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1.
Alex M. Zimmer C. Michele Nawata Chris M. Wood 《Journal of comparative physiology. B, Biochemical, systemic, and environmental physiology》2010,180(8):1191-1204
Recently, a “Na+/NH4
+ exchange complex” model has been proposed for ammonia excretion in freshwater fish. The model suggests that ammonia transport
occurs via Rhesus (Rh) glycoproteins and is facilitated by gill boundary layer acidification attributable to the hydration
of CO2 and H+ efflux by Na+/H+ exchanger (NHE-2) and H+-ATPase. The latter two mechanisms of boundary layer acidification would occur in conjunction with Na+ influx (through a Na+ channel energized by H+-ATPase and directly via NHE-2). Here, we show that natural ammonia loading via feeding increases branchial mRNA expression
of Rh genes, NHE-2, and H+-ATPase, as well as H+-ATPase activity in juvenile trout, similar to previous findings with ammonium salt infusions and high environmental ammonia
(HEA) exposure. The associated increase in ammonia excretion occurs in conjunction with a fourfold increase in Na+ influx after a meal. When exposed to HEA (1.5 mmol/l NH4HCO3 at pH 8.0), both unfed and fed trout showed differential increases in mRNA expression of Rhcg2, NHE-2, and H+-ATPase, but H+-ATPase activity remained at control levels. Unfed fish exposed to HEA displayed a characteristic reversal of ammonia excretion,
initially uptaking ammonia, whereas fed fish (4 h after the meal) did not show this reversal, being able to immediately excrete
ammonia against the gradient imposed by HEA. Exposure to HEA also led to a depression of Na+ influx, demonstrating that ammonia excretion can be uncoupled from Na+ influx. We suggest that the efflux of H+, rather than Na+ influx itself, is critical to the facilitation of ammonia excretion. 相似文献
2.
Gregg A. Kormanik James N. Cameron 《Journal of comparative physiology. B, Biochemical, systemic, and environmental physiology》1981,141(4):457-462
Summary A series of experiments was conducted to investigate whether ammonia is excreted across the seawater-acclimated blue crab's gills as ionized NH
4
+
or as the free base, NH3. The net excretion rate of ammonia was not changed by transfer of the crabs to reduced (150 mM) Na+ solutions, by transfer to Na+- and K+-free artificial sea water, or by the sodium transport inhibitor amiloride. Ammonia excretion, therefore, does not appear to be linked to Na+ uptake in these animals, and appears to take place by passive diffusion. Since ammonia could diffuse either as NH
4
+
or NH3, we examined two other kinds of evidence. The trans-epithelial potential was measured in sea water and the various artificial media. In spite of a 10 mV more negative potential in Na+-, K+-free medium, the ammonia excretion was not reduced. Also, in alkalinized seawater in which the partial pressure gradient of NH3 was reduced, but the concentration gradient of NH
4
+
increased, ammonia excretion was reduced by about 70%. These results are consistent with the conclusion that ammonia excretion takes place by diffusion of the free base, NH3.Abbreviations
SW
sea water
-
ASW
artificial sea water
-
t.e.p.
transepithelial potential
The University of Texas Marine Science Institute Contribution No. 461Supported by NSF Grant PCM77-24358 相似文献
3.
