The role of sodium-channel density in the natriferic response of the toad urinary bladder to an antidiuretic hormone

Effects of standard diuretics and RPH 2823 on transepithelial Na+ transport in isolated frog skin

Short-circuit current (SCC) techniques were used to monitor the effects of various diuretic agents on Na+ transport in isolated frog skin, a model for the late distal tubule and the collecting duct of the mammalian kidney. Acetazolamide, hydrochlorothiazide, torasemide, and ethacrynic acid did not affect sodium transport (as indicated by the SCC) or transepithelial electrical resistance when added either to the apical (outer) or to the inner (basolateral, corial) bathing solution of the tissue. However, Na+ transport was sensitive to amiloride, the triamterene derivate dimethylamino-hydroxypropoxytriamterene (RPH 2823), and to furosemide. Whereas apical amiloride, and RPH 2823 induced a dose-dependent decrease in SCC and increase in transepithelial electrical resistance, apical furosemide resulted in a dose-dependent increase in SCC and a decrease in electrical resistance. None of the three diuretic agents caused a significant change in SCC when applied to the inner bathing Ringer's solution. The small furosemide-induced decrease in resistance compared with the huge increase in SCC suggests that furosemide affects Cl- permeability as well as Na+ permeability. Evidence for this notion was achieved by the following findings: The decrease in resistance after furosemide was more pronounced in tissues bathed in Cl(-)-free solutions compared with Cl(-)-containing solutions. n contrast, SCC stimulation by apical furosemide is Cl(-)-ion independent, but strongly Na+-ion dependent. SCC stimulation by furosemide is amiloride-sensitive. With respect to the onset, locus, and reversibility of action, it seems reasonable to assume that amiloride, RPH 2823, and furosemide all influence transepithelial Na+ transport by interacting with the Na+ channel or a regulator site of it within the apical membrane. The stoichiometry of the amiloride (RPH 2823)-receptor site interaction revealed Hill-coefficient(s) of less than 1, indicating a negative cooperativity among the receptor sites. The interaction between Na+ ions and amiloride or RPH 2823 displayed mixed competitive-noncompetitive inhibition. Taken together, these results support the hypothesis that amiloride and Na+ as well as RPH 2823 and Na+ may act at different loci on the apical entry mechanism in Rana esculenta skin.

Detergent solubilization, functional reconstitution, and partial purification of epithelial amiloride-binding protein

The amiloride-binding protein from cultured toad kidney cells (A6) was solubilized in 3-[(3-cholamidopropyl)dimethylammonio]-1-propanesulfonate (CHAPS), functionally reconstituted into liposomes, and partially purified. The specific binding of [3H]methylbromoamiloride ([3H]CH3BrA) was measured in intact A6 epithelia, A6 cell homogenate (H), apical plasma membrane vesicle (V1), and CHAPS-solubilized V1 and on material obtained after affinity chromatography of CHAPS-solubilized plasma membrane vesicles on agarose-immobilized wheat germ agglutinin (WGA). Specific [3H]CH3BrA binding to H, V1, and WGA material reached equilibrium after 10 min. Scatchard analysis of [3H]CH3BrA binding to V1 and WGA material revealed a homogeneous class of binding sites with KD's of 130 and 128 nM, respectively. These KD values were similar to the apparent inhibitory dissociation constant determined from amiloride inhibition of 22Na+ influx in both intact A6 epithelia and V1. The total number of specific binding sites was 4 pmol/mg of V1 protein, which represented a 10-fold enrichment compared to H, and 66.6 pmol/mg of WGA material (a 148-fold enrichment). From association/displacement kinetic studies of specific [3H]CH3BrA binding to V1, the rate constants of association (ka) and dissociation (kd) were calculated to be 3.6 X 10(5) M-1 s-1 and 49.5 X 10(-3) s-1, respectively. These values yield an equilibrium dissociation constant of 138 nM. In solubilized V1 protein, binding activity was enriched approximately 20-fold over H and was markedly dependent upon the relative concentrations of detergent and phospholipid. CHAPS solubilization of V1 resulted in an average 44% recovery of protein with 90% retention of the total number of specific [3H]CH3BrA binding sites. After WGA chromatography 2.7% of the applied protein and 46% of the specific binding sites were recovered.(ABSTRACT TRUNCATED AT 250 WORDS)

Impairment of Na+ transport across frog skin by Tl+: effects on turnover, area density and saturation kinetics of apical Na+ channels

Na+ transport across abdominal skin of the frogs, Rana temporaria and Rana esculenta was followed by measuring Na+-dependent short-circuit, current (INa) kinetics and INa fluctuations induced by triamterene, a diuretic. Exposure of the skin to serosal Tl+ led to a pronounced and irreversible drop in INa and INa-blocker noise. At low serosal Tl+ concentrations, we observed mainly a decrease in the apparent Michaelis constant for INa saturation while, at larger [Tl+], the maximal INa dropped irreversibly. Tl+ acts even when serosal Tl+ "transporters" like the Na+-K+ pump, or the K+ channel are nonfunctional. The rate constants for the triamterene/Na+ channel reaction were unchanged after Tl+ whereas the relaxation noise from channel blockage decreased in amplitude. Noise analysis in terms of a two-state blocking model suggested that Tl+ poisoning results in a small decrease in single-channel current through apical Na+ pathways, as well as in a drastic and irreversible drop in channel density. The impairment of Na+ transport by Tl+ can be attributed to the above cited concerted events at the level of the apical membrane.

Binding of 3H-phenamil, an irreversible amiloride analog, to toad urinary bladder: effects of aldosterone and vasopressin

Phenamil, an analog of amiloride, has previously been shown to bind specifically to sodium channels in toad bladder (J.L. Garvin et al., J. Membrane Biol. 87:45-54, 1985). In this paper, 3H-phenamil was used to measure sodium channel density in both isolated epithelial cells and intact bladders. From the specific binding to intact bladders, a channel density of 455 +/- 102 channels/micron2 was calculated. No correlation between specific binding and the magnitude of irreversible inhibition of short-circuit current was found. Pretreatment of intact bladders with 1 mg/ml trypsin reduced specific binding to isolated cells by 82 +/- 5%. In isolated cells, neither aldosterone nor vasopressin had any significant effect on specific phenamil binding. It is inferred that phenamil binds to both open and closed channels which may be either in the mucosal membrane or in the submembrane space. Finally, and rather surprisingly, we found that 3H-phenamil binds irreversibly to the basolateral membrane at concentrations as low as 4 X 10(-7) M. Therefore, care must be used in interpreting binding studies with amiloride or its analog at such concentrations.

