Control of plasma arginine vasotocin in Kelp gulls (Larus dominicanus): roles of osmolality, volume, and plasma angiotensin II.

The physiological roles of plasma osmolality, volume, and angiotensin II (AII) in the control of plasma arginine vasotocin (AVT) have been evaluated in the Kelp gull. Plasma AVT (measured by radioimmunoassay) consistently followed the changes in plasma osmolality induced by dehydration, intravenous (iv) infusion of hypertonic saline, or iv infusion of hypotonic glucose solution. Osmoreceptor control of AVT was further characterized by the stimulatory effect of iv hypertonic mannitol but lack of effect of iv hypertonic urea. A direct volume effect on AVT release was demonstrated during hemorrhage, but blood volume reductions in excess of 10% were required. However, the volume of the extracellular fluid (ECF) compartment does have an important role in modulating the osmotic release of AVT in a way that enables contraction or expansion of ECF to be more expeditiously corrected than would occur if osmoreceptors alone regulated plasma AVT. Any physiological role of plasma AII in the control of plasma AVT is doubtful because after the iv infusion of AII for 60 min at 10, 25, 50, and 100 ng.min-1, only the two highest doses of AII, which produced supraphysiological plasma AII levels, caused elevations of plasma AVT. In addition, the osmotic reactivity of AVT release was not altered by exogenous administration of AII.