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Abstracts from VI International Symposium on Avian EndocrinologyMarch 31 - April 5, 1996 Chateau Lake Louise, Alberta An Overview of Osmoregulation in BirdsJ. Braun Department of Physiology, Arizona Health Sciences Center, P.O.
Box 245051, Tucson, AZ 85724-5051, USA The homeostasis of fluid and ions in birds involves several organ systems and may be a somewhat more complex phenomenon than in other vertebrates. In birds, the kidneys and lower gastrointestinal tract (cloaca, rectum, digestive ceca) are involved in the regulation of extracellular fluid composition. Many birds also have functional salt glands which makes the osmoregulatory organ axis a triad. The kidney has a limited capacity for the conservation of body water and electrolytes via elimination of hyperosmotic urine. Most commonly the ureteral urine is only slightly hyperosmotic to the plasma and at a maximum capacity the urinetoplasma osmolar ratios are about 2 to 2.5. With increasing water deprivation, the plasma osmolality of birds tends to increase, therefore skewing the U/Posm value downward. This low capacity to concentrate the urine is not a liability as in most birds the ureteral urine eventually enters the rectum where a strongly hyperosmotic fluid could lead to a negative fluid flow. Few species have been studied (domestic fowl, galah and emu), but the available data indicate that the rectal epithelium has a variable capacity to absorb ions and water against an osmotic gradient (relatively low in the fowl, much greater in the galah and emu). The predominate form in which nitrogen is excreted by birds (uric acid) requires little water for excretion, due to it low aqueous solubility. However, it does require a significant amount of protein to maintain it in a colloidal suspension in the urine. The source of some of this protein is the plasma, as significant amounts pass through the glomerular filtration barrier. This protein (energy) is not lost because it is broken down when the urine enters the lower GI tract. The coordinated action of the kidneys, lower GI tract, and the salt glands in the regulation of fluid and ion balance is a classic example of the integration of organs required to maintain homeostatic balance. No single organ appears to have an outstanding capacity to conserve ions and water, but instead they all function in concert to maintain total body fluid homeostasis to allow birds to inhabit a wide range of environments. This coordinated action is facilitated by control through both the nervous and endocrine systems. (Supported by NSF IBN9220241) Hypothalamic Control of OsmoregulationR. Gerstberger Max-Planck-Institute for Physiology and Clinical Res., WG.
Kerckhoff Institute, D-61231 Bad Nauheim, Germany To maintain body fluid homeostasis in birds, the hypothalamus represents the main central nervous entity to integrate afferent information concerning changes in extracellular fluid (ECF) tonicity or volume. Hypothalamic structures inside and outside the bloodbrain barrier (BBB) also serve as sensors for ECF tonicity. Signal integration subsequently leads to neural and hormonal readjustments of water intake, the neuroendocrine ADH and CRFACTH systems, the cardiovascular system and results in (extra)renal salt and water elimination. Localization and functional characteristics of hypothalamic osmosensors were evaluated in the duck, employing intracerebroventricular (icv) perfusion techniques with artificial ECF of varying ionic composition, neural tracing studies, and electrophysiological recordings in isolated brain slice preparations. Sodium sensitive neurons were located in a periventricular unit of the paraventricular nucleus (PVN) inside the BBB. Osmosensitive neurons were found outside the BBB in circumventricular organs (CVOs) of the lamina terminalis (SFO, OVLT), neurally connected to the ADHproducing magnocellular PVN. Within the hypothalamic osmoregulatory circuitry, angiotensin II (ANGII) and nitric oxide (NO) serve as neuromodulators, although acting differentially at the blood and brain side of the BBB. Bloodborne ANGII acts on receptors in the SFO to inhibit extrarenal sodium excretion and to stimulate thirst. Actions of ANGII inside the BBB are mediated by receptors in the PVN and periventricular AV3V region, and include increases in blood pressure and ADH release. Neuronal NO synthase, the key enzyme for NO formation, is present in the SFO, PVN and AV3V region. NO applied icv inhibits extrarenal sodium excretion and stimulates both the ADH and CRF ACTH systems at lowered arterial pressure. In the SFO, NO antagonizes the excitation of neurons by ANGII. Sexual Differences in OsmoregulationM.R. Hughes Department of Zoology, University of British Columbia,