Membrane potentials and intracellular Cl- activity of toad skin epithelium in relation to activation and deactivation of the transepithelial Cl- conductance

The potential dependence of unidirectional 36Cl fluxes through toad skin revealed activation of a conductive pathway in the physiological region of transepithelial potentials. Activation of the conductance was dependent on the presence of Cl or Br in the external bathing solution, but was independent of whether the external bath was NaCl-Ringer's, NaCl-Ringer's with amiloride, KCl-Ringer's or choline Cl-Ringer's. To partition the routes of the conductive Cl- ion flow, we measured in the isolated epithelium with double-barrelled microelectrodes apical membrane potential. Va, and intracellular Cl- activity, acCl, of the principal cells identified by differential interference contrast microscopy. Under short-circuit conditions, Isc = 27.0 +/- 2.0 microA/cm2, with NaCl-Ringer's bathing both surfaces, Va was -67.9 +/- 3.8 mV (mean +/- SE, n = 24, six preparations) and acCl was 18.0 +/- 0.9 mM in skins from animals adapted to distilled water. Both Va and acCl were found to be positively correlated with Isc (r = 0.66 and r = 0.70, respectively). In eight epithelia from animals adapted to dry milieu/tap water Va and acCl were measured with KCl Ringer's on the outside during activation and deactivation of the transepithelial Cl- conductance (GCl) by voltage clamping the transepithelial potential (V) at 40 mV (mucosa positive) and -100 mV. At V = 40 mV; i.e. when GCl was deactivated, Va was -70.1 +/- 5.0 mV (n = 15, eight preparations) and acCl was 40.0 +/- 3.8 mM. The fractional apical membrane resistance (fRa) was 0.69 +/- 0.03. Clamping to V = -100 mV led to an instantaneous change of Va to 31.3 +/- 5.6 mV (cell interior positive with respect to the mucosal bath), whereas neither acCl nor fRa changed significantly within a 2 to 5-min period during which GCl increased by 1.19 +/- 0.10 mS/cm2. When V was stepped back to 40 mV, Va instantaneously shifted to -67.8 +/- 3.9 mV while acCl and fRa remained constant during deactivation of GCl. Similar results were obtained in epithelia impaled from the serosal side. In 12 skins from animals adapted to either tap water or distilled water the density of mitochondria-rich (DMRC) cells was estimated and correlated with the Cl current (ICl through the fully activated (V = -100 mV) Cl conductance). A highly significant correlation ws revealed (r = -0.96) with a slope of -2.6 nA/m.r. (mitochondria-rich cell and an I-axis intercept not significantly different from zero.(ABSTRACT TRUNCATED AT 400 WORDS)

Amiloride-sensitive Na channels from the apical membrane of the rat cortical collecting tubule

Currents through individual Na channels in the apical membrane of the rat cortical collecting tubule were resolved by using the patch-clamp technique. In cell-attached patches, the channels had a conductance of 5 pS with 140 mM NaCl in the pipet. The conductance was a saturable function of external Na, with a maximal value of about 8 pS and a half saturation at about 75 mM Na. In excised inside-out patches, the selectivity of the channels for Na over K was estimated from reversal potentials to be at least 10:1. The channels underwent spontaneous transitions between open and closed states. Both states had mean lifetimes of 3-4 sec. Amiloride (0.5 microM) added to the pipet induced more frequent closures and openings of the channels and a reduction in the mean open time. These channels are presumed to mediate Na reabsorption by this nephron segment in vivo.

Sodium transport by rat cortical collecting tubule. Effects of vasopressin and desoxycorticosterone

We have used rat cortical collecting tubules perfused in vitro to study the effects of antidiuretic hormone (ADH) and desoxycorticosterone (DOCA) on the unidirectional fluxes of sodium. We found that in the basal state, lumen-to-bath flux (Jlb) and bath-to-lumen flux (Jbl) of 22Na were approximately equal, 39.5 +/- 3.9 and 41.8 +/- 11.0 pmol X min-1 X min-1, respectively, resulting in no net flux. Addition of 100 microU/ml ADH to the bath produced a stable increase in Jlb to 58.3 +/- 4.7 pmol X min-1 X mm-1. Pretreatment of the animal with DOCA for 4 to 7 d (20 mg/kg per d) increased baseline Jlb to 81.6 +/- 8.7 pmol X min-1 X mm-1. Addition of ADH to a tubule from a DOCA-pretreated rat caused an increase in Jlb to 144.1 +/- 12.0 pmol X min-1 X mm-1 X Neither hormone had an effect on Jbl X Thus ADH produced a greater absolute and fractional increase in Jlb when the animal was pretreated with DOCA, and the ADH-induced increase over baseline was greater than the DOCA-induced increase. Both the ADH-and DOCA-induced stimulation of Jlb were completely abolished by 10(-5) M luminal amiloride, suggesting that the route of sodium transport stimulated by both hormones involves apical sodium channels. However, ADH and DOCA have very different time courses of action; ADH acted within minutes, while aldosterone and DOCA are known to require 90-180 min. The facilitating action of ADH on DOCA-induced stimulation of sodium transport may be important for maximal sodium reabsorption and for the ability to achieve a maximally concentrated urine.

[3H]phenamil, a radiolabelled diuretic for the analysis of the amiloride-sensitive Na+ channels in kidney membranes

The interaction of amiloride and amiloride derivatives with the Na+ channels of pig kidney membranes was studied from 22Na+ uptake experiments. The order of potency of the different molecules tested is: phenamil greater than benzamil greater than amiloride, ethylisopropylamiloride. [3H]labelled phenamil was prepared and used to titrate Na+ channels in pig kidney membranes. Kinetics experiments, equilibrium binding studies and competition experiments between [3H]phenamil and unlabelled phenamil indicate that phenamil recognizes a single family of binding sites with a Kd value of 20 nM and a maximum binding capacity of 11.5 pmol/mg of protein. The order of potency of different amiloride analogs tested in [3H]phenamil competition experiments is identical to that found for the inhibition of 22Na+ uptake by apical Na+ channels.

