Effects of cholera toxin on cellular and paracellular sodium fluxes in rabbit ileum

The diarrhea observed in patients which cholera is known to be related to secretion of water and electrolytes into the intestinal lumen. However, the exact mechanisms involved in these secretory processes have remained unclear. Although it is clear that purified toxin acts on epithelial cell metabolism, its activity on Na⁺ transport across intestinal mucosa is equivocal: reported either to prevent net Na⁺ absorption or to cause net secretion of Na⁺ from serosa to mucosa. Since total transmural Na⁺ fluxes across “leaky” epithelia involve very significant movement via a paracellular shunt pathway, we studied the effects of cholera toxin on the cellular and paracellular pathways of Na⁺ movement. Unidirectional Na⁺ fluxes were examined as functions of applied potential in control tissues and in tissues from the same animal treated with purified cholera toxin. Treatment of rabbit ileum in vitro with toxin stimulated the cellular component of serosa-to-mucosa Na⁺ flux (from $\text{2.41 ± 0.49 μ}\text{equiv./h}$ per cm² under control conditions to $\text{4.71 ± 0.43 μ}\text{equiv./h}$ per cm² after treatment with toxin, $\text{P < 0.01}$). The effect of cholera toxin on Na⁺ movement through the cells from mucosa to serosa appeared to be insignificant. Finally, a marked decrease in the Na⁺ permeability ($\text{P < 0.01}$) and no detectable significant changes in transference number for Na⁺ of the paracellular shunt pathway were observed following treatment with cholera toxin. These results provide direct evidence for the hypothesis that purified cholera toxin stimulates active sodium secretion but has minimal effect on sodium absorption.     

Ion transport across the isolated intestinal mucosa of the winter flounder,Pseudopleuronectes americanus: II. Effects of cyclic AMP

Summary Addition of cyclic AMP and theophylline to the intestinal mucosa of the winter flounder,Pseudopleuronectes americanus decreased short-circuit current and net Na and Cl absorption and increased total conductance and the serosa-to-mucosa unidirectional Cl flux (J _sm ^Cl ). The last two changes were independent of the original rate of NaCl absorption and persisted even when net absorption of Na and Cl had been abolished by ouabain. Voltageclamp experiments revealed that the increment inJ _sm ^Emphasis>/Cl produced by cyclic AMP is PD-insensitive and therefore not due to an increase in the Cl conductance of the paracellular shunt. Cyclic AMP appears, therefore, both to inhibit net NaCl absorption and to increase the Cl permeability and total conductance of the intestinal epithelial cells; its failure to stimulate secretion (in contrast to its action on mammalian intestine) may be related to the absence of crypts in flounder intestinal epithelium.     

Nacl flux in the flounder (Pseudopleuronectes americanus) intestine: Effects of ph and transport inhibitors

• 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 J_ms^na (mucosal-to-serosal flux) and J_net^na (net flux) (r = 0.90) and J_net^Cl (r = 0.92) [due to an increase in J_sm^Cl, (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 J_ms.
• 4.4. Luminal barium (5mM) and luminal copper (100 μM) decreased J_ms^Cl and increased J_sm^Cl.
• 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.

     

Short circuit current in tight and leaky epithelia

The effect of short circuit current on the unidirectional fluxes of ions transported across tight and leaky epithelia was investigated. It was found that short circuiting of the frog gastric mucosa (classified as a tight epithelium) caused a decrease of the passive J_ms^C1 and a significant increase of the net Cl^? secretion. However, no significant change of H⁺ secretory rate was observed. On the other hand, short circuiting of the mouse intestine (a known leaky membrane) caused a simultaneous increase of both J_ms and J_sm fluxes of Na⁺ while the net fluxes of Na⁺ and Cl^? remained unchanged. Also, short circuiting did not change the water permeability of the mouse intestine. To explain some of these results a theoretical model is presented to demonstrate that while short circuiting can block the passive ionic movement, it will cause an increase in the energy consumption of the system and introduce certain important changes in the ionic barriers and e.m.fs. The simultaneous increase in the unidirectional fluxes of Na⁺ under short circuit conditions can best be explained by a decrease in the polarized nature of the transepithelial shunt, thereby increasing the diffusion coefficient of the ion(s). Such an increase is specially favorable to the Na⁺ rather than an anion.     

