Digitonin-permeabilized colonic cell layers. Demonstration of calcium-activated basolateral K+ and Cl- conductances

Direct access and control of the intracellular solution environment in single cells

Methods that can control and vary the solution environment around single cells are abundant. In contrast, methods that offer direct access to the intracellular proteome and genome in single cells with the control, flexibility, and convenience given by microfluidic methods are both scarce and in great demand. Here, we present such a method based on using a microfluidic device mounted on a programmable scanning stage and cells on-chip permeabilized by the pore-forming glycoside digitonin. We characterized the on-chip digitonin poration, as well as the solution exchange within cells. Intracellular solution exchange times vary with the dose of exposure to digitonin from less than a second to tens of seconds. Also, the degree of permeabilization obtained for cells treated with the same dose varies considerably, especially for low doses of digitonin exposure and low permeabilities. With the use of the presented setup, the degree of permeabilization can be measured during the permeabilization process, which allows for "on-line" optimization of the digitonin exposure time. Using this calibrated permeabilization method, we demonstrate the generation of intracellular oscillations, intracellular gradients, and the delivery of substrate to initiate enzymatic reactions in situ. This method holds the potential to screen and titrate intracellular receptors or enzymes or to generate intracellular oscillations, useful in the study of signaling pathways and oscillation decoding among other applications.

Basolateral Cl channels in primary airway epithelial cultures

Salt and water absorption and secretion across the airway epithelium are important for maintaining the thin film of liquid lining the surface of the airway epithelium. Movement of Cl across the apical membrane involves the CFTR Cl channel; however, conductive pathways for Cl movement across the basolateral membrane have been little studied. Here, we determined the regulation and single-channel properties of the Cl conductance ( GCl) in airway surface epithelia using epithelial cultures from human or bovine trachea and freshly isolated ciliated cells from the human nasal epithelium. In Ussing chamber studies, a swelling-activated basolateral GCl was found, which was further stimulated by forskolin and blocked by N-phenylanthranilic acid (DPC) = sucrose > flufenamic acid = niflumic acid = glibenclamide > CdCl2 = 5-nitro-2-(3-phenylpropylamino)benzoic acid (NPPB) = DIDS = ZnCl2 > tamoxifen > 4,4′-dinitro-2,2′-stilbene-disulfonate disodium salt (DNDS). In whole cell patch-clamp experiments, three types of GCl were identified: 1) a voltage-activated, DIDS- (but not Cd-) blockable and osmosensitive GCl; 2) an inwardly rectifying, hyperpolarization-activated and Cd-sensitive GCl; and 3) a forskolin-activated, linear GCl, which was insensitive to Cd and DIDS. In cell-attached patch-clamp recordings, the basolateral pole of isolated ciliated cells expressed three types of Cl channels: 1) an outwardly rectifying, swelling-activated Cl channel; 2) a strongly inwardly rectifying Cl channel; and 3) a forskolin-activated, low-conductance channel. We propose that, depending on the driving force for Cl across the apical membrane, basolateral Cl channels confine Cl− secretion or support transcellular Cl− absorption.

The flavonol quercetin activates basolateral K(+) channels in rat distal colon epithelium

1. The flavonol quercetin has been shown to activate a Cl(-) secretion in rat colon. Unlike the secretory activity of the related isoflavone genistein, quercetin's secretory activity does not depend on cyclic AMP; instead, it depends on Ca(2+). We investigated the possible involvement of Ca(2+) dependent basolateral K(+) channels using apically permeabilized rat distal colon epithelium mounted in Ussing chambers. 2. In intact epithelium, quercetin induced an increase in short-circuit current (I(sc)), which was diminished by the Cl(-) channel blockers NPPB and DPC, but not by glibenclamide, DIDS or anthracene-9-carboxylic acid. The effect of the flavonol was also inhibited by several serosally applied K(+) channel blockers (Ba(2+), quinine, clotrimazole, tetrapentylammonium, 293B), whereas other K(+) channel blockers failed to influence the quercetin-induced increase in I(sc) (tetraethylammonium, charybdotoxin). 3. The apical membrane was permeabilized by mucosal addition of nystatin and a serosally directed K(+) gradient was applied. The successful permeabilization was confirmed by experiments demonstrating the failure of bumetanide to inhibit the carbachol-induced current. 4. In apically permeabilized epithelium, quercetin induced a K(+) current (I(K)), which was neither influenced by ouabain nor by bumetanide. Whereas DPC, NPPB, charybdotoxin and 293B failed to inhibit this I(K), quinine, Ba(2+), clotrimazole and tetrapentylammonium were effective blockers of this current. 5. We conclude from these results that at least part of the quercetin-induced Cl(-) secretion can be explained by an activation of basolateral K(+) channels.

