NH3 Transport Research Articles

Carbonic anhydrase facilitates CO2 and NH3 transport across the sarcolemma of trout white muscle

An isolated, perfused tail preparation was used to study the role of carbonic anhydrase (CA) in CO2 and NH3 transport across the sarcolemma of white muscle in the rainbow trout. Tissue was perfused with either control saline or saline containing the CA inhibitors quaternary ammonium sulfanilamide (QAS) or acetazolamide (Az). Inhibition of extracellular CA by QAS reduced CO2 efflux by approximately 30% and caused a significant increase in intracellular PCO2. Inhibition of total muscle CA activity (extracellular and intracellular) by Az also caused a reduction in CO2 efflux, but selective inhibition of intracellular CA only had no effect. Inhibition of both extracellular and intracellular CA activity resulted in increases in total intracellular ammonia concentrations, intracellular NH3 partial pressure (PNH3) and an increased PNH3 gradient across the sarcolemma. This suggests that both extracellular and intracellular CA function in normal NH3 transport out of the muscle. We suggest that CA in the extracellular boundary layer facilitates CO2 transport via the catalyzed hydration of CO2, thus maintaining the PCO2 gradient across the sarcolemma. H ions produced by that reaction serve to protonate excreted NH3, which helps maintain the PNH3 gradient. Thus CO2 and NH3 excretion are linked by the action of CA.

Ammonia Flux between Oilseed Rape Plants and the Atmosphere in Response to Changes in Leaf Temperature, Light Intensity, and Air Humidity (Interactions with Leaf Conductance and Apoplastic NH4+ and H+ Concentrations).

NH3 exchange between oilseed rape (Brassica napus) plants and the atmosphere was examined at realistic ambient NH3 levels under controlled environmental conditions. Different leaf conductances to NH3 diffusion were obtained by changing leaf temperature (10 to 40[deg]C), light intensity (0 to 600 [mu]mol m-2 s-1), and air humidity (20 to 80%), respectively. NH3 adsorption to the cuticle with subsequent NH3 transport through the epidermis had no significant effect on the uptake of atmospheric NH3, even at 80% relative air humidity. NH3 fluxes increased linearly with leaf conductance when light intensities were increased from 0 to 600 [mu]mol m-2 s-1. Increasing leaf temperatures from 10 to 35[deg]C caused an exponential increase in NH3 emission from plants exposed to low ambient NH3 concentrations, indicating that leaf conductance was not the only factor responding to the temperature increase. The exponential relationship between NH3 emission and temperature was closely matched by the temperature dependence of the mole fraction of gaseous NH3 above the leaf apoplast (NH3 compensation point), as calculated on the basis of NH4+ and H+ concentrations in the leaf apoplast at the different leaf temperatures. NH3 fumigation experiments showed that an increase in leaf temperature may cause a plant to switch from being a strong sink for atmospheric NH3 to being a significant NH3 source. In addition to leaf temperature, the size of the NH3 compensation point depended on plant N status and was related to plant ontogeny.

An apical permeability barrier to NH3/NH4+ in isolated, perfused colonic crypts.

Fermentation of nonabsorbed nutrients in the colon generates high concentrations of NH3/NH4+ in the colonic lumen. NH3 is a small, lipophilic neutral weak base that readily permeates almost all cell membranes, whereas its conjugate weak acid NH4+ generally crosses membranes much more slowly. It is not known how colonocytes maintain intracellular pH in the unusual acid-base environment of the colon, where permeant acid-base products of fermentation exist in high concentration. To address this issue, we hand dissected and perfused single, isolated crypts from rabbit proximal colon, adapting techniques from renal-tubule microperfusion. Crypt perfusion permits control of solutions at the apical (luminal) and basolateral (serosal) surfaces of crypt cells. We assessed apical- vs. basolateral-membrane transport of NH3/NH4+ by using fluorescent dyes and digital imaging to monitor intracellular pH of microvacuolated crypt cells as well as luminal pH. We found that, although the basolateral membranes have normal NH3/NH4+ permeability properties, there is no evidence for transport of either NH3 or NH4+ across the apical borders of these crypt cells. Disaggregating luminal mucus did not increase the transport of NH3/NH4+ across the apical border. We conclude that, compared to the basolateral membrane, the apical border of crypt colonocytes has a very low permeability-area product for NH3/NH4+. This barrier may represent an important adaptation for the survival of crypt cells in the environment of the colon.