Na and Cl transport across the isolated turtle colon: Parallel pathways for transmural ion movement

Summary Transmural fluxes of³H-mannitol and²²Na or³⁶Cl were measured simultaneously in portions of isolated turtle colon stripped of serosal musculature. The relationships between mannitol flux and the flux of Na or Cl are characteristic of simple diffusion and suggest that transmural mannitol flow is largely confined to a paracellular pathway where Na, Cl and mannitol move much as in free solution. The contribution of edge damage to the transmural mannitol flow appears to be minimal. Mucosal hyperosmolarity causes blisters in epithelial tight junctions and increases the diffusional permeability to Na and mannitol, suggesting that the rate-limiting barrier in the shunt path is the tight junction. If the total mucosa to serosa flux of Na is corrected for the portion traversing the shunt pathway it is apparent that changes in the short-circuit current are completely accounted for by the mucosa to serosal movement of Na through a cellular path. In addition, the serosa to mucosa flux of Na appears to be restricted to the shunt. These observations suggest that there is no appreciable backflux of Na through the active, cellular path. In the presence of 10^–4 m amiloride the short-circuit current is markedly reduced and the mucosa to serosa Na flux is restricted to the shunt, so that the net Na flux is abolished. The small amiloride-insensitive short-circuit current is consistent with HCO₃ secretion. Mucosa to serosa and serosa to mucosa fluxes of Cl appear to be largely restricted to the paracellular shunt path and there is no evidence for any net flow of Cl under short-circuit conditions. The total tissue conductance can be described as the sum of three components: a shunt conductance which is linearly related to the transmural mannitol flow, an active conductance which is linearly related to the short-circuit current and a small residual conductance. The shunt conductance is attributable to the diffusive movements of Na and Cl through the paracellular path. Variations in the active Na transport from tissue to tissue are largely attributable to variations in the apparent conductance of the active Na transport path. The driving force for active Na transport can be described as an apparent emf of approximately 130 mV. These results suggest that transmural mannitol flux provides a quantitative estimate of the ion permeability and electrical conductance of a paracellular shunt path across the isolated turtle colon and thereby facilitates the study of the transport characteristics and electrical properties of cellular paths for transepithelial solute movement.     

Evidence for electrogenic bicarbonate transport in Amphiuma small intestine

Isolated segments of small intestine of Amphiuma were short-circuited in buffer containing bicarbonate. Theophylline (10 mM) increased short circuit current (I_sc) in proportion to the bicarbonate concentration in the bath. The theophylline-stimulated I_sc was rapidly reduced, though not abolished, in the presence of acetazolamide at concentrations as low as 10^?6 M. Unidirectional fluxes of ²²Na and ²⁴Na in paired intestinal segments in Cl-free buffer reveal that the increase in I_sc produced by theophylline is not accounted for by an increase in net sodium flux. These results suggest that theophylline stimulates an electrogenic secretion of bicarbonate.     

Effect of theophylline and Na+ on methionine influx in Na+-depleted intestine

The unidirectional influx of methionine into the brush border epithelium of chicken jejunum has been studied. Tissues leached of Na⁺ transport methionine from a medium devoid of Na⁺ with reduced apparent affinity (K_t) and maximal flux (J_max). Addition of Na⁺ to the medium during a 1-min incubation with substrate, or during a 30-min preincubation, restored K_t but affected J_max slightly. Theophylline was found to maintain J_max in the absence of Na⁺. Essentially complete restoration of K_t and J_max could be attained when theophylline-treated tissue was exposed to Na⁺ for 30 min. Influx from a Na⁺ medium was unaffected by theophylline pretreatment in Na⁺-containing buffer. K_t was increased without an effet upon J_max when influx was studied from choline medium following preincubation in Na⁺.Modifiers of tissue cyclic AMP levels were investigated in conjunction with theophylline. Histamine and carbachol were found to inhibit theophylline-stimulated transport. Secretin was found to stimulate influx in Na⁺-leached tissue, but did not potentiate the theophylline effect. Amino acids in the incubation medium inhibited theophylline-stimulated influx, whereas preloaded lysine or methionine had no effect.The results are interpreted in terms of a model which envisions roles for cellular and external Na⁺ and for cyclic AMP in the activation and regulation of amino acid transport in intestine.     

