Vasopressin Metabolism in the Amniotic Sac of the Fetal Guinea Pig*

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Abstract
A route for fetal 8-arginine vasopressin (AVP) clearance has not yet been established. The results of this study indicate that in the fetal guinea pig, the amniotic sac is a site for AVP metabolism. When fetal plasma, urine, and amniotic fluid were fractionated on HPLC, AVP was identified only in fetal plasma and urine. In addition, when AVP and inulin were injected into the amniotic sac in vivo during the last week of gestation, the AVP to inulin ratio decreased (P < 0.05) over a 2-h period, indicating a swallowing-independent disappearance of AVP. Furthermore, when tritiated AVP was similarly injected, the radioactivity to insulin ratio remained constant, suggesting that AVP was not diffusing out of the amniotic sac. Two hours after the injection into the amniotic sac, amniotic fluid was collected and fractionated by HPLC, and only 42% of the total radioactivity was intact AVP. The remainder was identified in two other products (M1 and M2). Tritiated AVP incubated with amniotic fluid and amnionic membrane in vitro produced both M1 and M2. Triated AVP incubated with amnionic membrane in artificial amniotic solution produced only M1. Tritiated AVP incubated in only amniotic fluid produced M2, with only slight M1 production. This, neither metabolite was necessary for the production of the other; M1 was amnionic membrane dependent, and M2 was amniotic fluid dependent. In conclusion, the amniotic sac may play a role in the clearance of AVP from the fetal system.

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