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During the first ten minutes after the beginning of a continuous exposure of rat thymocyte populations (maintained in vitro) to epinephrine, there is an increase in the cellular concentration of cyclic AMP. The hormone also increases the activity of a crude preparation of the thymocyte's cyclic AMP-forming enzyme, adenyl cyclase. Between 30 and 45 minutes after the beginning of exposure to epinephrine, an additional part of the cell population begins to synthesize deoxyribonucleic acid (DNA). These changes are finally followed two to four hours later by an increase of the flow of cells into mitosis. Since cyclic AMP itself is known to stimulate both the initiation of DNA synthesis and thymocyte proliferation, and since the mitogenic action of epinephrine is shown to be potentiated by caffeine and inhibited by imidazole, it is concluded that the mitogenic action of this hormone is mediated by the cyclic nucleotide. (Author), Revision of report dated 23 Jun 70.

Cortisol (hydrocortisone) at a concentration of 0.27 micromoles causes the complete loss of nuclear structure ('pycnosis') in rat thymocytes maintained in vitro by the initiation during the first hour of exposure of a phosphate-dependent, respiration-linked reaction. The proportion of cells which lose their nuclear structure is directly and linearly proportional to the phosphate concentration in the medium. Under anoxic conditions, cells treated with cortisol cannot develop structures nuclei. Dinitrophenol can also reduce or prevent the cortisol-induced loss of nuclear structure even at concentrations which do not reduce respiration. These properties of the action of cortisol are identical to those of the actions of ionising radiation, parathyroid hormone and valinomycin. Possible mechanisms underlying the action of cortisol are discussed in the light of available information on the effects of these other agents. (Author)

Incubation of thymocytes in the anoxic thymus of the dead rat for periods longer than 30 min damages them. This damage causes no visible change in nuclear structure if the cells remain in the anoxic thymus, but when they are released therefrom and suspended in vitro their nuclei become structurally homogeneous ('pycnotic'). The fraction of cells which subsequently lose their nuclear structure in vitro is proportional to the concentration of inorganic phosphate in the medium. The amount of damage inflicted upon the cells during incubation in the anoxic thymus can be reduced by introducing tris(hydroxymethyl)aminomethane (Tris) buffer into the animal before its death. The establishment of primary radiation damage in thymocytes can be considerably reduced by incubation in the anoxic thymus of a Tris-treated, dead aminal during the first 2 h after irradiation. The development of radiation damage cannot be reduced if the 2-h sojourn in the anoxic thymus is delayed until 30 min after irradiation. The beneficial effect of postirradiation oxygen deprivation is at least equal to that produced by the classical 'oxygen effect'. (Author), Revision of rept. dated 27 Feb 68.