ABSTRACT. Pituitary folliculo-stellate (FS) cells were able to modify the effect of activin-A on gonadotropes through the paracrine factor, follistatin. The present study was aimed to examine whether a hypothalamic peptide, pituitary adenylate cyclase activating polypeptide (PACAP), could be a regulator of this paracrine interaction. Co-culture of FS cell-originated cell line TtT/GF cells with rat anterior pituitary cells showed faint inhibitory effect on the stimulatory action of activin-A on FSH secretion. When PACAP was added to the culture during the co-culture period, however, the presence of TtT/GF cells caused significant suppression of the effect of activin-A on FSH secretion. Conditioned-media (CM) from TtT/GF cells, obtained by incubation of TtT/GF cells in the presence or absence of PACAP, were next added to the cultures of anterior pituitary cells alone. CM from TtT/GF cells without PACAP treatment revealed slight, but not significant, suppressive effect on activin-induced increases in FSH secretion and the percentage of FSH cells. Meanwhile, CM from PACAP-treated TtT/GF cells attenuated both effects of activin-A. Furthermore, the inhibitory effect of the CM was neutralized when follistatin antibody was present in the culture. These results suggest that PACAP is able to regulate the paracrine action of FS cells on pituitary gonadotropes. Besides expressing direct actions on pituitary endocrine cells, PACAP may have roles as a regulator of cell-tocell interactions within the pituitary gland.KEY WORDS: activin, conditioned-media, gonadotrope, PACAP, TtT/GF cell.J. Vet. Med. Sci. 62 (7): [731][732][733][734][735][736] 2000 Gonadotropins, follicle-stimulating hormone (FSH) and luteinizing hormone (LH), play important roles in the regulation of reproduction and fertility. Studies on the regulatory mechanisms of pituitary gonadotropes, the source of gonadotropins, are therefore essential to understand the physiology, pathology, as well as to develop remedies, of reproductive functions and disorders. Hypothalamic gonadotropin-releasing hormone (GnRH) and gonadal steroid hormones are classically known regulators of the gonadotrope functions, and today the inhibin, activin and follistatin have also obtained recognition as the regulators [7]. Activin not only stimulates the secretion of FSH [22,37] but also enlarges the population of FSH gonadotropes in primary cultures of rat anterior pituitary cells [17,18]. Our previous study demonstrated that the latter effect of activin-A (one of three forms of dimeric activin; -A, -AB and -B) was negatively controlled by pituitary folliculo-stellate (FS) cells through a paracrine factor, follistatin [16]. This and other studies concerning interactions of activin and follistatin in various tissues [3,6,23] have led us to the understanding that the balance of these two factors are important in the regulation of various biological functions. In the anterior pituitary gland, follistatin expression has been shown to vary under different conditions [8,12]. Although hypothal...

Abstract.Activin-A induces increases in FSH secretion, as well as the number of immunoreactive FSH cells, in cultured rat pituitary cells. In this study, we examined whether mechanisms involved in these two actions of activin-A are identical or not, with respect to the involvement of cellular proliferation and of Cat+-dependent signaling pathways.Treatment with activin-A (25 ng/ml) for 48 h caused increases in the number of cultured rat anterior pituitary cells that incorporated BrdU, a thymidine analog. The stimulatory effects of activin-A on FSH secretion and on the percentage of immunoreactive FSH cells were, however, not inhibited by the presence of the mitotic inhibitor cytosine arabinoside.On the other hand, the stimulatory effect of activin-A on the percentage of immunoreactive FSH cells was completely blocked in the presence of the Ca2+/calmodulin kinase inhibitor KN-62 or the L-type Ca2+ channel blocker nicardipine.Neither of these inhibitors, however, revealed significant influence on the effect of activin-A on FSH secretion. These results suggest that activin-A exhibits its dual effect on FSH cells without causing cellular proliferation.Furthermore, activin-A appears to induce increases in FSH secretion and enlargement of FSH cell population through distinct intracellular signaling pathways, the former through Cat-independent and the latter through Cat+-dependent mechanisms.