In vitro DNA synthesis as indicator of mammary epithelial cell division: [14C]thymidine uptake versus flow cytometry cell cycle analysis

Mammary and adipose explants from eight mid-lactation Holstein cows were co-cultured for 24 h in the presence or absence of liver explants, 1 μg/ml pituitary bovine somatotrophin, or 100 ng/ml insulinlike growth factor-I. Liver explants in the media significantly depressed DNA and protein synthesis by mammary tissue as measured by ¹⁴C]-thymidine and amino acid incorporation. As measured by flow cytometry, the concentration of DNA in the G₀G₁ and G₂M cells and the percentage of cells in the G₀G₁ population of mammary tissue was also significantly depressed by liver tissue. Changes in the percentage of cells in the S and G₂M phases were not significant. Insulinlike growth factor-I in the presence of liver explants depressed protein synthesis, thymidine incorporation, and the concentration of DNA in the G₀G₁ and G₂M cells compared to control but did not affect the percentage of cells in the G₀G₁, S, or G₂M phases. Previously it was assumed that changes in ¹⁴C]thymidine incorporation indicated that changes in cell division were occurring. Flow cytometry revealed that changes in DNA content of mammary cells as a result of liver or hormonal stimulation were not due to changes in cell division. Indications are that differences in cellular DNA content result from changes in the rate of amplification of individual genes responsible for milk protein synthesis.