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Water & Solute Balance
Comparative Physiology
Chapter 16
Selective Pressures for Stringent
Regulation of Water and Solutes
• Protein biophysical properties are strongly
dependent upon the composition of the
intracellular fluid
• Many environments subject animal cells to
osmotic stress
– High, low, or fluctuating salinity (e.g. eurohaline
osmoconformers)
– Desiccation
– Freezing
Major Intracellular Organic Osmolytes
• Carbohydrates (i.e. trehalose, sucrose)
• Polyhydric alcohols (i.e. glycerol, mannitol)
• Free amino acids and amino acid derivates
(i.e. glycine, proline, taurine, ß-alanine)
• Urea and methyl amines (i.e. trimethyl
amine oxide, betaine) in combination
These Observations Raise the
Questions...
• Why do cells accumulate expensive energy-rich
organic metabolites rather than more readily
available inorganic ions, such as Na+ and K+?
• Why do some organic osmolytes occur in certain
combinations and often in fairly invariant
proportions?
• What properties of compatible osmolytes make
them compatible with cell function?
High Concentrations of Some Osmolytes
Inhibit Enzymes
• Perterbing osmolytes alter vmax and KM of enzymes
• Accumulation of compatible osmolytes matches the
osmolarity of ECF and prevents volume change w/o
altering the concentrations of pertubating osmolytes
Methyl Amines Counteract Denaturating
Effects of Urea
• Elasmobranchs: [Urea] = 350 - 450 mM
• Inner medulla of mammalian kidney [Urea] up
to 3 M!
• Methylamines (e.g. TMAO, betaine) are
powerful counteractants of urea
• Urea and methylamines are almost always
present in a 2:1 ratio which is optimal
Two Separate Mechanisms Make
Incompatible Osmolytes Incompatible
1 Direct cation interference with catalysis
Many enzymes have sites for cations, such as Ca2+ and
Zn2+. Other cations, such as K+ and Na+, have weak but
significant affinity for the same sites. [K+]i and [Na+]i
>> [Ca2+]i and [Zn2+]i. Organic solutes are neutral or
zwitterionic and do not interfere with enzymes.
Two Separate Mechanisms Make
Incompatible Osmolytes Incompatible
2 Compatible and pertubating osmolytes affect hydration,
solubility, and charge interactions of various protein
groups (e.g. peptide backbone; side chains).
Transporting Epithelia
• Located at the interface between the internal space
of the organism and the external space, the
environment
• Adjacent epithelial cells are sealed together by
tight junctions
– Tight junction effectiveness varies
– Actively transported solutes must follow the
transcellular pathway
– Only passive movement of materials occur through the
paracellular pathway
Function of Transporting Epithelia
Depends on
• the transporter set-up of the two different
cell membranes (apical and basolateral) in
series, and
• the properties of the tight junctions
FW Fish Gill is a Tight Epithelium
[Na] ~ 0.6 mM
[Na] ~ 130 mM
Physiological Characteristics of
Tight Epithelia
• High transepithelial potential (TEP)
• High transepithelial electric resistance
• Steep ion gradients maintained by ion
transport
The Epithelium of the Small
Intestine is Leaky
Paracellular Ion Fluxes Across
Leaky Epithelia were Discovered by
Voltage Scanning
Physiological Characteristics of
Leaky Epithelia
• Low transepithelial potential
• Low transepithelial resistance
• Small to moderate solute gradients
Evidence for Active Na+ Transport in
Frog Skin
• Net Na+ fluxes from apical to basolateral side can occur
against an oppising electrochemical gradient
• Transport is inhibited by general metabolic inhibitors (CN-,
iodoacetate)
• Transport inhibited by specific Na/K-ATPase blocker
(oubain) but only when applied to basolateral side ==>
Na/K-ATPase in basolateral membrane only
• Strong temperature dependence
• Saturation kinetics for Na+ transport (only apically)
Benefits & Costs of UreoOsmoconformation
• Energy savings
• Less of diffusive ion
influx
• Necessety for urea
tolerance
• Other???