Estimation of glucose-alanine-lactate-glutamine cycles in postabsorptive humans: role of skeletal muscle.
To evaluate transfer of carbon between plasma glucose and plasma alanine (glucose-alanine cycle) and lactate (Cori cycle), to assess the contribution of skeletal muscle to these cycles, and to determine whether a glucose-glutamine cycle exists in postabsorptive humans, we infused 11 normal overnight-fasted volunteers with [2-3H]glucose, [6-14C]glucose, and [3-13C]alanine to isotopic steady state and in 7 of these simultaneously measured forearm net balance, uptake, and release of labeled and unlabeled glucose, lactate, and alanine. We found that 40.9 +/- 3.3, 66.8 +/- 3.2, and 13.4 +/- 1.1%, respectively, of plasma alanine, lactate, and glutamine carbon came from plasma glucose. More plasma glucose was converted to plasma alanine than could be derived from plasma alanine (1.89 +/- 0.20 vs. 1.48 +/- 0.15 mumol.kg-1.min-1, P < 0.001). A similar direction of net carbon flux was found for lactate (8.5 vs. 4.2 mumol.kg-1.min-1), with only glutamine adding more carbon to plasma glucose than was received from it (1.0 vs. 0.75 mumol.kg-1.min-1). Skeletal muscle accounted for 50.2 +/- 3.9 and 45.5 +/- 5.7% of the overall appearance of alanine and lactate in plasma and 54.2 +/- 5.4 and 36.4 +/- 4.2% of their respective origins from plasma glucose. Skeletal muscle release of alanine and lactate that had been formed from plasma glucose accounted for 19.1 +/- 2.1 and 48.4 +/- 4.8%, respectively, of muscle glucose uptake and 42.4 +/- 5.5 and 49.9 +/- 5.8% of the overall release of alanine and lactate from muscle.(ABSTRACT TRUNCATED AT 250 WORDS)
Perriello
G
G
Division of Endocrinology and Metabolism, Scripps Clinic, La Jolla, California 92037, USA.
Jorde
R
R
Nurjhan
N
N
Stumvoll
M
M
Dailey
G
G
Jenssen
T
T
Bier
D M
DM
Gerich
J E
JE
D000328:Adult; D000409:Alanine / Q000378:metabolism*; D001769:Blood / Q000378:metabolism; D004435:Eating; D005260:Female; D005542:Forearm; D005947:Glucose / Q000378:metabolism*; D005973:Glutamine / Q000378:metabolism*; D006801:Humans; D007773:Lactates / Q000378:metabolism*; D019344:Lactic Acid; D008297:Male; D018482:Muscle, Skeletal / Q000378:metabolism*
D007773:Lactates; D005973:Glutamine; D019344:Lactic Acid; D005947:Glucose; D000409:Alanine