I'm not certain I understand the question completely but will give some information that I hope clears it up for you.
Oxytocin causes smooth muscle contractions. It is used when older sows or those with prolonged labor become exhausted to help uterine contractions (smooth muscles of the uterus).
Oxytocin has a short half life (~10 minutes) which means any effect it has directly is rapid because it is rapidly cleared from the bloodstream. It can, however, have more long-term impacts because it can down-regulate receptors.
Oxytocin is also sometimes used to help with milk let down. Some effect may be seen with an intramuscular injection, though the effect is more pronounced with IV treatment. Massaging the udders will also stimulate oxytocin release and can help with milk let down. The advantage of this approach is that the release is more coordinated. In either case, oxytocin still has a short half-life, so it is not prudent to treat often. Normal milk let down in sows is about once per hour for about 30 seconds at a time.
If that did not answer your question let me know and I will try again.
State Swine Breeding Specialist
University of Missouri