CHARACTERISTICS OF PROLONGED DOMINANT VERSUS CONTROL FOLLICLES - FOLLICLE CELL NUMBERS, STEROIDOGENIC CAPABILITIES, AND MESSENGER-RIBONUCLEIC-ACID FOR STEROIDOGENIC ENZYMES

Cattle with low (subluteal) levels of plasma progesterone develop it p ersistent dominant follicle; plasma estradiol and LH pulse frequency a re elevated, and fertility subsequent to the ovulation of a prolonged dominant follicle is compromised. The hypotheses were 1) that prolonge d dominant follicles produce more estradiol because they have theca an d granulosa cells with an enhanced capacity to produce androgen and es tradiol, respectively, and 2) that these changes in steroidogenic capa city are paralleled by concomitant changes in mRNA for the appropriate steroidogenic enzymes. Prolonged dominant follicles were induced by t reating Holstein heifers with exogenous progesterone via an intravagin al controlled internal drug-release device (CIDR) from Day 14 to 28 of the cycle. Prolonged dominant follicles were collected just before (C IDRb, Day 28; n = 4) or 24 h after (CIDRa, Day 29; n = 4) CIDR removal , and their steroidogenic capacity was compared to that of growing, co ntrol dominant follicles obtained just before (CONTb, n = 4) or 24 h a fter (CONTa, n = 4) a luteolytic injection of prostaglandlin F-2 alpha during the late luteal phase. After natural luteolysis, CIDR heifers maintained subluteal concentrations of progesterone (1-2 ng/ml) and ha d higher estradiol and LH pulse frequency than control heifers, as exp ected. In CIDR heifers, prolonged dominant follicles were present on t he ovary for a longer time, reached a larger diameter, and had more gr anulosa cells and a larger mass of theca than dominant follicles from control heifers (p < 0.05). Concentrations of steroids in follicular f luid, estradiol secretion by granulosa cells in vitro, and levels of m RNA for steroidogenic enzymes in theca and granulosa cells provided no evidence for greater capacity of theca and granulosa cells of CIDR fo llicles to produce androgen and estradiol. In fact, follicular fluid e stradiol and mRNA for P450 aromatase were higher after luteolysis than before in control animals (p < 0.05) but not after CIDR removal in tr eated animals. Therefore, the data do not support the hypotheses. Rath er it is suggested that prolonged dominant follicles produce more estr adiol because they have more granulosa cells and a larger mass of thec a than control dominant follicles. In contrast, progesterone concentra tions in the follicular fluid increased in CIDRa relative to CIDRb fol licles (p < 0.05), a change that did not occur in control follicles; a nd granulosa cells from CIDRa follicles secreted more progesterone tha n granulosa tells from any other group. The increased capacity of CIDR a follicles to secrete progesterone suggests premature luteinization, which could contribute to decreased fertility in cattle that ovulate a prolonged dominant follicle.