Basolateral potassium channels of rabbit colon epithelium: role in sodium absorption and chloride secretion

In order to assess the role of different classes of K(+) channels in recirculation of K(+) across the basolateral membrane of rabbit distal colon epithelium, the effects of various K(+) channel inhibitors were tested on the activity of single K(+) channels from the basolateral membrane, on macroscopic basolateral K(+) conductance, and on the rate of Na(+) absorption and Cl(-) secretion. In single-channel measurements using the lipid bilayer reconstitution system, high-conductance (236 pS), Ca(2+)-activated K(+) (BK(Ca)) channels were most frequently detected; the second most abundant channel was a low-conductance K(+) channel (31 pS) that exhibited channel rundown. In addition to Ba(2+) and charybdotoxin (ChTX), the BK(Ca) channels were inhibited by quinidine, verapamil and tetraethylammonium (TEA), the latter only when present on the side of the channel from which K(+) flow originates. Macroscopic basolateral K(+) conductance, determined in amphotericin-permeabilised epithelia, was also markedly reduced by quinidine and verapamil, TEA inhibited only from the lumen side, and serosal ChTX was without effect. The chromanol 293B and the sulphonylurea tolbutamide did not affect BK(Ca) channels and had no or only a small inhibitory effect on macroscopic basolateral K(+) conductance. Transepithelial Na(+) absorption was partly inhibited by Ba(2+), quinidine and verapamil, suggesting that BK(Ca) channels are involved in basolateral recirculation of K(+) during Na(+) absorption in rabbit colon. The BK(Ca) channel inhibitors TEA and ChTX did not reduce Na(+) absorption, probably because TEA does not enter intact cells and ChTX is 'knocked off' its extracellular binding site by K(+) outflow from the cell interior. Transepithelial Cl(-) secretion was inhibited completely by Ba(2+) and 293B, partly by quinidine but not by the other K(+) channel blockers, indicating that the small (<3 pS) K(V)LQT1 channels are responsible for basolateral K(+) exit during Cl(-) secretion. Hence different types of K(+) channels mediate basolateral K(+) exit during transepithelial Na(+) and Cl(-) transport.

Electrolyte transport in the mammalian colon: mechanisms and implications for disease

The colonic epithelium has both absorptive and secretory functions. The transport is characterized by a net absorption of NaCl, short-chain fatty acids (SCFA), and water, allowing extrusion of a feces with very little water and salt content. In addition, the epithelium does secret mucus, bicarbonate, and KCl. Polarized distribution of transport proteins in both luminal and basolateral membranes enables efficient salt transport in both directions, probably even within an individual cell. Meanwhile, most of the participating transport proteins have been identified, and their function has been studied in detail. Absorption of NaCl is a rather steady process that is controlled by steroid hormones regulating the expression of epithelial Na(+) channels (ENaC), the Na(+)-K(+)-ATPase, and additional modulating factors such as the serum- and glucocorticoid-regulated kinase SGK. Acute regulation of absorption may occur by a Na(+) feedback mechanism and the cystic fibrosis transmembrane conductance regulator (CFTR). Cl(-) secretion in the adult colon relies on luminal CFTR, which is a cAMP-regulated Cl(-) channel and a regulator of other transport proteins. As a consequence, mutations in CFTR result in both impaired Cl(-) secretion and enhanced Na(+) absorption in the colon of cystic fibrosis (CF) patients. Ca(2+)- and cAMP-activated basolateral K(+) channels support both secretion and absorption of electrolytes and work in concert with additional regulatory proteins, which determine their functional and pharmacological profile. Knowledge of the mechanisms of electrolyte transport in the colon enables the development of new strategies for the treatment of CF and secretory diarrhea. It will also lead to a better understanding of the pathophysiological events during inflammatory bowel disease and development of colonic carcinoma.

