The beginning of fluctuation analysis of epithelial ion transport

Apical potassium channels in the rat connecting tubule

Apical membrane K channels in the rat connecting tubule (CNT) were studied using the patch-clamp technique. Tubules were isolated from the cortical labyrinth of the kidney and split open to provide access to the apical membrane. Cell-attached patches were formed on presumed principal and/or connecting tubule cells. The major channel type observed had a single-channel conductance of 52 pS, high open probability and kinetics that were only weakly dependent on voltage. These correspond closely to the "SK"-type channels in the cortical collecting duct, identified with the ROMK (Kir1.1) gene product. A second channel type, which was less frequently observed, mediated larger currents and was strongly activated by depolarization of the apical membrane voltage. These were identified as BK or maxi-K channels. The density of active SK channels revealed a high degree of clustering. Although heterogeneity of tubules or of cell types within a tubule could not be excluded, the major factor underlying the distribution appeared to be the presence of channel clusters on the membrane of individual cells. The overall density of channels was higher than that previously found in the cortical collecting tubule (CCT). In contrast to results in the CCT, we did not detect an increase in the overall density of SK channels in the apical membrane after feeding the animals a high-K diet. However, the activity of amiloride-sensitive Na channels was undetectable under control conditions but was increased after both 1 day (90 +/- 24 pA/cell) or 7 days (385 +/- 82 pA/cell) of K loading. Thus one important factor leading to an increased K secretion in the CNT in response to increased dietary K is an increased apical Na conductance, leading to depolarization of the apical membrane voltage and an increased driving force for K movement out into the tubular lumen.

Mechanism of C2-toxin Inhibition by Fluphenazine and Related Compounds: Investigation of their Binding Kinetics to the C2II-channel using the Current Noise Analysis

The binding component C2II of the binary actin ADP-ribosylating C2-toxin from Clostridium botulinum is essential for intoxication of target cells. Activation by a protease leads to channel formation and this is presumably required for the transport of the toxic C2I component into cells. The C2II-channel is cation selective and contains a binding site for fluphenazine and structurally related compounds. Ion transport through C2II and in vivo intoxication is blocked when the sites are occupied by the ligands. C2II was reconstituted into artificial lipid bilayer membranes and formed ion permeable channels. The binding constant of chloroquine, primaquine, quinacrine, chloropromazine and fluphenazine to the C2II-channel was determined using titration experiments, which resulted in its block. The ligand-induced current noise of the C2II-channels was investigated using fast Fourier transformation. The noise of the open channels had a rather small spectral density, which was a function of the inverse frequency up to about 100 Hz. Upon addition of ligands to the aqueous phase the current through C2II decreased in a dose-dependent manner. Simultaneously, the spectral density of the current noise increased drastically and its frequency dependence was of Lorentzian type, which was caused by the on and off-reactions of the ligand-mediated channel block. The ligand-induced current noise of C2II was used for the evaluation of the binding kinetics for different ligands to the channel. The on-rate constant of ligand binding was between 10(7) and 10(9) M(-1) s(-1) and was dependent on the ionic strength of the aqueous phase. The off-rate varied between about 10 s(-1) and 3900 s(-1) and depended on the structure of the ligand. The role of structural requirements for the effective block of C2II by the different ligands is discussed.

CymA of Klebsiella oxytoca outer membrane: binding of cyclodextrins and study of the current noise of the open channel

CymA, the outer membrane component of the cyclodextrin (CD) uptake and metabolism system of Klebsiella oxytoca, was reconstituted into lipid bilayer membranes. The channel properties of this unusual porin were studied in detail. The binding of CDs to the channel resulted in its complete block for ion transport. This result allowed the detailed investigation of carbohydrate binding, and the stability constants for the binding of cyclic and linear carbohydrates to the binding site inside the channel were calculated from titration experiments of the membrane conductance with the carbohydrates. Highest stability constant was observed for alpha-cyclodextrin (alpha-CD; K = 32,000 1/M) followed by beta-cyclodextrin (beta-CD; K = 1970 1/M) and gamma-cyclodextrin (gamma-CD; K = 310 1/M). Linear maltooligosaccharides bound also to CymA but with much smaller stability constants as compared to cyclic ones. The noise of the current through CymA in multi- and single-channel experiments was investigated using fast Fourier transformation. The current through the open channels had a rather high spectral density, which was a Lorentzian function of the frequency up to 2000 Hz. Upon addition of cyclic dextrins to the aqueous phase the spectral density decreased in a dose-dependent manner, which made it impossible to evaluate the binding kinetics. Experiments with single CymA-channels demonstrated the channel is highly asymmetric concerning channel flickers and current noise.

