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)

The type I adenylyl cyclase is directly stimulated by Ca2+ and calmodulin in vivo (Choi, E. J., Wong, S. T., Hinds, T. R. and Storm, D. R. (1992) J. Biol. Chem. 267, 12440-12442; Wu, Z., Wong, S. T., and Storm, D. R. (1993) J. Biol. Chem. 268, 23766-23768). In this study, we examined the sensitivity of the type I adenylyl cyclase expressed in HEK-293 cells to beta-adrenergic agonists or glucagon when intracellular Ca2+ was elevated by Ca2+ ionophore or carbachol. Although previous studies have shown that this enzyme can be directly stimulated by activated Gs in vitro, we demonstrate that it is not stimulated by Gs-coupled receptors in vivo. However, the enzyme was stimulated by Gs-coupled receptors in vivo when it was activated by intracellular Ca2+. For example, the Ca2+ ionophore A23187 stimulated the enzyme 3 +/- 0.5-fold (n = 9) and isoproterenol alone did not stimulate the enzyme, but the combination of the two stimulated type I adenylyl cyclase 13 +/- 2-fold (n = 9) in vivo. Similarly, 500 nM glucagon alone did not stimulate the enzyme but the combination of A23187 and glucagon activated the enzyme 90 +/- 8-fold (n = 4). Synergistic stimulation of type I adenylyl cyclase activity was also obtained with combinations of carbachol and isoproterenol or glucagon. This phenomenon was not observed with a mutant enzyme that is insensitive to Ca2+ and calmodulin, suggesting that conformational changes caused by binding of calmodulin to the type I adenylyl cyclase enhance binding or coupling to activated Gs. These data illustrate that this adenylyl cyclase can couple Ca2+ and neurotransmitter signals to generate optimal cAMP levels, a property of the enzyme that may be important for its role in learning and memory in mammals.
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PMID:Synergistic activation of the type I adenylyl cyclase by Ca2+ and Gs-coupled receptors in vivo. 792 37

Anion-exchange chromatography of solubilized pig liver cell membranes on DEAE-Sepharose gave a fraction with high affinity binding proteins for VIP and glucagon distinct from each other. Scatchard analysis indicated the presence of one binding site for VIP (Kd 1.5 +/- 0.6 nM and Bmax 1.3 +/- 0.4 pmol/mg). The order of potency for VIP-related peptides to inhibit [125I]VIP binding was: VIP > peptide histidine isoleucine amide (PHI) > rat growth hormone releasing factor (rGRF) > secretin. GTP-gamma-S inhibited [125I]VIP binding and reduced the affinity of VIP binding sites to 6.5 nM. In the same isolated fraction, [125I]glucagon binding was displaced by glucagon preferentially to oxyntomodulin, and GTP did not affect this [125I]glucagon binding. Scatchard analysis indicated the presence of one binding site for glucagon (Kd 0.08 +/- 0.03 nM and Bmax 0.31 +/- 0.01 pmol/mg). A low-affinity VIP binding protein (IC50 0.7 microM) was detected in a fraction eluting later and exhibited a peptide specificity: rGRF > VIP > VIP(10-28) > secretin > PHI. This rGRF-preferring protein (18 kDa) was purified and had a partial amino-acid sequence identical to that of calmodulin. Its [125I]VIP binding was competitively inhibited by VIP and calmidazolium in a manner similar to that for pig brain calmodulin. Thus we have co-solubilized VIP and glucagon high affinity receptors from pig liver cell membranes and separated them from VIP-binding calmodulin.
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PMID:Simultaneous solubilization of high-affinity receptors for VIP and glucagon and of a low-affinity binding protein for VIP, shown to be identical to calmodulin. 838 69

Glucagon-producing pancreatic islet cells generate calcium-dependent action potentials. By the control of calcium influx through voltage-gated calcium channels, calcium is a tightly regulated second messenger in these cells. It is unknown whether calcium is a signal for glucagon gene transcription. Therefore, rat glucagon reporter fusion genes were transiently transfected into pancreatic islet cell lines. High potassium-induced membrane depolarization activated glucagon gene transcription. The effects of a calcium chelator, calcium channel blockers, calmodulin antagonists, and an inhibitor of calcium/calmodulin-dependent protein kinase II (CaM kinase II) indicate that depolarization-induced glucagon gene transcription depends on calcium influx and CaM kinase II. The depolarization-responsive element was mapped to the glucagon cAMP-responsive element (CRE). The CRE-binding protein CREB was shown, by using GAL4-CREB fusion proteins, to function as a depolarization-regulated transcription factor in pancreatic islet cells. Membrane depolarization and cAMP had synergistic effects on glucagon gene transcription. These results suggest that rat glucagon gene transcription is regulated by membrane electrical activity and calcium influx in pancreatic islet cells. This signal may be transmitted via CaM kinase II and CREB to the glucagon CRE.
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PMID:Membrane depolarization and calcium influx induce glucagon gene transcription in pancreatic islet cells through the cyclic AMP-responsive element. 838 30

