Stimulation of glycogenolysis in isolated hepatocytes by adenosine and one of its analogues is inhibited by caffeine.
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ABSTRACT:
SUBMITTER: Stanley JC
PROVIDER: S-EPMC1148479 | biostudies-other | 1987 Nov
REPOSITORIES: biostudies-other
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Project description:Addition of the opioid peptides, [Leu]enkephalin and [Met]enkephalin, to isolated hepatocytes was shown to produce a stimulation of glycogenolysis comparable with that observed in the presence of maximal concentrations of glucagon, adrenaline or angiotensin. This stimulation was demonstrated to be the result of an activation of phosphorylase by a rapid Ca2+-dependent mechanism and was not decreased by the presence or either alpha- or beta-adrenergic antagonists, although it was dependent on the presence of the N-terminal tyrosine residue in the enkephalin molecule. It is suggested that this may be further evidence for specific opioid receptors in the liver. Addition of [Leu]enkephalin also inhibited lactate formation, indicating that the opioid peptides exert a concerted effect on hepatic carbohydrate metabolism to enhance glucose output. The transient nature of the effect of the enkephalins was shown to be the result of a rapid breakdown of the peptides in the incubation as a result of aminopeptidase activity, the initial product being the inactive des-tyrosine derivative.
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Project description:The opioid peptides [Leu]enkephalin and dynorphin-(1-13) were shown to enhance glycogen breakdown when added directly to hepatocytes. This was the result of a concerted effect on the enzymes of glycogen metabolism, with a stimulation of glycogen phosphorylase activity and a simultaneous decrease in glycogen synthase I activity. The latter only became significant when the enzyme was activated by incubating the cells in presence of 20 mM- or 40 mM-glucose. The effect of the opioid peptides was independent of an increase in cyclic AMP or any change in the activity ratio of the cyclic AMP-dependent protein kinase and was abolished by depleting the cells of Ca2+. Both [Leu]enkephalin and dynorphin-(1-13) produced a significant decrease in cyclic AMP formation, suggesting that in liver, as in neuronal tissue, they may act by inhibiting adenylate cyclase activity.
| S-EPMC1147166 | biostudies-other
Project description:Infusion of adenosine into perfused rat livers resulted in transient increases in glucose output, portal-vein pressure, the effluent perfusate [lactate]/[pyruvate] ratio, and O2 consumption. 8-Phenyltheophylline (10 microM) inhibited adenosine responses, whereas dipyridamole (50 microM) potentiated the vasoconstrictive effect of adenosine. The order of potency for adenosine analogues was: 5'-N-ethylcarboxamidoadenosine (NECA) greater than L-phenylisopropyladenosine greater than cyclohexyladenosine greater than D-phenylisopropyladenosine greater than 2-chloroadenosine greater than adenosine, consistent with adenosine actions modulated through P1-purine receptors of the A2-subtype. Hepatic responses exhibited homologous desensitization in response to repeated infusion of adenosine. Adenosine effects on the liver were attenuated at lower perfusate Ca2+ concentrations. Indomethacin decreased hepatic responses to both adenosine and NECA. Whereas adenosine stimulated glycogen phosphorylase activity in isolated hepatocytes, NECA caused no effect in hepatocytes. The response to adenosine in hepatocytes was inhibited by dipyridamole (50 microM), but not 8-phenyltheophylline (10 microM). The present study indicates that, although adenosine has direct effects on parenchymal cells, indirect effects of adenosine, mediated through the A2-purinergic receptors on another hepatic cell type, appear to play a role in the perfused liver.
| S-EPMC1148497 | biostudies-other
Project description:The opioid agonists [leucine]enkephalin, [D-Ala2,D-Leu5]enkephalin and dynorphin-(1-13)-peptide, but not morphine, stimulated the conversion of [2-14C]pyruvate into glucose and glycogenolysis when added directly to isolated hepatocytes. Naloxone produced a small but significant inhibition of both the basal and stimulated rate of incorporation of label into glucose but had no effect on the total glucose output by the cells. The effects of the opioid peptides were mediated by a cyclic AMP-independent mechanism.
