12.1.3 Osmotic Functions of Small Nitrogenous Compounds
407
acid spectrum, for which one had great hopes
30 years ago, eventually proved to be unsuitable.
Of the factors which can influence the concentration and variety of free amino acids, nutrition
plays a significant role, in that it directly changes
the input into the pool. Fundamental changes in
the amino acid pool are observed following
changes in the physiological situation (e.g. during
metamorphosis of insects [46] and amphibians,
and in the ecdysis cycle of the crustaceans), at
times of increased protein synthesis (e.g. silk production, and formation of sex substances), or in
response to parasite infection. Changes in internal conditions are the basis of seasonal and diurnal fluctuations in the amino acid pool. External
factors that may influence the concentrations of
free amino acids include temperature, oxygen
partial pressure and osmolality.
12.1.3 Osmotic Functions
of Small Nitrogenous Compounds
Inorganic ions constitute usually more than 98 %
of the osmotically active substances of the extracellular fluids. Organic molecules make significant contributions to the extracellular osmolality
in only a few animal groups, e.g. amino acids or
sugars in insect haemolymph or urea, and trimethylamine oxide in the blood of certain fishes. In
tissues, inorganic ions usually contribute about
half the osmolality and the rest mainly arises from
the presence of nitrogenous substances such as
aminocarbonic acids, taurine, glycinebetaine, trimethylamine oxide and urea. The dominant role
of urea as an osmotic substance in the marine
elasmobranchs and crossopterygians is an interesting problem. Here, the urea concentrations
in the blood plasma are generally about
300-450 mmolll, and in the case of the ray Trygonorhina as high as 600 mmolll; the intracellular
concentrations are even higher. Such high urea
concentrations should, in fact, disturb the functional and structural integrity of the cell proteins.
It appears, however, that the simultaneous presence of methylamino compounds, such as trimethylamine oxide, glycinebetaine and Nmethylglycine (sarcosine), compensates for the
destabilizing effects of urea [253].
The osmolality of metazoan cells always agrees
more or less with that of the extracellular environment, and the cell must react to all changes in
the extracellular osmolality. Adaptation (isosmotic intracellular regulation) results above all
from changes in the concentration of nitrogenous
compounds. This has the advantage that the concentrations of inorganic ions, which have other
specific biological effects in addition to their
osmotic functions, are subject to minimal change
[90, 149, 253]. Comparable intracellular regulation has been observed following dehydration,
e.g. in the edible snail Helix pomatia. Exactly
which nitrogenous compound is most involved in
osmoregulation depends upon both the species
and the tissue (Table 12.2), and may also be
determined by the speed at which the changes in
salinity of the external medium occur. In various
mussels, long-lasting changes in salinity lead to
regulatory changes in taurine and amino acid concentration; this does not happen, for example, in
response to the 12-hour rhythm of the tides in
river estuaries.
In view of the variety and number of factors
that affect the intracellular pools of small nitrogenous compounds, it is not surprising that there
is no general explanation for the mechanisms of
isosmotic intracellular regulation. Osmoregulatory increases in intracellular nitrogenous substances (hyperosmotic regulation) can occur by
uptake from the extracellular space, through
increases in de novo synthesis, reduced degradation, or a shift in the rate of protein turnover in
favour of free amino acids. Several of these
mechanisms are probably always involved simultaneously, depending upon the species and the
tissue [82]. That some amino acids have their origins in cell proteins is suggested by the reported
increases in essential amino acids, although the
role of protein hydrolysis in intracellular regulation is still controversial. Interesting data have
been reported in this connection for the edible
mussel Mytilus edulis. Here, aminopeptidase I is
encoded by the polymorphic Lap locus; animals
with the Lap94 allele have a 20 % higher proteolytic enzyme activity and do, in fact, show more
rapid increases in the amount of free amino acids
following hyperosmotic stress [63].
Evidence for regulatory changes in the de novo
synthesis of amino acids has been sought, on the
one hand, in tracer experiments, which unfortunately often give ambigous results, and, on the
other hand, in studies of the enzymes of amino
acid metabolism in vitro. Glutamate dehydrogenase, which is without doubt a key enzyme of
amino acid metabolism, is activated by NaCI and
other inorganic salts. However, the ion sensitivity
of this enzyme in an animal with a marked capacity for intracellular osmoregulation, such as the
shore crab Carcinus maenas, has in fact been
found to be lower than in the much less adaptable
407
acid spectrum, for which one had great hopes
30 years ago, eventually proved to be unsuitable.
