Glucagon-like peptide-1 (GLP-1) has been shown to have insulin-like effects upon the metabolism of glucose in rat liver, muscle and fat, and on that of lipids in rat and human adipocytes. These actions seem to be exerted through specific receptors which, unlike that of the pancreas, are not - at least in liver and muscle - cAMP-associated. Here we have investigated the effect, its characteristics, and possible second messengers of GLP-1 on the glucose metabolism of human skeletal muscle, in tissue strips and primary cultured myocytes. In muscle strips, GLP-1, like insulin, stimulated glycogen synthesis, glycogen synthase a activity, and glucose oxidation and utilization, and inhibited glycogen phosphorylase a activity, all of this at physiological concentrations of the peptide. In cultured myotubes, GLP-1 exerted, from 10(-13) mol/l, a dose-related increase of the D-[U-(14)C]glucose incorporation into glycogen, with the same potency as insulin, together with an activation of glycogen synthase a; the effect of 10(-11) mol/l GLP-1 on both parameters was additive to that induced by the equimolar amount of insulin. Synthase a was still activated in cells after 2 days of exposure to GLP-1, as compared with myotubes maintained in the absence of peptide. In human muscle cells, exendin-4 and its truncated form 9-39 amide (Ex-9) are both agonists of the GLP-1 effect on glycogen synthesis and synthase a activity; but while neither GLP-1 nor exendin-4 affected the cellular cAMP content after 5-min incubation in the absence of 3-isobutyl-1-methylxantine (IBMX), an increase was detected with Ex-9. GLP-1, exendin-4, Ex-9 and insulin all induced the prompt hydrolysis of glycosylphosphatidylinositols (GPIs). This work shows a potent stimulatory effect of GLP-1 on the glucose metabolism of human skeletal muscle, and supports the long-term therapeutic value of the peptide. Further evidence for a GLP-1 receptor in this tissue, different from that of the pancreas, is also illustrated, suggesting a role for an inositolphosphoglycan (IPG) as at least one of the possible second messengers of the GLP-1 action in human muscle.
MA Luque, N Gonzalez, L Marquez, A Acitores, A Redondo, M Morales, I Valverde, and ML Villanueva-Penacarrillo
A Acitores, N Gonzalez, V Sancho, I Valverde, and ML Villanueva-Penacarrillo
Glucagon-like peptide-1 (GLP-1), an incretin with glucose-dependent insulinotropic and insulin-independent antidiabetic properties, has insulin-like effects on glucose metabolism in extrapancreatic tissues participating in overall glucose homeostasis. These effects are exerted through specific receptors not associated with cAMP, an inositol phosphoglycan being a possible second messenger. In rat hepatocytes, activation of phosphatidylinositol 3-kinase (PI3K)/protein kinase B (PKB), protein kinase C (PKC) and protein phosphatase 1 (PP-1) has been shown to be involved in the GLP-1-induced stimulation of glycogen synthase. We have investigated the role of enzymes known or suggested to mediate the actions of insulin in the GLP-1-induced increase in glycogen synthase a activity in rat skeletal muscle strips. We first explored the effect of GLP-1, compared with that of insulin, on the activation of PI3K, PKB, p70s6 kinase (p70s6k) and p44/42 mitogen-activated protein kinases (MAPKs) and the action of specific inhibitors of these kinases on the insulin- and GLP-1-induced increment in glycogen synthase a activity. The study showed that GLP-1, like insulin, activated PI3K/PKB, p70s6k and p44/42. Wortmannin (a PI3K inhibitor) reduced the stimulatory action of insulin on glycogen synthase a activity and blocked that of GLP-1, rapamycin (a 70s6k inhibitor) did not affect the action of GLP-1 but abolished that of insulin, PD98059 (MAPK inhibitor) was ineffective on insulin but blocked the action of GLP-1, okadaic acid (a PP-2A inhibitor) and tumour necrosis factor-alpha (a PP-1 inhibitor) were both ineffective on GLP-1 but abolished the action of insulin, and Ro 31-8220 (an inhibitor of some PKC isoforms) reduced the effect of GLP-1 while completely preventing that of insulin. It was concluded that activation of PI3K/PKB and MAPKs is required for the GLP-1-induced increment in glycogen synthase a activity, while PKC, although apparently participating, does not seem to play an essential role; unlike in insulin signaling, p70s6k, PP-1 and PP-2A do not seem to be needed in the action of GLP-1 upon glycogen synthase a activity in rat muscle.
Concentrations of thyroxine above 10-7 m inhibited the activity of the 'usual' and 'atypical' human plasma cholinesterase in vitro. The 'atypical' enzy mewas more readily inhibited and the ratio of the I50 atypical: I50 usual indicates that the hormone can be used as a differential inhibitor to identify the two phenotypes. Similar results were obtained with thiourea, but the action of thiouracil appeared to differ in so far as this inhibited both enzymes to the same extent. Neither glucagon nor thyroid stimulating hormone had any effect.
