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G J Cooney

Publications and source records attributed to G J Cooney.

18 recordsLinked to original sources
PubMed1992

The effects of fasting and refeeding on liver glycogen synthase and phosphorylase in obese and lean mice.

The responses of hepatic glycogen synthase and phosphorylase to fasting and refeeding were assessed as part of an investigation into possible sites of insulin resistance in gold thioglucose (GTG) obese mice. The active forms glycogen synthase and phosphorylase (synthase I and phosphorylase a) and the total activity of these enzymes were estimated in lean and GTG mice over 48 h of food deprivation, and for 120 min after glucose gavage (1 g/kg wt). In lean mice there was a maximal reduction in hepatic glycogen content after 12 h of starvation and the activity of phosphorylase a decreased from 23.8 +/- 1.9 to 6.8 +/- 0.7 mumol/g protein/min. These changes were accompanied by an increase in the activity of synthase I (from 0.14 +/- 0.01 to 0.46 +/- 0.04 mumol/g protein/min). In obese mice, similar changes in enzyme activity occurred after 48 h of starvation. These changes were accompanied by a significant reduction in the hyperinsulinemia and hyperglycemia of the GTG mice. After glucose gavage in both lean and obese mice, the activity of synthase I further increased over the first 30 min and declined thereafter. The activity of phosphorylase a increased progressively after refeeding. Results from this study suggest that despite increased hepatic glycogen deposition, the responses of glycogen synthase and phosphorylase, in livers of obese mice, to fasting and refeeding are similar to those of control mice even in the presence of insulin resistance.

Animals↗
PubMed1992

Increased gluconeogenesis in hepatocytes from GTG-obese mice is insensitive to inhibition by insulin.

The effect of a supraphysiological concentration of insulin on gluconeogenesis from L-[U14C] lactate was studied in hepatocytes isolated from control mice and mice made obese by a single injection of gold-thioglucose (GTG). At the time of experimentation (10-12 weeks post GTG injection) the obese mice weighted significantly more than controls (41.7 +/- 0.5 vs. 29.6 +/- 0.8 g respectively; P < 0.001), and exhibited fasting hyperinsulinaemia (35.9 +/- 4.6 vs. 21.3 +/- 4.2 microU/ml; P < 0.05) and hyperglycaemia (16.4 +/- 1.2 vs. 9.2 +/- 1.1 mmol/l; P < 0.001). The amount of lactate converted to glucose by hepatocytes isolated from GTG-obese mice was significantly greater than from lean controls (322 +/- 44 vs. 209 +/- 20 nmol/30 min/10(6) cells; P < 0.05). The addition of 10(-6)M insulin to the incubations significantly reduced lactate conversion to glucose by hepatocytes isolated from control mice (209 +/- 20 vs. 123 +/- 22 nmol/30 min/10(6) cells; P < 0.02), but there was no effect of insulin on glucose production from lactate by hepatocytes isolated from GTG-obese mice (322 +/- 44 vs. 294 +/- 47 nmol/30 min/10(6) cells). Glycogen production and triacylglycerol glycerol production from L-[U14C] lactate were also significantly increased in hepatocytes from GTG-obese mice compared with controls. There was no effect of 10(-6)M insulin on glycogen or triacylglycerol glycerol production from lactate by hepatocytes from GTG-obese mice but the addition of 10(-6)M insulin to the incubations of control hepatocytes significantly reduced the amount of lactate converted to glycogen and triacylglycerol glycerol.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗
PubMed1992

Diurnal rhythms of glycogen metabolism in the liver and skeletal muscle in gold thioglucose induced-obese mice with developing insulin resistance.

The circadian rhythm of glycogen metabolism in liver and skeletal muscle was studied in lean and gold thioglucose (GTG) induced-obese mice. The active forms of glycogen synthase (GSI) and phosphorylase (GPa) and the total activity of these enzymes were measured every three hours over a 24 h period in mice fed ad libitum. Hepatic and muscle glycogen content displayed a marked diurnal rhythm that was similar in lean and obese mice. In skeletal muscle the glycogen content, GSI and GPa were not significantly different in lean and obese animals over the 24 h period. The activities of muscle GSI and GPa were constant in both groups despite the diurnal variation in the muscle glycogen content. The absence of an increase in the glycogen content of skeletal muscle despite the pronounced hyperinsulinemia and hyperglycemia in the obese mice, may indicate the degree of insulin resistance in this tissue or the maximal capacity of muscle tissue to store glycogen. In liver, glycogen concentration and total glycogen storage were higher in obese mice. Unlike muscle, both hepatic GSI and GPa underwent significant changes in activity over the 24 h period. Hepatic GSI was lower and GPa was higher in obese mice. The circadian rhythm in enzyme activities was independent of both blood glucose and insulin levels. The total glycogen storage and the activities of total phosphorylase and GPa were significantly increased in the liver from GTG obese mice over a 24 h period and could be implicated in the development of insulin resistance and glucose intolerance in this model of obesity.

