Gene/Protein Disease Symptom Drug Enzyme Compound
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Query: UNIPROT:P01275 (glucagon)
 26,492 document(s) hit in 31,850,051 MEDLINE articles (0.00 seconds)

Glucagon causes a rapid activation of cAMP-dependent protein kinase in rat liver parenchymal cells which correlates well with the accumulation of cAMP. Full activation of phosphorylase or inactivation of glycogen synthase is achieved with half-maximal or less activation of protein kinase. Epinephrine stimulates glycogen breakdown in these cells mainly by mechanisms involving alpha-adrenergic receptors and not beta-receptors. Activition of alpha-receptors results in rapid activation of phosphorylase and inactivation of glycogen synthase without accumulation of cAMP or activation of cAMP-dependent protein kinase. Activation of beta-receptors causes a transient rise in cAMP and a short-lived activation of protein kinase with correspondingly little stimulation of glycogenolysis.
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PMID:Studies on the role of cAMP-dependent protein kinase in the actions of glucagon and catecholamines on liver glycogen metabolism. 18 93

1. The administration of insulin to anaesthetized rabbits caused the inactivation of liver phosphorylase and phosphorylase kinase, but did not change either the hepatic concentration of cyclic AMP or the activity of cyclic AMP-dependent histone kinase. All measured parameters were increased by the subsequent administration of glucagon. 2. Activation of glycogen synthase by insulin was only observed when phosphorylase had been strongly inactivated.
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PMID:The effect of insulin on the glycogenolytic cascade and on the activity of glycogen synthase in the liver of anaesthetized rabbits. 19 7

A number of enzymatic methods have been developed to prepare hepatocytes using collagenase and hyaluronidase. However, best cell preparations are obtained by using only low concentrations of collagenase and exposing the liver to the enzyme for a very short period of time. These isolated cells with intact cell membranes and large numbers of microvilli on the cell surface respond to hormones at physiological concentrations suggesting that these microvilli contain hormone receptors. In addition, high glycogen content is essential to maintain the in vivo metabolic characteristics of the hepatocytes suggesting that intracellular glycogen plays an important role in the hormonal regulation of metabolism in hepatocytes. Studies with glucagon and insulin on carbohydrate metabolism show that the molar ratios of these hormones control gluconeogenesis and glycogenolysis. Furthermore, in vitro addition of insulin stimulates glycogen synthesis and activates glycogen synthase. Insulin also stimulates protein synthesis in cells containing high glycogen and maintains more normal parallel strands of polyribosomes. Studies with isolated hepatocytes from diabetic, hypophysectomized and adrenalectomized animals show a reduced glucagon response to glycogenolysis. This lack of glucagon response was not due to reduction in glycogen levels. Other hormones such as somatostatin and parathyroid also give rise to alterations in carbohydrate metabolism in isolated hepatocytes.
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PMID:Studies of hormonal regulation of metabolism using isolated hepatocytes. 19 66

The effects of starvation on the hepatic glycogen synthase and phosphporylase systems were sequentially assessed in fed and 24-120-hr-fasted rats. Enzymic changes before and after glucose were correlated with simultaneous measurements of hepatic cyclic AMP and glycogen concentrations and glucose, insulin, and glucagon concentrations in the portal vein plasma. Fasting caused parallel changes in plasma glucose and hepatic glycogen concentrations with decreases by 24 hr and subsequent increases, which correlated with increases in hepatic synthase l and decreases in phosphorylase activites. Hepatic cyclic AMP levels increased as 24-48 hr, decreased below fed levels at 96 hr, and increased again at 120 hr. Fasting caused progressive impairment of glucose disposal, decreased basal and postglucose insulin concentrations, and decreased basal glucagon levels at 48-72 hr. Hepatic synthase l increments following glucose were exaggerated in 48-120-hr-fasted rats, although consistent phosphorylase decrements were seen only in fed rats. There was no clearcut relationship between synthase activation and phosphorylase inactivation following glucose in fed or fasted rats.
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PMID:Effect of starvation on hepatic glycogen metabolism and glucose homeostasis. 20 23

Angiotensin II, catecholamines, and vasopressin are thought to stimulate hepatic glycogenolysis and gluconeogenesis via a cyclic AMP-independent mechanism that requires calcium ion. The present study explores the possibility that angiotensin II and vasopressin control the activity of regulatory enzymes in carbohydrate metabolism through Ca2+-dependent changes in their state of phosphorylation. Intact hepatocytes labeled with [32P]PO43- were stimulated with angiotensin II, glucagon, or vasopressin and 30 to 33 phosphorylated proteins resolved from the cytoplasmic fraction of the cell by electrophoresis in sodium dodecyl sulfate polyacrylamide slab gels. Treatment of the cells with angiotensin II or vasopressin increased the phosphorylation of 10 to 12 of these cytosolic proteins without causing measurable changes in cyclic AMP-dependent protein kinase activity. Glucagon stimulated the phosphorylation of the same set of 11 to 12 proteins through a marked increase in cyclic AMP-dependent protein kinase activity. The molecular weights of three of the protein bands whose phosphorylation was increased by these hormones correspond to the subunit molecular weights of phosphorylase (Mr = 93,000), glycogen synthase (Mr = 85,000), and pyruvate kinase (Mr = 61,000). Two of these phosphoprotein bands were positively identified as phosphorylase and pyruvate kinase by affinity chromatography and immunoprecipitation, respectively. Incubation of hepatocytes in a Ca2+-free medium completely abolished the effects of angiotensin II and vasopressin on protein phosphorylation but did not alter those of glucagon. Treatment of hepatocytes with angiotensin II, glucagon, or vasopressin stimulated phosphorylase activity by 250 to 260%, inhibited glycogen synthase activity by 50%, and inhibited pyruvate kinase activity by 30 to 35% (peptides) to 70% (glucagon). The effects of angiotensin II and vasopressin on the activity of all three enzymes were completely abolished if the cells were incubated in a Ca2+-free medium while those of glucagon were not altered. The results imply that angiotensin II, catecholamines, and vasopressin control hepatic carbohydrate metabolism through a Ca2+-requiring, cyclic AMP-independent pathway that leads to the phosphorylation of important regulatory enzymes.
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PMID:The role of calcium ion as a mediator of the effects of angiotensin II, catecholamines, and vasopressin on the phosphorylation and activity of enzymes in isolated hepatocytes. 22 57

