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     

Intracellular and luminal ion concentrations in sea turtle salt glands determined by x-ray microanalysis

Summary The lachrymal salt glands of hatchlings of the green sea turtle (Chelonia mydas) secrete a hyperosmotic (up to 2000 mosmol·kg^–1) NaCl solution. X-ray microanalysis of frozen-hydrated glands showed that during secretion intracellular Na⁺ concentration in the principal cells increased from 13 to 34 mmol·l^–1 of cell water, whilst Cl^– and K⁺ concentrations remained unchanged at 81 mmol·l^–1 and 160–174 mmol·l^–1, respectively. The high Cl^– concentration and the change in Na⁺ concentration are consistent with the prevailing paradigm for secretion by the structurally and functionally similar elasmobranch rectal gland. Concentrations of Na⁺, Cl^– and K⁺ in the lumina of secretory tubules of secreting (Na⁺ 122, Cl^– 167, K⁺ 38 mmol·l^–1) and non-secreting (Na⁺ 114, Cl^–1 174, K⁺ 44 mmol·l^–1) glands were similar and the fluid was calculated to be approximately isosmotic with blood. In the central canals Na⁺ and Cl^– concentrations were similar but K⁺ concentration was lower (11–15 mmol·l^–1). It is concluded that either a high transepithelial NaCl gradient in secretory tubules and central canals is very rapidly dissipated during the short time between gland excision and freezing, or that ductal modification of an initial isosmotic secretion occurs.     

Passive sodium movements across the opercular epithelium: The paracellular shunt pathway and ionic conductance

Summary The unidirectional Na⁺, Cl^–, and urea fluxes across isolated opercular epithelia from seawater-adaptedFundulus heteroclitus were measured under different experimental conditions. The mean Na⁺, Cl^–, and urea permeabilities were 9.30×10^–6 cm·sec^–1, 1.24×10^–6 cm·sec^–1, and 5.05×10^–7 cm·sec^–1, respectively. The responses of the unidirectional Na⁺ fluxes and the Cl^– influx (mucosa to serosa) to voltage clamping were characteristic of passively moving ions traversing only one rate-limiting barrier. The Na⁺ conductance varied linearly with, and comprised a mean 54% of, the total tissue ionic conductance. The Cl^– influx and the urea fluxes were independent of the tissue conductance. Triaminopyrimidine (TAP) reduced the Na⁺ fluxes and tissue conductance over 70%, while having no effect on the Cl^– influx or urea fluxes. Mucosal Na⁺ substitution reduced the Na⁺ permeability 60% and the tissue conductance 76%, but had no effect on the Cl^– influx or the urea fluxes. Both the Na⁺ and Cl^– influxes were unaffected by respective serosal substitutions, indicating the lack of any Na⁺/Na⁺ and Cl^–/Cl^– exchange diffusion.The results suggest that the unidirectional Na⁺ fluxes are simple passive fluxes proceeding extracellularly (i.e., movement through a cation-selective paracellular shunt). This pathway is dependent on mucosal (external) Na⁺, independent of serosal (internal) Na⁺, and may be distinct from the transepithelial Cl^– and urea pathways.     

Functional Characterization of Na<Superscript>+</Superscript>/H<Superscript>+</Superscript> Exchangers in Primary Cultures of Prairie Dog Gallbladder

Gallbladder Na⁺ absorption is linked to gallstone formation in prairie dogs. We previously reported Na⁺/H⁺ exchanger (NHE1-3) expression in native gallbladder tissues. Here we report the functional characterization of NHE1, NHE2 and NHE3 in primary cultures of prairie dog gallbladder epithelial cells (GBECs). Immunohistochemical studies showed that GBECs grown to confluency are homogeneous epithelial cells of gastrointestinal origin. Electron microscopic analysis of GBECs demonstrated that the cells form polarized monolayers characterized by tight junctions and apical microvilli. GBECs grown on Snapwells exhibited polarity and developed transepithelial short-circuit current, I_sc, (11.6 ± 0.5 µA · cm^–2), potential differences, V_t (2.1 ± 0.2 mV), and resistance, R_t (169 ± 12 · cm²). NHE activity in GBECs assessed by measuring dimethylamiloride-inhibitable ²²Na⁺ uptake under a H⁺ gradient was the same whether grown on permeable Snapwells or plastic wells. The basal rate of ²²Na⁺ uptake was 21.4 ± 1.3 nmol · mg prot^–1 · min^–1, of which 9.5 ± 0.7 (~45%) was mediated through apically-restricted NHE. Selective inhibition with HOE-694 revealed that NHE1, NHE2 and NHE3 accounted for ~6%, ~66% and ~28% of GBECs total NHE activity, respectively. GBECs exhibited saturable NHE kinetics (V_max 9.2 ± 0.3 nmol · mg prot^–1 · min^–1; K_m 11.4 ± 1.4 mM Na⁺). Expression of NHE1, NHE2 and NHE3 mRNAs was confirmed by RT-PCR analysis. These results demonstrate that the primary cultures of GBECs exhibit Na⁺ transport characteristics similar to native gallbladder tissues, suggesting that these cells can be used as a tool for studying the mechanisms of gallbladder ion transport both under physiologic conditions and during gallstone formation.     

