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.