Glycogen metabolism hormones

VITAMIN Be IS IMPORTANT IN AMINO ACID GLYCOGEN METABOLISM IN STEROID HORMONE ACTION... 

Hormones and neuronal activity affect brain glycogen metabolism 538... 

Hormones and neuronal activity affect brain glycogen metabolism. Glycogen is affected by hormones endogenous to the brain including vasoactive intestinal peptide and noradrenaline, as well as circulating hormones, such as insulin [61, 63, 64]. The mechanism whereby insulin exerts an effect on glycogen metabolism in brain has not been determined [63]. Glycogen metabolism in brain, unlike in other tissues, is controlled locally, due to differential local metabolic rates. 

Diacylglycerol, on the other hand, is lipid soluble and remains in the lipid bilayer of the membrane. There it can activate protein kinase C (PKC), a very important and widely distributed enzyme which serves many systems through phosphorylation, including neurotransmitters (acetylcholine, a,- and P-adrenoceptors, serotonin), peptide hormones (insulin, epidermal growth hormone, somatomedin), and various cellular functions (glycogen metabolism, muscle activity, structural proteins, etc.), and also interacts with guanylate cyclase. In addition to diacylglycerol, another normal membrane lipid, phos-phatidylserine, is needed for activation of PKC. The DG-IP3 limbs of the pathway usually proceed simultaneously. 

Fig. 7.18. Regulation of glycogen metabolism in muscle. Phosphorylase kinase stands at the center of regulation of glycogen metabolism. Phosphorylase kinase may exist in an active, phosphorylated form and an inactive, unphosphorylated form. Phosphorylation of phosphorylase kinase is triggered by hormonal signals (e.g. adrenahne) and takes place via an activation of protein kinase A in the cAMP pathway. In the absence of hormonal stimulation, phosphorylase kinase can also be activated by an increase in cytosolic Ca. The active phosphorylase kinase stimulates glycogen degradation and inhibits glycogen synthesis, in that, on the one side, it activates glycogen phosphorylase by phosphorylation, and on the other side, it inactivates glycogen synthase by phosphorylation.

Hormonal control Glycogen metabolism is tightly controlled by hormones. When blood glucose by epinephrine levels fall, glucagon is secreted by the a cells of the pancreas and acts on the and glucagon liver to stimulate glycogen breakdown to glucose which is then released into... 

Recently Exton and co-workers [93] have proposed that adrenergic responsiveness in skeletal muscle is regulated by thyroid hormones at two levels, i.e., 1) stimulation of /3-adrenergic receptors and adenylate cyclase activity and 2) increased activity of phosphoprotein phosphatases. Such results would explain the effect of thyroid hormones on glycogen metabolism in muscle although the primary mechanism of these actions remains unknown. 

Figure 21.17 Coordinate control of glycogen metabolism. Glycogen metabolism is regulated, in part, by hormone triggered cyclic AMP cascades. The sequence of reactions leading to the activation of protein kinase A ultimately activates glycogen degradalion. At the same time, protein kinase A also inactivates glycogen synthase, shutting down glycogen synthesis.

Finally, no attempt will be made to review the extensive literature on the influence of various hormones on glycogen metabolism with regard to insulin, the view that its major action is to regulate the permeability of cells to D-glucose, rather than to control the activity of certain enzymes, should not be overlooked. 

Glycogen Metabolism Is Regulated by Hormone-Induced Activation of Protein Kinase A... 

Short-term hormonal control of protein phosphatases involved in hepatic glycogen metabolism, W. Stalmans, M. Bollen, B. Toth, and P. Gergely, 1990, Adv. Enzyme Reg., 305327. 

The opposing effects of the hormones insulin and glucagon on glycogen metabolism. 

Begins with hydrolysis of fat to glycerol and free fatty acids (is hormonally regulated - comparable to glycogen metabolism - Figure 18.11)... 

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