THE ROLE OF PREGNENOLONE-METABOLIZING ENZYMES IN THE REGULATION OF ESTRADIOL BIOSYNTHESIS DURING DEVELOPMENT OF THE FIRST WAVE DOMINANT FOLLICLE IN THE COW

During the luteal phase in the cow, a first-wave dominant follicle gro ws to reach ovulatory size, but then ceases to grow, becomes no longer dominant and enters a phase of slow regression. During this growth tr ansition, the concentration of oestradiol has been shown to decrease i n follicular fluid. The objective of this study was to determine if fo llicular fluid oestradiol concentrations are regulated by the activity of three major steroidogenic enzymes, namely P450-aromatase (P450-aro m), 3 beta-hydroxysteroid dehydrogenase/Delta 5-Delta 4 isomerase (3 b eta-HSD) and 17 alpha-hydroxylase C-17,20 lyase cytochrome P450 enzyme (P450-17 alpha) measured in granulosa and theca cells isolated from i ndividual first-wave dominant follicles. Follicle growth and state of dominance was assessed by ultrasonography and follicles were classifie d as growing-dominant (GD, n=6), non-growing-dominant (NGD, n=8) or no n-growing-non-dominant (NGND, n=6). Mean follicular fluid concentratio ns of oestradiol were higher in GD than in NGD or NGND follicles (511 +/- 98 versus 136 +/- 16 and 20 +/- 11 nmol/l respectively). Oestradio l was not correlated with P450-arom in any of the three groups. In GD follicles, oestradiol was positively correlated with pregnenolone conc entration but neither was correlated with granulosa or theca 3 beta-HS D activity or with theca P450-17 alpha activity. In NGD follicles, oes tradiol was negatively correlated with theca 3 beta-HSD activity and p regnenolone was negatively correlated with granulosa 3 beta-HSD activi ty. In NGND follicles, oestradiol was positively correlated, and pregn enolone was negatively correlated with theca 3 beta-HSD and P450-17 al pha activities. These studies demonstrated that pregnenolone supply is the principal regulating factor of oestradiol output during follicle dominance and during the loss of dominance but that the levels of P450 -17 alpha and 3 beta-HSD activity become rate-limiting when the follic le is no longer dominant.