Hormone concentration changes temporally associated with the hour of transition from preluteolysis to luteolysis in mares

The temporal associations of cortisol, estradiol-17β, and oxytocin with pulses of PGFM at the common hour of transition between preluteolysis and luteolysis was studied in plasma from hourly blood samples in mares (n = 8). The transitional hour was determined from progesterone concentrations and occurred between 2 PM and 2 AM in all mares. Pulses of PGFM were grouped into those occurring at the last pulse of preluteolysis (preluteolytic pulse), at the hour of transition (transitional), and during luteolysis (luteolytic). The preluteolytic PGFM pulse (45 ± 16 pg/ml at peak) and transitional pulse (42 ± 7 pg/ml) are reportedly less prominent than the first luteolytic pulse (193 ± 36 pg/ml). Cortisol increased (P < 0.05) between −1 h and 0 h (peak) and then decreased (P < 0.05) within the hours of the luteolytic PGFM pulse but did not change within the preluteolytic and transitional pulses. Estradiol increased (P < 0.006) during −3 to 2 h of the luteolytic pulse but not for the other pulses. Oxytocin differed for the hours of the transitional PGFM pulse (P < 0.02) and the luteolytic pulse (P < 0.03) but did not differ significantly during the hours of the last preluteolytic pulse. Oxytocin increased (P < 0.05) between −3 h and 0 h and then decreased (P < 0.05) within each of the transitional and the luteolytic pulses. The oxytocin results are novel and support the hypothesis that on a temporal basis oxytocin in association with PGF2α accounts for the transition between preluteolysis and luteolysis within a single hour in mares, despite the small transitional PGFM pulse.