CELL LOSS AND INFLUX OF LABELED HOST CELLS IN THREE TRANSPLANTABLE MOUSE TUMORS USING [125I]UdR RELEASE

The ¹²⁵I]UdR loss technique was used to estimate cell loss from RIF-1, EMT6 and KHJJ tumors in order to determine the length of the delay between labeling and the beginning of the loss of labeled cells, and also to calculate a value for ø, the cell loss factor. To determine the importance of reutilization of label released from the gut and/or the influx of labeled host cells, the blood flow to some tumors was occluded during and for 30 min after injection of the label. Relatively small amounts of radioactivity entered occluded RIF-1 tumors during 9 days after injection of ¹²⁵I]UdR, indicating that reutilization of systemic label and influx of labeled host cells are not significant in this system. In contrast, substantial amounts of radioactivity entered occluded EMT6 and KHJJ tumors, reaching 40% of the total activity in non-occluded tumors during 6 days following injection. After corrections were made for this influx of label, the ¹²⁵I]UdR loss curves from RIF-1 and EMT6 tumors were essentially exponential from the first day following injection of label. This was interpreted as indicating the loss of proliferating as well as non-proliferating cells from both tumors. The cell loss factor derived from the ¹²⁵I]UdR loss curves corrected for influx appeared to agree well with published values derived from analysis of percent labeled mitoses curves. In contrast, the corrected ¹²⁵I]UdR loss curves from KHJJ tumors showed that loss of activity began three days after injection of label, indicating that primarily nonproliferating cells are lost from this tumor.