Of the factors which can influence the concentration and variety of free amino acids, nutrition
plays a significant role, in that it directly changes
the input into the pool. Fundamental changes in
the amino acid pool are observed following
changes in the physiological situation (e.g. during
metamorphosis of insects [46] and amphibians,
and in the ecdysis cycle of the crustaceans), at
times of increased protein synthesis (e.g. silk production, and formation of sex substances), or in
response to parasite infection. Changes in internal conditions are the basis of seasonal and diurnal fluctuations in the amino acid pool. External
factors that may influence the concentrations of
free amino acids include temperature, oxygen
partial pressure and osmolality.
12.1.3 Osmotic Functions
of Small Nitrogenous Compounds
Inorganic ions constitute usually more than 98 %
of the osmotically active substances of the extracellular fluids. Organic molecules make significant contributions to the extracellular osmolality
in only a few animal groups, e.g. amino acids or
sugars in insect haemolymph or urea, and trimethylamine oxide in the blood of certain fishes. In
tissues, inorganic ions usually contribute about
half the osmolality and the rest mainly arises from
the presence of nitrogenous substances such as
aminocarbonic acids, taurine, glycinebetaine, trimethylamine oxide and urea. The dominant role
of urea as an osmotic substance in the marine
elasmobranchs and crossopterygians is an interesting problem. Here, the urea concentrations
in the blood plasma are generally about
300-450 mmolll, and in the case of the ray Trygonorhina as high as 600 mmolll; the intracellular
concentrations are even higher. Such high urea
concentrations should, in fact, disturb the functional and structural integrity of the cell proteins.
It appears, however, that the simultaneous presence of methylamino compounds, such as trimethylamine oxide, glycinebetaine and Nmethylglycine (sarcosine), compensates for the
destabilizing effects of urea [253].
The osmolality of metazoan cells always agrees
more or less with that of the extracellular environment, and the cell must react to all changes in
the extracellular osmolality. Adaptation (isosmotic intracellular regulation) results above all
from changes in the concentration of nitrogenous
compounds. This has the advantage that the concentrations of inorganic ions, which have other
specific biological effects in addition to their
osmotic functions, are subject to minimal change
[90, 149, 253]. Comparable intracellular regulation has been observed following dehydration,
e.g. in the edible snail Helix pomatia. Exactly
which nitrogenous compound is most involved in
osmoregulation depends upon both the species
and the tissue (Table 12.2), and may also be
determined by the speed at which the changes in
salinity of the external medium occur. In various
mussels, long-lasting changes in salinity lead to
regulatory changes in taurine and amino acid concentration; this does not happen, for example, in
response to the 12-hour rhythm of the tides in
river estuaries.
In view of the variety and number of factors
that affect the intracellular pools of small nitrogenous compounds, it is not surprising that there
is no general explanation for the mechanisms of
isosmotic intracellular regulation. Osmoregulatory increases in intracellular nitrogenous substances (hyperosmotic regulation) can occur by
uptake from the extracellular space, through
increases in de novo synthesis, reduced degradation, or a shift in the rate of protein turnover in
favour of free amino acids. Several of these
mechanisms are probably always involved simultaneously, depending upon the species and the
tissue [82]. That some amino acids have their origins in cell proteins is suggested by the reported
increases in essential amino acids, although the
role of protein hydrolysis in intracellular regulation is still controversial. Interesting data have
been reported in this connection for the edible
mussel Mytilus edulis. Here, aminopeptidase I is
encoded by the polymorphic Lap locus; animals
with the Lap94 allele have a 20 % higher proteolytic enzyme activity and do, in fact, show more
rapid increases in the amount of free amino acids
following hyperosmotic stress [63].
Evidence for regulatory changes in the de novo
synthesis of amino acids has been sought, on the
one hand, in tracer experiments, which unfortunately often give ambigous results, and, on the
other hand, in studies of the enzymes of amino
acid metabolism in vitro. Glutamate dehydrogenase, which is without doubt a key enzyme of
amino acid metabolism, is activated by NaCI and
other inorganic salts. However, the ion sensitivity
of this enzyme in an animal with a marked capacity for intracellular osmoregulation, such as the
shore crab Carcinus maenas, has in fact been
found to be lower than in the much less adaptable