151
Chapter seven: Water balance
maintained in freshwater compared to those in seawater suggesting that freshwater consumption was a significant component of their water flux (Ortiz et al. 1999).
7.2.4 Infusion experiments: Water, electrolytes, and organic molecules
The infusions of water, electrolytes, or organic molecules provide important experimental
protocols to help elucidate mechanisms that regulate body water and salts. With respect
to osmoregulatory mechanisms, the intravenous infusion of hyper- or hypo-osmotic solutions and the subsequent monitoring of the humoral, excretory, and/or renal responses
provide insight to the physiological mechanisms and adjustments to recover homeostasis.
Infusion studies with freshwater, hypertonic saline, mannitol, and isotonic gelatin among
others have been performed in a variety of marine mammals to help reveal their osmoregulatory capabilities.
7.2.4.1 Loading of hypertonic/hyperosmotic saline
In terrestrial mammals, the ingestion of seawater typically induces excretion of excess salts
and water resulting in dehydration without the ability of producing urine that is more concentrated than the ingested seawater. Excessive salt intake (sodium chloride; NaCl) has
consequences in the form of diarrhea, vomiting, edema, and elevated arterial blood pressure (Boyd et al. 1966; Meneely and Battarbee 1976; Thompson 2011). Generally, marine
mammals are thought to be resistant to seawater ingestion because they have adapted to
living in the marine habitats.
Hypertonic saline infusion of various concentrations induces urine production
with excretion of Na + and Cl − in bottlenose dolphin (Fetcher and Fetcher 1942) and seals
(Albrecht 1950; Bradley et al. 1954; Tarasoff and Toews 1972; Hong et al. 1982; Skog and
Folkow 1994; Storeheier and Nordøy 2001; Ortiz et al. 2002; How and Nordøy 2007).
Although these marine mammals can produce concentrated urine with a relatively high
osmolality (Bester 1975; Costa 1982; Maluf 1989; Ortiz 2001), they do not usually concentrate Na + and Cl − above that of seawater. At least in pinnipeds, the tolerance to salt
loading seems to vary among species. For example, in the harbor seal, vomiting and
diarrhea were induced by the loading of 2.6 ml seawater/kg of body weight and the
continuous intravenous infusion of hyperosmotic saline resulted in death in two harbor
seals (Albrecht 1950). In the California sea lion, similar consequences were reported with
the loading of 1.5 ml/kg (Ridgway 1972). However, the acute intravenous bolus infusion
of iso- and hypertonic saline with equivalent amounts of Na + (310 mEq) had no apparent consequences on northern elephant seal pups suggesting that some pinnipeds are
resistant to excessive salt loading or that the differences in the experimental protocols
accounted for the disparate physiological responses (Ortiz et al. 2002). Similarly, in the
bottlenose dolphin, given 4 ml/kg (Ridgway 1972) or 4 l (Ridgway and Venn-Watson
2010) of seawater by gavage did not demonstrate any clinical signs of seawater toxicosis.
In the dolphins, seawater ingestion initially induced a diuresis with high urine concentrations of Na + , Cl − , and K + , with a relatively hyperosmotic urine continuously produced
(Ridgway and Venn-Watson 2010). Collectively, these observations suggest that the tolerance to a hyperosmotic load is species-specific among marine mammals and depends on
the extent of the Na + load.
Infusion of mannitol, which can increase osmolality of a solution independent of
changes in electrolyte concentrations, has also been performed in some marine mammals.
Mannitol infusion in gray seals decreased plasma electrolyte and urea concentrations without changing plasma osmolality (Skog and Folkow 1994) indicative of a hemodilution effect.
Chapter seven: Water balance
maintained in freshwater compared to those in seawater suggesting that freshwater consumption was a significant component of their water flux (Ortiz et al. 1999).
7.2.4 Infusion experiments: Water, electrolytes, and organic molecules
The infusions of water, electrolytes, or organic molecules provide important experimental
protocols to help elucidate mechanisms that regulate body water and salts. With respect
to osmoregulatory mechanisms, the intravenous infusion of hyper- or hypo-osmotic solutions and the subsequent monitoring of the humoral, excretory, and/or renal responses
provide insight to the physiological mechanisms and adjustments to recover homeostasis.
Infusion studies with freshwater, hypertonic saline, mannitol, and isotonic gelatin among
others have been performed in a variety of marine mammals to help reveal their osmoregulatory capabilities.
7.2.4.1 Loading of hypertonic/hyperosmotic saline
In terrestrial mammals, the ingestion of seawater typically induces excretion of excess salts
and water resulting in dehydration without the ability of producing urine that is more concentrated than the ingested seawater. Excessive salt intake (sodium chloride; NaCl) has
consequences in the form of diarrhea, vomiting, edema, and elevated arterial blood pressure (Boyd et al. 1966; Meneely and Battarbee 1976; Thompson 2011). Generally, marine
mammals are thought to be resistant to seawater ingestion because they have adapted to
living in the marine habitats.
Hypertonic saline infusion of various concentrations induces urine production
with excretion of Na + and Cl − in bottlenose dolphin (Fetcher and Fetcher 1942) and seals
(Albrecht 1950; Bradley et al. 1954; Tarasoff and Toews 1972; Hong et al. 1982; Skog and
Folkow 1994; Storeheier and Nordøy 2001; Ortiz et al. 2002; How and Nordøy 2007).
Although these marine mammals can produce concentrated urine with a relatively high
osmolality (Bester 1975; Costa 1982; Maluf 1989; Ortiz 2001), they do not usually concentrate Na + and Cl − above that of seawater. At least in pinnipeds, the tolerance to salt
loading seems to vary among species. For example, in the harbor seal, vomiting and
diarrhea were induced by the loading of 2.6 ml seawater/kg of body weight and the
continuous intravenous infusion of hyperosmotic saline resulted in death in two harbor
seals (Albrecht 1950). In the California sea lion, similar consequences were reported with
the loading of 1.5 ml/kg (Ridgway 1972). However, the acute intravenous bolus infusion
of iso- and hypertonic saline with equivalent amounts of Na + (310 mEq) had no apparent consequences on northern elephant seal pups suggesting that some pinnipeds are
resistant to excessive salt loading or that the differences in the experimental protocols
accounted for the disparate physiological responses (Ortiz et al. 2002). Similarly, in the
bottlenose dolphin, given 4 ml/kg (Ridgway 1972) or 4 l (Ridgway and Venn-Watson
2010) of seawater by gavage did not demonstrate any clinical signs of seawater toxicosis.
In the dolphins, seawater ingestion initially induced a diuresis with high urine concentrations of Na + , Cl − , and K + , with a relatively hyperosmotic urine continuously produced
(Ridgway and Venn-Watson 2010). Collectively, these observations suggest that the tolerance to a hyperosmotic load is species-specific among marine mammals and depends on
the extent of the Na + load.
Infusion of mannitol, which can increase osmolality of a solution independent of
changes in electrolyte concentrations, has also been performed in some marine mammals.
Mannitol infusion in gray seals decreased plasma electrolyte and urea concentrations without changing plasma osmolality (Skog and Folkow 1994) indicative of a hemodilution effect.