A modest glucokinase overexpression in the liver promotes fed expression levels of glycolytic and lipogenic enzyme genes in the fasted state without altering SREBP-1c expression

Hepatic genes crucial for carbohydrate and lipid homeostasis are regulated by insulin and glucose metabolism. However, the relative contributions of insulin and glucose to the regulation of metabolic gene expression are poorly defined in vivo. To address this issue, adenovirus-mediated hepatic overexpression of glucokinase was used to determine the effects of increased hepatic glucose metabolism on gene expression in fasted or ad libitum fed rats. In the fasted state, a 3 fold glucokinase overexpression was sufficient to mimic feeding-induced increases in pyruvate kinase and acetyl CoA carboxylase mRNA levels, demonstrating a primary role for glucose metabolism in the regulation of these genes in vivo. Conversely, glucokinase overexpression was unable to mimic feeding-induced alterations of fatty acid synthase, glucose-6-phosphate dehydrogenase, carnitine palmitoyl transferase I or PEPCK mRNAs, indicating insulin as the primary regulator of these genes. Interestingly, glucose-6-phosphatase mRNA was increased by glucokinase overexpression in both the fasted and fed states, providing evidence, under these conditions, for the dominance of glucose over insulin signaling for this gene in vivo. Importantly, glucokinase overexpression did not alter sterol regulatory element binding protein 1-c mRNA levels in vivo and glucose signaling did not alter the expression of this gene in primary hepatocytes. We conclude that a modest hepatic overexpression of glucokinase is sufficient to alter expression of metabolic genes without changing the expression of SREBP-1c.

Insulin stimulation of glutamine: fructose-6-phosphate amidotransferase occurs via an insulin-like growth factor-1 pathway in rat fibroblasts

The biosynthetic pathway for hexosamine mediates some of the adverse effects of high glucose. The rate limiting enzyme in this pathway is glutamine:fructose-6-phosphate amidotransferase (GFA). Using HPLC, the regulation of GFA activity by glucose and insulin was studied in wild type and rat-1 fibroblasts overexpressing human insulin receptors (HIRcB cells). In wild type cells only maximal doses of insulin (580 ng/ml) resulted in an increase in GFA activity (51.0 +/- 40.6%). In HIRcB cells insulin led to a dose dependent increase in GFA activity that was enhanced when compared to wild type (89 +/- 5% (p<0.001) increase at 580 ng/ml). Insulin's action was glucose dependent and required prolonged serum deprivation. HIRcB's cultured in 0 mM glucose had a 58.2% (p<0.001) decrease in insulin stimulation. However, when present the concentration of glucose (2-20 mM) did not affect insulin stimulation of GFA activity. Most of insulin's effects occur by way of the IGF-1 receptor as a two-fold stimulation of GFA activity was seen with significantly lower doses (10 ng/ml) of IGF-1. We conclude that GFA enzyme activity is upregulated by insulin and this may occur via a IGF-1 receptor mediated pathway.

Overexpression of glutamine: fructose-6-phosphate amidotransferase in the liver of transgenic mice results in enhanced glycogen storage, hyperlipidemia, obesity, and impaired glucose tolerance

To examine the effect of increased hexosamine flux in liver, the rate-limiting enzyme in hexosamine biosynthesis (glutamine:fructose-6-phosphate amidotransferase [GFA]) was overexpressed in transgenic mice using the PEPCK promoter. Liver from random-fed transgenic mice had 1.6-fold higher GFA activity compared with nontransgenic control littermates (276 +/- 24 pmol x mg(-1) x min(-1) in transgenic mice vs. 176 +/- 18 pmol x mg(-1) x min(-1) in controls, P < 0.05) and higher levels of the hexosamine end product UDP-N-acetyl glucosamine (288 +/- 11 pmol/g in transgenic mice vs. 233 +/- 10 pmol/g in controls, P < 0.001). Younger transgenic mice compared with control mice had lower fasting serum glucose (4.8 +/- 0.5 mmol/l in transgenic mice vs. 6.5 +/- 0.8 mmol/l in controls, P < 0.05) without higher insulin levels (48.0 +/- 7.8 pmol/l in transgenic mice vs. 56.4 +/- 5.4 pmol/l in controls, P = NS); insulin levels were significantly lower in transgenic males (P < 0.05). At 6 months of age, transgenic animals had normal insulin sensitivity by the hyperinsulinemic clamp technique. Hepatic glycogen content was higher in the transgenic mice (108.6 +/- 5.2 pmol/g in transgenic mice vs. 32.8 +/- 1.3 micromol/g in controls, P < 0.01), associated with an inappropriate activation of glycogen synthase. Serum levels of free fatty acids (FFAs) and triglycerides were also elevated (FFAs, 0.67 +/- 0.03 mmol/l in transgenic mice vs. 0.14 +/- 0.01 in controls; triglycerides, 1.34 +/- 0.15 mmol/l in transgenic mice vs. 0.38 +/- 0.01 in controls, P < 0.01). Older transgenic mice became heavier than control mice and exhibited relative glucose intolerance and insulin resistance. The glucose disposal rate at 8 months of age was 154 +/- 5 mg x kg(-1) x min(-1) in transgenic mice vs. 191 +/- 6 mg x kg(-1) x min(-1) in controls (P < 0.05). We conclude that hexosamines are mediators of glucose sensing for the regulation of hepatic glycogen and lipid metabolism. Increased hexosamine flux in the liver signals a shift toward fuel storage, resulting ultimately in obesity and insulin resistance.

