Effect of continuous infusion of oxytocin on prostaglandin F2 alpha secretion and luteolysis in the cyclic ewe

Abstract

Circulating concentrations of 13,14-dihydro-15-keto PGF2 alpha (DHKF2 alpha), luteinizing hormone (LH) and prolactin (PRL) have been measured in cyclic ewes treated with continuous infusions of oxytocin, in order to investigate the mechanism by which the treatment delays luteal regression. Continuous infusion of oxytocin reduced prostaglandin F2 alpha (PGF2 alpha) secretion but had no detectable direct effect on LH or PRL. Oxytocin (3 nmol h-1 i.v.) given from Day 12 or 13 until Day 18 after oestrus delayed luteolysis, eight out of nine treated ewes not returning to behavioural oestrus until Day 29.1 +/- 3.2 (mean +/- s.e.m.; cycle length of control ewes 16.7 +/- 0.3 days). In the ewe in which oxytocin failed to prevent luteolysis, luteal regression had commenced before oxytocin treatment was started. In three ewes undergoing delayed luteolysis (cycle lengths, 21, 24 and 25 days) basal concentrations of PGF2 alpha (measured as DHKF2 alpha) were unchanged, but there was only one episode of PGF2 alpha secretion compared with 20 episodes in three control ewes. Prolactin secretion was pulsatile during oxytocin infusion, and levels were low following infusion in ewes with cycle length greater than 25 days while the corpora lutea were maintained. Circulating PRL concentrations were high in ewes undergoing delayed luteolysis but there was not discrete episode of PRL secretion associated with the pre-ovulatory LH surge in these animals. To investigate the possibility that the pattern of PGF2 alpha secretion was affected by depletion of oxytocin from corpora lutea, ewes previously treated with oxytocin to delay luteolysis were given a luteolytic dose of cloprostenol on Day 21 after oestrus. The amount of oxytocin secreted in response to cloprostenol was less than 10% of that seen in ewes similarly treated on Days 11-13 after oestrus. Low levels of luteal oxytocin may therefore reduce PGF2 alpha secretion in ewes undergoing delayed luteolysis.

Effect of intra-ovarian infusion of oxytocin on plasma progesterone concentrations in pregnant ewes

“…In nonpregnant women (Richardson & Masson, 1985) and sheep (Rodgers et al, 1985), basal as well as gonadotrophin-stimulated pro¬ gesterone secretion from the corpus luteum, in vitro, remained unaffected by oxytocin. Further¬ more, there is evidence that oxytocin infusion has no effect on progesterone secretion in cyclic sheep (Sheldrick & Flint, 1990), cattle (Gilbert et al, 1989) or monkeys (Wilks, 1983). These results are in agreement with our earlier study on oxytocin receptors in sheep (Sernia et ai, 1989).…”

“…The jugular plasma concen¬ tration of oxytocin did not change significantly from a control value of 2-5 + 1-5 pg OT/ml (Fig. 1), while in the oxytocin-infused group the concentration increased from 9-6 + 12 to 202 + 50 pg OT/ ml at 1 h and to 268 ± 80 pg OT/ml at 4 h. A striking, but not unusual (Flint et ai, 1989;Sheldrick & Flint, 1990) groups. In the saline-infused group, no significant changes in concentration were observed over the 4h of infusion, while a significant decrease (anova, < 001) occurred in the oxytocin-infused group.…”

“…Current evidence suggests that oxytocin stimulates rather than inhibits gonadotrophin secretion (Robinson & Evans, 1990). Moreover, in recent experiments in which cyclic ewes were infused with oxytocin for 5 days, there was no indication of a direct effect on luteinizing hormone (LH) or progesterone secretion (Sheldrick & Flint, 1990). Yet another expla¬ nation of our results would be vasoconstriction of the ovarian vasculature by the high concen¬ tration of oxytocin.…”

Effect of oxytocin infusion on secretion of progesterone and luteinizing hormone and the concentration of uterine oxytocin receptors during the periovulatory period in cloprostenol-treated ewes

“…These studies suggest that endogenous oxytocin could be of physiological importance in the regulation of the LH surge, but other workers have treated rats with either oxytocin or an oxytocin antiserum and reported no change in LH release (Lumpkin et al, 1983;Sarker, 1988). This relationship has not been investigated in detail in ewes, although Sheldrick & Flint (1990) found that a relatively low dose (3 nmol h~1) oxytocin infusion had no effect on LH secretion during the luteal phase. In their study one ewe that underwent luteolysis during treatment had a normal LH surge.…”

Absence of oxytocin-neurophysin messenger RNA in the Day-18 bovine conceptus

“…Continuous infusion of oxytocin into the uterus of cyclic ewes and cows (Gilbert et ai, 1989) prolongs the oestrous cycle, and in the ewe results in decreased oxytocin receptor concentrations and a decrease in PGF-2a concentrations (Sheldrick & Flint, 1987).…”