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)

Hepatocytes respond to stimulation by glycogenolytic agonists acting via phosphoinositide (PI) breakdown through oscillations of the free cytosolic concentration of Ca2+ ([Ca2+]cyt.). Since the second-messenger repertoire of hepatocytes includes many other factors besides Ca2+, we investigated to what degree the regulation of [Ca2+]cyt. oscillations is integrated into these other signalling systems. [Ca2+]cyt. was recorded in single rat hepatocytes by using the Ca(2+)-indicator fura-2. Parallel stimulation with phenylephrine (an alpha 1-adrenergic agonist of PI breakdown) and glucagon resulted in a synergistic stimulation of [Ca2+]cyt. oscillations. Direct activation of the cyclic-AMP-dependent pathway with several stimuli (forskolin, 8-bromo cyclic AMP, 8-CPT cyclic AMP) mimicked the response to glucagon. In contrast, [Ca2+]cyt. oscillations induced by various combinations of these agonists could be antagonized by the glycogenic hormone insulin. As one of the options in the insulin-signalling network, we tested a diacylglycerol activator of protein kinase C, DiC8. It also acted as an inhibitor of [Ca2+]cyt. oscillations. We investigated how these observations could be reconciled with our previously introduced model of [Ca2+]cyt. oscillations in hepatocytes [Somogyi and Stucki (1991) J. Biol. Chem. 266, 11068-11077]. First of all, the effect of calmodulin inhibitors (calmidazolium and CGS 9343 B), acting at the core of our model on the feedback of Ca2+ on Ins(1,4,5)P3-induced Ca2+ release, was not altered by the new modulators. In addition, all agonists and antagonists could be used interchangeably in combination and introduced no significant change in the oscillatory pattern or spike shape. Since the response was solely limited to frequency modulation, over- or understimulation of the oscillatory system, there is no need to create a new oscillator or to introduce further reaction steps into the core of the model. We conclude that the regulation of [Ca2+]cyt. via the explored second-messenger pathways can be embedded into the oscillatory system as modulation of rate constants already present in this model.
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PMID:Modulation of cytosolic-[Ca2+] oscillations in hepatocytes results from cross-talk among second messengers. The synergism between the alpha 1-adrenergic response, glucagon and cyclic AMP, and their antagonism by insulin and diacylglycerol manifest themselves in the control of the cytosolic-[Ca2+] oscillations. 132 20

Interactions between the different signaling roles of myo-inositol 1,4,5-trisphosphate and 1,2-diacylglycerol, the products of agonist-stimulated phosphatidylinositol 4,5-bisphosphate breakdown, are assessed in isolated rat hepatocytes. Measurements of the kinetics of accumulation of individual [3H]inositol phosphates after the addition of different Ca2+-mobilizing agonists in general support the role of inositol 1,4,5-trisphosphate as the second messenger responsible for release of sequestered intracellular Ca2+. Various agonists, when added at maximal concentrations, however, produce qualitatively and quantitatively different responses, which reflect varying abilities of the agonists to activate phospholipase C. Qualitative differences are revealed by a pronounced biphasic pattern to the Ins(1,4,5)P3 accumulation after vasopressin and phenylephrine stimulation, which is indicative of negative feedback. It is suggested that this effect is mediated by a partial diacylglycerol activation of protein kinase C, which in vitro causes an activation of inositol phosphate 5-phosphatase and hence promotes removal of Ins(1,4,5)P3 to Ins(1,4)P2. An alternative mechanism proposed by Biden and Wollheim (1986) of a secondary Ca2+ activation of Ins(1,4,5)P3 3-kinase is considered less likely as a general mechanism, since highly purified kinase prepared from rat brain shows only an inhibition by Ca2+. Glucagon, 8-Br-cAMP, and EGF induce small increases of Ins(1,4,5)P3 in hepatocytes, together with slower and smaller increases of cytosolic free Ca2+ than those produced by vasopressin or phenylephrine, with Ca2+ being mobilized from the same intracellular pools with each of the agonists. The Ca2+-mobilizing effect of glucagon, therefore, may be entirely due to a cAMP-dependent process, although a direct receptor-mediated activation of phospholipase C, as suggested by Wakelam et al. (1986), remains a possibility. The EGF receptor appears to be coupled to phospholipase C, presumably via a G-protein. It is speculated that the mechanism by which cAMP increases Ins(1,4,5)P3 levels in hepatocytes could either be by phosphorylation and inhibition of inositol phosphate 5-phosphatase or by phosphorylation and facilitation of the coupling between the G-protein and phospholipase C. When protein kinase C is maximally activated by pretreatment of hepatocytes with PMA, the stimulatory effects of phenylephrine, glucagon, 8-Br-cAMP, and EGF on the accumulation of inositol phosphates and increase of cytosolic free Ca2+ are largely inhibited.(ABSTRACT TRUNCATED AT 400 WORDS)
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PMID:Mechanisms involved in receptor-mediated changes of intracellular Ca2+ in liver. 285 Jun 13

