156
Marine Mammal Physiology: Requisites for Ocean Living
from the bottlenose dolphin in dogs. A positive correlation between plasma renin activity (PRA; an indicator of Ang I generation) and plasma aldosterone concentration was
observed in the California sea lion (Malvin et al. 1978), fasting elephant seal pups (Ortiz
et al. 2000), the bottlenose dolphin (Malvin et al. 1978), and manatees (Ortiz et al. 1998).
When sodium availability is reduced, the sensitivity of RAAS appears to be increased in
the case of elephant seals (Ortiz et al. 2000). Aldosterone concentration correlated negatively to water efflux rate in fasting elephant seals suggesting contribution of RAAS to
water conservation during fasting (Ortiz et al. 2006). Because manatees that inhabit freshwater have reduced availability to Na + (Best 1981; Ortiz et al. 1999), manatees are more
sensitive to RAAS than in pinnipeds and cetaceans (Ortiz 2001). Collectively, RAAS is
a significant endocrine mechanism in the regulation of water and electrolytes in those
marine mammals studied.
7.2.5.3 Natriuretic peptides
The family of natriuertic peptides consists of atrial natriuertic peptide (ANP), brain natriuertic
peptide (BNP), and C-type natriuretic peptide (CNP), and the peptides defend against excessive salt and water retention, and also influence renal hemodynamic and direct tubular
actions resulting in diuretic and natriuretic actions (Levin et al. 1998). ANP is secreted
mainly from the atrial wall in response to increased cardiac pressure and distention
(Stanton 1991). ANP promotes natriuresis by increasing renal filtration, inhibiting Na +
absorption by suppressing renin and aldosterone release and antagonizing the actions of
Ang II (Light et al. 1989). While ANP may be thought to be a highly functional and relevant
hormone in marine mammals because of the abundance of Na + in their environment, only
a few reports of changes in natriuretic peptides and their potential osmoregulatory function exist.
Precursors of ANP and BNP were cloned in the Pacific white-sided dolphin
(Lagenorhynchus obliquidens), bottlenose dolphin, and Dall’s porpoise (Phocoenoides dalli),
and their presence in the plasma of bottlenose dolphins were also detected (Naka et al.
2007). Zenteno-Savin and Castellini (1998a) reported plasma concentrations of an ANPlike substance in six pinniped species. Sleep apnea, consistent with how animals hold
their breath during diving, increased plasma ANP concentration in the northern elephant and Weddell seals suggesting that the bradycardia associated with breath-holding decreased cardiac output and increased intra-cardiac pressure (Zenteno-Savin and
Castellini 1998b). Urinary ANP excretion was more than doubled in response to a hyperosmotic saline infusion in fasting elephant seal pups, but levels were not significantly
increased with an iso-osmotic infusion (Ortiz et al. 2002). However, only the hyperosmotic infusion was associated with a sustained elevation in plasma Na + suggesting that
ANP is responsive to the typical mammalian stimulus in pinnipeds (Ortiz et al. 2002).
Further studies to ascertain the functional relevance of ANP and the other natriuretic
peptides in marine mammals are warranted.
7.3 Toolbox
7.3.1 General tools
The large body masses and strictly aquatic life styles in many groups (cetaceans and
sirenians) of marine mammals make investigations into the osmoregulatory mechanisms in these animals extremely challenging. Early studies were limited to comparisons of hormones and electrolytes among different species as well as descriptions of
Marine Mammal Physiology: Requisites for Ocean Living
from the bottlenose dolphin in dogs. A positive correlation between plasma renin activity (PRA; an indicator of Ang I generation) and plasma aldosterone concentration was
observed in the California sea lion (Malvin et al. 1978), fasting elephant seal pups (Ortiz
et al. 2000), the bottlenose dolphin (Malvin et al. 1978), and manatees (Ortiz et al. 1998).
When sodium availability is reduced, the sensitivity of RAAS appears to be increased in
the case of elephant seals (Ortiz et al. 2000). Aldosterone concentration correlated negatively to water efflux rate in fasting elephant seals suggesting contribution of RAAS to
water conservation during fasting (Ortiz et al. 2006). Because manatees that inhabit freshwater have reduced availability to Na + (Best 1981; Ortiz et al. 1999), manatees are more
sensitive to RAAS than in pinnipeds and cetaceans (Ortiz 2001). Collectively, RAAS is
a significant endocrine mechanism in the regulation of water and electrolytes in those
marine mammals studied.
7.2.5.3 Natriuretic peptides
The family of natriuertic peptides consists of atrial natriuertic peptide (ANP), brain natriuertic
peptide (BNP), and C-type natriuretic peptide (CNP), and the peptides defend against excessive salt and water retention, and also influence renal hemodynamic and direct tubular
actions resulting in diuretic and natriuretic actions (Levin et al. 1998). ANP is secreted
mainly from the atrial wall in response to increased cardiac pressure and distention
(Stanton 1991). ANP promotes natriuresis by increasing renal filtration, inhibiting Na +
absorption by suppressing renin and aldosterone release and antagonizing the actions of
Ang II (Light et al. 1989). While ANP may be thought to be a highly functional and relevant
hormone in marine mammals because of the abundance of Na + in their environment, only
a few reports of changes in natriuretic peptides and their potential osmoregulatory function exist.
Precursors of ANP and BNP were cloned in the Pacific white-sided dolphin
(Lagenorhynchus obliquidens), bottlenose dolphin, and Dall’s porpoise (Phocoenoides dalli),
and their presence in the plasma of bottlenose dolphins were also detected (Naka et al.
2007). Zenteno-Savin and Castellini (1998a) reported plasma concentrations of an ANPlike substance in six pinniped species. Sleep apnea, consistent with how animals hold
their breath during diving, increased plasma ANP concentration in the northern elephant and Weddell seals suggesting that the bradycardia associated with breath-holding decreased cardiac output and increased intra-cardiac pressure (Zenteno-Savin and
Castellini 1998b). Urinary ANP excretion was more than doubled in response to a hyperosmotic saline infusion in fasting elephant seal pups, but levels were not significantly
increased with an iso-osmotic infusion (Ortiz et al. 2002). However, only the hyperosmotic infusion was associated with a sustained elevation in plasma Na + suggesting that
ANP is responsive to the typical mammalian stimulus in pinnipeds (Ortiz et al. 2002).
Further studies to ascertain the functional relevance of ANP and the other natriuretic
peptides in marine mammals are warranted.
7.3 Toolbox
7.3.1 General tools
The large body masses and strictly aquatic life styles in many groups (cetaceans and
sirenians) of marine mammals make investigations into the osmoregulatory mechanisms in these animals extremely challenging. Early studies were limited to comparisons of hormones and electrolytes among different species as well as descriptions of