Osmoregulation in Birds

The article discusses advances in osmoregulation and excretion with emphasis on how multicellular animals in different osmotic environments regulate their milieu interieur. Mechanisms of energy transformations in animal osmoregulation are dealt with in biophysical terms with respect to water and ion exchange across biological membranes and coupling of ion and water fluxes across epithelia. The discussion of functions is based on a comparative approach analyzing mechanisms that have evolved in different taxonomic groups at biochemical, cellular and tissue levels and their integration in maintaining whole body water and ion homeostasis. The focus is on recent studies of adaptations and newly discovered mechanisms of acclimatization during transitions of animals between different osmotic environments. Special attention is paid to hypotheses about the diversity of cellular organization of osmoregulatory and excretory organs such as glomerular kidneys, antennal glands, Malpighian tubules and insect gut, gills, integument and intestine, with accounts on experimental approaches and methods applied in the studies. It is demonstrated how knowledge in these areas of comparative physiology has expanded considerably during the last two decades, bridging seminal classical works with studies based on new approaches at all levels of anatomical and functional organization. A number of as yet partially unanswered questions are emphasized, some of which are about how water and solute exchange mechanisms at lower levels are integrated for regulating whole body extracellular water volume and ion homeostasis of animals in their natural habitats. © 2014 American Physiological Society.

Osmoregulation and excretion

1. Introduction.- 1.1 The Comparative Approach.- 1.2 Environmental Requirements in Regulating the Water and Solute Composition of the Internal Environment.- 1.3 Possible Roles of Kidneys in Regulating Water and Solute Composition of the Internal Environment.- 1.4 Useful Reviews on Comparative Renal Physiology.- 2. Renal Morphology.- 2.1 Introduction.- 2.2 Gross Internal and External Morphology.- 2.2.1 General Considerations.- 2.2.2 Fishes.- 2.2.3 Amphibians.- 2.2.4 Reptiles.- 2.2.5 Birds.- 2.2.6 Mammals.- 2.2.7 General Blood Vessel Patterns.- 2.3 Fine Internal Structure.- 2.3.1 General Considerations.- 2.3.2 Glomerulus.- 2.3.3 Proximal Tubule.- 2.3.4 Early Distal Tubule.- 2.3.5 Late Distal and Collecting Tubules.- 2.3.6 Juxtaglomerular Apparatus.- 3. Initial Process in Urine Formation.- 3.1 Introduction.- 3.2 Filtration of Fluid by Glomeruli.- 3.2.1 Process of Ultrafiltrate Formation at Renal Glomeruli.- 3.2.2 Values for Single-Nephron Glomerular Filtration Rates.- 3.2.3 Changes in Whole-Kidney Glomerular Filtration Rates.- 3.2.4 Changes in Single-Nephron Filtration Rates and in Number of Filtering Nephrons.- 3.2.5 Regulation of Single-Nephron Filtration Rates and Number of Filtering Nephrons.- 3.3 Secretion of Fluid by Tubules.- 4. Transport of Inorganic Ions by Renal Tubules.- 4.1 Introduction.- 4.2 Sodium and Chloride.- 4.2.1 Direction, Magnitude, and Sites of Net Transport.- 4.2.2 Mechanism of Transport.- 4.2.3 Control of Transport.- 4.3 Potassium.- 4.3.1 Direction, Magnitude, and Sites of Net Transport.- 4.3.2 Mechanism of Transport.- 4.3.3 Hormonal Control.- 4.4 Hydrogen Ion.- 4.4.1 Magnitude and Sites of Net Transport.- 4.4.2 Mechanism of Transport.- 4.5 Calcium.- 4.5.1 Direction, Magnitude, and Sites of Net Transport.- 4.5.2 Mechanism of Transport.- 4.5.3 Hormonal Control.- 4.6 Phosphate.- 4.6.1 Direction, Magnitude, and Sites of Net Transport.- 4.6.2 Mechanism of Transport.- 4.6.3 Control by Parathyroid Hormone.- 4.7 Magnesium.- 4.7.1 Direction, Magnitude, and Sites of Net Transport.- 4.7.2 Mechanism of Transport.- 4.8 Sulfate.- 4.8.1 Direction, Magnitude, and Sites of Net Transport.- 4.8.2 Mechanism of Transport.- 5. Transport of Fluid by Renal Tubules.- 5.1 Introduction.- 5.2 Fluid Absorption.- 5.2.1 Magnitude and Sites of Net Absorption.- 5.2.2 Mechanism and Control of Fluid Absorption.- 5.3 Fluid Secretion.- 6. Transport of Organic Substances by Renal Tubules.- 6.1 Introduction.- 6.2 Glucose.- 6.2.1 Direction, Magnitude, and Sites of Net Transport.- 6.2.2 Mechanism of Transport.- 6.3 Bicarbonate.- 6.3.1 Direction, Magnitude, and Sites of Net Transport.- 6.3.2 Mechanism of Transport.- 6.4 Amino Acids.- 6.4.1 Direction, Magnitude, and Sites of Net Transport.- 6.4.2 Mechanism of Transport.- 6.5 Urea.- 6.5.1 Direction, Magnitude, and Sites of Net Transport.- 6.5.2 Mechanism of Transport.- 6.6 Ammonia.- 6.6.1 Magnitude and Sites of Net Secretion.- 6.6.2 Process of Production and Secretion.- 6.7 Organic Acids and Anions (Except Amino Acids, Urate, and Lactate).- 6.7.1 Direction and Sites of Net Transport.- 6.7.2 Mechanism of Transport.- 6.8 Urate.- 6.8.1 Direction, Magnitude, and Sites of Net Transport.- 6.8.2 Mechanism of Transport.- 6.9 Lactate.- 6.10 Organic Cations.- 6.10.1 Direction and Sites of Net Transport.- 6.10.2 Mechanism of Transport.- 7. Diluting and Concentrating Processes.- 7.1 Introduction.- 7.2 Range of Urine Osmolality.- 7.3 Process and Sites of Dilution.- 7.4 Process of Concentration.- 7.5 Regulation of Urine Osmolality.- 8. Integrative Summary of Renal Function.- 8.1 Introduction.- 8.2 Integrative Summary Within Each of the Major Vertebrate Groups.- 8.2.1 Fishes.- 8.2.2 Amphibians.- 8.2.3 Reptiles.- 8.2.4 Birds.- 8.2.5 Mammals.- 8.3 Summary Comparisons and Contrasts of Renal Function Among Groups of Vertebrates.- 8.3.1 Glomerular Filtration Rate - Stability Versus Lability.- 8.3.2 Diluting and Concentrating Process.- 8.3.3 Sites and Mechanisms of Tubular Transport.- References.