Renal function at steady state in a toad (Bufo viridis) acclimated in hyperosmotic NaCl and urea solutions

Kidney function of the euryhaline toad Bufo viridis was studied in animals acclimated to tap water and solutions of NaCl (230 and 500 mosmol · kg^-1 H₂O) and urea (500 mmol · l^-1) in steady-state conditions. An ureter was eatheterized for continuous urine collection and blood was sampled from an iliac artery. A single injection of ³H-inulin served for estimation of glomerular filtration rate: this was in the range of 15–27 ml · kg^-1 · h^-1 and did not differ significantly in any of the acclimation conditions. Urine flow, on the other hand, varied considerably and was highest in tap water (18.2±3.2 ml · kg^-1 · h^-1; urine/plasma inulin ratio=0.88), lower in 230 mosmol · kg^-1 H₂O NaCl solution (13.5±3.9 ml · kg^-1 · h^-1; u/p inulin ratio=1.73) and lowest in 500 mosmol · kg^-1 H₂O NaCl or urea acclimation solutions (5–7 ml · kg^-1 · h^-1; u/p inulin=3.7–4.2). Clearance of free water was high in the tap water group, lower in 230 mosmol · kg^-1 H₂O NaCl solution, and much lower in the hyperosmotic acclimation conditions. Clearances of both Na⁺ and Cl^- were similar under our experimental conditions, but changed independently in accordance to the composition of the acclimation solution. Potassium clearance was similar in all acclimation conditions, and a constant plasma K⁺ concentration was maintained. Urea clearance was high in tap water and 500 mmol · l^-1 urea acclimation groups and low in the NaCl acclimations. The experiments show that the glomerular filtration rate remains more or less unchanged in all acclimation conditions, and suggest that the different rates of urine flow at steady state must be due mostly to tubular processes. The final composition of the urine is the result of specific and highly selective tubular processes.Abbreviations %T fractional reabsorbance - AVT argine vasotocin - C _{free water} free water clearance - C _osmol osmolyte clearance - GFR glomerular filtration rate - MS-222 methanetricaine sulphonate - U/P urine to plasma inulin ratio - V volume     

Sidium-dependent ion cotransport in steady-state Ehrlich ascites tumor cells

Summary The Ehrlich tumor cell possesses and anion-cation cotransport system which operates as a bidirectional exchanger during the physiological steady state. This cotransport system, like that associated with the volume regulatory mechanism (i.e. coupled net uptake of Cl^–+Na⁺ and/or K⁺) is Cl^–-selective and furosemide-sensitive, suggesting the same mechanism operating in two different modes. Since Na⁺ has an important function in the volume regulatory response, its role in steady-state cotransport was investigated. In the absence of Na⁺, ouabain-insensitive K⁺ and DIDS-insensitive Cl^– transport (KCl cotransport) are low and equivalent to that found in 150mm Na⁺ medium containing furosemide. Increasing the [Na⁺] results in parallel increases in K⁺ and Cl^– transport. The maximum rate of each (18 to 20 meq/(kg dry wt)·min) is reached at about 20mm Na⁺ and is maintained up to 55mm. Thus, over the range 1 to 55mm Na⁺ the stoichiometry of KCl cotransport is 11. In contrast to K⁺ and Cl^–, furosemide-sensitive Na⁺ transport is undetectable until the [Na⁺] exceeds 50mm. From 50 to 150mm Na⁺, it progressively rises to 7 meq/(kg dry wt)·min, while K⁺ and Cl^– transport decrease to 9 and 16 meq/(kg dry wt)·min, respectively. Thus, at 150mm Na⁺ the stoichiometric relationship between Cl^–, Na⁺ and K⁺ is 211. These results are consistent with the proposal that the Cl^–-dependent cation cotransport system when operating during the steady state mediates the exchange of KCl for KCl or NaCl for NaCl; the relative proportion of each determined by the extracellular [Na⁺].     

