Osmotic inhibition of prolactin secretion in rats

Chronic hyponatremia is known to cause inhibition of pituitary vasopressin (AVP) and oxytocin (OT) secretion in response to most physiological stimuli, as well as a marked inhibition of synthesis of these peptides. Because many studies have implicated neurohypophyseal peptides in the regulation of pituitary prolactin (PRL) secretion, we investigated the effects of chronic hyponatremia on basal and stimulus-induced PRL secretion in rats. Hyponatremia was induced by subcutaneous infusion of 1-deamino-[8-D-arginine]-vasopressin (dDAVP) (5 ng/h) to rats fed a nutritionally balanced liquid diet, and plasma [Na+] was maintained < or = 115 mmol/l for 10-12 days. After this period, hyponatremic rats and normonatremic controls fed the same diet without dDAVP were subjected to one of the following stimuli known to stimulate PRL release in rats: 3 min exposure to ether, hemorrhage (20 ml/kg), intravenous injection of 5-hydroxytryptophane (5-HTP, 10 mg/kg), or intravenous injection of estradiol (5 micrograms/kg). A baseline blood sample was collected before each stimulus, and 3-6 additional blood samples were collected at selected intervals after the stimulus. Baseline levels of plasma PRL were not different between normonatremic and hyponatremic rats. However, PRL responses induced by either or estradiol, but not those induced by hemorrhage or 5-HTP, were very significantly blunted in the chronically hyponatremic rats. Plasma AVP and OT responses were measured as an index of magnocellular secretion, but did not correlate with the PRL responses for any of the stimuli tested.(ABSTRACT TRUNCATED AT 250 WORDS)