Quantitative analysis of mRNA synthesis during early cortisol action on rat thymocytes: restricted size of a possible...

Complete inhibition of cortisol-induced pycnosis was seen by actinomycin D and cycloheximide only when added during the initial period of hormone action. This phenomenon, being a characteristic of steroid hormone effects in general, is often taken as indirect evidence for early steroid-induced mRNA synthesis. The lack of direct evidence for this theory has been tested for significance. Approximately 133 newly synthesized mRNA molecules were found to accumulate in the cytoplasm/min/cell, suggesting an average synthesis rate for individual mRNA species of about 1 copy/h/cell. Electrophoretic fractionation of double labelled RNA failed to reveal any changes of the isotope ratio of single fractions during the first 15-45 min of cortisol action, within an experimental error corresponding to +/- 2- +/- 20 molecules/cell. Possible effects of cortisol are thus restricted to changes in the range of constitutive mRNA synthesis rates. In contrast, the RNA labelling pattern was differentially changed after 45 min treatment with 10(-5) M cycloheximide.