Alternate Fluctuations of Leucine and Thymidine Incorporation by Mammary Tumors in Rats during the Estrous Cycle

Rates of [³H]leucine and [³H]thymidine incorporation by 7,12-dimethylbenz(a)anthracene-induced rat Mammary tumors were examined during different days of the host's estrous cycle. Actively growing tumors were surgically removed from regularly 5-day-cycling hosts and were cut into 1 to 2-cu mm pieces for incubation. the incubation was carried out in 4 ml of Medium 199 containing either [³H]leucine for 2 hr or [³H]thymidine for 1 hr at 37° under 95% 0₂ and 5% CO₂. By scintillation counting the rate of [³H]leucine incorporation was the highest at proestrus and dropped significantly to its lowest point on the next day at estrus. During the remaining 3 days of the cycle at metestrus, diestrus 1, and diestrus 2, the rate increased slightly but was still significantly lower than that observed at proestrus. Unlike [³H]leucine incorporation, the rate of [³H]thymidine incorporation was the lowest at proestrus and remained low at estrus. the rate began to rise by metestrus, peaked at diestrus 1, and declined by diestrus 2. Rates of [³H]thymidine incorporation correlated significantly with Tissue mitotic counts, which also peaked at diestrus 1. Radioactivities in the acid-soluble fraction of the incubated Tissues were not significantly different for either labeled precursors throughout the estrous cycle. Autoradiographic studies of Tissues collected after incubation with [³H]thymidine showed that silver grains were localized over Cell nuclei. the data indicate that the growth of 7,12-dimethylbenz(a)anthracene-induced Mammary tumors in rats during the estrous cycle is associated with a peak of [³H]leucine incorporation at proestrus and a wave of [³H]thymidine incorporation during metestrus-diestrus.