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1.
Summary Measurements of the unidirectional influxes of Na and Cl from the mucosal solution into the epithelium (J me ) of flounder intestine under short-circuit conditions reveal the presence of a coupled NaCl influx process at the brush border membrane which appears to be essential for the absorption of these ions.J me Cl andJ me Na were inhibited by replacing Na or Cl, respectively, in the bathing media with nontransported ions which also reduced the short-circuit current (I sc) to near-zero values. Addition of furosemide to the mucosal solution alone inhibited theI sc and reducedJ me Cl andJ me Na under control conditions, but not in the absence of Na or Cl, respectively. The reductions inJ me Cl andJ me Na elicited by ion replacement or furosemide were approximately equal, suggesting that the coupled influx mechanism mediates a one-for-one entry of these ions into the cell from the mucosal solution. Furosemide inhibited Cl absorption by reducing the unidirectional Cl flux from mucosa to serosa, consistent with its inhibition of the influx process. As in other epithelia, coupled NaCl influx is inhibited by cyclic AMP, which accounts for the decrease in Cl absorption elicited by cyclic nucleotides. These results support the notion thattranscellular NaCl transport is a neutral process and that the serosa-negative transepithelial electrical potential difference and preponderance of Cl over Na absorption under short-circuit conditions result from dissimilar permeabilities of the paracellular pathway to Na and Cl.  相似文献   

2.
Ion transport by rabbit colon   总被引:7,自引:0,他引:7  
Summary Descending rabbit colon, stripped ofmuscularis externa, absorbs Na and Cl under short-circuit conditions and exhibits a residual ion flux, consistent with HCO3 secretion, whose magnitude is approximately equal to the rate of active Cl absorption. Net K transport was not observed under short-circuit conditions. The results of ion replacement studies and of treatment with ouabain or amiloride suggest that the short-circuit currentI sc is determined solely by the rate of active Na transport and that the net movements of Cl and HCO3 are mediated by a Na-independent, electrically-neutral, anion exchange process. Cyclic AMP stimulates an electrogenic Cl secretion, abolishes HCO3 secretion but does not affect the rate of Na absorption under short-circuit conditions. Studies of the effect of transepithelial potential difference on the serosa-to-mucosa fluxesJ sm i of Na, K and Cl suggest thatJ sm Na ,J sm K and one-third ofJ sm Cl may be attributed to ionic diffusion. The permeabilities of the passive conductance pathway(s) are such thatP KP NaP Cl=1.00.070.11. Electrolyte transport byin vitro rabbit colon closely resembles that reported fromin vivo studies of mammalian colon and thus may serve as a useful model for the further study of colonic ion transport mechanisms.  相似文献   

3.
  • 1.1. The effects of extracellular pH on Na+ and Cl absorption were studied in vitro in the small intestine of the winter flounder, Pseudopleuronectes americanus.
  • 2.2. Reductions in bathing solution pH inhibited Jmsna (mucosal-to-serosal flux) and Jnetna (net flux) (r = 0.90) and JnetCl (r = 0.92) [due to an increase in JsmCl, (serosal-to-mucosal)] and decreased short circuit current (Isc).
  • 3.3. Luminal bumetanide (0.1 mM) and amiloride (1 mM) inhibited Na+ and Cl absorption by reducing Jms.
  • 4.4. Luminal barium (5mM) and luminal copper (100 μM) decreased JmsCl and increased JsmCl.
  • 5.5. We conclude that reductions in extracellular pH inhibit a luminal membrane NaCl absorptive process (Na+-K+-2Cl) and stimulate an electrogenic Cl secretory process.
  相似文献   

