Circadian host and tumor rhythms in Balb/C mice, rhythm induction in Harding-passey Melanoma

The Harding-Passey melanoma in Bal/C or CD2F1 female mice shows, in light synchronized (LD12:12) undisturbed animals, no detectable circadian rhythm in cell proliferation as gauged by 3H-thymidine uptake in DNA. Manipulation of the animals and saline injection (.2 ml/20 gm), hydroxyurea injection (10 mg/ .2 ml/20 gm), and ACTH-17 (HOE 433) (.4 IU/ .2 ml/20 gm) induced a statistically significant circadian variation detected in several studies 4-24 and 16-36 hours after the treatment, only if the injection is given at the beginning of the light phase (LD12:12). Thus, tumor synchronization in this model is critically dependent upon the circadian stage of administration of the synchronizing agent. Endogenous and exogenous ACTH seem to be the synchronizing agent. Hydroxyurea in the dose given shows no additional effect.