Metabolic support of Na+ pump in apically permeabilized A6 kidney cell epithelia: role of creatine kinase

The contribution of ATP-generating systems to Na+ pump (Na+-K+-ATPase) function was studied in Xenopus laevis A6 kidney epithelia apically permeabilized with digitonin. The ouabain-inhibitable Na+ pump current (I(P)) was measured in the presence of otherwise impermeant inhibitors and/or substrates at Na+ and K+ concentrations that allowed near-maximal pump function. Confocal fluorescence microscopy after apical addition of sulfosuccinimidobiotin (molecular weight of 443) showed that all cells were permeabilized. Less than 15% of the endogenous lactate dehydrogenase and creatine kinase (CK) were released into the apical medium. The I(P) was approximately 5 microA/cm2 in the presence of D-glucose. Blocking glycolysis with 2-deoxy-D-glucose or oxidative phosphorylation with antimycin A decreased it by > or = 50%. Exogenously added ATP prevented these decreases fully or partially, respectively. Two CK isoforms were detected, one likely being mitochondrial and the other corresponding to mammalian B isoform of CK. Phosphocreatine partially restored Na+ pump activity during inhibition of either ATP synthesis pathway. In conclusion, the ATP used by Na+ pumps of apically digitonin-permeabilized A6 epithelia is generated to a similar extent by glycolysis and oxidative phosphorylation. The CK system can partially support the ATP supply to the Na+ pumps.

Ca2+ and cAMP activate different K+ conductances in the human intestinal goblet cell line HT29-Cl.16E

The mechanism of regulated Cl- secretion was evaluated in the mucin-secreting cell line HT29-Cl.16E by transepithelial electrophysiology and fura 2 measurements of cytosolic Ca2+. Carbachol by itself was a weak secretagogue, but augmented adenosine 3',5'-cyclic monophosphate (cAMP)-mediated secretion more than twofold, consistent with activation of a rate-limiting K+ conductance. To characterize this conductance, monolayers were apically permeabilized with amphotericin B. At least two types of K+ conductances were identified. One type was activated by elevated cytosolic cAMP levels and inhibited by Ba2+ (inhibitor constant 0.3 mM) in the basolateral solution but was not affected by quinidine or elevated cytosolic Ca2+. The other type was activated by carbachol via cytosolic Ca2+ and was partially inhibited by quinidine (60% inhibition by 2.5 mM quinidine) but was not affected by Ba2+ up to 1 mM. Both conductances appear to be involved in active, transepithelial Cl- secretion in intact monolayers but under different conditions because 1) the cAMP-stimulated short-circuit current (Isc) can be partially inhibited by 1 mM Ba2+ (50%) but not quinidine, 2) the Ba2+ inhibition does not affect the carbachol-induced increase in Isc in cells with elevated cAMP levels, and 3) the carbachol-dependent Isc can be inhibited by quinidine. Therefore, the contribution of the cAMP-dependent K+ conductance appears important for maintaining the membrane potential and therewith Cl- secretion when cAMP is the only messenger for secretion signals, whereas the Ca(2+)-dependent K+ conductance is responsible for the carbachol-stimulated increase in Isc.