Nitrogen Dynamics in Irrigated Corn: Soil‐Plant Nitrogen and Atmospheric Ammonia Transport

AbstractAmmonia transport in cropping systems can affect atmospheric pollution, soil‐plant N relationships, and crop N‐use efficiency. The purpose of this study was to evaluate the N dynamics of a corn (Zea mays L., Pioneer hybrid 3379) cropping system and determine the effect of NH3 transport and crop NH3 compensation point on N relations in the crop. Soil and plant N measurements were made during the growing season along with plant‐atmosphere NH3 transport using flux‐gradient micrometeorological techniques. The crop generally emitted NH3 throughout the season when the atmospheric NH3 concentrations were near background levels. During periods of high atmospheric NH3 concentrations, the crop absorbed significant amounts of NH3 from the atmosphere. Net seasonal NH3 transport was determined to be a small loss, based on the number of measurement periods throughout the growing season; however, N isotope studies in a nearby field estimated significant NH3 losses. Nitrogen isotope studies cannot account for the substitution of 14NH3 absorbed from the atmosphere for volatilized 15NH3. These results indicate that care must be taken when using N isotopes to evaluate N losses from cropping systems when plants are actively growing. Nitrous oxide flux was measured late in the cropping season, but showed small denitrification losses during the measurement period. The crop NH3 compensation point varied during the time of day and with respect to soil‐plant‐atmosphere influence. Net NH3 transport accounted for little of the N lost from this cropping system.

A kinetically defined Na+/H+ antiporter within a mathematical model of the rat proximal tubule.

The luminal membrane antiporter of the proximal tubule has been represented using the kinetic formulation of E. Heinz (1978. Mechanics and Engergetics of Biological Transport. Springer-Verlag, Berlin) with the assumption of equilibrium binding and 1:1 stoichiometry. Competitive binding and transport of NH+4 is included within this model. Ion affinities and permeation velocities were selected in a least-squares fit to the kinetic parameters determined experimentally in renal membrane vesicles (Aronson, P.S., M.A. Suhm, and J. Nee. 1983. Journal of Biological Chemistry. 258:6767-6771). The modifier role of internal H+ to enhance transport beyond the expected kinetics (Aronson, P.S., J. Nee, and M. A. Suhm. 1982. Nature. 299:161-163) is represented as a velocity effect of H+ binding to a single site. This kinetic formulation of the Na+/H+ antiporter was incorporated within a model of the rat proximal tubule (Weinstein, A. M. 1994. American Journal of Physiology. 267:F237-F248) as a replacement for the representation by linear nonequilibrium thermodynamics (NET). The membrane density of the antiporter was selected to yield agreement with the rate of tubular Na+ reabsorption. Simulation of 0.5 cm of tubule predicts that the activity of the Na+/H+ antiporter is the most important force for active secretion of ammonia. Model calculations of metabolic acid-base disturbances are performed and comparison is made among antiporter representations (kinetic model, kinetic model without internal modifier, and NET formulation). It is found that the ability to sharply turn off Na+/H+ exchange in cellular alkalosis substantially eliminates the cell volume increase associated with high HCO3- conditions. In the tubule model, diminished Na+/H+ exchange in alkalosis blunts the axial decrease in luminal HCO3- and thus diminishes paracellular reabsorption of Cl-. In this way, the kinetics of the Na+/H+ antiporter could act to enhance distal delivery of Na+, Cl-, and HCO3- in acute metabolic alkalosis.

