INHIBIN AND ESTRADIOL-17-BETA IN ANTRAL FOLLICLES OF VARIOUS SIZE CLASSES OF CYCLIC RATS

On the various days of the 5-day oestrous cycle of the rat, ovarian an tral follicles were dissected out and grouped in five size classes. Fo ur follicles of the same size class were homogenized jointly in medium , after which inhibin-like bioactivity, inhibin immunoreactivity and o estradiol-17 beta content were measured. In general, there was a signi ficant correlation between immunologically and biologically active inh ibin levels in the different size classes; overall correlation was 0.8 5 (n=87, P<0.00001). In the smallest antral follicles (classes 1 and 2 ) inhibin bioactivity was detected only during the first three days of the cycle. With increasing follicle size, inhibin bioactivity and imm unreactivity increased, with maximal activity present in the largest, i.e. preovulatory, follicles (class 5) during the last three days of t he cycle (the day of oestrus denotes day 1 of the cycle). These result s indicate that only follicles which reach the antral stage at oestrus , and are known to be recruited by the periovulatory FSH peak, acquire the potency to produce biologically active inhibin. This is the cohor t of follicles from which selection of ovulatory follicles will normal ly take place. In contrast to inhibin, follicular oestradiol-17 beta c oncentrations were negligible until the last days of the cycle when oe stradiol-17 beta was present in follicles larger than class 2; levels increased with increasing follicle size and a maximal level was found in preovulatory follicles at pro-oestrus. It is concluded that (1) the re is a good correlation between follicular content of inhibin-like bi oactivity and inhibin immunoreactivity and (2) there are differences b etween patterns of follicular levels of inhibin immunoreactivity and o estradiol-17 beta during the ovarian cycle: follicles destined to ovul ate start to produce inhibin in volume classes 1 and 2 and only therea fter also contain oestradiol-17 beta. Follicles entering classes 1 and 2 at dioestrus-2 and -3 or at pro-oestrus do not produce inhibin and become atretic.