Ion Secretion and Isotonic Transport in Frog Skin Glands

The aim of this study was to clarify the mechanism of isotonic fluid transport in frog skin glands. Stationary ion secretion by the glands was studied by measuring unidirectional fluxes of ²⁴Na⁺, ⁴²K⁺, and carrier-free ¹³⁴Cs⁺ in paired frog skins bathed on both sides with Ringer's solution, and with 10⁻⁵ m noradrenaline on the inside and 10⁻⁴ m amiloride on the outside. At transepithelial thermodynamic equilibrium conditions, the ¹³⁴Cs⁺ flux ratio, J ^out _Cs/J ⁱⁿ _Cs, varied in seven pairs of preparations from 6 to 36. Since carrier-free ¹³⁴Cs⁺ entering the cells is irreversibly trapped in the cellular compartment (Ussing & Lind, 1996), the transepithelial net flux of ¹³⁴Cs⁺ indicates that a paracellular flow of water is dragging ¹³⁴Cs⁺ in the direction from the serosal- to outside solution. From the measured flux ratios it was calculated that the force driving the secretory flux of Cs⁺ varied from 30 to 61 mV among preparations. In the same experiments unidirectional Na⁺ fluxes were measured as well, and it was found that also Na⁺ was subjected to secretion. The ratio of unidirectional Na⁺ fluxes, however, was significantly smaller than would be predicted if the two ions were both flowing along the paracellular route dragged by the flow of water. This result indicates that Na⁺ and Cs⁺ do not take the same pathway through the glands. The flux ratio of unidirectional K⁺ fluxes indicated active secretion of K⁺. The time it takes for steady-state K⁺ fluxes to be established was significantly longer than that of the simultaneously measured Cs⁺ fluxes. These results allow the conclusion that — in addition to being transported between cells — K⁺ is submitted to active transport along a cellular pathway.Based on the recirculation theory, we propose a new model which accounts for stationary Na⁺, K⁺, Cl⁻ and water secretion under thermodynamic equilibrium conditions. The new features of the model, as compared to the classical Silva-model for the shark-rectal gland, are: (i) the sodium pumps in the activated gland transport Na⁺ into the lateral intercellular space only. (ii) A barrier at the level of the basement membrane prevents the major fraction of Na⁺ entering the lateral space from returning to the serosal bath. Thus, Na⁺ is secreted into the outside bath. It has to be assumed then that the Na⁺ permeability of the basement membrane barrier (P ^BM _Na) is smaller than the Na⁺ permeability of the junctional membrane (P ^JM _Na), i.e., P ^JM _Na/P ^BM _Na > 1. The secretory paracellular flow of water further requires that the Na⁺ reflection coefficients (σ_Na) of the two barriers are governed by the conditions, σ^BM _Na > 0, and σ^BM _Na > σ^JM _Na. (iii) Na⁺ channels are located in the apical membrane of the activated gland cells, so that a fraction of the Na⁺ outflux appearing downstream the lateral intercellular space is recirculated by the gland cells. Based on measured unidirectional fluxes, a set of equations is developed from which we estimate the ion fluxes flowing through major pathways during stationary secretion. It is shown that 80% of the sodium ions flowing downstream the lateral intercellular space is recycled by the gland cells. Our calculations also indicate that under the conditions prevailing in the present experiments 1.8 ATP molecule would be hydrolyzed for every Na⁺ secreted to the outside bath. Received: 30 January 1996/Revised: 12 March 1996     

Luminal hyperosmolarity decreases Na transport and impairs barrier function of sheep rumen epithelium

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 (pH_i) 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.     

