REGULATION OF NEUROHYPOPHYSEAL HORMONE-SECRETION IN AN AUSTRALIAN MARSUPIAL

The brushtail possum secretes the typically reptilian mesotocin (MT) a nd arginine vasopressin (AVP) as its neurohypophysial hormones. In thi s study we have looked at the regulation of MT and AVP secretion in co nscious possums by studying the effects of surgical stress and handlin g of animals, hypertonic saline infusion, hemorrhage, and angiotensin II (ANG II) infusion on the plasma concentrations of MT and AVP. Surgi cal insertion of a jugular catheter and handling stress increased MT s ecretion for 3 days after surgery without affecting plas ma AVP concen trations. Hypertonic saline infusion induced a gradual increase in pla sma osmolarity and Na+ concentration throughout the infusion, which st eadily increased plasma AVP without affecting plasma K+ or hematocrit. The relationship between plasma osmolarity and AVP was exponential. P lasma MT was stimulated only by supraphysiological plasma osmolarities . ANG II increased plasma MT and AVP concentrations equipotently throu ghout the infusion. Hemorrhage was a relatively specific stimulus for AVP secretion, but MT secretion was highly stimulated during severe hy povolemia. It was concluded that MT and AVP secretion is differentiall y regulated in the possum.