Mineralocorticoid regulation of epithelial Na+ channels is maintained in a mouse model of Liddle's syndrome

Currents through epithelial Na channels (ENaCs) were measured in the cortical collecting tubule (CCT) of mice expressing truncated beta-subunits of ENaC, reproducing one of the mutations found in human patients with Liddle's syndrome. Tubules were isolated from mice homozygous for the Liddle mutation (L/L) and from wild-type (WT) littermates. Amiloride-sensitive currents (INa) from single cells were recorded under whole cell clamp conditions. CCTs from mice kept under control conditions and fed a diet with normal levels of Na had very small INas (WT: 18 +/- 13 pA; L/L: 22 +/- 8 pA at Vm = -100 mV) that were not different in WT and L/L animals. However, the L/L mice had much larger currents when the animals were fed a low-Na diet (WT: 256 +/- 127 pA; L/L: 1,820 +/- 330 pA) or infused with aldosterone (WT: 285 +/- 63 pA; L/L: 1,600 +/- 280 pA). Currents from L/L mice were also larger when animals were pretreated with a high-K diet but not when the CCTs were stimulated in vitro with 8-CTP-cAMP. Noise analysis of amiloride-induced fluctuations in INa showed that single-channel currents at Vm = 0 mV were slightly smaller in L/L mice (WT: 0.33 pA; L/L: 0.24 pA). This difference could be attributed to a decrease in driving force since current-voltage analysis indicated that intracellular Na was increased in the L/L animals. Analysis of spontaneous channel noise indicated that the open probability was similar in the two genotypes(WT: 0.77; L/L: 0.80). Thus the increase in whole cell current is attributed to a difference in the density of conducting channels.

On translocation through a membrane channel via an internal binding site: kinetics and voltage dependence

Here we present a model for maltodextrin translocation through maltoporin channels. In a first step, our theoretical analysis does consider the case of a single binding site for a given substrate in a structurally unaffected channel with a possibly different entrance barrier on either side. It is shown how by means of conventional electrical conductance measurements (including current noise analysis) the basic equilibrium and rate constants can be determined as functions of the applied voltage. Then also the net translocation rate of the substrate becomes accessible quantitatively. This most simple model mechanism has been extended to include a voltage-dependent fast conformational change of the channel that prevents the binding process. The so developed approach has been tested with experimental data for a single maltoporin trimer being reconstituted in black lipid membranes when studied in the presence of maltohexaose as the substrate. The experimental results turned out to be clearly incompatible with binding alone. They are, however, very satisfactorily fitted by pertinent theoretical curves if also inhibition of binding by a conformational transition is taken into account. Accordingly, quantitative evaluations of the underlying parameters and eventually of the translocation rate have been carried out successfully. Our analysis reveals a set of parameters necessary for an optimal translocation that nicely corresponds to natural conditions.

Site-directed mutagenesis of tyrosine 118 within the central constriction site of the LamB (maltoporin) channel of Escherichia coli. II. Effect on maltose and maltooligosaccharide binding kinetics

The 3-D structure of the maltooligosaccharide-specific LamB channel of Escherichia coli (also called maltoporin) is known from x-ray crystallography. The central constriction of the channel formed by the external loop 3 is controlled by tyrosine 118. Y118 was replaced by site-directed mutagenesis by 10 other amino acids (alanine (A), isoleucine (I), asparagine (N), serine (S), cysteine (C), aspartic acid (D), arginine (R), histidine (H), phenylalanine (F), and tryptophan (W)) including neutral ones, negatively and positively charged amino acids to study the effect of their size, their hydrophobicity index, and their charge on maltose and maltooligosaccharide binding to LamB. The mutants were reconstituted into lipid bilayer membranes and the stability constants for binding of maltose, maltotriose, maltopentaose, and maltoheptaose to the channel were measured using titration experiments. The mutation of Y118 to any other non-aromatic amino acid led to a substantial decrease of the stability constant of binding by factors between about two and six. The highest effect was observed for the mutant Y118A. Replacement of Y118 by the two other aromatic amino acids, phenylalanine (F) and tryptophan (W), resulted in a substantial increase of the stability constant maximally by a factor of almost 400 for the Y118W mutant. The carbohydrate-induced block of the channel function was used for the study of current noise through the different mutant LamB channels. The analysis of the power density spectra allowed the evaluation of the on- and off-rate constants (k(1) and k(-1)) of sugar binding. The results suggest that both rate constants were affected by the mutations. For most mutants, with the exception of Y118F and Y118W, k(1) decreased and k(-1) increased, whereas the opposite was found for the aromatic amino acid mutants. The results suggest that tyrosine 118 has a crucial effect on carbohydrate transport through LamB.

