Osmoregulation by Birds Eldon J. BraunDepartment of PhysiologyUniversity of ArizonaMaintenance of the Internal Environment Osmoregulation means the maintenanceof the homeostasis of internal environment.What constitutes the internal environment?Birds osmoregulated wellBirds inhabit all environmentsAquatic Fresh waterMarineEstuaries Terrestrial PolarTemperateDesertIn terms of osmoregulation, mammals are the unusual groupKidneys are only osmoregulatory organOsmoregulation among other vertebratesFish, amphibians, reptiles, and birdsMultiple organs function in osmoregulationORGANS THAT CONTRIBUTE TO OSMOREGULATION IN VERTEBRATES Group Osmoregulatory Organs Fish Kidneys Gills Bladder Intestine Amphibians Kidneys Gills Bladder Skin Intestine Reptiles Kidneys Salt Glands Intestine Birds Kidneys Salt Glands Intestines Mammals KidneysOsmoregulation by birds: Organs InvolvedKidneysLower gastrointestinal tractSalt glandsAvian salt glandsThe avian renal and gastrointestinal systems must function in concert in the regulation of ion and fluid balance.As birds do not have urinary bladders, the ureteral urine is refluxed from the cloaca into colonEvolutionary Rationale for this Type of Arrangement Excess mass of urinary bladder GFRs of Birds and Mammals Do NotDiffer Fraction of Filtered Water Reabsorbed by Kidney Less by Avian Kidney Urine of Birds in Constant “Flux”(i.e. urine entering lower GI tract)Argument does not “hold water”Urine to plasma osmolar ratioHow well kidneys of animals concentrate urine isUsually expressed as the ratio of the urine osmolalityTo the plasma osmolality.Or simply the U/PosmBirds or the avian kidney does not concentrate urine to a high degreeMaximum U/Posms of some mammalsValues range from about 1 in the Mountain Beaver to ca. 25 in some of the small desert Rodents.Humans U/Posm?Urine-to-Plasma Osmolar Ratios for Birds (U/Posm)Ring-necked Pheasant 1.5Senegal Dove 1.7Savannah Sparrow 1.7King Quail 1.8White-crowned Sparrow 1.8Domestic Fowl 2.0Budgerigar 2.3House Finch 2.4Singing Honeyeater 2.4Stubble Quail 2.6Mean 2.05Comparison of U/Posms between birds and mammals Not valid comparison to make Urine in lower GI tract End products of nitrogen metabolism Uric acid vs. ureaEffects of conc. fluid in lower GI tractUrea ca. 50% of solutes in urineUric acid not in solution Plasma and urine osmolality of Desert QuailNitrogen Excretion in BirdsCompound PercentUrea 4Ammonium 20Uric Acid 76Solubilites of Nitrogen-Containing CompoundsCompound Solubility (mmol/L)Uric Acid 0.381Ammonium Urate 3.21Sodium Urate 8.32Potassium Urate 14.75Urea 16,650Structure of Uric AcidEvolutionary Rationale for this Type of Arrangement(i.e. urine entering lower GI tract) Crystal of Uric AcidEvolutionary Rationale for this Type of Arrangement(i.e. urine entering lower GI tract) Physical form of uric acid in avian urineSmall spherical structuresSpheres ca. 65%uric acidUric acid boundTo a matrix proteinPrevention of Sphere CoalescenceProtein in avian ureteral urine Avian urine contains 5 mg/ml proteinProtein conc. in human urineca. 0.05 mg/mlNature of Protein in Urine of BirdsSDS PAGE of avian Urine and plasmaEnergy in Avian Ureteral UrineMale FemaleKcal/Day 5.3 12.4% BMR 5.4 11.3SDS PAGE of Avian Excreted FluidModification of Urine in Lower GI Tract of BirdsAnderson & BraunDegradation of Uric Acid in Lower GI Tract 68% of uric acid in ureteral urine Bacterial action• Fate of liberated nitrogeno Glutamic acid9 Renal tubules--Buffer H ions9 Gluconeogensis 9 Citric acid cycleo Short chain volatile fatty acidsProducts Formed From the Breakdown of Uric Acid in Avian Lower GI tract77% of [15N]uric acid introduced into ceca of cockerelsdisappeared in 60 minAnd nitrogen appeared as ammonia and rapidly absorbedKarasawa, 1989Labelled nitrogen appeared in plasma within glutamine Where do these product go?Vasculature Surrounding the Avian KidneyAkesterCoccygomesenteric veindrains into renal portal system Birds have a functional renal portal systemTubule LumenCellsPeritubule SideBlood fromrenal portalsystemGlutamineGlutamineNH3+NH3+Na+H+H+NH4+Use of glutamine by renal tubules (To buffer hydrogen ions)ProteinsAmino AcidsPyruvateAcetyl CoACitric Acid cycleGlyceraldehyde3-phoasphateGlucoseFatty AcidsAmmonia Possible Pathway for Utilization of Uric Acid Also, deamination of glutamine produces ketoglutaric acid that can enter the krebs