Day of cycle affects changes in equine intrauterine pressure in response to teasing.

Oxytocin is released in response to teasing during both estrus and diestrus in mares, and at least during estrus, teasing results in an increase in electromyographic activity in the uterus. Exogenous oxytocin causes an increase in intrauterine pressure and prior studies have shown that this response is correlated to the day of the estrous cycle. To determine if teasing causes an increase in intrauterine pressure and if this response varies by day of the cycle, intrauterine pressure was measured while mares were teased with a stallion 2 days before ovulation, on the day ovulation was detected and 2 days after ovulation. A significant increase in intrauterine pressure was observed in response to teasing both 2 days before ovulation and on the day of ovulation, when plasma concentrations of progesterone were low. No significant increase in intrauterine pressure was observed in response to teasing 2 days after ovulation when progesterone concentrations were elevated. Management practices that include teasing or stallion exposure may be beneficial in stimulating uterine clearance mechanisms in mares during the preovulatory period.

Uterine responses to exogenous oxytocin before and after pre-partum luteolysis in the cow.

The aim of this study was to test the functional status of uterine oxytocin receptors in cows in vivo around parturition. The animals received consecutive, intra-arterial injections of 800, 1,600 and 3,200 mU of oxytocin at three different stages: during late gestation (days 260-274), at 12 h and at 24 h after intramuscular injection of a prostaglandin F2alpha analogue at day 275 to induce parturition. Cows (n = 6) had been provided with myometrial electrodes and a catheter had been installed in the aorta and in a branch of the uterine vein (UV). Regular blood samples were obtained from the UV from 5 min before until 45 min after each oxytocin injection to measure plasma levels of prostaglandin F2alpha (PGF2alpha) and oxytocin. Uterine electromyographic (EMG) activity was registered continuously during each experiment. The increase of oxytocin levels in UV plasma after intra-arterial injections was dose dependent (p < 0.02). Pre- and post treatment oxytocin levels at 24 h after induction of parturition were significantly increased (p = 0.0313). Both during late pregnancy and at 12 h after induction of parturition, oxytocin caused a significant increase in EMG activity (p = 0.022). After the 3,200 mU dose the increase was significantly higher than with the other 2 doses (p = 0.004). After each dose, EMG activity returned to baseline levels within some 15 min. At 24 h after induction of parturition, the pre-treatment level of EMG activity had increased. Doses of 800 mU and 1,600 mU of oxytocin produced a significant (p = 0.022) increment of EMG activity, which was of the same magnitude as during the preceding stages; after 3,200 mU of oxytocin the response was significantly higher than before (p = 0.008). No significant increases of PGF2alpha levels in UV plasma could be measured after oxytocin injections at any of the three stages. It is concluded that the myometrium of the pregnant cow responds in vivo to physiological doses of oxytocin. At 24 h after induction of parturition, when luteolysis has occurred and a parturient pattern of parturient myometrial activity has already started to develop, the response is enhanced. Physiological doses of oxytocin did not evoke a spurt release of PGF2alpha in uterine venous blood during the peripartal period.

Effect of dose and day of treatment on uterine response to oxytocin in mares.

To determine the effect of dose and day of oxytocin treatment on intrauterine pressure, 6 normal mares were treated with 10 or 25 IU oxytocin 2 days before ovulation, on the day of ovulation and 2 days after ovulation. Intrauterine pressure (IUP) was measured using micro-tip-catheters (one placed intrauterine, a second and third serving as reference sensors in the vagina and external to the mare) and transmitted by telemetry for 30 min to establish a baseline before saline was administered, iv, and for an additional 30 min after saline administration. Oxytocin was then given, iv, and IUP was recorded for 60 min. No change in IUP was observed after saline injection. The administration of both 10 (n=16) and 25 (n=10) IU oxytocin induced a response (P<0.01). The intensity of response depended on the day of administration (P<0.01) and the dose of oxytocin (P<0.001). The variation of response was significantly greater after 10 IU oxytocin (CV 15.78%) compared with 25 IU oxytocin (CV 6.42%). The uterine response was greatest on Day 2 prior to ovulation and lowest on Day 2 after ovulation. The response was negatively correlated to increasing plasma progesterone (10 IU oxytocin: r = -0.435, 25 IU oxytocin: r = -0.265). There was no correlation between the uterine response and plasma estradiol-17beta concentration (P<0.01). In conclusion the results of this study show that oxytocin administration to mares before ovulation provides a greater response than after ovulation. A decline in the intensity of response after ovulation can be compensated for with a higher dose of oxytocin. Furthermore, the use of the multiple catheter technique is an effective method for assessing changes in uterine pressure.

PGFM response to exogenous oxytocin and determination of the half-life of oxytocin in nonpregnant mares.

We investigated the half-life of oxytocin in reproductively normal mares and the prostaglandin response after oxytocin administrations. Mares were given oxytocin, 10 or 25 iu, i.v., on the day of, or 2 days after, ovulation, and frequent jugular blood samples were collected for analysis of oxytocin and Prostaglandin F metabolite (PGFM) by RIA. Neither dose of oxytocin nor day of treatment affected the half-life of the exogenous oxytocin, which was determined to be 6.8 min. A significant increase in PGFM was observed within 6 min of oxytocin administration and peak values were observed within 10 min. PGFM response after oxytocin administration on the day of ovulation appeared elevated compared to the response 2 days after ovulation.