4.
Summary Models for active Cl transport across epithelia are often assumed to be universal although they are based on detailed studies of a relatively small number of epithelia from vertebrate animals. Epithelial Cl transport is also important in many invertebrates, but little is known regarding its cellular mechanisms. We used short-circuit current, tracer fluxes and ion substitutions to investigate the basic properties of Cl absorption by locust hindgut, an epithelium which is ideally suited for transport studies. Serosal addition of 1mm adenosine 35-cyclic monophosphate (cAMP), a known stimulant of Cl transport in this tissue, increased short-circuit current (I sc) and net reabsorptive36Cl flux (J net Cl ) by 1000%. Cl absorption did not exhibit an exchange diffusion component and was highly selective over all anions tested except Br. Several predictions of Na- and HCO3-coupled models for Cl transport were tested: Cl-dependentI sc was not affected by sodium removal (<0.05mm) during the first 75 min. Also, a large stimulation ofJ net Cl was elicited by cAMP when recta were bathed for 6 hr in nominally Na-free saline (<0.001 to 0.2mm) and there was no correlation between Cl transport rate and the presence of micromolar quantities of Na contamination. Increased unidirectional influx of36Cl into rectal tissue during cAMP-stimulation was not accompanied by a comparable uptake of22Na.J net Cl was independent of exogenous CO2 and HCO3, but was strongly dependent on the presence of K. These results suggest that the major fraction of Cl transport across this insect epithelium occurs by an unusual K-dependent mechanism that does not directly require Na or HCO3.  相似文献   

5.
Summary The effect of changes in Cl concentration in the external and/or serosal bath on Cl transport across short-circuited frog skin was studied by measurements of transepithelial Cl influx (J 13 Cl ) and efflux (J 31 Cl ), short-circuit current, transepithelial potential, and conductance (G m).J 13 Cl as well asJ 31 Cl were found to have a saturating component and a component which is apparently linear with Cl concentration. The linear component ofJ 31 Cl appears only upon addition of Cl to external medium, and about 3/4 of this component does not contribute toG m. The saturating component ofJ 31 Cl is only 5% of totalJ 31 Cl with 115mm Cl in the serosal medium. Replacement of 115mm Cl in external medium by SO 4 = , NO 3 , HCO 3 or I results in 87–97% reduction ofJ 31 Cl , whereas replacement with Br has no effect. As external Cl concentration is raised in steps from 2 to 115mm,J 13 Cl andJ 31 Cl increase by the same amount butJ 13 Cl is persistently 0.15 eq/cm2 hr larger thanJ 31 Cl . These results indicate that at least 3/4 of linear components ofJ 13 Cl andJ 31 Cl proceed via an exchange diffusion mechanism which seems to be located at the outer cell border. The saturating component ofJ 13 Cl is involved in active Cl transport in an inward direction, and there is evidence suggesting that Cl uptake across outer cell border, which proceeds against an electrochemical gradient, is electroneutral but not directly linked to Na.Reprinted from The Journal of Membrane Biology, Vol. 54, No. 3, pages 191–202. Our apologies for deleting the author's names on the original version.  相似文献   

6.
Summary The effect of changes in Cl concentration in the external and/or serosal bath on Cl transport across short-circuited frog skin was studied by measurements of transepithelial Cl influx (J 13 Cl ) and efflux (J 31 Cl ), short-circuit current, transepithelial potential, and conductance (G m).J 13 Cl as well asJ 31 Cl were found to have a saturating component and a component which is apparently linear with Cl concentration. The linear component ofJ 31 Cl appears only upon addition of Cl to external medium, and about 3/4 of this component does not contribute toG m. The saturating component ofJ 31 Cl is only 5% of totalJ 31 Cl with 115mm Cl in the serosal medium. Replacement of 115mm Cl in external medium by SO 4 = , NO 3 , HCO 3 or I results in 87–97% reduction ofJ 31 Cl , whereas replacement with Br has no effect. As external Cl concentration is raised in steps from 2 to 115mm,J 13 Cl andJ 31 Cl increase by the same amount butJ 13 Cl is persistently 0.15 eq/cm2 hr larger thanJ 31 Cl . These results indicate that at least 3/4 of linear components ofJ 13 Cl andJ 31 Cl proceed via an exchange diffusion mechanism which seems to be located at the outer cell border. The saturating component ofJ 13 Cl is involved in active Cl transport in an inward direction, and there is evidence suggesting that Cl uptake across outer cell border, which proceeds against an electrochemical gradient, is electroneutral but not directly linked to Na.  相似文献   

