Potassium channels in the apical membrane of the toad gallbladder

Kinetics of potassium transport across trout gills

1. Kinetics of potassium transport across trout gills was studied, using an isolated-head preparation. 2. Potassium exchanges were shown to take place across secondary lamellae only. 3. Influx of potassium was saturable and fitted satisfactorily the lineweaver-Burk linear plot. 4. Results suggest that these exchanges occur through potassium channel. 5. Kinetics of potassium exchanges is discussed in relation to the maintenance of the osmoregulation in fish.

Potassium channels in the basolateral membrane of the rectal gland of Squalus acanthias. Regulation and inhibitors

The present study examines the influences of pH and Ca2+ and several putative inhibitors on the basolateral K+ channel of the rectal gland of Squalus acanthias. Excised membrane patches were examined using the patch clamp technique. It is shown that reduction of the calcium activity on the cytosolic side to less than 10(-9) mol/l has no detectable inhibitory effect on this channel. Conversely, increase in calcium activity to some 10(-3) mol/l reduced the activity of this channel. Variations in cytosolic pH had only a moderate effect on the current amplitude: alkalosis by one pH unit increased and acidosis reduced the single current amplitude by some 15%. Several inhibitors were tested in excised patches when added to the cytosolic side. Ba2+ (approximately 5 X 10(-3) mol/l), quinine (approximately 10(-3) mol/l), quinidine (approximately 10(-4) mol/l), lidocaine (approximately equal to 1 mmol/l), tetraethylammonium (approximately 10 mmol/l), Cs+ (approximately 10 mmol/l), and Rb+ (approximately 20 mmol/l) all blocked this K+ channel reversibly. We conclude that the basolateral K+ channel of the rectal gland is distinct from other epithelial K+ channels inasmuch as it is not stimulated by Ca2+ directly, but that it is qualitatively similar to many other known K+ channels with respect to its sensitivity towards blockers.

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.

Ca2+- and voltage activated K+ channel in apical cell membrane of gallbladder epithelium from Triturus

The presence of Ca2+- and voltage-activated K+ channels was directly demonstrated in the apical cell membrane of gallbladder epithelium by patch-clamp single-channel current recording. In K+-depolarized epithelial cells, negative pipette potentials induced outward current steps when the patch-pipette was filled with Na+-rich solution and these current steps were not affected by the presence or absence of Cl-. When K+-rich solution was in the pipette and K+-depolarized cells were examined, the current-voltage relations were linear with a single-channel conductance of 140 pS and polarity was reversed at 0 mV. In excised inside-out membrane patches, raising the free Ca2+ concentration of the medium facing the inner side of the membrane from 10(-7) to 10(-6) M evoked a marked increase in open state probability of the channels without affecting the elementary current steps. This suggests that intracellular Ca2+ as a second messenger plays a crucial role in the regulatory mechanism of the membrane potential by modulating the high-conductance apical K+ channels.

Single anion-selective channels in basolateral membrane of a mammalian tight epithelium

Basolateral membrane chloride permeability of surface cells from rabbit urinary bladder epithelium was studied using the patch-clamp technique. Two types of anion-selective channel were observed. One channel type showed inward rectification and had a conductance of 64 pS at-50 mV when bathed symmetrically by saline solution containing 150 mM chloride; the other resembled high-conductance voltage-dependent anion channels (VDACs). Both channels had the selectivity sequence Cl-approximately equal to Br-approximately equal to I- approximately equal to SCN- approximately equal to NO3- greater than F- greater than acetate greater than gluconate greater than Na+ approximately equal to K+ and were sensitive to the anion exchange inhibitor 4,4'-diisothiocyanostilbene-2,2'-disulfonic acid. Basolateral chloride conductance in urinary bladder is apparently due to the 64 pS anion channel, which is active at physiological potentials. Imperfect selectivity of this channel against cations might also account for the low, but finite, sodium permeability of the basolateral membrane.

