|MadSci Network: General Biology|
Dear Vince, That's a good question. You can find more details in any good college-level biochemistry text. Here goes: Both adrenaline and noradrenaline (catecholamine hormones) are secreted by the adrenal medulla into the bloodstream and by sympathetic neurons in response to low blood glucose levels. Like glucagon, they stimulate the mobilization of glycogen and triacylglycerols by triggering the production of cyclic AMP (cAMP). When adrenaline (or glucagon) binds its receptor (beta-adrenergic receptors or glucagon receptors), it activates the enzyme responsible for the formation of cAMP. cAMP in turn, activates an enzyme called protein kinase A, which phosphorylates other proteins (that is, covalently attaches a phosphate group to certain amino acid residues in specific proteins). Phosphorylation alters the enzymatic activity of these enzymes. In our case, the end result is the activation of glycogen phosphorylase which causes the breakdown of glycogen and simultaneously the inhibition of glycogen synthase which is responsible for the synthesis of glycogen. Adrenaline and noradrenaline differ from glucagon in that their glycogenolytic effect is greater in muscle than in liver. They also inhibit the uptake of glucose by muscle. Instead, fatty acids released from adipose tissue are used as fuel. Adrenaline also stimulates the secretion of glucagon and inhibits the secretion of insulin. Thus, catecholamines such as adrenaline and noradrenaline increase the amount of glucose released into the blood by the liver and decrease the utilization of glucose by muscle. Hope this helps, Terry
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