Glucagon and dibutyryl cyclic AMP (Bt2cAMP) stimulate Na+/taurocholate (TC) cotransport and increase the intracellular Ca2+ concentration ([Ca2+]i) of hepatocytes. Whether the effect of cAMP is mediated via increases in [Ca2+]i, cAMP-dependent protein kinase (PKA), and/or protein kinase C (PKC) was investigated in this study. TC uptake and [Ca2+]i were determined in isolated rat hepatocytes using [14C]TC and the fluorescent dye quin-2, respectively. Bt2cAMP, forskolin, and 8-bromo-cAMP stimulated Na(+)-dependent, but not Na(+)-independent TC uptake. Bt2cAMP increased the maximal rate of Na+/TC cotransport without affecting the apparent Km. Increases in TC uptake and [Ca2+]i by Bt2cAMP were inhibited in hepatocytes preloaded with bis-(2-amino-5-methylphenoxy)-ethane-N,N,N',N'-tetraacetic acid (MAPTA) or preincubated with 8-diethylaminooctyl 3,4,5-trimethoxybenzoate (TMB8). Calmodulin antagonists inhibited Bt2cAMP-induced increases in TC uptake, but not [Ca2+]i. Other Ca(2+)-mobilizing agents (thapsigargin, vasopressin, phenylephrine, and ionomycin) increased [Ca2+]i but failed to stimulate TC uptake, indicating that an increase in [Ca2+]i alone is not a sufficient stimulus for TC uptake. However, increases in TC uptake by 1 and 10 microM Bt2cAMP were further increased by thapsigargin, indicating a permissive role for Ca2+/calmodulin. Bt2cAMP-induced increases in TC uptake and [Ca2+]i were inhibited by known inhibitors of PKA and by an activator of PKC, but they remained unaffected by a specific inhibitor of PKC. Unlike thapsigargin, vasopressin inhibited Bt2cAMP-induced increases in TC uptake. Taken together these results indicate that stimulation of hepatic Na+/TC cotransport by cAMP 1) is mediated via PKA; 2) is potentiated, but not mediated, by Ca2+/calmodulin-dependent processes; and 3) may be down-regulated by PKC.
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PMID:Role of intracellular calcium and protein kinases in the activation of hepatic Na+/taurocholate cotransport by cyclic AMP. 839 49

Glucagon enhances the electrical activity of pancreatic beta-cells. The mechanism of the glucagon-evoked enhancement of electrical activity was investigated in terms of glucose metabolism. ICR mice aged 6-12 weeks were used for experiments. Intracellular Ca2+ increased in parallel with the enhancement of electrical activity. The stimulating effect of glucagon on Ca2+ oscillation was suppressed by calmodulin-antagonists (Chlorpromazine, W-7, and trifluoperazine). To trace the glucagon-evoked change in glucose metabolism, the reduced pyridine nucleotide (NAD(P)H) fluorescence was monitored using the microfluorometry with the excitation of 360 nm and the emission of 465 nm in islet cell clusters mainly consisting of beta-cells. In the presence of 2.5 mM Ca2+ glucagon (8.6 X 10(-8) M) increased the NAD(P)H fluorescence, while in the absence of Ca2+ the hormone had no effect on the fluorescence. Extracellular Ca2+ removal from the glucagon-containing perifusion solution decreased the fluorescence to the level which had been attained before glucagon was added. Chlorpromazine (10 microM) reversed the glucagon-induced increase of NAD(P)H fluorescence as well as removing Ca2+ W-7 (15 microM) and trifluoperazine (30 microM) also suppressed the glucagon-induced increase of NAD(P)H. These results suggest that Ca2+/calmodulin system is involved in the acceleration of glycogenolysis by glucagon in beta-cells. On the basis of these observations, the mechanism of glucagon-induced enhancement of electrical activity and the relative ineffectiveness of glucagon at low glucose concentrations were discussed.
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PMID:Glucagon induces Ca2+-dependent increase of reduced pyridine nucleotides in mouse pancreatic beta-cells. 859 11