Effect of crude extract of roots of Bredemeyera floribunda Willd. II. Effect on glomerular filtration rate and renal tubular function of rats

The effect of crude extract of roots of Bredemeyera floribunda Willd., Polygalaceae used by Brazilian popular medicine as a potent diuretic, on renal function in antidiuresis or water diuresis in rats, was studied. During intravenous infusion of the extract (0.05 mg/min/100 g), mean arterial pressure did not change significantly but urine flow, glomerular filtration rate, fractional water and sodium excretion and solute clearance increased significantly, in both groups of animals. In antidiuresis rats the extract significantly increased reabsorption of water by the collecting duct and in water diuresis animals the extract significantly increased free water clearance. Our findings indicate a direct effect of extract on glomerular filtration rate, possibly by detergent like interactions with structural components of glomerular membranes and/or by decreasing renal perfusion pressure. The study on the concentrating and diluting mechanisms suggests preferential action of extract in the proximal tubular cells, possibly on the (Na(+)-K+)-ATPase countertransport system and/or on other proteins components of tubular cell membranes involved with salt transport mechanisms.

Effect of crude extract of roots of Bredemeyera floribunda Willd. III. Effect on hormone-stimulated water transport in isolated frog skin

The diuretic activity of the crude extract of roots of Bredemeyera floribunda Willd. (Polygalaceae) was investigated on basal and hormone-stimulated absorptive water transport in the isolated frog skin. The extract, added to the mucosal skin surface, at the peak of oxytocin (Oxy) action induces steady and dose-dependent reduction of water transport. Pre-treatment of the skins during 2 h with the extract, added to mucosal skin surface, renders the epithelium non-responsive to oxytocin. These effects are not observed when the extract is added to the serosal skin surface. Basal water transport is not altered by the extract. The results, apart from indicating inhibition of the oxytocin-stimulated water transport in frog skin epithelium, support the folk therapeutic use of infusion of the root extract as a diuretic.

Noise analysis of ion current through the open and the sugar-induced closed state of the LamB channel of Escherichia coli outer membrane: evaluation of the sugar binding kinetics to the channel interior

LamB, a sugar-specific channel of Escherichia coli outer membrane was reconstituted into lipid bilayer membranes and the current noise was investigated using fast Fourier transformation. The current noise through the open channels had a rather small spectral density, which was a function of the inverse frequency up to about 100 Hz. The spectral density of the noise of the open LamB channels was a quadratic function of the applied voltage. Its magnitude was not correlated to the number of channels in the lipid bilayer membrane. Upon addition of sugars to the aqueous phase the current decreased in a dose-dependent manner. Simultaneously, the spectral density of the current noise increased drastically, which indicated interaction of the sugars with the binding site inside the channel. The frequency dependence of the spectral density was of Lorentzian type, although the power of its frequency dependence was not identical to -2. Analysis of the power density spectra using a previously proposed simple model (Benz, R., A. Schmid, and G. H. Vos-Scheperkeuter. 1987. J. Membr. Biol. 100: 12-29), allowed the evaluation of the on- and the off-rate constants for the maltopentaose binding to the binding site inside the LamB channels. This means also that the maltopentaose flux through the LamB channel could be estimated by assuming a simple one-site, two-barrier model for the sugar transport from the results of the noise analysis.