Ca2+-sensitive, spontaneously fluctuating, cation channels in the apical membrane of the adult frog skin epithelium

The fluctuations in transepithelial current through the abdominal skin of bullfrogs (Rana catesbeiana) were analysed while the transepithelial voltage was clamped to zero. A Lorentzian component in the power spectrum was recorded when the skin was bathed with Ca2+ free NaCl Ringer's on both sides. After replacement of all mucosal Na+ by choline the Lorentzian component disappeared. The application of mucosa positive potentials enhanced the plateau of the relaxation noise component while it was depressed by mucosa negative potentials. These observations showed that the current associated with the relaxation noise, was carried by Na+ moving in the inward direction. Divalent cations added to the mucosal solution in micromolar concentrations depressed the relaxation noise immediately, which is indicative for an apical localization of the fluctuating channels. The relaxation noise depended strongly on the pH of the mucosal medium: alkalinization enhanced the relaxation noise while acidification depressed the fluctuations. Micromolar concentrations of the diuretic amiloride, which is known to block the Na+ entry into the cellular compartment, enhanced the Na+-dependent relaxation noise while at higher concentrations an inhibitory effect was observed. From these observations it was concluded that the relaxation noise is caused by inward Na+ movement through fluctuating channels which are localized in the apical membrane. These channels seem to constitute a pathway in parallel with the amiloride-blockable channels. Ionic substitution of Na+ by other monovalent cations showed that these channels are also permeable for K+, Rb+, NH4+, Cs+ and Tl+, but not for Li+. Divalent cations in micromolar concentrations completely occlude these fluctuating channels. Therefore, this pathway will be blocked for monovalent cations when normal Ca2+ containing Ringer's are used as mucosal bathing medium.

Single channel recordings from basolateral and apical membranes of renal proximal tubules

A new method is described, which enables the recording of single ionic channels from the basolateral as well as the luminal membrane of renal proximal tubules with the patch-clamp technique. Segments of late proximal tubules of rabbit kidney are cannulated and perfused from one end. The other end is open and freely accessible to a patch pipette. The patch electrode can be moved against lateral cell membranes or can be inserted through the open end into the lumen and brought to contact with the brush-border membrane. Both, in the basolateral as well as in the luminal membrane, giga seals can be achieved. In both membranes, K+ selective channels could be identified.

Voltage-dependent K conductance at the apical membrane of Necturus gallbladder

The epithelial and cellular effects of clamping the transepithelial potential (Vt, mucosa reference) have been investigated in the Necturus gallbladder. Following initial equilibration at short circuit, tissue conductance gt was 4.1 +/- 1.2 (SD) mS/cm2, the apical potential Va was -76 +/- 8 mV, and the apical fractional voltage on brief voltage perturbation (fa = delta Va/delta Vt, reflecting the ratio of apical membrane to transcellular resistance) was 0.72 +/- 0.11 (21 gallbladders, 34 impalements). On clamping Vt at positive values, Va depolarized and fa decreased; at the same time gt decreased. Clamping Vt at negative values produced converse effects. All of the above changes were related directly to the magnitude of the clamping potential Vt and were reversed on return to the short circuit state. Effects of Vt on fa are not due to changes in the extracellular pathway resistances (which, however, contribute to gt). Furthermore, the effects of Vt on fa were abolished by the mucosal application of TEA or Ba, or acidification of the mucosal solution. Thus, these experiments disclose the presence of a voltage-dependent apical K conductance that increases with apical membrane depolarization. The calculated dose-response curve of TEA inhibition of apical conductance and the values of the apparent dissociation constant were in good agreement with those found for K channels in excitable tissues. Mucosal application of the Ca ionophore A23187 shifted the voltage dependence curve of fa to more negative values of Va without altering its shape. The effect of A23187 suggests a possible role of intracellular Ca in the modulation of the apical K channels.

The interaction of "K+-like" cations with the apical K+ channel in frog skin

The apparent permeability of the apical K+ channel in the abdominal skin of the frog (Rana temporaria) for different monovalent cations was tested by comparing the short-circuit current (SCC) obtained after imposition of serosally directed ionic concentration gradients. Furthermore, the SCC was subjected to noise analysis. Of various cations tested, only the "K+-like" ions NH+4, Rb+ and Tl+, besides K+, were found to permeate the apical K+ channel, as reflected by SCC- and fluctuation analysis: (i) The SCC could be depressed by addition of the K+-channel blocker Ba2+ to the mucosal solution. (ii) With the K+-like ions (Ringer's concentration), a spontaneous Lorentzian noise was observed. Plateau values were similar for K+ and Tl+, and smaller for NH+4 and Rb+. The corner frequencies clearly increased in the order K+ less than NH+4 less than Tl+ much less than Rb+. The SCC dose-response relationships revealed a Michaelis-Menten-type current saturation only for pure K+- or Tl+-Ringer's solutions as mucosal medium, whereas a more complicated SCC behavior was seen with Rb+ and especially, NH+4. For K+-Tl+ mixtures an anomalous mole-fraction relationship was observed: At low [Tl+]/[K+] ratios, Tl+ ions appeared to inhibit competitively the K+ current while, at high [Tl+]/[K+] ratios, Tl+ seemed to be a permeant cation. This feature was also detected in the noise analysis of K+-Tl+ mixtures. Long-term exposure to mucosal Tl+ resulted in an irreversible deterioration of the tissue. The SCC depression by Ba2+ was of a simple saturation-type characteristic with, however, different half-maximal doses (NH+4 less than K+ less than Rb+). Ba2+ induced a "blocker noise" in presence of all permeant cations with corner frequencies that depended on the Ba2+ concentration. A linear increase of the corner frequencies of the Ba2+-induced noise with increasing Ba2+ concentration was seen for NH+4, Rb+ and K+. With the assumption of a pseudo two-state model for the Ba2+ blockade the on- and off-rate constants for the Ba2+ interaction with the NH+4/Rb+/K+ channel were calculated and showed marked differences, dependent on the nature of the permeant ion. The specific problems with Tl+ prevented such an analysis but SCC- and noise data indicated a comparably poor efficiency of Ba2+ as Tl+-current inhibitor. We attempted a qualitative analysis of our results in terms of a "two-sites, three-barriers" model of the apical K+ channel in frog skin.

