Uterine secretion of prostaglandin F2alpha stimulated by different doses of oxytocin and released spontaneously during luteolysis in cattle.
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The objectives were to determine the involvement of oxytocin (OT) in the stimulation of prostaglandin F2alpha (PGF) secretion during luteolysis in cattle. On days 16-17 of the oestrous cycle, catheters were inserted into the aorta abdominalis of heifers for OT or saline infusion and into the jugular vein for blood sample collection. The following day, heifers were assigned to one of three experimental groups (Gr): Gr I - 10 IU OT (n=4); Gr II - 20 IU OT (n=4); Gr III - 50 IU OT (n=4). Blood samples were collected every 10 min during a 1-h control period before treatment and every 5-10 min for 2 h after OT treatment. In Gr IV (n=5), a catheter was inserted into the jugular vein on day 15 of the cycle and blood samples were collected every 15 min for 12 h on days 16-19. Plasma concentrations of progesterone, PGF metabolite, 13, 14-dihydro-15-keto-prostaglandin F2alpha (PGFM) and OT were determined. Within 5 min of infusion of 10 or 20 IU OT, peripheral concentrations of OT (7-12 pg/mL) increased by about 200 and 350-500 pg/mL, respectively. These doses did not affect plasma concentrations of PGFM or progesterone within 1.5 h. Fifty IU of OT increased its maximal peripheral concentration to 1500 pg/mL, which is over 20 times greater than that observed physiologically. Concentrations of plasma PGFM in Gr III increased from basal concentrations (5065 pg/mL) to 150-250 pg/mL (P < 0.01) within 10-30 min. During luteolysis, PGFM pulses ranged between 250 and 600 pg/mL on days 16-19 of the cycle (Gr IV), whereas coincident pulses of OT, and those appearing between spikes of PGFM, were never above 75 pg/mL. Only 50% of OT pulses coincided with pulses of PGFM, and 54 % of PGFM pulses coincided with a pulse of OT. Results indicate that luteolytic PGF secretion in cattle is not directly dependent upon ovarian OT.