The synchronization of the circadian rhythms of [3H]thymidine uptake (as gauge of DNA synthesis and presumably of cell proliferation) in colon, thymus, and Harding-Passey melanoma were studied in 456 male BALB/c × DBA/2 F1 mice under a 12-hr-light, 12-hr-dark regimen. In two groups of animals, the feeding time was restricted to 4 hr/day (either at the beginning of the light span or at the beginning of the dark span).

The circadian rhythms in body temperature and [3H]thymidine uptake in the colon were determined in their timing primarily by the time of food intake. In contrast, the circadian rhythm of [3H]thymidine uptake in the thymus and in the transplanted melanoma remained synchronized with the lighting regimen, and under the conditions of this study, was not altered in its timing by the change in feeding time. It thus appears feasible to alter the phase relations between certain circadian rhythms of host and tumor. If applicable to the human situation, this observation might be of interest for the scheduling of chemo- and radiotherapy, in an attempt to obtain maximal effects upon the tumor with minimal undesired side effects upon vital functions of the host.


Supported by the St. Paul-Ramsey Medical Education and Research Foundation (Grant 8277).

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