D-valine medium maintains prolactin production in primary culture

Prolactin secretion by bovine pituitary cells in L-valine-containing medium decreases approximately 96% from day 3 to day 11 of culture. We hypothesized that this decrease was caused by overgrowth of these cultures by fibroblasts. Our present objective was to maintain the synthesis and secretion of PRL by bovine pituitary cells in culture. We attempted this by growing pituitary cells in D-valine-containing medium to achieve selective suppression of fibroblast growth. Substitution of D-valine for L-valine in Earle's or Swim's medium resulted in undiminished PRL synthesis and release over a 30-day culture period. In contrast, comparable measures for cells maintained in medium with L-valine decreased more than 90% from day 5 to day 20 of culture and remained low thereafter. Cells cultured in medium containing D-valine retained their ability to release PRL in response to thyrotropin-releasing hormone throughout the 30-day culture period, although there was a decrease in magnitude of response with time. Similarly, estradiol increased PRL release by pituitary cells maintained in D-valine, but this stimulatory effect was no longer demonstrable by day 20 of culture. The amount of growth hormone (GH) and luteinzing hormone (LH) released into the medium decreased with time and this decrease was independent of the valine isomer contained in the medium. We conclude that substituting D-valine for L-valine in culture medium allows PRL synthesis and release to persist undiminished for at least 30 days in culture.