Glucose as fuel

To provide the nutrients for proliferation of the spermatogonia and spermatocytes and maturation of the sperm. These are essential amino acids, glutamine and glucose as fuels and also as precursors for formation of nucleotides essential fatty acids for formation of phospholipids (for discussion of these topics, see Chapters 20 and 21). It is presumed that Sertoli cells extract these compounds from the blood. 

Figure 5-5. Metabolic activities of major organs during a short-term fast. The importance of the liver in providing glucose to support the brain and other glucose-requiring organs in the post-absorptive state is illustrated. The body relies on available glycogen stores as a ready source for glucose as fuel. PPP, pentose phosphate pathway FA, fatty adds TAG, triacylglycerol.

Skeletal muscle can use free fatty acids, ketone bodies, or glucose as fuel, depending on the degree of muscular activity (Fig. 23-17). In resting muscle, the primary fuels are free fatty acids from adipose tissue and... 

The brain does not use fatty acids as a fuel, as fetty acids are transported in the blood bound to albumin and do not cross the blood brain barrier. The brain prefers glucose as fuel, but does not manufacture it. Instead it acquires it from the diet or from the liver. In starvation, the brain may use ketones as fuel. The liver largely obtains its energy from lactate (after a meal) or from fatty acids (after lasting). 

Fatty acids do not serve as fuel for the brain, because they are bound to albumin in plasma and so do not traverse the blood-brain barrier. In starvation, ketone bodies generated by the liver partly replace glucose as fuel for the brain. 

Skeletal muscle utilizes glucose as a fuel, forming both lactate and CO2. It stores glycogen as a fuel for its use in muscular contraction and synthesizes muscle protein from plasma amino acids. Muscle accounts for approximately 50% of body mass and consequently represents a considerable store of protein that can be drawn upon to supply amino acids for gluconeogenesis in starvation. 

A PET scan requires a substance called a tracer. A suitable tracer must accumulate in the target organ, and it must be modified to contain unstable radioactive atoms that emit positrons. Glucose is used for brain imaging, because the brain processes glucose as the fuel for mental and neural activities. A common tracer for PET brain scans is glucose modified to contain radioactive fluorine atoms. Our molecular inset shows a simplified model of this modified glucose molecule. 

Diabetes results from a lack of insulin secretion by the pancreas. Without insulin, cells take up glucose very slowly. The lack of insulin results in an inability to use blood glucose for fuel. Consequently, the body behaves as if it were starving even though food is available. The metabolic responses of the untreated insulin-dependent diabetic are essentially the metabolic responses of starvation. 

Tabernero, A., Vicario, C. and Medina, J. M. Lactate spares glucose as a metabolic fuel in neurons and astrocytes from primary culture. Neurosci. Res. 26 369-376,1996. 

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