Quantification of ammonium sorption in acid forest soils by sorption isotherms

Quantity/Intensity relations of NH4 + were established to evaluate the role of NH4 + sorption as a N-sink as well as for the transport of NH4 + within the soil profile. Two haplic podzols (originating from Phyllite and Granite) from the German Fichtelgebirge were used. Exchange isotherms were obtained from batch experiments with NH4 + concentrations between 0.01 to 0.6 mmol NH4 + L-1. Background solutions had a composition close to the natural soil solutions to identify the cations displaced by NH4 +. Organic layers sorbed a maximum amount of 0.3–0.5 mmolc NH4 + 100 g-1 A horizons 0.1–0.2 mmolc NH4 + 100 g-1 and B horizons 0.09 mmolc 100 g-1, corresponding to less than 1% of the cation exchange capacities. NH4 + displaced K+ and Ca2+ throughout the whole profile, and additionally Al in the subsoil horizons and H3O+ in the organic and A horizons. “Initial mass isotherms” (Quantity/Quantity isotherms) were established with distribution coefficients ranging from 0.17 to 0.35 for NH4 + with highest values in the O horizons. While exchangeable NH4 + in acid forest soils is considered to be only a minor long-term sink for deposited NH4 +, it has its relevance in the seasonal dynamics and transport of NH4 +.

The mass accommodation coefficient of ammonia on water

The mass accommodation coefficient γ of ammonia gas on water has been determined by measuring the absorption rate of 50–200 ppm NH3 in one atm of air or helium into a liquid jet of 97 µm diameter as function of the exposed jet length, and comparing the results with numerical simulations which treat γ as the only free parameter. The model considers in detail transport of NH3 by molecular diffusion, penetration of the gas/water interface, hydrolysis in the acidified water, and transport of the solutes from the surface into the jet. A correction is applied for the time evolution of the jet surface speed, using literature data on the fluid mechanics of liquid jets. The result of nine sets of independent measurements is $$\gamma NH_3 = 0.04_{ - 0.005 \cdot }^{ + 0.03} $$ A sensitivityS=∂Caq/∂ log γ is defined for mass accommodation measurements by the jet technique. It may be utilized to optimize experimental conditions, and to assign error ranges to measured values of γ. The atmospheric implications of γNH3=0.04 are briefly discussed.

Transport distance of ammonia and ammonium in Northern Europe: 1. Model description

A Lagrangian model for the emission, transport, transformation, and deposition of NH3 and species derived from NH3 emissions is described. This model also involves SO2 and NOx, since the chemistries of NH3, NOx, and SO2 are interlinked. The model is evaluated by comparing it with measurements of sulfur and nitrogen compounds in air and precipitation during an intensive 3‐month period in August–October 1985 at six Scandinavian European Monitoring and Evaluation Programme sites. The concentrations of NO2 are underestimated at all sites except one, and they almost follow the curve for a 1:2 ratio when they are compared to the measured values. This is a consistent feature which is not strongly linked to meteorological variability or emission changes but rather is caused by the chemical formulation of the model and its physical assumptions. For nitrate aerosol there is a systematic overestimation in the model, while for the sum of ammonia and ammonium aerosol the observed average concentration is twice the calculated one, with particularly large discrepancies where there are local ammonia sources. The average of all observations of SO2 is matched by the calculations, but the regression line has a wrong slope and a nonzero intercept. For sulfate aerosol the calculations give concentrations which are about 50% of the measured values, while for nitrate, ammonium, and sulfate in precipitation, the correlations between observations and calculated values are lower than those for the airborne species. This difference arises in part from the differences between precipitation in the model and as measured at the sites.

Ammonia and ammonium fluxes between photolithotrophs and the environment in relation to the global nitrogen cycle

summaryAmmonium (used here to signify NH4+ plus NH3) is the immediate inorganic precursor of organic nitrogen in photolithotrophs. In marine habitats ammonium is also the major exogenous nitrogen source, and the same is probably true of terrestrial habitats. In addition to this major role of ammonium as exogenous nitrogen source, it is also quantitatively very important as an endogenous nitrogen source with nitrate or N2 as exogenous nitrogen source, and as a recycled nitrogen compound in photorespiration and in phenylpropanoid synthesis. The shoots of terrestrial plants have higher NH3 compensation partial pressures than most natural soils, and especially higher than that of the sea. However, long‐distance transport of gaseous NH3 (e.g. from continents to oceans) is a negligible component of the ‘natural’ global nitrogen cycle. Current concepts of evolution of the atmosphere and biosphere do not involve high partial pressure of NH3 in the atmosphere: any ammonium produced in inorganic or biological processes is removed from the atmosphere by rain‐out, or, over larger time‐scales, by the action of ultraviolet radiation (especially before the ozone screen came into being) and of hydroxyl radicals (especially after oxygenation of the atmosphere). In addition to posing problems for the origin of life and arguing for an early rather than a late evolution of diazotrophy, a low NH3 partial pressure renders implausible arguments that phenylpropanoid synthesis was restricted early in evolution by the effect of high ammonia in reversing phenylalanine ammonia‐lyase.