Noise analysis reveals K+ channel conductance fluctuations in the apical membrane of rabbit colon

In this paper we describe current fluctuations in the mammalian epithelium, rabbit descending colon. Pieces of isolated colon epithelium bathed in Na+ or K+ Ringer's solutions were studied under short-circuit conditions with the current noise spectra recorded over the range of 1-200 Hz. When the epithelium was bathed on both sides with Na+ Ringer's solution (the mucosal solution contained 50 microM amiloride), no Lorentzian components were found in the power spectrum. After imposition of a potassium gradient across the epithelium by replacement of the mucosal solution by K+ Ringer's (containing 50 microM amiloride), a Lorentzian component appeared with an average corner frequency, fc = 15.6 +/- 0.91 Hz and a mean plateau value So = (7.04 +/- 2.94) x 10(-20) A2 sec/cm2. The Lorentzian component was enhanced by voltage clamping the colon in a direction favorable for K+ entry across the apical membrane. Elimination of the K+ gradient by bathing the colon on both sides with K+ Ringer's solutions abolished the noise signal. The Lorentzian component was also depressed by mucosal addition of Cs+ or tetraethylammonium (TEA) and by serosal addition of Ba2+. The one-sided action of these K+ channel blockers suggests a cellular location for the fluctuating channels. Addition of nystatin to the mucosal solution abolished the Lorentzian component. Serosal nystatin did not affect the Lorentzian noise. This finding indicates an apical membrane location for the fluctuating channels. The data were similar in some respects to K+ channel fluctuations recorded from the apical membranes of amphibian epithelia such as the frog skin and toad gallbladder. The results are relevant to recent reports concerning transcellular potassium secretion in the colon and indicate that the colon possesses spontaneously fluctuating potassium channels in its apical membranes in parallel to the Na+ transport pathway.

Poorly selective cation channels in the skin of the larval frog (stage less than or equal to XIX)

The abdominal skin of bullfrog larvae (Rana catesbeiana) was placed in an Ussing-type chamber, and its transepithelial electrical parameters were recorded with mucosal solutions of different ionic composition. With "K+-like" cations (K+, NH+4, RB+, Cs+) the power spectra of the fluctuations in short-circuit current displayed a Lorentzian component (fc = 30 - 40 Hz). The relaxation noise could be suppressed by addition of the K+ -channel blockers Ba2+ and TEA to the mucosal solution. Also, in presence of the ionophore antibiotic nystatin the Lorentizian noise was abolished. The Na+ -channel probes amiloride and benzimidazolyl-2-guanidine (BIG) both enhanced the relaxation noise obtained with the K+-like cations but, with Na+ and Li+, also caused the rise of a relaxation component above the background noise. In presence of amiloride or BIG, the addition of Ba2+, TEA and nystatin still abolished the Lorentizian noise. It can be concluded that the relaxation-noise source is located in the apical cell membranes of the tadpole skin. These spontaneously fluctuating cation channels do not seem to strictly discriminate between K+-like ions (K+, NH+4, Rb+, Cs+) and Na+-like ions (Na+, Li+). On the other hand, well-known specific probes for K+ channels (Ba2+, TEA) and for Na+ channels (amiloride, BIG) interact with this apical cation channel. It is possible that the poorly selective channel plays a role in the ontogenesis of the specific Na+ transport in the maturing frog skin.

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.