7.
Summary Unidirectional fluxes of35SO4 across and into rabbit ileal epithelium were measured under short-circuit conditions, mostly at a medium SO4 concentration of 2.4mm. Unidirectional mucosa (m)-to-serosa (s) ands-to-m fluxes (J ms,J sm) were 0.456 and 0.067 moles hr–1 cm–2, respectively.J ms was 2.7 times higher in distal ileum than in mid-jejunum. Ouabain abolished net SO4 transport (J net) by reducingJ ms. Epinephrine, a stimulus of Cl absorption, had no effect on SO4 fluxes. Theophylline, a stimulus of Cl secretion, reducedJ ms without affectingJ sm, causing a 33% reduction inJ net. Other secretory stimuli (8-Br-cAMP, heat-stable enterotoxin, Ca-ionophore A23187) had similar effects. Replacement of all Cl with gluconate markedly reducedJ net through both a decrease inJ ms and an increase inJ sm. The anion-exchange inhibitor, 4-acetoamido-4-isothiocyano-2,2-sulfonic acid stilbene (SITS), when added to the serosal side, reducedJ ms by 94%, nearly abolishingJ net. SITS also decreasedJ sm by 75%. Mucosal SITS (50 m) was ineffective. 4,4-diisothiocyano-2,2-sulfonic acid stilbene (DIDS) had effects similar to SITS but was less potent. Measurements of initial rates of epithelial uptake from the luminal side (J me) revealed the following: (1)J me is a saturable function of medium concentration with aV max of 0.94 moles hr–1 cm–2 and aK 1/2 of 1.3mm; (2) replacing all Na with choline abolishedJ me; (3) replacing all Cl with gluconate increasedJ me by 40%; (4) serosal SITS had no effect onJ me; and (5) stimuli of Cl secretion had no effect onJ me or increased it slightly. Determination of cell SO4 with35SO4 indicated that, at steady-state, the average mucosal concentration is 1.1 mmoles per liter cell water, less than half the medium concentration. Cell SO4 was increased to 3.0mm by adding SITS to the serosal side. Despite net transport rates greater than 1.4 Eq hr–1 cm–2, neither addition of SO4 to the SO4-free medium nor addition of SITS to SO4-containing medium altered short-circuit current. The results suggest that (1) ileal SO4 absorption consists of Na-coupled influx (symport) across the brush border and Cl-coupled efflux (antiport) across the basolateral membrane; (2) the overall process is electrically neutral; (3) the medium-to-cell Cl concentration difference may provide part of the driving force for net SO4 absorption; and (4) since agents affecting Cl fluxes (both absorptive and secretory) have little effect on SO4 fluxes, the mechanisms for their transcellular transports are under separate regulation.  相似文献   

8.
The effects of luminal hyperosmolarity on Na and Cl transport were studied in rumen epithelium of sheep. An increase of luminal osmotic pressure with mannitol (350 and 450 mosm/l) caused a significant increase of tissue conductance, G T, which is linearly correlated with flux rates of 51Cr-EDTA and indicates an increase of passive permeability. Studies with microelectrodes revealed, that an increase of the osmotic pressure caused a significant increase of the conductance of the shunt pathway from 1.23±0.10 (control) to 1.92±0.14 mS cm−2 (450 mosm/l) without a change of fractional resistance. Hyperosmolarity significantly increased J sm and reduced J net Na. The effect of hyperosmolarity on J ms Na is explained by two independent and opposed effects: increase of passive permeability and inhibition of the Na+/H+ exchanger. Hypertonic buffer solution induced a decrease of the intracellular pH (pHi) of isolated ruminal cells, which is consistent with an inhibition of Na+/H+ exchange, probably isoform NHE-3, because NHE-3-mRNA was detectable in rumen epithelium. These data are in contrast to previous reports and reveal a disturbed Na transport and an impaired barrier function of the rumen epithelium, which predisposes translocation of rumen endotoxins and penetration of bacteria.  相似文献   

