The sodium concentration of the lateral intercellular spaces of MDCK cells: a microspectrofluorimetric study

Propionibacteria as promising tools for the production of pro-bioactive scotta: a proof-of-concept study

Dairy propionibacteria are Gram positive Actinomycetota, routinely utilized as starters in Swiss type cheese making and highly appreciated for their probiotic properties and health promoting effects. In this work, within the frame of a circular economy approach, 47 Propionibacterium and Acidipropionibacterium spp. were isolated from goat cheese and milk, and ewe rumen liquor, and characterized in view of their possible utilization for the production of novel pro-bioactive food and feed on scotta, a lactose rich substrate and one of the main by-products of the dairy industry. The evaluation of the Minimum Inhibitory Concentration (MIC) of 13 among the most common antibiotics in clinical practice revealed a general susceptibility to ampicillin, gentamycin, streptomycin, vancomycin, chloramphenicol, and clindamycin while confirming a lower susceptibility to aminoglycosides and ciprofloxacin. Twenty-five isolates, that proved capable of lactose utilization as the sole carbon source, were then characterized for functional and biotechnological properties. Four of them, ascribed to Propionibacterium freudenreichii species, and harboring resistance to bile salts (growth at 0.7-1.56 mM of unconjugated bile salts), acid stress (>80% survival after 1 h at pH 2), osmostress (growth at up to 6.5% NaCl) and lyophilization (survival rate > 80%), were selected and inoculated in scotta. On this substrate the four isolates reached cell densities ranging from 8.11 ± 0.14 to 9.45 ± 0.06 Log CFU mL-1 and proved capable of producing different vitamin B9 vitamers after 72 h incubation at 30°C. In addition, the semi-quantitative analysis following the metabolomics profiling revealed a total production of cobalamin derivatives (vitamin B12) in the range 0.49-1.31 mg L-1, thus suggesting a full activity of the corresponding biosynthetic pathways, likely involving a complex interplay between folate cycle and methylation cycle required in vitamin B12 biosynthesis. These isolates appear interesting candidates for further ad-hoc investigation regarding the production of pro-bioactive scotta.

Long-Term Prolactin Exposure Differentially Stimulated the Transcellular and Solvent Drag-Induced Calcium Transport in the Duodenum of Ovariectomized Rats

Prolactin, having been shown to stimulate transcellular active and solvent drag-induced calcium transport in the duodenum of female rats, was postulated to improve duodenal calcium transport in estrogen-deficient rats. The aim of the present study was, therefore, to demonstrate the effects of long-term prolactin exposure produced by anterior pituitary (AP) transplantation on the duodenal calcium transport in young (9-week-old) and adult (22-week-old) ovariectomized rats. We found that ovariectomy did not alter the transcellular active duodenal calcium transport in young and adult rats fed normal calcium diet (1.0% w/w Ca) but decreased the solvent drag-induced duodenal calcium transport from 75.50 ± 10.12 to 55.75 ± 4.77 nmol·hr–1 cm–2 (P < 0.05) only in adult rats. Long-term prolactin exposure stimulated the transcellular active calcium transport in young and adult AP-grafted ovariectomized rats fed with normal calcium diet by more than 2-fold from 7.56 ± 0.79 to 16.54 ± 2.05 (P < 0.001) and 9.78 ± 0.72 to 15.99 ± 1.75 (P < 0.001) nmol·hr–1 cm–2, respectively. However, only the solvent drag-induced duodenal calcium transport in young rats was enhanced by prolactin from 95.51 ± 10.64 to 163.20 ± 18.03 nmol·hr–1 cm–2 (P < 0.001) whereas that in adult rats still showed a decreased flux from 75.50 ± 10.12 to 47.77 ± 5.42 nmol·hr–1 cm–2 (P < 0.05). Because oral calcium supplement has been widely used to improve calcium balance in estrogen-deficient animals, the effect of a high-calcium diet (2.0% w/w Ca) was also investigated. The results showed that stimulatory action of long-term prolactin on the transcellular active duodenal calcium transport in both young and adult rats was diminished after being fed a high-calcium diet. The same diet also abolished prolactin-enhanced solvent drag-induced duodenal calcium transport in young and further decreased that in adult AP-grafted ovariectomized rats. We concluded that the solvent drag-induced duodenal calcium transport in adult rats was decreased after ovariectomy. Long-term prolactin exposure stimulated the transcellular active duodenal calcium transport in both young and adult rats whereas enhancing the solvent drag-induced duodenal calcium transport only in young rats. Effects of prolactin were abolished by a high-calcium diet.

