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Excretion

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Chapter 17 Regulation of ... Energy requirement met by consumption of floral nectar At ... Default Design Microsoft Word Document Excretion Regulation of Vesicle ... – PowerPoint PPT presentation

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Title: Excretion


1
Excretion
  • Comparative Physiology
  • Chapter 17

2
Regulation of Vesicle Trafficking by SNAREs and
VAMPs
  • Vesicle targeting receptors, SNAREs, in plasma
    membrane provide docking sites and mediate
    exocytosis of specific vesicles.
  • Syntaxins
  • SNAP-25 and its homologues
  • VAMPs (Vesicle Associated Membrane Proteins
    synaptobrevins) in membrane exocytotic vesicles
    bind to SNAREs when stimulated (e.g. by cAMP).

3
Long-Term Regulation of Transepithelial Water
Permeability
  • Research in 50s Ability to concentrate urine in
    response to vasopressin administration was
    enhanced in patients subjected to 48-h water
    restriction.
  • Later Research (1991) In vitro preparations of
    rat IMCD showed that prolonged thirsting
    increased water permeability of the IMCD even if
    no vasopressin was present in incubation medium.
  • Conclusion Long-term thirsting produces a
    conditioning effect

4
Transcriptional Regulation of Aquaporins
AQP-2 (WCH-CD)
5
CRE
AP-1
AQP-3 (GLIP)
5
SP-1
AP-2
5
The Major Nitrogenous Waste Products
  • Ammonia No extractable energy gt cheap High
    water sulubility High permeability through lipid
    bilayers (NH3) 1N/molecule Highly toxic
  • Urea Expensive to synthesize (2ATP) High water
    solubility Intermediate permeability in lipid
    bilayers 2N/molecule Moderate toxicity
  • Uric acid Very expensive to synthesize Low
    water solubility Low permeability in lipid
    membranes 4N/molecule Low toxicity

6
N-Excretion in Amphibians
  • Tadpoles generally ammonotelic
  • Adult FW amphibians (e.g. Xenopus) often
    ammonotelic (total N 60 - 80 as NH3 20 - 40
    as urea)
  • Xenopus and Rana cancrivora in SW use urea as
    osmolyte and are ureotelic
  • Semiterrestrial and terrestial amphibians
    typically strongly ureotelic (total N gt80 urea)
  • Some tree frogs from Africa (Chiromantis) and
    South America (Phyllomedusa) are uricotelic

7
N-Excretion in Chondrichtyes
8
N-Ecretion in Reptiles
9
N-Excretion in Birds
  • Uricotely demonstrated in chicken gt200 yrs ago
  • Only 6 other species have been investigated
    scientifically - all during conditions that would
    favour uric acid excretion (e.g., dehydration,
    salt loading, protein-loading)
  • Above data and casual observations of white
    precititate in excretia lead to the general dogm
    that birds are uricotelic

10
Hummingbirds
  • Highest metabolic rates of any vertebrate
  • Energy requirement met by consumption of floral
    nectar
  • At low temperatures, metabolic cost is high gt
    extreme rates of nectar consumption gt very high
    water turn-over
  • At higher temperatures, evaporative water loss is
    high due to panting
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