[Possible reasons for decrease in the proliferating activity of the rat thymus after a single injection of glucocorticoids].

A single injection of triamcinolone-acetonide evoked multiple changes in adrenectomized rat thymuses, e.g. decrease of DNA polymerase and thymidine kinase activities, incorporation of [3H]thymidine into DNA and fragmentation of chromatin. The decrease of proliferating activity of thymus was preceded by inactivation of cytoplasmic DNA polymerase responsible for DNA replication. Fragmentation of DNA caused by double strand ruptures at the linkage sites affected primarily the non-proliferating thymocytes. Incubation of nuclei isolated from the thymuses of control and hormone-treated animals in Ca2+ and Mg2+-containing media increased the level of low molecular weight compounds in them. It was assumed that the inhibiting action of triamcinolone-acetonide on cytoplasmic DNA polymerase prevents the involvement of proliferating thymocyte subpopulation in a new generation cycle and provides for their terminal differentiation and decay, eventually resulting in fragmentation of DNA.