leucine incorporation as an index of protein synthesis. High glucose (HG, 25 mmol/L) increased leucine incorporation (167% +/- 0.2% over normal glucose, n=4, P<.01) compared with a physiological glucose concentration (5.5 mmol/L, normal glucose). The HG-induced increase in leucine incorporation was time- and dose-dependent and was not due to osmotic changes because 25 mmol/L mannitol did not change leucine incorporation. High glucose also significantly reduced elongation factor 2 phosphorylation, an effect known to result in increased protein synthesis at the elongation step. Western blot analysis showed that HG-activated protein kinase B (PKB), also called Akt (PKB/Akt), at 18 hours. High glucose-induced leucine incorporation was attenuated with phosphatidylinositol 3-kinase (PI3K) inhibition using wortmannin and LY294002 and by rapamycin, a mammalian target of rapamycin (mTOR) inhibitor, 72%, 64%, and 65% (P<.05), respectively. High glucose also activated extracellular signal-regulated kinase 1/2 activity with peak stimulation at 5 minutes. In addition, PD98059, an inhibitor of mitogen-activated protein kinase kinase, attenuated HG-induced leucine incorporation. These data show for the first time that elevated glucose increases protein synthesis in cardiac myocytes. The increase appears to be mediated by activation of PI3K-PKB/Akt and/or PI3K-mTOR as well as extracellular signal-regulated kinase 1/2. These results provide new evidence for a direct effect of glucose independent of insulin on cardiac myocyte growth.     

Regulation of sarcolemmal glucose and fatty acid transporters in cardiac disease

Circulating long-chain fatty acids (LCFA) and glucose are the main sources for energy production in the heart. In the healthy heart the ratio of glucose and LCFA oxidation is sensitively balanced and chronic alterations in this substrate mix are closely associated with cardiac dysfunction. While it has been accepted for several years that cardiac glucose uptake is mediated by facilitated transport, i.e. by means of the glucose transport proteins GLUT1 and GLUT4, only in the last few years it has become clear that proteins with high-affinity binding sites to LCFA, referred to as LCFA transporters, are responsible for bulk LCFA uptake. Similar to the GLUTs, the LCFA transporters CD36 and FABP(pm) can be recruited from an intracellular storage compartment to the sarcolemma to increase the rate of substrate uptake. Permanent relocation of LCFA transporters, mainly CD36, from intracellular stores to the sarcolemma is accompanied by accumulation of lipids and lipid metabolites in the heart. As a consequence, insulin signalling and glucose utilization are impaired, leading to decreased contractile activity of the heart. These observations underline the particular role and interplay of substrate carriers for glucose and LCFA in modulating cardiac metabolism, and the development of heart failure. The signalling and trafficking pathways and subcellular machinery regulating translocation of glucose and LCFA transporters are beginning to be unravelled. More knowledge on substrate transporter recycling, especially the similarities and differences between glucose and LCFA transporters, is expected to enable novel therapies aimed at changing the subcellular distribution of glucose and LCFA transporters, thereby manipulating the substrate preference of the diseased heart to help restore cardiac function.     

The ability of heat stress and metabolic preconditioning to protect primary rat cardiac myocytes

Primary rat cardiocytes were subjected to either thermal preconditioning for 30 min at 43°C or 20 min metabolic preconditioning (10 mM deoxyglucose, 20 mM lactate, pH 6.5). Eighteen hours later cells were analysed either for hsp 70i expression or subjected to a subsequent lethal heat stress or simulated ischaemia (10 mM deoxyglucose, 20 mM lactate, 0.75 mM sodium dithionite, 12 mM potassium chloride, pH 6.5) for 2 hours and assessed for survival by trypan blue exclusion.Hsp 70i was induced over 100 fold by thermal preconditioning and 30 fold by metabolic preconditioning (p<0.001, p<0.05), hsp 90 was induced 2.71 fold and 2.24 fold (p<0.001, p<0.001) by thermal and metabolic preconditioning respectively, while hsp 60 was not induced by either treatment. Preconditioned cultures had improved survival against subsequent lethal heat stress or simulated ischaemia: Thermal preconditioning reduced death from 69.22% to 52.46% upon subsequent lethal heat stress and from 49.13% to 36.66% upon subsequent lethal simulated ischaemia. Metabolic preconditioning reduced cell death from 51.29% to 33.8% against subsequent lethal heat stress, and from 69.09% to 55.61% upon subsequent lethal simulated ischaemia. A second marker of cell death, the release of lactate dehydrogenase activity into the culture media, was reduced to 65% and 60% of control values for thermally preconditioned cells subjected to lethal heat or lethal simulated ischaemia respectively. Metabolically preconditioned cells demonstrated lactate dehydrogenase activity of 59% and 51% that of control values, when subjected to lethal heat or lethal simulated ischaemia respectively.Abbreviations hsp heat stress protein - hsp 70i inducible 70 kDa heat stress protein - LDH lactate dehydrogenase - PBS phosphate buffered saline     

