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’ CREAP-1 AQP-3 (GLIP) 5’ SP-1AP-2

5. The Major Nitrogenous Waste Products Ammonia: No extractable energy ==> cheap; High water sulubility; High permeability through lipid bilayers (NH 3 ); 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 NH 3 ; 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

7. N-Excretion in Chondrichtyes

8. N-Ecretion in Reptiles

9. 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

10. 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