Oxytocin Release During Vaginal Distention in the Goat

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Abstract
A highly sensitive and specific radioimmunoassay (RIA) for the neurohypophyseal peptide, oxytocin, was developed using antibody to oxytocin raised in rabbits injected with a conjugate of oxytocin and rabbit serum albumin (RSA). Between 39 and 50 percent of added 1 2 5I-oxytocin (label) was bound at a 1:16,000 dilution of antibody in the absence of reference standard oxytocin. 1 2 5I-labeled oxytocin was prepared by a modification of the chloramine-T procedure and an average of 93 percent of the label was bound in antibody excess. Extraction of oxytocin from acidified goat plasma was accomplished by adsorption to Fuller’s earth. Extracts from goat plasma rendered hormone-deficient with charcoal were reconstituted in buffer and the assay components were incubated in this milieu. Basal concentrations of jugular plasma oxytocin in female goats were generally at the threshold of sensitivity of the RIA (2 µU oxytocin/ml plasma). Vaginal distention significantly (P<0.01) elevated the plasma concentration of oxytocin in one pregnant and two nonpregnant goats. However, pretreatment of nonpregnant goats with dexamethasone phosphate (8 mg daily for 4 days) attenuated the reflexive surge of oxytocin. In addition, progesterone (P) infusion was not always inhibitory to oxytocin release after dexamethasone or during pregnancy. It is suggested that inhibition of oxytocin release may play a role in the placental retention associated with corticoid-induced parturition.