American journal of veterinary research
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Comparative Study
Actions of oxytocin and a long-acting carba oxytocin analog on the porcine myometrium in vitro and in vivo.
The contractile actions of oxytocin and its analog 1-desamino-1-monocarba-[2-Tyr(OMe)]-oxytocin were compared on in vitro (experiment 1) and in vivo (experiment 2) preparations of porcine uterine horns. In both experiments, sows in estrus were selected for maximal sensitivity. Despite the loss of peak amplitude associated with the substitution at sequence position 2 of O-methyltyrosine for tyrosine, the two remaining alterations (N-terminal desamination and replacement of the 1-6 S-S bridge by CH2S) prolonged the action of single-dose presentations. Thus, instead of having a response lasting minutes when the parent hormone is given, an IV dose of about 5 micrograms of the analog/kg of body weight in the intact anesthetized sow resulted in a response lasting two hours.
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The long-acting oxytocin (OT) analog 1-desamino-1-monocarba-E12-Tyr(OMe)]-OT(dCOMOT) was given IV to 13 pregnant cows near to term, but not in actual labor. The animals were para 1 to 5. Of these cows, four were treated with 20 mg of dexamethoasone 48 hours before the peptide was injected; the remaining nine animals were given no other medication. ⋯ The difference between the two values was significant. For both groups separately, and all data together, there was a linear inverse relation between the size of the external ostium uteri at injection and the duration of labor after peptide injection. All calves were healthy with no signs of hypoxia and the dose rates used did not result in any instance of uterine tetany or tachyphylaxis.