Vancouver, BC V6T 1Z4 Canada Marine birds ingest excessive amounts of sodium chloride (NaCl). The NaCl is absorbed in the gut and filtered by the kidneys, where most is reabsorbed for secretion via the cephalic salt glands. The effects of NaCl stress on these organs is usually evaluated in Peking ducks, Anas platyrhynchos, which have lower osmoregulatory capacity than truly marine birds. Further, the osmoregulatory organs of male and female ducks differ in size and use. Relative to body mass, the gut is heavier and has longer intestinal caeca in females; females have heavier kidneys containing more nephrons; and males have larger salt glands. At drinking water [NaCl] exceeding 300 mM, cloacal fluid [NaCl] is relatively higher in females and salt gland secretion [NaCl] is relatively higher in males. When drinking water [NaCl] is 450 mM, plasma osmolality and angiotensin II concentration increase more in males than in females and more males die. Ducks reabsorb NaCl in the rectum, ileum, and caeca. When the caeca are ligated before the ducks drink saline, water flux is increased only in males and salt gland size decreased only in females. Saline also alters the putative melatonin receptors in the intestinal tract, which tend to be more abundant and of greater binding affinity in females. Female ducks also tend to have higher plasma melatonin concentrations. Osmoregulatory Systems in Free Living BirdsD.L. Goldstein Department of Biological Sciences, Wright State University,
Dayton, OH, USA Osmoregulation is a fundamental homeostatic demand on free-living birds. As such, one could operationally define an osmotically challenging ("stressful") environment by measuring two physiological variables in wild-caught animals: those which are homeostatically regulated (is the animal regulating successfully?), and those which achieve homeostasis (is the animal attempting to regulate?). With respect to osmoregulation, the former category (regulated variables) includes total body water,and the volume and composition of the extracellular fluid. The latter category (regulating variables) includes water and sodium turnover rates, renal function (rates of filtration and reabsorption, urine flow rate and composition), intestinal transport, and, importantly, circulating levels of osmoregulatory hormones such as arginine vasotocin (which regulates renal water excretion) and aldosterone (aldo, which regulates intestinal, and probably renal, sodium transport). To date, there is but a single published report (aldo concentrations in Passer domesticus) for either hormone in freshly-captured birds. My recent studies of Australian honeyeaters (Meliphagidae) indicate significant variation in aldo concentrations both among species [New Holland honeyeaters (nectar/insect diet; 134 pg/ml aldo) > yellow throated miners (insect diet; 60 pg/ml) > red wattlebirds (insect/nectar diet; 15 pg/ml) and among seasons (for New Hollands, higher in winter (199 pg/ml) than summer (70 pg/ml)]. Among these species, variation in aldo concentrations was not related to differences in urine or plasma Na concentration. Laboratory studies of each species may be necessary to establish patterns of hormonal regulation. Field osmoregulatory endocrinology remains a promising, yet largely uncharted, field. Nitrogen Excretion in BirdsG. Casotti, E.J. Braun Department of Physiology, Arizona Health Sciences Center, P.O.
Box 245051, Tucson, AZ 85724-5051, USA Birds are uricotelic, excreting metabolized nitrogen mainly in the form of urate. Urate is present in the nephron as a colloidal suspension of small spheres ranging in size from 1 to 15µm. Previous studies have found that the spheres consist of plasma protein, urate and inorganic ions. This study examined how protein may enter the avian nephron and the mechanism of sphere formation within the nephron. Transmission electron microscopy (TEM) was used to examine the charge and pore sizes of the avian glomerular filtration membrane (GFM), in Gallus gallus. The anionic charge of the GFM was examined by staining the basement membrane with ruthenium red. The sizes of the endothelial and epithelial pores were measured directly from the electron microscope. No anionic barrier was present within the basement membrane of the GFM. However, birds did possess a thick glycocalyx (surrounding the podocytes), and this may function as a charge barrier. The pores on the endothelium of the GFM are elliptical and up to 30% larger than those found in mammals. The epithelial pores of the GFM were four times larger than those in mammals. These results indicate that in G. gallus, entry of plasma proteins into the nephron maybe by filtration through the GFM. Once in the nephron, the protein combines with urate and inorganic ions to form spheres. Ureteral urine was collected and passed through a system of filters to separate the spheres into different size categories. The spheres were placed on stubs, dried and coated with carbon. Xray microanalysis was used to determine the identity and the amount of