Transport-dependent alterations of membrane properties of mammalian colon measured using impedance analysis

Parathyroid hormone (PTH) rapidly enhances CFTR-mediated HCO₃⁻ secretion in intestinal epithelium-like Caco-2 monolayer: a novel ion regulatory action of PTH

Besides being a Ca²-regulating hormone, parathyroid hormone (PTH) has also been shown to regulate epithelial transport of certain ions, such as Cl, HCO₃, and Na, particularly in the kidney. Although the intestinal epithelium also expressed PTH receptors, little was known regarding its mechanism in the regulation of intestinal ion transport. We investigated the ion regulatory role of PTH in intestinal epithelium-like Caco-2 monolayer by Ussing chamber technique and alternating current impedance spectroscopy. It was found that Caco-2 cells rapidly responded to PTH within 1 min by increasing apical HCO₃- secretion. CFTR served as the principal route for PTH-stimulated apical HCOV efflux, which was abolished by various CFTR inhibitors, namely, NPPB, glycine hydrazide-101 (GlyH-101), and CFTRinh-172, as well as by small interfering RNA against CFTR. Concurrently, the plasma membrane resistance was decreased with no changes in the plasma membrane capacitance or paracellular permeability. HCOV was probably supplied by basolateral uptake via the electrogenic Na⁺-HCO₃⁻ cotransporter and by methazolamide-sensitive carbonic anhydrase, while the resulting intracellular H⁺ might be extruded by both apical and basolateral Na/H exchangers. Furthermore, the PTH-stimulated HCO₃-secretion was markedly reduced by protein kinase A (PKA) inhibitor (PKI 14-22 amide) and phosphoinositide 3-kinase (PI3K) inhibitors (wortmannin and LY-294002), but not by intracellular Ca²⁺ chelator (BAPTA-AM) or protein kinase C inhibitor (GF-109203X). In conclusion, the present study provided evidence that PTH directly and rapidly stimulated apical HCO₃- secretion through CFTR in PKA- and PI3K-dependent manner, which was a novel noncalciotropic, ion regulatory action of PTH in the intestinal epithelium.

Electrogenic bicarbonate secretion by prairie dog gallbladder

Pathological rates of gallbladder salt and water transport may promote the formation of cholesterol gallstones. Because prairie dogs are widely used as a model of this event, we characterized gallbladder ion transport in animals fed control chow by using electrophysiology, ion substitution, pharmacology, isotopic fluxes, impedance analysis, and molecular biology. In contrast to the electroneutral properties of rabbit and Necturus gallbladders, prairie dog gallbladders generated significant short-circuit current (I(sc); 171 +/- 21 microA/cm(2)) and lumen-negative potential difference (-10.1 +/- 1.2 mV) under basal conditions. Unidirectional radioisotopic fluxes demonstrated electroneutral NaCl absorption, whereas the residual net ion flux corresponded to I(sc). In response to 2 microM forskolin, I(sc) exceeded 270 microA/cm(2), and impedance estimates of the apical membrane resistance decreased from 200 Omega.cm(2) to 13 Omega.cm(2). The forskolin-induced I(sc) was dependent on extracellular HCO(3)(-) and was blocked by serosal 4,4'-dinitrostilben-2,2'-disulfonic acid (DNDS) and acetazolamide, whereas serosal bumetanide and Cl(-) ion substitution had little effect. Serosal trans-6-cyano-4-(N-ethylsulfonyl-N-methylamino)-3-hydroxy-2,2-dimethyl-chroman and Ba(2+) reduced I(sc), consistent with the inhibition of cAMP-dependent K(+) channels. Immunoprecipitation and confocal microscopy localized cystic fibrosis transmembrane conductance regulator protein (CFTR) to the apical membrane and subapical vesicles. Consistent with serosal DNDS sensitivity, pancreatic sodium-bicarbonate cotransporter protein pNBC1 expression was localized to the basolateral membrane. We conclude that prairie dog gallbladders secrete bicarbonate through cAMP-dependent apical CFTR anion channels. Basolateral HCO(3)(-) entry is mediated by DNDS-sensitive pNBC1, and the driving force for apical anion secretion is provided by K(+) channel activation.

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.

System for dynamic measurements of membrane capacitance in intact epithelial monolayers

Dynamic measurements of exocytosis have been difficult to perform in intact epithelial monolayers. We have designed a system that estimates with +/-1% accuracy (99% confidence) the total membrane capacitance of monolayers represented by a lumped model. This impedance measurement and analysis system operates through a conventional transepithelial electrophysiology clamp, performing all signal measurements as frequently as every 5 s. Total membrane capacitance (the series combination of apical and basolateral membranes) is the inverse of one of three unique coefficients that describe the monolayer impedance. These coefficients are estimated using a weighted, nonlinear, least-squares algorithm. Using the estimated coefficients, solution ranges for individual membrane parameters are calculated, frequently providing results within +/-20% of true values without additional electrophysiological measurements. We determined the measurement system specifications and statistical significance of estimated parameters using 1) analytical testing with circuit simulation software and equation-generated data; 2) a system noise analysis combined with Monte Carlo simulations; and 3) analog model circuits for calibration of the electronic system and to check equation-generated results. Finally, the time course of capacitance changes associated with purinergically stimulated mucin exocytosis are quantified in monolayers of the colonic goblet cell-like cell line HT29-CI.16E.