During the first two thirds of gestation, the concentrations of UDPG, ATP, ADP, and Mg++ in human fetal liver remain constant, whereas the concentration of Pi decreases twofold and the G-6-P and AMP concentrations increase. Incubation of human fetal liver explants with glucagon or insulin did not alter the concentrations of any of these intermediates. ATP, ADP, and Pi are inhibitors of human fetal liver glycogen synthase D-form activity, while G-6-P and AMP and Mg++ are stimulators. Ca++ at concentrations of less than 0.1 mM was found to stimulate glycogen synthase D activity. This effect of Ca++ was also observed in "physiologic" mixtures containing UDPG, G-6-P, ATP, ADP, AMP, Pi, and Mg++ at concentrations found either in liver in utero or in explants. 45Ca++ efflux from perifused (rat) fetal liver explants was stimulated by glucagon. These data provide a picture of the metabolite regulation of human fetal liver glycogen synthase activity in which the D-form may largely control glycogen synthesis in utero and hormonal effects on glycogen synthase may be induced by effects of Ca++ on the D-form.
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PMID:Hormonal regulation of glycogen metabolism in human fetal liver. II. Regulation of glycogen synthase activity. 81 98

In rat hepatocytes, molybdate and tungstate inactivate glycogen synthase by a mechanism independent of Ca2+ and activate glycogen phosphorylase by a Ca(2+)-dependent mechanism. On the other hand, both molybdate and tungstate increase fructose 2,6-bisphosphate levels and counteract the decrease in this metabolite induced by glucagon. These effectors do not directly modify 6-phosphofructo-2-kinase activity, even though they partially counteract the inactivation of this enzyme induced by glucagon. These effects are related to an increase on the glycolytic flux, as indicated by the increase in L-lactate and CO2 production and the decrease in glucose 6-phosphate levels in the presence of glucose. All these effects are similar to those previously reported for vanadate, although molybdate and tungstate are less effective than vanadate. These results could indicate that molybdate, tungstate and vanadate act on glucose metabolism in isolated hepatocytes by a similar mechanism of action.
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PMID:Molybdate and tungstate act like vanadate on glucose metabolism in isolated hepatocytes. 131 28

1. Extracellular UTP and ATP show obvious similarities in their control of several metabolic functions of rat isolated hepatocytes. 2. They have a similar time-course and concentration-dependency for the activation of glycogen phosphorylase, the generation of inositol trisphosphate (IP3), the inhibition of glycogen synthase and the lowering of adenosine 3':5'-cyclic monophosphate (cyclic AMP) levels. 3. There is a similar synergism of the nucleotides with glucagon in activating phosphorylase. 4. They undergo a similar inhibition by phorbol myristic acid of their glycogenolytic effect. 5. The ATP and UTP effect on IP3 levels are not additive. 6. It is tentatively concluded that UTP and ATP use a common receptor.
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PMID:Extracellular ATP and UTP exert similar effects on rat isolated hepatocytes. 155 36

Adrenalin and glucagon inhibit glycogen, fatty acid and cholesterol synthesis by elevation of cyclic AMP, activation of cyclic AMP-dependent protein kinase and increased phosphorylation of the rate-limiting enzymes of these pathways. Here, we review recent evidence which indicates that inhibition of these biosynthetic pathways in muscle, adipose tissue and liver is much more indirect than has previously been supposed. In particular, cyclic AMP-dependent protein kinase does not appear to inhibit glycogen synthase, acetyl-CoA carboxylase and HMG-CoA reductase by phosphorylating them directly. It appears to achieve the same end result by inactivation of the protein phosphatases which dephosphorylate these regulatory enzymes in vivo, although this has only been established definitively in the case of glycogen synthesis.
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PMID:The actions of cyclic AMP on biosynthetic processes are mediated indirectly by cyclic AMP-dependent protein kinase. 165 40

Incubation of hepatocytes in conditions known to increase their volume, i.e. with amino acids or in hypo-osmotic media, resulted in the parallel activation of glycogen synthase and acetyl-CoA carboxylase. The activation of both enzymes by glutamine was antagonized by the addition of raffinose to prevent cell swelling, or by glucagon and microcystin. The findings are consistent with the involvement of a common mechanism for the activation of the two enzymes.
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PMID:Swelling of rat hepatocytes activates acetyl-CoA carboxylase in parallel to glycogen synthase. 168 Mar 22


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