A 55-year-old man with alcoholic cirrhosis is admitted to the hospital for routine evaluation before liver transplantation. The physician asks the patient to stop eating 10 hours before surgery. Which of the following structures contributes directly to preventing fasting hypoglycemia by producing glucose in this patient?
|A.||Red blood cells|
Answer B) Intestine
Gluconeogenesis is the biosynthesis of glucose from pyruvate, lactate, lipids (glycerol), and amino acids (alanine and glutamine). In this patient with liver failure, hepatic gluconeogenesis may be impaired.
The intestinal epithelium is one of the primary extrahepatic sites of gluconeogenesis. It responds to the same hypoglycemic cues as the liver, including glucagon, glucocorticoids (cortisol), and acidosis (typically from starvation ketosis). Another important site is the renal cortex, and recent evidence indicates that astrocytes of the central nervous system perform gluconeogenesis as well.
Red blood cells
Erythrocytes lack the enzymes necessary to carry out gluconeogenesis. However, they are a major site for production of reducing equivalents (e.g., NADPH) via the hexose monophosphate shunt.
Skeletal muscle does not carry out gluconeogenesis because it lacks the final enzyme glucose-6-phosphatase. Absence of this enzyme prohibits the conversion of glucose-6-phosphate to glucose so that skeletal muscle cannot produce glucose capable of entering the bloodstream and influencing serum glucose concentration. Instead, glucose-6-phosphate produced by skeletal muscle is used to store energy as glycogen for later use by the skeletal muscle itself.
The skin is the predominant location of vitamin D biosynthesis. Vitamin D3 (cholecalciferol) is synthesized in the stratum basale upon exposure to ultraviolet light. It is then converted to 25-hydroxycholecalciferol (calcidiol) in the liver. Final conversion to the active form 1,25-dihydroxycholecalciferol (calcitriol) occurs in the kidneys. The enzymes needed for gluconeogenesis are not found in the skin.