Impedance analysis of MDCK cells measured by electric cell-substrate impedance sensing

Transepithelial impedance of Madin-Darby canine kidney cell layers is measured by a new instrumental method, referred to as electric cell-substrate impedance sensing. In this method, cells are cultured on small evaporated gold electrodes, and the impedance is measured in the frequency range 20-50,000 Hz by a small probing current. A model for impedance analysis of epithelial cells measured by this method is developed. The model considers three different pathways for the current flowing from the electrode through the cell layer: (1) in through the basal and out through the apical membrane, (2) in through the lateral and out through the apical membrane, and (3) between the cells through the paracellular space. By comparing model calculation with experimental impedance data, several morphological and cellular parameters can be determined: (1) the resistivity of the cell layer, (2) the average distance between the basal cell surface and substratum, and (3) the capacitance of apical, basal, and lateral cell membranes. This model is used to analyze impedance changes on removal of Ca2+ from confluent Mardin-Darby canine kidney cell layers. The method shows that reduction of Ca2+ concentration causes junction resistance between cells to drop and the distance between the basal cell surface and substratum to increase.

Effects of aldosterone on the impedance properties of cultured renal amphibian epithelia

The cultured renal amphibian cell line A6 has proven advantageous for studies of Na+ transport regulation. In the present study, the effects of aldosterone action on the transepithelial electrical properties of this epithelium were assessed. Specifically, the time course of aldosterone action was determined and the effects of chronic (10-18 day) aldosterone elevation were assessed using transepithelial equivalent circuit methods and impedance analysis techniques. Short-term (< 4 hr) exposure to aldosterone (0.1 microM) stimulated the amiloride-sensitive short-circuit current (Isc) by over twofold and increased the transepithelial conductance (GT) by approximately 12%. The increases in Isc and GT were maintained in epithelia subjected to chronic aldosterone exposure. In contrast to previous reports, paracellular resistance (Rj) was not altered by aldosterone. This difference may be related to the longer time of exposure or different basal Na+ transport rates in the present study. The apical membrane conductance was significantly increased for aldosterone-treated epithelia compared to aldosterone-depleted (i.e., serum-deprived) controls. Apical membrane area (capacitance) was not significantly affected. This finding is consistent with a higher density (number of channels per membrane area) of conducting Na+ channels in this membrane following aldosterone stimulation. Basolateral membrane properties were not significantly altered for aldosterone-treated tissues compared to serum-treated control tissues. In contrast, basolateral membrane-specific conductance (i.e., basolateral membrane conductance normalized to basolateral membrane capacitance) was significantly lower for serum-deprived epithelia than for serum-treated controls or aldosterone-treated tissues. The effects of chronic aldosterone exposure were also evaluated for the A6 subclonal cell line, 2F3. Similar to A6 epithelia, Isc was essentially doubled following aldosterone stimulation while Rj and cellular driving force (Ec) were not affected. Apical membrane conductances under control conditions for 2F3 epithelia were higher than those for A6, but were not significantly different from A6 following aldosterone exposure or serum deprivation. These findings suggest possible differences in the regulation of apical membrane Na+ channels for 2F3 and A6 epithelia.

Transmitter-induced changes of the membrane voltage of HT29 cells

The colonic carcinoma cell line HT29 was used to examine the influence of agonists increasing cytosolic cAMP and Ca2+ activity on the conductances and the cell membrane voltage (Vm). HT29 cells were grown on glass cover-slips. Cells were impaled by microelectrodes 4-10 days after seeding, when they had formed large plaques. In 181 impalements Vm was -51 +/- 1 mV. An increase in bath K+ concentration from 3.6 mmol/l to 18.6 mmol/l or 0.5 mmol/l Ba2+ depolarized the cells by 10 +/- 1 mV (n = 49) or by 9 +/- 2 mV (n = 3), respectively. A decrease of bath Cl- concentration from 145 to 30 mmol/l depolarized the cells by 11 +/- 1 mV (n = 24). Agents increasing intracellular cAMP such as isobutylmethylxanthine (0.1 mmol/l), forskolin (10 mumol/l) or isoprenaline (10 mumol/l) depolarized the cells by 6 +/- 1 (n = 13), 15 +/- 3 (n = 5) and 6 +/- 2 (n = 3) mV, respectively. In hypoosmolar solutions (225 mosmol/l) cells depolarized by 9 +/- 1 mV (n = 6). Purine and pyrimidine nucleotides depolarized the cells dose-dependently with the following potency sequence: UTP greater than ATP greater than ITP greater than GTP greater than TTP greater than CTP = 0. The depolarization by ATP was stronger than that by ADP and adenosine. The muscarinic agonist carbachol led to a sustained depolarization by 27 +/- 6 mV (n = 5) at 0.1 mmol/l, and to a transient depolarization by 12 +/- 4 mV (n = 5) at 10 mumol/l. Neurotensin depolarized with a half-maximal effect at around 5 nmol/l.(ABSTRACT TRUNCATED AT 250 WORDS)

