146
Marine Mammal Physiology: Requisites for Ocean Living
renal ducts. Accumulated urea contributes to increased osmolality in the medullary interstitium resulting in reabsorption of water and the concentration of the urine. Urea transporters
(UT) function to reabsorb urea, and two subtypes are known, UT-A (SLC14A2) and UT-B
(SLC14A1), of which UT-A2 is essential for urine concentrating ability. Janech et al. (2002)
first demonstrated the presence of two UT-A2 transcripts in the kidney of a short-finned
pilot whale (Globicephala macrorhynchus). The transporters had only one protein kinase C site,
different from that of terrestrial mammals, which have two sites, suggesting that unique
cellular mechanisms regulate this transporter. Birukawa et al. (2008) reported UT-A2 structures with variation in the number of protein kinase C sites in baleen whales. In addition,
plasma urea concentrations are comparatively higher in cetaceans than those in terrestrial
mammals (Artiodactyla), which may reflect the carnivorous feeding habits of cetaceans
(Birukawa et al. 2005). When compared to the mouse, plasma urea concentration in cetaceans is markedly elevated as a result of lower urea cycling suggesting that the regulation
of urea may be an important adaption for the conservation of body water (Miyaji et al. 2010).
Acidic glucolipids include gangliosides and sulfoglycolipids, and the latter may contribute to the maintenance of ionic homeostasis because the expression of sulfoglycolipids
is upregulated in the renal-collecting duct in response to increased environmental osmolality (Niimura and Ishizuka 1990). The higher total concentrations of sulfoglycolipids in
the kidneys of the Steller sea lion (Eumetopias jubatus), melon-headed whale (Peponocephala
elecrta), and rough-toothed dolphin (Steno bredanensis) are thought to contribute to their
adaptation to a marine habitat (Nagai et al. 2008). Sulfoglycolipids may also serve as a
counter-ion for interstitial ammonium accumulated in the medulla and papilla and may
be necessary for ammonium secretion into the collecting duct, resulting in the urinary
excretion of H + (Stettner et al. 2013). This process is important for the regulation of blood
pH and provides evidence for the importance of the kidney in pH in marine mammals.
Immersion in water generally induces diuresis (increased excretion of urine) (Šrámek
et al. 2000). Water immersion increases cardiac output and arterial pressure (Epstein 1992;
Wilcock et al. 2006), and ultimately, the acute distention of arteries induces a baro-reflex
that suppresses the anti-diuretic hormone, vasopressin, resulting in a diuresis. This reflex
arc is known as the Henry-Gauer reflex (Gauer and Henry 1976). However, this reflex may
be permanently suppressed and/or non-existent in marine mammals as an adaptation to
conserve water during chronic and/or frequent immersion (Ortiz 2001). During immersion of harbor seal (Phoca vitulina), urine flow ceased (Murdaugh et al. 1961b). This dramatic reduction in urine flow is likely the result of greatly reduced glomerular filtration rate
(GFR: the rate at which a volume of blood passes through the glomeruli) because GFR was
reported to decrease with induction of the diving reflex in seals (Ladd et al. 1951; Davis
et al. 1983). The immersion- and/or diving-induced decrease in GFR corresponds well with
a decrease in blood flow (ml/min) to the kidneys (Zapol et al. 1979). Therefore, the shunting of regional blood flow to the kidneys appears to be the principal hemodynamic alteration that evolved to allow marine mammals to conserve water during diving/immersion.
