A NOVEL HYPOTHALAMIC PEPTIDE, PITUITARY ADENYLATE CYCLASE-ACTIVATING PEPTIDE, REGULATES THE FUNCTION OF RAT GRANULOSA-CELLS IN-VITRO

Pituitary adenylate cyclase-activating peptide (PACAP) is a novel pept ide that was isolated from ovine hypothalamic tissue on the basis of i ts ability to stimulate cAMP accumulation in cultured rat pituitary ce lls. Recently we demonstrated that PACAP can stimulate cAMP accumulati on and secretory function in cultured rat Sertoli cells. Since ovarian granulosa cells share many properties with Sertoli cells, we have exa mined the effect of PACAP (consisting of 38 or 27 amino acid residues) on cultured granulosa cell function. Granulosa cells were obtained fr om the ovaries of 25-day-old rats implanted with a silastic capsule co ntaining diethylstilbestrol 5 days prior to culture. PACAP 38 (0.1 mu M-0.01 pM), both alone and in the presence of the phosphodiesterase in hibitor, methylisobutylxanthine, stimulated cAMP accumulation 4-8-fold with an ED(50) of similar to 100 pM. Maximal PACAP 38 or PACAP 27 sti mulation of granulosa cell cAMP was significantly greater than that pr oduced by a maximally effective concentration of FSH. Because PACAP 38 and 27 have 68% sequence homology with vasoactive intestinal peptide (VIP), and since VIP stimulates granulosa cell cAMP accumulation and e stradiol and progesterone secretion, we examined the possibility that PACAP could be acting via the VIP receptor. VIP stimulated cAMP only a t concentrations of 10 nM or greater, whereas the PACAP stimulation wa s evident at 10 pM. Moreover, only one of th ree potent VIP antagonist s inhibited VIP stimulation of cAMP accumulation, and only at 1 mu M o r greater. This VIP antagonist did not inhibit PACAP 38 action at 2000 -fold excess concentration. Interestingly PACAP 38 was more effective than PACAP 27 with regard to steroid secretion and the ability to indu ce LH responsiveness. PACAP and VIP stimulation of granulosa cell cAMP accumulation or estradiol or progesterone secretion was not additive. Thus, these data support the hypothesis that granulosa cells have spe cific PACAP 38 receptors and that VIP acts via these receptors. In add ition, PACAPs 38 and 27 are more potent stimulators of cAMP accumulati on in luteinized granulosa cells than LH. These results both pre- and postovulation, along with previous data indicating that the PACAPs are found in the ovaries, suggest a role for PACAP in the regulation of o varian function.