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

Characterization of vasopressin and oxytocin receptors in an Australian marsupial

In this study arginine vasopressin (AVP) and oxytocin (OT) receptors have been characterized in the brushtail possum. AVP receptors were characterized using [3H]AVP and the radioiodinated AVP V1a receptor antagonist 125I-labelled [C6H5-CH2CO)-O-methyl-D-Tyr-Phe-Gln-Asn-Arg-Pro-Arg-Tyr-NH2] while OT receptors were characterized using the radioiodinated OT receptor antagonist 125I-labelled d(CH2)5[Tyr(Me)2,Thr4,Orn8,Tyr-NH2(9)]-vasotocin. The receptor affinities and densities have been compared with the rat AVP and OT receptors. Low densities of OT receptors were present in the possum ovary and kidney. High densities of AVP-binding sites were found in the possum adrenal, testis, mesenteric artery, ovary and renal medulla and lower densities in the possum liver. The AVP-binding sites showed marked differences in ligand-binding characteristics from the rat AVP V1a and V2 receptors. Receptor affinities were similar between tissues, except for a distinctly lower value in the renal medulla. It is concluded that the brushtail possum expresses AVP receptors with distinct ligand specificities from those of the rat AVP V1a and V2 receptors