Fractal analysis of a voltage-dependent potassium channel from cultured mouse hippocampal neurons

The kinetics of ion channels have been widely modeled as a Markov process. In these models it is assumed that the channel protein has a small number of discrete conformational states and the kinetic rate constants connecting these states are constant. In the alternative fractal model the spontaneous fluctuations of the channel protein at many different time scales are represented by a kinetic rate constant k = At1-D, where A is the kinetic setpoint and D the fractal dimension. Single-channel currents were recorded at 146 mM external K+ from an inwardly rectifying, 120 pS, K+ selective, voltage-sensitive channel in cultured mouse hippocampal neurons. The kinetics of these channels were found to be statistically self-similar at different time scales as predicted by the fractal model. The fractal dimensions were approximately 2 for the closed times and approximately 1 for the open times and did not depend on voltage. For both the open and closed times the logarithm of the kinetic setpoint was found to be proportional to the applied voltage, which indicates that the gating of this channel involves the net inward movement of approximately one negative charge when this channel opens. Thus, the open and closed times and the voltage dependence of the gating of this channel are well described by the fractal model.

Structure-activity relationship of amiloride analogs as blockers of epithelial Na channels: II. Side-chain modifications

The overall on- and off-rate constants for blockage of epithelial Na channels by amiloride analogs were estimated by noise analysis of the stationary Na current traversing frog skin epithelium. The (2-position) side chain structure of amiloride was varied in order to obtain structure/rate constant relationships. Hydrophobic chain elongations (benzamil and related compounds of high blocking potency) increase the stability of the blocking complex (lowered off-rate), explained by attachment of the added phenyl moiety to a hydrophobic area near the site of side chain interaction with the channel protein. Some other chain modifications show that the on-rate, which is smaller than a diffusion-limited rate, varies with side chain structure. In several cases this effect is not attributable to steric hindrance on encounter, and implies that the side chain interacts briefly with the channel protein (encounter complex) before the main blocking position of the molecule is attained. The encounter complex must be labile since the overall rate constants of blockage are not concentration-dependent. In two cases, changes at the 2-position side chain and at other ring ligands, with known effects on the blocking rate constants, could be combined in one analog. The rate constants of blocking by the resulting compounds indicate that the structural changes have additive effects in terms of activation energies. Along with other observations (voltage dependence of the rate constants and competition with the transported Na ion), these results suggest a blocking process of at least two steps. It appears that initially the 2-position side chain invades the outward-facing channel entrance, establishing a labile complex. Then the molecule is either released completely (no block) or the 6-ligand of the pyrazine ring gains access to its receptor counterpart, thus establishing the blocking complex, the lifetime of which is strongly determined by the electronegativity of the 6-ligand.

Recent advances in the characterization of epithelial ionic channels

Physiologists have long recognized the importance of channels in the functioning of neurons and excitable membranes. This brief review has been an attempt to illustrate how channel properties are also essential to an understanding of epithelial transport physiology. Among their more important functions, channels influence membrane potentials and serve as conduits for ion movements. As the need to understand the molecular basis for ion transport continues to develop, it is crucial to be able to distinguish between different channel properties. For example, apparent voltage-dependent properties can arise because of a voltage-dependent gating process, or alternatively, because of a rectification of channel conductance. Voltage-dependent effects can also be only indirect, mediated by changes in cell volume, intracellular ion levels, the levels of secondary intracellular messengers such as Ca2+ (perhaps through voltage-dependent membrane Ca2+ channels), or possibly even by morphological changes. An important area for future research is to differentiate mechanisms which modulate the activity of open channels. For example, a decrease in channel number, a reduction in open-channel conductance or a decline in the probability of channel opening can all underlie changes in macroscopic permeability. The factors which mediate hormonal activation of epithelial channels particularly need to be understood. Specifically, the mechanisms of aldosterone and anti-diuretic hormone activation of apical membrane Na+ channels need to be identified. In conclusion, we are witnessing a new era in epithelial electrophysiology which promises to resolve many issues concerning the cellular regulation of ion transport and open new, unanticipated avenues of inquiry.