Mechanisms of ammonium excretion: role of the renal medulla.

E XCRETION of ammonium* by the kidney plays a central role in the regulation of systemic acid-base balance. Ammonium excreted in the urine is derived almost entirely from its renal synthesis, with the bulk of ammonium production occurring in the cortex in cells of the proximal tubules. The biochemical reactions involved in proximal tubule ammonium production, and the factors controlling proximal tubule ammonium production and secretion, have been studied extensively and widely discussed.‘-‘j In contrast, much less attention has been focused on ammonium transport processes in the renal medulla, which channel the ammonium produced and secreted by the proximal tubules to the final urine. Over the past few years, considerable progress has been made in understanding the pathways and mechanisms of ammonium transport in the renal medulla. These studies have shown a complex pattern of ammonium movement within the medulla that includes active transport of NH4+ by the thick ascending limb, concentration of ammonium in the medullary interstitial fluid by countercurrent multiplication, and generation of large NH3 concentration gradients across the medullary collecting ducts. Furthermore, the control of urinary ammonium excretion has been found to depend

Physiological effects of long‐term exposure to low and moderate concentrations of atmospheric NH3 on poplar leaves

Abstract. Poplar shoots (Populus euramericana L.) obtained from cuttings were exposed for 6 or 8 weeks to NH3 concentrations of 50 and 100 μgm−3 or filtered air in fumigation chambers. After this exposure the rates of NH3 uptake, transpiration, CO2 assimilation and respiration of leaves were measured using a leaf chamber. During the long‐term exposure also modulated chlorophyll fluorescence measurements were carried out to obtain information about the photosynthetic performance of individual leaves. Both fluorescence and leaf chamber measurements showed a higher photosynthetic activity of leaves exposed to 100 μg NH3 m−3. These leaves showed also a larger leaf conductance and a larger uptake rate of NH3 than leaves exposed to 50 μg m−3 NH3 or filtered air. The long‐term NH3 exposure did not induce an internal resistance against NH3 transport in the leaf, nor did it affect the leaf cuticle. So, not only at a short time exposure, but also at a long‐term exposure NH3 uptake into leaves can be calculated from data on the boundary layer and stomatal resistance for H2O and ambient NH3‐concentration. Furthermore, the NH3 exposure had no effect on the relation between CO2‐assimilation and stomatal conductance, indicating that NH3 in concentrations up to 100 μg m−3 has no direct effect on stomatal behaviour; for example, by affecting the guard or contiguous cells of the stomata.

Ammonia Transport in the Proximal Tubule In Vivo

Studies were performed to characterize the determinants of proximal tubule ammonia entry (and retention) in vivo. Rat proximal tubules were studied in vivo using in situ microperfusion. In both normal animals and animals with metabolic acidosis, increasing luminal flow rate significantly enhanced luminal ammonia entry. In contrast, luminal pH was not as important in determining ammonia entry. Analysis of the levels of luminal NH3 in these studies was not consistent with simple diffusion equilibrium of NH3. In animals with chronic metabolic acidosis, additional studies demonstrated that inhibition of the Na+-H+ exchanger had no direct effect on luminal ammonia entry. However, studies of ammonia efflux from tubules perfused with 10 mmol/L ammonia demonstrated significant transport of both NH3 and NH4+. Studies of luminal glutamine deamidation via gamma-glutamyltransferase in control conditions did not indicate a significant role for luminal ammoniagenesis in the superficial proximal tubule in vivo. These and other recent studies of proximal tubule ammonia transport significantly modify the traditional diffusion equilibrium (of NH3) model of ammonia transport. Luminal flow rate is an important determinant of luminal ammonia entry. Transport of NH4+, both into and out of the tubule lumen, represents a major component of total ammonia transport.