Animals↗
PubMed1991

Differences in lipogenesis in tissues of control and gold-thioglucose obese mice after an isocaloric meal.

Lipogenesis was measured in 2 and 5 week gold-thioglucose (GTG) obese mice after a single meal of 0.5 g of standard chow. Compared to control mice the rate of lipogenesis in GTG obese mice, was 4-fold higher in liver and 10-fold higher in white adipose tissue (WAT). In brown adipose tissue (BAT) of GTG-injected mice the lipogenic rate was only 50% of that of controls. These results indicate that the increased lipid synthesis observed in GTG-injected mice is not due solely to hyperphagia and that some other stimuli, such as increased basal insulin levels and/or decreased thermogenesis and insulin resistance in BAT, contribute to the high rates of fat synthesis in this animal model of obesity.

Adipose Tissue↗
PubMed1991

Heterogeneity of response to exercise of rat muscle pyruvate dehydrogenase complex.

Muscle glucose uptake is greatly stimulated by moderate exercise, but full oxidation of the glucose to CO2 depends on the activity of the pyruvate dehydrogenase (PDH) complex. Our aim was to determine how PDH complex in different muscle groups responds to varying periods of moderate exercise. Rats were run on a motor-driven treadmill for 5-30 min and muscle PDH complex activity was determined in heart, diaphragm and red quadriceps muscles after isolation of mitochondria in the presence of inhibitors of PDH complex interconversion. In heart and diaphragm muscle, exercise caused an increase in PDH complex activity after 5 min, but this was followed by a significant decrease in activity as exercise progressed. In red quadriceps muscle, PDH complex activity was reduced after 5 min of exercise and was decreased further as exercise continued. We conclude that increased duration of exercise can lead to reduced PDH complex activity in rat muscles. We propose that this is a consequence of elevated fatty acid oxidation, the products of which stimulate PDH kinase. This implies that increased glycolysis to lactate and increased fatty acid oxidation can simultaneously provide energy for contracting muscle.

Animals↗
PubMed1991

Cation-induced restoration of insulin action in insulin-desensitized HTC cells.

Insulin desensitization of amino acid uptake in HTC cells was induced by preincubation with 4 or 10 micrograms/ml insulin. Insulin binding after desensitisation was decreased by both insulin concentrations due to a 45-49% decrease in insulin receptor numbers. Desensitization with 4 micrograms/ml insulin increased the ED50 for half-maximal stimulation of amino acid uptake from 19.5 +/- 9.2 ng/ml in control cells to 84.0 +/- 8.3 ng/ml (P less than 0.0001). It also decreased the maximal insulin response of amino acid uptake from 1.40 +/- 0.10 to 1.14 +/- 0.10 nmol/mg protein, indicating the production of a mild postreceptor defect. Desensitization with 10 micrograms/ml insulin completely abolished this insulin response. When cellular receptors were down-regulated with 4 micrograms/ml insulin and restimulated with insulin in the presence of 0.03 mM ruthenium red (RR) or 10 mM Ca2+, both the insulin response and insulin binding were increased. Insulin binding was restored to levels comparable to those observed in control cells by an increase in receptor affinity. The ED50 of amino acid uptake decreased to 20.5 +/- 7.3 ng/ml insulin in the presence of RR and to 42.2 +/- 8.9 ng/ml in the presence of 10 mM Ca2+ (both P less than 0.0001 from down-regulated cells). In addition, the maximal insulin response increased from 1.14 +/- 0.10 to 1.40 +/- 0.10 and 1.45 +/- 0.10 nmol/mg protein, respectively. Preincubation with 10 micrograms/ml insulin prevented the effect of RR and Ca2+ on the recovery of insulin responses. These experiments suggest that insulin-desensitized cells undergo a progressive loss of their insulin response and that RR and Ca2+ provide useful reagents to investigate the mechanisms of this process because they can counteract the decrease in insulin response by increasing receptor affinity and receptor-effector coupling.

Amino Acids↗
PubMed1991

Insulin-like growth factor-binding protein-1 modulates blood glucose levels.