7.2.2.3 Other organs—Synergies to conserve body water
The epidermis (skin) serves as a boundary between the internal body and the external environment. The cetacean epidermis is composed of lipokeratinocytes that contain not only
keratin filaments but also lipid droplets, likely serving as a barrier to a hypertonic environment (Menon et al. 1986). Semi-aquatic and aquatic mammals have high numbers of lipid
vesicles in the cells of the strata granulosum and the strata corneum, which may function as a
barrier and possess a rich distribution of Na + /K + exchanger-1 (Meyer et al. 2011). In addition,
sweat glands are absent in marine mammals, and this is likely because there is no need for
Marine Mammal Physiology: Requisites for Ocean Living
renal ducts. Accumulated urea contributes to increased osmolality in the medullary interstitium resulting in reabsorption of water and the concentration of the urine. Urea transporters
(UT) function to reabsorb urea, and two subtypes are known, UT-A (SLC14A2) and UT-B
(SLC14A1), of which UT-A2 is essential for urine concentrating ability. Janech et al. (2002)
first demonstrated the presence of two UT-A2 transcripts in the kidney of a short-finned
pilot whale (Globicephala macrorhynchus). The transporters had only one protein kinase C site,
different from that of terrestrial mammals, which have two sites, suggesting that unique
cellular mechanisms regulate this transporter. Birukawa et al. (2008) reported UT-A2 structures with variation in the number of protein kinase C sites in baleen whales. In addition,
plasma urea concentrations are comparatively higher in cetaceans than those in terrestrial
mammals (Artiodactyla), which may reflect the carnivorous feeding habits of cetaceans
(Birukawa et al. 2005). When compared to the mouse, plasma urea concentration in cetaceans is markedly elevated as a result of lower urea cycling suggesting that the regulation
of urea may be an important adaption for the conservation of body water (Miyaji et al. 2010).
Acidic glucolipids include gangliosides and sulfoglycolipids, and the latter may contribute to the maintenance of ionic homeostasis because the expression of sulfoglycolipids
is upregulated in the renal-collecting duct in response to increased environmental osmolality (Niimura and Ishizuka 1990). The higher total concentrations of sulfoglycolipids in
the kidneys of the Steller sea lion (Eumetopias jubatus), melon-headed whale (Peponocephala
elecrta), and rough-toothed dolphin (Steno bredanensis) are thought to contribute to their
adaptation to a marine habitat (Nagai et al. 2008). Sulfoglycolipids may also serve as a
counter-ion for interstitial ammonium accumulated in the medulla and papilla and may
be necessary for ammonium secretion into the collecting duct, resulting in the urinary
excretion of H + (Stettner et al. 2013). This process is important for the regulation of blood
pH and provides evidence for the importance of the kidney in pH in marine mammals.
Immersion in water generally induces diuresis (increased excretion of urine) (Šrámek
et al. 2000). Water immersion increases cardiac output and arterial pressure (Epstein 1992;
Wilcock et al. 2006), and ultimately, the acute distention of arteries induces a baro-reflex
that suppresses the anti-diuretic hormone, vasopressin, resulting in a diuresis. This reflex
arc is known as the Henry-Gauer reflex (Gauer and Henry 1976). However, this reflex may
be permanently suppressed and/or non-existent in marine mammals as an adaptation to
conserve water during chronic and/or frequent immersion (Ortiz 2001). During immersion of harbor seal (Phoca vitulina), urine flow ceased (Murdaugh et al. 1961b). This dramatic reduction in urine flow is likely the result of greatly reduced glomerular filtration rate
(GFR: the rate at which a volume of blood passes through the glomeruli) because GFR was
reported to decrease with induction of the diving reflex in seals (Ladd et al. 1951; Davis
et al. 1983). The immersion- and/or diving-induced decrease in GFR corresponds well with
a decrease in blood flow (ml/min) to the kidneys (Zapol et al. 1979). Therefore, the shunting of regional blood flow to the kidneys appears to be the principal hemodynamic alteration that evolved to allow marine mammals to conserve water during diving/immersion.
7.2.2.3 Other organs—Synergies to conserve body water
The epidermis (skin) serves as a boundary between the internal body and the external environment. The cetacean epidermis is composed of lipokeratinocytes that contain not only
keratin filaments but also lipid droplets, likely serving as a barrier to a hypertonic environment (Menon et al. 1986). Semi-aquatic and aquatic mammals have high numbers of lipid
vesicles in the cells of the strata granulosum and the strata corneum, which may function as a
barrier and possess a rich distribution of Na + /K + exchanger-1 (Meyer et al. 2011). In addition,
sweat glands are absent in marine mammals, and this is likely because there is no need for