Noise analysis of the K+ current through the apical membrane of Necturus gallbladder

Current noise power spectra of the voltage-channel (V = 0) Necturus gallbladder, exposed to NaCl-Ringer's on both sides contained a relaxation noise component, which overlapped with a 1/f alpha noise component, with alpha being about 2. Substitution of all Na+ by K+ on either the serosal or mucosal side increased the relaxation as well as the 1/f alpha noise component considerably. In Necturus gallbladder both noise components are reduced by addition of 10 mM, 2,4,6-triaminopyrimidine (TAP) or 5 mM BA2+ to the mucosal side, as well as by acidification of the mucosal solution to pH 5 and lower. Five mM of tetraethylammonium (TEA+) added to the mucosal solution, abolished K+ relaxation noise and decreased the 1/f alpha noise component. Applying a Cs+ concentration gradient across the epithelium did not yield relaxation noise. However, if Rb+ was substituted for all Na+ on one side, a Lorentzian noise component appeared in the spectrum. Its plateau was smaller than with KCl-Ringer's on the respective side. These data confirm the existence of fluctuating K+ channels in the apical membrane of the Necturus gallbladder. Furthermore it can be concluded that these channels have the permeability sequence K+ greater than Rb+ greater than Cs+. The inhibition of the fluctuations by mucosal acidification indicates the existence of acidic sites in the channel. The single-channel conductance was estimated to be between 6.5 and 40 pS.

Noise analysis of the K+ current through the apical membrane of Necturus gallbladder

Current noise power spectra of the voltage-clamped (V = 0) Necturus gallbladder, exposed to NaCl-Ringer's on both sides contained a relaxation noise component, which overlapped with a 1/f alpha noise component, with alpha being about 2. Substitution of all Na+ by K+ on either the serosal or mucosal side increased the relaxation as well as the 1/f alpha noise component considerably. In Necturus gallbladder both noise components are reduced by addition of 10mM 2,4,6-triaminopyrimidine (TAP) or 5 mM of tetraethylammonium (TEA+) added to ification of the mucosal solution to pH 5 and lower. Five mM of tetraethylammonium (TEA+) added to the mucosal solution, abolished K+ relaxation noise and decreased the 1/f alpha noise component. Applying a Cs+ concentration gradient across the epithelium did not yield relaxation noise. However, if Rb+ was substituted for all Na+ on one side, a Lorentzian noise component appeared in the spectrum. Its plateau was smaller than with KCl-Ringer's on the respective side. These data confirm the existence of fluctuating K+ channels in the apical membrane of the Necturus gallbladder. Furthermore it can be concluded that these channels have a permeability sequence K+ greater than Rb+ greater than Cs+. The inhibition of the fluctuations by mucosal acidification indicates the existence of acidic sites in the channel. The single-channel conductance was estimated to be between 6.5 and 40 pS.

Mechanisms of cation permeation across apical cell membrane of Necturus gallbladder: effects of luminal pH and divalent cations on K+ and Na+ permeability

Conventional microelectrode techniques were combined with unilateral mucosal ionic substitutions to determine the effects of luminal pH and luminal alkali-earth cation concentrations on apical membrane cation permeability in Necturus gallbladder epithelium. Acidification of the mucosal solution caused reversible depolarization of both cell membranes and increase of transepithelial resistance. Low pH media also caused: (a) reduction of the apical membrane depolarization induced by high K, and (b) increase of the apical membrane hyperpolarization produced by Na replacement with Li or N-Methyl-D-glucamine. These results, in conjunction with estimates of cell membrane conductances, indicate that acidification of the luminal solution produces a reduction of apical membrane K permeability (PK). Addition of alkali earth cations (Mg2+, Ca2+, Sr2+, or Ba2+) produced cell membrane depolarization, increase of relative resistance of the luminal membrane and reduction of the apical membrane potential change produced by a high-K mucosal medium. These results, as those produced by low pH, can be explained by a reduction of apical membrane PK. The effects of Ba2+ on membrane potential and relative apical membrane PK were larger than those of all other four cations at all concentrations tested (1-10 mM). The effect of Sr2+ was significantly larger than those of Mg2+ and Ca2+ at 10 mM, but not different at 5 mM. The reduction of PK produced by mucosal acidification appears to be mediated by: (a) nonspecific titration of membrane fixed negative charges, and (b) an effect of luminal proton activity on the apical K channel. Divalent cations reduce apical membrane PK probably by screening negative surface charges. The larger magnitude of the effects of Ba2+ and Sr2+ can be explained by binding to membrane sites, in the surface or in the K channel, in addition to their screening effect. We suggest that the action of luminal pH on K secretion in some segments of the renal tubule could be mediated in part by this pH-dependent K permeability of the luminal membrane.