Environmental Effects on Nitrogen Dynamics in Soybean under Conservation and Clean Tillage Systems

AbstractSoybean [Glycine max (L.) Merr.) obtains required N from the soil via root uptake of mineralized organic matter and fertilizer, and from the air by symbiotic gaseous N2 fixation and ammonia (NH3) absorption. The purpose of this study was to evaluate environmental and tillage effects on dynamic N transport under conservation (NT) and clean tillage (CT) management systems. Soil, plant, and microclimate measurements were made concurrently with estimates of gaseous N2 (using 15N2 techniques) and NH3 uptake from the atmosphere. Soybean growth and N uptake characteristics were similar in both tillage systems, except during a 14‐d drought period, which caused water stress in the CT system, but not the NT system. Nitrogen fixation decreased in the CT system to less than half that of the NT system fixation rate, resulting in a seasonal reduction of 29% in the CT system. Net NH3 transport varied from net absorption by the canopy during periods of adequate soil water to net evolution by the CT system during the drought‐stress period. Integration of the net NH3 flux density curves indicated that approximately 3% of the NT system and less than 1% of the CT system plant N was derived from the atmosphere. Also during the drought‐stress period, there was a net loss of plant N to the soil in the CT system indicated by soil and plant mass balance. Insect infestation was much greater in the NT system, reflecting insects' preference for a higher N crop, resulting in a significant N removal from the system. This study showed that soybean in these systems decreased soil N by about the amount of harvested N yield.

METABOLIC REGULATION OF AMMONIUM UPTAKE BY ULVA RIGIDA (CHLOROPHYTA): A COMPARTMENTAL ANALYSIS OF THE RATE-LIMITING STEP FOR UPTAKE

Non-linear time courses of ammonium (NH4+) depletion from the medium and internal accumulation of soluble nitrogen (N) in macroalgae imply that the rate-limiting step for ammonium uptake changes over time. We tested this hypothesis by measuring the time course of N accumulation in N-limited Ulva rigida C. Agardh. Total uptake was measured as removal of NH4+ from medium. Rates for the component processes (transport of NH4+ across the membrane = Rv assimilation of tissure NH4+ into soluble N compounds = Ra, assimilation of tissue NH4+ into soluble N compounds = Ra and incorporation of soluble N compounds into macromolecules = R1) were determined by measuring the rate of labelling of the major tissue N pools after the addition of 15N-ammonium. The results indicate that nitrogen-specific rates (mass N taken up / mass N present / unit time) are ranked in the order of Rt < Ra < R1 Absolute uptake rates (μmol N. mg dry wt−1. h−1) showed a different relationship. Membrane transport appears to be inhibited when NH4+ accumulates in the tissue. Maximum uptake rates occur when assimilation of NH4+ into soluble N compounds begins. Assimilation of NH4+ into soluble N compounds was initially faster than incorporation of soluble N compounds into macromolecules. Implications of rate limitations caused by differences in maximal rates and maximal pool sizes are discussed.

Processes of nitrogen loss from fertilizers applied to flooded rice fields on a calcareous soil in north-central China

Losses of nitrogen were investigated after applications of ammonium bicarbonate and urea to flooded rice at transplanting. Ammonia (NH3) volatilization was determined by direct micrometeorological methods, and total loss of fertilizer nitrogen (N) was measured by15N balance. All the loss appeared to be in gaseous forms, since there was no evidence of leaching and runoff was prevented. The difference between N loss and NH3 loss was thus assumed to be denitrification loss. Both NH3 volatilization and denitrification losses were large, being 39% and 33%, respectively, of the ammonium bicarbonate N, and 30% and 33%, respectively, of the urea N applied by farmers' methods. Ammonia fluxes from the field fertilized with ammonium bicarbonate were very high for two days, and then declined rapidly as the NH3 source in the floodwater diminished. Moderate fluxes from the field fertilized with urea continued over 6 days, but calculations showed that NH3 transfer from floodwater to atmosphere was retarded during the middle period of the experiment, particularly on day 2 when a thick algal scum appeared on the water surface. The results indicate that this algal mass obstructed the transport of NH3 across the water-air interface until the scum was dispersed by wind action. Nevertheless, the prolonged NH3 losses on the urea treatment were due primarily to high floodwater pH values promoted by the strong algal growth during the daylight hours. Nitrogen-15 balance studies showed that incorporation of fertilizer into drained soil substantially increased recoveries of fertilizer N in rice plants and soil compared with incorporation of fertilizer in the presence of standing floodwater. Ammonia loss measurements on these treatments when urea was applied suggested that the improvement in fertilizer N efficiency was due mainly to reductions in NH3 loss.

