Carbohydrate Metabolism
The liver plays an important role in the homeostasis of blood
glucose, especially after a carbohydrate meal, diurnal fast, or prolonged exercise.
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Whether the liver consumes or produces glucose depends on (1) the glucose concentration
in the sinusoidal blood and (2) hormonal influences—primarily insulin, catecholamines,
and glucagon.[13]
The rate at which the liver takes
up or
releases glucose relates directly to the degree of hyperglycemia or hypoglycemia.
A reciprocal relation exists between the hepatic synthesis of glucose and glycogen.
In the fed state, the liver converts glucose to glycogen. Conversely, carbohydrate
deprivation prompts the liver to increase glucose production. This initially involves
breaking hepatic glycogen down to glucose and releasing it into the blood. Two rate-limiting
enzymes control hepatic glycogen metabolism: (1) glycogen synthase promotes polymer
formation from uridine diphosphate (UDP) glucose, and (2) glycogen phosphorylase
helps cleave glycogen to glucose-1-phosphate, one residue at a time.
When hepatic glycogen stores have been drained (e.g., as a result
of starvation for 24 to 48 hours or prolonged exercise), gluconeogenesis is the sole
pathway for producing glucose. Substrates for this pathway include lactate, glycerol
(from hydrolysis of triglycerides), and glucogenic amino acids (alanine, glutamine)
supplied by muscle.[13]
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Glucagon stimulates gluconeogenesis by activating cyclic adenosine monophosphate
(cAMP)-dependent protein kinase; catecholamines stimulate this pathway by cAMP-dependent
and -independent mechanisms.[92]
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In contrast, insulin inhibits hepatic gluconeogenesis and antagonizes the effects
of both catecholamines and glucagon on this pathway.