NF-kappa B inhibits glucocorticoid and cAMP-mediated expression of the phosphoenolpyruvate carboxykinase gene

Transcription of the phosphoenolpyruvate carboxykinase (PEPCK) gene is regulated by a variety of agents. Glucocorticoids, retinoic acid, and glucagon (via its second messenger, cAMP) stimulate PEPCK gene transcription, whereas insulin, phorbol esters, cytokines, and oxidative stress have an opposing effect. Stimulation of PEPCK gene expression has been extensively studied, and a number of important DNA elements and binding proteins that regulate the transcription of this gene have been identified. However, the mechanisms utilized to turn off expression of this gene are not well-defined. Many of the negative regulators of PEPCK gene transcription also stimulate the nuclear localization and activation of the transcription factor NF-kappaB, so we hypothesized that this factor could be involved in the repression of PEPCK gene expression. We find that the p65 subunit of NF-kappaB represses the increase of PEPCK gene transcription mediated by glucocorticoids and cAMP in a concentration-dependent manner. The mutation of an NF-kappaB binding element identified in the PEPCK gene promoter fails to abrogate this repression. Further analysis suggests that p65 represses PEPCK gene transcription through a protein.protein interaction with the coactivator, CREB binding protein.

Effects of glucosamine infusion on insulin secretion and insulin action in humans

Glucose toxicity (i.e., glucose-induced reduction in insulin secretion and action) may be mediated by an increased flux through the hexosamine-phosphate pathway. Glucosamine (GlcN) is widely used to accelerate the hexosamine pathway flux, independently of glucose. We tested the hypothesis that GlcN can affect insulin secretion and/or action in humans. In 10 healthy subjects, we sequentially performed an intravenous glucose (plus [2-3H]glucose) tolerance test (IVGTT) and a euglycemic insulin clamp during either a saline infusion or a low (1.6 micromol x min(-1) x kg(-1)) or high (5 micromol x min(-1) x kg(-1) [n = 5]) GlcN infusion. Beta-cell secretion, insulin (SI*-IVGTT), and glucose (SG*) action on glucose utilization during the IVGTT were measured according to minimal models of insulin secretion and action. Infusion of GlcN did not affect readily releasable insulin levels, glucose-stimulated insulin secretion (GSIS), or the time constant of secretion, but it increased both the glucose threshold of GSIS (delta approximately 0.5-0.8 mmol/l, P < 0.03-0.01) and plasma fasting glucose levels (delta approximately 0.3-0.5 mmol/l, P < 0.05-0.02). GlcN did not change glucose utilization or intracellular metabolism (glucose oxidation and glucose storage were measured by indirect calorimetry) during the clamp. However, high levels of GlcN caused a decrease in SI*-IVGTT (delta approximately 30%, P < 0.02) and in SG* (delta approximately 40%, P < 0.05). Thus, in humans, acute GlcN infusion recapitulates some metabolic features of human diabetes. It remains to be determined whether acceleration of the hexosamine pathway can cause insulin resistance at euglycemia in humans.

Transcriptional regulation of transforming growth factor beta1 by glucose: investigation into the role of the hexosamine biosynthesis pathway

BACKGROUND

The hexosamine biosynthesis pathway (HBP) is hypothesized to mediate many of the adverse effects of hyperglycemia. We have shown previously that increased flux through this pathway leads to induction of the growth factor transforming growth factor-alpha (TGF-alpha) and to insulin resistance in cultured cells and transgenic mice. TGF-beta is regulated by glucose and is involved in the development of diabetic nephropathy. We therefore hypothesized that the HBP was involved in the regulation of TGF-beta by glucose in rat vascular and kidney cells.