Evidence for plasmid-mediated salt tolerance in the human gut microbiome and potential mechanisms

The human gut microbiome is critical to health and wellbeing. It hosts a complex ecosystem comprising a multitude of bacterial species, which contributes functionality that would otherwise be absent from the host. Transient and commensal bacteria in the gut must withstand many stresses. The influence of mobile genetic elements such as plasmids in stress adaptation within the ecosystem is poorly understood. Using a mobilomic approach we found evidence for plasmid-mediated osmotolerance as a phenotype amongst the Proteobacteria in healthy faecal slurries. A transconjugant carrying multiple plasmids acquired from healthy faecal slurry demonstrated increased osmotolerance in the presence of metal salts, particularly potassium chloride, which was not evident in the recipient. Pyrosequencing and analysis of the total plasmid DNA demonstrated the presence of plasmid-borne osmotolerance systems (including KdpD and H-NS) which may be linked to the observed phenotype. This is the first report of a transferable osmotolerance phenotype in gut commensals and may have implications for the transfer of osmotolerance in other niches.

There is evidence of a transferable osmotolerance phenotype in gut commensals which may have implications for the transfer of osmotolerance in other niches.

Skp Trimer Formation Is Insensitive to Salts in the Physiological Range

Biogenesis of the Gram-negative outer membrane involves the chaperone seventeen kilodalton protein (Skp). A Skp trimer is currently thought to bind its unfolded outer membrane protein (uOMP) substrates. Using sedimentation equilibrium, we discovered that Skp is not an obligate trimer under physiological conditions and that Na(+), Cl(-), Mg(2+), and PO4(3-) ions are not linked to Skp trimerization. These findings suggest that electrostatics play a negligible role in Skp assembly. Our results demonstrate that Skp monomers are populated at biologically relevant concentrations, which raises the idea that kinetic formation of Skp-uOMP complexes likely involves Skp monomer assembly around its substrate. In addition, van't Hoff analysis of Skp self-association does not support a previously proposed coupled folding and trimerization of Skp.

Prolactin is an important regulator of intestinal calcium transport

Prolactin has been shown to stimulate intestinal calcium absorption, increase bone turnover, and reduce renal calcium excretion. The small intestine, which is the sole organ supplying new calcium to the body, intensely expresses mRNAs and proteins of prolactin receptors, especially in the duodenum and jejunum, indicating the intestine as a target tissue of prolactin. A number of investigations show that prolactin is able to stimulate the intestinal calcium transport both in vitro and in vivo, whereas bromocriptine, which inhibits pituitary prolactin secretion, antagonizes its actions. In female rats, acute and long-term exposure to high prolactin levels significantly enhances the (i) transcellular active, (ii) solvent drag-induced, and (iii) passive calcium transport occurring in the small intestine. These effects are seen not only in pregnant and lactating animals, but are also observed in non-pregnant and non-lactating animals. Interestingly, young animals are more responsive to prolactin than adults. Prolactin-enhanced calcium absorption gradually diminishes with age, thus suggesting it has an age-dependent mode of action. Although prolactin's effects on calcium absorption are not directly vitamin D-dependent; a certain level of circulating vitamin D may be required for the basal expression of genes related to calcium transport. The aforementioned body of evidence supports the hypothesis that prolactin acts as a regulator of calcium homeostasis by controlling the intestinal calcium absorption. Cellular and molecular signal transductions of prolactin in the enterocytes are largely unknown, however, and still require investigation.

Chronic metabolic acidosis stimulated transcellular and solvent drag-induced calcium transport in the duodenum of female rats

