Circadian rhythm of RNA and DNA in some organs of fed rats.

Circadian changes in RNA and DNA contents in the thymus, spleen, liver and myocardium were studied in rats fed ad libitum (maintained in a room illuminated from 07.00 to 19.00 and darkened from 19.00 to 07.00 hours). The values for the thymus, liver and heart muscle fluctuated broadly over the 24-hour period: between 25% and 42% for RNA, and between 46% and 58% for DNA. However, no significant changes in RNA and DNA contents were detected in the spleen. In general the lowest values of nucleic acid content in the thymus, spleen and myocardium were recorded aroung 23.00-02.00 hours, and were replaced by a quick increase in the following hours (mainly till 05.00) up to a maximum reached around 11.00 in the thymus and liver, and around 17.00 in myocardium. The changes in the amount of RNA and DNA did not always run in parallel; consequently the ratio RNA/DNA varied, with the widest fluctuations found in the thymus.