J. Moreira-Silva T.K.N. Tsui J. Coimbra M.M. Vijayan Y.K. Ip J.M. Wilson 《Comparative biochemistry and physiology. Toxicology & pharmacology : CBP》2010,151(1):40-50
The weatherloach, Misgurnus anguillicaudatus, is a freshwater, facultative air-breathing fish that lives in streams and rice paddy fields, where it may experience drought and/or high environmental ammonia (HEA) conditions. The aim of this study was to determine what roles branchial Na+/K+-ATPase, H+-ATPase, and Rhcg have in ammonia tolerance and how the weatherloach copes with ammonia loading conditions. The loach's high ammonia tolerance was confirmed as was evident from its high 96 h LC50 value and high tissue tolerance to ammonia. The weatherloach does not appear to make use of Na+/NH4+-ATPase facilitated transport to excrete ammonia when exposed to HEA or to high environmental pH since no changes in activity were observed. Using immunofluorescence microscopy, distinct populations of vacuolar (V)-type H+-ATPase and Na+/K+-ATPase immunoreactive cells were identified in branchial epithelia, with apical and basolateral staining patterns, respectively. Rhesus C glycoprotein (Rhcg1), an ammonia transport protein, immunoreactivity was also found in a similar pattern as H+-ATPase. Rhcg1 (Slc42a3) mRNA expression also increased significantly during aerial exposure, although not significantly under ammonia loading conditions. The colocalization of H+-ATPase and Rhcg1 to the similar non-Na+/K+-ATPase immunoreactive cell type would support a role for H+-ATPase in ammonia excretion via Rhcg by NH4+ trapping. The importance of gill boundary layer acidification in net ammonia excretion was confirmed in this fish; however, it was not associated with an increase in H+-ATPase expression, since tissue activity and protein levels did not increase with high environmental pH and/or HEA. However the V-ATPase inhibitor, bafilomycin, did decrease net ammonia flux whereas other ion transport inhibitors (amiloride, SITS) had no effect. H+-ATPase inhibition also resulted in a consequent elevation in plasma ammonia levels and a decrease in the net acid flux. In gill, aerial exposure was also associated with a significant increase in membrane fluidity (or increase in permeability) which would presumably enhance NH3 permeation through the plasma membrane. Taken together, these results indicate the gill of the weatherloach is responsive to aerial conditions that would aid ammonia excretion. 相似文献
4.
Active excretion of ammonia across the gills of the shore crab Carcinus maenas and its relation to osmoregulatory ion uptake 总被引:1,自引:0,他引:1
D. Weihrauch W. Becker U. Postel S. Riestenpatt D. Siebers 《Journal of comparative physiology. B, Biochemical, systemic, and environmental physiology》1998,168(5):364-376
The mechanism of transbranchial excretion of total ammonia of brackish-water acclimated shore crabs, Carcinus maenas was examined using isolated, perfused gills. Applying physiological gradients of NH4Cl (100–200 μmol · l−1) directed from the haemolymph space to the bath showed that the efflux of total ammonia consisted of two components. The
saturable component (excretion of NH4
+) greatly exceeded the linear component (diffusion of NH3). When an outwardly directed gradient (200 μmol · l−1) was applied, total ammonia in the perfusate was reduced by more than 50% during a single passage of saline through the gill.
Effluxes of ammonia along the gradient were sensitive to basolateral dinitrophenol, ouabain, and Cs+ and to apical amiloride. Acetazolamide (1 mmol · l−1 basolateral) or Cl−-free conditions had no substantial effects on ammonia flux, which was thus independent of both carbonic anhydrase mediated
pH regulation and osmoregulatory NaCl uptake. When an inwardly directed gradient (200 μmol · l−1) was employed, influx rates were about 10-fold smaller and unaffected by basolateral ouabain (5 mmol · l−1) or dinitrophenol (0.5 mmol · l−1). Under symmetrical conditions (100 μmol · l−1 NH4Cl on both sides) ammonia was actively excreted against the gradient of total ammonia, which increased strongly during the
experiment and against the gradient of the partial pressure of NH3. The active excretion rate was reduced to 7% of controls by basolateral dinitrophenol (0.5 mmol · l−1), to 44% by basolateral ouabain (5 mmol · l−1), to 46% by Na+-free conditions and to 42% by basolateral Cs+ (10 mmol · l−1), indicating basolateral membrane transport of NH4
+ via the Na+/K+-ATPase and K+-channels and a second active, apically located, Na+ independent transport mechanism of NH4
+. Anterior gills, which are less capable of active ion uptake than posterior gills, exhibited even increased rates of active
excretion of ammonia. We conclude that, under physiological conditions, branchial excretion of ammonia is a directed process
with a high degree of effectiveness. It even allows active extrusion against an inwardly directed gradient, if necessary.