Chronic metabolic acidosis results in a negative calcium balance as a result of bone resorption and renal calcium loss. However, reports on the changes in intestinal calcium transport have been controversial. The present investigation therefore aimed to study the effects of chronic metabolic acidosis induced by 1.5% NH(4)Cl administration on the three components of duodenal calcium transport, namely, solvent drag-induced, transcellular active, and passive paracellular components, in rats using an in vitro Ussing chamber technique. The relative mRNA expression of genes related to duodenal calcium transport was also determined. We found that 21-day chronic metabolic acidosis stimulated solvent drag-induced and transcellular active duodenal calcium transport but not passive paracellular calcium transport. Our results further demonstrated that an acute direct exposure to serosal acidic pH, in contrast, decreased solvent drag-induced calcium transport in a pH-dependent fashion but had no effect on transcellular active calcium transport. Neither the transepithelial resistance nor duodenal permeability to Na(+), Cl(-), and Ca(2+) via the passive paracellular pathway were altered by chronic metabolic acidosis, suggesting that widening of the tight junction and changes in the charge-selective property of the tight junction did not occur. Thus the enhanced duodenal calcium transport observed in chronic metabolic acidosis could have resulted from a long-term adaptation, possibly at the molecular level. RT-PCR study revealed that chronic metabolic acidosis significantly increased the relative mRNA expression of duodenal genes associated with solvent drag-induced transport, i.e., the beta(1)-subunit of Na(+)-K(+)-ATPase, zonula occludens-1, occludin, and claudin-3, and with transcellular active transport, i.e., transient receptor potential vanilloid family Ca(2+) channels 5 and 6 and plasma membrane Ca(2+)-ATPase isoform 1b. Total plasma calcium and free ionized calcium and magnesium concentrations were also increased, whereas serum parathyroid hormone and 1alpha,25-dihydroxyvitamin D(3) levels were not changed. The results indicated that 21-day chronic metabolic acidosis affected the calcium metabolism in rats partly through enhancing the mRNA expression of crucial duodenal genes involved in calcium absorption, thereby stimulating solvent drag-induced and transcellular active calcium transport in the duodenum.

Prolactin directly enhanced Na+/K+- and Ca2+-ATPase activities in the duodenum of female rats

Prolactin has recently been shown to directly stimulate 2 components of the active duodenal calcium transport in female rats, i.e., solvent drag-induced and transcellular-active calcium transport. Since the basolateral Na+/K+- and Ca2+-ATPases, respectively, play important roles in these 2 transport mechanisms, the present study aimed to examine the direct actions of prolactin on the activities of both transporters in sexually mature female Wistar rats. The results showed that 200, 400, and 800 ng/mL prolactin produced a significant increase in the total ATPase activity of duodenal crude homogenate in a dose-dependent manner within 60 min (i.e., from a control value of 1.53 ± 0.13 to 2.29 ± 0.21 (p < 0.05), 2.68 ± 0.19 (p < 0.01), and 3.92 ± 0.33 (p < 0.001) µmol Pi·(mg protein)–1·min–1, respectively). Activity of Na+/K+-ATPase was increased by 800 ng/mL prolactin from 0.17 ± 0.03 to 1.18 ± 0.29 µmol Pi·(mg protein)–1·min–1 (p < 0.01). Prolactin at doses of 400 and 600 ng/mL also significantly increased the activities of Ca2+-ATPase in crude homogenate from a control value of 0.84 ± 0.03 to 1.75 ± 0.29 (p < 0.05), and 2.30 ± 0.37 (p < 0.001) µmol Pi·(mg protein)–1·min–1. When the crude homogenate was purified for the basolateral membrane, the Na+/K+-ATPase activities were elevated 10-fold. In the purified homogenate, 800 ng/mL prolactin increased Na+/K+-ATPase activity from 1.79 ± 0.38 to 2.63 ± 0.44 µmol Pi·(mg protein)–1·min–1 (p < 0.05), and Ca2+-ATPase activity from 0.08 ± 0.14 to 2.03 ± 0.23 µmol Pi·(mg protein)–1·min–1 (p < 0.001). Because the apical calcium entry was the first important step for the transcellular active calcium transport, the brush border calcium uptake was also investigated in this study. We found that, 8 min after being directly exposed to 800 ng/mL prolactin, the brush border calcium uptake into the duodenal epithelial cells was increased from 0.31 ± 0.02 to 0.80 ± 0.28 nmol·(mg protein)–1 (p < 0.05). It was concluded that prolactin directly and rapidly enhanced the brush border calcium uptake as well as the activities of the basolateral Na+/K+- and Ca2+-ATPases in the duodenal epithelium of female rats. These findings explained the mechanisms by which prolactin stimulated duodenal active calcium absorption.