9.
Summary This paper reports experiments designed to assess the relations between net salt absorption and transcellular routes for ion conductance in single mouse medullary thick ascending limbs of Henle microperfusedin vitro. The experimental data indicate that ADH significantly increased the transepithelial electrical conductance, and that this conductance increase could be rationalized in terms of transcellular conductance changes. A minimal estimate (G c min ) of the transcellular conductance, estimated from Ba++ blockade of apical membrane K+ channels, indicated thatG c min was approximately 30–40% of the measured transepithelial conductance. In apical membranes, K+ was the major conductive species; and ADH increased the magnitude of a Ba++-sensitive K+ conductance under conditions where net Cl absorption was nearly abolished. In basolateral membranes, ADH increased the magnitude of a Cl conductance; this ADH-dependent increase in basal Cl conductance depended on a simultaneous hormone-dependent increase in the rate of net Cl absorption. Cl removal from luminal solutions had no detectable effect onG e , and net Cl absorption was reduced at luminal K+ concentrations less than 5mm; thus apical Cl entry may have been a Na+,K+,2Cl cotransport process having a negligible conductance. The net rate of K+ secretion was approximately 10% of the net rate of Cl absorption, while the chemical rate of net Cl absorption was virtually equal to the equivalent short-circuit current. Thus net Cl absorption was rheogenic; and approximately half of net Na+ absorption could be rationalized in terms of dissipative flux through the paracellular pathway. These findings, coupled with the observation that K+ was the principal conductive species in apical plasma membranes, support the view that the majority of K+ efflux from cell to lumen through the Ba++-sensitive apical K+ conductance pathway was recycled into cells by Na+,K+,2Cl cotransport.  相似文献   

10.
Summary Na and Cl fluxes and short-circuit current (I sc) in rabbit ileum have been studied as a function of ionic concentrations in HCO3-free solutions. Both net Na flux (J net Na ) andI sc show similar saturation functions of [Na] at fixed [Cl]. They show no significant difference between zero and 112mm Na but at 140mm NaI sc is significantly greater than theJ net Na . Net Cl transport, secretion, is observed only at 140mm Na and is approximately equivalent to the difference between theI sc andJ net Na . The transcellular mucosa-to-serosa Na fluxes measured at 140 and 70mm Na do not differ significantly from the correspondingI sc. The net Cl flux varies with [Cl] at fixed [Na] whileI sc is virtually not affected by [Cl]. These results suggest that the absorptive Na transport process is electrogenic and responsible for theI sc and that the secretory fluxes of Na and Cl are coupled, require high [Na], vary with [Cl], and do not contribute toI sc. K-free solution abolishes theI sc after a prolonged lag. Finally, the effect of a low resistance shunt pathway on active Na absorption is examined with a four-compartment model.Deceased (October 16, 1974).  相似文献   

