Prolactin storage in a clonal strain of rat pituitary tumor cells is cell-cycle dependent.

GH4C1 cells (CH cells) are a clonal strain of rat pituitary tumor cells which secrete prolactin. We measured intracellular prolactin at different stages of the cell cycle using flow microfluorometry. Prolactin was stained by an indirect immunocytochemical technique using fluorescein isothiocyanate (FITC)-conjugated antiserum, and DNA was stained simultaneously with propidium iodide. We found that prolactin storage in GH cells was cell-cycle dependent; prolactin storage increased as cells passed from G1 to S to G2 + M. We have shown previously that insulin and 17 beta-estradiol act synergistically to increase intracellular prolactin three- to sevenfold and slow the rate of cell growth to approximately 70% of control cells. In this study we observed that insulin and estradiol increased prolactin storage at each stage of the cell cycle but did not affect the cell-cycle distribution of the population even though cell growth was slowed. We conclude that insulin and estradiol did not increase prolactin storage by affecting the cell-cycle distribution of the population.