Basolateral membrane potential and conductance in frog skin exposed to high serosal potassium

In studies of apical membrane current-voltage relationships, in order to avoid laborious intracellular microelectrode techniques, tight epithelia are commonly exposed to high serosal K concentrations. This approach depends on the assumptions that high serosal K reduces the basolateral membrane resistance and potential to insignificantly low levels, so that transepithelial values can be attributed to the apical membrane. We have here examined the validity of these assumptions in frog skins (Rana pipiens pipiens). The skins were equilibrated in NaCl Ringer's solutions, with transepithelial voltage Vt clamped (except for brief perturbations delta Vt) at zero. The skins were impaled from the outer surface with 1.5 M KCl-filled microelectrodes (Rel greater than 30 M omega). The transepithelial (short-circuit) current It and conductance gt = -delta It/delta Vt, the outer membrane voltage Vo (apical reference) and voltage-divider ratio (Fo = delta Vo/delta Vt), and the microelectrode resistance Rel were recorded continuously. Intermittent brief apical exposure to 20 microM amiloride permitted estimation of cellular (c) and paracellular (p) currents and conductances. The basolateral (inner) membrane conductance was estimated by two independent means: either from values of gt and Fo before and after amiloride or as the ratio of changes (-delta Ic/delta Vi) induced by amiloride. On serosal substitution of Na by K, within about 10 min, Ic declined and gt increased markedly, mainly as a consequence of increase in gp. The basolateral membrane voltage Vi (= -Vo) was depolarized from 75 +/- 4 to 2 +/- 1 mV [mean +/- SEM (n = 6)], and was partially repolarized following amiloride to 5 +/- 2 mV.(ABSTRACT TRUNCATED AT 250 WORDS)

Sodium flux in the apical membrane of the toad skin: aspects of its regulation and the importance of the ionic strength of the outer solution upon the reversibility of amiloride inhibition

Injection of small pulses of concentrate solutions of salts or drugs into the outer bathing fluid led to sudden increases of its solute concentration. Vigorous stirring of the outer bathing solution was used to minimize the thickness of the unstirred layer adjacent to the outer skin surface. Pulses of 1 M NaCl injected into the outer compartment induced sharp increases of the SCC following a time course variable with the magnitude of the pulse and the particular condition of each skin. Comparison of the spontaneous decline of the SCC with the decline induced by a small dose of amiloride, where an increase in R was observed, indicates that the spontaneous decline cannot be explained simply as a reduction of the Na permeability of the apical membrane by self-inhibition of feedback inhibition of the apical membrane Na channels. Reduction of the driving force for Na movement into the epithelial cells must play an important role in the process. Reversibility of the amiloride inhibition of the SCC was highly dependent upon the ionic strength of the solution used to rinse and wash out the inhibitor from the outer skin surface. With H2O, the amiloride molecules washed out slowly as compared to NaCl or KCl solutions. Na or K have the same ability to dislodge the amiloride molecules from their binding sites. This effect is apparently of a purely electrostatic nature.

Defense strategies against hypoxia and hypothermia

Because aerobic metabolic rates decrease in hypoxia-sensitive cells under oxygen-limiting conditions, the demand for glucose or glycogen for anaerobic glycolysis may rise drastically as a means of making up for the energetic shortfall. However, ion and electrical potentials typically cannot be sustained because of energy insufficiency and high membrane permeabilities; therefore metabolic and membrane functions in effect become decoupled. In hypoxia-tolerant animals, these problems are resolved through a number of biochemical and physiological mechanisms; of these metabolic arrest and stabilized membrane functions are the most effective strategies for extending tolerance to hypoxia. Metabolic arrest is achieved by means of a reversed or negative Pasteur effect (reduced or unchanging glycolytic flux at reduced O2 availability); and coupling of metabolic and membrane function is achievable, in spite of the lower energy turnover rates, by maintaining membranes of low permeability (probably via reduced densities of ion-specific channels). The possibility of combining metabolic arrest with channel arrest has been recognized as an intervention strategy. To date, the success of this strategy has been minimal, mainly because depression of metabolism through cold is the usual arrest mechanism used, and hypothermia in itself perturbs controlled cell function in most endotherms.

Single-channel recordings from two types of amiloride-sensitive epithelial Na+ channels

We report here the first evidence in intact epithelial cells of unit conductance events from amiloride-sensitive Na+ channels. The events were observed when patch-clamp recordings were made from the apical surface of cultured epithelial kidney cells (A6). Two types of channels were observed: one with a high selectivity to Na+ and one with relatively low selectivity. The characteristics of the low-selectivity channel are as follows: single-channel conductance ranged between 7 and 10 pS (mean = 8.4 +/- 1.3), the current-voltage (I-V) relationship displayed little if any nonlinearity over a range of +/- 80 mV (with respect to the patch pipette) and the channel Na+/K+ selectivity was approximately 3-4:1. Amiloride, a cationic blocker of the channel, reduced channel mean open time and increased channel mean closed times as the voltage of the cell interior was made more negative. Amiloride induced channel flickering at increased negative potentials (intracellular potential with respect to the patch) but did not alter the single-channel conductance or the I-V relationship from that observed in control patches. The characteristics of the high-selectivity channel are: a single-channel conductance of 1-3 pS (mean = 2.8 +/- 1.2), the current-voltage relationship is markedly nonlinear with a Na+/K+ selectivity greater than 20:1. The mean open and closed times for the two types of channels are quite different, the high-selectivity channel being open only about 10% of the time while the low-selectivity channel is open about 30% of the time.

Electrophysiology and noise analysis of K+-depolarized epithelia of frog skin

Epithelia of frog skin bathed either symmetrically with a sulfate-Ringer solution or bathed asymmetrically and depolarized with a 112 mM K+ basolateral solution (Kb+) were studied with intracellular microelectrode techniques. Kb+ depolarization caused an initial decrease of the short-circuit current (Isc) with a subsequent return of the Isc toward control values in 60-90 min. Whereas basolateral membrane resistance (Rb) and voltage were decreased markedly by high [Kb+], apical membrane electrical resistance (Ra) was decreased also. After 60 min, intracellular voltage averaged -27.3 mV, transcellular fractional resistance (fRa) was 86.8%, and Ra and Rb were decreased to 36.1 and 13.0%, of their control values, respectively. Amiloride-induced noise analysis of the apical membrane Na+ channels revealed that Na+ channel density was increased approximately 72% while single-channel Na+ current was decreased to 39.9% of control, roughly proportional to the decrease of apical membrane voltage (34.0% of control). In control and Kb+-depolarized epithelia, the Na+ channel density exhibited a phenomenon of autoregulation. Inhibition of Na+ entry (by amiloride) caused large increases of Na+ channel density toward saturating values of approximately 520 X 10(6) channels/cm2 in Kb+-depolarized tissues.