11.
Summary Cl transport in apical membrane vesicles derived from bovine tracheal epithelial cells was studied using the Cl-sensitive fluorescent indicator 6-methoxy-N-(3-sulfopropyl) quinolinium. With an inwardly directed 50 mM Cl gradient at 23°C, the initial rate of Cl entry (J Cl) was increased significantly from 0.32±0.12 nmol · sec–1 · mg protein–1 (mean±sem) to 0.50±0.07 nmol · sec–1 · mg protein–1 when membrane potential was changed from 0 to +60 mV with K/valinomycin. At 37°C, with membrane potential clamped at 0 mV, there was a 34±7% (n=5) decrease inJ Cl from a control value of 0.37±0.03 nmol · sec–1 · mg protein–1 upon addition of 0.2mm diphenylamine-2-carboxylate. The following did not alterJ Cl significantly (J Cl values gives as percent change from control): 50mm cis Na (–1±5%), 0.1mm furosemide (–3±4%), 0.1mm furosemide in the presence of 50mm cis Na (–5±2%), 0.1mm H2DIDS (–18±9%), a 1.5 pH unit inwardly directed H gradient (–7±7%), and 0.1mm H2DIDS in the presence of a 1.5 unit pH gradient (4±18%). With inward 50mm anion gradients, the initial rates of Br and I entry (J Br andJ 1, respectively) were not significantly different fromJ Cl.J Cl was a saturable function of Cl concentration with apparentK d of 24mm and apparentV max of 0.54 nmol · sec–1 · mg protein–1. Measurement of the temperature dependence ofJ Cl yielded an activation energy of 5.0 kcal/mol (16–37°C). These results demonstrate that Cl transport in tracheal apical membrane vesicles is voltage-dependent and inhibited by diphenylamine-2-carboxylate. There is no significant contribution from the Na/K/2Cl, Na/Cl, or Cl/OH(H) transporters. The conductive pathway does not discriminate between Cl, Br, and I and is saturable. The low activation energy supports a pore-type mechanism for the conductance.  相似文献   

12.
Summary Bioelectrical parameters and unidirectional sodium and chloride fluxes were measured under voltageclamp conditions in groups of lizards submitted to single or chronic aldosterone treatment. Both acute (AT) and chronic (CT) treatment induced significant increases in the short-circuit current (I sc), as well as in the mucosa-to-serosa (J m-s Na ) and net sodium flux (J net Na ). In AT tissues, aldosterone did not change net chloride flux (J net Cl ) but did so in CT tissues. Amiloride reduced the aldosterone-increased I sc in AT and CT tissues, inhibited J net Na in AT tissues and abolished it in CT colons. J net Cl was also reduced by the diuretic in the group of AT colons, whereas no changes were observed in the CT tissues. Addition of luminal DIDS reduced Na+ absorption and totally inhibited Cl- absorption in the AT tissues, but did not change I sc. However, in CT tissues neither Na+ nor Cl- transport were affected by DIDS. A good relationship between I sc and J m-s Na was apparent after DIDS treatment in AT tissues. In this group, simultaneous addition of DIDS and amiloride totally abolished J net Na and reduced I sc to untreated control values. Addition of serosal ouabain abolished I sc and Na+ absorption in AT and CT colons, but Cl- absorption was only altered in AT tissues. These results support the hypothesis that aldosterone induces an electrogenic, amiloride-sensitive sodium absorption, and in a dose-dependent fashion suppresses electroneutral NaCl absorption in the lizard colon.Abbreviations AT acutely treated - CT chronically treated animals - DIDS 4-4-diisothiocyanatostibene-2-2-disulfonic acid - DMSO dimethylsulphoxide - G t tissue conductance - I sc short circuit current - PD transepithelial potential difference - SITS 4-acetamido-4-isothiocyanatostilbene-2-2-disulfonic acid - UC untreated controls Preliminary results of this paper were presented at the X th meeting of the European Intestinal Transport Group (EITG), Askov Hojskole, Denmark, 16–19 September 1990  相似文献   