Cardiotrophin-1 can protect cardiac myocytes from injury when added both prior to simulated ischaemia and at reoxygenation

OBJECTIVE: The cytokine cardiotrophin-1 (CT-1) has previously been shown to protect cultured cardiocytes from cell death induced by serum removal or hypoxia when administered prior to the damaging stimulus. We wished to test whether a similar protective effect could be observed if CT-1 was added after the ischaemic period and to investigate the signalling pathways involved in the protective effect when CT-1 is given prior to or after ischaemia. METHODS: We therefore examined the protective effect of CT-1 in cultured rat cardiocytes exposed to simulated ischaemia followed by reoxygenation when CT-1 was administered either prior to simulated ischaemia or at reoxygenation. RESULTS: We show that CT-1 can exert a protective effect against the damaging effects of simulated ischaemia/reoxygenation both when added after the simulated ischaemia at reoxygenation (P<0.05 in trypan blue, TUNEL and annexin V assays) or when added prior to the simulated ischaemia (P<0.05). In both cases, these protective effects are blocked by an inhibitor of the p42/p44 MAPK pathway (P<0.05 in all assays). CONCLUSION: CT-1 can protect cardiac cells when added either prior to simulated ischaemia or at the time of reoxygenation following simulated ischaemia and these effects are dependent upon its ability to activate the p42/p44 MAPK pathway. Hence CT-1 may have therapeutic potential when added at the time of reperfusion following ischaemic damage.     

Translocation of the glucose transporter GLUT4 in cardiac myocytes of the rat.

The insulin-regulated glucose transporter GLUT4 was immunolocalized in rat cardiac muscle under conditions of basal and stimulated glucose uptake, achieved by fasting and a combined exercise/insulin stimulus, respectively. In basal myocytes there was very little (less than 1%) GLUT4 in the different domains of the plasma membrane (sarcolemma, intercalated disk, and transverse tubular system). GLUT4 was localized in small tubulo-vesicular elements that occur predominantly near the sarcolemma and the transverse tubular system and in the trans-Golgi region. Upon stimulation approximately 42% of GLUT4 was found in the plasma membrane. Each domain of the plasma membrane contributed equally to this effect. GLUT4-positive, clathrin-coated pits were also present at each cell surface domain. The remainder of the labeling was in tubulo-vesicular elements at the same sites as in basal cells and in the intercalated disk areas. The localization of GLUT4 in cardiac myocytes is essentially the same as in brown adipocytes, skeletal muscle, and white adipocytes. We conclude that increased glucose transport in muscle and fat is accounted for by translocation of GLUT4 from the intracellular tubulo-vesicular elements to the plasma membrane. The labeling of coated pits indicates that in stimulated myocytes, as in adipocytes, GLUT4 recycles constantly between the endosomal compartment and the plasma membrane and that stimulation of the exocytotic rate constant is likely the major mechanism for GLUT4 translocation.     

Direct effect of thyroid hormones on glucose oxidation by isolated rat cardiac myocytes.

The incubation of isolated heart muscle cells in phosphate buffered saline in the presence of triiodothyronine (10^?5 to 10^?9m) stimulated the oxidation of ¹⁴C-U-glucose. L-thyroxine was equally effective in stimulating glucose oxidation, while the D-isomer was ineffective. No stimulation of oxidation of either ¹⁴C-U-labelled lactate or ¹⁴C-1 labelled octanoate was observed. There was no preferential oxidation of the C-1 carbon versus the C-6 carbon of glucose. Examination of the glycolytic intermediates of triiodothyronine treated cells showed a significant decrease in the level of fructose-6-phosphate and a slight increase in the level of fructose-1, 6-diphosphate. From these results it is postulated that the observed effect is due to a stimulation of phosphofructokinase.     