Accepted: 11 March 1998 相似文献
5.
Summary Although it is generally believed thatChara and some fresh-water angiosperms transport bicarbonate ions inwards across their plasma membranes, there has been no direct demonstration of such transport in these plants. The (indirect) arguments for their transporting HCO
3
–
are arguments against the inward diffusion of CO2 at the observed rates. They rest on calculations of the equilibrium concentration of CO2 or of the maximum rate at which CO2 might be produced from HCO
3
–
at the pH of the medium outside the cells. SinceChara acidifies the medium over about half the cell surface during C assimilation, these calculations have been based on questionable premises.We propose a model forChara in which the acidification is attributed to active efflux of H+, and we calculate that both the equilibrium concentration of CO2 and its rate of production outside the cell can be high enough to support the observed rates of C assimilation, without postulating transport of the species HCO
3
–
or H2CO3.Calculations are presented also for alternative models in which there is membrane transport of HCO
3
–
. The first includes symport of H+ with HCO
3
–
, again dependent on active H+ efflux. In the second, there is active electrogenic transport of HCO
3
–
. In this case the low pH in the medium outside the cell is caused by the dissociation of H2CO3 produced by hydration of CO2 which leaks from the cell cytoplasm.All three models are consistent with the observations to date, but the first is more economical of postulates. It can also explain the apparent transport of HCO
3
–
by fresh-water angiosperms such asEgeria. 相似文献
6.
D. Weihrauch W. Becker U. Postel S. Luck-Kopp D. Siebers 《Journal of comparative physiology. B, Biochemical, systemic, and environmental physiology》1999,169(1):25-37
Isolated perfused gills of stenohaline crabs Cancer pagurus adapted to seawater, brackish water-adapted euryhaline shore crabs Carcinus maenas and freshwater-adapted extremely euryhaline Chinese crabs Eriocheir sinensis were tested for their capacity to excrete ammonia. Gills were perfused with haemolymph-like salines and bathed with salines
equal in adaptation osmolality. Applying 100 μmol · l−1 NH4Cl in the perfusion saline and concentrations of NH4Cl in the bath that were stepwise increased from 0 to 4000 μmol · l−1 allowed us to measure transbranchial fluxes of ammonia along an outwardly as well as various inwardly directed gradients.
The gills of all three crab species were capable – to different extents – of active excretion of ammonia against an inwardly
directed gradient. Of the three crab species, the gills of Cancer pagurus revealed the highest capacity for active excretion of ammonia, being able to excrete it from the haemolymph (100 μmol · l−1 NH+
4) through the gill epithelium against ambient concentrations of up to 800 μmol · l−1, i.e. against an eightfold gradient. Carcinus maenas and E. sinensis were able to actively excrete ammonia against approximately fourfold gradients. Within the three crab species, the gills
of E. sinensis exhibited the greatest capacity to resist influx at very high external concentrations of up to 4000 μmol · l−1. We consider the observed capacities for excretion of ammonia against the gradient as ecologically meaningful. These benthic
crustaceans protect themselves by burying themselves in the sediment, where, in contrast to the water column, concentrations
of ammonia have previously been reported that greatly increase haemolymph levels. Electrophysiological results indicate that
the permeabilities of the gill epithelia are a clue to understanding the species-specific differences in active excretion
of ammonia. During the invasion of brackish water and freshwater, the permeabilities of the body surfaces greatly decreased.
The gills of marine Cancer pagurus exibited the greatest permeability (ca. 250 mS cm−2), thus representing practically no influx barrier for ions including NH+
4. We therefore assume that C. pagurus had to develop the strongest mechanism of active excretion of ammonia to counteract influx. On the other hand, freshwater-adapted
E. sinensis exhibited the lowest ion permeability (ca. 4 mS cm−2) which may reduce passive NH+
4 influxes at high ambient levels.
