Osmolality and volume factors in salt gland control of pekin ducks after adaptation to chronic salt loading

Summary Pekin ducks were adapted to permanent osmotic stress by rearing them on a NaCl solution of increasing concentration up to 2% as drinking water. Their salt and water balance was compared with that of non-adapted ducks maintained on tap water. Amounts and osmolalities of salt gland secretion and cloacal discharges, plasma osmolality and electrolytes were measured during stepwise osmotic loading by intravenous infusion of NaCl solution of about 740 mosm·kg^–1, at rates of 0.25, 0.45 and 0.65 ml·min^–1. Before loading, the plasma osmolality of the adapted ducks was about 22 mosm·kg^–1 higher than in non-adapted animals. The initial step of loading induced salt gland secretion in the adapted ducks after an average rise of plasma osmolality of 3.6 mosm·kg^–1 and in the non-adapted animals after a rise of 7.8 mosm·kg^–1. The method of osmotic loading enabled both groups of animals to balance their water input and output. However, only the adapted ducks were able to balance NaCl input and output, predominantly by salt gland secretion, thus maintaining a stable plasma osmolality. The nonadapted ducks retained 42% of the salt load which resulted in a rise of plasma osmolality of 49 mosm·kg^–1, more salt being excreted by the kidneys than by the salt glands.In the salt-adapted ducks, salt gland activity, plasma osmolality and Na⁺ concentration did not correlate during balanced states of salt input and output. The involvement of tonicity receptors in salt gland control was confirmed by the stimulating effects of various hypertonic solutions. On the other hand, continuous loading by a constant infusion of NaCl solution of 1,300 mosm·kg^–1 induced a steady salt gland secretion at a rising plasma osmolality and thus suggested that a volume factor is involved in salt gland control. Inhibition of salt gland activity by withdrawing blood and activation by blood infusion confirmed this assumption. While a direct cause and effect relationship between volume changes and salt gland secretion cannot be demonstrated, the results indicate that volume changes in one or more extracellular compartments do affect salt gland secretion.Supported by Deutsche Forschungsgemeinschaft (Si 320/2)     

Adrenocortical response in the duck exposed to corticosteroid administration and salt loading

The functional zonation in the adrenocortical tissue of the duck was experimentally investigated after chronic administration of corticosteroids of different types (corticosterone, desoxycorticosterone, dexamethasone) and salt loading (chlorides of sodium and potassium). The cytomorphology of the interrenal cells belonging to subcapsular and central zones was explored by light- and electron microscopy and by biochemical analysis of plasma corticosterone. Corticoid-induced involution of the interrenal tissue, or hyperactivity elicited by salt loading were evident in both subcapsular and central regions of the gland. In the duck the adrenocortical tissue appears to be devoid of steroidogenic functional differentiation. The modifications of different cellular organelles and inclusions can be explained in the light of known concepts about corticosteroid metabolism. The depression of plasma corticosterone titre in corticosteroid-treated animals agrees with the image of cytological inhibition. The lowered corticosterone value in KCl-loaded ducks signifies increased peripheral metabolism of this hormone. The hypercorticosteronemia in NaCl-loaded ducks is probably related to activation of the nasal gland under osmotic stress.     

ANG II-induced attenuation of duck salt gland secretion does not depend upon the release of adrenal catecholamines

Pekin ducks (Anas platyrhynchos) were bilaterally adrenalectomized (ADX) using a two-stage procedure and given daily i.m. injections of 1 mg kg bw⁻¹ of dexamethasone (DEXA), a steroid lacking mineralocorticoid activity, and 0.9% saline drinking water ad libitum to counterbalance renal losses of salt and water. Mean arterial blood pressure (mmHg) fell from 161±3.7 (intact controls) to 116±6.9 (bilateral ADX+DEXA), a decrease of 27%, but heart rates (HR) were unchanged. The nasal salt glands were fully active after ADX+DEXA. Rates of fluid secretion and electrolyte and osmolal concentrations were unchanged. Secretion stopped, then rebounded several minutes later if ADX+DEXA ducks were injected i.v. with 1 μg of [Asn¹,Val⁵]-angiotensin II (ANG II) kg bw⁻¹ which showed that attenuation was not adrenal catecholamine-dependent.     