Interactions of amiloride and other blocking cations with the apical Na channel in the toad urinary bladder

A simple model of the action of amiloride to block apical Na channels in the toad urinary bladder was tested. According to the model, the positively charged form of the drug binds to a site in the lumen of the channel within the electric field of the membrane. In agreement with the predictions of the model: (1) The voltage dependence of amiloride block was consistent with the assumption of a single amiloride binding site, at which about 15% of the transmembrane voltage is sensed, over a voltage range of +/- 160 mV. (2) The time course of the development of voltage dependence was consistent with that predicted from the rate constants for amiloride binding previously determined. (3) The ability of organic cations to mimic the action of amiloride showed a size dependence implying a restriction of access to the binding site, with an effective diameter of about 5 angstroms. In a fourth test, divalent cations (Ca, Mg, Ba and Sr) were found to block Na channels with a complex voltage dependence, suggesting that these ions interact with two or more sites, at least one of which may be within the lumen of the pore.

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.

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 amiloride-sensitive epithelial sodium channel

We report here the first evidence in intact epithelial cells of unit conductance events from an amiloride-sensitive Na+ channel. The events were observed when patch-clamp recordings were made from the apical surface of cultured epithelial kidney cells (A6). The channel characteristics are as follows. Single-channel conductance ranged between 7 and 10 pS (mean = 8.4 +/- 1.3), the current-voltage (I-V) relationship displayed little if any nonlinearity over a range of +/- 80 mV (with respect to the patch pipette), and the channel Na+/K+ selectivity was approximately 3-4:1. Amiloride, a cationic blocker of the channel, reduced channel mean open time and increased channel mean closed time as the voltage of the cell interior was made more negative. Amiloride induced channel flickering at increased negative potentials (intracellular potential with respect to the patch) but did not alter the single-channel conductance or the I-V relationship from that observed in control patches.

Effects of anions on cellular volume and transepithelial Na+ transport across toad urinary bladder

The effects of complete substitution of gluconate for mucosal and/or serosal medium Cl- on transepithelial Na+ transport have been studied using toad urinary bladder. With mucosal gluconate, transepithelial potential difference (VT) decreased rapidly, transepithelial resistance (RT) increased, and calculated short-circuit current (Isc) decreased. Calculated ENa was unaffected, indicating that the inhibition of Na+ transport was a consequence of a decreased apical membrane Na+ conductance. This conclusion was supported by the finding that a higher amiloride concentration was required to inhibit the residual transport. With serosal gluconate VT decreased, RT increased and Isc fell to a new steady-state value following an initial and variable transient increase in transport. Epithelial cells were shrunken markedly as judged histologically. Calculated ENa fell substantially (from 130 to 68 mV on average). Ba2+ (3 mM) reduced calculated ENa in Cl- Ringer's but not in gluconate Ringer's. With replacement of serosal Cl- by acetate, transepithelial transport was stimulated, the decrease in cellular volume was prevented and ENa did not fall. Replacement of serosal isosmotic Cl- medium by a hypo-osmotic gluconate medium (one-half normal) also prevented cell shrinkage and did not result in inhibition of Na+ transport. Thus the inhibition of Na+ transport can be correlated with changes in cell volume rather than with the change in Cl-per se. Nystatin virtually abolished the resistance of the apical plasma membrane as judged by measurement of tissue capacitance. With K+ gluconate mucosa, Na+ gluconate serosa, calculated basolateral membrane resistance was much greater, estimated basolateral emf was much lower, and the Na+/K+ basolateral permeability ratio was much higher than with acetate media. It is concluded the decrease in cellular volume associated with substitution of serosal gluconate for Cl results in a loss of highly specific Ba2+-sensitive K+ conductance channels from the basolateral plasma membrane. It is possible that the number of Na+ pump sites in this membrane is also decreased.

Effects of aldosterone and dexamethasone on apical membrane properties and Na-transport of rabbit distal colon in vitro

The effects of pre-treatment in vivo with aldosterone and dexamethasone were investigated on rabbit distal colon. Apical Na-permeability and net sodium transport were measured in vitro. In this epithelium, Na-transport is entirely electrogenic. It can therefore be measured electrically as the fraction of short circuit current which is blockable by amiloride. The epithelia were studied in an Ussing chamber and the electrical values recorded by a computerized digital voltage clamp. Transepithelial parameters, and the transapical membrane parameters (in preparations depolarized from the serosal side) were investigated after treatment with the two hormones. Under transepithelial conditions, aldosterone and dexamethasone stimulated the short circuit current (Isc) from control (17.4 microA/cm2) to a similar degree (86.6 and 93.8 microA/cm2). However, whereas aldosterone did not alter the transepithelial resistance (RT) significantly, dexamethasone reduced RT from 357 to 167 omega X cm2. The stimulation of the potential difference (VT) under control condition (6.6 mV) was therefore significantly different between aldosterone (28.7 mV) and dexamethasone (16 mV). Mucosal amiloride (0.1 mM) inhibited Isc and VT completely under all conditions. Steady state current-voltage relations were obtained by voltage clamping the tissues in "staircase" increments before and after mucosal treatment with amiloride. As measured by the difference between these two states, Na-currents were calculated both for the transepithelial and the transapical condition. Intracellular Na-activity and apical Na-permeability were then calculated by the Nernst and Goldman-Hodgkin-Katz equations. These values were found to be increased after treatment with both hormones. Dexamethasone was a more potent stimulator of both values.(ABSTRACT TRUNCATED AT 250 WORDS)

Modulation of apical Na permeability of the toad urinary bladder by intracellular Na, Ca, and H