Endogenous prolactin modulated the calcium absorption in the jejunum of suckling rats

Prolactin has been reported to stimulate intestinal calcium absorption in young and mature, but not aging rats. The present study was performed on suckling rats to elucidate the actions of endogenous prolactin on calcium absorption in various intestinal segments. Before measuring the calcium fluxes, 9-day-old rats were administered for 7 days with 0.9% NaCl, s.c. (control), 3 mg/kg bromocriptine, i.p., twice daily to abolish secretion of endogenous pro lac tin, or bromocriptine plus exogenous 2.5 mg/kg prolactin, s.c. Thereafter, the 16-day-old rats were experimented upon by instilling the45Ca-containing solution into the intestinal segments. The results showed that, under a physiological condition, the jejunum had the highest rate of calcium absorption compared with other segments (1.4 ± 0.35 µmol·h–1·cm–1, p < 0.05). The duodenum and ileum also manifested calcium absorption, whereas the colon showed calcium secretion. Lack of endogenous prolactin decreased lumen-to-plasma and net calcium fluxes in jejunum from 2.07 ± 0.31 to 1.19 ± 0.12 and 1.40 ± 0.35 to 0.88 ± 0.18 µmol·h–1·cm–1(p < 0.05), respectively, and exogenous prolactin restored the jejunal calcium absorption to the control value. Endogenous prolactin also had an effect on the duodenum but, in this case, exogenous prolactin did not reverse the effect of bromocriptine. However, neither ileal nor colonic calcium fluxes were influenced by prolactin. Because luminal sodium concentration has been demonstrated to affect calcium absorption in mature rats, the effect of varying luminal sodium concentrations on calcium fluxes in suckling rats was evaluated. The jejunum was used due to its highest rate of calcium absorption. After filling the jejunal segments with 124 (control), 80, 40 mmol/L Na+-containing or Na+-free solution, increases in calcium absorption were found to be inversely related to luminal sodium concentrations in both control and bromocriptine-treated rats. The plasma concentration of45Ca under luminal sodium free condition was also higher than that of the control condition (2.26% ± 0.07% vs. 2.01% ± 0.09% administered dose, p < 0.05). However,3H-mannitol, a marker of the widening of tight junction that was introduced into the lumen, had a stable level in the plasma during an increase in plasma45Ca, suggesting that the widening of tight junction was not required for enhanced calcium absorption. In conclusion, calcium absorption in suckling rats was of the highest rate in the jejunum where endogenous prolactin modulated calcium absorption without increasing the paracellular transport of mannitol. Key words: calcium absorption, intestinal segments, prolactin, suckling rats.

Prolactin directly stimulated the solvent drag-induced calcium transport in the duodenum of female rats

Prolactin has been reported to stimulate the calcium absorption of the duodenum where three components of the active calcium transport, namely transcellular active, voltage-dependent and solvent drag-induced calcium transport, were identified. It was known that the transcellular active, but not the voltage-dependent, duodenal calcium transport was directly stimulated by prolactin. The present study thus aimed to evaluate the direct action of prolactin on the solvent drag-induced duodenal calcium transport by using the Ussing chamber technique. The jejunum was used as a reference for the existence of solvent drag and the widening of tight junction induced by cytochalasin E. Results showed that the solvent drag-induced calcium transport existed in both intestinal segments, but the magnitude was significantly greater in the duodenum (29.27+/-2.27 vs. 17.31+/-1.65 nmol h(-1) cm(-2), P<0.001). We further demonstrated that 200, 600 and 800, but not 1000 ng/ml, prolactin significantly promoted the solvent drag-induced duodenal calcium transport in a dose-response manner, i.e. from the control value of (nmol h(-1) cm(-2)) 24.31+/-2.36 to 45.42+/-3.47 (P<0.01), 63.82+/-5.28 (P<0.001) and 53.93+/-5.41 (P<0.01), respectively. However, prolactin did not manifest any effect on the jejunum. Because the paracellular transport was suggested to be size-selective as well as charge-selective, further experiments were designed to evaluate the mechanism by which prolactin stimulated the solvent drag-induced calcium transport. The duodenum was exposed to 20 microM cytochalasin E, 600 ng/ml prolactin or the combination of both in the presence of a paracellular marker 3H-mannitol, while the jejunum was a positive reference. The results showed that, in the jejunum, cytochalasin E alone and cytochalasin E plus prolactin significantly increased the mannitol fluxes from (micromol h(-1) cm(-2)) 0.29+/-0.04 to 0.49+/-0.03 (P<0.05) and 0.48+/-0.05 (P<0.05), respectively, while having no effect on the calcium fluxes. Prolactin alone had no effect on the jejunal calcium flux. In the duodenum, neither mannitol nor calcium fluxes were enhanced by cytochalasin E, however, prolactin still increased the solvent drag-induced calcium flux from 27.74+/-2.41 to 51.03+/-4.35 nmol h(-1) cm(-2) (P<0.001). It was concluded that prolactin directly stimulated the solvent drag-induced duodenal calcium transport in a dose-response and biphasic manner without the widening of tight junction.

