Effect of saline intake on water flux and osmotic homeostasis in Pekin ducks (<Emphasis Type="Italic">Anas platyrhynchos</Emphasis>)

The physiological regulation of body water volume and concentration was evaluated in Pekin ducks, Anas platyrhynchos, slowly acclimated to increasingly saline drinking water (six equal 75 mM NaCl increments). Body mass, total body water (TBW), water flux, plasma osmolality (Osm(pl)), and ionic and osmoregulatory hormone concentrations were measured at the end of each increment. The salinity at which each variable deviates from its homeostatic set point was calculated by continuous two-phase linear regression. We hypothesized that, as drinking water salinity increases: (1) body water increases in concentration before it decreases in volume and (2) that regulating variables that help determine homeostatically set values (plasma hormone concentrations and water flux) deviate from values of freshwater ducks at lower drinking water salinities than the variables they regulate (Osm(pl), hematocrit, TBW). Osm(pl) was the first variable for which we could calculate a deviation from its homeostatically controlled value. It increases at much lower drinking water salinity than that at which TBW decreases, supporting our first hypothesis, but not our second hypothesis. We further hypothesized that, because the concentration of Pekin duck salt gland secretion is only slightly higher than that of their drinking water, they increase water flux (drinking) as salinity of drinking water increases, until the latter exceeds the secretion concentration and then they drink less. There was no change in water flux until it decreases when TBW decreases, 329 mM NaCl and 335 mM NaCl, respectively. The results do not support our hypothesis that Pekin ducks increase drinking as the salinity of their drinking water increases, but do indicate that, at tolerable salinities, Pekin ducks maintain body water volume while allowing body water osmolality to increase. At higher salinities, ducks decrease drinking and use body water to get rid of the excess salt.     

Evidence for the presence of nasal salt glands in the roadrunner and the Coturnix quail.

The purpose of this investigation was to determine whether or not the nasal glands of the roadrunner and the Coturnix quail show cytological specializations for salt secretion. In addition, the Na-K ATPase content of the quail gland was determined before and after drinking of saline solutions, in an effort to evaluate the functional status of the gland. The ability to maintain weight while drinking salt water was also measured as a general index of tolerance to saline conditions. The ultrastructure of the nasal glands of the roadrunner injected with salt and of quail drinking 200 mM NaCl was similar to that of salt glands in reptiles and the fresh-water acclimated duck. Numerous lateral cell evaginations and abundant mitochondria were present in the principal cell types. There was a significant increase in quail nasal gland Na-K ATPase when young birds were offered only saline solutions to drink. The ability of Coturnix quail to maintain weight while drinking saline solutions improves with age and at adulthood is comparable to that of some North American desert quail. Roadrunners were previously known to possess functional salt glands whereas quail were not. However the characteristic fine structure and the high Na-KATPase content of the quail nasal gland suggest that it is a salt gland.     

Redistribution of extracellular water and sodium may contribute to saline tolerance in wild ducks

The compartmentalization of body fluids was measured in three species of ducks that differ in saline tolerance. Half of the birds of each species drank freshwater, while the other half drank saline (300 mM NaCl). Among ducks that drank freshwater, total body water (TBW) was similar among all species, but Barrow's goldeneyes (Bucephala islandica), the most marine species, had larger extracellular fluid volume (ECFV) than freshwater mallards (Anas platyrhynchos) or estuarine canvasbacks (Aythya valisineria). When acclimated to saline, only goldeneyes shifted extracellular water and Na+ into the intracellular compartment. ECFV was correlated with plasma aldosterone concentration in goldeneyes, but not in canvasbacks (aldosterone was not measured in mallards). Data summarized from the literature showed that TBW does not differ among terrestrial, freshwater, or marine species, but marine species have a larger part of their TBW in the extracellular compartment. Saline induced movement of extracellular water and Na+ into the cells only in goldeneyes. ECFV and redistribution of extracellular water and Na+ into the cells may be important components in saline tolerance of marine birds.     

