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Osmoregulation in Marine Mammals, Otter, Manatees and Dugong …

- - - - Order Cetacea
        (dolphins and whales) and
        Pinnipedia (seals, sea lions and walruses) :
        • Possess reniculate kidneys similar to those
        of bears and otters.
        • Made up of hundreds
        of individual lobes, or reniculi, each of which contains discrete
        cortical tissue and a single medullary pyramid inserted in a
        single calyx
      - Manatees (order Sirenia)
        • Kidney is considered superficially
        lobulate since it lacks true reniculi and the cortex is continuous
      - Histological dissection of the cetacean
        • Specialized glycogen stores in the proximal
        convoluted tubule epithelial cells
        • Highly concentrated
        bundles of medullary blood vessels (vasa recta bundles) and
        • The presence of a sporta perimedullaris musculosa, a layer
        of collagen, elastic fibers and smooth muscle separating the
        cortex from the medulla
      - Sirenians - marine dugong (Dugong dugon),
        Amazonian manatee (Trichechus inunguis) to the West Indian
        (T. manatus) and West African (T. senagalensis) manatees.
        • Externally elongated, with features similar to those found in the kidneys of the camel and
        horse
        • Kidney of West Indian
        manatees has several large lobes with a continuous cortex
      - Kidney of marine mammals
        • Increased medullary thickness) - produce a highly concentrated urine
        • The reniculate
        kidney does not attain the concentrating ability expected from
        a kidney with such excellent anatomical credentials
        • Extremely poor concentrators of urine
    - - Adaptation of marine mammals to a hyperosmotic environment was
        accomplished via more conventional mechanisms such as
        hormonal regulation of urine concentration and/or the rate of
        urine formation
  - - - West Indian manatees
        • Possess the ability to concentrate their urine to an osmolality
        greater than that of sea water
        • Urine osmolalities of approximately
        1200 mosmol l−1
        • Manatees (held in freshwater, then expose to saltwater) - urine osmolalities of approximately 217 mosmol l−1
    - - • Marine mammals do
        not regularly concentrate Na+ and Cl− to levels above that of
        sea water.
        • Do not rely on
        the consumption of sea water to maintain their fluid balance.
        • Urine osmolality correlate with increased plasma osmolality
- - - - Harbor seals
        
        Urine flow increases, urine osmolality decreases within 2h, a decrease in plasma osmolality but, no changes in plasma vasopressin levels with freshwater loads. Also, they exhibit a decrease in excreted Na+ and K+ with no change in GFR. (Without induced diuresis)
        
        A decrease in tubular, free water resorption, increase in GFR. (With induced diuresis)
        
        Baikal (Phoca sibirica) and ringed (P. hispida) seals
        
        Increased diuresis resulted in an increased clearance of Na+, K+, Cl- and urea with reduction in the filtered load that is reabsorbed and GFR increases as side effects.
        
        Conclusion: Seals produced hypo-osmotic urine following infusion of the freshwater Loading of freshwater results in an increase in urine volume and fractional clearance of water and electrolytes, attributed to an increase in GFR, and a decrease in plasma and urine osmolality.
      - Harbor seals
        
        Infusion of hyperosmotic saline caused copious vomiting and diarrhea which resulted in a net loss of body water and then with continuous intravenous infusion, death.
        
        Increases in urine flow and excreted Na+ and Cl (or urine osmolality in the case of mannitol) occur as soon as 40 minutes following hyperosmotic solution infusions of different concentrations. This resulted in an increase in urine flow to be found associated with infusions of fresh water and not with the increase of GFR.
        
        Conclusion: volume receptor of harbor seals is regulated differently from humans as its urine output is dose dependent with urine flow did not associate with GFR but, with infusions of fresh water.