Apical Na+/H+ exchange near the base of mouse colonic crypts

Colonic crypts can absorb fluid, but the identity of the absorptive transporters remains speculative. Near the crypt base, the epithelial cells responsible for vectorial transport are relatively undifferentiated and often presumed to mediate only Cl- secretion. We have applied confocal microscopy in combination with an extracellular fluid marker [Lucifer yellow (LY)] or a pH-sensitive dye (2',7'-bis(2-carboxyethyl)-5(6)-carboxyfluorescein) to study mouse colonic crypt epithelial cells directly adjacent to the crypt base within an intact mucosal sheet. Measurements of intracellular pH report activation of colonocyte Na+/H+ exchange in response to luminal or serosal Na+. Studies with LY demonstrate the presence of a paracellular fluid flux, but luminal Na+ does not activate Na+/H+ exchange in the nonepithelial cells of the lamina propria, and studies with LY suggest that the fluid bathing colonocyte basolateral membranes is rapidly refreshed by serosal perfusates. The apical Na+/H+ exchange in crypt colonocytes is inhibited equivalently by luminal 20 microM ethylisopropylamiloride and 20 microM HOE-694 but is not inhibited by luminal 20 microM S-1611. Immunostaining reveals the presence of epitopes from NHE1 and NHE2, but not NHE3, in epithelial cells near the base of colonic crypts. Comparison of apical Na+/H+ exchange activity in the presence of Cl- with that in the absence of Cl- (substitution by gluconate or nitrate) revealed no evidence of the Cl--dependent Na+/H+ exchange that had been previously reported as the sole apical Na+/H+ exchange activity in the colonic crypt. Results suggest the presence of an apical Na+/H+ exchanger near the base of crypts with functional attributes similar to those of the cloned NHE2 isoform.

A mathematical model of solute coupled water transport in toad intestine incorporating recirculation of the actively transported solute

A mathematical model of an absorbing leaky epithelium is developed for analysis of solute coupled water transport. The non-charged driving solute diffuses into cells and is pumped from cells into the lateral intercellular space (lis). All membranes contain water channels with the solute passing those of tight junction and interspace basement membrane by convection-diffusion. With solute permeability of paracellular pathway large relative to paracellular water flow, the paracellular flux ratio of the solute (influx/outflux) is small (2-4) in agreement with experiments. The virtual solute concentration of fluid emerging from lis is then significantly larger than the concentration in lis. Thus, in absence of external driving forces the model generates isotonic transport provided a component of the solute flux emerging downstream lis is taken up by cells through the serosal membrane and pumped back into lis, i.e., the solute would have to be recirculated. With input variables from toad intestine (Nedergaard, S., E.H. Larsen, and H.H. Ussing, J. Membr. Biol. 168:241-251), computations predict that 60-80% of the pumped flux stems from serosal bath in agreement with the experimental estimate of the recirculation flux. Robust solutions are obtained with realistic concentrations and pressures of lis, and with the following features. Rate of fluid absorption is governed by the solute permeability of mucosal membrane. Maximum fluid flow is governed by density of pumps on lis-membranes. Energetic efficiency increases with hydraulic conductance of the pathway carrying water from mucosal solution into lis. Uphill water transport is accomplished, but with high hydraulic conductance of cell membranes strength of transport is obscured by water flow through cells. Anomalous solvent drag occurs when back flux of water through cells exceeds inward water flux between cells. Molecules moving along the paracellular pathway are driven by a translateral flow of water, i.e., the model generates pseudo-solvent drag. The associated flux-ratio equation is derived.

Fluorescence lifetime microscopy of the sodium indicator sodium-binding benzofuran isophthalate in HeLa cells

The behavior of the sodium indicator sodium-binding benzofuran isophthalate (SBFI) is investigated in HeLa cells by time-resolved fluorescence microscopy. The fluorescence relaxation of SBFI in HeLa cells can be described by a triexponential for intracellular sodium concentration ([Na(+)](i)) between 0 and 90 mM. Changes in [Na(+)](i) affect neither the fluorescence relaxation times (0.21, 0. 60, and 2.7 ns) nor the average decay time (2.2 ns). The preexponential factor of the shortest decay time is negative. However, the ratio of the fluorescence excitation signal at 340 nm to that at 380 nm increases with [Na(+)](i). To elucidate the behavior of SBFI in cells, experiments are performed on SBFI in buffer at various concentrations of sodium, potassium, and bovine serum albumin (BSA) and at various viscosities. The fluorescence decay is triexponential only in the presence of BSA. The relaxation times are independent of [Na(+)] and [BSA]. The preexponential factor of the shortest decay time is negative from a certain [BSA] on, which depends on [Na(+)]. The data indicate that interactions with intracellular components rather than microviscosity influence the SBFI behavior in cells. A model is suggested in which the fluorescence intensities are mainly determined by the signals from the Na(+) subsetSBFI and SBFI subsetprotein complexes.