Commitment and differentiation of cardiac myocytes

This article reviews what is known about the earliest stages of heart development focusing on the periods of commitment and differentiation of cardiac progenitor cells and their molecular regulation. The pathway from precursor to differentiated cardiac myocyte is crucial to forming a normal, functional heart. Congenital cardiac abnormalities are some of the most common, estimated at 5-8 per 1000 live births worldwide. These conditions affect mortality and morbidity of patients as infants, children, and adults. Knowledge of what steps are critical to normal heart development would lead to earlier diagnosis and possibly repair of these defects.     

Direct effects of high glucose and insulin on protein synthesis in cultured cardiac myocytes and DNA and collagen synthesis in cardiac fibroblasts

The present study examined the direct effects of high glucose and insulin on protein synthesis in cardiac myocytes and DNA and collagen synthesis in cardiac fibroblasts. Cultured rat cardiac myocytes and fibroblasts were grown in media containing normal glucose, high glucose, or osmotic control, and incubated with or without insulin. In cardiac myocytes, high glucose had no effect, but insulin increased protein synthesis and atrial natriuretic peptide (ANP) secretion and gene expression. The extracellular signal-regulated protein kinase (ERK)/mitogen-activated protein kinase (MAPK) inhibitor and the protein kinase C (PKC) inhibitor blocked insulin-induced protein synthesis. In cardiac fibroblasts, high glucose and osmotic control media increased DNA synthesis. Collagen synthesis and fibronectin and transforming growth factor-beta1 (TGF-beta1) mRNA expression were stimulated by high glucose, but not by osmotic control. Insulin increased DNA and collagen synthesis in fibroblasts, and the insulin-induced increase in DNA synthesis was blocked by the phosphatidylinositol 3 kinase (PI3K) inhibitor. Our findings suggest that cardiomyocyte protein synthesis is mainly regulated by insulin rather than high glucose and both high glucose and insulin contribute to fibroblast DNA and collagen synthesis. High glucose accelerates fibroblast DNA synthesis and collagen synthesis, and fibronectin and TGF-beta1 mRNA expression, dependent or independent of osmotic stress. Insulin regulates myocyte protein synthesis and fibroblast DNA synthesis through different intracellular mechanisms.     

Surgical resection of esophagogastric junction stromal tumor: How to protect the cardiac function

BACKGROUND Various surgical procedures have been described for gastrointestinal stromal tumors(GISTs)at the esophagogastric junction(EGJ)close to the Z-line.However,surgery for EGJ-GIST involving Z-line has been rarely reported.AIM To introduce a novel technique called conformal resection(CR)for open resection of EGJ-GIST involving Z-line.METHODS In this retrospective study,43 patients having GISTs involving Z-line were included.The perioperative outcomes of patients receiving CR(n=18)was compared with that of proximal gastrectomy(PG)(n=25).RESULTS CR was successfully performed in all the patients with negative microscopic margins.The mean operative time,time to first passage of flatus,and postoperative hospital stay was significantly shorter in the CR group(P<0.05),while the intraoperative blood loss was similar in the two groups.The postoperative gastroesophageal reflux as diagnosed by esophageal 24-h pH monitoring and quality of life at 3 mo were significantly in favor of CR compared to PG(both P<0.001).The 5-year disease-free survival between the two groups was similar(P=0.163).The cut-off value for the determination of CR or PG was 7.0 mm above the Z-line(83.33%sensitivity,84.00%specificity,83.72%accuracy).CONCLUSION CR is safe and feasible for EGJ-GIST located within 7.0 mm above the Z-line.     

Stimulation of glucose transport in rat cardiac myocytes by guanosine 3',5'-monophosphate