The Na conductance of the apical membrane of the toad urinary bladder was measured at different concentrations of Na both in the external medium and in the cell. Bladders were bathed in high K-sucrose medium to reduce basal-lateral resistance and voltage, and the transepithelial currents measured under voltage-clamp conditions. Amiloride was used as a specific blocker of the apical Na channel. At constant external Na, the internal Na concentration was increased by blocking the basal-lateral Na pump with ouabain. With high Na activity in the mucosal medium (86 mM), increases in intracellular Na activity from 10 to over 40 mM increased the amiloride-sensitive slope conductance at zero voltage while apical Na permeability, estimated from current-voltage plots using the constant field equation, decreased by less than 20%. Lowering the serosal Ca concentration from 1 to 0.1 mM had no effect on the change in PNa with increasing Nac, but increasing serosal Ca to 5 mM enhanced the reduction in PNa with increasing Nac, presumably by increasing Ca influx into the cell. PNa was also reduced by serosal vanadate (0.5 mM), a putative blocker of ATP-dependent Ca extrusion from the cell, and by acute exposure to CO2, which presumably acidifies the cytoplasm. Current-voltage relationships of the amiloride-sensitive transport pathway were also measured in the absence of a Na gradient across the apical membrane. These plots show that outward current passes through the channels somewhat less easily than does inward current. The shape of the I-V relationships was not significantly altered by changes in cellular Na, Ca or H, indicating that the effects of these ions on PNa are voltage independent.

Comparison between bretylium and diphenylhydantoin interaction with mucosal sodium-channels

The antifibrillatory drug bretylium and the antiepileptic drug diphenylhydantoin cause an increase in the potential different and in the short-circuit current (SCC) across frog skin when added to the outer surface. The effect of both drugs depends upon the presence of sodium ions in the outer medium and is blocked by the specific sodium channel blocker, amiloride. Quantitative analysis shows that amiloride binds to open as well as closed mucosal sodium channel with the same affinity. The effects of diphenylhydantoin and bretylium differ with respect to their dependence on external pH. The diphenylhydantoin or the bretylium stimulatory effects are additive to the effects of oxytocin. In most cases the diphenylhydantoin and bretylium effects are also additive. It is concluded that the external side of the mucosal Na+ channels contains sites which interact specifically with either bretylium or diphenylhydantoin and thus remove the sodium induced closure of the channels.

Effects of vasopressin on electrolyte transport across isolated colon from normal and dexamethasone-treated rats

Vasopressin enhanced the absorption of Na+ and Cl- across the short-circuited colon descendens from normal rats. This effect of vasopressin results from an increase in the mucosal to serosal movement of Na+ and Cl- and a decrease in the serosal to mucosal movement of Cl- and was accompanied with a decrease in the short-circuit current (ISC). Neither the base-line absorption of Na+ and Cl-, the vasopressin-induced increase in Na+ and Cl- absorption nor the decrease in ISC were inhibited by amiloride in the colon from normal rats. Colon descendens from rats treated for 3 days with dexamethasone had remarkably higher transmural potential difference (p.d.), tissue conductance (Gt) and ISC. The absorption of Na+ across the short-circuited colon descendens from dexamethasone-treated rats was increased 3-fold when compared to colon from normal rats. The absorption of Cl- in normal rats was reversed to Cl- secretion in treated rats. Amiloride rapidly and reversibly decreased the p.d., Gt and ISC in colon from dexamethasone-treated rats. The transport of Na+ was nearly completely inhibited by amiloride in treated rats. In contrast to its enhancing effects on Na+ absorption in colon from normal rats vasopressin did not enhance Na+ absorption in colon from dexamethasone-treated rats. This enhancement of Cl- absorption by vasopressin was retained in colon from treated rats. This enhancement of Cl- transport was due solely to a decrease in the serosal to mucosal movement of Cl- and was accompanied with a decrease in ISC and Gt. The results support the hypothesis that vasopressin causes inhibition of the electrogenic secretion of Cl- in colon from dexamethasone-treated rats. Furthermore, the results suggest that the increase in the mucosal to serosal movement of Na+ and Cl- and the decrease in the serosal to mucosal movement of Cl- in colon from normal rats are caused by independent effects of vasopressin.

Effects of antidiuretic hormone on cellular conductive pathways in mouse medullary thick ascending limbs of Henle: II. determinants of the ADH-mediated increases in transepithelial voltage and in net Cl-absorption

Cellular impalements were used in combination with standard transepithelial electrical measurements to evaluate some of the determinants of the spontaneous lumen-positive voltage, Ve, which attends net Cl- absorption, JnetCl, and to assess how ADH might augment both JnetCl and Ve in the mouse medullary thick ascending limb of Henle microperfused in vitro. Substituting luminal 5 mM Ba++ for 5 mM K+ resulted in a tenfold increase in the apical-to-basal membrane resistance ratio, Ra/Rbl, and increasing luminal K+ from 5 to 50 mM in the presence of luminal 10(-4)M furosemide resulted in a 53-mV depolarization of apical membrane voltage, Va. Thus K+ accounted for at least 85% of apical membrane conductance. Either with or without ADH, 10(-4) M luminal furosemide reduced Ve and JnetCl to near zero values and hyperpolarized both Va and Vbl, the voltage across basolateral membranes; however, the depolarization of Vbl was greater in the presence than in the absence of hormone while the hormone had no significant effect on the depolarization of Va. Thus ADH-dependent increases in Ve were referable to greater depolarizations of Vbl in the presence of ADH than in the absence of ADH. 68% of the furosemide-induced hyperpolarization of Va was referable to a decrease in the K+ current across apical membranes, but, at a minimum, only 19% of the hyperpolarization of Vbl could be accounted for by a furosemide-induced reduction in basolateral membrane Cl- current. Thus an increase in intracellular Cl- activity may have contributed to the depolarization of Vbl during net Cl- absorption, and the intracellular Cl- activity was likely greater with ADH than without hormone. Since ADH increases apical K+ conductance and since the chemical driving force for electroneutral Na+, K+, 2Cl- cotransport from lumen to cell may have been less in the presence of ADH than in the absence of hormone, the cardinal effects of ADH may have been to increase the functional number of both Ba++-sensitive conductance K+ channels and electroneutral Na+, K+, 2Cl- cotransport units in apical plasma membranes.

