Pulsatile arginine vasopressin release from the rat hypothalamo neurohypophyseal system during osmotic stimulation

Arginine vasopressin (AVP) was released in vitro in a pulsatile pattern from the hypothalamo-neurohypophyseal system (HNS) and from the hypothalamus during continuous hyperosmotic stimuli with NaCl or fructose. No significant difference was found in the AVP pulse frequency between the two kinds of hyperosmotic agents. AVP was released from the HNS in a dose-related manner under NaCl stimulation. When the neural lobe was stimulated with NaCl or fructose, a clear AVP pulse pattern was not apparent. Urea failed to evoke a significant AVP release from the neural lobe or HNS. A stepwise increase in NaCl stimulation from 5 to 25 mEq induced a AVP response from the HNS and hypothalamus similar to that under constant stimulation at 25 mEq NaCl. This phenomenon was also found with fructose or sucrose. These results suggest that AVP release from the HNS during continuous osmotic stimulation has a pulsatile pattern regardless of the hyperosmotic substance or osmotic pressure. This AVP release accurately reflects the physiological function of the hypothalamus without modulation in the neural lobe. These results also suggest that the total amount of AVP was related to the osmotic pressure or the osmotic substance but that the frequency of the pulse release was not, moreover, that the AVP release depends not only on the absolute osmotic pressure, but also on the changing rate of osmotic pressure.