Abstract
The effect of glucagon on hepatic protein systhesis and proteolysis has been investigated. The intraperitoneal administration of 200 mug of glucagon produced an increase of the polypeptide chains completion time which was maximal 5 min after its administration and approached control values at 20 min. The increase of the polypeptides chains completion time observed at 5 min after the hormone administration represents a 38% inhibition of the hepatic protein synthetic rate. When glucagon was continuously supplied by intravascular infusion, maximal inhibition was attained throughout the experiment. This inhibition of protein synthesis brought about by glucagon was accompanied by an increase in the polyribosomal state of aggregation, indicating that the hormone acts mainly if not exclusively, on the elongation or termination step, or both. The administration of glucagon produced also a progressive increase in the hepatic valine concentration. This increase could not be accounted for the the decrease in plasma valine levels, suggesting that the rise in haptic valine concentration is an expression of hepatic proteolysis rather than the result of an accelerated transport of amino acids across the hepatocyte plasma membrane. The different time sequence in the glucagon-induced effects of protein synthesis and proteolysis suggests that both effects are independent and probably mediated by different mechanisms.
Highlights
The effect of glucagon on hepatic protein synthesis and proteolysis has been investigated
When glucagon was continuously supplied by intravascular infusion, maximal inhibition was attained throughout the experiment. This inhibition of protein synthesis brought about by glucagon was accompanied by an increase in the polyribosomal state of aggregation, indicating that the hormone acts mainly if not exclusively, on the elongation or termination step, or both
The different time sequence in the glucagon-induced effects on protein synthesis and proteolysis suggests that both effects are independent and probably mediated by different mechanisms
Summary
The effect of glucagon on hepatic protein synthesis and proteolysis has been investigated. The administration of glucagon to animals [1, 2] and man [3,4,5] is followed by an increased nitrogen excretion and a concomitant rise in plasma urea levels The latter finding made many authors focus their attention on the liver as a main site of the hormone action. Ayuso-Parrilla and Parrilla [7] reported a predominant, if not exclusive, effect of glucagon on proteolysis according to their findings in the perfused rat liver as well as in uiuo This conflict between the findings of different authors may be at least in part a consequence of the glucagon effect varying the intracellular amino acids pools. Such approach originally described by Haschemeyer [13] and Mathews et al [14] and modified by Scornik [15] is based on the determination of the average transit time of a radioactive precursor along the messenger RNA
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