Accepted: 14 October 1998 相似文献
7.
Loong AM Chew SF Wong WP Lam SH Ip YK 《Journal of comparative physiology. B, Biochemical, systemic, and environmental physiology》2012,182(4):491-506
The freshwater climbing perch, Anabas testudineus, is an obligatory air-breathing teleost which can acclimate to seawater, survive long period of emersion, and actively excrete
ammonia against high concentrations of environmental ammonia. This study aimed to clone and sequence the Na+:K+:2Cl− cotransporter (nkcc) from the gills of A. testudineus, and to determine the effects of seawater acclimation or exposure to 100 mmol l−1 NH4Cl in freshwater on its branchial mRNA expression. The complete coding cDNA sequence of nkcc from the gills of A. testudineus consisted of 3,495 bp, which was translated into a protein with 1,165 amino acid residues and an estimated molecular mass
of 127.4 kDa. A phylogenetic analysis revealed that the translated Nkcc of A. testudineus was closer to fish Nkcc1a than to fish Nkcc1b or Nkcc2. After a progressive increase in salinity, there were significant
increases in the mRNA expression and protein abundance of nkcc1a in the gills of fish acclimated to seawater as compared with that of the freshwater control. Hence, it can be concluded that
similar to marine teleosts, Cl− excretion through basolateral Nkcc1 of mitochondrion-rich cells (MRCs) was essential to seawater acclimation in A. testudineus. Exposure of A. testudineus to 100 mmol l−1 NH4Cl for 1 or 6 days also resulted in significant increases in the mRNA expression of nkcc1a in the gills, indicating a functional role of Nkcc1a in active ammonia excretion. It is probable that NH4
+ enter MRCs through basolateral Nkcc1a before being actively transported across the apical membrane. Since the operation of
Nkcc1a would lead to an increase in the intracellular Na+ concentration, it can be deduced that an upregulation of basolateral Na+/K+-ATPase (Nka) activity would be necessary to compensate for the increased influx of Na+ into MRCs during active NH4
+ excretion. This would imply that the main function of Nka in active NH4
+ excretion is to maintain intracellular Na+ and K+ homeostasis instead of transporting NH4
+ directly into MRCs as proposed previously. In conclusion, active salt secretion during seawater acclimation and active NH4
+ excretion during exposure to ammonia in freshwater could involve similar transport mechanisms in the gills of A. testudineus. 相似文献
8.
The hypothesis that ambient CO2 levels determine the end-products of energy metabolism excreted by Hymenolepis diminuta was tested by incubating the parasite in a range of CO2 concentrations and measuring internal concentrations of adenine nucleotides and the excretion of organic acids. The strain of H. diminuta used was found to excrete mainly lactic acid and acetic acid. Succinic acid production was generally less than 5–10% of the total. At high CO2 concentrations, the rate of excretion of lactic acid decreased while that of succinic acid increased, which conforms with the hypothesis. Acetic acid excretion did not vary significantly over the range of CO2 concentrations used. Other results did not support the hypothesis. High CO2 levels reduced the total amounts of acids excreted and the rate of succinic acid excretion was so small as to be ineffective in preventing the accumulation of H+ ions. When present in the incubation medium, succinic acid was taken up by H. diminuta. Lactic and acetic acid excretion was always sufficient to limit the accumulation of H+ ions. The conditions of incubation were shown not to be responsible for the low rates of succinic acid excreted. Incubation conditions and metabolic end-products were found to affect the rates of excretion of organic acids. There is thus a need, in work of this nature, to regulate and specify experimental conditions and to stipulate the strain of parasite used. The hypothesis was rejected and it was suggested that the energy metabolism of parasitic helminths is adapted to fluctuating O2 and CO2 tensions. 相似文献
9.
