In vitro labeling and gold activation autoradiography for determination of labeling index and DNA synthesis times of solid tumors.

In vitro labeling and gold activation autoradiography were used to determine the [3H]thymidine ([3H]TdR)-labeling indices and DNA synthesis times for C3H/He spontaneous mammary tumors. Three variations of the [3H]TdR, [14C]thymidine ([14C]TdR) double-labeling method, together with double-emulsion autoradiography, were used to determine the DNA synthesis times (TS). Tumors labeled totally in vivo (in vivo-in vivo method) and tumors labeled with [3H]TdR in vivo and subsequently labeled with [14C]TdR in vitro showed similar TS values. DNA synthesis times for tumors determined totally in vitro by double labeling (in vitro-in vitro method) were significantly longer than those observed in vivo; however, identical samples subjected to Hypaque-Ficoll gradient separation after double labeling showed TS's similar to those found in vivo. Furthermore, the interval between [3H]TdR and [14C]TdR administration had no effect on TS estimates in vitro. Gold activation autoradiography was used in the present experiments to reduce autoradiographic exposure times. This method, together with in vitro labeling, permits [3H]TdR labeling index and TS determinations after 6-hr and 7-day exposures, respectively.