Osmotic regulation of vasopressin and oxytocin release is rate sensitive in hypothalamoneurohypophysial explants

Explants of the hypothalamoneurohypophysial system (HNS) from rats were maintained in perifusion culture and exposed to 15-18 mosmol/kgH₂O changes in the osmolality of the culture medium achieved by increasing or decreasing the NaCl concentration. The rate of change in osmolality was either 0.14 ± 0.01 mosmol/min (2%/h), 0.27 ± 0.02 mosmol/min (5%/h), or 1.7 ± 0.2 mosmol/min (30%/h). Vasopressin (VP) and oxytocin (OT) release into the culture medium was determined. Increased VP release was detected after all three rates of increase, but the peak response occurred sooner and in response to a smaller increment in osmolality during the 5% rate of increase compared with the 2% rate of increase. Peak VP release occurred during the first 10 min of the 30% pulse and was significantly greater than the response to the 2 and 5% rates (P < 0.05). The increase in VP release was sustained throughout the 3 h of hypertonicity during the 2% pulse, but not during the 5 and 30% osmotic pulses. A significant decrease in VP release was observed on returning the osmolality to basal at both the 2 and 5% rate, but this inhibition was followed by a rebound in VP release that started during the decrease in osmolality and significantly exceeded basal VP release. A significant inhibition of VP release also was observed when explants were exposed to hypotonic pulses at the 2 and 5% rate. At both rates, the inhibition of VP release corresponded to a 5- to 7-mosmol/kgH₂O decrease in osmolality. Again, both rates of decrease elicited a rebound stimulation of VP release that significantly exceeded basal release. The OT responses to both the hypertonic and hypotonic pulses were comparable with the VP responses. These data suggest that VP and OT release is sensitive to the rate of increase in osmolality, but the response to a decrease in osmolality is not rate sensitive. The mechanisms underlying the rebound stimulation of VP and OT release after a decrease in osmolality remain to be elucidated.