Membrane pores: a computer simulation of interacting pores analyzed by g1(tau) and g2(tau) correlation functions

Ion channels in a cell membrane were modeled by a computer simulation of fluctuating pores distributed in a spatial array, a cellular automata. The sum of the currents through such a set of pores models a noise analysis experiment. These currents were analyzed by using the optical correlation functions gn(tau) = (fn(t)fn(t + tau))/(f2(t))n, where f(t) are the current deviations around the mean current and () denotes the time average. These functions can be easily used to determine if the noise is Gaussian. If the noise is not Gaussian, they provide additional information not already contained in the power spectrum. When the pores do not interact with each other, the noise is Gaussian and the power spectrum a Lorentzian. When the pores interact in a strongly cooperative way the noise was still Gaussian and the power spectrum still a Lorentzian, but the usual analysis applied to such a case would over-estimate the single channel conductance. If the kinetics of the pore opening and closing vary on the time scale of the experiment then the relationship g2(tau) = 1 + 2[g1(tau)]2 is no longer satisfied.

The amiloride-sensitive sodium channel

Net Na+ movement across the apical membrane of high-electrical resistance epithelia is driven by the electrochemical potential energy gradient. This entry pathway is rate limiting for transepithelial transport, occurs via a channel-type mechanism, and is specifically inhibited by the diuretic drug amiloride. This channel is selective for Na+, Li+, and H+, saturates with increasing extracellular Na+ concentration, and is not affected, at least in frog skin epithelium, by changes in apical membrane surface potential. There also appears to be multiple inhibitory regions associated with each Na+ channel. We discuss the possible implications of a voltage-dependent block by amiloride in terms of macroscopic inhibitory phenomena. We describe the use of cultured epithelial systems, in particular, the toad kidney-derived A6 cell line, and the preparation of apical plasma membrane vesicles to study the Na+ entry process. We discuss experiments in which single, amiloride-sensitive channel activity has been detected and summarize current experimental approaches directed at the biochemical identification of this ubiquitous Na+ transport system.

Voltage dependence of Na channel blockage by amiloride: relaxation effects in admittance spectra

Amiloride, present in the mucosal solution, causes the appearance of a distinct additional dispersion in the admittance spectrum of the apical membrane of toad urinary bladder. The parameters of this dispersion (characteristic frequency, amplitude) change with amiloride concentration and with membrane voltage. They allow the calculation of the overall rate constants for Na channel blockage by the positively charged form of amiloride, and the voltage dependence of these rate constants. The on-rate of blockage increases and the off-rate decreases when the membrane surface to which cationic amiloride has access, is made more positive. This result is suggestive of a blocking model where the cationic amidino group of amiloride, depending on its charge, senses 10 to 13% of the membrane voltage while invading the channel entrance by a single-step process, and rests at an electrical distance corresponding to 24 to 30% of membrane voltage while occupying the blocking position.

Voltage dependence of the blocking rate constants of amiloride at apical Na channels

The overall rate constants of blockage of apical Na channels by amiloride, previously determined by noise analysis, were obtained in macroscopic relaxation experiments with toad urinary bladders exposed to a mucosal Na activity of 60 mM. By the use of step voltage perturbations and by admittance analysis we show that the on-rate constant of blockage increases, and the off-rate constant decreases when the outer membrane surface is made more positive. In the frame-work of a plug-type blocking model the results imply that the cationic amidino group of amiloride senses about 10% of the membrane voltage while invading the channel entrace and slightly more than 10% while leaving the entrance.

Amiloride-sensitive epithelial Na+ channels reconstituted into planar lipid bilayer membranes

High resistance epithelia actively transport sodium from the luminal side to the blood. Aldosterone and vasopressin stimulate this sodium transport system; the diuretic drug amiloride inhibits it in a reversible fashion. The first step in the transepithelial transport of Na+ is the facilitated diffusion of Na+ across the apical membrane via Na+-specific, amiloride-sensitive channels. We report here the first direct measurements of single, amiloride-sensitive Na+ channel activity. The channel was isolated after incorporation of purified apical membrane vesicles from A6 cells into planar lipid bilayers. The channel had the following characteristics: single-channel conductance ranged from 4 to 80 pS at 200 mM NaCl; it was perfectly cation-selective; amiloride reduced the open-state conductance in a dose-dependent fashion when present in the cis compartment, and induced flickering when present in the trans chamber; channel conductance and gating were voltage-independent; and the Na+/K+ selectivity ratio of the channel was 2:1.

