Changes in ovarian oxytocin secretion as an indicator of corpus luteum response to prostaglandin F 2α treatment in cattle

Abstract

Exogenous prostaglandin F 2α (PGF 2α) rapidly increases ovarian oxytocin (OT) release and decreases progesterone (P4) secretion in cattle. Hence, the measurement of OT secretion (the area under the curve and the height of the peak) after different doses of Oestrophan — PGF 2α analogue (aPGF 2α) on Days 12 and 18 of the estrous cycle (estrus = day 0), could be a suitable indicator of corpus luteum (CL) sensitivity to PGF 2α treatment. Mature heifers (n = 36) were used in this study. Blood samples were collected from the jugular vein for the estimation of OT, P4 and 13, 14-dihydro-15-keto-prostaglandin F 2α (PGFM). In Experiment 1, different doses of aPGF 2α (400, 300, 200 and 100 μg) given on Day 12 of the estrous cycle (n = 8) shortened (P < 0.05) the cycle duration (15.2 ± 0.6 d) compared with that of the control (21.7 ± 0.4 d). Successive heifers were also treated on Day 12 with 200 (n = 2), 100 (n = 2), 75 (n = 2) or 50 μg aPGF 2α (n = 2). Only the 50 μg aPGF 2α dose did not cause CL regression, although it increased OT concentrations to levels comparable to those observed during spontaneous luteolysis (50 to 70 pg/ml). In Experiment 2, on Day 18 of the cycle heifers (n = 8) were treated with 50, 40, 30 and 20 μg aPGF 2α. There was a dose-dependent effect of aPGF 2α on OT secretion on Day 18 of the estrous cycle (r = 0.77; P < 0.05). In Experiment 3, an injection of 500 μg aPGF 2α on Day 12 (n = 4) and 50 μg aPGF 2α on Day 18 (n = 4) caused a similar (P > 0.05) increase in the OT concentration (288.5 ± 23.0 and 261.5 ± 34.7 pg/ml, respectively). Thus the effect of the same dose of aPGF 2α (50 μg) on OT secretion was different on Days 12 and 18 of the cycle. To evoke similar OT secretion on Days 12 and 18 the dose of aPGF 2α on Day 18 could be reduced 10-fold, confirming that CL sensitivity to PGF 2α appears to increase in the late luteal phase.