Kidney In Long-Term Fasting

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Chapter: Biochemistry : The Feed-Fast Cycle

As fasting continues into early starvation and beyond, the kidney plays important roles. The kidney expresses the enzymes of gluconeogenesis, including glucose 6-phosphatase, and in late fasting about 50% of gluconeogenesis occurs here.


KIDNEY IN LONG-TERM FASTING

As fasting continues into early starvation and beyond, the kidney plays important roles. The kidney expresses the enzymes of gluconeogenesis, including glucose 6-phosphatase, and in late fasting about 50% of gluconeogenesis occurs here. [Note: A portion of this glucose is used by the kidney itself.] The kidney also provides compensation for the acidosis that accompanies the increased production of ketone bodies (organic acids). The glutamine released from the muscle’s metabolism of BCAAs is taken up by the kidney and acted upon by renal glutaminase and glutamate dehydrogenase, producing α-ketoglutarate that can be used as a substrate for gluconeogenesis, plus ammonia (NH3). The NH3 picks up protons from ketone body dissociation and is excreted in the urine as ammonium (NH4+), thereby decreasing the acid load in the body (Figure 24.18). In long-term fasting, then, there is a switch from nitrogen disposal in the form of urea to disposal in the form of ammonia. [Note: As ketone body concentration rises, enterocytes, typically consumers of glutamine, become consumers of ketone bodies. This allows more glutamine to be available to the kidney.]


Figure 24.18 Use of glutamine from BCAA catabolism in muscle to generate ammonia (NH3) used for the excretion of protons (H+) as ammonium (NH+4) in kidney.

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