J C Parker, K S Lavery, N Irwin, B D Green, B Greer, P Harriott, F P M O’Harte, V A Gault, and P R Flatt
Introduction Glucose-dependent insulinotrophic polypeptide (GIP) and glucagon-like peptide-1 (GLP-1) are important gastrointestinal-releasing hormones involved in the regulation of postprandial nutrient homeostasis ( Meier et al
R. D. GILL and I. C. HART
A method is described for the isolation of viable hepatocytes from sheep liver. The characteristics of insulin and glucagon binding to the cells were investigated by the use of mono-iodinated hormone, and from these data the optimum in-vitro incubation conditions for hormone-receptor binding were established. Glucagon and insulin receptors were examined in relation to plasma concentrations of hormones and metabolites in non-mated, and 20- and 50-day-lactating ewes (six animals/group). Measurements of insulin, growth hormone and non-esterified fatty acids in the circulation, together with a fall in body weight, suggested that at peak lactation (20 days) the ewes were in energy-deficit and were mobilizing body tissue. The percentage binding of insulin was higher in hepatocytes after 50 days of lactation when compared with that in both the unmated (P < 0·05) and 20-day-lactating animals. No changes in insulin binding were found between the unmated and 20-day-lactating groups. Glucagon binding was reduced in the 20- (P < 0·02) and increased in the 50-day-lactating group (P < 0·001) when compared with the unmated control animals. The binding of glucagon was higher at 50 days as compared with 20 days of lactation (P < 0·001). The changes in insulin binding resulted primarily from altered receptor numbers whereas changes in the binding of glucagon were due to alterations in both receptor numbers and affinity. Our results indicated that the binding of insulin and glucagon to isolated hepatocytes was altered during lactation in sheep and that these changes might modulate the sensitivity of the cells to the actions of the hormones.
J. Oben, L. Morgan, J. Fletcher, and V. Marks
The effect of gastric inhibitory polypeptide (GIP), glucagon-like peptide-1(7–36) amide, (GLP-1(7–36) amide), glucagon-like peptide-2 (GLP-2), glucagon and insulin on fatty acid synthesis in explants of rat adipose tissue from various sites was investigated. GIP, GLP-1(7–36) amide and insulin stimulated fatty acid synthesis, as determined by measuring the incorporation of [14C]acetate into saponifiable fat, in a dose-dependent manner, over the concentration range 5–15 ng/ml (0·87–2·61 nmol/l) for insulin and 0·5–7·5 ng/ml for GIP (0·10–1·50 nmol/l) and GLP-1(7–36) amide (0·15–2·27 nmol/l). Insulin and GIP caused a significantly greater stimulation of [14C]acetate incorporation into fatty acids in omental adipose tissue than in either epididymal or subcutaneous adipose tissue. Both GIP and GLP-1(7–36) amide had the ability to stimulate fatty acid synthesis within the physiological range of the circulating hormones. At lower concentrations of the hormones, GLP-1(7–36) amide was a more potent stimulator of fatty acid synthesis than GIP in omental adipose tissue culture; the basal rate of fatty acid synthesis was 0·41±0·03 pmol acetate incorporated/mg wet weight tissue per 2 h; at 0·10 nmol hormone/l 1·15±0·10 and 3·40±0·12 pmol acetate incorporated/mg wet weight tissue per 2 h for GIP and GLP-1(7–36) amide respectively (P < 0·01). GLP-2 and glucagon were without effect on fatty acid synthesis in omental adipose tissue. The study indicates that GIP and GLP-1(7–36) amide, in addition to stimulating insulin secretion, may play a direct physiological role in vivo, in common with insulin, in promoting fatty acid synthesis in adipose tissue.
Journal of Endocrinology (1991) 130, 267–272
Intravenous injection of 8·5 nmol (1–24)ACTH increased the plasma levels of glucagon, insulin, glucose and free fatty acids in rabbits. The (1–24)ACTH-induced hyperglucagonaemia and hyperinsulinaemia started 3 and 20 min after the injection respectively. Similar increases in the plasma levels of glucagon, insulin and free fatty acids were found with 5·3 nmol (1–39)ACTH, whereas (1–4)ACTH, (4–10)ACTH, (1–10)ACTH, (11–24)ACTH, (7–38)ACTH and (18–39)ACTH (corticotrophin-like intermediate lobe peptide) injected at doses of approximately 8 nmol were inactive. Infusions with the alpha-adrenergic blocking drug, phentolamine, reduced the (1–24) ACTH-induced hyperglucagonaemia and hypergly-caemia, and augmented the (1–24)ACTH-induced hyperinsulinaemia, which now became significant after 5 min. Infusions with the beta-adrenergic blocking drug, propranolol, did not diminish the (1–24)ACTH-induced effects, but killed the rabbits after 2–4 h.