Glucose transport in isolated rat cardiomyocytes is stimulated by insulin, catecholamines, and anoxia approximately 2- to 3-fold over basal rates. The molecular mechanisms controlling these responses are unknown. In our search for possible cellular mediators of glucose transport stimulation, we examined the effects of a number of nucleotides on 3-O-methylglucose transport in heart cells. The nucleotides and/or permeable analogs (monosuccinyl, 8-bromo, and dibutyryl derivatives) included cUMP, cIMP, cCMP, cAMP, and cGMP at concentrations ranging from 10 nM to 1 mM. Of all the nucleotides tested only cGMP analogs induced a significant stimulation of transport at concentrations as low as 100 nM. This effect was observed in both the 8-bromo- and dibutyryl derivatives and with 1 mM cGMP itself. The effect was concentration dependent for both analogs and produced a maximal response equivalent to that of 100 nM insulin. This insulinomimetic effect of cGMP was examined in more detail in order to evaluate its role as a potential mediator of this response. Agents that are known to stimulate guanylate cyclase in the heart produced a clear stimulation of transport when added to cardiomyocytes. These include insulin, aminophylline, histamine, beta-estradiol, and biotin-nitrophenyl ester. Methylene blue, an inhibitor of guanylate cyclase, blocked the insulin response when added to cells before insulin, but was ineffective when added after insulin. In addition, agents that raise intracellular cGMP levels by inhibiting cyclic nucleotide phosphodiesterases were also examined for effects on glucose transport. Out of several phosphodiesterase inhibitors tested, only Zaprinast (which selectively increases cGMP in heart) stimulated transport in a concentration-dependent manner to within 80% of the maximal insulin effect. These results are consistent with the notion that cGMP may be involved in glucose transport stimulation.     

Surface cables of cardiac myocytes

Heart muscle cells prepared by mechanical disaggregation were seen by transmission electron microscopy (TEM) to possess an intact glycocalyx. Scanning electron microscopy (SEM) studies of the surface of these cells revealed longitudinally oriented cables, 10 to 12 nm thick. Each surface cable appeared to be less than one sarcomere long, extending longitudinally from the glycocalyx close to one M-line to the glycocalyx close to an adjacent M-line. Cables having similar orientation, distribution, thickness, and attachment positions were present in SEM preparations of mechanically disaggregated myocytes from hamster, rat, and dog. Unbanded tubular fibrils, similar to the surface cables in size and location, were demonstrable by TEM. Tubular fibrils indistinguishable from those in contact with the glycocalyx were seen in the interstitial space at a distance from the cell surface. Similar fibrils were also seen in association with interstitial elastin that occasionally remained attached to the isolated myocytes. It is postulated that the tubular filaments and the surface cables are the same structure, and may be an important determinant of the mechanical properties of myocardium.     

辛伐他汀对高浓度葡萄糖诱导心肌细胞炎症反应的干预作用及机制研究

目的 （1）观察高浓度葡萄糖对心肌细胞的作用,探讨高浓度葡萄糖损伤心肌的可能机制;（2）观察辛伐他汀对高浓度葡萄糖刺激下心肌细胞炎症因子表达的影响,并探讨其可能机制。方法 （1）取SD乳鼠心肌细胞做原代培养。（2）阴性对照组（blank）:培养基中不加任何刺激物,培养72小时。（3）阳性对照组（positive control）:培养基中加入终浓度为25 mmol/L的高浓度葡萄糖刺激乳鼠心肌细胞,培养72小时。（4）甲羟戊酸对照组（MVA）:培养基中加入终浓度为25 mmol/L的高浓度葡萄糖和终浓度为200 umol/L的甲羟戊酸,培养72小时。（5）辛伐他汀干预组（Sim）:培养基内加入终浓度为25 mmol/L的高浓度葡萄糖,同时分为三组并分别加入浓度为10^-5 mol/L、10^-6mol/L、10^-7mol/L辛伐他汀培养72小时。（6）辛伐他汀+甲羟戊酸干预组（Sim+MVA）:培养基中加入终浓度为25mmol/L的高浓度葡萄糖,终浓度为10-5 mol/L的辛伐他汀和终浓度为200 umol/L的甲羟戊酸,培养72小时。（7）收集各组心肌细胞培养基上清液,用ELISA法测各组TNF-α、ICAM-1、MCP-1的表达;收集各组心肌细胞,用MTT法测定心肌细胞活力,RT-PCR法测心肌细胞AT1RmRNA、AT2R mRNA表达。结果 （1）与空白对照组比较,阳性对照组、MVA组心肌细胞活力显著下降（P<0.01）,TNF-α、ICAM-1、MCP-1表达均明显增加（P<0.01）。（2）与阳性对照组比较,辛伐他汀组心肌细胞活力明显提高（P<0.01）,TNF-oα、ICAM-1、MCP-1表达明显降低（P<0.05或P<0.01）,且呈浓度依赖性;加入甲羟戊酸后（Sim+MVA组）,上述指标与阳性对照组比较无统计学差异。（3）与阳性对照组比较,辛伐他汀组[Sim(10^-5mol/L)组和Sim(10^-6mol/L)组]心肌细胞AT1R mRNA表达显著降低（P<0.05）,AT2R mRNA表达轻度增加（P<0.05）。（4）AT1RmRNA表达与TNF-α表达呈正相?     