Kinetic studies on the stimulation of Na+−H+ exchange activity in renal brush border membranes isolated from thyroid hormone-treated rats

Summary Na⁺–H⁺ exchange activity in renal brush border membrane vesicles isolated from hyperthyroid rats was increased. When examined as a function of [Na⁺], treatment altered the initial rate of Na⁺ uptake by increasingV _m (hyperthyroid, 18.9±1.1 nmol Na⁺ · mg^–1 · 2 sec^–1; normal, 8.9±0.3 nmol Na⁺ · mg^–1 · 2 sec^–1), and not the apparent affinityK _Na ⁺ (hyperthyroid, 7.3±1.7mm; normal, 6.5±0.9mm). When examined as a function of [H⁺] and at a subsaturating [Na⁺] (1mm), hyperthyroidism resulted in the proportional increase in Na⁺ uptake at every intravesicular pH measured. A positive cooperative effect on Na⁺ uptake was found with increased intravesicular acidity in vesicles from both normal and hyperthyroid rats. When the data were analyzed by the Hill equation, it was found that hyperthyroidism did not change then (hyperthyroid, 1.2±0.06; normal, 1.2±0.07) or the [H⁺]_0.5 (hyperthyroid, 0.39±0.08 m; normal, 0.44±0.07 m) but increased the apparentV _m (hyperthyroid, 1.68±0.14 nmol Na⁺ · mg^–1 · 2 sec^–1; normal 0.96±0.10 nmol Na⁺ · mg^–1 · 2 sec^–1). The uptake of Na⁺ in exchange for H⁺ in membrane vesicles from normal and hyperthyroid animals was not influenced by membrane potential. H⁺ translocation or debinding was rate limiting for Na⁺–H⁺ exchange since Na⁺–Na⁺ exchange activity was greater than Na⁺–H⁺ exchange activity. Hyperthyroidism caused a proportional increase and hypothyroidism caused a proportional decrease in Na⁺–Na⁺ and Na⁺–H⁺ exchange. We conclude that hyperthyroidism leads to either an increase in the number of functional exchangers in the membrane or exactly proportional increases in the rate-limiting steps for Na⁺–Na⁺ and Na⁺–H⁺ exchange activity.     

Brief pre- and post-irrigation sprinkling with fresh water reduces foliar salt uptake in maize and barley sprinkler irrigated with saline water

Brief pre- and post-irrigation sprinkling treatments using freshwater were tested to determine if these practices could reduce the uptake of salts through leaves when saline water is used to sprinkler irrigate crops. Maize and barley were sprinkler irrigated 2 to 3 times per week for 30 min with saline water (4.2 dS m^–1, 30 mmol L^–1 NaCl and 2.8 mmoles L^–1 CaCl₂ for maize and 9.6 dS m^–1, 47 mmoles L^–1 NaCl and 23.5 mmoles L^–1 CaCl₂ for barley) in separate experiments with plants grown in pots outdoors. The soil surface of all pots was covered to prevent salinization of the soil by the sprinkling water. One half of the sprinkled plants was grown in nonsaline soil to study the effects of pre-wetting and post-washing when ion uptake was primarily through leaves. The other half of the sprinkled plants was grown in soil salinized by drip irrigation, in order to evaluate the effects of pre-wetting and post-washing when Na⁺ and Cl^- uptake was through both leaves and roots.Post-washing with freshwater (5 min) reduced the leaf sap concentrations of Cl^- in saline-sprinkled plants from 56 to 43 mmol L^–1 in maize and from 358 to 225 mmol L^–1 in barley (averages for plants grown in nonsaline and saline soil). Na⁺ concentrations in leaf sap were reduced from 93 to 65 mmoles L^–1 (maize) and from 177 to 97 mmoles L^–1 (barley) by the post-washing. Pre-wetting had a small effect on ion uptake through leaves, the only significant reduction in seasonal means being in leaf Na⁺ concentrations for plants grown in nonsaline soil. Pre-wetting and post-washing, when combined, reduced leaf Cl^- concentrations to levels similar to those of nonsprinkled plants grown in saline soil; however, Na⁺ concentrations in leaves remained 3.5 times (maize) and 1.5 times (barley) higher than those of nonsprinkled plants. When pre-wetting and post-washing were not applied, sprinkled barley plants grown in saline soil had grain yields which were 58% lower than nonsprinkled plants grown in saline soil, but the reduction in grain yield was only 17% when the freshwater treatments were given. We conclude that a brief period of post-washing with freshwater is essential when saline water is employed in sprinkler irrigation. By comparison, the benefits from pre-wetting were small in these experiments. ei]T J Flowers     

