Glucocorticoid Action on Rat Thymus Cells

In rat thymus cells incubated with carbohydrate substrate the steady state rate of incorporation of uridine into RNA generally parallels the rate of incorporation of amino acids into protein. In both instances glucose is required for continued maximal incorporation rates, a requirement that is only partially satisfied by other readily metabolized substrates such as pyruvate, lactate, or β-hydroxybutyrate. The inhibitory effects of cortisol on the incorporation of uridine into RNA parallel those on the incorporation of amino acids into protein and appear to be largely on the carbohydrate-dependent portion. Substantial differences in relative rates of incorporation of RNA and protein precursors are revealed, however, by a study of the rapid changes that occur after the delayed addition of carbohydrate; an immediate burst in the incorporation of uridine into RNA, maximal at 1 to 2 min, precedes the slower rise in the incorporation of amino acids into protein. Moreover, in cells previously treated for 2 hours with cortisol there emerges from the moment of carbohydrate addition a maximal cortisol inhibition on uridine incorporation, about 40% of the carbohydrate-induced rise, whereas cortisol inhibition of amino acid incorporation emerges gradually after a 10-min lag. Studies with actinomycin and puromycin reveal that, for both the stimulatory effects of carbohydrates and the inhibitory effects of cortisol that emerge after delayed carbohydrate addition, the changes in RNA and protein metabolism are independent of each other since the influences of both carbohydrates and cortisol on the incorporation of precursors into either macromolecular species persist when the synthesis of the other species has been blocked by antibiotic. It is concluded that the late, 2-hour, effects of cortisol on the labeling of both RNA and protein are secondary to early hormonal inhibition of carbohydrate metabolism, possibly mediated through alterations in adenine nucleotide metabolism.