Glucose in muscle

Figure 15-5. Transport and fate of major carbohydrate and amino acid substrates and metabolites. Note that there is little free glucose in muscle, since it is rapidly phosphorylated upon entry.

Any pyruvate produced from glucose in muscle, which is not oxidised, will be converted to lactate and released into the blood. 

Increased uptake of glucose in muscle and adipose tissue ... 

Recall How does the yield of ATP from complete oxidation of one molecule of glucose in muscle and brain differ from that in liver, heart, and kidney What is the underlying reason for this difference ... 

Going for the burn in a workout refers to the sensation that accompanies lactic acid buildup. This in turn arises from anaerobic metabolism of glucose in muscle. 

The inhibitory action of adrenaline on phosphorylation of glucose in muscle has been attributed to inhibition of hexokinase by the elevated concentration of glucose-6-phosphate which the hormone induces by accelerating glycogen breakdown (cf. Cori and Cori, 1931-1932 Hegnauer and Cori, 1934 Sutherland, 1952 Kipnis, 1959 Newsholme and Randle, 1961). 

The effects of thyroidectomy and of hyperthyroidism on uptake of glucose and on rates of membrane transport and phosphorylation of glucose in muscle have yet to be investigated. Since hyperthyroidism is associated with impaired efficiency of respiratory chain phosphorylation, it might be expected to accelerate membrane transport and phosphorylation of glucose (see Section IV, A, 1). 

Any excess lactate is taken up by the liver, converted into pyruvate and then into glucose. This, along with the production of lactate from glucose in muscle cells, constitutes the Cori cycle. 

However, it has been found that the hexokinases of different organisms and of different tissues within an organism may differ with respect to the specificity of the enzyme and the reaction catalyzed. For instance, the phosphorylation of glucose in muscle is effected by a specific glucokinase which forms glucose-6-phosphate. This enzyme does not phosphorylate fructose, which has its own transphosphorylase, fructokinase. 

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