Characterization of esophageal desalination in the seawater eel,Anguilla japonica

To characterize mechanisms of esophageal desalination, osmotic water permeability and ion fluxes were measured in the isolated esophagus of the seawater eel. The osmotic permeability coefficient in the seawater eel esophagus was 2·10^-4 cm·s^-1. This value was much lower than those in tight epithelial, although the eel esophagus is a leaky epithelium with a tissue resistance of 77 ohm·cm^-2. When the esophagus was bathed in normal Ringer solutions on both sides no net ion and water fluxes were observed. However, when mucosal NaCl concentration was increased by a factor of 3, Na⁺ und Cl^- ions were transferred from mucosa to serosa (desalination). If only Na⁺ or Cl^- concentration in the mucosal fluid was increased by a factor of 3, net Na⁺ and Cl^- fluxes were reduced to 30–40%, indicating that 60–70% of the net Na⁺ and Cl^- fluxes are coupled mutually. The coupled NaCl transport seems to be effective in desalting the luminal high NaCl. The remaining 30–40% of the total Na⁺ and Cl^- fluxes seems to be due to a simple diffusion, because these components are independent of each other and follow their electrochemical gradients, and also because these fluxes remain even after treatment with NaCN or ouabain. A half of the coupled NaCl transport could be explained by a Na⁺/H⁺–Cl^-/HCO ₃ ^- double exchanger on the apical membrane of the esophageal epithelium, because mucosal amiloride and 4.4-diisothiocyanatostilbene-2,2-disulphonic acid inhibited the net Na⁺ and Cl^- fluxes by approximately 30%. The other half of the coupled NaCl transport, which follows their electrochemical gradients, still remains to be explained.Abbreviations DIDS 4,4-diisothiocyanatostilbene-2,2-disulphonic acid - NMDG N-methyl-d-glucosamine - P _Cl Cl^- permeability coefficient - PD transepithelial potential difference - P _Na Na⁺ permeability coefficient - P _osm osinotic permeability coefficient - TALH thick ascending limb of Henle's loop     

Function and regulation of the avian caecal bulb: influence of dietary NaCl and aldosterone on water and electrolyte fluxes in the hen (Gallus domesticus) perfused in vivo

Summary The function of the caecal bulb, and its adaptation to chronic high- or low-Na⁺ intake, was investigated by in vivo perfusion of anaesthetised birds. Effects of acute aldosterone injection (125 g·kg^–1 body mass) were also measured.Evidence was found for primary active net absorption of Na⁺, inducing parallel Na-linked absorption of water and Cl^– and secretion of K⁺. Around 20–35% of total Cl^– absorption and K⁺ secretion were independent of Na⁺ fluxes, and these components appear to be driven by passive processes with apparent conductances of 6.3×10^–3 (G _Cl) and 1.1×10^–3 (G _K) S·cm^–2.Acetate (40mM) stimulated Na⁺ fluxes (8.5–9.9 Eq·cm^–2·h^–1) and Na-linked water fluxes (27–44 l·cm^–2·h^–1). Increased coupling ratios (2.9–4.6 l·Eq^–1) and other data indicate that these effects may be due to increased osmotic permeabilities of barriers involved in the Na-linked water transfer pathway.Low-Na⁺ maintenance enhanced EPD (49–69 mV, serosa positive) and all net fluxes:J _Na (6.8–11.6);J _K (–3.2––4.3);J _Cl (4.3–5.6 Eq·cm serosal area^–2·h^–1);J _v (28–43 l·cm^–2·h^–1) (mucosal-serosal fluxes positive).Acute aldosterone enhancedJ _Na (10.8–14.0 Eq·cm^–2·h^–1) and EPD (54–66 mV) by 3 h after injection, but had no effect on the Na-linked components ofJ _K orJ _Cl.Abbreviations ECPD, EPD Electrochemical or electrical potential difference - G _Cl ,G _K ionic conductances (Cl^–, K⁺) - J _v ,J _ion net volume or ion flux rate, mucosa-serosa positive;P _d (Cl) diffusive permeability coefficient (of Cl^–) - SEDM standard error of difference between means     