Recent immunohistochemical findings suggested that a constitutive nitric oxide synthase (cNOS) resides in endocrine pancreas. Here we provide direct biochemical evidence for the presence of cNOS activity in isolated islets. The regulating influence of this nitric oxide synthase (NOS) activity for islet hormone release was also investigated. We observed that cNOS activity could be quantitated in islet homogenates by monitoring the formation of L-citrulline from L-arginine using an Amprep CBA cation-exhange minicolumn before derivatization with o-phthaldialdehyde and subsequent high-performance liquid chromatography analysis. The islet NOS was dependent on both Ca2+ and calmodulin and suppressed by the NOS inhibitor NG-nitro-L-arginine methyl ester (L-NAME). This effect was enantiomerically specific. Islet insulin release induced by a mixture of L-arginine and glucose was enhanced by L-NAME, whereas L-arginine-induced glucagon release was inhibited. The effect of L-NAME on insulin release was dose dependently potentiated by increasing glucose concentrations, suggesting that glucose is an important regulator of islet NO production. Complementary in vivo studies showed similar results, i.e., the insulin secretory response to a mixture of glucose and L-arginine was extremely enhanced by pretreatment with L-NAME, whereas L-arginine-stimulated glucagon response was suppressed. Finally, in isolated islets, the intracellular nitric oxide (NO) donor hydroxylamine suppressed insulin release and increased glucagon release. In summary, the islets of Langerhans contain a constitutive, Ca2+/calmodulin-dependent isoform of NOS. Islet NO suppressed insulin but enhanced glucagon secretion. The data also suggest a negative feedback by NO on glucose-induced insulin release. The islet NO system is a novel and important regulatory factor in insulin and glucagon secretion.
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PMID:Islet constitutive nitric oxide synthase: biochemical determination and regulatory function. 876 45

In a previous study we demonstrated that the administration of 20 micrograms/kg b.wt. of glucagon to rats caused a significant diminution of hepatic cytosolic glutathione S-transferase (GST) activity. This inhibition was non-competitive and reversible. We suggested that the effect would be mediated by cytosolic effectors. The present work was performed to characterize the mechanism involved in this inhibition. Liver tissue slices (170 to 200 mg) were incubated during different periods of time (0, 5, 10, 15, 20 and 30 min.) with several concentrations of glucagon (10(-5) M, 10(-8) M and 10(-10) M), dibutiryl cyclic AMP (10(-4) M, 10(-6) M and 10(-9) M), divalent cation ionophore A23187 (10(-4) M, 10(-6) M and 10(-9) M) or vasopressin (10(-7) M, 5 x 10(-7) M and 10(-8) M). The incubation was done with or without calcium in the medium. In all cases the cytosolic GST activity were determined in liver slices. The percentage of inhibition of GST activity was directly related to the increase of concentration of the test substances. An inhibition between 40% to 45% after 10 min. of incubation with the highest concentrations was observed (except vasopressin which caused 10% of inhibition). 10(-10) M glucagon did not produce a decrease of GST activity. The inhibition disappeared in calcium-free incubated slices, but direct relationship between plasma-membrane calcium influx and inhibition of GST activity (r = 0.950, P < 0.001, n = 24) could be obtained. By using calmodulin antagonists, we conclude that the inhibition process of the enzyme was mediated by calmodulin. In summary, we propose that plasma-membrane calcium influx induced by high concentrations of glucagon activates calmodulin, which promotes a modification (actually a methylation, according to other authors) on GST, thereby causing a decrease in its activity.
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PMID:Role of calcium fluxes in the action of glucagon on cytosolic glutathione S-transferase activity in rat liver slices. 877 42

Cold-induced expression of heat-shock proteins (HSPs) has been suggested to facilitate thermogenesis in brown adipose tissue (BAT). However, the regulation of this response and the mechanism supporting this facilitation have not been established. Because of the significant role of insulin in maintaining BAT thermogenesis, we employed a transgenic mouse model of diabetes to investigate the regulation and function of HSPs in BAT thermogenesis. These transgenic mice overexpress a calmodulin minigene regulated by the rat insulin II promotor, resulting in severe diabetes characterized by elevated blood glucose and glucagon that coincides with reduced serum and pancreatic insulin. Body temperature (Tb) of diabetic mice dropped significantly faster during a 3-h cold exposure (6 degrees C) than Tb of similarly treated control littermates. Cold exposure resulted in increased levels of constitutive and inducible HSP70 transcripts in control mice, but only constitutive HSP70 mRNA transcripts were induced in diabetic mice. Diabetes did not affect uncoupling protein induction, but cold-induced expression of members of other HSP families was reduced. Correspondingly, heat-shock regulatory factors were not activated in diabetic mice even though these factors were present. Phenylephrine induced HSP70 expression in control and diabetic animals, indicating that alpha-receptor-coupled HSP induction remained intact in BAT of diabetic mice. Insulin replacement restored the Tb response of diabetic mice as well as the HSP response. From these results it is clear that physiological signals that regulate cold-induced activation of BAT also regulate HSP expression in this tissue. This diabetic model provides a novel system in which the HSP response to cold has been selectively knocked out, making it a useful tool for the study of HSP regulation and function in BAT.
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PMID:Thermoregulatory and heat-shock protein response deficits in cold-exposed diabetic mice. 878 Feb 16

