Hypophysial portal blood was collected from pentobarbital-anesthetized male rats, and the content of oxytocin (OT) was measured by RIA. The concentration of OT in portal plasma was 1.88 ± 0.48 ng/ml (SEM) and was at least 15 times higher than the concentration of OT in peripheral plasma samples from the same rats. Dilutions of portal plasma produced a displacement curve parallel to those of synthetic OT and posterior lobe extract. The data support a possible role for OT as a hypothalamic-releasing hormone.
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