A comparison of oxygen,nitrate, and sulfate respiration in coastal marine sediments

Aerobic respiration with oxygen and anaerobic respiration with nitrate (denitrification) and sulfate (sulfate reduction) were measured during winter and summer in two coastal marine sediments (Denmark). Both aerobic respiration and denitrification took place in the oxidized surface layer, whereas sulfate reduction was most significant in the deeper, reduced sediment. The low availability of nitrate apparently limited the activity of denitrification during summer to less than 0.2 mmoles NO ₃ ^– m^–2 day^–1, whereas activities of 1.0–3.0 mmoles NO ₃ ^– m^–2 day^–1 were measured during winter. Sulfate reduction, on the contrary, increased from 2.6–7.6 mmoles SO ₄ ^2– m^–2 day^–1 during winter to 9.8–15.1 mmoles SO ₄ ^2– m^–2 day^–1 during summer. The aerobic respiration was high during summer, 135–140 mmoles O₂ m^–2 day^–1, as compared to estimated winter activities of about 30 mmoles O₂ m^–2 day^–1. The little importance of denitrification relative to aerobic respiration and sulfate reduction is discussed in relation to the availability and distribution of oxygen, nitrate, and sulfate in the sediments and to the detritus mineralization.     

Contribution of Na/Na and Cl/Cl exchanges to sodium and chloride fluxes in the crabGoniopsis cruentata

Summary Sodium or chloride efflux and transepithelial potentials (TEP) were measured in crabs exposed to seawater concentrations ranging from 150 to 25% SW. In crabs acclimated to 150% SW the Na⁺ efflux (3.8 mmol/h·100 g) was significantly higher than the Cl^– efflux (2.1 mmol/h·100 g), but both fluxes decreased to about 0.6 mmol/h·100 g in crabs from 50 or 25% SW. The TEP varied linearly from –1 mV (blood negative) in 150% SW, to –11 mV in 25% SW. In 150 and 100% SW the calculated components of the ion fluxes (i.e., diffusive, urinary, active uptake or extrusion) added up to less than one-half of the isotopically measured values. In 50 and 25% SW the measured effluxes were fully accounted for by their calculated components. In crabs transferred from 150% SW to low-Na 150% SW (=TRIS ASW), the Na⁺ efflux decreased abruptly, from 3.7 to 0.6 mmol/h; the Cl^– efflux decreased much less, from 1.9 to 1.5 mmol/h. A large fraction of the Na⁺ (or Cl^–) fluxes in crabs from concentrated SW meets the criteria for exchange diffusion, which decreases or disappears as the external concentration of each ion is lowered. This suggests that changes of the permeability to ions, in response to alterations of environmental salinity, may not constitute an important adaptive strategy in this species.Abbreviations SW seawater - TEP transepithelial potential - TRIS ASW artificial seawater 150%     

Na+/H+ exchange in crayfish neurons: dependence on extracellular sodium and pH

Summary Intracellular pH (pH_i) regulation was studied in crayfish neurons with pH-, and Na⁺-sensitive microelectrodes. It was confirmed to involve both a HCO ₃ ^– -dependent and a HCO ₃ ^– -independent mechanism. The latter was identified as the amiloride-sensitive Na⁺/H⁺ exchange described in vertebrate cells. Its dependence on extracellular pH (pH_e) and Na⁺ concentration ([Na⁺]_e) was studied in CO₂-free external solutions at 20°C. The steady state pH_i and the rate constant (k) of the exponential pH_i recovery following an acid load were determined. At pH_e=7.5 and [Na⁺]_e=200 mM, the average steady state pH_i was 7.09±0.12 (as compared to 7.30±0.10 in the presence of 5 mM bicarbonate). The dependence of the rate constant of recovery on [Na⁺]_e could be described by Michaelis-Menten kinetics; at pH_e=7.5 the apparentK _m andK _max were 39 mM and 1.4 mmol·l^–1·min^–1, respectively. Decreasing pH_e reduced the rate of recovery, the variations ofk with pH_e conforming to a simple titration curve with an apparent pK of 7.05±0.21. These kinetic properties of the Na⁺/H⁺ exchange in crayfish neurons are similar to those described in vertebrate cells.Preliminary results were presented at the First International Congress of Comparative Physiology and Biochemistry (Liège, Belgium, 1984)     