13.
Summary Prostaglandins are known to stimulate the active sodium absorption in frog skin. In this paper it is shown that prostaglandin E2 (PGE2) stimulates an active secretion of Cl, Na+, and K+ from the skin glands inRana esculenta. The active Cl secretion is enhanced more than the Na and K secretion. Therefore, in skins where the Na absorption is inhibited by amiloride, the addition of PGE2 results in an increase in the short-circuit current (SCC). The PGE2-stimulated Cl secretion could be inhibited by the presence of ouabain or furosemide in the basolateral solution or diphenylamine-2-carboxylate in the apical solution. The PGE2-stimulated Cl secretion was enhanced by the phosphodiesterase inhibitor, theophylline, indicating that the effect of PGE2 was caused by an increase in the intracellular cAMP level in the gland cells. The calcium ionophore A23187, which increases the PGE2 synthesis in frog skin, stimulated the glandular Cl secretion. This secretion could be blocked by the prostaglandin synthesis inhibitor indomethacin, indicating that A23187 acts by increasing the prostaglandin synthesis and not by a direct action of Ca2+ ionsper se. The net water flow (J w) and the Cl secretion were measured simultaneously under the conditions outlined above. The stimulation, inhibition, and the time-course of the outward-directedJ w were similar to the change observed for the Cl secretion. These results show that PGE2 stimulates a glandular secretion of Cl and water in frog skin, probably by increasing the cAMP level in the gland cells.  相似文献   

14.
Summary Porcine distal colon epithelium was mounted in Ussing chambers and bathed in plasma-like Ringer solution. Tissue conductances ranged from 10 to 15 mS and the short-circuit current (Isc) ranged from-15 to 220 A·cm-2. Variations in basal Isc resulted from differences in the amount of amiloride (10M mucosal addition)-sensitive Na+ absorption. Ion substitution and transepithelial flux experiments showed that 10 M amiloride produced a decrease in the mucosal-to-serosal (M-S) and net Na flux, and that this effect on Isc was independent of Cl- and HCO 3 - replacement. When the concentration of mucosal amiloride was increased from 10 to 100 M, little change in Isc was observed. However, increasing the concentration to 1 mM produced a further inhibition, which often reversed the polarity of the Isc. The decrease in Isc due to 1 mM amiloride was dependent on both Cl- and HCO 3 - , and was attributed to reductions in the M-S and net Na+ fluxes as well as the M-S unidirectional Cl- flux. Ion replacement experiments demonstrated that Cl- substitution reduced the M-S and net Na fluxes, while replacement of HCO 3 - with HEPES abolished net Cl- absorption by reducing the M-S unidirectional Cl- flux. From these data it can be concluded that: (1) Na+ absorption is mediated by two distinct amiloride-sensitive transport pathways, and (2) Cl- absorption is completely HCO 3 - -dependent (presumably mediated by Cl-/HCO 3 - exchange) and occurs independently of Na+ absorption.Abbreviations Gt tissue conductance - HEPES tris (hydroxymethyl) aminomethane - (tris) N-2-hydroxyethylpiperazine-N-2-ethanesulfonic acid - Isc short-circuit current - Jr residual flux - M-S mucosal-to-scrosal - S-M serosal-to-mucosal - TTX tetrodotoxin  相似文献   

15.
Summary The effect of addition of FeCl3 to the media bathing the isolated skin ofRana pipiens was studied by measuring short-circuit current, transepithelial potential, and resistance, and by determining the influx and efflux of sodium (J 13 Na andJ 31 Na , respectively) and the influx and efflux of chloride (J 13 Cl andJ 31 Cl , respectively) across the epithelium. With normal Ringer's solution on both sides of the skin, addition of 10–3 m FeCl3 to the external medium resulted in nearly complete inhibition of active Na transport (J 13 Na decreased from 1.30±0.14 to 0.10±0.04 eq/cm2 hr (N=8)) and in appearance of active chloride transport in outward direction due to an 80% increase inJ 31 Cl . Average (J 31 ClJ 13 Cl ) obtained from means of 8 skins in 6 consecutive control and last 3 experimental periods was –0.17±0.04 and 0.38±0.05 eq/cm2 hr, respectively. FeCl3 added to external medium also induced substantial net chloride movement in outward direction when external medium contained Na-free choline chloride Ringer's or low ionic strength solution. Under the latter condition net Na movement was virtually eliminated by external FeCl3. After addition of FeCl3 to serosal medium there was delayed inhibition ofJ 13 Na but no change in chloride fluxes. Immediate and profound changes in Na and Cl transport systems seen after external application of FeCl3 indicate charge effects of Fe3+ on surface of apical cell membranes, possibly close to or in ion channels.  相似文献   