We have determined the consequences of insulin-like growth factor-binding protein-1 (IGFBP-1) administration alone and in combination with insulin-like growth factor-I (IGF-I). Human recombinant IGF-I, infused as a bolus into male Wistar rats, induced a fall in plasma glucose to 72 +/- 3% of baseline 15 min after injection. Co-infusion of equimolar concentrations of human IGFBP-1 abolished the IGF-I-induced fall (P less than 0.001). Injection of IGFBP-1 alone caused a rise in plasma glucose levels (P less than 0.002). The half life of human IGFBP-1, measured using a primate-specific RIA, was 12.5 +/- 0.7 min and was not influenced by the co-infusion of IGF-I. This study demonstrates that, in the rat, human IGFBP-1 blocks the hypoglycemic response to intravenous IGF-I and increases blood glucose levels when administered alone. Since IGFBP-1 concentrations are dependent on metabolic status, we suggest that fluctuating IGFBP-1 levels might modulate the hypoglycemic activity of unbound IGFs in the circulation.

Animals↗
PubMed1990

Pyruvate dehydrogenase-complex activity in brown adipose tissue of gold thioglucose-obese mice.

The activity of pyruvate dehydrogenase (PDH) complex and PDH kinase were measured in brown adipose tissue (BAT) of 4-week-gold thioglucose (GTG)-obese mice. The proportion of PDH complex in the active dephosphorylated form was 2-fold higher in BAT of post-absorptive obese mice compared with lean controls. This result was consistent with the higher circulating insulin concentration observed in GTG-obese mice. In both obese and lean mice the PDH-complex activity in BAT decreased after 24 h starvation and increased in response to supraphysiological insulin injection, indicating that the PDH complex is insulin-responsive in BAT of GTG-obese mice. There was no difference in the PDH kinase activity of BAT in post-absorptive or insulin-injected lean and obese mice, suggesting that the higher PDH-complex activity in obese mice was not due to decreased PDH kinase activity. There is no evidence for a decreased activity of PDH complex contributing to insulin resistance in BAT of 4-week-GTG-obese mice.

Adipose Tissue, Brown↗
PubMed1990

High affinity insulin binding and insulin receptor-effector coupling: modulation by Ca2+.

Insulin binding and insulin stimulated amino acid and glucose uptake were determined in cultured HTC hepatoma cells in the presence of Ca2+ and ruthenium red (RR) in order to further characterise the putative calcium binding site on the receptor. These ions increased insulin receptor high affinity binding and the sensitivity of these responses to insulin. The insulin concentration required to half-maximally stimulate amino acid uptake decreased significantly from 26.9 +/- 5.8 ng/ml to 6.0 +/- 1.3 ng/ml in the presence of 10 mM Ca2+ and to 1.3 +/- 0.5 ng/ml in the presence of RR. The effect of Ca2+ and RR was more pronounced on insulin stimulated glucose uptake. These agents also increased receptor-effector coupling, reducing the percentage of occupied receptors required for maximal insulin stimulation of amino acid uptake from 10.8% in control cells to 3.4 and 1.4% in the presence of Ca2+ and RR respectively. The receptor occupancy required to produce maximal insulin responses on glucose uptake decreased from 20% (control) to 3.8% (Ca2+ and RR). We hypothesize that since Ca2+ and RR have similar effects, that occupation of Ca2+ binding sites on the receptor produces a conformational change in the insulin receptor which increases insulin receptor affinity, insulin sensitivity and acts on an early post-receptor event responsible for coupling binding to insulin action.

Amino Acids↗
PubMed1989

Effect of starvation and insulin in vivo on the activity of the pyruvate dehydrogenase complex in rat skeletal muscles.

The in vivo responses of pyruvate dehydrogenase (PDH) complex to starvation and insulin was assessed in heart, diaphragm and red quadriceps muscle. PDH complex activity was decreased by starvation (3.4-10.2-fold), the magnitude of change depending on muscle type. Insulin increased PDH activity in all muscle types. In fed rats, this effect was relatively small (1.25-1.29-fold). In starved rats there were effects in heart (4.3-fold) and red quadriceps (1.7-fold) but no effect in diaphragm. These results demonstrate that PDH complex in different groups of muscle has different insulin sensitivity (particularly in tissues from starved animals).

Adipose Tissue, Brown↗
PubMed1989

Changes in the lipogenic response to feeding of liver, white adipose tissue and brown adipose tissue during the development of obesity in the gold-thioglucose-injected mouse.

