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 Ureo-Osmoconformation

• Energy savings• Less of diffusive ion

influx

• Necessety for urea tolerance

• Other???