Transport of butyrate across the isolated bovine rumen epithelium — interaction with sodium, chloride and bicarbonate

The Ussing chamber technique was used for studying unidirectional fluxes of ¹⁴C-butyrate across the bovine rumen epithelium in vitro. Significant amounts of butyrate were absorbed across the bovine rumen epithelium in vitro, without any external driving force. The paracellular pathway was quantitatively insignificant. The transcellular pathway was predominately voltage-insensitive. The serosal to mucosal (SM) pathway was regulated by mass action, whereas the mucosal to serosal (MS) pathway further includes a saturable process, which accounted for 30 to 55% of the MS flux. The studied transport process for ¹⁴C-butyrate across the epithelium could include metabolic processes and transport of ¹⁴C-labelled butyrate metabolites. The transport of butyrate interacted with Na⁺, Cl⁻ and HCO₃⁻, and there was a linear relationship between butyrate and sodium net transport. Lowering the sodium concentration from 140 to 10 mmol l⁻¹ decreased the butyrate MS flux significantly. Amiloride (1 mmol l⁻¹) did, however, not reduce the butyrate flux significantly. Chloride concentration in itself did not seem to influence the transport of butyrate, but chloride-free conditions tended to increase the MS and SM flux of butyrate by a DIDS-sensitive pathway. DIDS (bilateral 0.5 mmol l⁻¹) did further decrease the butyrate SM flux significantly at all chloride concentrations. Removing bicarbonate from the experimental solutions decreased the MS and increased the SM flux of butyrate significantly, and abolished net butyrate flux. There were no significant effects of the carbonic anhydrase inhibitor Acetazolamide (bilateral 1.0 mmol l⁻¹). The results can be explained by a model where butyrate and butyrate metabolites are transported both by passive diffusion and by an electroneutral anion-exchange with bicarbonate. The model couples sodium and butyrate via CO₂ from metabolism of butyrate, and intracellular pH.     

Coupled sodium-chloride influx across brush border of flounder intestine

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.     

Characterization of Na+/H+ Exchanger Isoform (NHE1, NHE2 and NHE3) Expression in Prairie Dog Gallbladder

Gallbladder Na⁺ absorption is linked to gallstone formation in prairie dogs. Na⁺/H⁺ exchange (NHE) is one of the major Na⁺ absorptive pathways in gallbladder. In this study, we measured gallbladder Na⁺/H⁺ exchange and characterized the NHE isoforms expressed in prairie dogs. Na⁺/H⁺ exchange activity was assessed by measuring amiloride-inhibitable transepithelial Na⁺ flux and apical ²²Na⁺ uptake using dimethylamiloride (DMA). HOE-694 was used to determine NHE2 and NHE3 contributions. Basal J ^Na _ms was higher than J ^Na _sm with J ^Na _net absorption. Mucosal DMA inhibited transepithelial Na⁺ flux in a dose-dependent fashion, causing J ^Na _ms equal to J ^Na _sm and blocking J ^Na _net absorption at 100 μm. Basal ²²Na⁺ uptake rate was 10.9 ± 1.0 μmol · cm⁻²· hr⁻¹ which was inhibited by ∼43% by mucosal DMA and ∼30% by mucosal HOE-694 at 100 μm. RT-PCR and Northern blot analysis demonstrated expression of mRNAs encoding NHE1, NHE2 and NHE3 in the gallbladder. Expression of NHE1, NHE2 and NHE3 polypeptides was confirmed using isoform-specific anti-NHE antibodies. These data suggest that Na⁺/H⁺ exchange accounts for a substantial fraction of gallbladder apical Na⁺ entry and most of net Na⁺ absorption in prairie dogs. The NHE2 and NHE3 isoforms, but not NHE1, are involved in gallbladder apical Na⁺ uptake and transepithelial Na⁺ absorption. Received: 9 February 2001/Revised: 11 April 2001     

Effect of oxytocin on transepithelial transport of water and Na+ in distinct ventral regions of frog skin (Rana catesbeiana)

