Increase in progesterone and luteal blood flow without a luteolytic response after prostaglandin F2α treatment in early luteal-phase heifers

Acute changes in circulating progesterone concentration and luteal blood flow in heifers after a conventional dose of prostaglandin F2α (PGF; 25 mg dinoprost, i.m.) were compared between the early luteal phase (Day 3) and midluteal phase (Day 10; Day 0 = ovulation), using four groups (Day-3 control, Day-3 PGF, Day-10 control, and Day-10 PGF; n = 6 heifers/group). Blood samples were collected at 0, 2, 5, 10, 15, 30, 60, and 120 min (0 min = treatment). Percentage of luteal area with color-Doppler blood-flow signals was estimated at 0, 10, and 30 min. In the Day-3 and Day-10 PGF groups, progesterone increased to a peak at 15 min. In the Day-3 PGF group, progesterone decreased to the pretreatment concentration by 60 min but did not decrease to below the pretreatment concentration during the 2-h experimental period. In the Day-10 PGF group, progesterone decreased to below pretreatment concentration by 30 min, indicating a luteolytic response. In the Day-3 and Day-10 PGF groups, luteal blood flow increased within 10 min and remained elevated until the last examination at 30 min. The absence of a decrease in progesterone to below pretreatment concentrations in the Day-3 PGF group indicated that luteolysis does not necessarily follow a transient increase in progesterone and a concomitant increase in luteal blood flow. The immediate transient increase in progesterone and an increase in luteal blood flow without a subsequent decrease in progesterone to below pretreatment concentrations after PGF treatment in early luteal-phase heifers are novel findings.