I'm hoping for some insight/info regarding use of Oxytocin.
Here's the situation... I am attempting to breed (via frozen semen) an aged mare who typically (for 8 years) has been very regular in her estus and timing of ovulation. I know...I know...you can't count on them not to change....
However, she recently received Oxytocin to enhance clearance of a small amount of uterine fluid. This is the second cycle this year in which she has received Oxytocin during estrus. Last cycle she she received it Q6H x 3 days. Including 1 day after ovulation. Uterine cytology is normal. She was inseminated that cycle but no pregnancy at 15 days.
The concern is...there have been some changes in her normal pattern and behavior as well as in her follicle growth.
This cycle she has exhibited strong estrus and receptivity (5 days) but has only 20-25 follicles which are not progressing. Her diet and hay source has not been changed for 3 years. She produced a foal last year.
Any ideas, guesses, suppositions about this change? Could it be the Oxytocin? She also has moderate udder development.
Also wondering if anyone can speak further on this quote from McKinnon & Voss: (wondering about pregnancy maintenance after oxytocin use post-ov)
"Thus evidence shows that oxytocin may lead to uterine secretion of PGF2a and luteolysis in horses in a manner similar to that observed in other domestic species."
I have no doubt that the changes you are seeing are not related to the oxytocin use.
Oxytocin is a naturally occurring hormone that is secreted by the mare during estrus in response to the stallion's teasing and the presence of sperm in the uterus. There is therefore nothing abnormal in it's use exogenously to treat mares with delayed uterine clearance.
The half-life of oxytocin in the mare is 6.8 minutes. 5 half-lives pharmacologically speaking represent a complete elimination of circulating levels from the body, hence within about 35-40 minutes circulating levels are essentially zero. There will still be some residual action at tissue level but that will be dissipated by 2 hours after treatment (that is why we have to give it so often when treating with the oxytocin protocol).
Treatment with oxytocin may be continued through to 3½ days after ovulation. The conceptus does not descend from the oviduct into the uterus until about 5½ days post-ovulation, and the oviducts are not affected by the oxytocin.
The cervix is not closed sufficiently until about 3½-4 days after ovulation to reliably prevent passage of the conceptus if it descends into the uterus early, so the pregnancy would be lost anyway if it were to descend earlier than that for some reason, and not as a result of oxytocin treatment.
Treatment with oxytocin during diestrus may result in luteolysis as a result of the PGF2a release, but it is more likely to result in a colicky mare, and is therefore not (shouldn't be) given during diestrus!
I hope this answers your questions satisfactorily!
Yeah... but remember that a lighthouse is put there because there's rocks around it...!!!!
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