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B Leighton

Publications and source records attributed to B Leighton.

At least 37 records · Page 2Linked to original sources
PubMed1997

More marked stimulation by lithium than insulin of the glycogenic pathway in rat skeletal muscle.

Lithium's impact on glucose metabolism was compared with that of insulin in isolated rat soleus muscle. Lithium chloride (20 mmol/l) induced a 4.8-fold more pronounced increment over basal glycogen synthase activity than insulin (10 nmol/l) (nmol UDP-glucose into glycogen in synthase activity assay.g-1.min-1: lithium, +22.1 +/- 1.8 vs. insulin, +4.6 +/- 3.9; P < 0.01). In parallel, lithium was less efficient than insulin in stimulating glucose transport (counts per minute 2-deoxy-D-[3H]glucose.mg-1.h-1: lithium, +211 +/- 19 vs. insulin, +311 +/- 57; P < 0.05) and lactate release (mumol.g-1.h-1: lithium, +1.0 +/- 0.5 vs. insulin, +3.9 +/- 0.5; P < 0.01), and similar increments were induced in glycogen synthesis (mumol glucose into glycogen.g-1.h-1: lithium, +3.32 +/- 0.43 vs. insulin, +3.46 +/- 0.47; not significant). Full additivity of glycogenic effects and divergent dependency on phosphatidylinositol 3-kinase activation provided further evidence for different mechanisms of action. In muscle from insulin-resistant obese Zucker rats (fa/fa), failure of lithium to reverse deficits in glucose metabolism suggested a primary deficit in muscle glucose uptake rather than glycogen synthesis. Hence lithium distinctly stimulates glycogen synthase activity in skeletal muscle and may therefore be regarded as a candidate for the treatment of disorders associated with primary deficits in the glycogenic pathway.

Androstadienes↗
PubMed1996

Activation of glycogen phosphorylase and glycogenolysis in rat skeletal muscle by AICAR--an activator of AMP-activated protein kinase.

We determined whether the cell permeable molecule AICAR, whose metabolite activates AMP-activated protein kinase (AMPK) in cells, affected glycogen metabolism in rat soleus muscle preparations in vitro. The basal and insulin-stimulated rates of radiochemical lactate formation, net lactate release and glycogen synthesis were determined. AICAR stimulated net lactate release (but not radiochemical lactate formation) only at a basal concentration of insulin. An increased rate of glycogenolysis was the likely cause of increased net lactate release as glycogen phosphorylase activity was significantly increased by AICAR. AICAR-stimulated net lactate release and phosphorylase activity were potently inhibited by insulin.

AMP-Activated Protein Kinases↗
PubMed1996

Abnormal regulation of hexokinase in insulin-resistant skeletal muscle.

The present study has assessed the potential involvement of hexokinase in the control of insulin-mediated glucose metabolism in insulin-sensitive and -resistant skeletal muscle. Soleus muscle strips from lean (insulin-sensitive) and obese (insulin-resistant) Zucker rats were incubated with 10 or 10,000 microU insulin.ml-1 and then homogenized using a protocol to maintain the location of hexokinase in situ. Hexokinase is inhibited by glucose 6-phosphate, a metabolic intermediate which may have a central role in the regulation of glycogen synthesis. Two separate measurements of hexokinase activity were made on each muscle homogenate: the total hexokinase activity (glucose 6-phosphate was metabolized immediately by glucose 6-phosphate dehydrogenase) and the fractional hexokinase activity (glucose 6-phosphate accumulated so as to regulate the enzyme as in vivo). The total hexokinase activity was equal in insulin-sensitive and -resistant muscle and was unaffected by the extracellular insulin concentration. The fractional hexokinase activity was significantly increased by insulin (10,000 microU.ml-1) in all muscles (lean, 82%; obese, 52%; P < 0.05) although the stimulated fractional hexokinase activity was lower in the muscle from obese Zucker rats compared to lean (P < 0.05). These results provide evidence that insulin decreases the inhibition of hexokinase by glucose 6-phosphate in insulin-sensitive but not in insulin-resistant muscle. This study has revealed short-term regulation of hexokinase by insulin which is defective in insulin-resistant skeletal muscle. Thus, the study has identified hexokinase as a potential regulatory site of insulin action that is abnormal in insulin resistance. The altered regulation of hexokinase may be a major contributing factor to the reduced insulin-mediated glucose fluxes in insulin-resistant skeletal muscle.