Cell Cl and transepithelial na transport in toad urinary bladder

Relationships between short-circuit current (Isc), cell Cl and the mechanism(s) of Cl accumulation in toad bladder epithelial cells were investigated. In serosal Cl-free gluconate Ringer, 80% of the cell Cl (measured by x-ray microanalysis) was lost over 30-60 min with an associated decrease in cell water content. concomitantly, Isc fell to 20% of its initial value within 10 min but then recovered to 45% of its initial value despite continued Cl loss. With the reintroduction of Cl, cell Cl and Isc both recovered within 10 min. Serosal SITS (4-acetamido-4'-isothiocyano-stilbene-2,2'-disulfonate; 0.5 mM) plus bumetanide (0.1 mM), did not prevent the fall in Isc or the loss of cell Cl in gluconate medium, although they did inhibit subsequent recovery of Isc in this medium. They also prevented the recovery of Isc in Cl medium but not the reaccumulation of Cl by the cells. Although SITS and bumetanide did not prevent the loss or recovery of Cl, they modified the pattern of the ion changes. In their absence, changes in cellular Cl were twice that of the changes in measured cellular cations implicating basolateral Cl/HCO3 exchange in Cl movement. With SITS plus bumetanide present, changes of similar magnitude in Cl were associated with equivalent changes in cation, consistent with the inhibition of Cl/HCO3 exchange.

Swelling-induced anion and cation conductances in human epididymal cells

1. Activation of both anion and cation conductances was observed in primary cultured human epididymal cells during osmotic swelling under the patch-clamp whole-cell configuration. The swelling-induced anion conductance was 25.66 +/- 4.70 nS and the cation conductance was 7.35 +/- 1.40 nS. The permeability ratio of K+ to Cl- (PK/PCl) was calculated to be 0.40. Known anion or cation channel blockers could inhibit both conductances simultaneously. 2. When the major permeant ion species in the pipette and bath solution was Cl-, the mean conductance was found to be 17.06 +/- 1.8 nS, significantly smaller than that obtained in the presence of intracellular K+, 25.66 +/- 4.70 nS (P < 0.05). No significant current activation was observed when solutions containing only K+ as the permeant ion were used. 3. When the anionic amino acids glutamate and aspartate were used to replace extracellular Cl-, the permeability ratios were calculated to be PGlut/PCl = 0.20 and PAsp/PCl = 0.17. 4. The cation conductance was found to be non-selective since its permeability to other cations such as Na+ and choline, an organic compound highly concentrated in epididymal fluid, was similar to that of K+. 5. Regulatory volume decrease (RVD) was observed after initial osmotic swelling; this could be inhibited by either anion or cation channel blockers. 6. The results of this study suggest that both anion and cation conductances are activated during cellular swelling, and indicate the existence of an interdependent relationship between the swelling-induced cation and anion conductances. Both swelling-induced cation and anion conductances are involved in the volume regulatory process and may be responsible for transporting amino acids or organic compounds in human epididymal cells.

Inward-rectifier potassium channels in basolateral membranes of frog skin epithelium

Inward-rectifier K channel: using macroscopic voltage clamp and single-channel patch clamp techniques we have identified the K+ channel responsible for potassium recycling across basolateral membranes (BLM) of principal cells in intact epithelia isolated from frog skin. The spontaneously active K+ channel is an inward rectifier (Kir) and is the major component of macroscopic conductance of intact cells. The current-voltage relationship of BLM in intact cells of isolated epithelia, mounted in miniature Ussing chambers (bathed on apical and basolateral sides in normal amphibian Ringer solution), showed pronounced inward rectification which was K(+)-dependent and inhibited by Ba2+, H+, and quinidine. A 15-pS Kir channel was the only type of K(+)-selective channel found in BLM in cell-attached membrane patches bathed in physiological solutions. Although the channel behaves as an inward rectifier, it conducts outward current (K+ exit from the cell) with a very high open probability (Po = 0.74-1.0) at membrane potentials less negative than the Nernst potential for K+. The Kir channel was transformed to a pure inward rectifier (no outward current) in cell-attached membranes when the patch pipette contained 120 mM KCl Ringer solution (normal NaCl Ringer in bath). Inward rectification is caused by Mg2+ block of outward current and the single-channel current-voltage relation was linear when Mg2+ was removed from the cytosolic side. Whole-cell current-voltage relations of isolated principal cells were also inwardly rectified. Power density spectra of ensemble current noise could be fit by a single Lorentzian function, which displayed a K dependence indicative of spontaneously fluctuating Kir channels.

CONCLUSIONS

under physiological ionic gradients, a 15-pS inward-rectifier K+ channel generates the resting BLM conductance in principal cells and recycles potassium in parallel with the Na+/K+ ATPase pump.

