Natural changes in the cell division rate in normal tissues and the possible role of adrenalin concentration

Mitotic incidence (MI) and DNA labeling index (LI) were measured in the liver, lung and kidney of rats at different times throughout the day. There were significant variations in MI and LI in all organs and there were also significant differences between organs in respect of the pattern of variation. It was found that adrenalin had no consistent inhibitory action on the MI and LI of these tissues maintained in organ culture. These results are not consistent with the view that diurnal (or circadian) rhythms in mitotic activity are directly mediated by changes in adrenalin secretion. In organ culture the rate of entry into DNA synthesis was higher than the rate of mitosis, leading to significant increases in the proportions of cells with reduplicated DNA content.