Urine concentration and avian aquaporin water channels

Abstract

Although birds and mammals have evolved from primitive tetrapods and advanced divergently, both can conserve water by producing hyperosmotic urine. Unique aspects in the avian system include the presence of loopless and looped nephrons, lack of the thin ascending limb of Henle's loop, a corticomedullary osmotic gradient primarily consisting of NaCl without contribution of urea, and significant postrenal modification of final urine. The countercurrent multiplier mechanism operates between the descending and ascending limbs of Henle via recycling of a single solute (NaCl) with no water accompaniment, forming an osmotic gradient along the medullary cone. Bird kidneys and developing rat kidneys share morphological and functional characteristics. Avian kidneys express aquaporin (AQP) 1, 2, and 4 homologues that share considerable homology with mammalian counterparts, but their distribution and function may not be the same. AQP2 expression in Japanese quail (q) evolves in the collecting duct of early metanephric kidneys and continues to increase in intensity and distribution during nephrogenesis and maturation. qAQP2 mRNA and protein are increased by arginine vasotocin (avian ADH), but vasotocin-induced enhancement of cAMP production and water permeability are less marked than in mammalian kidneys. Nephrogenesis is delayed by insufficient nutrition in avian embryos and newborns and results in fewer nephrons and an impaired water balance in adults. Diabetes insipidus quail with homozygous autosomal recessive mutation and an unaffected vasotocin system have low AQP2 expression, underdeveloped medullary cones. Comparative studies will provide important insight into integrative, cellular, and molecular mechanisms of epithelial water transport and its control by humoral, neural, and hemodynamic mechanisms.

Aquaporin‐2 regulation in health and disease

“…Localization of AQP2 in the principal cells of the collecting duct is conserved across mammalian species, including dogs, rats, mice, cynomolgus monkeys, cats, horses, sheep, dolphins, and whales . In birds, AQP2 has been demonstrated in collecting ducts from both the mammalian‐type (possessing loops of Henle) and reptilian‐type (nonlooped) nephrons . Expression of AQP2 and response to water deprivation appear greater in the medullary region, consistent with the conclusion that the mammalian‐type nephrons are more sensitive to vasotocin, the avian equivalent of vasopressin, than are the reptilian‐type nephrons.…”

Gene expression and localization of two types of AQP5 inXenopus tropicalisunder hydration and dehydration

“…3). Third, if this gene is aqp2, it should be expressed in the kidney, as in mammals (37), quails (32), and tree frogs (34). However, RT-PCR analysis did not detect the expression of this gene in the kidney, or rather it was expressed in the skin glands.…”

Aquaporins in avian kidneys: function and perspectives

“…Many AQPs and aquaGLPs have a wide tissue distribution, and recent evidence suggests that, in addition to permeability to water, glycerol, and other small molecules, they have previously unappreciated physiological functions in skin elasticity, gastrointestinal function, neural signaling, cell migration/proliferation, and fat metabolism, and some pathophysiological role in diabetes mellitus (100,122). AQPs 1, 2, and 3 are expressed in avian kidneys (80,132, and Yang Y, Yamamoto K, Nishimura H; unpublished observations), whereas…”

“…An evolutionary analysis of AQPs in vertebrates using the Clustal X program based on similarity scores is shown in Fig. 1 (80,118,138); the resulting multiple alignments were refined by visual inspection. The length of the horizontal connecting bars is inversely proportional to the pair-wise similarity scores calculated from the alignment of sequences with use of the Clustal algorithm for PC/GENE.…”

“…The length of the horizontal connecting bars is inversely proportional to the pair-wise similarity scores calculated from the alignment of sequences with use of the Clustal algorithm for PC/GENE. The unrooted phylogenetic tree of AQPs in vertebrates suggests a divergence (dichotomy) from the ancestor of the AQP superfamily (2,80,123,138). The AQP family contains subcellular AQPs and membrane-bound AQPs.…”