Temporal dynamics of pituitary prolactin depletion and release in three strains of rats

Previous reports have shown that pituitary prolactin is rapidly transformed to a less soluble, but much more releasable, form prior to release from the lactotroph. One manifestation of this transformation is that pituitary prolactin depletion is significantly greater than concurrent release both in vivo and in vitro. The objective of this study was to compare the magnitude and temporal dynamics of depletion and release from pituitaries of ovariectomized estrogen-treated rats of three different strains in vitro to assess the effect of strain on the transformation process. Mature ovariectomized Wistar-Furth (WF), Sprague-Dawley (SD) and Long-Evans (LE) rats (7-10/group) were killed by decapitation 7 days after a single s.c. injection of 100 micrograms of polyestradiol phosphate. The anterior pituitaries were quickly removed and cut into quarters which were incubated for up to 4 hrs in the absence of dopamine or other prolactin secretagogues. Representative fragments from each strain were not incubated but were snap frozen to measure pre-incubation content. Fragments from each strain were removed from incubation at 30, 60, 120, 180 and 240 min for prolactin content measurement. Medium was collected at 30 min intervals and replaced with fresh medium. The experiments were repeated twice. Prolactin in medium and pituitary homogenates was measured by radioimmunoassays using NIAMDD-RP-1 as standard. In all three stains release of prolactin was approximately 30-50% of the prolactin depleted from the pituitary in 4 hrs. Strains varied in the magnitude of this difference and the time course over which it occurred. WF and SD rats showed significantly greater depletion and release of prolactin than did LE rats when the data were expressed as micrograms prolactin/mg pituitary. When the data were expressed as a percentage of prolactin available for release, the differences in depletion between strains disappeared and the LE rats released a significantly greater percentage of the prolactin available for release than did the other two strains. We conclude that pituitary prolactin undergoes a process of transformation prior to release which causes it to disappear from the pituitary but not appear in culture medium. We further conclude that the magnitude and temporal dynamics of this process are not equivalent across all strains of rats.