STIMULATION OF OVARIAN ADENYLYL-CYCLASE ACTIVITY BY GONADOTROPINS DURING THE NATURAL AND GONADOTROPIN-INDUCED CYCLES IN THE HAMSTER

In this study we utilized the hamster ovary as a model to investigate the effects of ovulation induction with gonadotrophin on the activatio n of the signal transducer effector system, adenylyl cyclase (AC). For this purpose, we prepared membrane particles from the ovary and analy sed both gonadotrophin-sensitive AC and non-receptor-mediated activati on during a cycle in which ovulation and luteinization were achieved b y pregnant mare's serum gonadotrophin (PMSG)/human chorionic gonadotro phin (HCG) administration. Results were directly compared with AC acti vation in similarly prepared membranes obtained at different stages of the natural unstimulated cycle. AC activity was quantified by the dir ect conversion of ATP substrate into cyclic adenosine monophosphate (c AMP). Measurements of ovarian weights, serum oestradiol and progestero ne concentrations provided a solid base from which to evaluate the fun ctional status of the ovary at each time period during the natural and stimulated cycles. We found that ovarian membranes contain functional components of the AC system and demonstrated that AC is highly depend ent on hormonal changes and the functional state of the ovary. Thus, d uring the natural cycle, ovarian AC showed relatively constant respons iveness to follicle-stimulating hormone (FSH) throughout the cycle, wh ereas responsiveness to luteinizing hormone (LH)/HCG reached its peak during the luteal phase. On the other hand, during the stimulated cycl e, sensitivity to FSH and LH/HCG varied considerably, being absent dur ing the peri-ovulatory period. AC responsiveness to gonadotrophins was only regained 48 h after ovulation. Also during the peri-ovulatory pe riod of the gonadotrophin-induced cycle, stimulation of ovarian AC wit h non-hormonal activators declined. However, the rate of cAMP producti on in response to these activators remained very high, indicating that despite refractoriness to gonadotrophins, ovarian AC retained the cap acity to generate cAMP at near maximal efficiency. Basal (non-stimulat ed) activity, guanine nucleotide activation, hormone responsiveness an d stimulation by the non-hormonal activators NaF and forskolin were al l significantly increased in comparison with the natural cycle. Basal activity alone was similar to 7-fold higher than the activity observed during the unstimulated cycle. These results suggest that subsequent to exogenous gonadotrophin administration, the transmembrane effector AC system must be primed for a higher level of activity in the ovarian tissue. This priming of the ovarian AC system by exogenous gonadotrop hin was also evident when the enzyme was measured under conditions all owing maximal activity, i.e. in the presence of a combination of NaF a nd forskolin. Maximal AC activity increased 4- to 5-fold compared with the natural cycle. We conclude that gonadotrophin administration indu cing ovulation causes profound alterations in the expression of AC in ovarian membranes. Gonadotrophin treatment increased the enzyme activi ty and induced a temporal desensitization to FSH and LH/HCG in the per i-ovulatory period of the stimulated cycle. Because the gonadotrophin- sensitive AC system represents the capacity of FSH and LH/HCG receptor s to couple and elicit a biological response, our results provide new insights into the cellular mechanisms that regulate ovarian activity d uring induction of ovulation.