Basolateral potassium membrane permeability of A6 cells and cell volume regulation

The K+ permeabilities (86Rb(K) transport) of the basolateral membranes (JbK) of a renal cell line (A6) were compared under isosmotic and hypo-osmotic conditions (serosal side) to identify the various components involved in cell volume regulation. Changing the serosal solution to a hypo-osmotic one (165 mOsm) induced a fast transient increase in Cai (max < 1 min) and cell swelling (max at 3-5 min) followed by a regulatory volume decrease (5-30 min) and rise in the SCC (stabilization at 30 min). In isosmotic conditions (247 mOsm), the 86Rb(K) transport and the SCC were partially blocked by Ba2+, quinidine, TEA and glibenclamide, the latter being the least effective. Changing the osmolarity from isosmotic to hypo-osmotic resulted in an immediate (within the first 3-6 min) stimulation of the 86Rb(K) transport followed by a progressive decline to a stable value higher than that found in isosmotic conditions. A serosal Ca(2+)-free media or quinidine addition did not affect the initial osmotic stimulation of JbK but prevented its "secondary regulation", whereas TEA, glibenclamide and DIDS completely blocked the initial JbK increase. Under hypo-osmotic conditions, the initial JbK increase was enhanced by the presence of 1 mM of barium and delayed with higher concentrations (5 mM). In addition, cell volume regulation was fully blocked by quinidine, DIDS, NPPB and glibenclamide, while partly inhibited by TEA and calcium-free media. We propose that a TEA- and glibenclamide-sensitive but quinidine-insensitive increase in K+ permeability is involved in the very first phase of volume regulation of A6 cells submitted to hypo-osmotic media. In achieving cell volume regulation, it would play a complementary role to the quinidine-sensitive K+ permeability mediated by the observed calcium rise.

Tetraethylammonium-sensitive apical K+ channels mediating K+ secretion by turtle colon

1. Apical membrane K+ channels in turtle colon were identified and characterized using current fluctuation analysis. 2. Under short-circuit conditions in NaCl-Ringer solution, the power density spectrum (PDS) of the short-circuit current (Isc) sometimes exhibited a clearly discernible Lorentzian component, indicating spontaneous fluctuations produced by a population of apical ion channels. The Lorentzian component had a characteristic corner frequency (fc) which averaged 10.2 +/- 0.9 Hz (mean +/- S.E.M., n = 20). 3. The power of the spontaneous fluctuations was enhanced (So increased) by manoeuvres that depolarize the apical membrane electrical potential (Va). Discernible fluctuations were enhanced or induced by raising the serosal K+ concentration ([K+]s = 50-115 mM, Na+ replacement), by clamping the transepithelial potential (Vt) to serosa-positive values, or by blocking basolateral K+ channels with Ba2+. 4. Mucosal amiloride (100 microM) attenuated the spontaneous fluctuations observed in NaCl-Ringer solution but had no effect in the presence of serosal high K+, indicating that amiloride did not block the K(+)-permeable channels but these channels resided in the same cells as the amiloride-sensitive Na+ channels. 5. Raising the mucosal K+ concentration attenuated spontaneous fluctuations. 6. In the presence of serosal high K+ and mucosal amiloride, the spontaneous fluctuations were often accompanied by a reversed Isc consistent with K+ secretion. These conditions were used to test the effects of putative channel blockers. 7. Mucosal Ba2+ and tetraethylammonium (TEA+) were effective inhibitors of the spontaneous fluctuations and the reversed Isc. At a concentration of 10 mM, TEA+ was maximally effective but the TEA+ analogues tetramethylammonium (TMA+) and tetrapropylammonium (TPrA+) were much less effective. Mucosal Rb+ or Cs+ did not inhibit at a concentration of 10 mM. 8. Mucosal lidocaine (200 microM), quinidine (200 microM), or diphenylamine-2-carboxylate (DPC, 1 mM) had little or no effect on the spontaneous fluctuations and reversed Isc. Quinine (100 microM), 4-aminopyridine (1 mM), and apamin (100 nM) were also without effect. 9. Mucosal TEA+ (10 mM) abolished the active secretory K+ flux measured in the presence of serosa-positive transepithelial potentials. 10. These experiments identified a population of TEA(+)-sensitive, apical K+ channels which mediate active K+ secretion in turtle colon. Sensitivity to external TEA+ distinguishes these channels from basolateral K+ channels in turtle colon and demonstrates similarity to apical K+ channels in mammalian colon.