We have augmented our previous studies [Storey, Shears, Kirk & Michell (1984) Nature (London) 312, 374-376] on the subcellular location and properties of Ins(1,4,5)P3 (inositol 1,4,5-trisphosphate) phosphatases in rat liver and human erythrocytes. We also investigate Ins(1,3,4)P3 (inositol 1,3,4-trisphosphate) metabolism by rat liver. Membrane-bound and cytosolic Ins(1,4,5)P3 phosphatases both attack the 5-phosphate. The membrane-bound enzyme is located on the inner face of the plasma membrane, and there is little or no activity associated with Golgi apparatus. Cytosolic Ins(1,4,5)P3 5-phosphatase (Mr 77,000) was separated by gel filtration from Ins(1,4)P2 (inositol 1,4-bisphosphate) and inositol 1-phosphate phosphatases (Mr 54,000). Ins(1,4,5)P3 5-phosphatase activity in hepatocytes was unaffected by treatment of the cells with insulin, vasopressin, glucagon or dibutyryl cyclic AMP. Ins(1,4,5)P3 5-phosphatase activity in cell homogenates was unaffected by changes in [Ca2+] from 0.1 to 2 microM. After centrifugation of a liver homogenate at 100,000 g, Ins(1,3,4)P3 phosphatase activity was largely confined to the supernatant. The sum of the activities in the supernatant and the pellet exceeded that in the original homogenate. When these fractions were recombined, Ins(1,3,4)P3 phosphatase activity was restored to that observed in unfractionated homogenate. Ins(1,3,4)P3 was produced from Ins(1,3,4,5)P4 (inositol 1,3,4,5-tetrakisphosphate) and was metabolized to a novel InsP2 that was the 3,4-isomer. Ins(1,3,4)P3 phosphatase activity was not changed by 50 mM-Li+ or 0.07 mM-Ins(1,4)P2 alone, but when added together these agents inhibited Ins(1,3,4)P3 metabolism. In Li+-treated and vasopressin-stimulated hepatocytes, Ins(1,4)P2 may reach concentrations sufficient to inhibit Ins(1,3,4)P3 metabolism, with little effect on Ins(1,4,5)P3 hydrolysis.
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PMID:Dephosphorylation of myo-inositol 1,4,5-trisphosphate and myo-inositol 1,3,4-triphosphate. 303 88

In amphibian liver, signal transduction of [Arg8]vasotocin (AVT), a "classical" Ca(2+)-dependent hormone in rat liver, is mediated via the generation of adenosine 3',5'-cyclic monophosphate (cAMP) and not via inositol 1,4,5-trisphosphate [Ins(1,4,5)P3]. In isolated hepatocytes from axolotl, hormones that stimulated cAMP formation (the order of efficacy was glucagon > isoprenaline > epinephrine > or = AVT) also provoked a pronounced increase in cytosolic Ca2+, as indicated from changes in fura 2 fluorescence. 8-Bromoadenosine 3',5'-cyclic monophosphate at 100 microM was as potent as maximally effective concentrations of glucagon. Ins(1,4,5)P3 mobilized Ca2+ from the endoplasmic reticulum of saponin-permeabilized axolotl hepatocytes with a half-maximal effect at 0.65 microM, as did GTP (20 microM), even in the absence of polyethylene glycol. However, the hormonally induced increase in cytosolic Ca2+ was not due to a mobilization of the cation from internal stores by Ins(1,4,5)P3, but to an increased inflow from the extracellular medium. We conclude that in axolotl liver, in contrast to rat liver, hormones stimulate the production of cAMP that, in addition to stimulating processes such as glycogenolysis, also regulates the opening of an ion gate in the plasma membrane, which allows the inflow of Ca2+. To our knowledge this is the first demonstration of a second messenger-operated Ca2+ channel in a splanchnic tissue.
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PMID:Hormone-induced rise in cytosolic Ca2+ in axolotl hepatocytes: extracellular origin and control by cAMP. 823 80

We have investigated the transduction pathways mediating the contractile effect of two glucagon-containing peptides, glicentin (GLIC) and oxyntomodulin (OXM), on smooth muscle cells isolated from rabbit antrum. Low concentrations of GLIC induced a biphasic and rapid (first phase at 5-8 sec) Ins(1,4,5)P3 production. By comparison, higher concentrations of OXM or OXM(19-37) were required to obtain biphasic time-courses of Ins(1,4,5)P3 production. In a Ca2+ free medium, the first phase of Ins(1,4,5)P3 production induced by GLIC or OXM was maintained, while the second phase disappeared. In saponin-permeabilized cells, all three peptides induced cell contraction with similar efficacies and potencies. Exogenous Ins(1,4,5)P3 mimicked the contractile effect of the peptides and heparin, which inhibits the Ins(1,4,5)P3 binding to its receptor, prevented contraction stimulated by each effector. We conclude that a Ca2+ mobilization from the intracellular stores is essential in the contractile effects of GLIC and OXM. Using the fluo-3 probe, a [Ca2+]i increase was observed in the presence of GLIC, OXM, or OXM(19-37). The three peptides reduced by 30-40% the cAMP content of cells stimulated by forskolin. This effect was pertussis toxin sensitive as demonstrated with OXM(19-37). Our data constitute important clues for the existence in smooth muscle cells of receptor(s) specific for the GLIC/OXM hormones, coupled via G protein(s) to both Ca2+ and cAMP pathways.
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PMID:Glicentin and oxyntomodulin modulate both the phosphoinositide and cyclic adenosine monophosphate signaling pathways in gastric myocytes. 988 2

We highlight a recent paper which documents the important role that Ca2+ release through type-1 Inositol 1,4,5-trisphosphate receptor (IP3R1) plays in the acute regulation by glucagon of gluconeogenesis in hepatocytes. The specificity is likely the result of discrete localization close to mitochondria and PKA-dependent phosphorylation of IP3R1 which enhances Ca2+ release.
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PMID:Pivotal role of type-1 inositol 1,4,5-trisphosphate receptor for glucagon-induced gluconeogenesis. 3256 74