Hormone-dependent dissociation of blood flow and secretion rate in the lingual salt glands of the estuarine crocodile, Crocodylus porosus

Salt and water balance in the estuarine crocodile, Crocodylus porosus, involves the coordinated action of both renal and extra-renal tissues. The highly vascularised, lingual salt glands of C. porosus excrete a concentrated sodium chloride solution. In the present study, we examined the in vivo actions of vasoactive intestinal peptide (VIP), B-type natriuretic peptide (BNP) and angiotensin II (ANG II) on the secretion rate and blood perfusion of the lingual salt glands. These peptides were selected for their vasoactive properties in addition to their reported actions on salt gland activity in birds and turtles and rectal gland activity in elasmobranchs. The femoral artery was cannulated in seven juvenile crocodiles for delivery of peptides and measurement of mean blood pressure and heart rate. In addition, secretion rate of, and blood flow to, the salt glands were recorded simultaneously using laser Doppler flowmetry. VIP stimulated salt secretion was coupled to an increase in blood flow and vascular conductance of the lingual salt glands. BNP was a potent stimulant of salt gland secretion, resulting in a maximal secretion rate of more than 15-fold higher than baseline; however, this was not coupled to an increase in perfusion rate, which remained unchanged. ANG II failed to stimulate salt gland secretion and there was a transient decrease in salt gland blood flow and vascular conductance. It is evident from this study that blood flow to, and secretion rate from, the lingual salt glands of C. porosus are regulated independently; indeed, it is apparent that maximal secretion from the salt glands may not require maximal blood flow.     

Role of plasma ANG II in the excretion of acute sodium load in a bird with salt glands (Anas platyrhynchos)

This study was designed to further examine the role of plasma ANG II in the excretion of sodium in the Pekin duck, a bird with salt glands. Renal and extrarenal (salt gland) excretion of an intravenously administered isotonic saline load was monitored over a 4-h period in a group of eight birds under two conditions: the control condition, in which isotonic saline infusion decreased endogenous plasma ANG II from 102.6 to 16.5 pg/ml, and the experimental condition, in which ANG II suppression was prevented by intravenous infusion of a 3.5 ng. kg(-1). min(-1) dose of synthetic ANG II. ANG II infusion significantly decreased the total sodium excretion (by 15%), primarily via an inhibition of salt gland output. The results suggest that ANG II suppression facilitates the excretion of an administered sodium load in birds with salt glands.     

Catecholamine hormones and the control of avian salt glands

Hypertonic saline loading (0.5 M NaCl, 15 ml.kg-1 i.v.) increased cardiac frequency and elicited nasal salt gland secretion in control ducks. Partial depletion of catecholamines by prior treatment with reserpine decreased body weight, lowered arterial pressure and abolished the tachycardiac responses to saline loading. Reserpine also increased plasma concentrations of Na, K and total osmolytes, yet altered neither the composition nor the flow rate of nasal fluid secretion. The preservation of the normal secretory responses to hypertonic stress in hypotensive, reserpine-treated ducks indicates that the nasal salt glands can function independently of changes in circulating catecholamine hormones.     

Osmoregulatory responses of glaucous-winged gulls (Larus glaucescens) to dehydration and hemorrhage

The effects of dehydration and hemorrhage on plasma ionic, osmotic, and antidiuretic hormone (arginine vasotocin) concentrations and of hemorrhage on salt gland secretion and glomerular filtration rate were evaluated in glaucous-winged gulls, Larus glaucescens. Dehydration for 24 h did not affect plasma ionic, osmotic or arginine vasotocin concentrations; 72 h dehydration significantly elevated plasma osmolality, plasma sodium and chloride concentrations, and plasma arginine vasotocin concentration, but did not affect plasma potassium concentration. Constant infusion of 0.8 mol·l^-1 NaCl increased plasma arginine vasotocin concentration and produced salt gland secretion in seven gulls; four secreted well, while three secreted less well. Removal of 20% blood volume during saline infusion immediately reduced (P<0.001) salt gland secretion rate in all gulls. After bleeding, good secretors maintained glomerular filtration rate and urine flow rate; the poorer secretors increased glomerular filtration rate and became diuretic. Blood replacement returned salt gland secretion rate to the prebleeding level (P<0.05) without affecting salt gland secretions sodium concentration in gulls which secreted well, but did not restimulate salt gland secretion in gulls which secreted poorly. Reinfusion of blood had no effect on glomerular filtration rate. Bleeding and blood replacement did not affect plasma arginine vasotocin concentration.Abbreviations AVT arginine vasotocin - ECF extracellular fluid - ECFV extracellular fluid volume - EDTA ethylenediaminetetra-acetate - EWL evaporative water loss - GFR glomerular filtration rate - Hct hematocrit - LB large blood sample - [Na⁺]_pl plasma sodium concentration - Osm_pl plasma osmolality - PEG polyethylene glycol - RH relative humidity - RIA radioimmunoassay - SB small blood sample - SGS salt gland secretion - T _a ambient temperature - TFA trifluoroacetic acid - UFR urine flow rate     

