Plasma vasopressin levels and water conservation in fasting, postweaned northern elephant seal pups (Mirounga angustirostris)

Northern elephant seal pups (Mirounga angustirostris) use several physiological mechanisms to conserve water during their 8-12-wk postweaning fast. Urine was analyzed from five animals and plasma from four of those animals was sampled serially throughout the postweaning period to quantify changes in urine concentrating ability and electrolyte homeostasis at various stages of the fast (Early = 1-4 wk, Mid = 4-7 wk, Late = 7-10 wk). Plasma osmolarities stayed relatively constant at 301.6 +/- 13.1 mOsm/kg. Urine osmolarities ranged from 1,053.5 +/- 78.6 mOsm/kg (Early), to 1,585.0 +/- 136.7 mOsm/kg after eight weeks, dropping to 1,214.3 +/- 349.6 mOsm/kg (Late). Urine concentrations for Na+ and K+ early in the fast were 55.0 +/- 14.6 mM and 180.8 +/- 48.6 mM, respectively, declining to 8.2 +/- 3.3 mM and 91.4 +/- 29.0 mM later. Plasma vasopressin concentrations ranged from 34.8 +/- 18.2 pg/ml (Early) to 4.8 +/- 1.3 pg/ml (Late). The pups appear to conserve their body water by producing a concentrated urine, thus reducing urinary water loss. The significance of the antidiuretic role of vasopressin in the conservation of body water in these pups could not be conclusively determined from the results.