It is concluded that the acute in-vivo effects of ACTH in rabbits are modulated by the involvement of alpha-adrenergic receptors, which increased the plasma levels of glucagon and glucose, and delayed and diminished the ACTH-induced increases in the plasma levels of insulin. The (1–24)ACTH-induced increases in the plasma levels of free fatty acids were not influenced by the adrenergic blocking drugs.
J. Endocr. (1984) 100, 345–352
Ch. Foltzer-Jourdainne, S. Harvey, H. Karmann, and P. Mialhe
Human pancreatic GH-releasing factor (hpGRF) increased the concentrations of plasma GH when infused i.v. into immature ducks. A dose-dependent increase in plasma GH was observed within 10 min of the start of infusion and was maintained during the 30-min infusion period. Simultaneous infusion of somatostatin S-14 prevented the increase in plasma GH induced by hpGRF, but when the infusion had finished there was a rebound increase in plasma GH. Infusion of the highest dose of hpGRF (800 ng/kg per min) in adult ducks had no significant effect on plasma GH.
Plasma somatostatin concentrations were reduced during the infusion of hpGRF in young but not in adult ducks. This observation suggests that the stimulatory effect of hpGRF on GH secretion may be partly due to its inhibitory effect on somatostatin secretion. Infusion of hpGRF in ducklings also increased the concentrations of glucagon and decreased levels of insulin in the plasma. Peripheral plasma glucagon and insulin levels in adult ducks were unaffected by hpGRF infusion. These results indicate that in ducklings, hpGRF increases plasma GH and glucagon concentrations and lowers plasma somatostatin and insulin levels. In the adult, these hormonal responses to hpGRF are not maintained. The highly stimulatory effect of hpGRF on GH secretion in ducklings may explain why plasma GH concentrations are high in these birds.
J. Endocr. (1987) 114, 25–32
D. J. Gawler, A. Wilson, and M. D. Houslay
Glucagon stimulated adenylate cyclase activity some 21-fold in liver membranes from lean (Fa/Fa) and some 20-fold in membranes from obese (fa/fa) Zucker rats, with constants yielding half-maximal activation (K a values) of 12·6 and 120·1 nmol/l respectively. Treatment of animals with the biguanide drug metformin (N′,N′-dimethylbiguanide) decreased the ability of glucagon to stimulate this enzyme to some 16-fold for both the lean and obese animals and reduced the K a values for activation of this enzyme by glucagon to 6·3 and 60·9 nmol/l respectively. Insulin inhibited glucagon-stimulated adenylate cyclase activity by some 24% in liver membranes from lean animals and some 17% in liver membranes from obese animals, with constants yielding half-maximal inhibition (K i values) of 110 and 160 nmol/l respectively. The ability of insulin to inhibit the adenylate cyclase activity, from obese but not lean animals, was attenuated when insulin concentrations over 5 nmol/l were employed. Treatment of animals with metformin profoundly altered the sensitivity of adenylate cyclase to inhibition by insulin, with inhibition being increased to some 32% using liver membranes from either lean or obese animals. Values of K i for this inhibitory action of insulin were 520 and 500 nmol/l using membranes from the lean and obese animals respectively, and no reduction in the ability of insulin, at concentrations over 5 nmol/l, to inhibit adenylate cyclase activity was observed using membranes from obese animals. Metformin also changed the kinetics of inhibition of adenylate cyclase by insulin. These were apparently negatively co-operative, with Hill coefficients of 0·76 and 0·89 using membranes from the untreated lean and obese animals respectively, and positively co-operative, with Hill coefficients of 1·45 and 1·20 for the metformin-treated lean and obese animals respectively.
Journal of Endocrinology (1989) 122, 207–212
J. R. Attali, D. Darnis, P. Valensi, C. Weisselberg, and J. Sebaoun
Perifusion of rat thyroid fragments was performed to study short-term effects of TSH, theophylline and glucagon on thyroid hormone secretion. This technique proved to be relatively convenient and sensitive, and gave reproducible results for at least 3 h, permitting precise kinetic studies of response to hormonal and pharmacological agents without any interference. There was a significant (P < 0·001) linear correlation between the log TSH concentrations over the range 20–150 mu./ml and thyroid response. A second stimulation, using the same concentration of TSH, did not differ from the first stimulation if they were separated by an active 'washing' period of only 15 min. Theophylline also had a stimulating effect and like TSH induced an early release of the hormone fraction with a peak between 2 and 4 min, but it did not potentiate the TSH effect.
Perifusion of rat thyroid fragments was found to be a useful tool for analysing dynamic effects of various substances. These effects were significant for periods of time as short as 20 min. Each thyroid preparation could be used a second time for another pharmacological or hormonal test. Our preliminary results also suggested that there was a direct glucagon effect on thyroid hormone secretion with a dose–response correlation.
J. Endocr. (1984) 102, 43–48