T-tubule function in mammalian cardiac myocytes

The transverse tubules (t-tubules) of mammalian cardiac ventricular myocytes are invaginations of the surface membrane. Recent studies have suggested that the structure and function of the t-tubules are more complex than previously believed; in particular, many of the proteins involved in cellular Ca2+ cycling appear to be concentrated at the t-tubule. Thus, the t-tubules are an important determinant of cardiac cell function, especially as the main site of excitation-contraction coupling, ensuring spatially and temporally synchronous Ca2+ release throughout the cell. Changes in t-tubule structure and protein expression occur during development and in heart failure, so that changes in the t-tubules may contribute to the functional changes observed in these conditions. The purpose of this review is to provide an overview of recent studies of t-tubule structure and function in cardiac myocytes.     

Excitation-contraction coupling in voltage-clamped cardiac myocytes

Summary Myocytes isolated from guinea pig ventricles were voltage-clamped using patch pipettes in the whole-cell configuration. For proper voltage control fast Na⁺ current was blocked by TTX or inactivated by an appropriate prepulse. Zero-load cell shortening was monitored by a photoelectric device. The mechanical response to a short depolarizing clamp was mainly a phasic (transient) contraction. Long-lasting depolarizations caused a tonic (sustained) shortening of a cell. Different clamp patterns were used to study the mode of activation of phasic contraction. 1) With a constant Ca²⁺ preload established by a train of conditioning pulses, the shortening-voltage relation measured with test pulses of varying height was a bell-shaped curve reflecting the slow inward current (I_Ca)-voltage relation. The test pulse had a striking influence on the first contraction of the following conditioning series, resulting in an S-shaped relation between post-test contraction and test potential. 2) With series of identical clamps of varying height, steady-state contraction was maximal around 40 mV and not in proportion to I_Ca. In these measurements Ca²⁺ preload was likely to increase with increasing potential. It is concluded that I_Ca initiates phasic contraction by inducing a release of Ca²⁺ from internal stores while replenishment of the stores is largely determined by an electrogenic transsarcolemmal Na⁺–Ca²⁺ exchange. The data suggest that Na⁺–Ca²⁺ exchange is not only involved in long-term changes of cardiac contractility but also in beat-to-beat regulation.     

Regulation of glutathione in cardiac myocytes

Reduced glutathione (GSH) is an essential, multifunctional tripepetide that controls redox-sensitive cellular processes, but its regulation in the heart is poorly understood. The present study used a pharmocological model of GSH depletion to examine cellular mechanisms controlling cardiac GSH. Inhibition of GSH metabolism was elicited in normal rats by daily injections of buthionine sulfoximine (BSO), a blocker of gamma-glutamylcysteine synthetase, plus 1,3-bis-(2-chloroethyl)-1-nitrosourea (BCNU), an inhibitor of glutathione reductase. After 3 d of BSO/BCNU treatment, intracellular [GSH] was measured in isolated-ventricular myocytes by fluorescence microscopy using the probe monochlorobimane. Basal [GSH] in left-ventricular myocytes from BSO/BCNU-treated rats (2.0 +/- 0.05 amol/microm(3), n = 146) was 50% less than control (4.0 +/- 0.13 amol/microm(3), n = 116; P < 0.05). Incubation of myocytes from BSO/BCNU rats with 0.1 microM insulin normalized [GSH] after a delay of 3-4 h (3.6 +/- 0.29 amol/microm(3), n = 66). This effect of insulin was blocked by pre-treating myocytes with cycloheximide. A protein tyrosine phosphatase inhibitor, bis-peroxovanadium-1,10-phenanthroline (bpV(phen), 1 microM), elicited a similar effect as insulin, while neither agent altered [GSH] in myocytes from control rats. Moreover, the effect of insulin and bpV(phen) to up-regulate GSH was blocked by inhibitors of PI 3-kinase (wortmannin, LY294002), MEK (PD98059) and p38 MAP kinases (SB203580). These data suggest that the insulin-signaling cascade regulates [GSH] in ventricular myocytes by a coordinated activation of PI 3-kinase and MAP kinase pathways. These signaling mechanisms may play essential roles in controlling intracellular redox state and normal function of cardiac myocytes.