10.
Kevin V. Brix Chris M. Wood Martin Grosell 《Comparative biochemistry and physiology. Part A, Molecular & integrative physiology》2013,164(1):221-228
In this study, Na+ uptake and acid–base balance in the euryhaline pupfish Cyprinodon variegatus variegatus were characterized when fish were exposed to pH 4.5 freshwater (7 mM Na+). Similar to the related cyprinodont, Fundulus heteroclitus, Na+ uptake was significantly inhibited when exposed to low pH water. However, it initially appeared that C. v. variegatus increased apparent net acid excretion at low pH relative to circumneutral pH. This result is opposite to previous observations for F. heteroclitus under similar conditions where fish were observed to switch from apparent net H+ excretion at circumneutral pH to apparent net H+ uptake at low pH. Further investigation revealed disparate observations between these studies were the result of using double endpoint titrations to measure titratable alkalinity fluxes in the current study, while the earlier study utilized single endpoint titrations to measure these fluxes (i.e.,. Cyprinodon acid–base transport is qualitatively similar to Fundulus when characterized using single endpoint titrations). This led to a comparative investigation of these two methods. We hypothesized that either the single endpoint methodology was being influenced by a change in the buffer capacity of the water (e.g., mucus being released by the fish) at low pH, or the double endpoint methodology was not properly accounting for ammonia flux by the fish. A series of follow-up experiments indicated that buffer capacity of the water did not change significantly, that excretion of protein (a surrogate for mucus) was actually reduced at low pH, and that the double endpoint methodology does not properly account for NH3 excretion by fish under low pH conditions. As a result, it overestimates net H+ excretion during low pH exposure. After applying the maximum possible correction for this error (i.e., assuming that all ammonia is excreted as NH3), the double endpoint methodology indicates that net H+ transport was reduced to effectively zero in both species at pH 4.5. However, significant differences between the double endpoint (no net H+ transport at low pH) and single endpoint titrations (net H+ uptake at low pH) remain to be explained. 相似文献
11.
This paper summarizes investigations on the enzyme carbonic anhydrase (CA) in the gills of the osmoregulating shore crabCarcinus maenas. Carbonic anhydrase, an enzyme catalyzing the reversible hydration of CO2 to HCO3
− and H+, is localized with highest activities in the posterior salt-transporting gills of the shore crab- and here CA activity is
strongly dependent on salinity. Contrary to the earlier hypothesis established for the blue crabCallinectes sapidus that cytoplasmic branchial CA provides the counter ions HCO3
− and H+ for apical exchange against Na+ and Cl−, the involvement of CA in NaCl uptake mechanisms can be excluded inCarcinus. Differential and density gradient centrifugations indicate that branchial CA is a predominantly membrane-associated protein.
Branchial CA was greatly inhibited by the sulfonamide acetazolamide (AZ) Ki=2.4·10−8 mol/l). Using the preparation of the isolated perfused gill, application of 10−4 mol/l AZ resulted in an 80% decrease of CO2/HCO3
− excretion. Thus we conclude that CA is localized in plasma membranes, maintaining the CO2 gradient by accelerating adjustment of the pH-dependent CO2/HCO3
− equilibrium. 相似文献
12.
Yung-Che Tseng Jia-Jiun Yan Fumiya Furukawa Pung-Pung Hwang 《BioEssays : news and reviews in molecular, cellular and developmental biology》2020,42(5):1900161
How vertebrates evolved different traits for acid excretion to maintain body fluid pH homeostasis is largely unknown. The evolution of Na+/H+ exchanger (NHE)-mediated NH4+ excretion in fishes is reported, and the coevolution with increased ammoniagenesis and accompanying gluconeogenesis is speculated to benefit vertebrates in terms of both internal homeostasis and energy metabolism response to acidic stress. The findings provide new insights into our understanding of the possible adaptation of fishes to progressing global environmental acidification. In human kidney, titratable H+ and NH4+ comprise the two main components of net acid excretion. V-type H+-ATPase-mediated H+ excretion may have developed in stenohaline lampreys when they initially invaded freshwater from marine habitats, but this trait is lost in most fishes. Instead, increased reliance on NHE-mediated NH4+ excretion is gradually developed and intensified during fish evolution. Further investigations on more species will be needed to support the hypothesis. Also see the video abstract here https://youtu.be/vZuObtfm-34 . 相似文献
13.