METHODS

A plasmid containing the promoter region of TGF-beta1 cloned upstream of the firefly luciferase gene was electroporated into rat aortic smooth muscle, mesangial, and proximal tubule cells. Luciferase activity was measured in cellular extracts from cells cultured in varying concentrations of glucose and glucosamine.

RESULTS

Glucose treatment of all cultured cells led to a time- and dose-dependent stimulation in TGF-beta1 transcriptional activity, with high (20 mM) glucose causing a 1.4- to 2.0-fold increase. Glucose stimulation did not occur until after 12 hours and disappeared after 72 hours of treatment. Glucosamine was more potent than glucose, with 3 mM stimulating up to a 4-fold increase in TGFbeta1-transcriptional activity. The stimulatory effect of glucosamine was also dose-dependent but was slower to develop and longer lasting than that of glucose.

CONCLUSIONS

The metabolism of glucose through the HBP mediates extracellular matrix production, possibly via the stimulation of TGF-beta in kidney cells. Hexosamine metabolism therefore, may play a role in the development of diabetic nephropathy.

Regulation of glutamine:fructose-6-phosphate amidotransferase by cAMP-dependent protein kinase

Glutamine:fructose-6-phosphate amidotransferase (GFA) is the rate-limiting enzyme in hexosamine biosynthesis, an important pathway for cellular glucose sensing. Human GFA has two potential sites for phosphorylation by cAMP-dependent protein kinase A (PKA). To test whether GFA activity is regulated by cAMP-dependent phosphorylation, rat aortic smooth muscle cells were treated in vivo with cAMP-elevating agents, 10 micromol/l forskolin, 1 mmol/l 8-Br-cAMP, or 3-isobutyl-1-methylxanthine. All treatments resulted in rapid and significant increases (2- to 2.4-fold) in GFA activity assayed in cytosolic extracts. Maximal effects of forskolin were observed at 10 micromol/l and 60 min. Preincubation of cells with cycloheximide did not abolish the effect of forskolin. Incubation of cytosolic extracts at 37 degrees C for 10 min in a buffer without phosphatase inhibitors led to a 79% decrease of GFA activity. This loss of activity was inhibited by the addition of phosphatase inhibitors (5 mmol/l sodium orthovanadate, 50 mmol/l sodium fluoride, or 5 mmol/l EDTA, but not 100 nmol/l okadaic acid), suggesting that GFA undergoes rapid dephosphorylation by endogenous phosphatases. Purified GFA is phosphorylated in vitro by purified PKA, resulting in a 1.7-fold increase in GFA activity. Treatment of GFA with purified protein kinase C had no effect. We conclude that GFA activity may be modulated by cAMP-dependent phosphorylation.

Insulin and glucosamine infusions increase O-linked N-acetyl-glucosamine in skeletal muscle proteins in vivo

O-linked N-acetylglucosamine (O-GlcNAc) is an abundant posttranslational modification of serine/threonine residues of nuclear and cytoplasmic proteins. We determined whether insulin or coinfusion of glucosamine (GlcN) with insulin alters O-GlcNAc of skeletal muscle proteins. Three groups of conscious fasted rats received 6-hour infusions of either saline (BAS), insulin 18 mU/kg.min and saline (INS), or insulin and GlcN 30 micromol/kg.min (GLCN) during maintenance of normoglycemia. At 6 hours, the concentrations of muscle UDP-GlcNAc, UDP-N-acetylgalactosamine (UDP-GalNAc), UDP-glucose (UDP-Glc), UDP-galactose (UDP-Gal), glycogen, and N and O-linked GlcNAc (galactosyltransferase labeling followed by beta elimination) were measured in freeze-clamped abdominis muscle. Insulin increased whole-body glucose uptake from 49 +/- 5 to 239 +/- 8 micromol/kg.min (P < .001) and glycogen in abdominis muscle from 138 +/- 11 to 370 +/- 26 mmol/kg dry weight (P < .001). Insulin increased the amount of cytosolic N - and O-linked GlcNAc by 56% from 362 +/- 30 to 564 +/- 45 dpm/microg protein . 100 min (P < .02), and O-GlcNAc from 221 +/- 16 to 339 +/- 27 dpm/microg . 100 min (P < .02). Glycogen content was positively correlated with the amount of total (r = .90, P < .005) and O-linked GlcNAc in insulin-infused animals. Coinfusion of GlcN with insulin increased muscle UDP-GlcNAc about fourfold (100 +/- 6 nmol/g) compared with insulin (27 +/- 1, P < .001) or saline (25 +/- 1, P < .001) infusion. GlcN also decreased glucose uptake over 6 hours by 30% to 168 +/- 8 micromol/kg . min (P < .001 for GLCN v INS) and muscle glycogen to 292 +/- 24 mmol/kg dry weight (P < .05 for GLCN v INS). Both total (635 +/- 60 dpm/microg . 100 min, P < .002) and O-linked GlcNAc (375 +/- 36 dpm/microg . 100 min, P < .002) in the cytosol were significantly higher in GLCN rats (635 +/- 60 dpm/microg) versus BAS rats (P < .002). As in INS rats, muscle glycogen and O-GlcNAc were positively correlated in GLCN rats (r = .54, P < .05). Variation in total and O-linked GlcNAc in GLCN rats was due both to GlcN (P < .02) and to variation in the glycogen content (P < .005).