Ammonia entry along rat proximal tubule in vivo: effects of luminal pH and flow rate.

The roles of luminal pH and flow rate in determining ammonia entry along the rat proximal tubule were examined using in vivo microperfusion. With perfusion rate constant at 15 nl/min, perfusate bicarbonate concentration was varied. Collected fluid ammonia concentration correlated with collected fluid bicarbonate concentration, consistent with nonionic diffusion (r = 0.726; P less than 0.001). Hence ammonia entry was dependent on luminal pH. With perfusate bicarbonate constant at 5 or 25 mM, perfusion rate was varied. In all groups, there was little change in collected fluid ammonia concentration with flow rate. Thus ammonia entry was also highly dependent on flow rate. With paired collections using a 25 mM bicarbonate perfusate, collected fluid bicarbonate was higher at a 30 nl/min perfusion rate than at 15 nl/min (16.8 +/- 1.1 vs. 10.3 +/- 1.1 mM), whereas total ammonia concentrations were similar (0.54 +/- 0.1 and 0.55 +/- 0.1). Thus the NH3 concentration was higher at 30 than at 15 nl/min (6.1 +/- 1.2 vs. 3.4 +/- 0.5 microM; P less than 0.025), a result not predicted by simple nonionic diffusion. Thus these studies demonstrate the importance of nonionic diffusion in determining ammonia entry along the proximal tubule. However, the results also demonstrate that flow rate importantly determines ammonia entry in vivo in a manner not predicted by simple nonionic diffusion of NH3. This augmentation of ammonia entry with increasing flow rate may involve flow-dependent alterations in ammonia synthesis or transport of NH+4.

Ammonium and methylammonium transport in Rhodobacter sphaeroides.

Rhodobacter sphaeroides maintained intracellular ammonium pools of 1.1 to 2.6 mM during growth in several fixed nitrogen sources as well as during diazotrophic growth. Addition of 0.15 mM NH4+ to washed, nitrogen-free cell suspensions was followed by linear uptake of NH4+ from the medium and transient formation of intracellular pools of 0.9 to 1.5 mM NH4+. Transport of NH4+ was shown to be independent of assimilation by glutamine synthetase because intracellular pools of over 1 mM represented NH4+ concentration gradients of at least 100-fold across the cytoplasmic membrane. Ammonium pools of over 1 mM were also found in non-growing cell suspensions in nitrogen-free medium after glutamine synthetase was inhibited with methionine sulfoximine. In NH4+-free cell suspensions, methylammonium (14CH3NH3+) was taken up rapidly, and intracellular concentrations of 0.4 to 0.5 mM were maintained. The 14CH3NH3+ pool was not affected by methionine sulfoximine. Unlike NH4+ uptake, 14CH3NH3+ uptake in nitrogen-free cell suspensions was repressed by growth in NH4+. These results suggest that R. sphaeroides may produce an NH4+-specific transport system in addition to the NH4+/14CH3NH3+ transporter. This second transporter is able to produce normal-size NH4+ pools but has very little affinity for 14CH3NH3+ and is not repressed by growth in high concentrations of NH4+.