Induction of the catalytic protein of (Na+ plus K+)-ATPase in the salt gland of the duck.

The (Na+ plus K+)-ATPase activities in salt gland homogenates increased 3- to 4-fold after saline treatment of ducks for 3 weeks. The ATPase was purified to a specific activity of 460 and 1015 mumol Pi/mg protein per h, respectively, in control and saline-treated ducks. The catalytic protein was identified on polyacrylamide electrophoresis gels by phosphorylating the enzyme with (32P)ATP. The molecular weight of the protein was estimated to be 98 000. The amount of catalytic unit increased commensurately with the enzyme activity after saline treatment. It is therefore concluded that the increased enzyme activity is due to a de novo enzyme synthesis and is not an activation effect. Phospholipid concentration in the salt gland tissue increased 1.7-fold after the saline treatment. Significant increases occurred in the percentage of the total phospholipids as phosphatidylserine and sphingomyelin. In the partially purified (Na+ plus K+)-ATPase preparation, the percentage composition of phosphatidylserine and phosphatidylethanolamine increased after saline treatment.     

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.     

Effects of caecal ligation and saline acclimation on plasma concentration and organ mass in male and female Pekin ducks,Anas platyrhynchos

Summary The intestinal caeca reabsorb urinary sodium chloride (NaCl) and water (Rice and Skadhauge 1982). Free water may be generated if the reabsorbed NaCl is secreted via salt gland secretion (Schmidt-Nielsen et al. 1958). Therefore ceacal ligation should (a) reduce hingut NaCl and water reabsorption, (b) enhance the increase in plasma osmolality during saline acclimation, and (c) affect drakes more than ducks. Twelve Pekin drakes and 13 Pekin ducks, Anas platyrhynchos, were caecally ligated or sham operated before acclimation to 450 mmol · 1 NaCl. Body mass, hematocrit, plasma osmolality, and inonic concentrations of plasma, cloacal fluid, and salt gland secretion were measured after each increase in drinking water salinity. Osmoregulatory organ masses were determined. Caecal ligation did not effect plasma osmolality or ion concentrations of plasma, cloacal fluid, or salt gland secretion, but reduced salt gland size in ducks. Drakes and ducks drinking fresh water had the same hematocrit, plasma osmolality, and plasma concentrations of Na⁺ and Cl^–. In both sexes exposure to 75 mmol · 1^-1 NaCl significantly decreased plasma [Na⁺] and doubled cloacal fluid [Na⁺]. Exposure to 450 mmol · 1^-1 NaCl decreased body mass and increased hematocrit, plasma [Na⁺], [Cl^–], and plasma osmolality (more in drakes than in ducks); cloacal fluid osmolality nearly doubled compared to freshwater-adapted ducks, due mainly to osmolytes other than Na⁺ and Cl^–. The [Cl^–] in salt gland secretion only slightly exceeded drinking water [Cl^–].Abbreviations AVT antiduretic hormone - CF cloacal fluid - ECFV extraoellular fluid volume - FW freshwater acclimated - Hct hematocrit - MDWE mean daily water flux - [Na ⁺]_cf cloacal fluid sodium concentration - [Na ⁺]_pl plasma sodium concentration - Osm _cf cloacal fluid osmolality - Osm _pl plasma osmolality - SGS salt gland secretion - TBW total body water     