| S-EPMC1152532 | biostudies-other
Project description:Adenosine signaling is involved in glucose metabolism in hepatocytes and myocytes in vitro. However, no information is available regarding the effect of adenosine on glucose metabolism in vivo. Thus, we examined how extracellular adenosine acts on glucose metabolism using mice. Subcutaneous injections of adenosine (10, 25, and 50 mg/kg bodyweight) dose-dependently increased blood glucose levels, with the peak occurring at 30 min post injection. At 30 min after adenosine injection (25 mg/kg bodyweight), glycogen content in the liver, but not the skeletal muscle, was significantly decreased. Hepatic glycogen depletion by fasting for 12 h suppressed the increase of blood glucose levels at 30 min after adenosine injection. These results suggest that adenosine increases blood glucose levels by stimulating hepatic glycogenolysis. To investigate the effect of adenosine on the adrenal gland, we studied the glycogenolysis signal in adrenalectomized (ADX) mice. Adenosine significantly increased the blood glucose levels in sham mice but not in the ADX mice. The decrease in hepatic glycogen content induced by adenosine in the sham mice was partially suppressed in the ADX mice. The level of plasma corticosterone, the main glucocorticoid in mice, was significantly increased in the sham mice by adenosine but its levels were low in ADX mice injected with either PBS or adenosine. These results suggest that adenosine promotes secretion of corticosterone from the adrenal glands, which causes hepatic glycogenolysis and subsequently the elevation of blood glucose levels. Our findings are useful for clarifying the physiological functions of adenosine in glucose metabolism in vivo.
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Project description:The glycogenolytic potency of adenosine and ATP was studied in adult rat hepatocytes and compared with the action of glucagon and noradrenaline. In cells cultured for 48 h, adenosine and ATP as well as their analogues 2-chloroadenosine, phenylisopropyladenosine, N-ethylcarboxamidoadenosine and beta-gamma-methylene-substituted ATP (p[CH2]ppA) increased glycogen phosphorylase alpha to levels indistinguishable from those obtained by the addition of glucagon or noradrenaline. The P1 receptor antagonist 8-phenyltheophylline abolished the activation of phosphorylase by adenosine and by p[CH2]ppA, but not that by ATP. Protein kinase A was activated by p[CH2]ppA and ATP via their breakdown to adenosine. [14C]Glucose production from glycogen was stimulated only 3-fold by ATP and adenosine, compared with a 7-fold increase produced by the hormones. Stimulation of glucose production by glucagon or noradrenaline was almost completely abolished by ATP or adenosine, with half-maximal effects at around 10 microM. The non-degradable adenosine analogues were equipotent with glucagon with respect to stimulation of glucose production, and their action was also inhibited by adenosine. ATP and p[CH2]ppA, which were both degraded to adenosine, showed comparable metabolic effects, whereas the alpha, beta-methylene analogue was without biological action and also was not degraded to adenosine. In the presence of the adenosine transport inhibitor nitrobenzyl thioinosine (NBTI), adenosine exerted an increased glycogenolytic potency, reaching 80% of the maximal stimulation obtained by glucagon. The glucagon-antagonistic effect of adenosine could be completely abolished by NBTI, but was not affected by phenyltheophylline. It is concluded that, in the hepatocyte culture system, adenosine and ATP decrease the catalytic efficiency of phosphorylase alpha through signals arising from their uptake into the cell.
