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Inhibition of protein synthesis by glucagon in different rat muscles and protein fractions in vivo and in the perfused rat hemicorpus.

ABSTRACT: The effect of glucagon on the rate of muscle protein synthesis was examined in vivo and in the isolated perfused rat hemicorpus. An inhibition of protein synthesis in skeletal muscles from overnight-fasted rats at various plasma concentrations of glucagon was demonstrated in vivo. The plantaris muscle (Type II, fibre-rich) was more sensitive than the soleus (Type I, fibre-rich). Myofibrillar and sarcoplasmic proteins were equally sensitive in vivo. However, protein synthesis in mixed protein and in sarcoplasmic and myofibrillar fractions of the heart was unresponsive to glucagon in vivo. In isolated perfused muscle preparations from fed animals, the addition of glucagon also decreased the synthesis of mixed muscle proteins in gastrocnemius (Type I and II fibres) and plantaris, but not in the soleus. The sarcoplasmic and myofibrillar fractions of the plantaris were also equally affected in vitro. Similar results were observed in vitro with 1-day-starved rats, but the changes were less marked.

SUBMITTER: Preedy VR

PROVIDER: S-EPMC1149064 | biostudies-other | 1988 May

REPOSITORIES: biostudies-other

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Project description:Compartmentalization of amino acid was investigated to define conditions required for accurate measurements of rates of protein synthesis in rat lungs perfused in situ. Lungs were perfused with Krebs-Henseleit bicarbonate buffer containing 4.5% (w/v) bovine serum albumin, 5.6mm-glucose, normal plasma concentrations of 19 amino acids, and 8.6-690mum-[U-(14)C]phenylalanine. The perfusate was equilibrated with the same humidified gas mixture used to ventilate the lungs [O(2)/CO(2) (19:1) or O(2)/N(2)/CO(2) (4:15:1)]. [U-(14)C]Phenylalanine was shown to be a suitable precursor for studies of protein synthesis in perfused lungs: it entered the tissue rapidly (t((1/2)), 81s) and was not converted to other compounds. As perfusate phenylalanine was decreased below 5 times the normal plasma concentration, the specific radioactivity of the pool of phenylalanine serving as precursor for protein synthesis, and thus [(14)C]phenylalanine incorporation into protein, declined. In contrast, incorporation of [(14)C]histidine into lung protein was unaffected. At low perfusate phenylalanine concentrations, rates of protein synthesis that were based on the specific radioactivity of phenylalanyl-tRNA were between rates calculated from the specific radioactivity of phenylalanine in the extracellular or intracellular pools. Rates based on the specific radioactivities of these three pools of phenylalanine were the same when extracellular phenylalanine was increased. These observations suggested that: (1) phenylalanine was compartmentalized in lung tissue; (2) neither the extracellular nor the total intracellular pool of phenylalanine served as the sole source of precursor for protein; (3) at low extracellular phenylalanine concentrations, rates of protein synthesis were in error if calculated from the specific radioactivity of the free amino acid; (4) at high extracellular phenylalanine concentrations, the effects of compartmentalization were negligible and protein synthesis could be calculated accurately from the specific radioactivity of the free or tRNA-bound phenylalanine pool.

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Inhibition of protein synthesis by glucagon in different rat muscles and protein fractions in vivo and in the perfused rat hemicorpus.

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