14.
Summary The cellular mechanisms responsible for rectal acidification in the desert locust, Schistocerca gregaria, were investigated in isolated recta mounted as flat sheets in modified Ussing chambers. Previous studies conducted in the nominal absence of exogenous CO2 and HCO
3
–
suggested that the acidification was due to a proton-secretory rather than bicarbonate-reabsorptive mechanism (Thomson, R.B., Speight, J.D., Phillips, J.E. 1988. J. Insect Physiol.
34:829–837). This conclusion was confirmed in the present study by demonstrating that metabolic CO2 could not contribute sufficient HCO
3
–
to the lumen to account for the rates of rectal acidification observed under the nominally CO2/ HCO
3
–
-free conditions used in these investigations.Rates of luminal acidification (J
H
+) were completely unaffected by changes in contraluminal pH, but could be progressively reduced (and eventually abolished) by imposition of either transepithelial pH gradients (lumen acid) or transepithelial electrical gradients (lumen positive). Under short-circuit current conditions, the bulk of J
H
+ was not dependent on Na+, K+, Cl–,Mg2+, or Ca2+ and was due to a primary electrogenic proton translocating mechanism located on the apical membrane. A small component (10–16%) of J
H
+ measured under these conditions could be attributed to an apical amiloride-inhibitable Na+/H+ exchange mechanism.This work was supported by operating grants to J.E.P. and postgraduate scholarships to R.B.T. from Natural Sciences & Engineering Research Council, Canada. 相似文献
15.
The chemistry of the lowland rice rhizosphere 总被引:1,自引:1,他引:0
Models and experimental studies of the rhizosphere of rice plants growing in anaerobic soil show that two major processes lead to considerable acidification (1–2 pH units) of the rhizosphere over a wide range of root and soil conditions. One is generation of H+ in the oxidation of ferrous iron by O2 released from the roots. The other is release of H+ from roots to balance excess intake of cations over anions, N being taken up chiefly as NH4
+. CO2 exchange between the roots and soil has a much smaller effect. The zone of root-influence extends a few mm from the root surface. There are substantial differences along the root length and with time. The acidification and oxidation cause increased sorption of NH4
+ ions on soil solids, thereby impeding the movement of N to absorbing root surfaces. But they also cause solubilization and enhanced uptake of soil phosphate. 相似文献
16.
In a preparation of isolated gills of the shore crabCarcinus mediterraneus perfused with dilute sea water (pH 8.1, 200 mM Na+) which was identical to the bathing solution of the gill, acidification of the collected perfusate was observed. Acidification
was not affected by 10−4 M EIPA (5-[N-ethyl-N-isopropyl]amiloride), a strong inhibitor of Na+/H+ exchange. However, in the presence of 10−4 M acetazolamide, acidification was greatly blocked. The significant decrease of the acid load of the perfusate is considered
to be a result of inhibition of the branchial intracellular carbonic anhydrase catalyzing the formation of H+ ions. 相似文献
17.
