Water and Solutes I

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Water and Solutes I

• Concepts
• Water and ion balance in aquatic organisms
• Why it is important
• Osmoregulatory cells
• Diadromous animals
• Nitrogenous waste
• Its forms
• Who makes what
• Why does it matter to water balance?
• The MIGHTY kidney
• How it works
• Its role in hydration state and blood pressure
• Comparative renal physiology

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Aquatic animals
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Osmoregulation general concepts

• Water always flows toward higher ion
  concentration
• There are no water transporters
• To move water, animals create areas of high ion
  concentration
• Ions always flow toward lower concentrations
• Maintaining ionic differences (gradients)
  requires

ATP!
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Osmoregulation terminology

• Osmolarity total moles solute per liter of
  solution
• Osmolality total moles solute per kg. of
  solvent
• Isoosmotic same solute concentration
• Hyperosmotic higher solute concentration
• Hypoosmotic lower solute concentration
• Stenohaline tolerates a narrow range of
  salinities
• Euryhaline tolerates a wide range of salinities

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Osmoregulation vertebrates

• Vertebrate blood is generally 300 400 mOsm
• Freshwater is a hypoosmotic environment
• Gain water
• Lose ions
• Saltwater is a hyperosmotic environment
• Lose water
• Gain ions
• Some exceptions are isoosmotic with seawater
• elasmobranchs (sharks, rays skates)
• some amphibians
• Lots of marine invertebrates

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Marine fish
1000
300
Q Do marine mammals have the same problem?
A Yes and no have to deal with salty food and
water, However,dont have diffusive surfaces
(gills) in constant contact w/H2O
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Osmoregulation role of gills

• Its a particular challenge for animals with
  gills
• Gills are great surfaces for exchange!
• This is good for gas exchange
• Problematic for maintaining ion or water balance
• Gills are primary site of ion exchange
• Chloride cells
• Many aquatic animals without gills have salt
  glands
• Iguanas and Seabirds have them in their noses
• Crocodiles have them on their tounges
• Salt glands also contain chloride cells

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Both salt and freshwater fish are active
osmoregulators
Drinks seawater Pees as little as possible (to
get rid of certain ions) Actively exports Cl-
Passively exports Na
Drinks as little as possible Pees dilute
urine Actively imports Cl- and Na
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Getting rid of salt chloride cells
Gill membrane
Seawater
Apical surface
Blood/Extracellular fluid
Basolateral surface
High basolateral surface area (villi) Lots of
mitochondria (produce ATP) Lots of Na/K ATPase
(uses ATP)
builds huge Na gradient
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Chloride cells
Na
Na
Na
Cl-
Na
Na
Na
Cl-
Na
Na
Na/K ATPase pump
Na
Cl-
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ATP-driven salt pumping
1) Na/K ATPase builds whopping Na
gradient 2) Cl- transported using Na gradient
(Cl-/Na symporters) 3) Cl- builds up and
escapes through apical Cl- channels 4) Na
follows Cl- charge by sneaking through cell
junctions
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In freshwater fish gills
Super-high affinity Na Cl- transporters
Tight junctions no ion leak
ATP
Na
Very low Na
Na
Na
Cl-
Cl-
Also have basolateral Na/K ATPase
Na
Na
Na
Na
Cl-
Na
Na
Na
Na
Cl-
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Chloride cells in freshwater fish

• Also have lots of mitochondria and Na/K ATPase
• No Na/Cl- symporters
• Cell cytoplasm very low in Na and Cl-
• Apical membranes have ion pumps to pump ions IN
• Cell junctions very tight (no ion leak)

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What about them?
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Diadromous animalsmigrate between salt and fresh
Question How would gills have to change? How
wouldnt they?
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Getting the right chloride cells Anadromous fish

• Stem cells in gill epithelium
• Develop into salt water or fresh water chloride
  cells
• Cell type production is hormonally signalled
• Cortisol triggers proliferation of chloride cells
• Cortisol Growth Hormone seawater type
• Animal is growing and heading out to sea
• Cortisol Prolactin freshwater type
• Animal is preparing for reproduction
• Prolactin may play a role in fish reproduction
  not well known

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Intertidal habitats
Salt marsh
Sandy intertidal with tidepools
Animals must be euryhaline
Water can be a mix of fresh and salt
(brackish Or can evaporate in exposed pools
(hypersaline)
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Euryhaline animalsOsmoregulators vs.
Osmoconformers
Body fluid concentration (mOsm)
Water concentration (mOsm)
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Invertebrates in brackish water
All are euryhaline some osmoregulate, some
osmoconform
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What does all this have to do with kidneys???
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N excretion

• Animals take in C, O, H and N in their food (and
  S)
• C, O and H are lost as water and carbon dioxide
• N is more complicated to get rid of
• How is waste N is generated?
• If there is excess in diet
• Simply through tissue breakdown and turnover
• Proteins and enzymes have a life-span!
• Some animals are better at N recycling than others

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Deamination removing N from amino acids
a - Ketoglutarate
Glutamate
Krebs cycle intermediate
Amino acid common acceptor of amine groups from
other aas
Ammonia (NH3)
Waste product
-- toxic!!! Binds H ions Screws up
neurotransmitters Goes through membranes
Ammonium ion (usually present at physiological
pH)
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Ammonia cant exceed of 0.05 mmol/l !
Animals ability to concentrate it is very limited
water NH3 ltlt 0.05 mmol/l
For most aquatic organisms, ammonia diffuses into
environment before reaching toxic internal s
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Ammonotely
When an animal excretes most of its excess N in
the form of ammonia ( animal is ammonotelic)
WHO?
Aquatic animals Animals with high rates of water
intake and loss high internal dilution
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Evolution of terrestrial life required a better
way to get rid of ammonia
Crap!
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Two solutions Urea and Uric acid
Both are (1) less toxic and (2) less soluble than
ammonia
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N excretion overview
Ammonia Urea Uric acid
Toxic Not toxic Not toxic must stay low
can build up can build up until
precipitation
Toxicity
Soluability
Highly soluable Soluable Low soluability Cant
concentrate Can concentrate Can void as a
solid Lots of H2O to void Less H20 to void/N paste
Cost
FREE! Requires some E Requires most E 1.5
ATP/N 5 ATP/N
Terminology
Ammonotelic Ureotelic Uricotelic
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Who uses different N waste products?
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Least conservative of water
Aquatic animals Some amphibians
Mammals, Some amphibians, Some fish
The white paste birds deposit on your car
Birds, Reptiles Insects
Most conservative of water
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Uricotely
Uricotelic (uric acid producing) animals conserve
H2O well but pay an energetic price WHY?
Theres a hint in WHO
BIRDS REPTILES INSECTS
In egg, uric acid precipitates out of solution
without building up to high . Remains in
eggshell after hatching.
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Mammals cant synthesize uric acid from ammonia
BUT Can form it from the breakdown of purine
nucleotides
excess
Uric acid
If something goes wrong with uric acid breakdown
to urea Or if diet is very purine rich Uric
acid crystals can precipitate in joints Gout
afflicts kings and dalmations
Urea
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Least conservative of water
Aquatic animals Some amphibians
Mammals, Some amphibians, Some fish
The white paste birds deposit on your car
Birds, Reptiles Insects
Most conservative of water
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Ureotely Who and when?
Enzymes in urea cycle get expressed during
metamorphosis or as conditions get dry