Transport-related modulation of the membrane properties of toad urinary bladder epithelium

Impedance analysis and transepithelial electrical measurements were used to assess the effects of the apical membrane Na+ channel blocker amiloride and anion replacement on the apical and basolateral membrane conductances and areas of the toad urinary bladder (Bufo marinus). Mucosal amiloride addition decreased both apical and basolateral membrane conductances (Ga and Gbl, respectively) with no change in membrane capacitances (Ca and Cbl). Consequently, the specific conductances of these membranes decreased without significant changes in membrane area. Following amiloride removal, an increase was obtained in the steady-state rate of sodium transport compared to values before amiloride addition. This increase was independent of the initial transport rate, suggesting activation of a quiescent pool of apical sodium channels. Chloride replacement by acetate or gluconate had no significant effects on apical or basolateral membrane capacitances. The effects of these replacements on membrane conductances depended on the anion species. Gluconate (which induces cell shrinkage) decreased both membrane conductances. In contrast, acetate (which induces cell swelling) increased Ga and had no effect on Gbl. The increase in the apical membrane conductance was due to an increase in the amiloride-sensitive Na+ conductance of this membrane. In summary, mucosal amiloride addition or chloride replacements led to changes in membrane conductances without significant effects on net membrane areas.

Amiloride-sensitive Na+ transport across cultured renal (A6) epithelium: evidence for large currents and high Na:K selectivity

Electrical techniques were used to determine the Na:K selectivity of the amiloride-sensitive pathway and to characterize cellular and paracellular properties of A6 epithelium. Under control conditions, the mean transepithelial voltage (VT) was -57 +/- 5 mV, the short-circuit current (Isc) averaged 23 +/- 2 microA/cm2 and the transepithelial resistance (RT) was 2.8 +/- 0.3 k omega cm2 (n = 13). VT and Isc were larger than reported in previous studies and were increased by aldosterone. The conductance of the amiloride-sensitive pathway (Gamil) was assessed before and after replacement of Na+ in the mucosal bath by K+, using two independent measurements: (1) the slope conductance (GT), determined from current-voltage (I-V) relationships for control and amiloride-treated tissues and (2) the maximum amiloride-sensitive conductance (Gmax) calculated from the amiloride dose-response relationship. The ratio of Gamil in mucosal Na+ solutions to Gamil for mucosal K+ solutions was 22 +/- 6 for GT measurements and 15 +/- 2 for Gmax data. Serosal ion replacements in tissues treated with mucosal nystatin indicated a potassium conductance in the basolateral membrane. Equivalent circuit analyses of nystatin and amiloride data were used to resolve the cellular (Ec) and paracellular (Rj) resistances (approximately 5 k omega cm2 and 8-9 k omega cm2, respectively). Analysis of I-V relationships for tissues depolarized with serosal K+ solutions revealed that the amiloride-sensitive pathway could be described as a Na+ conductance with a permeability coefficient (PNa) = 1.5 +/- 0.2 x 10(-6) cm/s and the intracellular Na+ concentration (Nai) = 5 +/- 1 mM (n = 5), similar to values from other tight epithelia. We conclude that A6 epithelia are capable of expressing large amiloride-sensitive currents which are highly Na+ selective.

Aldosterone regulates paracellular pathway resistance in rabbit distal colon

Regulation of the paracellular pathway in rabbit distal colon by the hormone aldosterone was investigated in vitro in Ussing chambers by means of transepithelial and microelectrode techniques. To evaluate the cellular and paracellular resistances an equivalent circuit analysis was used. For the analysis the apical membrane resistance was altered using the antibiotic nystatin. Under control conditions two groups of epithelia were found, each clearly dependent on the light: dark regime. Low-transporting epithelia (LT) were observed in the morning and high-transporting epithelia (HT) in the afternoon. Na+ transport was about 3-fold higher in HT than in LT epithelia. Incubating epithelia of both groups with 0.1 mumol.l-1 aldosterone on the serosal side nearly doubled in LT epithelia the short circuit current and transepithelial voltage but the transepithelial resistance was not influenced. Maximal values were reached after 4-5 h of aldosterone treatment. In HT epithelia due to the effect of aldosterone all three transepithelial parameters remained constant over time. Evaluation of the paracellular resistance revealed a significant increase after aldosterone stimulation in both epithelial groups. This increase suggests that tight junctions might have been regulated by aldosterone. The hormonal effect on electrolyte transport was also dependent on the physiological state of the rabbit colon. Since net Na+ absorption in distal colon is, in addition to transcellular absorption capacity, also dependent on the permeability of the paracellular pathway, the regulation of tight junctions by aldosterone may be a potent mechanism for improving Na+ absorption during hormone-stimulated ion transport.