Activation of the hexosamine pathway by glucosamine in vivo induces insulin resistance in multiple insulin sensitive tissues

We determined the effect of infusion of glucosamine (GlcN), which bypasses the rate limiting reaction in the hexosamine pathway, on insulin-stimulated rates of glucose uptake and glycogen synthesis in vivo in rat tissues varying with respect to their glutamine:fructose-6-phosphate amidotransferase (GFA) activity. Three groups of conscious fasted rats received 6-h infusions of either saline (BAS), insulin (18 mU/kg x min) and saline (INS), or insulin and GlcN (30 micromol/ kg x min, GLCN). [3-(3)H]glucose was infused to trace whole body glucose kinetics and glycogen synthesis, and rates of tissue glucose uptake were determined using a bolus injection of [1-(14)C]2-deoxyglucose at 315 min. GlcN decreased insulin-stimulated glucose uptake (315-360 min) by 49% (P < 0.001) at the level of the whole body, and by 31-53% (P < 0.05 or less) in the heart, epididymal fat, submandibular gland and in soleus, abdominis and gastrocnemius muscles. GlcN completely abolished glycogen synthesis in the liver. GlcN decreased insulin-stimulated glucose uptake similarly in the submandibular gland (1.3 +/- 0.2 vs. 2.0 +/- 0.3 nmol/mg protein x min, GLCN vs. INS, P < 0.05) and gastrocnemius muscle (1.4 +/- 0.3 vs. 3.1 +/- 0.5 nmol/mg protein x min), although the activity of the hexosamine pathway, as judged from basal GFA activity, was 10-fold higher in the submandibular gland (286 +/- 35 pmol/mg protein x min) than in gastrocnemius muscle (27 +/- 3 pmol/mg protein x min, P < 0.001). These data raise the possibility that overactivity of the hexosamine pathway may contribute to glucose toxicity not only in skeletal muscle but also in other insulin sensitive tissues. They also imply that the magnitude of insulin resistance induced between tissues is determined by factors other than GFA.

UDP-N-acetylglucosamine transferase and glutamine: fructose 6-phosphate amidotransferase activities in insulin-sensitive tissues

Glutamine:fructose 6-phosphate amidotransferase (GFA) is rate-limiting for hexosamine biosynthesis, while a UDP-GlcNAc beta-N-acetylglucosaminyltransferase (O-GlcNAc transferase) catalyses final O-linked attachment of GlcNAc to serine and threonine residues on intracellular proteins. Increased activity of the hexosamine pathway is a putative mediator of glucose-induced insulin resistance but the mechanisms are unclear. We determined whether O-GlcNAc transferase is found in insulin-sensitive tissues and compared its activity to that of GFA in rat tissues. We also determined whether non-insulin-dependent diabetes mellitus (NIDDM) or acute hyperinsulinaemia alters O-GlcNAc transferase activity in human skeletal muscle. O-GlcNAc transferase was measured using 3H-UDP-GlcNAc and a synthetic cationic peptide substrate containing serine and threonine residues, and GFA was determined by measuring a fluorescent derivative of GlcN6P by HPLC. O-GlcNAc transferase activities were 2-4 fold higher in skeletal muscles and the heart than in the liver, which had the lowest activity, while GFA activity was 14-36-fold higher in submandibular gland and 5-18 fold higher in the liver than in skeletal muscles or the heart. In patients with NIDDM (n = 11), basal O-GlcNAc transferase in skeletal muscle averaged 3.8 +/- 0.3 nmol/mg.min, which was not different from that in normal subjects (3.3 +/- 0.4 nmol/mg.min). A 180-min intravenous insulin infusion (40 mU/m2.min) did not change muscle O-GlcNAc transferase activity in either group. We conclude that O-GlcNAc transferase is widely distributed in insulin-sensitive tissues in the rat and is also found in human skeletal muscle. These findings suggest the possibility that O-linked glycosylation of intracellular proteins is involved in mediating glucose toxicity. O-GlcNAc transferase does not, however, appear to be regulated by either NIDDM or acute hyperinsulinaemia, suggesting that mass action effects determine the extent of O-linked glycosylation under hyperglycaemic conditions.

