Abstract. Adenylate cyclase activity was measured in membranes prepared of granulosa cells isolated from PMSG-treated immature rats and the effects of ovine, highly purified human and rat gonadotrophins were compared. Furthermore, a comparison of the effects of the human preparations (hLH, hFSH) on the adenylate cyclase activity in membranes prepared from granulosa cells isolated at different stages of follicular maturation, was performed.
The adenylate cyclase in membranes of immature granulosa cells was stimulable with FSH but not with LH, while in pre-ovulatory granulosa cell membranes, both gonadotrophins were stimulatory with FSH generally being more effective than LH. Surprisingly, the dose-response curve for ovine LH (oLH) was biphasic with a plateau at a level of adenylate cyclase activity corresponding to the maximal stimulatory effect of hCG. With increasing oLH concentrations the response resumed and the maximal stimulation corresponded to that of oFSH. With highly purified rat gonadotrophins the FSH response was significantly higher than the response to LH at all concentrations tested. Using highly purified human gonadotrophins the maximal FSH response was 50% higher than the maximal LH response and by adding increasing concentrations of hFSH to a maximally stimulatory concentration of hLH it was possible to mimic the biphasic dose-response curve for oLH. When the membranes were prepared from ganulosa cells isolated after the pro-oestrus LH/FSH surge there was clear increase in the sensitivity of the adenylate cyclase to stimulation with LH although the maximal response was unaffected. The sensitivity to FSH was not altered.
These data indicate that FSH gives a higher maximal stimulation of the adenylate cyclase in rat granulosa cells than does LH, and the biphasic dose-response curve seen with oLH most probably is due to a FSH contamination in the LH preparation used. After in vivo exposure to the pro-oestrus LH/FSH surge no desensitization was observed but instead a clear increase in LH sensitivity. It thus seems as if the primary FSH sensitive cell under the influence of the LH/FSH surge turns to a primary LH sensitive one.