3H-thymidine and 3H-deoxycytidine were incorporated in vivo into the deoxyribonucleic acid (DNA) of thymus, spleen, peripheral and mesenteric lymph nodes of saline and cortisol-treated adrenalectomized rats. Employing 3H-thymidine as a label, the DNA of mesenteric lymph nodes displayed comparatively the highest specific activity and greatest sensitivity to the depressing action of cortisol injection. Incorporation of 3H-deoxycytidine into the thymus DNA was extensive and was depressed by cortisol administration under conditions that revealed no effect on a normally low level of 3H-thymidine incorporation. A dose-response relationship was obtained between the level of cortisol injected and the extent of depression of 3H-deoxycytidine incorporation into thymus DNA. These inhibitions of labeled precursor incorporation were not observed following injection of cholesterol, progesterone or deoxycorticosterone. Specific activity of the thymus DNA was depressed by the simultaneous administration of 5.0 or 50.0 μmoles/100 g body wt of unlabeled deoxycytidine with a tracer dose of 3H-deoxycytidine. Abolishment of the inhibitory response following cortisol injection occurred if the specific activity of the injected isotope was decreased by unlabeled deoxycytidine. These studies of the influence of cortisol administration on 3H-deoxycytidine uptake into thymus DNA are consistent with the thymolytic action of cortiosteroids, but do not necessarily suggest that inhibition of DNA synthesis in lymphoid tissue is an early primary event following administration of these hormones. (Endocrinology82: 767, 1968)
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Hofert et al. (1968) studied this question.