Overexpression of glutamine:fructose-6-phosphate amidotransferase in transgenic mice leads to insulin resistance

The hexosamine biosynthetic pathway has been hypothesized to be involved in mediating some of the toxic effects of hyperglycemia. Glutamine:fructose-6-phosphate amidotransferase (GFA), the first and rate limiting enzyme of the hexosamine biosynthetic pathway, was overexpressed in skeletal muscle and adipose tissue of transgenic mice. A 2.4-fold increase of GFA activity in muscle of the transgenic mice led to weight-dependent hyperinsulinemia in random-fed mice. The hyperinsulinemic-euglycemic clamp technique confirmed that transgenic mice develop insulin resistance, with a glucose disposal rate of 68.5 +/- 3.5 compared with 129.4 +/- 9.4 mg/kg per min (P < 0.001) for littermate controls. The decrease in the glucose disposal rate of the transgenic mice is accompanied by decreased protein but not mRNA levels of the insulin-stimulated glucose transporter (GLUT4). These data support the hypothesis that excessive flux through the hexosamine biosynthesis pathway mediates adverse regulatory and metabolic effects of hyperglycemia, specifically insulin resistance of glucose disposal. These mice can serve as a model system to study the mechanism for the regulation of glucose homeostasis by hexosamines.

Increased glutamine:fructose-6-phosphate amidotransferase activity in skeletal muscle of patients with NIDDM

Overactivity of the hexosamine pathway mediates glucose-induced insulin resistance in rat adipocytes. Glutamine:fructose-6-phosphate amidotransferase (GFA) is the rate-limiting enzyme of this pathway. We determined GFA activity in human skeletal muscle biopsies and rates of insulin-stimulated whole-body, oxidative, and nonoxidative glucose disposal using the euglycemic insulin clamp technique combined with indirect calorimetry (insulin infusion rate (1.5 mU x kg-1 x min-1)) in 12 male patients with NIDDM (age 54 +/- 2 years, BMI 27.5 +/- 0.9 kg/m2, fasting plasma glucose 8.5 +/- 0.6 mmol/l) and 9 matched normal men. GFA activity was detectable in human skeletal muscles and completely inhibited by uridine-5'-diphospho-N-acetylglucosamine (UDP-GlcNAc) in all subjects. GFA activity was 46% increased in the NIDDM patients compared with the normal subjects (9.5 +/- 1.3 vs. 6.5 +/- 1.2 pmol, P < 0.05). Whole-body glucose uptake was 58% decreased in patients with NIDDM (20 +/- 3 micromol x kg body wt-1 x min-1) compared with normal subjects (47 +/- 4 micromol x kg body wt-1 x min-1, P < 0.001). This decrease was attributable to decreases in both glucose oxidation (9 +/- 1 vs. 15 +/- 1 micromol x kg-1 x min-1, NIDDM patients vs. control subjects, P < 0.002) and nonoxidative glucose disposal (11 +/- 2 vs. 31 +/- 4 micromol x kg-1 x min-1, P < 0.001). In patients with NIDDM, both HbA1c (r= 0.51, P < 0.05) and BMI (r= -0.57, P < 0.05) correlated with whole-body glucose uptake. HbA1c but not BMI or insulin sensitivity was correlated with basal GFA activity (r = -0.57,P < 0.01) in NIDDM patients and control subjects. We conclude that GFA is found in human skeletal muscle and that all this activity is sensitive to feedback inhibition by UDP-GlcNAc. Chronic hyperglycemia is associated with an increase in skeletal muscle GFA activity, suggesting that increased activity of the hexosamine pathway may contribute to glucose toxicity and insulin resistance in humans.