Animals↗
PubMed1996

Effects of treatment of spontaneously hypertensive rats with the angiotensin-converting enzyme inhibitor trandolapril and the calcium antagonist verapamil on the sensitivity of glucose metabolism to insulin in rat soleus muscle in vitro.

We measured the sensitivity of glucose metabolism to insulin in soleus muscle preparations isolated from spontaneously hypertensive (SH) rats and normotensive age-matched Wistar-Kyoto (WKY) rats. SH rats were treated with the angiotensin-converting enzyme (ACE) inhibitor trandolapril (1 mg/kg) and/or a second antihypertensive drug, the calcium antagonist verapamil, alone (100 mg/kg) or as combination therapy (50 mg/kg). Treatment of SH rats with trandolapril or trandolapril in combination with verapamil for 6 weeks normalized the blood pressure. The estimated concentration of insulin required for half-maximal stimulation of glycogen synthesis (i.e., EC50 values) was approximately 500 microU/ml for muscles from both WKY and SH rats. This value is five times higher than the value obtained from soleus muscle preparations isolated from insulin-sensitive Wistar rats. This indicates that glycogen synthesis is insensitive to insulin in SH and WKY rat soleus muscle. Treatment of SH rats with trandolapril with or without verapamil improved the sensitivity of glycogen synthesis to insulin in soleus muscle. Further experiments investigated whether acute exposure (1 h) of insulin-sensitive skeletal muscle with either trandolaprilat (the active metabolite of trandolapril) or bradykinin (levels of which may be raised by ACE inhibition) could affect the insulin-stimulated rate of glucose metabolism. These results show that both trandolaprilat and bradykinin caused a small but significant increase in the rates of glucose metabolism. In conclusion, 1) SH and WKY rat skeletal muscle was insulin resistant, 2) chronic treatment of SH rats with trandolapril with or without verapamil normalized blood pressure and improved the response of glycogen metabolism to insulin, and 3) bradykinin and trandolaprilat acutely caused a small but significant increase in the rate of glycogen synthesis to a submaximal physiological concentration of insulin.

Angiotensin-Converting Enzyme Inhibitors↗
PubMed1995

The role of the sensory peptide calcitonin-gene-related peptide(s) in skeletal muscle carbohydrate metabolism: effects of capsaicin and resiniferatoxin.

1. The content of calcitonin-gene-related-peptide-like immunoreactivity (CGRP-LI) in various rat muscles was measured. Starvation for 24 h did not affect the content of CGRP-LI in these muscles, except for a decreased level in the starved-rat diaphragm. Higher contents of CGRP-LI were observed in well-vascularized muscles. 2. Capsaicin (at 1, 10 and 100 microM) inhibited insulin-stimulated rates of glycogen synthesis in isolated stripped incubated soleus muscle preparations by a mechanism independent of catecholamine release, since the effects of capsaicin were not altered by the beta-adrenoreceptor antagonist DL-propranolol. 3. Resiniferatoxin (10 nM), which is a potent capsaicin agonist, also significantly inhibited the insulin-stimulated rate of glycogen synthesis. Furthermore, the concentration of resiniferatoxin required to inhibit glycogen synthesis was 100 times less than the concentration of capsaicin needed for the same effect. 4. Capsaicin (10 microM) decreased the content of CGRP-LI in isolated stripped incubated soleus muscle preparations by about 40%. 5. Neonatal treatment of rats with capsaicin, which causes de-afferentation of some sensory nerves such, we hypothesize, that CGRP can no longer be released to counteract the effects of insulin in vivo, caused increased rates of glycogen synthesis and increased glycogen content in stripped soleus muscle preparations in vitro when muscles were isolated from the adult rats. 6. These findings support the hypothesis that capsaicin and resiniferatoxin elicit an excitatory response on sensory nerves in skeletal muscle in vitro to cause the efferent release of CGRP. Consequently, CGRP is delivered to skeletal muscle fibres to inhibit insulin-stimulated glycogen synthesis. The role of CGRP in recovery of blood glucose levels during hypoglycaemia is discussed.

