Comparing how peptide and nonpeptide oxytocin blockers reduce hormone binding and uterine contractions in pregnant rats, tested in the lab and in the body

The number of tritiated oxytocin binding sites in myometrial membranes increased markedly at day 21 of gestation compared with day 17.

• Dissociation constants for tritiated oxytocin did not differ significantly between gestational days 17 and 21.
• Nonpeptidyl oxytocin exhibited inhibition constant values that were 79 and 351 times larger than those of peptidyl oxytocin at pregnancy days 17 and 21, respectively.
• Both peptidyl and nonpeptidyl oxytocin significantly inhibited oxytocin-induced uterine contractions in a dose-dependent manner.
• Peptidyl oxytocin did not affect spontaneous and prostaglandin F2 alpha-induced contractions, except for spontaneous contractions at pregnancy day 21.
• Nonpeptidyl oxytocin suppressed spontaneous and prostaglandin F2 alpha-induced contractions both in vivo at pregnancy day 17 and in vitro at pregnancy day 21.

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