STEROIDOGENESIS IN THE PREOVULATORY PORCINE FOLLICLE

The expression of four major steroidogenic enzymes in porcine theca an d granulosa cell layers of preovulatory follicles was related to the l evels of steroids in follicular fluid and gonadotropin concentrations in peripheral serum at slaughter. Ovaries were collected during proest rus, early estrus, and late estrus as evidenced by behavioral signs. F ollicles were dissected from the ovaries, and theca, granulosa, and fo llicular fluids were pooled for each of 24 sows. Cytochromes P450 17 a lpha-hydroxylase/17-20 lyase (P450(cl7)), aromatase (P450(arom)), and side-chain cleavage (P450(scc)), as well as 3 beta-hydroxysteroid dehy drogenase (3 beta HSD), were subjected to Northern and Western immunob lot analyses. The concentrations of estradiol-17 beta, testosterone, a ndrostenedione, and progesterone were determined in follicular fluid, and peripheral serum was assayed for estradiol-17 beta, LH, and FSH. S tages of preovulatory development were verified by plasma levels of LH , FSH, and estradiol-17 beta. Theca expressed P450(cl7), P450(arom), P 450(scc), and 3 beta HSD whereas granulosa expressed only P450(arom) a nd low levels of P450(scc). Thecal P450(cl7), thecal P450(arom), and g ranulosa P450(arom) expression decreased coincidentally as serum estra diol-17 beta and follicular fluid estradiol-17 beta, testosterone, and androstenedione levels declined after the presumed gonadotropin surge . Unlike P450(cl7) and P450(arom), P450(scc) and 3 beta HSD remained r elatively constant in theca and granulosa. From these data, we suggest that the theca interna may be the primary steroidogenic compartment o f the porcine follicle during its final stages of preovulatory develop ment. Moreover, preovulatory estrogen secretion appears to be controll ed by the coordinated expression of a triad of enzymes in the porcine follicle that includes theca P450(cl7), theca P450(arom), and granulos a P450(arom).