Valérie Toulon Hervé Sentenac Jean-Baptiste Thibaud André Soler David Clarkson Claude Grignon 《Planta》1989,179(2):235-241
The effect of HCO
3
-
on ion absorption by young corn roots was studied in conditions allowing the independent control of both the pH of uptake solution and the CO2 partial pressure in air bubbled through the solution. The surface pH shift in the vicinity of the outer surface of the plasmalemma induced by active H+ excretion was estimated using the initial uptake rate of acetic acid as a pH probe (Sentenac and Grignon (1987) Plant Physiol. 84, 1367). Acetic acid and orthophosphate uptake rates and NO
3
-
accumulation were slowed down, while 86Rb+ uptake and K+ accumulation rates were increased by HCO
3
-
. These effects were similar to those induced by 4-(2-hydroxyethyl)-1-piperazineethane sulfonic acid/2-amino-2-(hydroxymethyl)-1,3-propanediol (Hepes-Tris). They were more pronounced when the H+ excretion was strong, were rapidly reversible and were not additive to those of Hepes-Tris. The hypothesis is advanced that the buffering system CO2/H2CO3/HCO
3
-
accelerated the diffusion of equivalent H+ inside the cell wall towards the medium. This attenuated the surface pH shift in the vicinity the plasma membrane and affected the coupling between the proton pump and cotransport systems.Abbreviations FW
fresh weight
- Hepes
4-(2-hydroxyethyl)-1-piperazineethanesulfonic acid
- Jaa
acetic acid influx
- JK
+
K+ influx
- JPi
orthophosphate influx
- Mes
2-(N-morpholino)ethanesulfonic acid
- pCO2
CO2 partial pressure
- Tris
2-amino-2-(hydroxymethyl)-1,3-propanediol 相似文献
18.
Colleen R. Talbot Daniel F. Stiffler 《Journal of comparative physiology. B, Biochemical, systemic, and environmental physiology》1992,162(5):416-423
Summary The skin/gills and the kidneys of aquatic amphibians are potential sites of acid-base regulation. The roles of these organs in acid-base balance were examined in larval Ambystoma tigrinum following gastric infusion of ammonium salts. A single dose of 1.75 mEq NH4Cl·100 g-1 produced a mixed acidosis by 1 h after gavage. By 8 h after ingestion, pH and HCO
3
–
had increased and PCO2 had decreased as the animals recovered. A prolonged acidosis was developed in a second group by gavage of an initial dose (1.5 mEq·100 g-1), followed by periodic maintenance doses (0.25 mEq·100 g-1) to prolong the disturbance for 8 h. The magnitude of the acidosis during this period was similar to that seen at 1 h after ingestion in the time-course study. A third group of larvae were given NaCl as a control for salt loading, which induced a small but significant respiratory acidosis. Unidirectional fluxes of Na+ and Cl- were examined during these serial ingestions. Salt loading inhibited the influx of the ingested ion. Na+ influx increased during the NH4Cl-induced acidosis. A fourth group of larvae were used to partition acid and ammonia excretion between the skin and the kidneys. These animals were given (NH4)2SO4 to allow re-examination of Cl- flux rates under non-Cl--loaded conditions. The ensuing acidosis had a reduced respiratory component and, therefore, pH did not decrease as much. Cl- influx rates did decrease significantly under these conditions. In both control and acidotic conditions, the majority of the acid efflux was as ammonia and the skin was the primary site of acid excretion. However, both the skin and the kidneys increased total acid excretion, although the efflux across the skin showed a much greater increase. This suggests a primary role for the skin in acid-base regulation in aquatic amphibians.Abbreviations GFR
glomerular filtration rate
-
PO2
partial pressure of oxygen
-
PCO2
partial pressure of carbon dioxide
- SITS
4-acetamido-4-isothiocynanatostilbene-2,2-disulfonic acid
- TA
titratible acidity
Present address: Department of Organismal Biology and Anatomy, University of Chicago, 1025 E. 57th St., Chicago, IL 60637, USA 相似文献
19.
