Binding of thyrotropin releasing hormone and prolactin release by synchronized GH3 rat pituitary cell line.

GH3 cells were synchronized by growing them in a low serum concentration (1%). They were thereafter put back in normal medium (17.5% serum) (time 0 of synchronization). Four parameters were then examined every two hours for up to 40 hours : rate of ³H] thymidine incorporation, cell number, binding of ³H] Thyrotropin Releasing Hormone (TRH) after a 30 min exposure, and prolactin (PRL) content of culture medium and cell extract.The rate of thymidine incorporation presented a 10–20 fold increase in S phase, beginning on 12–16 hours and lasting at 26 hours. The cell population was doubled at 28 hours. ³H] TRH binding to attached cells was observed throughout the cell cycle, but presented a significant increase (40–80%) during the S phase. In contrast, the % increase of PRL release in response to TRH was optimum (300% of control) in G₁ phase. Variations of the PRL cell content as well as of the PRL spontaneous release ability of the cell do not account for the variations of TRH responsiveness. The discrepancy between the two parameters of the TRH-GH3 cells interaction strongly suggest a morphological or functional heterogeneity of the TRH-binding sites.