Glucose utilisation rates

For calculation of rate of ATP generation from glucose, assuming complete oxidation of glucose, multiply rate of glucose utilisation by 30. 

On the basis of the data in this table, the calculated rate of glucose utilisation by the brain of an adult is about 3 g/hr. This is consistent with a rate of about 80 g in 24 hours (Chapter 14). 

The concentration of glucose in the blood is maintained as a balance between rates of glucose utilisation and glucose supply and changes in one or both of these can lead to hypoglycaemia. Three situations are considered. 

Hypoglycaemia caused by stimulation of the rate of glucose utilisation and inhibition of the rate of release of glucose by the liver... 

To provide an alternative fuel to glucose during starvation. Indeed, fatty acid oxidation restricts the rate of glucose utilisation, which maintains the blood glucose level, via a regulatory mechanism known as the glucose/ fatty acid cycle (Chapter 16). 

Figure 13.27 A possible mechanism by which a low blood glucose level could give rise to central fatigue. A low blood glucose level reduces the rate of glucose utilisation in the brain which decreases the ATP/ADP concentration ratio in the presunaptic neurone. This reduces the energy available for synthesis of neurotransmitters, packaging of neurotransmitter molecules into vesicles and exocytosis of neurotransmitter into synaptic cleft. This decreases electrical activity in postsynaptic neurones and hence in the motor pathway.

The rise in the plasma level of hydroxybutyrate leads to an increase in its rate of oxidation by the brain which reduces the rate of glucose utilisation by this organ from about 100 g to about 40 g per day (see below). Of this 40 g, about half is produced from glycerol. 

The increased oxidation of fatty acids decreases the rate of glucose utilisation and oxidation by muscle, via the glucose/fatty acid cycle, which accounts for some of the insulin resistance in trauma. An additional factor may be the effect of cytokines on the insulin-signalling pathway in muscle. An increased rate of fatty acid oxidation in the liver increases the rate of ketone body production the ketones will be oxidised by the heart and skeletal muscle, which will further reduce glucose utilisation. This helps to conserve glucose for the immune and other cells. 

Figure 23.1 Rate of glucose utilisation during the five phases of glucose homeostasis. Adapted from Ruderman NB (1975) Muscle amino acid...

Organ or tissue metabolic rate kj/kg per day Contribution to whole-body REE (% total) Approx, oxygen utilisation of tissue for adult Equivalent rate glucose uptake ... 

Hypoglycaemia that arises when an increased rate of utilisation exceeds that of glucose release by the liver... 

The physiological usefulness of this method for identifying the fuels that are used and their rates of utilisation in different cells is discussed in other chapters (Chapter 3). For example, measurement of the activity of the enzymes hexokinase and glutaminase in immune cells showed, for the first time, that glucose and glutamine are the major fuels utilised by these cells. This finding has had clinical significance (Chapter 17). 

Blood-bome fuels are glucose, which is derived from liver glycogen, and fatty acids derived from adipose tissue. Uptake depends on the flow of blood through the muscle, the concentration of the fuel in the blood and the demand for ATP within the muscle. During sustained exercise the flow of blood to the muscle can increase up to 50-fold and the rate of utilisation of the fuel can increase to a similar extent, yet the concentration of the fuels in blood remains remarkably constant (Table 13.5). 

As intensity increases, the contributions of blood glucose and glycogen to ATP generation increase. Above about 50% of maximum power output, the rate of glycogen utilisation increases almost exponentially since some glycogen is converted to lactic acid at an increasing rate. 

In the fed state, the only fuel used by the brain is glucose, derived from the blood. In prolonged starvation or chronic hypoglycaemia, ketone bodies are nsed which rednce the rate of utilisation of glucose by the brain bnt, even so, glucose still provides about 50% of the energy. Consequently, under all conditions, maintenance of the blood glucose concentration is essential for the function of the brain the mechanisms that are responsible for this are discnssed in Chapters 6, 12 and 16. 

From arteriovenous difference, the utilisation of glucose by human brain is approximately 0.32 mmol/min which, if completely oxidised would consume (6 x 0.32) or 1.92 mmol/min of oxygen. The rate of oxygen uptake by the human brain is also measured from arteriovenous difference it is 2.1 mmol/min (i.e. very close to that calculated, 1.92). 

Arteriovenous differences show that after the overnight fast the liver of a lean adult releases glucose at a rate of about 8 g/h and the brain utilises more than half of this... 

Figure 16.11 Pattern of fuel utilisation during prolonged starvation. The major metabolic change during this period is that the rates of ketone body formation and their utilisation by the brain increases, indicated by the increased thickness of lines and arrows. Since less glucose is required by the brain, gluconeogenesis from amino acids is reduced so that protein degradation in muscle is decreased. Note thin line compared to that in Figure 16.9.

Fig. 8.20. (a) Age-dependent rate of utilisation of periodic acid-Schiff (PAS) positive material a (t) in oncospheres of Hymenolepis diminuta incubated in Tyrode+glucose at 25 C. Solid line represents the best fit for exponential model o(r)/S exp yt, where 0=0.3822 and y = - 0.3259. Dots, observed points. (b) Time-dependent loss of PAS positive material. Comparison of observed and predicted results. Solid line, predicted results dots, observed results. (After Anderson Lethbridge, 1975.)... 

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