Thoracic, abdominal, and pelvic fragments of ventral skin of Rana catesbeiana were analysed regarding the effect of oxytocin on: (1) transepithelial water transport; (2) short-circuit current; (3) skin conductance and electrical potential difference; (4) Na⁺ conductance and electrical potential difference; (4) Na⁺ conductance, the electromotive force of Na⁺ transport mechanism, and shunt conductance; (5) short-circuit current responses to fast Na⁺ by K⁺ replacement in the outer compartment, and (6) epithelial microstructure. Unstimulated water and Na⁺ permeabilities were low along the ventral skin. Hydrosmotic and natriferic responses to oxytocin increased from thorax to pelvis. Unstimulated Na⁺ conductance was greater in pelvis than in abdomen, the other electrical parameters being essentially similar in both skin fragments. Contribution of shunt conductance to total skin conductance was higher in abdominal than in pelvic skin. Oxytocin-induced increases of total skin conductance, Na⁺ conductance, and shunt conductance in pelvis were significantly larger than in abdomen. An oscillatory behaviour of the short-circuit current was observed only in oxytocin-treated pelvic skins. Decrease of epithelial thickness and increase of mitochondria-rich cell number were observed from thorax to pelvis. Oxytocin-induced increases of interspaces were more conspicuous in pelvis and abdomen than in thorax.Abbreviations E _Na electromotive force of sodium transport mechansim - G _KCI skin conductance with external KCI Ringer - G _Na sodium conductance (series conductance) - G _shunt shunt pathway conductance - G _total total skin conductance - J _v water flux (in units of volume per area per time) - MRC mitochondria-rich cells - PD potential difference across skin - R _shunt resistance of the shunt pathway - SCC short-circuit current     

Active potassium transport by rabbit descending colon epithelium

Summary Previous studies of rabbit descending colon have disagreed concerning potassium transport across this epithelium. Some authors reported active K⁺ secretion underin vitro short-circuited conditions, while others suggested that K⁺ transport occurs by passive diffusion through a highly potassium-selective paracellular route. For this reason, we re-examined potassium fluxes across the colon in the presence of specific and general metabolic inhibitors. In addition, electrochemical driving forces for potassium across the apical and basolateral membranes were measured using conventional and ion-sensitive microelectrodes. Under normal conditions a significant net K⁺ secretion was observed (J _net ^K =–0.39±0.081 eq/cm²hr) with⁴²K fluxes, usually reaching steady-state within approximately 50 min following isotope addition. In colons treated with serosal addition of 10^–4 m ouabain,J _sm ^K was lowered by nearly 70% andJ _ms ^K was elevated by approximately 50%. Thus a small but significant net absorption was present (J _net ^K =0.12±0.027 eq/cm²hr). Under control conditions, the net cellular electrochemical driving force for K⁺ was 17 mV, favoring K⁺ exit from the cell. Cell potential measurements indicated that potassium remained above equilibrium after ouabain, assuming that passive membrane permeabilities are not altered by this drug. Net K⁺ fluxes were abolished by low temperature.The results indicate that potassium transport by the colon may occur via transcellular mechanisms and is not solely restricted to a paracellular pathway. These findings are consistent with our previous electrical results which indicated a nonselective paracellular pathway. Thus potassium transport across the colon can be modeled as a paracellular shunt pathway in parallel with pump-leak systems on the apical and basolateral membranes.     

Regulation of amiloride-sensitive Na+ absorption in the lizard (gallotia galloti) colon by aldosterone

• 1.1. Acute (AT) or chronic (CT) administration of exogenous aldosterone brought about a considerable increase in transmural potential difference (PD), short-circuit current (Isc) and net sodium flux (J_net^Na).
• 2.2. A good relationship between Isc and J_m-s^Na was observed in CT but not in AT or UC (untreated controls).
• 3.3. Addition of mucosal amiloride reduced Isc in AT and CT colons, but did not cause any change in UC colons. The relationship between Isc and J_m-s^Na, observed in CT was totally suppressed by the diuretic.
• 4.4. J_net^Na was reduced in AT tissues and abolished in CT colons by amiloride.
• 5.5. The present results strongly suggest that aldosterone levels quantitatively and qualitatively modify sodium absorption across colonie mucosa in a dose-dependent fashion and that lower levels of the hormone are required to induce the electrogenic Na absorption than to suppress the electroneutral transport.

