The Effect of Prednisolone Injected in vivo on RNA Synthesis in Rat Thymus Cells

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Abstract
Three hours after the injection of prednisolone into rats a strong reduction in the incorporation of [3H]uridine and [3H]cytidine into the acid precipitable material of thymus cells suspended in a tissue culture medium is observed. The inhibition of the incorporation of RNA precursors appears to be a specific effect of glucocorticoids. It is not caused by a dilution of the precursor pool or by an acceleration of RNA breakdown. These conclusions can be drawn from experiments in which the precursor pool was expanded by a factor of 100 and from kinetic experiments with actinomycin D respectively. A series of DNA‐RNA hybridization experiments were conducted to answer the question whether prednisolone causes the loss of certain species of hybridizable RNA. No such effects could be found. However, inhibition experiments with low doses of actinomycin, hybridization experiments performed at low RNA/DNA ratios, and pulse‐label experiments with [32P]orthophosphate suggest that prednisolone, besides causing a general inhibition of RNA synthesis, exerts a more specific preferential reduction in the formation of ribosomal RNA.