INTRAPITUITARY DISTRIBUTION AND EFFECTS OF ANNEXIN-5 ON PROLACTIN-RELEASE

Annexin 5 is expressed by rat anterior pituitary cells and a depolariz ing stimulus results in increased extracellular display and, depending on local calcium concentrations, potential release into the extracell ular environment. In order to further investigate the role of annexin 5 in anterior pituitary function, we have examined the intracellular d istribution by immunocytochemistry and the effects of annexin 5 on the release of a major secretory product, prolactin. Prolactin was chosen because we could easily monitor effects on basal release and effects on the immediate and sustained phases of thyroid stimulating hormone r eleasing hormone (TRH)-stimulated release. Immunocytochemical localiza tion of annexin 5 showed staining of the majority of anterior pituitar y cells. Labeling was predominantly on the nuclear envelope and plasma membrane. For the chosen secretory product, prolactin, annexin 5 was found in most, but not all prolactin positive cells. When recombinant annexin 5 (50 ng/mL) was added to a 3 h static culture incubation of r at anterior pituitary cells, prolactin release was inhibited by about 30% (p < 0.05). A lower dose had a reduced effect and higher doses had no further inhibitory effect, indicating that the effect was specific to annexin 5 and not a nonspecific toxic effect of some contaminant i n the preparation. This interpretation was further strengthened in a t ime-course experiment demonstrating that when TRH and annexin 5 were a dded together, there was no effect of an nexin 5 on the amount of prol actin released. After a 3 h preincubation in annexin 5, however, prola ctin release, in response to TRH, was suppressed by about 30% in both the acute and sustained phases. These data suggest that annexin 5 may be a local regulator of release in the anterior pituitary, but a slow onset effect on both phases of TRH-stimulated release suggests that th is is not an effect at the plasma membrane such as local extracellular calcium depletion by plasma membrane-bound annexin 5.