Avian osmoregulation comprises a set of physiological controls that function to protect the amount and concentration of water and solutes in the intracellular and extracellular fluids (Table 19.1). Achieving that regulation requires balancing the input and output of water and electrolytes and involves the interacting contributions of the kidneys, intestinal tract, salt glands (when present), skin, and respiratory tracts (as routes of evaporative water loss). The integration of these organs is unique in birds among the vertebrates. Birds lack a urinary bladder, and avian urine enters the lower intestine, where it may be significantly modified in the coprodeum, colon, and ceca. Under some circumstances, in some species, the salt glands are critical for osmoregulatory balance, and the majority of total water loss may be evaporative, not excretory, with the relative roles of skin and respiration varying among species and conditions. The present chapter will focus most heavily on anatomical and functional organization of the kidneys and lower intestine, but will also examine aspects of those other organs of osmoregulation. 

Renal and extrarenal regulation of body fluid composition

Na/D-Glucose Cotransport and SGLT1 Expression in Hen Colon Correlates with Dietary Na+

In rat distal colon, aldosterone has little effect on Na+ channel (ENaC) α-subunit levels but increases the expression of the β- and γ-subunits and stimulates electrogenic Na+ transport. By contrast, the molecular basis of aldosterone's inability to stimulate electrogenic Na+ transport in the proximal colon is unclear. We therefore compared the effects of hyperaldosteronism secondary to 10 days dietary Na+ depletion on ENaC subunit expression in rat proximal and distal colon. Northern analyses revealed appreciable and similar levels of α-subunit mRNA throughout the colon in control and Na+-depleted animals. By contrast, Na+ depletion substantially enhanced β-subunit mRNA expression in the distal colon, but had no effect on the low expression levels of β-subunit mRNA in the proximal colon. Expression of the γ-subunit, evaluated by PCR, was also restricted to the distal colon of Na+-depleted animals. Western analyses demonstrated similar levels of α-subunit protein in the proximal and distal colon of both groups of animals, whereas β-subunit and γ-subunit proteins were detected solely or predominantly in the distal colon of the Na+-depleted animals. Immunocytochemistry confirmed that significant levels of all three subunit proteins only occurred in the apical membrane of surface cells in the distal colon of Na+-depleted animals. Our findings are consistent with previous studies demonstrating that aldosterone stimulates electrogenic Na+ transport in rat distal colon by increasing the expression of β- and γ-subunit mRNA and protein, and thus the amount of functional heteromeric ENaC protein in the apical domain. They also show that aldosterone is incapable of stimulating electrogenic Na+ transport in rat proximal colon (despite the presence of α-subunit mRNA and protein) because of its inability to enhance β- and γ-subunit expression in this segment.

Segmental variability of ENaC subunit expression in rat colon during dietary sodium depletion.

Birds are a diverse vertebrate class in terms of diet and habitat, but they share several common physiological features, including the use of uric acid as the major nitrogenous waste product and the lack of a urinary bladder. Instead, ureteral urine refluxes from the urodeum into the more proximal coprodeum and portions of the hindgut (colon or rectum and ceca). This presents a potential problem in that hyperosmotic ureteral urine in contact with the permeable epithelia of these tissues would counteract renal osmotic work. This review describes and provides examples of different strategies used by avian species to balance renal and post-renal changes in urine composition. The strategies described include: 1. a "reptilian" mode, with moderate renal concentrating ability, but high rates of post-renal salt and water resorption; 2. the "mammalian" strategy, in which the coprodeum effectively functions like a mammalian urinary bladder, preserving the osmotic concentrating work of the kidney; 3. an interaction strategy, in which post-renal transport processes are hormonally regulated in order to optimize renal function under varying conditions of salt or water stress; 4. the salt gland strategy seen in marine or estuarine birds with functional salt glands, in which post-renal transport mechanisms are used to conserve urinary water and to recycle excess NaCl to the nasal salt glands. Finally, we also describe some features of an as-yet unstudied group of birds, the birds of prey. At least some species in this group are relatively good renal concentrators, and would be predicted to have post-renal mechanisms to preserve this work. This new synthesis illustrates the marked diversity of adaptive mechanisms used by avian species to maintain osmotic homeostasis.

Adaptive strategies for post-renal handling of urine in birds.

Transport in the colon of the domestic fowl switches from sodium-linked hexose and amino acid cotransport on high-salt intake to amiloride-sensitive sodium channel expression on low-salt (LS) diets...

Aldosterone suppresses expression of an avian colonic sodium-glucose cotransporter.

RNA was isolated from chicken lower intestine (both colon and coprodeum) and injected into Xenopus oocytes. 22Na+ fluxes measured after 1-4 days demonstrated the induction of an amiloride-blockable pathway. The Na+ transporter expressed by the exogenous RNA had a high affinity to amiloride (inhibitory constant less than 0.1 microM), but was insensitive to ethylisopropyl amiloride, i.e., it is likely to be the apical Na+ channel. Functional channels were readily expressed in oocytes injected with RNA derived from chickens fed a low-NaCl diet. On the other hand, no channel activity was detected in oocytes injected with RNA isolated from chickens fed a high-NaCl diet. Thus the previously reported regulation of transport by the dietary NaCl intake involves modulations in the level of mRNA that codes either for the Na+ channel or a posttranscriptional regulator of the channel.

NaCl-dependent expression of amiloride-blockable Na+ channel in Xenopus oocytes

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NaCl-dependent expression of amiloride-blockable Na+ channel in Xenopus oocytes