Osmoregulation Part 4

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Osmoregulation Part 4

• Non-mammalian Vertebrate Kidneys
• Salt Glands
• Fish Gills

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Non-Mammalian Vertebrate Kidneyscomparative
differences

• Marine hagfish nephrons possess glomeruli, but
  no tubules BC empty directly into collection
  ducts thus kidneys used primarily to excrete
  divalent ions (e.g. Ca2, Mg2, etc)
• Fresh water teleosts have larger/more glomeruli
  than their marine relatives because their
  bodies are hyperosmotic to environment water
  diffuses into their bodies, they maintain osmotic
  balance by producing large volumes of dilute
  urine

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Non-Mammalian Vertebrate Kidneys comparative
differences

• Some marine teleosts nephrons have neither glom
  nor BC aglomerular kidneys, urine is formed
  entirely by secretion because there is not
  specialized mechanism for production of a
  filtrate these fish are hypo-osmotic to
  environment lose water continually across skin
  gills their problem is H2O conservation
  they produce small vol. of urine
• Amphibians reptiles appear incapable of
  producing hyperosmotic urine because they lack
  countercurrent multiplier system of Loop of H
  (I.e. only birds mammals)

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Non-Mammalian Vertebrate Kidneys comparative
differences

• Birds combo of reptilian-type mammal-type
  nephrons some have LofH and some the loop is
  oriented perpendicular to collecting duct less
  efficient countercurrent system many birds have
  salt glands that excrete concentrated salt
  solution allowing them to drink seawater
• Closer look as some extrarenal osmoregulatory
  organs

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Salt Glands

• Elasmobranch rectal gland
• NaCl content lower than sea water (but animal is
  slightly hyperosmotic to seawater due to urea
  TMO trimethylamine oxide continuous influx of
  NaCl into the animal excess NaCl removed by RG
  which produces excretes a salt soln
  containing only NaCl no filtration only active
  secretion of NaCl with H2O following Fig 14.14
  for transport mechanism
• Consists of lg. blind tubules surrounded by
  caps. tubules drain into a duct which opens
  into intestine near rectum

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Salt Glands cont

• Salt glands of birds reptiles
• Present in many sp. Of birds reptiles,
  especially those subjected to osmotic stresses of
  a marine or desert environments
• Occupy shallow depressions in skull above eyes
• In birds, consists of many lobes 1 mm in
  diamter, ea. Drain via branching secretory
  tubules central canal into a duct that runs
  through the beak empties into nostrils
• Active secretion of salt across epithelium of
  sec. tubules consisting of salt-secreting cells
  Fig 14.14
• No filtration

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Special Note

• Mammals have salt-secreting cells in thick
  ascending limb of LofH sim. to those in nasal
  gland of birds rectal gland of elasmobranchs
  also seem to be controlled by the same array of
  hormones BUT mammals cannot survive by drinking
  seawater
• Arrangement of salt-secreting cells in mammals
  simply does not allow the production of a
  hypertonic salt solution that can be excreted
• Great example of how anatomic, cellular, and
  molecuar organization of an animal determines its
  ability to survive in a particular environment

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Fish Gills

• Surface area of gill epithelium must be large if
  it is to function efficiently as a gas-exchange
  organ also contributes to suitability of gills
  for osmoregulation
• Fig 14.38 structure of gill differences between
  teleosts elasmobranchs H2O flow, cap.
  Arrangement, etc.
• Epithelium separating blood from external H2O
  consists of many cell types including mucous
  cells, chloride cells pavement cells
• Uptake of salt in seawater - Chloride cells
  deeply invaginated by infoldings of the
  basolateral membrane endowed heavily with mito
  enz relative to active salt transport

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Fish Gills cont

• Chloride cells function sim. to salt-secreting
  cells of Fig. 14.14 I.e. they have high levels of
  Na/K pumps associate with Na/2Cl-/K
  co-transport in the basolateral membrane Cl-
  channel in apical membrane ea. chloride cell is
  associated with an accessory cell
• Na diffuses from blood to seawater through
  paracellular channel between chloride cell
  accessory cell
• Marine teleosts secretion of salt occurs
  against osmotic gradient, so no movement of H2O
  follows with the salt

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Fish Gills cont

• Shark rectal gland, avian nasal salt gland,
  marine teleost gills thick ascending limb of
  LofH in mammalian kidney tubule all appear to
  contain salt-secreting cells that transport NaCl
  by same basic mechanisms also see 14.36 HOWEVER,
  remember that in mammalian kidney, the direction
  of salt transport is into the blood rather than
  into the environment

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Fish Gills cont

• Uptake of salt in freshwater gills of
  freshwater fishes have a proton pump
  (electrogenic) Na channels in apical membrane
  presumably, pumps protons out of gills,
  generating a potential that draws Na into the
  cell (sim to frog skin mam. kidney) Fig 14.31a
  (yet to be demonstrated)
• Na/K pump in basolateral membrane pumps Na out
  of cell into blood and K cycles through K
  channels proton pump appears to energize Na
  uptake across apical membrane whereas Na/K pump
  moves Na across basolateral membrane of gills
  into body fluid for some fish, Na uptake is
  coupled to H excretion via an antiporter

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Fish Gills cont

• Cl- cells also present but differ from marine
  teleost fish in that have an anion transporter on
  apical membrane high levels of proton pumps
  within cell
• Migrating fish (e.g. salmon eels)
  physiological acclimatization gill epithelium
  adjusts to changes in environmental salinity I.e.
  actively take up NaCl in freshwater actively
  excrete it in saltwater acclimatizing to new
  environment involves synthesis or destruction of
  molecular components of epithelial transport
  systems changes in morphology Cl- cells
  acclimatization is modulated by hormones which
  influence epithelial differentiation metabolism
  (Growth Hormone cortisol) review the process of
  smolting producing a smolt p. 620 see Table
  14.10 p. 621 - remember the debate
  structure-function or function-structure?