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)     

Different modes of electrogenic Na+ absorption in the coprodeum of the chicken embryo: role of extracellular Ca2+

Summary Transepithelial electrogenic Na⁺ transport (I_Na) was investigated in the coprodeum of 20-days-old chicken embryos in Ussing chambers. Short circuit current (I_sc) and transepithelial resistance (R_t) were 14.7±4.8 A · cm^-2 (n=12) and 0.53±0.09 k · cm^-2 (n=12), respectively. I_Na was calculated from changes in I_sc by substitution of mucosal Na⁺ by (N-methyl-d-glucamine) (NMDG). I_sc inversed during Na⁺ removal, and I_Na was found to be 27.8±4.7 A · cm^-2 (n=12). Amiloride (100 mol · l^-1) inhibited only about 60% of I_Na. Analysis of I_sc fluctuations revealed a Lorentzian component in the power density spectrum with a corner frequency of about 57 Hz. This component was not correlated to I_Na, and its origin is still unclear. Removal of mucosal Ca²⁺ increased I_Na about 2.5-fold due to an increase of the amiloride-insensitive component of I_Na in additionally investigated adult tissues. The results clearly show that this is due to a non-selective cation channel with an apparent order of selectivity Cs⁺>Na⁺=K⁺>Rb⁺>Li⁺. The Ca²⁺ concentration required to block 50% of the I_sc was about 18 mol · l^-1. The I _sc ^Ca could also be supressed by other divalent cations such as Mg²⁺ and Ba²⁺. Additionally, an I_Na-linked Lorentzian component occurred which dominated the control spectrum with a significantly higher corner frequency (about 88 Hz). The results indicate that Na⁺ absorption in the coprodeum of the chicken embryo is more complex than in adult hens. However, the Ca²⁺ sensitivity of I_Na is similar to comparable effects described for other epithelia. This possibly reflects the existence of two types of amiloride-insensitive apical cation channels as pathways for Na⁺ absorption, which may be involved to differing degrees in ontogenetic developments of nonselective channels to Na⁺-specific ion channels.Abbreviations DPL direct-linear-plot method - slope of the back-ground noise component - EGTA ethylene glycol-bi(2-amino-ethylether)-N,N,N,N-tetraacetic acid - f frequency - f _c corner frequency of the Lorentzian noise component - G _t transepithelial conductance - HEPES N-hydroxyethylpiperazine-N-ethanesulfonic acid - I _sc short-circuit current - I _Na transepithelial sodium current - I _sc ^Ca Ca²⁺-sensitive short-circuit current - K _m ^Ca Michaelis-Menten constant for Ca²⁺ - K _B power density of the background noise component at f=1Hz - m mucosal - NMDG N-methyl-D-glucamine - R _t transepithelial resistance - s serosal - SEM standard error of mean - S(f) power density of the Lorentzian noise component - S _o plateau value of the Lorentzian noise component     

Calcium absorption by fish intestine: The involvement of ATP-and sodium-dependent calcium extrusion mechanisms

Summary Measurements of unidirectional calcium fluxes in stripped intestinal epithelium of the tilapia,Oreochromis mossambicus, in the presence of ouabain or in the absence of sodium indicated that calcium absorption via the fish intestine is sodium dependent. Active Ca²⁺ transport mechanisms in the enterocyte plasma membrane were analyzed. The maximum capacity of the ATP-dependent Ca²⁺ pump (V _m :0.63 nmol·min^–1 mg^–1,K _m : 27nm Ca²⁺) is calculated to be 2.17 nmol·min^–1·mg^–1, correcting for 29% inside-out oriented vesicles in the membrane preparation. The maximum capacity of the Na⁺/Ca²⁺ exchanger with high affinity for Ca²⁺ (V _m :7.2 nmol·min^–1·mg^–1,K _m : 181nm Ca²⁺) is calculated to be 13.6 nmol·min^–1·mg^–1, correcting for 53% resealed vesicles and assuming symmetrical behavior of the Na⁺/Ca²⁺ exchanger. The high affinity for Ca²⁺ and the sixfold higher capacity of the exchanger compared to the ATPase suggest strongly that the Na⁺/Ca²⁺ exchanger will contribute substantially to Ca²⁺ extrusion in the fish enterocyte. Further evidence for an important contribution of Na⁺/Ca²⁺ exchange to Ca²⁺ extrusion was obtained from studies in which the simultaneous operation of ATP-and Na⁺-gradient-driven Ca²⁺ pumps in inside-out vesicles was evaluated. The fish enterocyte appears to present a model for a Ca²⁺ transporting cell, in which Na⁺/Ca²⁺ exchange activity with high affinity for Ca²⁺ extrudes Ca²⁺ from the cell.     