The sulfatides and some histochemical correlations of the lachrymal glands involved in salt secretion in Chelonia.

High concentrations of sulfolipids (four fractions having different hexose/sulfate ratio), intense enzyme activity (ATPase, oxoreductases) and evidence of mucines (staining with PAS and Alcian blue) in intercellular spaces were found in the lachrymal glands of Caretta caretta and Malaclemys terrapin adapted to sea water. In addition, the supranuclear region of the gland cells in Malaclemys terrapin is filled with mucin granules. These biochemical and histochemical observations indicate that these glands have a function in salt secretion in both species and are also consistent with a function of mucous secretion exclusively in Malaclemys terrapin. Limited signs of hypotrophy are not accompanied by changes in concentrations of sulfolipids in Malaclemys terrapin adapted to fresh water; only the reactions for enzyme activities are less intense. The mucous secretion is not affected, whereas, in correlation with changes in salt secretion, the change in ATPase activity is mot conspicuous. The correlations between the different components of the gland and salt secretion are compared with salt glands of birds and elasmobranchs.     

Excretory organ growth and implications for salt tolerance in hatchling American avocets Recurvirostra americana

The role of salt glands in avian osmoregulation has been widely studied. Acclimation to saline habitats in aquatic birds involves increases in the relative size and complexity of the salt glands, and it is generally agreed that salt gland size varies as a function of salt loading, and is broadly correlated with habitat salinity. We report here salt gland sizes in three age classes of American avocet Recurvirostra americana chicks. Mean relative (mg/g body mass) salt gland masses for newly hatched (<24 h old) avocet chicks collected at a wetland supplied with pumped fresh water was 0.48 mg salt gland/g body mass. This value is comparable to the low end of published values for some strictly marine birds and similar to values for adult American avocets collected at saline lakes. These results suggest that American avocets, which are frequently raised in saline environments, hatch with salt glands that are large enough to cope with the osmoregulatory demands of saline environments.     

Ion secretion by salt glands of desert iguanas (Dipsosaurus dorsalis)

Unlike the NaCl-secreting salt glands of many birds and reptiles, the nasal salt glands of lizards can secrete potassium as well as sodium, with either chloride or bicarbonate as the accompanying anion. The factors responsible for initiating secretion by the gland and the rates of cation and anion secretion were studied in the desert iguana, Dipsosaurus dorsalis. Lizards were given combinations of ions for several days, and secreted salt was collected daily and analyzed for sodium, potassium, chloride, and bicarbonate. Maximum total cation secretion rate was 4.4+/-0.38 micromol/g/d. Cation secretion ranged from 24% to 100% potassium; even high NaCl loads did not abolish potassium secretion. Maximum bicarbonate secretion was about 0.5 micromol/g/d; chloride was the predominant anion. Secretion rate increased only in response to those treatments that included potassium and/or chloride; sodium ions and other osmotic loads (e.g., sucrose) did not increase secretion. This is in contrast to birds and some other reptiles with salt glands, which initiate NaCl secretion in response to any osmotic load. The specificity of the response of the salt gland of Dipsosaurus may be related to the ecological importance of dietary potassium and chloride for herbivorous desert lizards.     

Excretory organ growth and implications for salt tolerance in hatchling American avocets Recurvirostra americana

The role of salt glands in avian osmoregulation has been widely studied. Acclimation to saline habitats in aquatic birds involves increases in the relative size and complexity of the salt glands, and it is generally agreed that salt gland size varies as a function of salt loading, and is broadly correlated with habitat salinity. We report here salt gland sizes in three age classes of American avocet Recurvirostra americana chicks. Mean relative (mg/g body mass) salt gland masses for newly hatched (<24 h old) avocet chicks collected at a wetland supplied with pumped fresh water was 0.48 mg salt gland/g body mass. This value is comparable to the low end of published values for some strictly marine birds and similar to values for adult American avocets collected at saline lakes. These results suggest that American avocets, which are frequently raised in saline environments, hatch with salt glands that are large enough to cope with the osmoregulatory demands of saline environments.     

