Metabolism of branched-chain amino acids in leg muscles from tail-cast suspended intact and adrenalectomized rats

Degradation of branched-chain amino acids was studied in muscles of unloaded hind limbs from rats subjected to six days of tail-cast suspension. The total production of ¹⁴CO₂ from uniformly labeled ¹⁴C-leucine, isoleucine, or valine, and the fluxes through leucine aminotransferase and α-ketoisocaproate dehydrogenase, which were measured using l-1-¹⁴C-leucine, were generally greater in the soleus and extensor digitorum longus muscles of unloaded than of weight-bearing hind limbs. Adrenalectomy abolished any difference in flux through the aminotransferase, whereas the administration of cortisol to adrenalectomized animals restored the greater flux in the unloaded soleus muscle. Adrenalectomy partially diminished the greater flux through α-ketoisocaproate dehydrogenase in the unloaded soleus, whereas cortisol (2 mg/100 g body weight) treatment increased this difference. In the extensor digitorum longus, adrenalectomy abolished the differences in both enzyme fluxes due to hind limb suspension. In this muscle, cortisol treatment increased these fluxes to a similar extent in both weight-bearing and suspended, adrenalectomized animals so that the normal difference was not restored. These results suggest that leucine catabolism in hind limb muscles of suspended rats was influenced primarily by increased circulating glucocorticoid hormones, which are elevated twofold to fourfold in these animals.