Renal salt and water excretion in the budgerygah (Melopsittacus undulatus)

Effect of water restriction on renal function in ostriches (Struthio camelus)

The production of a white urine by ostriches can be indicative of illness or water deprivation. A metabolic renal study was carried out by dehydrating young, healthy ostriches for 2 days, the period during which urine was still excreted. It was found that the normally copious colourless urine changed to a thick, white excretion after 2 days dehydration and no fluid was excreted after 3 days. From the blood and urine analyses it is concluded that the ostrich conserves body water by an excellent renal concentrating mechanism which excretes urates. This excretion is a useful sign to the fanner that for some reason the animals are not drinking, be it due to a lack of drinking water or to illness.

Comparison of renal and salt gland function in three species of wild ducks

Three processes central to osmoregulation of marine birds were compared in three species of ducks that differ in habitat affinity, diet and saline tolerance. These processes are filtration of Na+ and water from the plasma by the kidneys, their reabsorption along the renal tubules, and secretion by the salt glands. Barrow's goldeneyes Bucephala islandica, the most marine species, have the highest rates for all three processes and only this species can secrete all the infused salt via the salt glands. Rates of all three processes are lower in mallards Anas platyrhynchos, the most freshwater species. Following saline acclimation, mallards could excrete all the infused Na+ by a combined Na+ excretion of the kidneys and salt glands. Canvasbacks Aythya valisineria, despite being more saline tolerant than mallards, are unable to excrete all the infused Na+. They produce a large volume of urine (like mallards) that has a low [Na+] (like goldeneyes). Salt gland secretion Na+ concentration did not differ among the three species, but only goldeneyes secrete at a rate sufficient to eliminate all infused Na+ via the salt glands. Differences in saline tolerance of these ducks species cannot be fully explained by differences in their filtration, reabsorption and secretion of Na+ and water, suggesting that the intestinal tract plays an important role.

Renal anatomy in sparrows from different environments

The renal anatomy of three species of sparrows, two from mesic areas, the House Sparrow (Passer domesticus) and Song Sparrow (Melospiza melodia), and one salt marsh species, the Savannah Sparrow (Passerculus sandwichensis) was examined. Electron microscopy was used to describe the ultrastructure of the nephron. In addition, stereology was used to quantify the volumes of cortex, medulla, and major vasculature of the kidneys, and the volumes and surface areas occupied by individual nephron components. There appeared to be no differences in the ultrastructural anatomy of the nephrons among the sparrows. Proximal tubules contained both narrow and wide intercellular spaces filled with interdigitations of the basolateral membrane. The thin limbs of Henle contained very wide intercellular spaces which were absent in the thick limbs of Henle. The distal tubule cells contained short, apical microvilli and infoldings of the basolateral membrane. In cross section, the medullary cones of all birds display an outer ring of thick limbs of Henle which surround an inner ring of collecting ducts, which in turn surround a central core of thin limbs of Henle. The Savannah Sparrow has a significantly higher volume of medulla compared to the two more mesic species. Within the cortex, the Savannah Sparrow also has a significantly higher volume of proximal tubules but a significantly lower volume of distal tubules than the other species. Within the medulla, the Savannah Sparrow has a significantly higher volume and surface area of capillaries, and a significantly higher surface area of thick limbs of Henle and collecting ducts than the mesic species. These data suggest that the salt marsh Savannah Sparrow has the renal morphology necessary to produce a more highly concentrated urine than the mesic zone species.

Contributions of the kidneys and intestines to water conservation, and plasma levels of antidiuretic hormone, during dehydration in house sparrows (Passer domesticus)