1. Previous studies have shown that vitamin D3 deficiency impairs the insulin response to glucose via an alteration of signal transduction pathways, such as Ca2+ handling and the phosphoinositide pathway. In the present study the adenylyl cyclase pathway was examined in islets from 3 independent groups: normal rats, 4 weeks-vitamin D3 deficient rats and one week-1,25 dihydroxyvitamin D3 (1,25(OH)2D3) treated rats. 2. We found that the very low rate of insulin release observed in vitamin D3 deficient rats could be restored in vitamin D3 deficient islets only with high concentrations of dioctanoyl-cyclic AMP (DO-cyclic AMP), whereas 1,25(OH)2D3 improved the sensitivity of the islets to this exogenous cyclic AMP analogue. 3. The beneficial effect of 1,25(OH)2D3 observed with or without DO-cyclic AMP was protein kinase A-dependent, since the addition of N-[2-(p-bromocinnamylamino) ethyl]-5-isoquinolinesulphonamide (H-89), a specific inhibitor of cyclic AMP-dependent protein kinases, decreased the insulin release of treated rats back to the level seen in vitamin D3 deficient islets. 4. The low rate of insulin release could not be consistently related to an alteration in cyclic AMP content of the islets. Indeed, low insulin response to a barium+theophylline stimulus observed in vitamin D3 deficient islets was paradoxically associated with a supranormal cyclic AMP content in the islets. 5. This paradoxical increase in cyclic AMP observed in these conditions could not be attributed to a lower total phosphodiesterase (PDE) activity, although the portion of Ca(2+)-calmodulin-independent PDE was predominant in islets from vitamin D3 deficient rats. 6. On the other hand, the higher cyclic AMP content of vitamin D3 deficient islets could be related to an increase in glucagon-induced cyclic AMP synthesis in relation to the hyperglucagonaemia previously observed in vitamin D3 deficient rats. Since higher concentrations of exogenous glucagon and higher endogenous cyclic AMP concentrations were required in vitro to restore insulin release to normal values, the cyclic AMP-dependent pathways that usually potentiate insulin secretion appeared to be less efficient in relation to an alteration in the post cyclic AMP effector system. 7. 1,25(OH)2D3 exerted a stimulating effect on insulin release via protein kinase A activation but reduced the supranormal cyclic AMP synthesis, thus exerting a differential modulatory influence on biochemical disturbances in islets induced by vitamin D3 deficiency.
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PMID:Modulatory role of 1,25 dihydroxyvitamin D3 on pancreatic islet insulin release via the cyclic AMP pathway in the rat. 920 44

Calcium is known to be of critical importance for hormone secretion in the insulin-producing B-cells of the endocrine, pancreas. Calcium-mediated intracellular signal transduction and the regulation of the concentration of free calcium in B-cells probably involve calcium-binding proteins. In the present study, we have investigated the expression of the calcium/calmodulin-dependent phosphatase, calcineurin, and the EF-hand calcium-binding protein, calretinin, in pancreata of hamsters, gerbils, and rats by immunocytochemistry. Immunocytochemical investigations of serial semithin sections of plastic-embedded pancreata revealed that calcineurin and calretinin were constantly present in islet cells of all three species. In addition to B-cells, these proteins could also be detected in glucagon (A-), somatostatin (D-), and pancreatic polypeptide (PP-) cells. Non-B-cells, especially glucagon-producing A-cells, often exhibited a significantly higher degree of immunoreactivity for both calcium-binding proteins than B-cells. Thus, calcineurin and calretinin may play distinct roles in the regulation of calcium-dependent secretory activities of the different pancreatic endocrine cell types.
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PMID:Rodent pancreatic islet cells contain the calcium-binding proteins calcineurin and calretinin. 927 32


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