Glycogen tissue reserves

In starvation, glucose must be ptovided for the brain and erythrocytes initially, this is supphed from hver glycogen reserves. To spare glucose, muscle and other tissues reduce glucose uptake in response to lowered insuhn secretion they also oxidize fatty acids and ketone bodies preferentially to glucose. 

The fuel reserves of a healthy adult human are of three types glycogen stored in the liver and, in relatively small quantities, in muscles large quantities of triacylglycerols in adipose tissues and tissue proteins, which can be degraded when necessary to provide fuel (Table 23-5). 

When the blood glucose level falls and the liver s glycogen reserves are also exhausted, the liver still has the capacity to synthesize glucose via gluconeogenesis from amino acids that are supplied from protein breakdown. Under starvation conditions the liver forms increasing amounts of ketone bodies (see fig. 18.7). This is due to elevated concentrations of acetyl-CoA, which favor the formation of ketone bodies. The ketone bodies are secreted and used as a source of energy by other tissues, especially those tissues like the brain that cannot catabolize fatty acids directly. 

Muscle pyridoxal phosphate is released into the circulation (as pyridoxal) in starvation as muscle glycogen reserves are exhausted and there is less requirement for glycogen phosphorylase activity. Under these conditions, it is potentially available for redistribution to other tissues, especially the liver and kidneys, to meet the increased requirement for gluconeogenesis from amino acids (Black et al., 1978). However, during both starvation and prolonged bed rest, there is a considerable increase in urinary excretion of 4-pyridoxic acid, suggesting that much of the vitamin Be released as a result of depletion of muscle glycogen and atrophy of muscle is not redistributed, but rather is ca-tabolized and excreted (Cobum et al., 1995). 

Glycogen phosphorylase catalyzes the sequential phosphorolysis of glycogen to release glucose 1-phosphate it is thus the key enzyme in the utilization of tissue glycogen reserves. 

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