Excretion

Comparative Physiology

Chapter 17

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).

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’

Transcriptional Regulation of Aquaporins

AQP-2 (WCH-CD)

5’CRE AP-1

AQP-3 (GLIP)

5’SP-1 AP-2

The Major Nitrogenous Waste Products

• Ammonia: No extractable energy ==> 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

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: >80% urea)

• Some tree frogs from Africa (Chiromantis) and South America (Phyllomedusa) are uricotelic

N-Excretion in Chondrichtyes

Medium Amm.-N(%)

Urea-N(%)

Pristis microdon FW 20 80

Pomatotrygon sp. FW 90 10

Squalus acanthias SW 20 80

Raja erinacea SW 16 84

N-Ecretion in Reptiles

Amm. (%) Urea (%) Uric acid (%)

Crocodiles 25 4 68

Turtle, aquatic 24 23 1

Turtle, terrestrial 6 61 4

Turtle, arid 4 22 52

Lizard, moist 25 15 58

Lizard, desert 1 0 99

Snakes 2-20 ? >50

N-Excretion in Birds• Uricotely demonstrated in chicken >200 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

Hummingbirds

• Highest metabolic rates of any vertebrate• Energy requirement met by consumption of floral

nectar• At low temperatures, metabolic cost is high =>

extreme rates of nectar consumption => very high water turn-over

• At higher temperatures, evaporative water loss is high due to panting