     

Fluorescence measurement of intracellular sodium concentration in single Escherichia coli cells

The energy-transducing cytoplasmic membrane of bacteria contains pumps and antiports maintaining the membrane potential and ion gradients. We have developed a method for rapid, single-cell measurement of the internal sodium concentration ([Na⁺]_in) in Escherichia coli using the sodium ion fluorescence indicator, Sodium Green. The bacterial flagellar motor is a molecular machine that couples the transmembrane flow of ions, either protons (H⁺) or sodium ions (Na⁺), to flagellar rotation. We used an E. coli strain containing a chimeric flagellar motor with H⁺- and Na⁺-driven components that functions as a sodium motor. Changing external sodium concentration ([Na⁺]_ex) in the range 1–85 mM resulted in changes in [Na⁺]_in between 5–14 mM, indicating a partial homeostasis of internal sodium concentration. There were significant intercell variations in the relationship between [Na⁺]_in and [Na⁺]_ex, and the internal sodium concentration in cells not expressing chimeric flagellar motors was 2–3 times lower, indicating that the sodium flux through these motors is a significant fraction of the total sodium flux into the cell.     

Studies of the chloride transport in the gastric mucosa of the frog

The unidirectional fluxes of Cl^- and Na⁺ across the frog gastric mucosa in vitro were investigated with radioactive isotopes, and related to the secretory and electrical properties of the normal, and metabolically inhibited, mucosa. The flux of Cl^- from nutrient to secretory surface of the mucosa was observed to rise sharply with increasing acid secretion, while the corresponding flux of Na⁺ did not change appreciably. Lowering [NaCl] in the secretory solution caused a proportional drop in the fluxes from secretory to nutrient surface, of both Cl^- and Na⁺. Under the same conditions, the flux of Cl^- from nutrient to secretory surface fell by nearly the same amount as did the flux of Cl^- in the opposite direction, while the flux of Na⁺ from nutrient to secretory surface remained essentially unchanged. Electrical and hydrodynamic causes for this observation could be excluded. Metabolic inhibitors, including cyanide, azide, DNP, and anaerobiosis depressed Cl^- flux in both directions distinctly below the corresponding values observed with the normal, non-secreting mucosa. At the same time, a decrease in electrical potential difference and conductance was observed under inhibition. The flux of Na⁺ was little changed by metabolic inhibition. The relationship between fluxes and conductance of Cl^- during metabolic inhibition differs markedly from that observed under normal conditions, and is consistent with the view that during metabolic inhibition most of the Cl^- moving across the mucosa does so as a free ion. From the above data it is concluded that Cl^- is normally transported across the mucosa in combination with a carrier, the supply of which is impaired under metabolic inhibition. According to the behavior of the Na⁺ flux, the passive permeability of the mucosa appeared to be little affected by the metabolic inhibition applied, but seemed to rise considerably after death of the mucosa, probably due to structural damage.     

Ion transport by rabbit colon

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.     

Influence of Transepithelial Potential Difference on the Sodium Uptake at the Outer Surface of the Isolated Frog Skin

The unidirectional uptake of sodium across the outer surface of the isolated frog skin (J₁₂^Na) was measured in the presence of transepithelial potential difference (Δ_ψ) ranging from +100 to -100 mV. With a sodium concentration of 115 mM in the bathing solutions J₁₂^Na increases significantly when the spontaneous Δ_ψ is reduced to zero by short-circuiting the skin. With an Na concentration of 6 mM a progressive increase J₁₂^Na can be observed when Δ_ψ is decreased in several steps from +100 to -100 mV (serosal side positive and negative, respectively). The observed change J₁₂^Na amounts to a fraction only of that predicted from the shift in Δ_ψ. The results suggest that under open circuit conditions the potential step across the outside surface is at most one half of Δ_ψ and that the resistance across the outside and inside barrier of the skin is ohmic. This is in agreement with measurements of intracellular potentials in the frog skin and with resistance measurements carried out in the toad skin. The data strongly support the view that the saturating component of J_ψ proceeds via a charged carrier system. Exposure to negative values of Δ_ψ of 50 mV or more for times of 24 min or more result in a marked reduction of J₁₂^Na which shows only partial or no reversibility.