Metabolism
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Alanine released from muscle in fasting
Wahren and coworkers reported a series of studies in which catheters were inserted into a brachial artery, a femoral vein and a hepatic vein (via an antecubital vein) in fasting volunteers. This meant that they could withdraw blood samples from three sites simultaneously to measure arterio-venous differences in concentrations of metabolites across the leg muscles (arterio-femoral vein difference) and across the liver (arterio-hepatic vein difference).
The arterio-venous difference is the concentration of the metabolite in arterial blood minus the concentration in the venous blood.
A negative a-v difference represents output of the metabolite by the tissue
A positive a-v difference represents uptake of the metabolite by the tissue
glucose |
lactate |
|
| arterial - femoral vein difference, mmol /L | 0.19 ± 0.02 |
-0.14 ± 0.03 |
| arterial - hepatic vein difference, mmol /L | -0.80 ± 0.08 |
0.21 ± 0.02 |
(From data reported by Wahren J. et al Journal of Clinical Investigation 51: 1870 1972)
What conclusions can you draw from these results?
Even in the fasting state there is a small uptake of glucose by muscle, but much of the glucose uptake will presumably have come from red blood cells, as will the output of lactate.
There is a considerable output of glucose by the liver. Some of this will come from liver glycogen, but much will come from gluconeogenesis after an overnight fast.
What are the likely substrates for gluconeogenesis in liver in the fasting state?
There will be a small amount of lactate, mainly from red blood cells, although resting muscle takes up glucose and metabolises it to some extent anaerobically, putting out lactate. However, the main substrates for gluconeogenesis in the fasting state are amino acids.
The diagram shows the arterial - femoral vein differences in concentrations of amino acids
(From data reported by Wahren J. et al Journal of Clinical Investigation 51: 1870 1972)
What conclusions can you draw from these results?
In most cases the arterio-venous difference is negative, showing that there is an output of most amino acids form muscle in the fasting state, although it takes up moderate amounts of serine and cysteine.
The output of alanine is 3-fold higher than that of any other amino acid, amounting to ~60% of the total amino acid output.