Selective block of specific K(+)-conducting channels by diphenylamine-2-carboxylate in turtle colon epithelial cells

1. The conduction and gating properties of K(+)-conducting channels were studied in isolated turtle colon cells in an attempt to identify the single channels responsible for specific components of the macroscopic conductance of the basolateral membrane. Three types of Ca(2+)-activated channel were identified, two of which were selective for K+ over Na+ and a third which was selective for monovalent cations over anions, but did not discriminate between K+ and Na+. 2. One of the K(+)-selective channels was a large-conductance 'maxi' K+ channel. A second was characterized by a lower conductance and pronounced inward rectification. 3. The inward-rectifying K+ channel was selectively blocked by diphenylamine-2-carboxylate (DPC). Neither the maxi K+ channel nor a previously identified K(+)-selective channel thought to be activated by cell swelling was affected by this compound. DPC also blocked the non-selective cation channel. 4. An inward-rectifying, DPC-sensitive current was prominent in whole cell-recordings, and DPC blocked basolateral K+ currents in colonic cell layers apically permeabilized with amphotericin-B. In addition, the compound blocked active Na+ absorption. 5. The selective block of a class of epithelial K+ channels by DPC may be a useful tool for determining the contribution of this specific subpopulation to macroscopic conductance and transepithelial salt transport.

Verapamil blocks basolateral K+ channels in the larval frog skin

The short-circuit current (Isc) across isolated skin from larval frogs (Rana catesbeiana) was measured when the tissue was bathed with Na2SO4 Ringer solution on the serosal side and with a Ringer solution containing K+ as the primary cation on the mucosal side. When 150 U/ml nystatin was added to the mucosal solution, the Isc increased from 1.4 +/- 0.1 to 35.4 +/- 4.8 microA/cm2. When verapamil was added to the mucosal and serosal Ringer solutions in concentrations between 2.5 and 80 microM, Isc was inhibited in a stepwise manner. At 80 microM, Isc was reduced by 75.3% to 8.74 +/- 1.14 microA/cm2. Analysis of the inhibition of Isc with the direct linear plot method showed that the blockage of Isc could be described by pseudo-first-order kinetics with a Michaelis constant (Km) of 9.59 +/- 2.20 microM. Fluctuation analysis revealed a Lorentzian component in power spectra obtained from preparations treated with 10-80 microM verapamil. The corner frequency of these Lorentzian components increased in a linear manner over this range of verapamil concentrations. The Km calculated from the ratio of the dissociation and association rate constants (k10/k'01) was 39.5 microM. The single-channel currents (i) calculated from the fluctuation analysis parameters decreased significantly between verapamil concentrations of 10 and 80 microM. It appears that the inhibition of K+ channels in the basolateral membrane of this tissue has at least two components.(ABSTRACT TRUNCATED AT 250 WORDS)

Volume-activated calcium uptake: its role in cell volume regulation of Madin-Darby canine kidney cells

Immediately after osmotic swelling of Madin-Darby canine kidney (MDCK) cells, a transient (1-2 min) increase in Ca2+ influx and internal Ca2+ (Ca2+i) is observed. The normal Ca2+ influx appears to be mediated by the 3Na(+)-Ca2+ exchange system [Borle et al. Am. J. Physiol. 259 (Cell Physiol. 28): C19-C25, 1990], but the swelling-induced component is different in 1) Na+ dependence, 2) affinity for Ca2+, 3) inhibition by La3+, and 4) direction of net flux at low external Ca2+. Swelling appears to activate an uncoupled Ca2+ flow, perhaps through cation-nonspecific stretch-activated channels. The regulatory volume decrease (RVD) is dependent on the swelling-induced pulse of Ca2+ influx and associated rise in Ca2+i. Swelling also induces a biphasic change in membrane potential, a hyperpolarization followed by depolarization, reflecting sequential increases in K+ and Cl- permeabilities. The time dependence of the former corresponds closely with the transient peak in Ca2+i, but the latter does not. Ca2+i appears to have a direct activating effect on K+ channels but an indirect effect on Cl- channels, mediated via other Ca(2+)-triggered systems. The sequence of events following cell swelling appears to be transient increases in Ca2+ permeability, Ca2+ influx, Ca2+i, K+ permeability, followed by triggering of a mediating system that increases Cl- permeability. The net result is KCl, osmotic water loss, and volume adjustment.