Animals↗
PubMed1995

Effects of glucosamine on insulin-stimulated glucose metabolism in rat soleus muscle.

Intracellular accumulation of glucosamine metabolites (which can be achieved by pre-incubation of cells with glucosamine) during hyperglycaemia may decrease the rate of insulin-mediated glucose transport in cells. Soleus muscle preparations were pre-incubated in the presence or absence of glucosamine in media that contained glutamine (Dulbecco's modified Eagle medium, DMEM; Medium 199, M199) or devoid of glutamine (Krebs-Henseleit's buffer, KHB). Subsequently, muscles were transferred to fresh media, in the absence of glucosamine, but with various concentrations of insulin and the rates of 2-deoxyglucose transport or intracellular glucose metabolism were measured. Glucosamine pre-exposure decreased both insulin-stimulated (1000 microU/ml) glucose transport and phosphorylation. The percentage decreases for 3H-2-deoxyglucose transport after pre-incubation with 40 mM glucosamine compared with untreated muscles were: DMEM, 48%; KHB, 50%; M199, 29%. The percentage decreases for 3H-2-deoxyglucose-6-phosphate accumulation were: DMEM, 53%; KHB 60%; M199, 37%. In DMEM and KHB, glucosamine pre-treatment of soleus muscle preparations markedly decreased the rate of lactate release and stimulated the rate of 14C-glucose incorporation into glycogen. Thus, a distinct shift of glucosyl units from glycolysis to glycogenesis occurred with low and high insulin concentrations. For the latter (1000 microU of insulin/ml) the ratio of moles of glucose converted to lactate divided by moles of glucose incorporated into glycogen in muscles pre-incubated in the absence or presence of glucosamine (40 mM) was, respectively: DMEM, 4.34 + 0.52 vs 1.55 + 0.06, P < 0.001; KHB, 2.80 + 0.44 vs 0.76 + 0.03, P < 0.005). Glycogen synthesis was not stimulated in muscles pre-exposed to glucosamine in M199. In muscles pre-incubated to glucosamine and incubated in DMEM or KHB, there was a marked shift of glucose transported into the cell from glycolysis to glycogenesis. Thus, glucosamine or its metabolites had distinct effects on intracellular glucose handling.

Animals↗
PubMed1994

Modulation of Pi transport in skeletal muscle by insulin and IGF-1.

In vivo, skeletal muscle Pi uptake influences both muscle cellular [Pi] and plasma [Pi], and may mediate the hypophosphataemic effects of insulin and insulin-like growth factor 1 (IGF-1). These effects were investigated in the cultured mouse myoblast cell line G8 and the isolated incubated rat soleus. The low Km for Pi in G8 cells is consistent with in vivo evidence that muscle cell [Pi] is partially protected against changes in plasma [Pi]. Insulin and IGF-1 stimulated Na-dependent Pi influx: in G8 cells both increased Vmax, with no change in Km, but while the insulin response occurred within 15 min and rapidly reversed upon insulin withdrawal, the response to IGF-1 occurred only after 60 min and persisted at least 60 min following IGF-1 withdrawal. Furthermore, only the IGF-1 response was inhibited by cycloheximide. We suggest that IGF-1 operates through de novo protein synthesis, while insulin stimulates transporter recruitment to the cell surface.

Animals↗
PubMed1994

Studies on the effects of growth hormone administration in vivo on the rates of glucose transport and utilization in rat skeletal muscle.

The effects of growth hormone (GH) administration to rats in vivo on the sensitivity of the rate of glucose utilization to insulin were studied in soleus muscles isolated from these rats. A single injection of GH did not increase the rate of glucose transport within 1-2 h. However, 12 h after, the rate of glucose transport was increased at 10 mU insulin l-1 and was accompanied by a similar increase in the rate of lactate formation but no change in the rate of glycogen synthesis. Prolonged treatment with GH decreased the rate of glucose transport and glycogen synthesis and increased the content of glucose 6-phosphate at physiological levels of insulin but did not affect the rate of lactate formation. These results suggest that: (a) GH does not increase the rate of glucose transport acutely; however, after several hours, the sensitivity of glucose transport and glycolysis to insulin are increased; (b) prolonged elevations of the level of GH in plasma decrease the sensitivity of the rate of glucose transport and glycogen synthesis to insulin. However, redirection of glucose residues away from the pathway of glycogen synthesis towards that of glycolysis and a possible increase in the rate of glycogenolysis maintain a normal rate of lactate formation, although the rate of glucose transport is decreased.

