METABOLISM OF SPECIALIZED TISSUES

METABOLISM OF SPECIALIZED TISSUES

Metabolism in the liver:

• Most low molecular weight metabolites that appear in the blood after digestion are first carried to the liver from the intestine through the portal vein.

• Liver plays a central role in regulating the serum levels of glucose and other metabolic fuel.

• During the fed state, it takes up excess glucose for storage as glycogen or conversion to fatty acids.

• During fasting state, glycogenolysis and gluconeogenesis by the liver are major sources of glucose for the rest of the body.

•Liver serves as the major site of fatty acid synthesis.

• Liver synthesizes ketone bodies during starvation.

• Liver synthesizes plasma lipoproteins.

Role of liver in blood glucose regulation

• When blood glucose rises to a very high concentration following a meal, two-thirds of the glucose

• When blood glucose level falls, glycogenolysis starts by the activity of the enzyme glucose 6-phosphataseAbsorbed from the gut is immediately stored in the liver as glycogen (glycogenesis). Furthermore, the liver releases glucose in the blood.

• When glycogen reserve diminishes, gluconeogenesis starts in the liver from glucogenic amino acids of the body proteins to maintain blood glucose level.

Metabolism in skeletal muscle:

• Skeletal muscle is the most significant consumer of metabolic fuel and oxygen due to its high mass compared with other tissues.

• Skeletal muscle maintains abundant glycogen stores, which provide a source of glucose for energy during exertion.

• In resting muscle, the preferred fuel is fatty acids.

• Muscle protein may be mobilized as a fuel source if no other fuel is available.

• Pyruvate, the product of glycolysis may be converted to either lactate or alanine and exported to the liver where it is used to regenerate glucose by the process of gluconeogenesis.

Metabolism in adipose tissue:

• Adipose tissue stores metabolic fuel in the form of triacylglycerol. During the fed state, the adipose tissue synthesizes triacylglycerol from fatty acids and glucose.

 • During the fasting state, triacylglycerols are converted to glycerol and fatty acids, which are exported to the liver and other tissues.

Metabolism in the brain:

• The brain contains no fuel reserves and must be continuously supplied with fuel.

Brain tissue uses glucose as an exclusive fuel. During starvation, the brain adapts to use ketone bodies.