Regulation of salt gland, gut and kidney interactions

Marine birds can drink seawater because their cephalic 'salt' glands secrete a sodium chloride (NaCl) solution more concentrated than seawater. Salt gland secretion generates osmotically free water that sustains their other physiological processes. Acclimation to saline induces interstitial water and Na move into cells. When the bird drinks seawater, Na enters the plasma from the gut and plasma osmolality (Osm(pl)) increases. This induces water to move out cells expanding the extracellular fluid volume (ECFV). Both increases in Osm(pl) and ECFV stimulate salt gland secretion. The augmented intracellular fluid content should allow more rapid expansion of ECFV in response to elevated Osm(pl) and facilitate activation of salt gland secretion. To fully utilize the potential of the salt glands, intestinally absorbed NaCl must be reabsorbed by the kidneys. Thus, Na uptake at gut and renal levels may constrain extrarenal NaCl secretion. High NaCl intake elevates plasma aldosterone concentration of Pekin ducks and aldosterone stimulates intestinal and renal water and sodium uptake. High NaCl intake induces lengthening of the small intestine of adult Mallards, especially males. High NaCl intake has little effect on glomerular filtration rate or tubular sodium Na uptake of birds with competent salt glands. Relative to body mass, kidney mass and glomerular filtration rate (GFR) are greater in birds with salt glands than in birds that do not have them. Birds with salt glands do not change GFR, when they drink saline. Thus, their renal filtrate contains excess Na that is, in some species, almost completely renally reabsorbed and excreted in a more concentrated salt gland secretion. Na reabsorption by kidneys of other species, like mallards is less complete and their salt glands make less concentrated secretion. Such species may reflux urine into the hindgut, where additional Na may also be reabsorbed for extrarenal secretion. During exposure to saline, marine birds maintain elevated aldosterone levels despite high Na intake. Marine birds are excellent examples of physiological plasticity.     

Osmoregulatory actions of angiotensin II are independent of adrenergic receptor mechanisms in the duck, Anas platyrhynchos

1. Intravenous infusion of a physiological dose of fowl angiotensin II (ANG II) in salt-loaded ducks raised systemic arterial blood pressure, inhibited nasal salt gland fluid and solute secretion, and stimulated renal-cloacal urine production. 2. Beta-adrenergic receptor blockade by propranolol lowered arterial pressure but did not prevent the effects of ANG II on salt and water excretion. 3. Alpha-adrenergic receptor blockade by prazosin decreased both arterial pressure and plasma glucose levels, but it did not impair the osmoregulatory actions of ANG II. 4. These observations indicate that redistribution by ANG II of salt and water excretion is independent of adrenergic receptor mechanisms and therefore does not depend on the sympathomimetic activity of the hormone.     

Nasal glandular secretory response to cholinergic stimulation in humans and guinea pigs.

A guinea pig model of nasal secretory responses was developed to assess the contributions of vascular permeability and glandular secretion responsible for the production of cholinergically stimulated nasal secretions. The nasal secretory responses to provocation with saline, methacholine, and atropine on the ipsilateral (challenged) side and contralateral (reflex) side were analyzed by measurement of total protein (Lowry method), guinea pig albumin (enzyme-linked immunosorbent assay), 125I-labeled bovine serum albumin after intravenous injection, and alkaline phosphatase enzyme activity in nasal fluid. Alkaline phosphatase was found to be localized to submucosal glands by zymography. Topical methacholine challenge increased the secretion of total protein, alkaline phosphatase activity, and albumin on the ipsilateral challenged side, whereas the percentage of total protein represented by albumin was not increased. This response was totally prevented by atropine pretreatment. Serial provocation with methacholine resulted in progressively reduced amounts of both the total protein and alkaline phosphatase in secretions. The observation that repeated challenges produced progressively smaller responses was also examined employing human nasal provocation. Repeating methacholine (25 mg) challenges four times at 10-min intervals in six human volunteers revealed that the initial challenge produced the largest response as reflected in total protein, albumin, lysozyme, lactoferrin, immunoglobulin (Ig) G, IgA, and secretory IgA secretion. When the constituents in secretions were analyzed in relationship to the total protein, the two vascular proteins, IgG and albumin, demonstrated the greatest decrements with repeated methacholine challenges. The glandular proteins, lactoferrin, lysozyme, and secretory IgA, either remained constant or increased in their relative proportion to total protein. Thus, cholinergic stimulation causes glandular secretion from both the guinea pig and human nasal mucosa.(ABSTRACT TRUNCATED AT 250 WORDS)     