Animals↗
PubMed1994

Effects of calcium antagonists on insulin-mediated glucose metabolism in skeletal muscle.

The effect of three calcium antagonists (verapamil, diltiazem, and nifedipine) on insulin effects was investigated in isolated rat soleus muscles. Soleus muscles were incubated in the presence of insulin (100 microU/ml), a concentration that stimulates the rates of lactate formation and glycogen synthesis half-maximally and with and without a calcium antagonist. A decrease (48%; P < 0.001) was noted in the insulin-mediated rate of glycogen synthesis by verapamil at 100 microM; no effect was observed at lower concentrations of verapamil. Diltiazem decreased the insulin-mediated rates of glycogen synthesis by 36 (P < 0.001), 64 (P < 0.001), and 73% (P < 0.001) at 1, 10, and 100 microM, respectively. Nifedipine decreased the insulin-mediated rates of glycogen synthesis by 37% at 0.1 microM (P < 0.001), 36% at 1 microM (P < 0.001), 21% at 10 microM (P < 0.05), and 72% at 100 microM (P < 0.001). Verapamil at 100 microM decreased lactate formation by 48% (P < 0.001). However, diltiazem increased the rate of lactate formation by 22 (P < 0.01), 43 (P < 0.001), and 61% (P < 0.001) at 1, 10, and 100 microM, respectively. In contrast, nifedipine increased the insulin-mediated rate of lactate formation by 45% only at 100 microM (P < 0.01). The increased rate of lactate formation was probably caused by an increased rate of glycogenolysis, because high concentrations of all the calcium antagonists significantly decreased muscle glycogen content. The insulin-stimulated rate of 3-O-methyl-D-glucose transport or cAMP content was not affected by diltiazem at 1 or 10 microM. The results suggest that the calcium antagonists work by a mechanism, possibly by activating a calcium channel or an extracellular receptor, to influence markedly insulin-mediated intracellular glucose metabolism in skeletal muscle.

Animals↗
PubMed1992

The effects of orthovanadate, vanadyl and peroxides of vanadate on glucose metabolism in skeletal muscle preparations in vitro.

The insulin-like effects of various vanadium compounds (orthovanadate, vanadyl and peroxides of vanadate) on rates of glucose oxidation, lactate formation and glycogen synthesis were measured in isolated incubated epitrochlearis (mainly type II fibres) and soleus (mainly type I fibres) muscle preparations. There was a small stimulation of the rate of glucose utilisation in soleus muscle preparations in vitro by orthovanadate (1 mM). Orthovanadate or vanadyl, at 1 mM, had little effect on the rates of lactate formation or glycogen synthesis in isolated incubated epitrochlearis muscle preparations. In contrast, peroxides of vanadate (peroxovanadates, at 1 mM) significantly stimulated glucose utilisation in both soleus and epitrochlearis muscle preparations in vitro. The stimulation of the rate of glycogen synthesis was associated with an increase in the percentage of glycogen synthase in the I (or a) form. Peroxovanadates were administered in the drinking water to rats made insulin deficient by streptozotocin treatment. There was no decrease in the elevated level of blood glucose over an 8 day administration period.

Animals↗
PubMed1992

Biotinyl analogues of amylin as biologically active probes for amylin/CGRP receptor recognition.

Biotinyl analogues of rat amylin were synthesised with sulfosuccinimidyl 2-(biotinamido) ethyl-1,3-dithiopropionate (NHS-SS-Biotin). Biotinylated amylin peptides were purified by HPLC, quantitated, and the presence of the biotin group at Lys-1 confirmed by peroxidase-labelled avidin and FAB mass spectroscopy. Amylin-biotin retained a similar affinity for binding to rat liver plasma membranes compared with rat amylin and also completely inhibited insulin-stimulated glycogen synthesis in rat soleus muscle incubated in vitro. These biologically active amylin probes will enable a complete analysis of amylin/CGRP receptor expression in various cell types and facilitate the isolation and characterisation of the hormone-receptor complex.

Amyloid↗