Oxytocin modulates the pulsatile secretion of prostaglandin F-2 alpha in initiated luteolysis in cattle

Subluteolytic doses of prostaglandin F-2 alpha analogue (oestrophan) given i.m. and oxytocin (OT) antagonist (CAP) and noradrenaline (NA) infused into the abdominal aorta were used to test the importance of luteal OT in pulsa tile secretion of prostaglandin F-2 alpha (PGF) during luteolysis in heifer s (n = 17). In experiment 1, heifers were pre-infused for 30 minutes with s aline on either day 17 of the oestrous cycle (group 1; n = 4) or on day 18 of the oestrous cycle (group 2; n = 3), and with CAP (8 mg per animal) on d ay 17 of the oestrous cycle (group 3; n = 4). Next, heifers were injected w ith oestrophan (30 mu g per animal). Injection of oestrophan in Group 3 inc reased or concentrations (P < 0.001) to values similar to those observed du ring spontaneous luteolysis (50 to 70 pg ml(-1)). PGFM concentrations in th is group also increased (P < 0.001), but were lower (P < 0.05) than the val ues in groups 1 and 2. CAP given prior to oestrophan decreased both PGFM el evation (P < 0.06) and its area under the curve (P < 0.01), compared to the saline pretreated heifers. In experiment 2 NA (4 mg) was infused twice for 30 minutes at five hour intervals to release or on day 17 of the oestrous cycle (n = 6). However, during hormone analysis it appeared that three of s ix heifers had elevated PGFM concentrations (group 1) and three others did not (group 2). NA caused the correlated increase of progesterone and or sec retion (r = 0.68; P < 0.05) in both groups but it only influenced PGF secre tion in group 1 only (P < 0.05). We postulate that or can amplify and modulate the course of induced luteoly sis as a regulator of the amplitude of pulsatile PGF secretion. PGF analogu e stimulates secretion of endogenous PGF from the uterus in cattle and this may be an important component of the luteolytic response to exogenous PGF.