Quinidine-sensitive K+ channels in the basolateral membrane of embryonic coprodeum epithelium: regulation by aldosterone and thyroxine

Basolateral K⁺ channels and their regulation during aldosterone- and thyroxine-stimulated Na⁺ transport were studied in the lower intestinal epithelium (coprodeum) of embryonic chicken in vitro. Isolated tissues of the coprodeum were mounted in Ussing chambers and investigated under voltage-clamped conditions. Simultaneous stimulation with aldosterone (1 mol·l^-1) and thyroxine (1 mol·l^-1) raised short-circuit current after a 1- to 2-h latent period. Maximal values were reached after 6–7 h of hormonal treatment, at which time transepithelial Na⁺ absorption was more than tripled (77±11 A·cm^-2) compared to control (24±8 A·cm^-2). K⁺ currents across the basolateral membrane with the pore-forming antibiotic amphotericin B and application of a mucosal-to-serosal K⁺ gradient. This K⁺ current could be dose dependently depressed by the K⁺ channel blocker quinidine. Fluctuation analysis of the short-circuit current revealed a spontaneous and a blocker-induced Lorentzian noise component in the power density spectra. The Lorentzian corner frequencies increased linearly with the applied blocker concentration. This enabled the calculation of single K⁺ channel current and K⁺ channel density. Single K⁺ channel current was not affected by stimulation, whereas the number of quinidine-sensitive K⁺ channels in the basolateral membrane increased from 11 to 26·10⁶·cm^-2 in parallel to the hormonal stimulation transepithelial Na⁺ transport. This suggests that the basolateral membrane is a physiological target during synergistic aldosterone and thyroxine regulation of transepithelial Na⁺ transport for maintaining intracellular K⁺ homeostasis.Abbreviations f frequency - f _c Lorentzian corner frequency - g _K single K⁺ channel conductance - HEPES N-2-hydroxyethylpiperazin-N'-2-ethansulfonic acid - i _K single K⁺ channel current - I_Ampho amphotericin B induced K⁺ current - I _sc short-circuit current - I _K quinidine blockable K⁺ current - I _max maximally blocked current by quinidine - IC ₅₀ half-maximal blocker concentration - k _on, k _off on- and off-rate coefficients of reversible single channel block by quinidine - M _K number of conducting K⁺ channels - [Q] quinidine concentration - R _t transepithelial resistance - S spectral density - S _o Lorentzian plateau - TBM cells toad urinary bladder cell line Present address: University of California at Berkeley, Dept. of Molecular and Cell Biology Berkeley, CA 94720, USA     

Caecal water and electrolyte absorption and the effects of acetate and glucose, in dehydrated, low-NaCl diet hens

Summary In vivo electrolyte transport and water absorption from the caeca of dehydrated, low-NaCl diet hens are reported. In the absence of luminal glucose or acetate, net electrolyte transport rates and water absorption are small. When physiological concentrations of acetate (40 mM) are included in the perfusate, Na⁺ transport and water absorption increase significantly (P<0.01): 38±7 eqNa⁺/caecum kg·h and 256±33 l H₂O/caecum · kg · h.A similar increase in water absorption occurs with the inclusion of 15 mM glucose in the perfusate (219±30 l H₂O/caecum · kg · h), however both net Na⁺ and Cl^– absorption increase: 28±6 eq Na⁺/caecum · kg · h and 21±5 eq Cl^–/caecum kg · h.These pronounced increases in electrolyte and water absorption are not accompanied by any significant increase in transmural potential difference.The data presented establish caeca as important sites in the recuperation of water and electrolytes in dehydrated, low-NaCl diet hens.Abbreviations ECPD electrochemical potential difference - PD (transmural) potential difference - PEG polyethylene glycol     

