Two methods of inducing low plasma progesterone concentrations have different effects on dominant follicles in cows

The effects of two methods of inducing low progesterone concentrations on t he shape of the plasma progesterone curve and on follicular characteristics in lactating cows were studied. A low ascending progesterone curve was eli cited by three PGF(2 alpha) injections on d 3 to 4 of the estrous cycle; a low constant curve by induction of corpus luteum regression on d 6 and inse rtion of two progesterone-containing intravaginal devices from d 6 to 15 of the cycle. Plasma progesterone concentration was highest in the untreated control group, intermediate in low ascending group, and lowest in the low c onstant group. On d 15, both control and low ascending groups had one large healthy and one large atretic follicle, suggesting a turnover of follicula r waves; in the low constant group, the presence of only one very large hea lthy follicle indicated follicular persistence. Estradiol concentration in the follicular fluid and its production by granulosa cells were highest in the low constant, intermediate in the low ascending, and lowest in the cont rol group. Androstenedione concentration in the follicular fluid and its pr oduction by theca cells were higher in the low constant than in the low asc ending and control groups. The results indicate that the low ascending prog esterone curve affected follicular development and steroidogenesis differen tly from the low constant curve. We suggest that the low ascending curve mi mics the effects of naturally occurring low plasma progesterone concentrati ons better, and it might, therefore, be used as a model for studying the ef fects of low plasma progesterone on fertility.