Regulation Of Glycogen Phosphorylase Activity Research Articles

In Vitro Hormonal Regulation of Glycogen Phosphorylase Activity in Fat Body of the Tropical Cockroach, Blaberus discoidalis

Blaberus hypertrehalosemic hormone (Bld-HTH)-dependent glycogen phosphorylase activation was investigated using in vitro fat bodies from the cockroach, Blaberus discoidalis. Resting levels of active phosphorylase were decreased by the presence of trehalose and glucose. Phosphorylase activation was dose-responsive to Bld-HTH and increased ca. 3-fold over a range of 0.02 to 2 n M Bld-HTH. Maximum phosphorylase activation required only 5-min exposure to Bld-HTH; reversion to the inactive state began within 15 min after Bld-HTH removal and was completed by 60 min. Octopamine also activated phosphorylase but required 10 3-fold higher concentrations than did Bld-HTH. Concentrations of Bld-HTH and octopamine that increased active phosphorylase did not elevate fat body cAMP levels, although a high concentration of octopamine increased tissue cAMP levels. cAMP did not increase phosphorylase activity, but Ca 2+ was important for both Bld-HTH-and octopamine-dependent phosphorylase activation.

Regulation of glycogen phosphorylase activity in isolated human hepatocytes.

Hepatocytes were isolated from human liver tissue by a two-step perfusion technique. They were treated with vasopressin, angiotensin, ATP and phenylephrine, which are known to be Ca(2+)-mediated glycogenolytic agents in rat liver tissue, and as a control, they were treated with the cyclic AMP-mediated hormones glucagon and isoproterenol. All agonists induce a time-dependent activation of glycogen phosphorylase. Glucagon and isoproterenol induce a somewhat higher degree of phosphorylase activation compared with vasopressin, angiotensin, ATP and phenylephrine, which all increase inositol tris-phosphate levels and have no effect on the cyclic AMP levels. The total activity of glycogen phosphorylase (a + b), amounting to 30 to 35 mU/mg protein, is found to be much lower than that found in rat liver tissue. Because only minor differences could be found, we conclude that the regulation of glycogen phosphorylase in human liver tissue is basically the same as that found in rat liver tissue.

Temporal integration of α1-adrenergic responses in BC3H-1 muscle cells: Regulation of glycogen phosphorylase activity

Abstract Regulation of Ca2+-dependent glycogen phosphorylase activity by alpha 1-adrenergic and H1-histamine receptors has been examined in BC3H-1 muscle cells. Stimulation by either norepinephrine or histamine elevates the phosphorylase activity ratio within 5 s from a resting value of 0.37 +/- 0.03 to maximal values of 0.8-0.9. Phosphorylase activation by alpha-adrenergic agonists is sustained over 20-30 min of agonist exposure, whereas histamine exposure only transiently activates phosphorylase during the initial 5 min of stimulation. The initial activation of phosphorylase by either receptor is not attenuated by treated cells with Ca2+-deficient and [ethylenebis(oxyethylenenitrilo)]tetraacetic acid-supplemented buffer, whereas the response to sustained adrenergic stimulation depends largely, but not totally, upon extracellular Ca2+. The involvement of protein kinase C in agonist responses was tested by treating cells with phorbol 12-myristate 13-acetate. Phorbol 12-myristate 13-acetate inhibits receptor-mediated mobilization of intracellular Ca2+ (IC50 = 3.6 nM) yet activates phosphorylase independently of agonist. Phorbol 12-myristate 13-acetate has no effect on cellular 45Ca2+ fluxes in the absence of agonist. Thus, the two receptors coordinately regulate intracellular signaling through Ca2+- and protein kinase C-mediated pathways. alpha 1-Adrenergic receptors elicit sustained phosphorylase activation whereas H1-histaminergic receptors desensitize.

Regulation of glycogen phosphorylase activity in fat body of Locusta migratoria and Periplaneta americana

Saline extracts of the corpus cardiacum (CC) of Locusta migratoria activate glycogen phosphorylase in locust fat body. The response of phosphorylase to CC extracts and to synthetic adipokinetic hormone (AKH) suggests that the factor responsible for the activating effect of the CC on phosphorylase is AKH, supplemented to a minor degree with Compound II. Octopamine does not influence fat body phosphorylase activity in locusts, however, it elicits a rapid short-term hyperlipemia. In cockroaches, Periplaneta americana, injection of octopamine results in a strong activation of fat body phosphorylase within 1 min. Cockroach CC extract exerts a more prolonged effect on phosphorylase activity than does octopamine.

Mechanisms of regulation of glycogen phosphorylase activity in Saccharomyces carlsbergensis.

The content of glycogen phosphorylase (1,4-alpha-D-glucan: orthophosphate alpha-D-glycosyl-transferase, EC 2.4.1.1) in yeast (Saccharomyces carlsbergensis) cells depended on the growth phase. Cells of the early exponential phase under carbohydrate-limited conditions showed low, but significant, phosphorylase activity; the activity markedly increased in the late exponential growth phase, concomitant with the appearance of measurable phosphorylase antigen. This pointed to an induction of the enzyme. During the interexponential phase (the slow proliferation phase during the diauxic growth of the culture, when the cells switch to utilization of accumulated ethanol) and the stationary growth phase, phosphorylase concentration remained constant while its specific molecular activity increased further, probably caused by conversion of the enzyme to an active form. During transition of stationary phase cells to growth, phosphorylase activity and concentration slowly decreased in the cells at a rate compatible with dilution by newly synthesized proteins. A residual activity always remained, which could be attributed to the presence of active phosphorylase, detectable by activity staining after gel electrophoresis in the presence of glycogen. No direct correlation could be detected between the specific molecular activity of phosphorylase and glycogen metabolism. This indicated that covalent modification of the enzyme regulated the total capacity of the enzyme available to the cell, rather than the actual activity limiting glycogen breakdown.