16.
Unidirectional flux rates of Ca2+ across gastrointestinal tissues from sheep and goats were measured in vitro by applying the Ussing-chamber technique. Except for the sheep duodenum, mucosal to serosal Ca2+ flux rates (J ms) exceeded respective flux rates in the opposite direction (J sm) in both species and in all segments of the intestinal tract. This resulted in net Ca2+ flux rates␣(J net = J ms − J sm) ranging between −2 and 9 nmol · cm−2 · h−1 in sheep and between 10 and 15 nmol cm−2 · h−1 in goats. In sheep, only J net in jejunum, and in goats, J netin duodenum and jejunum were significantly different from zero. Using sheep rumen wall epithelia, significant J net of Ca2+ of around 5 nmol · cm−2 · h−1 could be detected. Since the experiments were carried out in the absence of an electrochemical gradient, significant net Ca2+ absorption clearly indicates the presence of active mechanisms for Ca2+ transport. Dietary Ca depletion caused increased calcitriol plasma concentrations and induced significant stimulations of net Ca2+ absorption in goat rumen. J net of Ca2+ across goat rumen epithelia was significantly reduced by 1 mmol · l −1 verapamil in the mucosal buffer solution. In conclusion, there is clear evidence for the rumen as a main site for active Ca2+ absorption in small ruminants. Stimulation of active Ca2+ absorption by increased plasma calcitriol levels and inhibition by mucosal verapamil suggest mechanistic and regulatory similarities to active Ca2+ transport as described for the upper small intestines of monogastric species. Accepted: 31 July 1996  相似文献   

17.
Summary Proximal, stripped segments of small intestine from the urodeleAmphiuma were short-circuited in media containing Na+, Cl and HCO 3 . Under these conditions there was a large net absorption of Cl, a small net absorption of Na+ and a residual flux (J Net R ) consistent with HCO 3 secretion. Net Cl absorption correlated with the short-circuit current (I sc); net Na+ absorption correlated negatively withJ Net R . Acetazolamide eliminated theI sc, lowered Cl absorption by 50%, and reduced net Na+ absorption without alteringJ Net R . Benzolamide inhibited theI sc without alteringJ Net R . Benzolamide inhibited theI sc more rapidly when applied on the mucosal surface. Replacement of Na+ or HCO 3 (and CO2) in the media eliminated theI sc, net Cl absorption and the residual flux. Likewise, inclusion of the stilbene SITS in the serosal media eliminated theI sc, net Cl absorption and the residual flux. The cytoplasmic activity of Cl (a ci a ) was determined with single and double-barreled microelectrodes. Thea ci a of villus absorptive cells in normal media was 21.0mm and in excess of that expected on the basis of electrochemical equilibrium of Cl at the mucosal membrane. Active Cl accumulation was also observed in the presence of acetazolamide but was eliminated upon replacement of media Na+ with choline. The mucosal membrane potential was depolarized upon replacement of media Na+. It is concluded that Cl is actively absorbed into intestinal cells ofAmphiuma by an electrogenic process located in the mucosal membrane. Depending on the level of intracellular HCO 3 , accumulated Cl may diffuse passively back into the mucosal media or undergo exchange with bath HCO 3 at the serosal membrane.  相似文献   