Glutamine:fructose-6-phosphate amidotransferase activity in cultured human skeletal muscle cells: relationship to glucose disposal rate in control and non-insulin-dependent diabetes mellitus subjects and regulation by glucose and insulin

We examined the activity of the rate-limiting enzyme for hexosamine biosynthesis, glutamine:fructose-6-phosphate amidotransferase (GFA) in human skeletal muscle cultures (HSMC), from 17 nondiabetic control and 13 subjects with non-insulin-dependent diabetes. GFA activity was assayed from HSMC treated with low (5 mM) or high (20 mM) glucose and low (22 pM) or high (30 microM) concentrations of insulin. In control subjects GFA activity decreased with increasing glucose disposal rate (r = -0.68, P < 0.025). In contrast, a positive correlation existed between GFA and glucose disposal in the diabetics (r = 0.86, P < 0.005). Increased GFA activity was also correlated with body mass index in controls but not diabetics. GFA activity was significantly stimulated by high glucose (22%), high insulin (43%), and their combination (61%). GFA activity and its regulation by glucose and insulin were not significantly different in diabetic HSMC. We conclude that glucose and insulin regulate GFA activity in skeletal muscle. More importantly, our results are consistent with a regulatory role for the hexosamine pathway in human glucose homeostasis. This relationship between hexosamine biosynthesis and the regulation of glucose metabolism is altered in non-insulin-dependent diabetes.

Triggered propagated contractions in human atrial trabeculae

OBJECTIVE

Mechanical stretch and rapid release of locally damaged regions of rat cardiac trabeculae due to contraction of undamaged cells during the regular twitch have been shown to trigger local aftercontractions in the damaged region. These aftercontractions appear to propagate along the length of the trabeculae and to precede triggered arrhythmias. The aim of this study was to determine whether triggered propagated contractions can also be elicited in human cardiac tissue.

METHODS

Trabeculae were dissected from a biopsy of the right atrial appendage which was obtained during cardiac surgery. They were mounted horizontally, superfused with a modified Krebs-Henseleit solution, and monitored with a video system. Force was measured with a silicon strain gauge; sarcomere length was measured with laser diffraction techniques. Triggered contractions were elicited in 10 trabeculae following trains of 15 stimuli at a rate of 2 Hz, separated by 15 s rest intervals, at 21 degrees C and a [Ca2+]o above 5.0 mM.

RESULTS

Both video analysis and sarcomere length recordings showed that the contractions started at one end of the trabeculae and travelled from there as a localised contraction along the muscle. Increasing [Ca2+]o or the number of conditioning stimuli increased force of the stimulated twitches up to a maximum; further increase of [Ca2+]o or the number of stimuli was accompanied by a decrease of force. Force of the triggered propagated contractions increased monotonically with increasing [Ca2+]o and number of stimuli, while the interval between last stimulus and the peak of the force transient (or latency) decreased progressively, and propagation velocity increased monotonically. Propagation velocity was calculated from the interval between peak sarcomere shortening at two sites and the distance between these sites, as well as from the ratio between the length of the muscle and the duration of the force transient in the remainder of the trabeculae. With increasing temperature at constant [Ca2+]o twitch force increased while latency, force, and duration of the force transient decreased. At 37 degrees C, localised contractions travelling along the muscle could still be induced at sufficiently increased [Ca2+]o. For all interventions, propagation velocity varied from 0.8 to 3.9 mm.s-1.

CONCLUSIONS

Contractions can be elicited in human atrial cardiac trabeculae. These contractions have the same basic characteristics as the triggered propagated contractions that have been described previously in rat ventricular trabeculae.

Regulation of insulin-stimulated glycogen synthase activity by overexpression of glutamine: fructose-6-phosphate amidotransferase in rat-1 fibroblasts

High glucose concentrations such as are seen in diabetes mellitus are known to have deleterious effects on cells, but the pathways by which glucose induces these effects are unknown. One hypothesis is that metabolism of glucose to glucosamine might be involved. For example, it has been shown that glucosamine is more potent than glucose in inducing insulin resistance in cultured adipocytes and in regulating the transcription of the growth factor transforming growth factor alpha in smooth muscle cells. The rate-limiting step in glucosamine synthesis is the conversion of fructose-6-phosphate to glucosamine-6-phosphate by the enzyme glutamine:fructose-6-phosphate amidotransferase. To test the hypothesis that this hexosamine biosynthesis pathway is involved in the induction of insulin resistance, we have overexpressed the enzyme glutamine:fructose-6-phosphate amidotransferase in Rat-1 fibroblasts and investigated its effects on insulin action in those cells. We electroporated Rat-1 fibroblasts with expression plasmids that did and did not contain the gene for glutamine:fructose-6-phosphate amidotransferase and measured glycogen synthase activity at varying insulin concentrations. Insulin stimulation was blunted in the glutamine:fructose-6-phosphate amidotransferase-transfected cells, resulting in decreased insulin sensitivity reflected by a rightward shift in the dose-response curve for activation of synthase (ED50 = 7.5 nM vs. 3.4 nM insulin, in glutamine:fructose-6-phosphate amidotransferase and control cells, respectively). Rat-1 fibroblasts incubated with 5.- mM glucosamine for 3 days exhibited a similar shift in the dose-response curve. The rightward shift in the dose-response curve is seen as early as 2 days after poration.(ABSTRACT TRUNCATED AT 250 WORDS)

