Regulation of neurohypophysial hormone secretion in an Australian marsupial

The brushtail possum secretes the typically reptilian mesotocin (MT) and arginine vasopressin (AVP) as its neurohypophysial hormones. In this study we have looked at the regulation of MT and AVP secretion in conscious possums by studying the effects of surgical stress and handling of animals, hypertonic saline infusion, hemorrhage, and angiotensin II (ANG II) infusion on the plasma concentrations of MT and AVP. Surgical insertion of a jugular catheter and handling stress increased MT secretion for 3 days after surgery without affecting plasma AVP concentrations. Hypertonic saline infusion induced a gradual increase in plasma osmolarity and Na+ concentration throughout the infusion, which steadily increased plasma AVP without affecting plasma K+ or hematocrit. The relationship between plasma osmolarity and AVP was exponential. Plasma MT was stimulated only by supraphysiological plasma osmolarities. ANG II increased plasma MT and AVP concentrations equipotently throughout the infusion. Hemorrhage was a relatively specific stimulus for AVP secretion, but MT secretion was highly stimulated during severe hypovolemia. It was concluded that MT and AVP secretion is differentially regulated in the possum.