Transepithelial ammonia concentration gradients in inner medulla of the rat

Transport of NH3 from loops of Henle to medullary collecting ducts has been proposed to play an important role in renal ammonia excretion. To determine whether transepithelial ammonia concentration gradients capable of driving this transport are present in the inner medulla, micropuncture experiments were performed in control rats and in rats with chronic metabolic acidosis. In situ pH and total ammonia concentrations were measured to calculate NH3 concentrations ([NH3]) for base and tip collecting duct, loop of Henle, and vasa recta. In control and acidotic rats, [NH3] in the loop of Henle was significantly greater than [NH3] in the collecting ducts. [NH3] did not differ in loop of Henle and adjacent vasa recta in either group of rats, indicating that NH3 concentration gradients between loop and collecting duct represent NH3 gradients that are present between medullary interstitium and collecting duct. During acidosis, an increase in collecting duct ammonia secretion was associated with an increase in the NH3 concentration difference between loop of Henle and collecting duct but occurred in the absence of a fall in collecting duct pH. The NH3 concentration gradient favoring diffusion of NH3 into the collecting ducts increased during acidosis because [NH3] in the loop of Henle and medullary interstitium increased more than [NH3] in the collecting duct. These findings indicate that transport processes involved in medullary ammonia accumulation play an important role in regulating ammonia secretion into the inner medullary collecting duct in vivo and that a fall in inner medullary collecting duct pH is not necessarily required for ammonia secretion by this segment to increase during chronic metabolic acidosis.

Atmospheric ammonium sulphate deposition and its role in the acidification and nitrogen enrichment of poorly buffered aquatic systems

The atmospheric transport of NH3, emitted from breeding farms and strongly manured cropland, causes a severe loading of neighboring non-agricultural environments. Particularly the wet deposition of ammonium sulphate is enhanced, while the pH of rainwater is only slightly acidic. Due to the variation of gaseous air pollutants over the Netherlands large differences are noticed in the relative contribution of each acidifying component in rainwater. In areas with high atmospheric NH3 concentrations, ammonium sulphate contributes the major part to the potential acidifying influence of wet deposition. Preliminary experiments show a causal relation between ammonium sulphate and water acidification, ammonium enrichment and the expansion of ammonium tolerant shore macrophytes. For the Dutch situation it is clear that, particularly in areas with enhanced atmospheric NH3 concentrations, the deposition of NH3 and SO2 derived compounds has provided the major contribution to acidification and N-enrichment of hydrologically isolated, poorly buffered pools. In all countries, particularly in areas with high NH3 emission, it will be useful to pay more attention to the environmental impact of airborne ammonium sulphate deposition.

Ammonia transport by the turtle urinary bladder.

Ammonia transport across the turtle bladder was examined by adding NH4Cl to the serosal (S) or mucosal (M) solution. With appropriately fixed levels of pH and/or NH4Cl concentration the transepithelial flow of ammonia parallels the extracellular concentration of NH+4 while that of NH3 is kept constant and parallels the extracellular concentration of NH3 while that of NH+4 is kept constant. This suggests that NH+4 as well as NH3 traverses the bladder wall. The apparent S----M permeability to NH3 was 15-18 times greater than that to NH+4. At pH 6.4 in both S and M solutions, the net flow of ammonia was from S----M, but at pH 8.4 in the S and 6.4 in the M or vice versa ammonia transport was of the same magnitude in both directions. The relative permeability of the M membrane to NH+4 was less than that to Na and nearly the same as that to K. The relative permeability of the S membrane to NH+4 was greater than that to K. At S pH of 8.4 and M pH of 6.4, ammonia transport was a linear function of NH4Cl concentration. At S pH of 6.4 and M pH of 6.4, ammonia transport was a saturation function of NH4Cl concentration in that it was linear up to 5 mM and constant and maximal in excess of 7.5 mM. The net transport of methylamine directed from S to M was competitively inhibited by NH4Cl, suggesting that the two substances are transported through a common carrier system. At pH 6.4 in both S and M, the S addition of NH4Cl induced an increase in reverse short-circuit current, the magnitude of which approximated the chemically determined rate of ammonia transport. This means that ammonia transport at pH 6.4 is, at least in part, electrogenic due to the flow of ionic NH+4 through a transbladder conductive path. However, when the S pH was raised to 8.4 the increase in ammonia transport was not associated with an increase in current. The present study demonstrates that the turtle bladder is capable of transporting ammonia with different characteristics of NH+4 and NH3 transport.