Basolateral membrane potassium conductance of A6 cells

To study the properties of the basolateral membrane conductance of an amphibian epithelial cell line, we have adapted the technique of apical membrane selective permeabilization (Wills, N.K., Lewis, S.A., Eaton, D.C. 1979b, J. Membrane Biol. 45:81-108). Monolayers of A6 cells cultured on permeable supports were exposed to amphotericin B. The apical membrane was effectively permeabilized, while the high electrical resistance of the tight junctions and the ionic selectivity of the basolateral membrane were preserved. Thus the transepithelial current-voltage relation reflected mostly the properties of the basolateral membrane. Under "basal" conditions, the basolateral membrane conductance was inward rectifying, highly sensitive to barium but not to quinidine. After the induction of cell swelling either by adding chloride to the apical solution or by lowering the osmolarity of the basolateral solution, a large outward-rectifying K+ conductance was observed, and addition of barium or quinidine to the basolateral side inhibited, respectively, 82.4 +/- 1.9% and 90.9 +/- 1.0% of the transepithelial current at 0 mV. Barium block was voltage dependent; the half-inhibition constant (Ki) varied from 1499 +/- 97 microM at 0 mV to 5.7 +/- 0.5 microM at -120 mV. Cell swelling induces a large quinidine-sensitive K+ conductance, changing the inward-rectifying basolateral membrane conductance observed under "basal" conditions into a conductance with outward-rectifying properties.

Whole-cell and single channel K+ and Cl- currents in epithelial cells of frog skin

Whole-cell and single channel currents were studied in cells from frog (R. pipiens and R. catesbiana) skin epithelium, isolated by collagenase and trypsin treatment, and kept in primary cultures up to three days. Whole-cell currents did not exhibit any significant time-dependent kinetics under any ionic conditions used. With an external K gluconate Ringer solution the currents showed slight inward rectification with a reversal potential near zero and an average conductance of 5 nS at reversal. Ionic substitution of the external medium showed that most of the cell conductance was due to K and that very little, if any, Na conductance was present. This confirmed that most cells originate from inner epithelial layers and contain membranes with basolateral properties. At voltages more positive than 20 mV outward currents were larger with K in the medium than with Na or N-methyl-D-glucamine. Such behavior is indicative of a multi-ion transport mechanism. Whole-cell K current was inhibited by external Ba and quinidine. Blockade by Ba was strongly voltage dependent, while that by quinidine was not. In the presence of high external Cl, a component of outward current that was inhibited by the anion channel blocker diphenylamine-2-carboxylate (DPC) appeared in 70% of the cells. This component was strongly outwardly rectifying and reversed at a potential expected for a Cl current. At the single channel level the event most frequently observed in the cell-attached configuration was a K channel with the following characteristics: inward-rectifying I-V relation with a conductance (with 112.5 mM K in the pipette) of 44 pS at the reversal potential, one open and at least two closed states, and open probability that increased with depolarization. Quinidine blocked by binding in the open state and decreasing mean open time. Several observations suggest that this channel is responsible for most of the whole-cell current observed in high external K, and for the K conductance of the basolateral membrane of the intact epithelium. On a few occasions a Cl channel was observed that activated upon excision and brief strong depolarization. The I-V relation exhibited strong outward rectification with a single channel conductance of 48 pS at 0 mV in symmetrical 112 mM Cl solutions. Kinetic analysis showed the presence of two open and at least two closed states. Open time constants and open probability increased markedly with depolarization.(ABSTRACT TRUNCATED AT 400 WORDS)