Adrenergic and osmotic responses to atrial natriuretic peptide and angiotensin II in the duck (Anas platyrhynchos)

There is evidence that analogues of atrial natriuretic peptide (ANP) and angiotensin II (ANG II) occur in birds. The present experiments studied the adrenergic and osmoregulatory responses to synthetic ANP and ANG II in salt-loaded ducks (Anas platyrhynchos). Excretion of water and salt through the nasal salt glands was abolished by ANG II. This extrarenal, salt-retaining effect of ANG II was not altered by ANP. However, ANP did augment the diuretic response to ANG II. ANP also potentiated the stimulatory effect of ANG II on plasma norepinephrine. The data are consistent with physiological roles for native analogues of ANP and ANG II in adrenergic and osmotic regulation in the duck.     

Fluoride excretion in adélie penguins (Pygoscelis adeliae) and mallard ducks (Anas platyrynchos)

• 1.1. Chronic F⁻ doses of NaF (40 and 80mg F⁻/kg per day) given to adélie penguins for 6–7 weeks led to a significant increase in F⁻ concentrations of nasal salt secretions with 1.16 and 2.22μg F⁻/g (wet wt) respectively as opposed to 0.66 μg F⁻/g in controls.
• 2.2. Single oral administration of 6 mg fluoride as NaF to mallard ducks led to increased F⁻ concentrations in the nasal salt secretions, with a mean of 0.67μg F⁻/g as opposed to 0.11 μg F⁻/g in controls.
• 3.3. In mallard ducks as well as in adélie penguins the amount of F⁻ excreted via the cloaca was 10³-10⁴ times that excreted via the nasal salt glands.

     

Salinity as a constraint affecting food and habitat choice of mussel-feeding diving ducks

Observations of freshwater drinking in Eiders feeding primarily on mussels led us to hypothesize that the highly saline sea water enclosed in mussels could cause salt-related dehydration problems in the ducks, since they consume entire mussels. The proportion of sea water increases with increasing mussel size. Smaller duck species are more sensitive to the higher salt content of larger mussels than are larger ducks; however, salt stress may be avoided by feeding in habitats with lower salinity, by feeding on less salty food items, by utilizing smaller mussels, by drinking fresh water, or by employing low energy foraging techniques. A possible evolutionary strategy for solving the salt problem might be to increase body mass, enabling ducks to utilize larger mussels without passing an upper salt consumption limit. At the same time, foraging on larger mussels is more economical. Although large size facilitates the utilization of brackish and marine environments, it may be selected against in ducks breeding in fresh water, where fish competition may reduce optimal food item size. In conclusion, salinity is an important habitat barrier in both breeding and overwintering diving ducks, but there are various ways of crossing this barrier. To understand better how ducks utilize their habitats, however, it is necessary to measure habitat salinity levels and the size of both ducks and their preferred and less-preferred food types.     

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.     

