153
Chapter seven: Water balance
Collectively, freshwater loading causes diuresis in marine mammals. However, as they
cannot excrete urine with a lower osmolality than that of plasma, this diuresis leads to a
loss of electrolytes, resulting in hyponatremia, hypocholoremia, and hemodilution. Also,
the few freshwater studies that exist suggest that marine mammals may not have especially well-developed mechanisms to excrete excessive water efficiently while conserving
electrolytes. If so, this provides some insight to the evolution of physiological mechanisms
necessary to tolerate life without access to freshwater.
7.2.4.4 Loading of organic molecules
The mechanisms necessary to maintain water balance are changing continuously in
response to the status and content of food consumption because the constitution of diets
(organic nutrients, electrolytes, pre-formed water, etc.) varies. High protein diets promote
urine concentrating ability and GFR in human and other mammals to excrete metabolites
(primarily nitrogenous) arising from the excessive protein (Epstein et al. 1957; Bouby et al.
1988; Brändle et al. 1996). As pinnipeds and cetaceans are carnivorous, the consequence
is the need to excrete the end-product of protein (urea/ammonia) and excessive electrolytes efficiently. Generally, in marine mammals, feeding leads to an increase in GFR, renal
plasma flow, and urine flow rate (Hiatt and Hiatt 1942; Schmidt-Nielsen et al. 1959; Malvin
and Rayner 1968; Ridgway 1972). These changes increase urine production, which can lead
to an increase in the excretion of Na + , K + , and urea/ammonia (Schmidt-Nielsen et al. 1959;
Malvin and Rayner 1968; Ridgway 1972). Ridgway and Venn-Watson (2010) demonstrated
the combined loading of seawater and protein to bottlenose dolphin increased plasma
osmolality, urine flow, and urine concentrations of Na + , K + , and urea. These changes were
enhanced and lasted longer than those in the group infused with seawater alone. These
data suggest that the ability to concentrate urine above seawater in marine mammals is
an important mechanism adapted to tolerate a carnivorous feeding behavior in a marine
habitat.
7.2.5 Hormonal control
Hormones are important extrinsic factors that regulate signaling pathways at the cellular
level to help maintain homeostasis of water and electrolyte balance. Generally, in terrestrial mammals, a suite of vasoactive hormones such as arginine vasopressin, angiotensin
II, atrial natriuretic peptide, vasoactive intestinal peptide, and urotensin II contribute to
the acute (minutes–hours) perturbations in water and electrolyte balance, whereas s teroids
(aldosterone and cortisol), prolactin, and growth hormones are more involved in longerterm responses (McCormick and Bradshaw 2006). In marine mammals, studies have
focused more on the acute responses of the hormones to hyperosmotic water ingestion or
fasting than on the chronic effects.
7.2.5.1 Arginine vasopressin
Thirst, primarily in response to the low availability of freshwater, stimulates a cascade
of neural and humoral responses that culminates in the prevention of water loss by concentrating the urine. In mammals, the volume of excreted water is regulated tightly by
the octapeptide, arginine vasopressin (AVP; previously, antidiuretic hormone [ADH]). The
secretion of AVP from the posterior pituitary is primarily in response to an increase in
plasma osmolality or plasma volume following the stimulation of osmo- or baroreceptors,
respectively (Dunn et al. 1973; Robertson et al. 1976). The binding of AVP to its receptor
(V2R) in the principal cells in the collecting duct initiates the insertion of water channels
Chapter seven: Water balance
Collectively, freshwater loading causes diuresis in marine mammals. However, as they
cannot excrete urine with a lower osmolality than that of plasma, this diuresis leads to a
loss of electrolytes, resulting in hyponatremia, hypocholoremia, and hemodilution. Also,
the few freshwater studies that exist suggest that marine mammals may not have especially well-developed mechanisms to excrete excessive water efficiently while conserving
electrolytes. If so, this provides some insight to the evolution of physiological mechanisms
necessary to tolerate life without access to freshwater.
7.2.4.4 Loading of organic molecules
The mechanisms necessary to maintain water balance are changing continuously in
response to the status and content of food consumption because the constitution of diets
(organic nutrients, electrolytes, pre-formed water, etc.) varies. High protein diets promote
urine concentrating ability and GFR in human and other mammals to excrete metabolites
(primarily nitrogenous) arising from the excessive protein (Epstein et al. 1957; Bouby et al.
1988; Brändle et al. 1996). As pinnipeds and cetaceans are carnivorous, the consequence
is the need to excrete the end-product of protein (urea/ammonia) and excessive electrolytes efficiently. Generally, in marine mammals, feeding leads to an increase in GFR, renal
plasma flow, and urine flow rate (Hiatt and Hiatt 1942; Schmidt-Nielsen et al. 1959; Malvin
and Rayner 1968; Ridgway 1972). These changes increase urine production, which can lead
to an increase in the excretion of Na + , K + , and urea/ammonia (Schmidt-Nielsen et al. 1959;
Malvin and Rayner 1968; Ridgway 1972). Ridgway and Venn-Watson (2010) demonstrated
the combined loading of seawater and protein to bottlenose dolphin increased plasma
osmolality, urine flow, and urine concentrations of Na + , K + , and urea. These changes were
enhanced and lasted longer than those in the group infused with seawater alone. These
data suggest that the ability to concentrate urine above seawater in marine mammals is
an important mechanism adapted to tolerate a carnivorous feeding behavior in a marine
habitat.
7.2.5 Hormonal control
Hormones are important extrinsic factors that regulate signaling pathways at the cellular
level to help maintain homeostasis of water and electrolyte balance. Generally, in terrestrial mammals, a suite of vasoactive hormones such as arginine vasopressin, angiotensin
II, atrial natriuretic peptide, vasoactive intestinal peptide, and urotensin II contribute to
the acute (minutes–hours) perturbations in water and electrolyte balance, whereas s teroids
(aldosterone and cortisol), prolactin, and growth hormones are more involved in longerterm responses (McCormick and Bradshaw 2006). In marine mammals, studies have
focused more on the acute responses of the hormones to hyperosmotic water ingestion or
fasting than on the chronic effects.
7.2.5.1 Arginine vasopressin
Thirst, primarily in response to the low availability of freshwater, stimulates a cascade
of neural and humoral responses that culminates in the prevention of water loss by concentrating the urine. In mammals, the volume of excreted water is regulated tightly by
the octapeptide, arginine vasopressin (AVP; previously, antidiuretic hormone [ADH]). The
secretion of AVP from the posterior pituitary is primarily in response to an increase in
plasma osmolality or plasma volume following the stimulation of osmo- or baroreceptors,
respectively (Dunn et al. 1973; Robertson et al. 1976). The binding of AVP to its receptor
(V2R) in the principal cells in the collecting duct initiates the insertion of water channels