Coping with excess salt: adaptive functions of extrarenal osmoregulatory organs in vertebrates

In all organisms, changing environmental conditions require appropriate regulatory measures to physiologically adjust to the altered situation. Uptake of excess salt in non-mammalian vertebrates having limited or no access to freshwater is balanced by extrarenal salt excretion through specialized structures called ‘salt glands’. Nasal salt glands of marine birds are usually fully developed in very early stages of their lives since individuals of these species are exposed to salt soon after hatching. In individuals of other bird species, salt uptake may occur infrequently. In these animals, glands are usually quiescent and glandular cells are kept in a fairly undifferentiated state. This is the situation in ‘naive’ ducklings, Anas platyrhynchos, which have never been exposed to excess salt. When these animals become initially osmotically stressed, the nasal glands start to secrete a moderately hypertonic sodium chloride solution but secretory performance is meager. Within 48 h after the initial stimulus, however, the number of cells per gland is elevated by a factor of 2–3, the secretory cells differentiate and acquire full secretory capacity. During this differentiation process, extensive surface specializations are formed. The number of mitochondria is increased and metabolic enzymes and transporters are upregulated. These adaptive growth and differentiation processes result in a much higher efficiency of salt excretion in acclimated ducklings compared with naive animals. Receptors and signal transduction pathways in salt gland cells controling the adaptive processes seem to be the same as those controling salt secretion, namely muscarinic acetylcholine receptors and receptors for vasoactive intestinal peptide. This review focusses on signal transduction pathways activated by muscarinic receptors which seem to fine-tune salt secretion in salt-adapted ducklings and may control adaptive growth and differentiation processes in the nasal gland of naive animals.     

不同阴离子对二色补血草盐腺Na＋分泌速率的影响

本文以二色补血草（Limonium bicolor）为实验材料,用Hoagland营养液和200mmol·L—NaCl、NaBr、NaNO3溶液分别处理12h,测定二色补血草盐腺的Na＋分泌速率、叶片Na＋含量和MDA（丙二醛）含量以及质膜透性,并利用非损伤微测技术探索可能与盐腺相关的转运蛋白,以探讨不同阴离子对二色补血草盐腺分泌Na＋的作用及其可能原因。结果表明：在NaCl处理12h时二色补血草叶片Na＋分泌速率达到最大,然后逐渐下降;不同钠盐处理下叶片Na＋分泌速率为NaCl〉NaBr=NaNO3〉Hoagland,而叶片Na＋含量NaBr〉NaCl〉NaNO3〉Hoagland;不同盐处理下叶片质膜透性和MDA含量无显著性差异;利用Na—K—C1共转运体专一性抑制剂bumetanide处理发现Na＋分泌速率显著降低。这些结果表明Na—K—Cl共转运体可能参与盐腺分泌Na＋。     

Osmoregulation in water-deprived rats drinking hypertonic saline: effect of area postrema lesions

Rats drank rapidly when 0.3 M NaCl was the only drinking fluid available after overnight water deprivation, consuming approximately 200 ml/24 h. Although such large intakes of this hypertonic solution initially elevated plasma osmolality, excretion of comparable volumes of urine more concentrated than 300 meq Na(+)/l ultimately appears to restore plasma osmolality to normal levels. Rats drank approximately 100 ml of 0.5 M NaCl after overnight water deprivation, but urine Na(+) concentration (U(Na)) did not increase sufficiently to achieve osmoregulation. When an injected salt load exacerbated the initial dehydration caused by water deprivation, rats increased U(Na) to void the injected load and did not significantly alter 24-h intake of 0.3 or 0.5 M NaCl. Rats with lesions of area postrema had much higher saline intakes and lower U(Na) than did intact control rats; nonetheless, they appeared to osmoregulate well while drinking 0.3 M NaCl but not while drinking 0.5 M NaCl. Detailed analyses of drinking behavior by intact rats suggest that individual bouts were terminated by some rapid postabsorptive consequence of the ingested NaCl load that inhibited further NaCl intake, not by a fixed intake volume or number of licks that temporarily satiated thirst.     