inorganic ions in the spheres, to aid in understanding how they are formed. The results showed that the spheres contained calcium (70% of the total ions present) and potassium (30%). Chloride and magnesium were also detected, but in small amounts. There were no significant differences in the ion content between spheres of different sizes. These data suggest that the ions calcium and potassium may play a role in the formation of the spheres. We suggest that the spheres are formed to prevent blockage of the nephrons and thus to facilitate the excretion of urate. (Supported by NSF IBN9220241, NIH 5R01DK 1629420) Changes in Osmoregulatory Hormones and Water Flux during Saline Acclimation in Peking DucksD.C. Bennett, E. Albas, D.a. Gray1, M.R. Hughes Department of Zoology, University of British Columbia,
Vancouver, BC V6T 1Z4 Canada; 1Department of Physiology,
University of Witwatersrand, Johannesburg, 2193 South Africa Saline acclimated Peking ducks, Anas platyrhynchos, which have salt glands, have higher water flux and [AII]pI concentration than freshwater ducks. These have not been measured during saline acclimation. We hypothesized that, as drinking water salinity (DWS) of ducks increased, drinking rate would increase. Mean daily water influx (MDWI) was not significantly affected by DWS. In birds drinking saline, but not in freshwater controls, MDWI was negatively related to [AII]pI, while [AVT]pI was positively related to plasma osmolality (OsmpI); and [ANP]pI was positively, but weakly, related to total body water and/or relative plasma volume (100-Hct, %). As DWS increased, experimental birds deviated from controls in the following sequence and at the indicated DWS: OsMpI increased at 240 mM NaCl; body mass decreased at 253 mM; [AII]pI increased at 295 mM; total body water decreased at 299 mM; [AVT]pI increased at 342 mM; and [ANP]pI decreased at 386 mM. Although exogenous AII increases drinking in birds, [AII]pI and water influx were negatively related in this study. In conclusion, the unusually high MDWI of Peking ducks, which was little affected by DWS, resulted in high NaCl loads in ducks drinking saline. Changes in Melatonin, Osmoregulatory Hormones, Organ Weights and [125I]Odomelatonin Binding Sites After Saline Acclimation in Male Peking DucksM.R. Hughes, E.a. Ayre1,2, K.M. Cheng1, D.a. Gray3, P.Lee2, Y. Song2, S.F. Pang2 Departments of Zoology and 1Animal Science, University of
British Columbia, Vancouver, BC V6T 1Z4 Canada; 2Department of
Physiology, Faculty of Medicine, University of Hong Kong, Hong
Kong; 3Department of General Physiology, School of Dentistry,
University of Witwatersrand, Johannesburg, South Africa Male Peking ducks were gradually saline-acclimated to 360 mM NaCl in the drinking water over 4 weeks. Saline acclimation increased plasma osmolality and concentrations of antidiuretic hormone, angiotensin II ([AII]pl), and atrial natriuretic hormone. While there was no significant difference in the plasma concentration of melatonin ([MEL]pl) between acclimated and control drakes, there was a significant correlation (r2 = 0.46) between [MEL]pl and [AII]pl during saline acclimation. Saline-acclimated drakes had a significantly higher density of [125I]iodomelatonin binding sites in the salt glands and ileum and the binding sites in the salt glands, Harderian glands, anterior gut and rectum had a higher binding affinity than the controls. Saline acclimation increased the weight of salt glands and tended to increase the weights of all gut sections. This study is the first to detect effects of dietary NaCl on the density and affinity of putative melatonin receptors in the osmoregulatory organs of birds. Dehydration up-Regulates Angiotensin II Receptors in the Zebrafinch HypothalamusR. Gerstberger, T. Wolf, G. Warncke1 Max-Planck-Institute for Physiology and Clinical Res., WG
Kerckhoff Institute, D-61231 Bad Nauheim; 1Institute of
Neurophysiology, University of Cologne, D-50931 Cologne, Germany Reduction of extracellular fluid volume (ECFV) represents the main driving force for activation of the reninangiotensin system. Zebrafinches living in arid zones respond to chronic water deprivation (6 months;15 min water every 10 days) (FD) with a rise in the plasma concentration of sodium from 84±7 to 121±9 mEq/l, as compared to control animals (FW). Hematocrit of the blood was augmented from 55.0±2.5 to 63.4±2.6 %, indicative of reduced ECFV. Body mass was not significantly reduced (from an average of 13.8 to 12.8 g). Daily water balance in FD proved to be negative (0.49 g), as compared to 0.01 g for FW. Experimentally induced reduction of water intake was mainly compensated by diminishing evaporative and cloacal and fecal water losses to 55 and 20 % of values obtained for FW, respectively. Due to dehydration, plasma concentrations of neurohypophysial Arg8 vasotocin (AVT) and Val5 angiotensin II (ANGII), determined by specific