S. Morris P. Greenaway 《Journal of comparative physiology. B, Biochemical, systemic, and environmental physiology》1990,160(2):217-221
Summary The activity of carbonic anhydrase (CA), which catalyses the equilibrium CO2H++HCO
3
-
, was investigated in various tissues implicated in the excretion of CO2 by Birgus latro. Carbonic anhydrase was detected in the water-soluble fraction of gill tissue but also occurred in association with lipids (membrane bound). This is consistent with a CO2 excretory role and an ion regulation function for the gills. In the lungs (branchial chamber lining) CA activity was found in the membrane bound fraction but was not detected in the soluble fraction, suggesting that the lung CA is not important for ion regulation. The specific CA activity of gill tissue homogenate (A=1.8±0.7·mg-1) was higher than that measured for lung homogenates (A=0.4±0.2·mg-1), but when the whole organ was considered the total CA activity in the lungs was not significantly different from total CA activity in the gills. In comparison to aquatic and amphibious crustaceans the specific activity of carbonic anhydrase in the lungs was high (25% cf. gill activity). This CA activity in the lungs could be correlated with significant CO2 excretion by the lungs. CA may be retained in the branchial tissue as an adjunct to ion reabsorption by the gills. 相似文献
20.
Greg G. Goss Chris M. Wood 《Journal of comparative physiology. B, Biochemical, systemic, and environmental physiology》1991,161(6):635-646
Summary The novel application of a two-substrate model (Florini and Vestling 1957) from enzymology to transport kinetics at the gills of freshwater trout indicated that Na+/acidic equivalent and Cl-/basic equivalent flux rates are normally limited by the availability of the internal acidic and basic counterions, as well as by external Na+ and Cl- levels. Adult rainbow trout fitted with dorsal aortic and bladder catheters were chronically infused (10–16 h) with isosmotic HCl to induce a persistent metabolic acidosis. Acid-base neutral infusions of isosmotic NaCl and non-infused controls were also performed. Results were compared to previous data on metabolic alkalosis in trout induced by either isosmotic NaHCO3 infusion or recovery from environmental hyperoxia (Goss and Wood 1990a, b). Metabolic acidosis resulted in a marked stimulation of Na+ influx, no change in Cl- influx, positive Na+ balance, negative Cl- balance, and net H+ excretion at the gills. Metabolic alkalosis caused a marked inhibition of Na+ influx and stimulation of Cl- influx, negative Na+ balance, positive Cl- balance, and net H+ uptake (=base excretion). Mean gill intracellular pH qualitatively followed extracellular pH. Classical one-substrate Michaelis-Menten analysis of kinetic data indicated that changes in Na+ and Cl- transport during acid-base disturbance are achieved by large increases and decreases in Jmax, and by increases in Km. However, one-substrate analysis considers only external substrate concentration and cannot account for transport limitations by the internal substrate. The kinetic data were fitted successfully to a two-substrate model, using extracellular acid-base data as a measure of internal HCO
3
-
and H+ availability. This analysis indicated that true Jmax values for Na+/acidic equivalent and Cl-/basic equivalent transport are 4–5 times higher than apparent Jmax values by one-substrate analysis. Flux rates are limited by the availability of the internal counterions; transport Km values for HCO
3
-
and H+ are far above their normal internal concentrations. Therefore, small changes in acid-base status will have large effects on transport rates, and on apparent Jmax values, without alterations in the number of transport sites. This system provides an automatic, negative feedback control for clearance or retention of acidic/basic equivalents when acid-base status is changing.Abbreviations Amm
total ammonia in water
- DMO
55-dimethyl-24-oxyzolidine-dione
- Jin
unidirectional inward ion movement across the gill
- Jout
unidirectional outward ion movement across the gill
- Jnet
net transfer of ions (sum of Jin and Jout) across the gill
- Jmax
maximal transport rate for ion
- Km
inverse of affinity of transporter for ion
-
PIO2
partial pressure of oxygen in inspired water
- PaCO2
partial pressure of carbon dixide in arterial blood
- TAlk
titratable alkalinity of the water
- PEG
polyethylene glycol
- NEN
New England Nuclear 相似文献