Lactate Cori cycle

Lactate produced by anaerobic metabolism in skeletal muscle passes to liver, which uses it to synthesize glucose, which can then return to muscle (the Cori cycle). 

Each mole of glucose that goes through the Cori cycle costs the liver 6 ATP equivalents. If the Cori cycle were perfect and there were an endless supply of ATP, liver could supply glucose equivalents forever just using the same carbon atoms (6 in as lactate, 6 out as glucose). However, some of the lactate (pyruvate) in the nonliver tissues is burned to C02 by the TCA cycle in die muscle and other tissues. It s this pool that must be replaced by fresh glucose from liver. 

The alanine cycle accomplishes the same thing as the Cori cycle, except with an add-on feature (Fig. 17-11). Under conditions under which muscle is degrading protein (fasting, starvation, exhaustion), muscle must get rid of excess carbon waste (lactate and pyruvate) but also nitrogen waste from the metabolism of amino acids. Muscle (and other tissues) removes amino groups from amino acids by transamination with a 2-keto acid such as pyruvate (oxaloacetate is the other common 2-keto acid). 

Figure 6.10 The extended Cori cycle. Tumour cells convert glucose to lactate at a high rate and could be incorporated into this figure (Chapter 21).

Lactic acid The normal concentration of lactate in the blood is about 1 mmol/L which is the balance between production and utilisation. Various tissues produce lactic acid and release it into the blood, from where it is taken up by liver and converted to glucose (or glycogen) (the Cori cycle, see above). 

Lactate as a precursor for gluconeogenesis is mainly derived from muscle (see Cori cycle, p. 338) and erythrocytes. LDH (see p. 98) oxidizes lactate to pyruvate, with NADH+H" formation. 

After a period of intense muscular activity, the individual continues breathing heavily for some time, using much of the extra 02 for oxidative phosphorylation in the liver. The ATP produced is used for gluconeogenesis from lactate that has been carried in the blood from the muscles. The glucose thus formed returns to the muscles to replenish their glycogen, completing the Cori cycle (Fig. 23-18 see also Box 15-1). 

Lactate is released into the blood by exercising skeletal muscle, and by cells that lack mitochondria, such as red blood cells. In the Cori cycle, blood-borne glucose is converted by exercising muscle to lac tate, which diffuses into the blood. This lactate is taken up by the liver and reconverted to glucose, which is released back into the circulation (Figure 10.2). 

The cycling of lactate and glucose between muscle and liver in the Cori cycle. Under conditions of intense activity the muscle operates anaerobically so that the end product of glucose breakdown is lactate. This product can pass through the bloodstream to the liver and be converted to glucose, which can be returned to the muscle. 

C. lactate is converted back to glucose in the liver completing the Cori cycle. 

Figure 18.4 The Cori cycle. Glucose is metabolized anaerobically to lactate, as in white muscle fibers or red blood cells. Lactate is exported into the bloodstream, extracted from there by the liver, and reconverted to glucose via gluconeogenesis. Glucose is then returned to the muscle.

The Cori cycle refers to the metabolic pathway in which lactate, produced by anaerobic glycolysis in the muscle, moves to the liver and is converted to glucose, through gluconeogenesis glucose can then return to supply the muscle. 

Figure 16.33. The Cori Cycle. Lactate formed hy active muscle is converted into glucose hy the liver. This cycle shifts part of the metabolic burden of active muscle to the liver.

---