Lipogenic response to feeding was measured in vivo in liver, epididymal white adipose tissue (WAT) and interscapular brown adipose tissue (BAT), during the development of obesity in gold-thioglucose (GTG)-injected mice. The fatty acid synthesis after a meal was higher in all tissues of GTG-treated mice on a total-tissue basis, but the magnitude of this increase varied, depending on the tissue and the time after the initiation of obesity. Lipogenesis in BAT from GTG mice was double that of control mice for the first 2 weeks, but subsequently decreased to near control values. In WAT, lipogenesis after feeding was highest 2-4 weeks after GTG injection, and in liver, lipid synthesis in fed obese mice was greatest at 7-12 weeks after the induction of obesity. The post-prandial insulin concentration was increased after 2 weeks of obesity, and serum glucose concentration was higher in fed obese mice after 4 weeks. These results indicate that increased lipogenesis in GTG-injected mice may be due to an increase in insulin concentration after feeding and that insulin resistance (assessed by lipogenic response to insulin release) is apparent in BAT before WAT and liver.

Adipose Tissue↗
PubMed1988

Inactivation of pyruvate dehydrogenase complex in heart muscle mitochondria of gold-thioglucose-induced obese mice is not due to a stable increase in activity of pyruvate dehydrogenase kinase.

The proportion of pyruvate dehydrogenase (PDH) complex in the active dephosphorylated form was decreased (compared with fed lean control mice) in heart muscle mitochondria after the induction of obesity with gold-thioglucose (by 54%) or starvation of lean mice for 48 h (by 81%). The effects of obesity to inactivate PDH complex were demonstrable 4 weeks after administration of gold-thioglucose, and occurred despite significant hyperinsulinaemia in obese animals. Phosphorylation and inactivation of PDH complex in mouse heart muscle in starvation was attributed to a stable increase (2.7-fold) in the activity of PDH kinase as measured in extracts of mitochondria mediated by increased specific activity of a protein activator of PDH kinase (KAP) [Denyer, Kerbey & Randle (1986) Biochem. J. 239, 347-354]. In obese mice no such increase in kinase activity was observed, and we conclude that phosphorylation and inactivation of PDH complex in heart muscle in obesity is not mediated by KAP, but rather is a consequence of increased lipid oxidation.

Animals↗
PubMed1987

The activity of the pyruvate dehydrogenase complex in heart and liver from mice during the development of obesity and insulin resistance.

The amount of pyruvate dehydrogenase in the active form (PDHa) was increased 1.7-fold compared with controls in heart muscle of mice 1 week after induction of obesity with a single injection of gold-thioglucose. At 4 weeks post injection, the amount of PDHa was decreased to 32% of control, a value which was observed in later stages of the obesity syndrome. In contrast, liver PDHa was increased and remained at an increased activity during the development of obesity. Despite normal post-prandial serum insulin contents, liver membrane insulin-receptor numbers were decreased 1 week after gold-thioglucose injection, and there was no change in receptor affinity. The decrease in heart PDHa in the obese animals was reversed by a single dose of 2-tetradecylglycidic acid, but this inhibitor of mitochondrial fatty acid oxidation did not affect liver PDHa in these animals. These early and diverse changes in PDHa argue for a multifactorial aetiology in the development of the whole-body insulin resistance seen in older gold-thioglucose-treated obese animals.

Animals↗
PubMed1987

Insulin response in individual tissues of control and gold thioglucose-obese mice in vivo with [1-14C]2-deoxyglucose.

The dose-response characteristics of several glucose-utilizing tissues (brain, heart, white adipose tissue, brown adipose tissue, and quadriceps muscle) to a single injection of insulin have been compared in control mice and mice made obese with a single injection of gold thioglucose (GTG). Tissue content of [1-14C]2-deoxyglucose 6-phosphate and blood disappearance rate of [1-14C]2-deoxyglucose (2-DG) were measured at nine different insulin doses and used to calculate rates of 2-DG uptake and phosphorylation in tissues from control and obese mice. The insulin sensitivity of tissues reflected in the ED50 of insulin response varied widely, and brown adipose tissue was the most insulin-sensitive tissue studied. In GTG-obese mice, heart, quadriceps, and brown adipose tissue were insulin resistant (demonstrated by increased ED50), whereas in white adipose tissue, 2-DG phosphorylation was more sensitive to insulin. Brain 2-DG phosphorylation was insulin independent in control and obese animals. The largest decrease in insulin sensitivity in GTG-obese mice was observed in brown adipose tissue. The loss of diet-induced thermogenesis in brown adipose tissue as a result of the hypothalamic lesion in GTG-obese mice could be a major cause of insulin resistance in brown adipose tissue. Because brown adipose tissue can make a major contribution to whole-body glucose utilization, insulin resistance in this tissue may have a significant effect on whole-animal glucose homeostasis in GTG-obese mice.

Adipose Tissue↗