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.