Arterial hypotension in ducks adapted to high salt intake

Summary A homogeneous group of 8-week-old Pekin ducks was divided into two groups: saltwater (SW) ducks received salt water of gradually increasing salinity (200–600 mOsm·kg^-1) from the 8th to 20th week of age; freshwater (FW) ducks were maintained on fresh water but otherwise treated identically. During the course of salt-adaptation SW ducks increased plasma osmolality, Na⁺ and Cl^- levels, and concentrations of the osmoregulatory peptide hormones arginine vasotocin and angiotensin II. The apparent volume of inulin distribution decreased in SW ducks, but blood volume was not reduced. SW ducks also developed arterial hypotension, bradycardia, and reduced cardiac output in the course of salt adaptation. This depressed cardiovascular performance was associated with enhanced vagal restraint of cardiac function and reduced plasma concentrations of norepinephrine. Salt water adaptation did not alter the degrees to which mean arterial pressure and heart rate changed in response to intravenous bolus injections of catecholamines. The same applied to the osmoregulatory peptides which were, however, effective only at supraphysiological concentrations. The Pekin duck, as a bird predisposed for adaptation to high salt loads, presumably adapts to chronic hypertonic saline intake by resetting the central autonomic control of blood pressure to a lower level.Abbreviations FW ducks fresh water ducks - SW ducks salt water ducks - ANGI angiotensin II - AVT arginine vasotocin - MAP mean arterial pressure - HR heart rate - IV intravenous - CO cardiac output - SV stroke volume - TPR total peripheral resistance - ISp virtual inulin space - ECFV extracellular fluid volume     

The salt gland of the incubating Eider duck Somateria mollissima: the effects of natural salt deprivation

Female Eider ducks, living under natural conditions, do not drink salt water during the 26 days for which they incubate their eggs. In the course of incubation the nasal salt glands decrease in size, show a marked reduction in Na⁺/K⁺-ATPase activity and in the rate at which they clear a salt load. The time course of these events is similar to that seen in laboratory studies on other species where salt intake is manipulated.     

The duct system of the avian salt gland as a transporting epithelium: structure and morphometry in the duck Anas platyrhynchos

Summary The duct system of the nasal salt gland of the duck comprises central canals, secondary ducts and main ducts. The secondary and main ducts consist of a layer of columnar cells overlying a layer of small cuboidal cells. The columnar cells have complex intercellular spaces showing evidence of Na⁺ K⁺ -ATPase at the apical regions. Approximately 70% of surface area of the duct system is external to the gland. During adaptation to salt water the duct system increases in size as does the gland. Although the components of the gland of adapted ducks, including the duct system within the gland, increase in size compared with normal ducks, the percentage volume densities of the components remain similar in both categories of ducks, i.e. the duct system increases in size in proportion to the glandular tissue. The volume of the duct system external to the gland is six to seven times larger than the volume within the gland. Thus, if ductal modification of secreted fluid occurs, it will be most likely to take place in the ducts external to the gland.Total surface areas of the duct system were measured from serial sections of glands and ducts from one normal and one adapted duck. These were used to calculate possible flux rates of water and sodium across the duct epithelium, assuming the occurrence of either water reabsorption or sodium secretion. Although these flux rates are high it is shown that they are similar to calculated flux rates across the luminal surface of the secretory tubules.     

Osmotic and volume control of diuresis in conscious ducks (Anas platyrhynchos)

Summary In conscious Pekin ducks made diuretic either by infusing hyposmotic glucose solution or isosmotic saline, osmotic and volume effects on renal water excretion were investigated. As in mammals, antidiuresis mediated by enhanced release of antidiuretic hormone was induced by increasing carotid blood osmolality while a decrease augmented diuresis, indicating cerebral osmotic control of renal water excretion in birds.In contrast to the situation in mammals, a sensitive diuretic response to isosmotic volume expansion, corresponding to 1% of the extracellular volume, can be demonstrated, with intracarotid and intravenous application of the isosmotic saline infusion having identical effects.Volume loading with isosmotic saline produced a greater diuretic response than loading with the same amount of autologous blood, thereby indicating a major contribution of volume changes in the interstitial compartment to the control of renal water excretion. This corresponds to the importance of the interstitial fluid compartment for the control of salt gland activity in this species.Abbreviations AVP arginine vasopressin - ECF extracellular fluid - i.c., i.v. intracarotid, intravenous - ECFV ECF volume     

Cardiovascular changes induced by central hypertonic saline are accompanied by glutamate release in awake rats

