Subclinical, chronic intramammary infection lowers steroid concentrations and gene expression in bovine preovulatory follicles

Various doses of estradiol-17β (E₂) were used in heifers to induce a pulse of 13,14-dihydro-15-keto-prostaglandin F_2α (PGFM). The effect of E₂ concentration on the prominence of PGFM pulses and the relationship between prominence and intrapulse concentration of progesterone (P₄), LH, and luteal blood flow were studied. A single dose of 0 (vehicle), 0.01, 0.05, or 0.1 mg of E₂ was given (n = six/group) 14 d after ovulation. Blood samples were collected, and luteal blood flow was evaluated hourly for 10 h after the treatment. The 0.05-mg dose increased and the 0.1-mg dose further increased the prominence of the induced PGFM pulse, compared with the 0.0-mg dose and the 0.01-mg dose. The PGFM pulses were subdivided into three different prominence categories (<50, 50 to 150, and >150 pg/mL at the peak). In the 50 to 150 category, P₄ concentration increased (P < 0.05) between −2 h and 0 h (0 h = peak of PGFM pulse). In the >150 category, P₄ decreased (P < 0.05) between −1 h and 0 h, LH increased (P < 0.05) at 1 h, and luteal blood flow apparently decreased (P < 0.05) at 2 h of the PGFM pulse. The novel results supported the following hypotheses: (1) an increase in E₂ concentration increases the prominence of a PGFM pulse, and (2) greater prominence of a PGFM pulse is associated with a greater transient intrapulse depression of P₄ at the peak of the PGFM pulse. In addition, the extent of the effect of prostaglandin F_2α on the increase in LH and changes in blood flow within the hours of a PGFM pulse was related positively to the prominence of the PGFM pulse.