Comparative physiology of the vertebrate kidney

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

Serum concentrations of arginine vasotocin (AVT), mesotocin and prolactin were determined by radioimmunoassay in Rhode Island Red chickens during and after dehydration, haemorrhage and oviposition. During dehydration increased circulating levels of AVT, mesotocin and prolactin were found. As water deprivation proceeded, marked differences were observed. After an initial rise in serum AVT, mesotocin and prolactin levels during mild and moderate dehydration, concentrations of both AVT and prolactin tended to normalize during continued water deprivation, while those of mesotocin remained high throughout the whole dehydration experiment with the highest at the end of the water-deprivation period. Removal of 5 ml blood at intervals of 10 min during six consecutive time-periods did not affect serum osmolality and circulating levels of AVT and prolactin, but slightly increased mesotocin. These results suggest an osmoregulatory role for AVT and prolactin, whereas mesotocin may be involved in volume control. Finally, 1 min after oviposition, control values of 19.5 +/- 3.4 pmol AVT/1 (n = 9) were raised more than sevenfold to 142.9 +/- 12.5 pmol/l (n = 11). Thereafter, a decline occurred with a half-life for AVT of 13 min with raised serum levels up to 31 min after oviposition. In contrast, the serum concentrations of mesotocin and prolactin remained unaffected by oviposition.

E Skadhauge

Papers

Effects of arginine vasotocin on renal excretion of Na+, K+, Cl-, and urea in the hydrated chicken