In the adrenal medulla of rats exposed intermittently to cold (+4°C) for 100 and 300 hours, a considerable decrease (24 to 40 per cent) of the DNA content per nucleus was observed, followed by restoration to normal or above normal values within 10 days after the withdrawal of the stimulus. The findings were obtained with a scanning integrating histophotometer, and confirmed by microinterferometric investigations (on the basis of the measurement of total dry mass of nuclei isolated in aqueous medium before and after treatment with DNase) and by microchemical determinations, combined with the count of the nuclei in the homogenates. The observed decrease of DNA content cannot be attributed to errors of the methods used, nor to consequences of cellular degeneration. The available evidence seems to indicate a real decrease rather than a change in the state of a part of DNA in the nucleus in vivo whereby it becomes extractable by aqueous solutions. The restoration cannot be due to mitotic processes, which were actually never detected even with the use of colchicine, since the adrenal medulla cells in the adult rat are known to be irreversible, postmitotic cells. A correlation between the functional activity of the adrenal medulla cells and the content or state of DNA in their nuclei is demonstrated.

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