Organophosphate inhibition of avian salt gland Na, K-ATPase activity

1. Adult black ducks (Anas rubripes) were given freshwater or saltwater (1.5% NaCl) for 11 days and half of each group was also given an organophosphate (17 p.p.m. fenthion) in the diet on days 6-11. 2. After 11 days, ducks drinking saltwater had lost more weight and had higher plasma Na and uric acid concentrations and osmolalities than birds drinking freshwater. 3. Saltwater treatment stimulated the salt gland to increased weight and Na, K-ATPase activity. 4. Fenthion generally reduced plasma and brain cholinesterase activity and depressed cholinesterase and Na, K-ATPase activities in salt glands of birds drinking saltwater.     

Control of renal and extrarenal salt and water excretion by plasma angiotensin II in the kelp gull (Larus dominicanus)

Summary The osmoregulatory effects of intravenously (i.v.) administered angiotensin II (AII) at dose rates of 5, 15 and 45 ng · kg^–1 · min^–1 were examined in kelp gulls utilizing salt glands and/or kidneys as excretory organs.In birds given i.v. infusion of 1200 mOsmolal NaCl at 0.3 ml · min^–1 and utilizing only the salt glands to excrete the load, infusion of AII for 30 min consistently inhibited salt gland function in a dose-dependent manner.In birds given i.v. infusion of 500 mOsmolal NaCl at 0.72 ml · min^–1 and utilizing both salt glands and kidneys to excrete the load, each dose of AII given for 2 h inhibited salt gland function but stimulated the kidney, so that the overall outputs of salt and water were enhanced and showed significant (2P<0.01) positive correlations with plasma AII.In birds given i.v. infusion of 200 mOsmolal glucose at 0.5 ml · min^–1 and utilizing only the kidneys to excrete the load, low doses of AII (5 and 15 ng · kg^–1 · min^–1) caused renal salt and water retention, whereas a high dose (45 ng · kg^–1 · min^–1) stimulated salt and water output.The actions of plasma AII in kelp gulls support the concept that this hormone plays a vital role in avian osmoregulation, having effects on both salt gland and kidney function. Elevation of plasma AII consistently inhibits actively secreting salt glands, but its effects upon renal excretion depend primarily on the osmotic status as well as on the plasma AII concentration. In conditions of salt and volume loading doses of AII stimulate sodium and water excretion. With salt and volume depletion, the action of AII is bi-phasic with low doses promoting renal sodium and water retention but high circulating levels causing natriuresis and diuresis.     

Adaptive strategies for post-renal handling of urine in birds

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.     

Presystemic influences on thirst, salt appetite, and vasopressin secretion in the hypovolemic rat

The present studies investigated the influence of presystemic signals on the control of thirst, salt appetite, and vasopressin (VP) secretion in rats during nonhypotensive hypovolemia. Rats were injected with 30% polyethylene glycol (PEG) solution, deprived of food and water overnight, and then allowed to drink water, 0.15 M NaCl, or 0.30 M NaCl. The PEG treatment, which produced 30-40% plasma volume deficits, elicited rapid intakes in an initial bout of drinking, but rats consumed much more 0.15 M NaCl than water or 0.30 M NaCl. In considering why drinking stopped sooner when water or concentrated saline was ingested, it seemed relevant that little or no change in systemic plasma Na(+) concentration was observed during the initial bouts and that the partial repair of hypovolemia was comparable, regardless of which fluid was consumed. In rats that drank 0.15 M NaCl, gastric emptying was fastest and the combined volume of ingested fluid in the stomach and small intestine was largest. These and other observations are consistent with the hypothesis that fluid ingestion by hypovolemic rats is inhibited by distension of the stomach and proximal small intestine and that movement of dilute or concentrated fluid into the small intestine provides another presystemic signal that inhibits thirst or salt appetite, respectively. On the other hand, an early effect of water or saline consumption on VP secretion in PEG-treated rats was not observed, in contrast to recent findings in dehydrated rats. Thus the controls of fluid ingestion and VP secretion are similar but not identical during hypovolemia.