Body water handling in response to hypertonic-saline induced diuresis in fasting northern elephant seal pups ( Mirounga angustirostris)

Abstract

During natural fasting conditions in postweaned northern elephant seal (NES) ( Mirounga angustirostris) pups, urinary water loss is minimized and percent total body water (TBW) is maintained constant. However, following infusion of hypertonic saline, glomerular filtration rate (GFR) and urine output increased in fasting pups. Therefore, we quantified the magnitude of the hypernatremia-induced diuresis relative to the animal's total body water (TBW) pool and the percentage of filtered water reabsorbed. Following a 24 h control period, naturally fasting NES pups ( n=7) were infused (4 ml min −1) with hypertonic saline (16.7%) at a dose of 3 mmol NaCl kg −1 body mass. Total body water was estimated prior to infusion by tritium dilution, GFR was estimated by standard creatinine clearance, and urine output (V) was measured for 24 h during the control and post infusion periods. Percentage of filtered water reabsorbed was calculated as (1−(V/GFR))×100. Twenty-four hours following the infusion, GFR (control: 69±12 ml min −1 and post-infusion: 118±19 ml min −1; mean±S.E.) increased 77±28% above control and the percentage of filtered water reabsorbed was decreased 0.4±0.1%. The increase in urine output (control: 218±47 ml d −1 and post-infusion: 883±92 ml d −1) accounted for 1.7±0.2% of the pups’ TBW. The hypernatremia-induced diuresis was accompanied by the loss of body water indicating the lack of water retention. Although the 77% increase in GFR was only associated with a 0.4% decrease in the percentage of filtered water reabsorbed, this decrease was significant enough to result in a 4-fold increase in urine output. Despite the observed diuresis, fasting NES pups appear to possess an efficient water recycling mechanism requiring only a small percentage of body water to excrete an excess salt load. This water recycling mechanism may allow pups to avoid negative perturbations in body water as they initiate feeding in a marine environment following the fast.