Electrophysiology of Necturus urinary bladder: II. Time-dependent current-voltage relations of the basolateral membranes

As reported previously (S.R. Thomas et al., J. Membrane Biol. 73:157-175, 1983) the current-voltage (I-V) relations of the Na-entry step across the apical membrane of short-circuited Necturus urinary bladder in the presence of varying mucosal Na concentrations are (i) time-independent between 20-90 msec and (ii) conform to the Goldman-Hodgkin-Katz constant field flux equation for a single cation over a wide range of voltages. In contrast, the I-V relations of the basolateral membrane under these conditions are (i) essentially linear between the steady-state, short-circuited condition and the reversal potential (Es); and (ii) are decidedly time-dependent with Es increasing and the slope conductance, gs, decreasing between 20 and 90 msec after displacing the transepithelial electrical potential difference. Evidence is presented that this time-dependence cannot be attributed entirely to the electrical capacitance of the tissue. The values of gs determined at 20 msec are linear functions of the short-circuit current, Isc, confirming the relations reported previously, which were obtained using a more indirect approach. The values of Es determined at 20 msec are significantly lower than any reasonable estimate of the electromotive force for K across the basolateral membrane, indicating that this barrier possesses a significant conductance to other ions which may exceed that to K. In addition, these values increase linearly with decreasing Isc and approach the value of the electrical potential difference across the basolateral membrane observed when Na entry across the apical membrane is blocked with amiloride or when Na is removed from the mucosal solution. A possible explanation for the time-dependence of Es and gs is offered and the implications of these findings regarding the interpretation of previous microelectrophysiologic studies of epithelia are discussed.

Studies of sodium channels in rabbit urinary bladder by noise analysis

Sodium channels in rabbit urinary bladder were studied by noise analysis. There are two components of short-circuit current (Isc) and correspondingly two components of apical Na+ entry, one amiloride-sensitive (termed IA and the A channel, respectively) and one amiloride-insensitive (IL and the leak pathway, respectively). The leak pathway gives rise to l/f noise, while the A channel in the presence of amiloride gives rise to Lorentzian noise. A two-state model of the A channel accounts well for how the corner frequency and plateau value of Lorentzian noise vary with amiloride concentration. The single-channel current is 0.64 pA, and the conducting channel density is on the order of 40 copies per cell. Triamterene blocks the A channel alone, and increasing external Na+ decreases the number but not the single-channel permeability of the A channel. Hydrostatic pressure pulses ("punching") increase the number of both pathways. Repeated washing of the mucosal surface removes most of the leak pathway without affecting the A channel. Properties of the A channel revealed by noise analysis of various tight epithelia are compared, and the mechanism of l/f noise is discussed. It is suggested that the A channel is synthesized intracellularly, stored in intracellular vesicles, transferred with or from vesicular membrane into apical membrane under the action of microfilaments, and degraded into the leak pathway, which is washed out into urine or destroyed. The A channel starts with PNa/PK approximately 30 and loses selectivity in stages until PNa/PK reaches the free-solution mobility ratio (approximately 0.7) for the leak pathway. This turnover cycle functions as a mechanism of repair and regulation for Na+ channels, analogous to the repair and regulation of most intracellular proteins by turnover. Vesicular delivery of membrane channels may be operating in several other epithelia.

Intracellular electrolyte concentrations in the frog skin epithelium: effect of vasopressin and dependence on the Na concentration in the bathing media

The intracellular electrolyte concentrations of the frog skin epithelium have been determined in thin freeze-dried cryosections using the technique of electron microprobe analysis. Stimulation of the transepithelial Na transport by arginine vasopressin (AVP) resulted in a marked increase in the Na concentration and a reciprocal drop in the K concentration in all epithelial cell layers. The effects of AVP were cancelled by addition of amiloride. It is concluded from these results that the primary mechanism by which AVP stimulates transepithelial Na transport is an increase in the Na permeability of the apical membrane. However, also some evidence has been obtained for an additional stimulatory effect of AVP on the Na pump. In mitochondria-rich cells and in gland cells no significant concentration changes were detected, supporting the view that these cells do not share in transepithelial Na transport. Furthermore, the dependence of the intracellular electrolyte concentrations upon the Na concentration in the outer and inner bathing solution was evaluated. Both in control and AVP-stimulated skins the intracellular Na concentration showed saturation already at low external Na concentrations, indicating that the self-inhibition of transepithelial Na transport is due to a reduction of the permeability of the apical membrane. After lowering the Na concentration in the internal bath frequently a Na increase in the outermost and a drop in the deeper epithelial layers was observed. It is concluded that partial uncoupling of the transport syncytium occurs, which may explain the inhibition of the transepithelial Na transport and blunting of the AVP response under this condition.

Voltage-dependent block by amiloride and other monovalent cations of apical Na channels in the toad urinary bladder

Inhibition of the Na conductance of the apical membrane of the toad urinary bladder by amiloride, alkali cations and protons was voltage dependent. Bladders were bathed with a high K-sucrose serosal medium to reduce series basal-lateral resistance and potential difference. Transepithelial current-voltage relationships were measured over a voltage range of +/- 200 mV with a voltage ramp of frequency 0.5 to 1 Hz. Na channel I-V relationships were obtained by subtraction of currents measured in the presence of maximal doses of amiloride (10 to 20 microM). With submaximal doses of amiloride (0.05 to 0.5 microM), the degree of inhibition of the Na channel current (INa) increased as the mucosal potential was made more positive. The data can be reasonably well explained by assuming that amiloride blocks Na transport by binding to a site which senses approximately 12% of the transmembrane voltage difference. INa was reduced in a qualitatively similar voltage-dependent manner by mucosal K, Rb, Cs and Tl (approximately 100 mM) and by mucosal H (approximately 1 mM). Block by these cations cannot be explained in terms of interactions with a single membrane-voltage-sensing site; a model in which there are two or more blocking sites in series provides a better description of the data. On the other hand, amiloride block was reduced competitively by mucosal Na and K, suggesting that occupation of the channel by one cation excludes occupancy by the others. ADH and ouabain also reduce the apparent affinity of amiloride for its blocking site. Thus, intracellular Na may also compete with amiloride for occupancy of the channel.