Glucose regulation of transforming growth factor-alpha expression is mediated by products of the hexosamine biosynthesis pathway

We have recently shown that glucose and glucosamine regulate the transcription of transforming growth factor-alpha (TGF alpha) in rat aortic smooth muscle (RASM) cells. Based on the increased potency of glucosamine compared to glucose, we hypothesized that stimulation of TGF alpha transcription by glucose is mediated through the hexosamine biosynthesis pathway. The yeast cDNA for the rate-limiting enzyme of this pathway, glutamine:fructose-6-phosphate amidotransferase (GFA), was therefore expressed in RASM cells. GFA-transfected cells showed an increase in GFA activity, exhibiting a 2.2-fold increase in the synthesis of glucosamine-6-phosphate, the first product of the hexosamine biosynthetic pathway. To test the effect of GFA overexpression on TGF alpha transcriptional activity, cells were transiently cotransfected with GFA along with a reporter plasmid containing the firefly luciferase gene under control of the TGF alpha promoter. GFA-transfected cells exhibited a glucose-dependent 2-fold increase in TGF alpha activity compared to control cells. Maximal stimulation of TGF alpha-luciferase activity by glucosamine, however, was equivalent in GFA-and control-transfected cells, confirming that the stimulation observed by both agents operated through the same pathway. This increase in TGF alpha activity was inhibited (85% at 0.5 mM glucose and 69% at 30 mM glucose) by the glutamine analog and inhibitor of GFA, 6-diazo-5-oxonorleucine (10 microM). Control studies confirmed that the increased TGF alpha-luciferase activity in the GFA-expressing cells was not an artifact of altered growth, survival, or transfection efficiency.(ABSTRACT TRUNCATED AT 250 WORDS)

Suppressive effects of R 56865 on triggered propagated contractions and triggered arrhythmias in rat cardiac trabeculae

Triggered arrhythmias in rat right ventricular trabeculae are induced by triggered propagated contractions (TPCs) that start in damaged regions of the muscle and propagate along the preparation. We analyzed the effects of the Na+/Ca2+ overload inhibiting agents R 56865 on both TPCs and triggered arrhythmias. This compound has been shown to prevent ultrastructural signs of intracellular calcium overload and arrhythmias caused by exposure to toxic concentrations of cardiac glycosides. TPCs were induced by trains of 15 stimuli (2 Hz, 15 s intervals) at 19-21 degrees C and a [Ca2+]o of 1.0-2.5 mM in the superfusate. Force was measured with a silicon strain gauge; length and shortening of sarcomeres were measured at two sites of the muscle using laser diffraction techniques. Exposure to 1.14 x 10(-7) M R 56865 for 30 min decreased the force of the last stimulated twitch (twitch force) to 89.7 +/- 4.7% (mean +/- SEM) of control, the force produced by TPCs to 39.4 +/- 9.8%, and the velocity of propagation of TPCs to 52.8 +/- 6.3%, while TPC latency increased not significantly to 104.7 +/- 2.8% of control. R 56865 suppressed TPC force, for the same small decrease in twitch force (10%), significantly more than 100 nM D-600 did (29.5 +/- 2.0 vs. 12.4 +/- 3.1%). Eventually, TPCs disappeared in 8 of 14 muscles, in 2 of them without any decrease in twitch force. At 5.7 x 10(-7) M, R 56865 abolished TPCs in five additional trabeculae. An increase in [Ca2+]o reintroduced TPCs. During stimulation of 0.5 Hz, 1.14 x 10(-7) M R 56865 increased the stimulus threshold by 21 +/- 4% in 6 of 14 muscles and decreased the twitch force by 26 +/- 3% in 7 of 14 trabeculae. Triggered arrhythmias were induced in six muscles with the use of 0.5 mM caffeine or 5 nM Bay K 8644; R 56865 rapidly terminated these arrhythmias in all muscles. Although the mechanism of the antiarrhythmic effects of R 56865 remains to be determined, we speculate that the drug raises the threshold for both generation of triggered action potentials and calcium-induced calcium release from the sarcoplasmic reticulum.

