After an overnight fast the liver is the main source of glucose (~9 g/hr in humans). (Figure1). The glucose is derived from hepatic glycogen stores (via glycogenolysis) and synthesis of new glucose (via gluconeogenesis). In humans over 60% (~6 g/hr) of the glucose released by the liver is metabolized by the central nervous system (which is largely dependent on glucose) and red blood cells (which are wholly reliant on glucose). Adipose tissue releases nonesterified fatty acids by hydrolysis of stored triglycerides. These fatty acids are the primary oxidizable fuel for many tissues (heart, muscle, liver). The liver can also synthesize ketone bodies from fatty acids to export to muscle and other tissues for oxidation. As liver glycogen reserves deplete, amino acids arising from net muscle protein breakdown with prolonged fasting and lactate derived from hydrolysis of stored muscle glycogen provide carbons to support gluconeogenesis and thus provide glucose to the glucose-dependent tissues .

Fig1. Overview of Glucose and lipid flux in overnight fasted and fed humans. In the fasted setting liver is the source of the glucose and a large fraction of the glucose production is taken up by the brain. Adipose releases nonesterified fatty acids (FFA) that are used by the liver and skeletal muscle. In the fed state, the intestine becomes the source of glucose. The liver switches to a glucose consumer, while brain glucose uptake is unaltered. Lipolysis is suppressed and muscle and adipose tissue glucose uptake is increased and muscle fatty acid uptake is decreased.

In the fed state, after a meal in which there is an ample supply of carbohydrate, the metabolic fuel for most tis sues switches to glucose (see Figure 1). In response to a carbohydrate-rich meal the liver switches to a glucose consumer storing the majority of the glucose carbon as glycogen, with a small amount used for lipid synthesis. In contrast, glucose uptake by the brain and red blood cells is unaltered (~6 g/hr). The release of fatty acids from adipose tissue lipolysis is suppressed and tissues primarily reliant on fatty acid oxidation switch to glucose in part due to the decrease in fatty acid supply and increased glucose availability. Any dietary glucose not taken up by the liver is taken up by peripheral tissues for oxidation or storage. It is stored in muscle as glycogen or in adipose tissue as triacylglycerol.

The formation and mobilization of reserves of triacylglycerol and glycogen, and the extent to which tissues take up and oxidize glucose, are largely controlled by the hormones insulin and glucagon that are made in the endocrine pancreas. Their effects can also be modulated by other neural and/or endocrine signals (eg, sympathetic nervous system, growth hormone). Plasma glucose concentration is a tightly controlled variable. Because of the absolute dependency of the central nervous system on glucose; we have neuroendocrine systems to protect against low blood glucose (ie, hypoglycemia).

مواضيع ذات صلة

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دورة الخلية وتخليق الدنا Cell Cycle & DNA Synthesis

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اضطرابات استقلاب النوكليوتيدات البيريميدينية

اضطرابات استقلاب النكليوتيدات البورينية

تخليق النوكليوتيدات منقوصة الاكسجين ( dNTPs )

تقويض النوكليوتيدات البيريميدينية