Capacitive and inductive low frequency impedances of Necturus gallbladder epithelium

The electrical impedance of Necturus gallbladder epithelium was analysed in the frequency range 0.24 Hz to 6,323 Hz. Under control conditions (NaCl-Ringer's on both sides), the impedance function yields a semicircle with depressed center. When serosal Na+ was replaced by K+, an inductive low frequency (LF) component appeared in the impedance locus. With KCl-Ringer on the mucosal side a second circular arc was observed at frequencies below 1 Hz. The resistive parts of the capacitive and inductive LF components increased after application of TAP+ to the mucosal side. Both LF features were abolished after application of 5 mM TEA+ to the mucosal medium as well as after acidification of the mucosal side. The LF components were depressed by addition of 5 mM Ba2+ to the mucosal solution. As TEA+ blocks apical K+ channels (Van Driessche and Gögelein 1978), it is concluded that the capacitive as well as the inductive LF components are related to transcellular K+ flow. With KCl-Ringer on the mucosal side, mucosa negative potentials increased the equivalent resistance and decreased the equivalent capacitance of the LF impedance. With serosal KCl-Ringer, negative potentials evoked a capacitive component which overlapped with the inductive component observed at open circuit conditions. Positive potentials, however, abolished the capacitive as well as the inductive LF component, elicited by mucosal or serosal KCl-Ringer, respectively. These results demonstrate that serosa to mucosa directed K+ flow causes an inductive LF feature and that mucosa negative potentials elicit a capacitive LF component.

Spontaneous fluctuations of potassium channels in the apical membrane of frog skin

1. The previously demonstrated K+-dependent short-circuit current through the skin of frog species Rana temporaria (Zeiske & Van Driessche, 1979), bathed with mucosal K+- and serosal Na+-Ringer solution, was investigated with current-fluctuation analysis. 2. The current-noise spectra were recorded in the frequency range from 1 to 800 Hz and showed a Lorentzian component with a mean plateau value S0 = (1.50 +/- 0.05).10(-20) A2.s.cm-2 and a corner frequency of fc=(81.0 +/- 3.4)Hz(n=14). 3. S0 increased with mucosal K+ concentration, [K]o, while fc remained almost unchanged. A decrease in S0 was observed when serosal Na+ was replaced by K+. 4. Mucosal Cs+ (10 mM) depressed, reversibly, the K+-dependent current noise to the level of the background noise. Moreover, a linear decrease in fc with increasing Cs+ concentration was observed. 5. Among the other tested alkali cations, Rb+ was the only blocker though less potent than Cs+. Tetraethylammonium, 4-aminopyridine, 2.4.6-triaminopyrimidine and amiloride had no effect. 6. Alterations in the transcellular transport of Na+ contained in a mucosal solution with high [K]o resulted in significant changes in K+ current noise. 7. The current-fluctuation intensities decreased with increasing contact time to high [K]o; these changes were concomitant with the previously reported time dependence of the short-circuit current (Zeiske & Van Driessche, 1979). 8. The K+-dependent fluctuations are thought to originate from K+-selective pathways in the apical cell membranes. The description of the K+-current noise by a single Lorentzian suggests that the "K+ channels" switch randomly between an open and closed state. 9. Assuming a two state model for the channel-kinetics, the single channel current i and the channel density M were calculated as i=(0.37 +/- 0.05)pA and M=(0.53 +/- 0.08) mu-2 (n=13).

Triaminopyrimidinium (TAP+) blocks luminal membrane K conductance in Necturus gallbladder epithelium

The effect of triaminopyrimidinium (TAP+) on the apical membrane of necturus gallbladder epithelial cells was investigated with intracellular microelectrode techniques. TAP+, added to the mucosal bathing solution only, produced the following effects (all rapid and reversible): (i) cell depolarization, (ii) increase of apical membrane resistance, and (iii) decrease of the apical membrane potential change produced by K for Na substitution on the mucosal side. These results can be explained by a decrease of apical membrane K conductance. The paracellular effects of TAP+ were similar to the ones previously described by Moreno (J.H. Moreno, 1974; Nature (London) 251:150; J.H. Moreno, 1975. J. Gen. Physiol. 66:97). These results indicate that the change of transepithelial potential produced by TAP+ cannot be ascribed solely to its effect on the paracellular pathway.