The contributions of the kidneys, the small intestine and the lower intestine (rectum plus cloaca) to water conservation during dehydration in unanaesthetized, unrestrained house sparrows (Passer domesticus) were assessed. Thirty hours of acute dehydration resulted in a 12% loss in body mass and a significant increase in plasma osmolality. Glomerular filtration rate declined by 55%, from 7.7 to 3.5 ml/h, and urine flow rate delined by more than 80%, from 0.2 to 0.03 ml/h. These changes are likely attributable to a large increase in plasma levels of arginine vasotocin during dehydration, from less than 26 pg/ml in hydrated birds to greater than 200 pg/ml after 30 h dehydration. Flow of water from the ileum to the lower intestine was reduced during dehydration, primarily because of a reduced flow of dry matter (with no significant reduction in water content). The rate of water loss in the excreta declined from 0.2 ml/h in hydrated birds to 0.04 ml/h in dehydrated birds. The rate of water reabsorption in the lower intestine (equal to the rate of water loss in the excreta minus the combined rates of inflow into the lower intestine from the urine and the ileal contents) slightly exceeded the rate of water flow from the ileum in both hydrated and dehydrated birds. We suggest that much of the water reabsorbed in the lower intestine of hydrated birds derives from the urine, but that primarily water from ileal contents is reabsorbed in dehydrated birds. That is, urine undergoes significant post-renal modification in hydrated but not dehydrated house sparrows.

A quantitative survey of salt and water excretion

The present series of papers on salt and water excretion in birds offers surveys of the state of the art in the different fields. To complete the picture this introduction considers the quantitative role of the various avenues of salt and ater excretion. First, the fraction of total water lost by evaporation; second, the role of salt glands in the elimination of a salt load in birds with functional salt glands; third, the interaction of kidney and cloaca in birds without functional glands.

Transepithelial transport of K+, NH4+, inorganic phosphate and water by hen (Gallus domesticus) lower intestine (colon and coprodeum) perfused luminally in vivo

The colon and coprodeum of anaesthetised laying hens was perfused luminally with solutions of varying compositions to elucidate the influence of three levels of dietary Na+ and of luminal fluid composition on transport of electrolytes and water. Net rates of secretion of K+ and absorption of NH4+ were increased in response to dietary Na+ depletion and increased luminal Na+ concentrations, but these fluxes were linked to Na+ transport and no dietary or perfusion treatment effects could be detected on them independently of effects on Na+ flux. Similar results were found for Na+-linked water absorption from isosmotic perfusates. Serosal-mucosal net osmotic flow with Na+-free perfusates was significantly reduced by Na+-depletion, but no significant dietary effect could be detected on the serosal-mucosal osmotic permeability coefficient. The reflexion coefficient was effectively unity. Mucosal-serosal inorganic phosphate flux was small. The flux was independent of Na+ flux and dietary Na+ levels, and apparently facilitated by serosalmucosal osmotic flow. The data allowed determination of the absorption/secretion in coprodeum and colon of the ions investigated, as compared to the renal excretion rate.

Cloacal resorption of salt and water in the Galah (Cacatua roseicapilla)

1. The transmural net flow of salt and water in the coprodeum and large intestine of the Galah (Cacatua roseicapilla), an Australian xerophilic parrot, was measured by an in vivo perfusion technique. The main goal of the study was to understand what happens when the hyperosmotic ureteral urine formed in the dehydrated state is regurgitated into the cloaca. Buffered perfusion fluids, hyper- and isosmotic to plasma, with varying NaCl and KCl concentrations, were used. [(14)C]polyethylene glycol served as an unabsorbable water marker.2. The cloacal Na(+) and Cl(-) absorption rates were nearly parallel and at low luminal concentrations roughly proportional to the intraluminal concentrations. At higher concentrations the Na(+) absorption rate showed saturation. The maximal Na(+) flow was 217 muequiv/kg.hr, the concentration at half maximal flow 181 m-equiv/l. The Na(+) absorption rate was not impaired by a high K(+) concentration. K(+) was secreted into the intestine, except at high intraluminal K(+) concentrations where resorption was observed.4. The apparent osmotic permeability coefficient was 0.85 mul./kg.hr.m-osmole at an average osmotic difference of 446 m-osmole between lumen and plasma; it was higher at lower differences. In the (near) absence of an osmotic difference across the cloacal epithelium the solute-linked water flow was 5.0 mul. H(2)O/muequiv Na(+).5. It is concluded that the hyperosmotic ureteral urine formed in the dehydrated state can pass into the cloaca without a water loss. A Na(+) absorption of around 70% of the ureteral output is likely.