IN-VITRO DNA-SYNTHESIS AS INDICATOR OF MAMMARY EPITHELIAL-CELL DIVISION - [C-14] THYMIDINE UPTAKE VERSUS FLOW-CYTOMETRY CELL-CYCLE ANALYSIS

Mammary and adipose explants from eight mid-lactation Holstein cows we re co-cultured for 24 h in the presence or absence of liver explants, 1 mug/ml pituitary bovine somatotrophin, or 100 ng/ml insulinlike grow th factor-1. Liver explants in the media significantly depressed DNA a nd protein synthesis by mammary tissue as measured by [C-14]-thymidine and amino acid incorporation. As measured by flow cytometry, the conc entration of DNA in the G0G1 and G2M cells and the percentage of cells in the G0G1 population of mammary tissue was also significantly depre ssed by liver tissue. Changes in the percentage of cells in the S and G2M phases were not significant. Insulinlike growth factor-I in the pr esence of liver explants depressed protein synthesis, thymidine incorp oration, and the concentration of DNA in the G0G1 and G2M cells compar ed to control but did not affect the percentage of cells in the G0G1, S, or G2M phases. Previously it was assumed that changes in [C-14]thym idine incorporation indicated that changes in cell division were occur ring. Flow cytometry revealed that changes in DNA content of mammary c ells as a result of liver or hormonal stimulation were not due to chan ges in cell division. Indications are that differences in cellular DNA content result from changes in the rate of amplification of individua l genes responsible for milk protein synthesis.