Insulin and 17 beta-estradiol increase the intracellular prolactin content of GH4C1 cells.

We investigated the effects of insulin and 17 beta-estradiol in PRL-producing rat pituitary tumor cells (GH cells) in culture. Incubation with 1.8 X 10(-7) M insulin and 1 X 10(-9) M estradiol always increased intracellular PRL significantly to a mean of 290% of control levels. Insulin or estradiol alone caused smaller increases in intracellular PRL that were significant in five out of six experiments with insulin and three out of five experiments with estradiol, producing average increases of 150% and 160% of controls, respectively. Combined treatment with insulin and estradiol increased PRL secreted into the medium to 160% of controls, which was smaller than the effect on intracellular PRL. The effects of insulin and estradiol were detectable after 2-4 days of treatment and were maximal between 6-8 days. The effects were dose dependent; the half-maximal dose of insulin was about 2 X 10(-8) M and of estradiol was about 5 X 10(-11) M. Treatment with both insulin and estradiol increased cellular uptake of [3H]leucine by 45% compared to controls, incorporation of [3H]leucine into PRL by 65%, and total protein by 50%. The increased incorporation into PRL may represent an increase in PRL synthesis, but it is not enough to account for the increased intracellular hormone. Insulin and estradiol did not change the time required for most newly synthesized PRL to be processed and released from the cells, which was 60 min. Therefore, the amount of intracellular PRL was not increased as a result of an increased intracellular transit time. We conclude that insulin and estradiol increase the amount of intracellular PRL as a result of either decreasing the rate of degradation or increasing the cells' capacity to store PRL.