Feedback inhibition of sodium uptake in K+-depolarized toad urinary bladders

Ouabain-blocked toad urinary bladders were maintained in Na+-free mucosal solutions, and a depolarizing solution of high K+ activity containing only 5 mM Na+ on the serosal side. Exposure to mucosal sodium (20 mM activity) evoked a transient amiloride-blockable inward current, which decayed to near zero within one hour. The apical sodium conductance increased in the initial phase of the current decay and decreased in the second phase. The conductance decrease required Ca2+ to be present on the serosal side and was more rapid when the mucosal Na+ activity was higher. At 20 mM mucosal Na+ and 3 mM serosal Ca2+ the initial (maximal) rate of inhibition amounted to 20% in 10 min. The conductance decrease could be accelerated by raising the serosal Ca2+ activity to 10 mM. The inhibition reversed on lowering the serosal Ca2+ to 3 microM and, in addition, the mucosal Na+ to zero. Exposure of the mucosal surface to the ionophore nystatin abolished the Ca2+ sensitivity of the transcellular conductance, showing that the Ca2+-sensitive conductance resides in the apical membrane. The data imply that in the K+-depolarized epithelia, cellular Ca2+, taken up from the serosal medium by means of a Na+-Ca2+ antiport, cause feedback inhibition by blockage of apical Na+ channels. However, the rate of inhibition is small, such that this regulatory mechanism will have little effect at 1 mM serosal Ca2+ and less than 20 mM cellular Na+.

Calcium-activated potassium channels and their role in secretion

Single-channel recording by the patch-clamp technique has now characterized three kinds of membrane potassium channels activated by intracellular calcium ions in animal cells. These play a crucial part in the regulation of membrane potential and of secretion.

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)

Apical sodium entry in split frog skin: current-voltage relationship

Apical Na+ entry into frog skin epithelium is widely presumed to be electrodiffusive in nature, as for other tight epithelia. However, in contrast to rabbit descending colon and Necturus urinary bladder, the constant field equation has been reported to fit the apical sodium current (INa)-membrane potential (psi mc) relationship over only a narrow range of apical membrane potentials or to be inapplicable altogether. We have re-examined this issue by impaling split frog skins across the basolateral membrane and examining the current-voltage relationships at extremely early endpoints in time after initiating pulses of constant transepithelial voltage. In this study, the rapid transient responses in psi mc were completed within 0.5 to 3.5 msec. Using endpoints to 1 to 25 msec, the Goldman equation provided excellent fits of the data over large ranges in apical potential of 300 to 420 mV, from approximately -200 to about +145 mV (cell relative to mucosa). Split skins were also studied when superfused with high serosal K+ in order to determine whether the INapsi mc relationship could be generated purely by transepithelial measurements. Under these conditions, the basolateral membrane potential was found to be -10 +/- 3 mV (cell relative to serosa, mean +/- SE), the basolateral fractional resistance was greater than zero, and the transepithelial current was markedly and reversibly reduced. For these reasons, use of high serosal K+ is considered inadvisable for determining the INa-psi mc relationship, at least in those tissues (such as frog skin) where more direct measurements are technically feasible. Analysis of the INa-psi mc relationships under baseline conditions provided estimates of intracellular Na+ concentration and of apical Na+ permeability of 9 to 14 mM and of approximately 3 X 10(-7) cm X sec-1, respectively, in reasonable agreement with estimates obtained by different techniques.

2-Mercapto-1-(beta-4-pyridethyl) benzimidazole inhibition of basal and aldosterone-stimulated sodium transport but prolongation of the transient theophylline-induced stimulation in the toad bladder

2-Mercapto-1-(beta-4-pyridethyl) benzimidazole (MPB) was originally introduced as a reversible inhibitor of RNA synthesis, but subsequent findings made this suggestion doubtful. We examined the effect of MPB on active sodium transport, measured as short-circuit current (scc), across the isolated urinary bladder of the toad (Bufo marinus). The drug caused a rapid, dose-dependent inhibition of baseline scc; 25 micrograms/ml MPB reduced it by 70%. Sensitivity to MPB was the same in the presence and absence of metabolizable substrate. The transport stimulation by aldosterone (7 X 10(-8)M) was abolished entirely when MPB was introduced 30 min before the hormone. In bladders incubated with MPB with or without aldosterone, removal of both agents resulted in a rise in scc, which was more rapid in the aldosterone-pretreated hemibladders; a significant difference was observed after 30 min. This suggests that MPB inhibited transport at a site distal to messenger RNA accumulation. The effect of 3 hr of pretreatment with MPB on the response of the bladders to antidiuretic hormone (ADH, 20 mU) and cyclic AMP (cAMP, 10 mM) was then examined. The absolute increment in scc due to these agents was the same as in the absence of MPB, though the baseline was much reduced by the drug. After challenging MPB-pretreated bladders with theophylline (22.5 mM), sodium transport rose continuously for 90 min, in contrast to the small, short-lived rise in the absence of MPB. It is proposed that, in the toad bladder, MPB may: (1) inhibit cAMP-dependent protein kinase, as found by us in other tissues; and (2) counteract the accumulation of a transport inhibitor, possibly calcium or cyclic GMP, in tissues treated with endogenous or exogenous cAMP.

Chemical stimulation of Na transport through amiloride-blockable channels of frog skin epithelium

The stimulation of apical Na permeability caused by a number of reagents effective from the outer side of the membrane was investigated by fluctuation analysis. In the epidermis of R. ridibunda, parachloromercuriphenyl sulfonate (PCMPS) and benzimidazolyl guanidine (BIG) increase the number (N0) of conducting Na channels by releasing channels from Na self-inhibition. As a consequence, the apparent macroscopic affinity for amiloride is increased. 5-dimethyl amiloride and trinitrobenzene sulfonate (TNBS) also cause reversible stimulation by increasing N0; here release from self-inhibition is less clear. With each of the four stimulators investigated, the Na channel current remained unaffected or was only marginally increased. In addition to its stimulatory effect, TNBS caused irreversible blockage of Na channels. Apart from their stimulatory effects, BIG and 5-dimethyl amiloride, both of which have a side-chain terminated with an amidino group, are high rate-blocking competitors of amiloride.

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.