Basolateral K conductance: role in regulation of NaCl absorption and secretion

In this review we explore the possible role of basolateral K conductance (gK) in the regulation of salt absorption and secretion. This inquiry is prompted by a growing body of evidence which, taken together, suggests that basolateral gK is very labile and that alterations in basolateral gK may be a key feature in both stimulatory and inhibitory regulatory mechanisms. We first consider the role of basolateral gK in relation to models for salt absorption and secretion, particularly in relation to the maintenance of cellular charge balance and the obligatory coupling between the apical and basolateral membranes that is produced by transcellular current flow. Next, we review some of the experimental evidence that suggests that changes in basolateral gK are associated with transport regulation. The cellular mechanisms that are known to impact on K channel regulation are considered in a general way, and finally, we consider the use of integrated models for understanding possible coordinate regulation of apical and basolateral cell membranes.

Volume-activated K+ and Cl- pathways of dissociated epithelial cells (MDCK): role of Ca2+

Osmotic swelling of dissociated Madin-Darby canine kidney (MDCK) cells in NaCl medium is followed by shrinking (regulatory volume decrease, or RVD) or in KCl medium by secondary swelling. The cation ionophore gramicidin has little effect on volumes of isotonic cells but accelerates volume-activated changes in either medium. Immediately after hypotonic exposure, the membrane becomes transiently hyperpolarized followed by depolarization. The depolarization phase is diminished by the anion transport inhibitor 4,4'-diisothiocyanostilbene-2,2'-disulfonic acid (DIDS). Swelling is also associated with an almost immediate increase in Ca2+ influx and elevation of cytoplasmic Ca2+ ([Ca2+]i) preceding RVD. In Ca2(+)-free medium, [Ca2+]i rapidly declines to a low level. Osmotic swelling, under these circumstances, is associated with a small transient increase in [Ca2+]i, but RVD or secondary swelling (in KCl) are minimal. Under these conditions, addition of gramicidin or the Ca2(+)-ionophore A23187 induces significant volume changes, although not as large as those found in the presence of Ca2+. Quinine inhibits RVD in the absence of gramicidin, but not in its presence; oligomycin C, DIDS, and trifluoperazine, on the other hand, inhibit in the presence of the ionophore. These findings suggest that in MDCK cells RVD involves activation of distinct conductive K+ and Cl- pathways which allow escape of KCl and osmotically obligated water and that activation of both pathways is associated with elevated [Ca2+]i derived largely from volume activation of a Ca2(+)-influx pathway.

Reconstitution of a calcium-activated potassium channel in basolateral membranes of rabbit colonocytes into planar lipid bilayers

A highly enriched preparation of basolateral membrane vesicles was isolated from rabbit distal colon surface epithelial cells employing the method described by Wiener, Turnheim and van Os (Weiner, H., Turnheim, K., van Os, C.H. (1989) J. Membrane Biol. 110:147-162) and incorporated into planar lipid bilayers. With very few exceptions, the channel activity observed was that of a high conductance. Ca2+-activated K+ channel. This channel is highly selective for K+ over Na+ and Cl-, displays voltage-gating similar to "maxi" K(Ca) channels found in other cell membranes, and kinetic analyses are consistent with the notion that K+ diffusion through the channel involves either the binding of a single K+ ion to a site within the channel or "single-filing" ("multi-ion occupancy"). Channel activity is inhibited by the venom from the scorpion Leiurus quinquestriatus, Ba2+, quinine, and trifluoperazine. The possible role of this channel in the function of these cells is discussed.

Calcium: a program in BASIC for calculating the composition of solutions with specified free concentrations of calcium, magnesium and other divalent cations

A BASIC program is presented which facilitates the formulation of biologically relevant chemical solutions containing specified free concentrations of as many as three divalent metal cations (Ca2+, Mg2+ and the choice of a third divalent cation) at any pH in the presence of as many as three ligands (EGTA, ATP and GTP). The program uses the law of mass action and the absolute stability (association) constants found in the literature to calculate the total concentration of divalent metal cation needed to achieve a desired free concentration. The user enters the pH, the concentrations of the ligands used and the desired free concentrations of the divalent cations. This program was developed for use in a wide range of biological applications, particularly the rapid design of solutions which mimic certain aspects of intracellular fluid.