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Project description:The motor symptoms of Parkinson's disease (PD) are primarily due to the degeneration of the dopaminergic neurons in the nigrostriatal pathway. However, several other brain areas and neurotransmitters other than dopamine such as noradrenaline, 5-hydroxytryptamine and acetylcholine are affected in the disease. Moreover, adenosine because of the extensive interaction of its receptors with the dopaminergic system has been implicated in the pathophysiology of the disease. Based on the involvement of these non-dopaminergic neurotransmitters in PD and the sometimes severe adverse effects that limit the mainstay use of dopamine-based anti-parkinsonian treatments, recent assessments have called for a broadening of therapeutic options beyond the traditional dopaminergic drug arsenal. In this review we describe the interactions between dopamine and adenosine receptors that underpin the pre-clinical and clinical rationale for pursuing adenosine A(2A) receptor antagonists as symptomatic and potentially neuroprotective treatment of PD. The review will pay particular attention to recent results regarding specific A(2A) receptor-receptor interactions and recent findings identifying urate, the end product of purine metabolism, as a novel prognostic biomarker and candidate neuroprotectant in PD.
| S-EPMC3102301 | biostudies-literature
Project description:The monohydroxy bile acid taurolithocholate (TLC) causes a rapid and transient increase in free cytosolic Ca2+ concentration ([Ca2+]i) in suspensions of rat hepatocytes similar to that elicited by the InsP3-dependent hormone vasopressin. The effect of the bile acid is due to a mobilization of Ca2+, independent of InsP3, from the endoplasmic reticulum (ER). Short-term preincubation of cells with the phorbol ester 4 beta-phorbol 12 beta-myristate 13 alpha-acetate (PMA), which activates protein kinase C (PKC), blocked the increase in [Ca2+]i induced by TLC, but did not alter that mediated by vasopressin. We obtained the following results, indicating that the effect of PMA is mediated by the activation of PKC. (1) Phorbol esters were effective over a concentration range where they activate PKC (IC50 = 0.5 nM); (2) phorbol esters that do not activate PKC did not inhibit the effects of TLC; (3) the permeant analogue oleoylacetylglycerol mimicked the inhibitory effect of PMA; (4) lastly, the inhibition of the TLC-induced Ca2+ mobilization by phorbol esters was partially prevented by preincubating the cells with the PKC inhibitors H7 and AMG-C16. Preincubating hepatocytes with PMA had no effect on the cell uptake of labelled TLC, indicating that the phorbol ester does not interfere with the transport system responsible for the accumulation of bile acids. In saponin-treated liver cells, PMA added before or after permeabilization failed to abolish TLC-induced Ca2+ release from the ER. The possibility is discussed that PMA, via PKC activation, may alter the intracellular binding or the transfer of bile acids in the liver.
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Project description:The newly isolated peptide PHI provoked a dose-dependent stimulation of glycogenolysis and gluconeogenesis in isolated rat hepatocytes; at 1 microM-PHI, both processes were increased 1.6-fold as compared with basal values. These PHI-mediated effects were accompanied by the activation of glycogen phosphorylase and the inactivation of pyruvate kinase. PHI (1 microM) also caused a 2-fold increase in hepatocyte cyclic AMP.
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Project description:1. The alpha-adrenergic agonists noradrenaline (in the presence of beta-blocker) and phenylephrine cause a transient stimulation of the respiration in isolated rat hepatocytes. After a lag period of 12s, this activation first attains its maximal value (+24%) for about 1 min and then falls to a sustained value (+15%). The effect is blocked by the alpha-antagonists phenoxybenzamine and phentolamine. It is dose-dependent, with an half-maximal stimulation by 16 nM-noradrenaline, which is similar to that found for other cell responses to the hormone. 2. Vasopressin and ATP, which in common with alpha-agonists are believed to increase intracellular [Ca2+], induce similar activation in the respiration rate. 3. The alpha-adrenergic-mediated respiration depends on extracellular Ca2+. The activation is decreased or abolished when extracellular [Ca2+] is decreased by adding EGTA, or when the Ca2+ antagonists Mn2+ and La3+ are present in the incubation medium. 4. It is suggested that the activation of the mitochondrial respiration rate results from the increase in cytosolic Ca2+ concentration, presumably via Ca2+ influx or Ca2+ release from the plasma membrane or endoplasmic reticulum.
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