18.
Summary Hard- and softwater acclimated adult rainbow trout were statically exposed to copper (12.5, 25, 50, 100, and 200 ppb) for two, 12 h periods at neutral and pH 5.0. Unidirectional Na+, and Cl, and net Na+, Cl, K+, and ammonia fluxes were monitored as a measure of branchial ionoregulatory disturbance. Copper concentrations as low as 12.5 ppb led to measurable ion losses. Net Na+, Cl, and K+ losses were concentration dependent and unaffected by prior acclimation to either hard- or softwater at both neutral pH and pH 5. From 12.5 to 50 ppb net NaCl losses arose primarily as a result of the inhibition ofJ in, and at higher concentrations,J out was also stimulated. In softwater,J in was more resistant to inhibition than in hardwater. However, in hardwater,J out recovered to normal levels during the second 12 h period, but no such recovery was found in softwater. Plasma NaCl was inversely correlated with [copper], while plasma glucose and ammonia increased with [copper]. At pH 5.0 and [copper] from 12.5 to 50 ppb, H+ contributed significantly to the total ion loss, while at 100 and 200 ppb, ion losses were no greater at pH 5.0 than at neutral pH. In no case were the effects of copper and H+ strictly additive.  相似文献   

19.
The effect of conjugated dihydroxy and trihydroxy bile salts on electrolyte transport across isolated rabbit jejunal mucosa was studied. Both taurochenodeoxycholic acid and taurocholic acid increased the short-circuit current (Isc) in bicarbonate-Ringer solution but not in a bicarbonate-free, chloride-free solution. Taurochenodeoxycholic acid was significantly more effective than taurocholic acid in increasing Isc. The presence of theophylline prevented the taurochenodeoxycholic acid-and taurocholic acid-induced increase in Isc. Transmural ion fluxes across jejunal mucosa demonstrated that 2 mM taurochenodeoxycholic acid decreased net Na+ absorption, increased net Cl secretion and increased the residual flux (which probably represents HCO3 secretion). These studies support the hypothesis that cyclic AMP may be a mediator of intestinal electrolyte secretion.  相似文献   

20.
Summary The ionic permeability of the basolateral membrane ofNecturus gallbladder epithelium was studied with intracellular microelectrode techniques. After removal of most of the subepithelial tissue (to reduce unstirred layer thickness), impalements were performed from the serosal side, and ionic substitutions were made in the serosal solution while a microelectrode was kept in a cell. Thus, it was possible to obtain continuous (and reversible) records of transepithelial and cell membrane potentials and to measure intermittently the transepithelial resistance and the ratio of cell membrane resistances. From these data and the mean value of the equivalent resistance of the cell membranes in parallel (obtained from cable analysis in a different group of tissues), absolute cell membrane and shunt resistances and equivalent electromotive forces (emf's) were calculated. From the changes of basolateral membrane emf (E b ) produced by the substitutions, the conductance (G) and permeability (P) of the membrane for K, Cl and Na were estimated. Potassium-for-sodium substitutions produced large reductions of both cell membrane potentials, ofE b , and of the resistance of the basolateral membrane (R b ), indicating highG K andP K . Chloride substitution with isethionate or sulfate resulted in smaller changes of cell membrane potentials andE b and in no significant change ofR b , indicating small but measurable values ofG Cl andP Cl . Sodium substitutions with N-methyl-d-glucamine (NMDG) resulted in cell potential changes entirely attributable to the biionic potential produced in the shunt pathway (P Na >P NMDG ), and in no significant changes ofP b orE b , indicating thatG Na andP Na are undetectable. The question of the mechanism of Cl transport across the basolateral membrane was addressed by comparing the mean rate of transepithelial Cl transport (J Cl net ) and the predicted passive Cl flux across the basolateral membrane (from the membrane Cl conductance, potential, and Cl equilibrium potential). The conclusion is that only a very small fraction of the Cl flux across the basolateral membrane can be electrodiffusional. Since the paracellular Cl conductance is also too low to account forJ Cl net , these results suggest the presence of a neutral mechanism of Cl extrusion from the cells. This could be a NaCl pump, a downhill KCl transport mechanism, or a Cl–HCO3 exchange mechanism.  相似文献   

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