To elucidate neurochemical mechanisms responsible for cardiovascular responses induced by central salt loading, we directly perfused the paraventricular nucleus (PVN) of the hypothalamus region with hypertonic saline (0.3 or 0.45 M) by using an in vivo brain microdialysis technique. We then measured the extracellular concentrations of glutamate in the PVN region in conscious rats along with the blood pressure and heart rate. Blood pressure, heart rate, and glutamate levels were increased by perfusion of 0.45 M saline; however, they did not change by perfusion of 0.3 M saline. Next, we examined the possible involvement of glutamate in the cardiovascular responses induced by hypertonic saline. Dizocilpine, a noncompetitive antagonist of the N-methyl-D-aspartate (NMDA) receptor, attenuated the increases of blood pressure and heart rate, although 6-cyano-7-nitroquinoxaline-2,3-dione, an antagonist of the non-NMDA receptor, did not affect the blood pressure and heart rate. Our results show that local perfusion of the hypothalamic PVN region with hypertonic saline elicits a local release of glutamate, which may act via NMDA-type glutamate receptors to produce cardiovascular responses.     

下丘脑室旁核内GABA在中枢高渗刺激诱发的心血管反应中的作用

目的：探讨下丘脑室旁核（PVN）内的γ-氨基丁酸（GABA）在中枢高渗刺激诱发的应激性心血管反应中的作用及其机制。方法：在清醒自由活动大鼠,用脑部微量透析法和高效液相色谱法观察中枢高渗刺激对PVN区域GABA含量的影响,并同时记录血压和心率的变化;用GABAA受体阻断剂Bicuculline或GABAB受体阻断剂Saelofen直接灌流PVN区并给予中枢高渗刺激,进一步探讨PVN区GABA在中枢高渗刺激诱发的应激性心血管反应中的作用。结果：①PVN局部灌流0．6mol／L盐水时,血压和心率都显著增加（均为P〈0．01）,同时,PVN区细胞外液中GABA水平也明显增加到刺激前的561．96％±173．96％（P〈0．05）;②PVN局部灌流Bicuculline或Saclofen的同时,给予0．6mol／L盐水的刺激,可使高渗刺激引起的血压增加幅度明显降低（均为P〈0．01）,而心率的增加幅度未受明显影响（均为P〈0．05）。结论：中枢高渗刺激可引起PVN内GABA的分泌,而后者可通过GABAA和GABAB受体产生血压的升高反应。     

Gastrointestinal osmoreceptors and renal sodium excretion in humans

The hypothesis that natriuresis can be induced by stimulation of gastrointestinal osmoreceptors was tested in eight supine subjects on constant sodium intake (150 mmol NaCl/day). A sodium load equivalent to the amount contained in 10% of measured extracellular volume was administered by a nasogastric tube as isotonic or hypertonic saline (850 mM). In additional experiments, salt loading was replaced by oral water loading (3.5% of total body water). Plasma sodium concentration increased after hypertonic saline (+3.1 +/- 0.7 mM), decreased after water loading (-3.8 +/- 0.8 mM), and remained unchanged after isotonic saline. Oncotic pressure decreased by 9.4 +/- 1.2, 3.7 +/- 1.2, and 10.7 +/- 1.3%, respectively. Isotonic saline induced an increase in renal sodium excretion (104 +/- 15 to 406 +/- 39 micromol/min) that was larger than seen with hypertonic saline (85 +/- 15 to 325 +/- 39 micromol/min) and water loading (88 +/- 11 to 304 +/- 28 micromol/min). Plasma ANG II decreased to 22 +/- 6, 35 +/- 6, and 47 +/- 5% of baseline after isotonic saline, hypertonic saline, and water loading, respectively. Plasma atrial natriuretic peptide (ANP) concentrations and urinary excretion rates of endothelin-1 were unchanged. In conclusion, stimulation of osmoreceptors by intragastric infusion of hypertonic saline is not an important natriuretic stimulus in sodium-replete subjects. The natriuresis after intragastric salt loading was independent of ANP but can be explained by inhibition of the renin-angiotensin system.