RIAs, were elevated from 15.2±2.6 to 98.5±24.2 pg/ml (AVT) and from 85±12.3 to 127±18 pg/ml (ANGII). To counteract depletion of ECFV, ANGII in birds acts both peripherally (rise in peripheral resistance, antinatriuresis) and centrally (AVT release, thirst). As targets for circulating ANGII, the subfornical organ (SFO) and median eminence (ME) in the hypothalamus, outside the bloodbrain barrier, express ANGII receptors and mediate thirst and AVT release. ANGII receptors are also present in the AVT-producing paraventricular nucleus (PVN), the periventricular AV3V region and amygdala. Increased ANGII receptor density (at constant affinity) occured in the SFO (250 %), ME (110 %) and PVN (90 %), but not AV3V region or amygdala, indicating upregulation of the ANGII receptor system during chronic ECFV depletion. Effects of Opioid Receptor Agonist and Antagonist on the Release of Arginine Vasotocin in the ChickenT. Sasaki, K. Shimada, N. Saito Department of Animal Physiology, Nagoya University, Nagoya
464-01, Japan Arginine vasotocin (AVT), one of avian neurohypophysial peptides, is released from the neurohypophysis in relation to osmoregulation and oviposition. In mammal, endogenous opioid peptides (OP) regulate the release of neurohypophysial peptides. But in birds, it is now known whether OP is involved in mechanisms of AVT release. Thus, we studied effects of OP agonist (morphine) and antagonist (naloxone) on the release of AVT by hyperosmotic stimulation and oviposition in the chicken. Hyperosmotic stimulation (18%NaCl, 2ml/kg BW, i.v.) increased plasma AVT levels, but simultaneous injection of morphine (1, 3.5, 7mg/kg BW, i.v.) suppressed the increase of plasma AVT levels. Simultaneous naloxone (10mg/kg BW, i.v.) injection induced higher AVT levels than those induced only by hyperosmotic stimulation. On the other hand, plasma AVT levels of morphine-injected (7mg/kg BW, i.v.) chickens increased at oviposition as in control chickens. These results suggested that in chickens, OPs may be involved in the osmoregulatory mechanism regulating AVT release but not in the mechanism regulating AVT release during oviposition. Endogenous Plasma Atrial Natriuretic Peptide and the Control of Salt Gland Function in the Peking DuckD.a. Gray Department of Physiology, Wits University, Johannesburg, South
Africa Avian salt glands (SG) possess atrial natriuretic peptide (ANP) binding sites and respond to iv ANP with enhanced SG secretion. There has, however, been some argument as to whether this represents a physiological action, in view of the fact that the ANP effect is transient. This study therefore examined the role of endogenous plasma ANP in the control of SG by evaluating the effect of blocking circulating ANP in animals given an iv infusion of hypertonic saline which drives the SG and simultaneously raises plasma ANP. The continuous infusion of hypertonic saline (1000 mosm/kg) at 0.4 ml/min increased plasma osmolality by 10% and induced the SG to secrete at steady-state, fluid with an osmolality of 956 mosm/kg, at a rate of 0.34 ml/min. Plasma ANP levels increased from a basal value of 64.5 to 127.4 pg/ml. Administration of 1 ml specific ANP antiserum reduced plasma ANP levels to 23.5 pg/ml but had no significant effect upon SG secretion rate or output. The results suggest that endogenous plasma ANP has a minor, if any, regulatory effect upon the avian SG. Metabolic Adaptive Strategies of Dry and Wet Adapted Zebrafinches (Taeniopygia Guttata) on Low and High Fat DietsG. Warncke, R. Gerstberger1 Institute of Neurophysiology of the University of Cologne,
Robert-Koch-Str.39, 50931 Cologne, Germany; 1W.G.
Kerckhoff-Institute, MPI, 61231 Bad Nauheim, Germany Zebrafinches live in arid zones. It is uncertain, how and to what extent water budget and nutrition and (above all) the fat content of the food, influence their adaptation to this extreme environment. It is also not clear, whether this has an effect on thermoregulation, muscle activity and energy balance. Four groups (n=15) of zebrafinches were therefore investigated under laboratory conditions (LD12:12, ambient temperature: daytime, 25°C; nighttime, 18°C): 1: low fat dry (1-d), 2: low fat wet (1-w), 3: high fat dry (h-d), 4: high fat wet (h-w). For six months the birds had been adapted to wet (water ad lib.) and dry conditions (15 min water ad lib. every 10 days) on low and high fat diets. Thereafter the total water intake (preformed and oxidative water, water ad lib.) and the total water loss (evaporation, faeces, urine) was evaluated in order to calculate the daily water balance. The results show that dry adapted zebrafinches can reach homeostasis in their water balance by increasing food intake, reinforced intestinal reabsorbence of water, decreasing evaporation, reducing the part of blood-plasma in full blood and by concentrating the urine and blood. A high fat content seems to be advantageous. |