Vasopressin and oxytocin responses to hypertonic saline infusion: effect of the opioid antagonist naloxone.

Endogenous opioids inhibit the release of oxytocin (OT) when vasopressin (AVP) is secreted in response to acute pharmacological stimuli in man and to a variety of physiological and pharmacological stimuli to animals. We have investigated the effect of naloxone on the AVP and OT responses to hypertonic saline in man. In two separate studies, six male subjects were infused with hypertonic saline (675 mmol/l, 0.05 ml/kg/min for 2 h) and either naloxone (4 mg bolus and 6 mg/h) or normal saline in random order. Hypertonic saline resulted in similar significant rises of plasma osmolality and AVP in both groups and a small but significant decrease in OT. Thirst sensation was not altered by naloxone. Endogenous opioids do not play an important role in the suppression of OT release when AVP is secreted in response to an osmotic stimulus in man.