Contribution of plasma proteins to muscular de novo synthesis of glutamine.
Amino acid balance across the muscle bed is known to be negative in the postabsorptive state. The explanation for such permanent amino acid loss by muscle tissue has to be sought in either its proteolytic breakdown, that of plasma polypeptides and proteins, increased rate of amino acid transport out of the cell, or de novo synthesis of amino acids. To evaluate the contribution of plasma proteins to muscle amino acid release in vitro, hindquarters of 48-hour-fasted rats were perfused with 6 mmol/L glucose Krebs-Ringer buffer containing 20% human erythrocytes and albumin or gamma-globulin at concentrations of 5%, 0.5%, and 0%, respectively. To maintain oncotic pressure, plasma proteins were replaced in the latter protocol by 60-kd dextran. Perfusion for 60 minutes without plasma proteins resulted in a fall in amino acid release by 32% and 23%, respectively, compared with amino acid concentrations in the venous hindquarter effluent during infusion of 5% albumin and gamma-globulin, respectively. The impairment of amino acid release was primarily due to decrease in glutamine generation, which dropped from 327.4 +/- 47.3 and 323.0 +/- 35.7 to 211.9 +/- 22.0 mumol/L, respectively (means +/- SE, n = 7). The present data suggest that the apparent negative nitrogen balance observed across the muscle bed in vitro is not caused solely by muscle degradation but also, to a considerable extent, by proteolytic breakdown of "arterial" plasma protein, whose role in in vivo experiments remains to be established.
Komjati
M
M
I Medizinische Universitätsklinik, Division of Clinical Endocrinology and Diabetology, Vienna, Austria.
Waldhäusl
W
W
D000596:Amino Acids / Q000378:metabolism; D000818:Animals; D001798:Blood Proteins / Q000494:pharmacology*; D004912:Erythrocytes / Q000378:metabolism; D005973:Glutamine / Q000096:biosynthesis*; D007700:Kinetics; D008297:Male; D009124:Muscle Proteins / Q000378:metabolism; D009132:Muscles / Q000187:drug effects / Q000378:metabolism*; D009584:Nitrogen / Q000378:metabolism; D051381:Rats; D011919:Rats, Inbred Strains; D012709:Serum Albumin / Q000494:pharmacology; D005719:gamma-Globulins / Q000494:pharmacology
D000596:Amino Acids; D001798:Blood Proteins; D009124:Muscle Proteins; D012709:Serum Albumin; D005719:gamma-Globulins; D005973:Glutamine; D009584:Nitrogen