Relation between intracellular sodium and active sodium transport in rabbit colon: current-voltage relations of the apical sodium entry mechanism in the presence of varying luminal sodium concentrations

The current-voltage relations of the amiloride-sensitive Na entry pathway across the apical membrane of rabbit descending colon, exposed to a high K serosal solution, were determined in the presence of varying mucosal Na activities, (Na)m, ranging from 6.2 to 99.4 mM. These relations could be closely fit to the "constant field" flux equation yielding estimates of the permeability of the apical membrane to Na, PmNa, and the intracellular Na activity, (Na)c. The following empirical relations emerged: (Na)c increased hyperbolically with increasing (Na)m; PmNa decreased hyperbolically with increasing (Na)m and linearly with increasing (Na)c; spontaneous variations in Na entry rate at constant (Na)m could be attributed entirely to parallel, spontaneous variations in PmNa; the rate of Na entry increased hyperbolically with increasing (Na)m obeying simple Michaelis-Menten kinetics; the relation between (Na)c and "pump rate," however, was sharply sigmoidal and could be fit by the Hill equation assuming strong cooperative interactions between Na and multiple sites on the pump; the Hill coefficient was 2-3 and the value of (Na)c at which the pump-rate is half-maximal was 24 mM. The results provide an internally consistent set of relations among Na entry across the apical membrane, the intracellular Na activity and basolateral pump rate that is also consistent with data previously reported for this and other Na-absorbing epithelia.

Turnover, membrane insertion, and degradation of sodium channels in rabbit urinary bladder

Noise analysis of rabbit bladder revealed two components: Lorentzian noise, arising from interaction of amiloride with the Na+ channel, and flicker noise (l/f, where f is frequency), as in other biological membranes. Hydrostatic pressure, which causes exchange between intracellular vesicular membrane and apical membrane, increases the number but not the single-channel current of the amiloride-sensitive channels. Flicker noise arises from degraded channels that have lost amiloride sensitivity and Na+ to K+ selectivity. The degraded channels were selectively removed by washing the mucosal surface. These results imply channel turnover by intracellular synthesis, transfer from vesicular to apical membrane, degradation, and elimination.

Competitive blocking of apical sodium channels in epithelia

Apical sodium-selective channels in frog skin, when blocked by amiloride or triamterene, exhibit fluctuations in current, the spectra of which are Lorentzian. These effects have been modeled previously with two-state and three-state models by Lindemann and Van Driessche. A recent observation by Hoshiko and Van Driessche that corner frequencies are lowered by increasing the apical sodium concentration cannot be accounted for by these models. We explore the possibility that sodium (S) and amiloride (A) compete for a site at the mouth of the channel. A new three-state channel model (sodium-occupied, open/unoccupied, open/amiloride-blocked) is analyzed. Its corner frequency is of the form fc = fco [1 + (A/KA)/(1 + S/KS)], consistent with the observed sodium dependence of the corner frequency. The minimum frequency, fco, and the inhibition constants, KA and KS, are expressed in terms of the rate constants of the model. To account for sodium self-inhibition, we postulate that two sodium ions in the channel may result in clogging--a fourth state. The two corner frequencies are calculated; so are the plateau values of the noise power. The noise power shows a maximum as a function of blocker concentration, as observed previously using triamterene. The four-state model predicts the observed suppression by small amounts of blocker of the low-frequency sodium (clogging) noise.

Conversion of sodium channels to a form sensitive to cyclic AMP by component(s) from red cells

Sodium transport has been measured in the isolated epithelium from colons of male Sprague-Dawley rats. Sodium transport in colons was induced by pretreating the animals with dexamethasone (6 mg kg-1) which caused the appearance of an amiloride-sensitive short circuit current within a few hours. Forskolin, a diterpene, which activates adenylate cyclase, was found to increase the cyclic adenosine monophosphate (cyclic AMP) content of rat colons and also to increase short circuit current at the same time. However, measurements of chloride and sodium fluxes across the epithelium indicated that forskolin activates chloride secretion but has no effect on sodium transport. In confirmation of (3) it was found that the amiloride-sensitive short circuit current was unchanged after the short circuit current had been increased by forskolin under a variety of conditions. The behaviour of the mammalian colon as indicated in (3) and (4) is unlike that of amphibian sodium transporting epithelia. It is shown that in toad urinary bladder forskolin increases amiloride-sensitive short circuit current. Procedures were investigated which might make sodium transport in the mammalian colon sensitive to cyclic AMP. Exposing the apical surface to sonicated suspensions of nucleated red cells (frog, toad and duck), followed by washing, gave preparations with amiloride-sensitive short circuit currents which were increased by forskolin or dibutyryl cyclic AMP. It would appear that the sodium channel in the mammalian colon, unlike that of amphibian tissues, has lost the ability to have its properties modified by cyclic AMP. Incubation of colons with sonicated suspensions of nucleated red cells apparently modifies the tissues such that sodium transport across the tissue becomes sensitive to the nucleotide.

Amiloride-sensitive trypsinization of apical sodium channels. Analysis of hormonal regulation of sodium transport in toad bladder

Incubation of the mucosal surface of the toad urinary bladder with trypsin (1 mg/ml) irreversibly decreased the short-circuit current to 50% of the initial value. This decrease was accompanied by a proportionate decrease in apical Na permeability, estimated from the change in amiloride-sensitive resistance in depolarized preparations. In contrast, the paracellular resistance was unaffected by trypsinization. Amiloride, a specific blocker of the apical Na channels, prevented inactivation by trypsin. Inhibition of Na transport by substitution of mucosal Na, however, had no effect on the response to trypsin. Trypsinization of the apical membrane was also used to study regulation of Na transport by anti-diuretic hormone (ADH) and aldosterone. Prior exposure of the apical surface to trypsin did not reduce the response to ADH, which indicates that the ADH-induced Na channels were inaccessible to trypsin before addition of the hormone. On the other hand, stimulation of short-circuit current by aldosterone or pyruvate (added to substrate-depleted, aldosterone-repleted bladders) was substantially reduced by prior trypsinization of the apical surface. Thus, the increase in apical Na permeability elicited by aldosterone or substrate involves activation of Na channels that are continuously present in the apical membrane in nonconductive but trypsin-sensitive forms.

Membrane potential plays a dual role for chloride transport across toad skin

The Cl- -current through toad skin epithelium depends on the potential in a way consistent with a potential-controlled Cl- permeability. Computer analysis of the Koefoed-Johnsen Ussing two-membrane model provided with constant membrane permeabilities indicates that the voltage- and time-dependent currents are not caused by a trivial Goldmand-type rectification and ion redistributions following transepithelial potential pertubations. Extended with a dynamic Cl- permeability in the apical membrane according to a Hodgkin-Huxley kinetic scheme, the model predicts voltage clamp data which closely resemble experimental observations. This extension of the classic frog skin model implies that the Cl- permeability is activated by a voltage change caused by the inward Na+ current through the apical membrane.