Role of the sarcolemma in triggered propagated contractions in rat cardiac trabeculae

We have recently described that after contractions propagate through multicellular cardiac muscle preparations. These propagating contractions are triggered in damaged regions of rat right ventricular trabeculae during relaxation of electrically stimulated twitches. Propagation of triggered contractions has been attributed to calcium ions that diffuse along the preparation, causing calcium-induced calcium release from the sarcoplasmic reticulum in adjacent cells. In the present study we have investigated a possible role of the sarcolemma and delayed afterdepolarizations (DADs) in the initiation and propagation of triggered propagated contractions (TPCs) in multicellular preparations. We studied whether 1) TPCs are accompanied by delayed sarcolemmal depolarizations, 2) such depolarizations mediate local contraction, and 3) an intact sarcolemma is required for propagation of contractions. TPCs that remained stable for prolonged periods of time could be induced by trains of 15 stimuli (2 Hz, 15-second intervals) at lowered temperature (19-21 degrees C) of the superfusing Krebs-Henseleit medium and a [Ca(2+)]o of 1.0-1.5 mM. Although TPCs could be induced at 38 degrees C and a [Ca2+]o of 3.0-4.0 mM, they disappeared within 10 minutes. Force was measured with a silicon strain gauge; length and shortening of sarcomeres were measured at two sites of the muscle using laser diffraction techniques. Membrane potential was measured with flexible microelectrodes. Saponin was used to selectively render the sarcolemma permeable to small ions and molecules. Propagation velocity of TPCs in intact trabeculae varied from 1.7 to 13.4 mm/sec at 19-21 degrees C. TPCs were accompanied by DADs that could reach threshold and induce triggered arrhythmias. Changes in latency, duration, and force of TPCs, induced by changing [Ca(2+)]o or the number of conditioning stimuli, were closely matched by changes in latency, duration, and amplitude of DADs; DADs consistently preceded TPCs, on average by 60 msec. Local heating of the muscle, by applying a current through an insulated platinum wire (diameter 100 microns) that touched the muscle, interrupted propagation of TPCs reversibly. DADs were, in the absence of a local contraction, still recorded distal to the heated site. In muscles that were treated with saponin and exposed to solutions approximating the intracellular milieu, spontaneously occurring local contractions that propagated in both directions (at velocities of 70-200 microns/sec) were elicited at a bathing calcium concentration of approximately 0.6 microM. Below this threshold, propagated contractions could be triggered by pressure ejection of a calcium-containing solution from a microelectrode positioned close to the trabecula.(ABSTRACT TRUNCATED AT 400 WORDS)

Spontaneous contractions in rat cardiac trabeculae. Trigger mechanism and propagation velocity

It has previously been observed that spontaneous contractions start in a region of damage of isolated right ventricular trabeculae of rat, propagate along the muscle, and induce triggered arrhythmias (Mulder, B.J.M., P.P. de Tombe, and H.E.D.J. ter Keurs. 1989. J. Gen. Physiol. 93:943-961). The present study was designed to analyze the mechanisms that lead to triggered propagated contractions (TPCs). TPCs were elicited in 29 trabeculae by stimulation with trains (2 Hz; 15-s intervals) at varied number of stimuli (n), lowered temperature (19-21 degrees C), and varied [Ca++]o (1.5-4 mM) in the superfusate. Length (SL) and shortening of sarcomeres in the muscle were measured at two sites using laser diffraction techniques; twitch force (Ft) was measured with a silicon strain gauge. Time between the last stimulus in the train and the onset of sarcomere shortening due to a TPC at a site close to the damaged end region (latency) and propagation velocity of the contraction (Vprop) were correlated with Ft. For 10 trabeculae, TPCs were calculated to start in the end region itself 586 +/- 28 ms (mean +/- 1 SEM) after the last stimulus of a train (n = 15; [Ca++]o: 1.5 mM), i.e., at the end of or after the rapid release of the damaged end during twitch relaxation. When Ft was increased by increasing either SL prior to stimulation or the afterload during twitches, methods that do not affect intracellular calcium levels, latency decreased, but Vprop remained constant. No TPC occurred when Ft was less than 20% of maximal Ft. Both increasing [Ca++]o and n increased Ft to a maximum, increased Vprop progressively (maximum Vprop, 17 mm/s), but decreased latency. These observations suggest that initiation of TPCs depends on the force developed by the preceding twitch, and therefore on the degree of stretch and subsequent rapid release of damaged areas in the myocardium, while Vprop along the trabeculae is determined by intracellular calcium concentration.