Diffusion coefficients in the lateral intercellular spaces of Madin-Darby canine kidney cell epithelium determined with caged compounds

The diffusion coefficients of two caged fluorescent dyes were measured in free solution and in the lateral intercellular spaces (LIS) of cultured Madin-Darby canine kidney (MDCK) cells after photoactivation by illumination with a continuous or pulsed UV laser. Both quantitative video imaging and a new photometric method were utilized to determine the rates of diffusion of the caged fluorescent dyes: 8-((4,5-dimethoxy-2-nitrobenzyl)oxy)pyrene-1,3,6-trisulfonic acid (DMNB-HPTS) and (4,5-dimethoxy-2-nitrobenzyl) fluorescein dextran (10,000 MW) (DMNB-caged fluorescein dextran). The diffusion coefficients at 37 degrees C in free solution were 3.3 x 10(-6) cm2/s (HPTS) and 0.98 x 10(-6) cm2/s (10,000 MW dextran). Diffusion of HPTS within nominally linear stretches of the LIS of MDCK cells grown on glass coverslips was indistinguishable from that in free solution, whereas dextran showed a 1.6 +/- 0.5-fold reduction in diffusivity. Measurements of HPTS diffusion within the LIS of multicellular regions also exhibited a diffusivity comparable to the free solution value. The restriction to diffusion of the dextran within the LIS may be due to molecular hindrance.

Routes and mechanism of fluid transport by epithelia

The mechanism of fluid transport by leaky epithelia and the route taken by the transported fluid are in dispute. A consideration of current mathematical models for coupling of solutes and water, as well as the methodologies for the study of fluid transport, shows that local osmosis best accounts for water movement. Although it seems virtually certain that the tight junctions are water permeable, the fraction of absorbed fluid that crosses the tight junction cannot yet be determined with confidence.

The chloride concentration in the lateral intercellular spaces of MDCK cell monolayers

We measured the Cl concentration of the lateral intercellular spaces (LIS) of MDCK cell monolayers, grown on glass coverslips, by video fluorescence microscopy. Monolayers were perfused at 37 degrees C either with HEPES-buffered solutions containing 137 mM Cl or bicarbonate/CO2-buffered solutions containing 127 mM Cl. A mixture of two fluorescent dyes conjugated to dextrans (MW 10,000) was microinjected into domes and allowed to diffuse into the nearby LIS. The Cl-sensitive dye, ABQ-dextran, was selected because of its responsiveness at high Cl concentrations; a Cl-insensitive dye, Cl-NERF-dextran, was used as a reference. Both dyes were excited at 325 nm, and ratios of the fluorescence intensity at spectrally distinct emission wavelengths were obtained from two intensified CCD cameras, one for ABQ-dextran the other for Cl-NERF-dextran. LIS Cl concentration was calibrated in situ by treating the monolayer with digitonin or ouabain and varying the perfusate Cl between 0 and 137 mM (HEPES buffer) or between 0 and 127 mM (bicarbonate/CO2 buffer). LIS Cl in HEPES-buffered solutions averaged 176 +/- 19 mM (n = 12), calibrated with digitonin, and 170 +/- 9 mM (n = 12), calibrated with ouabain. LIS Cl in bicarbonate/CO2-buffered solutions averaged 174 +/- 10 mM (n = 7) using the ouabain calibration. The Cl concentration of MDCK cell domes, measured with Cl-sensitive microelectrodes and by microspectrofluorimetry, did not differ significantly. Images of the LIS at 3 focal planes, near the tight junction, midway and basal, failed to reveal any gradients in Cl concentration along the LIS. LIS Cl changed rapidly in response to perfusate Cl with characteristic times of 0.8 +/- 0.1 min (n = 21) for Cl decrease and 0.3 +/- 0.04 min (n = 21) for Cl increase. In conclusion, (i) Cl concentration is higher in the LIS than in the bathing medium, (ii) no gradients of Cl along the depth of LIS are detectable, (iii) junctional Cl permeability is high.