The sensitivity of apical Na+ permeability in frog skin to hypertonic stress

Na+ transport across abdominal skins of the frog species Rana esculenta and Rana pipiens was analyzed by recording short-circuit current (Isc), transepithelial conductance (Gt), and the current noise generated by the random blockage of apical Na+ channels by the diuretic, amiloride. Specific Na+ current (INa) and conductance (GNa), as reflected by the amiloride-sensitive part of Isc and Gt, respectively, were markedly depressed after addition of some osmotically active substances, like sugars or alcohols to the mucosal Na+-Ringer solution. These hypertonicity-induced reactions were fast and fully reversible, even at mucosal osmolarities of 1 Osmol. With mucosal solutions of moderate hyperosmolarity a recovery of INa and GNa was observed in presence of the osmotic gradient. This "regulatory" current showed to be carried by Na+ through the Na+-specific apical channels. Contrary to the fast current drop during the initial phase of hyperosmotic shocks, the "osmoregulation" was considerably slower. The recovery of INa was only complete at smaller osmotic gradients but became more and more suppressed at higher osmolarities. Steady-state analysis of the kinetics of the Na+-specific current revealed that the current depression by osmotic shocks obeys Michaelis-Menten kinetics. This current depression at high osmolarities, as well as during the initial phase before "osmoregulation" with small osmotic gradients, can be described in terms of a non-competitive inhibition. This was also suggested by Na+-concentration jump experiments indicating a reduction of the maximal, apical Na+ permeability as mechanism of the hypertonicity-induced drop in INa. The INa kinetics after complete "osmoregulation" were, however, indistinguishable from the isotonic control condition.(ABSTRACT TRUNCATED AT 250 WORDS)

Competitive blocking of epithelial sodium channels by organic cations: the relationship between macroscopic and microscopic inhibition constants

Fluctuation analysis of Na current passing the apical membrane in the skin of Rana ridibunda was used to study the kinetics of Na-channel blocking by several organic cations present in the outer solution together with 60 mM Na. The ratios of the apparent off-rate and on-rate constants (the microscopic inhibition constants) thus obtained for triamterene, triaminopyrimidine (TAP), 5,6-diCl-amiloride, 5H-amiloride and amiloride itself are found to be in the mean about sevenfold smaller than the corresponding inhibition constants obtained from macroscopic dose-response curves. The apparent discrepancy is explicable by competition of the organic blocker with the channel block by Na ions (the self-inhibition effect). The type of interaction between extrinsic blockage and self-inhibition may be purely competitive or mixed. However, in case of mixed inhibition the competitive component must dominate the noncompetitive component by at least seven to one.

Calcium reduces the sodium permeability of luminal membrane vesicles from toad bladder. Studies using a fast-reaction apparatus

Regulation of the sodium permeability of the luminal membrane is the major mechanism by which the net rate of sodium transport across tight epithelia is varied. Previous evidence has suggested that the permeability of the luminal membrane might be regulated by changes in intracellular sodium or calcium activities. To test this directly, we isolated a fraction of the plasma membrane from the toad urinary bladder, which contains a fast, amiloride-sensitive sodium flux with characteristics similar to those of the native luminal membrane. Using a flow-quench apparatus to measure the initial rate of sodium efflux from these vesicles in the millisecond time range, we have demonstrated that the isotope exchange permeability of these vesicles is very sensitive to calcium. Calcium reduces the sodium permeability, and the half-maximal inhibitory concentration is 0.5 microM, well within the range of calcium activity found in cells. Also, the permeability of the luminal membrane vesicles is little affected by the ambient sodium concentration. These results, when taken together with studies on whole tissue, suggest that cell calcium may be an important regulator of transepithelial sodium transport by its effect on luminal sodium permeability. The effect of cell sodium on permeability may be mediated by calcium rather than by sodium itself.

Fluctuation analysis of short-circuit current in a warm-blooded sodium-retaining epithelium: site current, density, and interaction with triamterene

