The limitations and characterization of protein and water conservation in pups of a fasting adapted species, Mirounga angustirostris.

摘要

Measurements of protein catabolism and renal filtration were conducted in fasting and suckling elephant seal pups to determine if protein catabolism decreases in response to prolonged aphagia and whether glomerular filtration decreases in response to starvation and water deprivation. Protein catabolism was assessed in starving pups to determine the transition to terminal starvation and the mass specific limitation to fasting. The body composition of fasting animals was determined to assess changes in tissue compartments over time. Measurements of serum aldosterone were made to test the hypothesis that tubular reabsorption is correlated with aldosterone concentrations under fasting conditions. Protein catabolism was measured through {dollar}{bsol}sp{lcub}14{rcub}{dollar}C-urea turnover and renal filtration characterized using inulin clearance. Body composition was determined with the HTO dilution method and all plasma hormone concentrations were determined via radioimmunoassay.; Glomerular filtration rate (GFR) varied independent of total mass, lean mass, or blood metabolites, and mean GFR was similar between suckling and fasting periods. Aldosterone concentration significantly decreased as the fast progressed suggesting that other factors are responsible for the increased tubular reabsorption and concentrating ability of the elephant seal weanling. Protein catabolism contributed less than 4.0% to the average metabolic rate (AMR) within both suckling and fasting periods and decreased to 0.76% of the AMR by the eighth week of the fast. Though typical correlates of protein catabolism demonstrated no relationship with mass in healthy, fasting weanlings, mass was directly related to the plasma concentration of glucogenic amino acids. Protein catabolism increased in response to decreasing mass in terminal unmolted (black coat) starvelings, but not until mass approached or fell below the expected birth mass (43 kg). Molted starvelings departed to sea before expiration but at a mass approaching that at birth. Protein catabolism was slightly greater than that observed in healthy weanlings late in the fast and commonality existed between the urea pool half-times (T{dollar}{bsol}sb{lcub}1/2{rcub}){dollar} of departing starvelings and weanlings departing during the eighth week of the fast. Departed starvelings attempted foraging at sea but blood urea nitrogen (BUN) concentrations and rates of mass loss suggested that foraging success was minimal and that either highly hydrated non-adipose stores were catabolized or there was an increased energetic cost associated with the attempt at foraging.; Elephant seal pups demonstrate a fuel-specific metabolism defined by active protein sparing under conditions of fasting and feeding, supporting the hypothesis that suckling pups are pre-adapted to the post-weaning fast. Protein conservation across the fast guards protein for developmental purposes and decreases metabolic wastes associated with catabolism. It is hypothesized that low levels of metabolic wastes are concentrated via increased glomerular filtration and proximal tubular reabsorption so that minimal water is lost through the passage of urine and water balance is maintained. The biochemical cue to feed may interrupt the ontogeny of diving in critically under sized pups before diving ability adequate for survival in the marine environment has developed. The fast is terminated in weanlings when protein catabolism begins to increase, implicating protein degradation as the cue to feed. Large lipid stores improve protein sparing, and with adequate protein stores, prolong the fast such that the development of diving skills necessary for foraging in the marine environment can occur.