Short-chain fatty acids and CO2 as regulators of Na+ and Cl− absorption in isolated sheep rumen mucosa

Summary Unidirectional ²²Na⁺ and ³⁶Cl^– fluxes were determined in short-circuited, stripped rumen mucosa from sheep by using the Ussing chamber technique. In both CO₂/HCO _– ³ -containing and CO₂/HCO _– ³ -free solutions, replacement of gluconate by short-chain fatty acids (SCFA, 39 mM) significantly enhanced mucosal-toserosal Na⁺ absorption without affecting the Cl^– transport in the same direction. Short-chain fatty acid stimulation of Na⁺ transport was at least partly independent of Cl^– and could almost completely be abolished by 1 mM mucosal amiloride, while stimulation of Na⁺ transport was enhanced by lowering the mucosal pH from 7.3 to 6.5. Similar to the SCFA action, raising the PCO₂ in the mucosal bathing solution led to an increase in the amiloride-sensitive mucosal-to-serosal Na⁺ flux. Along with its effect on sodium transport, raising the PCO₂ also stimulated chloride transport. The results are best explained by a model in which undissociated SCFA and/or CO₂ permeate the cell membrane and produce a raise in intracellular H⁺ concentration. This stimulates an apical Na⁺/H⁺ exchange, leading to increased Na⁺ transport. The stimulatory effect of CO₂ on Cl^– transport is probably mediated by a Cl^–/HCO _– ³ exchange mechanism in the apical membrane. Binding of SCFA anions to that exchange as described for the rat distal colon (Binder and Mehta 1989) probably does not play a major role in the rumen.Abbreviations DIDS 4,4'-diisothiocyanatostilbene-2,2'-disulfonic acid - G _t transepithelial conductance (mS·cm^-2) - HSCFA undissociated short-chain fatty acids - J _ms mucosal-to-serosal flux (Eq · cm^-2 · h^-1) - J _net net flux (Eq · cm^-2 · h^-1) - J _sm serosal-to-mucosal flux (Eq · cm^-2 · h^-1) - PD transepithelial potential difference (mV) - SCFA dissociated short-chain fatty acids - SCFA short-chain fatty acids     

Mechanisms of phosphate uptake into brush-border membrane vesicles from goat jejunum

This study concerns the uptake of inorganic phosphate into brush-border membrane vesicles prepared from jejunal tissues of either control or Ca-and/or P-depleted goats. The brush-border membrane vesicles showed a time-dependent accumulation of inorganic phosphate with a typical overshoot phenomenon in the presence of an inwardly directed Na⁺ gradient. The Na⁺-dependent inorganic phosphate uptake was completely inhibited by application of 5 mmol·l^-1 sodium arsenate. Half-maximal stimulation of inorganic phosphate uptake into brush-border membrane vesicles was found with Na⁺ concentrations in the order of 5 mmol·l^-1. Inorganic phosphate accumulation was not affected by a K⁺ diffusion potential (inside negative), suggesting an electroneutral transport process. Stoichiometry suggested an interaction of two or more Na ions with one inorganic phosphate ion at pH 7.4. Na⁺-dependent inorganic phosphate uptake into jejunal brush-border membrane vesicles from normal goats as a function of inorganic phosphate concentration showed typical Michaelis-Menten kinetic with V _max=0.42±0.08 nmol·mg^-1 protein per 15 s^-1 and K _m=0.03±0.01 mmol·l^-1 (n=4, x ±SEM). Long-term P depletion had no effect on these kinetic parameters. Increased plasma calcitriol concentrations in Ca-depleted goats, however, were associated with significant increases of V _max by 35–80%, irrespective of the level of P intake. In the presence of an inwardly directed Na⁺ gradient inorganic phosphate uptake was significantly stimulated by almost 60% when the external pH was decreased to 5.4 (pH_out/pH_in=5.4/7.4). The proton gradient had no effect on inorganic phosphate uptake in absence of Na⁺. In summary, in goats Na⁺ and calcitriol-dependent mechanisms are involved in inorganic phosphate transport into jejunal brush-border membrane vesicles which can be stimulated by protons.Abbreviations AP activity of alkaline phosphatase - BBMV brush-border membrane vesicles - EGTA ethyleneglycol-triacetic acid - n _app apparent Hill coefficient - P _i inorganic phosphate - PTH parathyroid hormone