Density and conductance of the Na-site in hen coprodeum were studied by employing fluctuation analysis of short-circuit current at sodium concentrations from 26 to 130 mM. Fluctuations of current in the frequency range 2-800 Hz were induced by triamterene, a reversible blocker of conducting epithelial Na-sites. At 130 mM Na the site density was 5.8 +/- 1.0 micrometer-2 and the site conductance was 4 pS. This conductance is equal to that of the frog skin (W. Van Driessche and B. Lindemann, 1979, Nature (London) 282:519-520). Extrapolation of site density to zero sodium renders a total of 38 +/- 28 sites micrometer-2, which is compared with other estimates for the coprodeum. The site-triamterene association and dissociation constants were 9.5 +/- 0.4 rad sec-1 micrometer-1 and 255 +/- 20 rad sec-1 and they were independent of external sodium concentration. An analysis of the affinity constant for triamterene based on the DC-short-circuit current was found to be unrelated to the external sodium concentration and identical to that obtained from fluctuation analysis indicating a noncompetitive interaction between sodium and triamterene. Due to the oxygen demand of the epithelium we have developed an experimental method using short data processing times. A new analytical approach using integration of the power density spectrum proved necessary because of low signal-to-noise ratios.

Transepithelial Na+ transport and the intracellular fluids: a computer study

Computer simulations of tight epithelia under three experimental conditions have been carried out, using the rheogenic nonlinear model of Lew, Ferreira and Moura (Proc. Roy. Soc. London. B 206:53-83, 1979) based largely on the formulation of Koefoed-Johnsen and Ussing (Acta Physiol. Scand. 42: 298-308. 1958). First, analysis of the transition between the short-circuited and open-circuited states has indicated that (i) apical Cl- permeability is a critical parameter requiring experimental definition in order to analyze cell volume regulation, and (ii) contrary to certain experimental reports, intracellular Na+ concentration (ccNa) is expected to be a strong function of transepithelial clamping voltage. Second, analysis of the effects of lowering serosal K+ concentration (csK) indicates that the basic model cannot simulate several well-documented observations; these defects can be overcome, at least qualitatively, by modifying the model to take account of the negative feedback interaction likely to exist between the apical Na+ permeability and ccNa. Third, analysis of the strongly supports the concept that osmotically induced permeability changes in the apical intercellular junctions play a physiological role in conserving the body's stores of NaCl. The analyses also demonstrate that the importance of Na+ entry across the basolateral membrane is strongly dependent upon transepithelial potential, cmNa and csK; under certain conditions, net Na+ entry could be appreciably greater across the basolateral than across the apical membrane.

Apamin as a selective blocker of the calcium-dependent potassium channel in neuroblastoma cells: voltage-clamp and biochemical characterization of the toxin receptor

This paper describes the interaction of apamin, a bee venom neurotoxin, with the mouse neuroblastoma cell membrane. Voltage-clamp analyses have shown that apamin at low concentrations specifically blocks the Ca2+-dependent K+ channel in differentiated neuroblastoma cells. Binding experiments with highly radiolabeled toxin indicate that the dissociation constant of the apamin-receptor complex in differentiated neuroblastoma cells is 15-22 pM and the maximal binding capacity is 12 fmol/mg of protein. The receptor is destroyed by proteases, suggesting that it is a protein. The binding capacity of neuroblastoma cells for radiolabeled apamin dramatically increases during the transition from the nondifferentiated to the differentiated state. The number of Ca2+-dependent K+ channels appears to be at most 1/5th the number of fast Na+ channels in differentiated neuroblastoma. The binding of radiolabeled apamin to its receptor is antagonized by monovalent and divalent cations. Na+ inhibition of the binding of 125I-labeled apamin is of the competitive type (Kd(Na+) = 44 mM). Guanidinium and guanidinated compounds such as amiloride or neurotensin prevent binding of 125I-labeled apamin, the best antagonist being neurotensin.

Intra- and transepithelial analytical techniques

Epithelia transport a variety of solutes and water. Study of such transport requires a determination of the driving forces responsible for transport, of the pathways through which transport occurs, and of the factors controlling such transport. Transepithelial driving forces are readily determined where the composition of the bathing media can be altered and electrical forces negated. Where substances move only through a paracellular pathway such manipulations may be adequate to define the permeability and selectivity of the pathways. For substances utilizing a cellular pathway, driving forces and permeabilities across the two dissimilar apical and basolateral cellular membranes must be determined. Where a substance can be shown to move across a membrane against its electrochemical potential gradient, the source of the energy for such movement must be assessed. This review focuses on the applicability and validity of a variety of techniques utilized for the study of epithelial transport to answer these questions. These include microelectrode techniques, chemical analyses, microprobe analysis, microscopy, and techniques for assessing the coupling of metabolism to transport.