Na+ transport and flux ratio through apical Na+ channels in toad bladder

Aldosterone: Renal Action and Physiological Effects

Aldosterone exerts profound effects on renal and cardiovascular physiology. In the kidney, aldosterone acts to preserve electrolyte and acid-base balance in response to changes in dietary sodium (Na+ ) or potassium (K+ ) intake. These physiological actions, principally through activation of mineralocorticoid receptors (MRs), have important effects particularly in patients with renal and cardiovascular disease as demonstrated by multiple clinical trials. Multiple factors, be they genetic, humoral, dietary, or otherwise, can play a role in influencing the rate of aldosterone synthesis and secretion from the adrenal cortex. Normally, aldosterone secretion and action respond to dietary Na+ intake. In the kidney, the distal nephron and collecting duct are the main targets of aldosterone and MR action, which stimulates Na+ absorption in part via the epithelial Na+ channel (ENaC), the principal channel responsible for the fine-tuning of Na+ balance. Our understanding of the regulatory factors that allow aldosterone, via multiple signaling pathways, to function properly clearly implicates this hormone as central to many pathophysiological effects that become dysfunctional in disease states. Numerous pathologies that affect blood pressure (BP), electrolyte balance, and overall cardiovascular health are due to abnormal secretion of aldosterone, mutations in MR, ENaC, or effectors and modulators of their action. Study of the mechanisms of these pathologies has allowed researchers and clinicians to create novel dietary and pharmacological targets to improve human health. This article covers the regulation of aldosterone synthesis and secretion, receptors, effector molecules, and signaling pathways that modulate its action in the kidney. We also consider the role of aldosterone in disease and the benefit of mineralocorticoid antagonists. © 2023 American Physiological Society. Compr Physiol 13:4409-4491, 2023.

Ion conduction and selectivity in acid-sensing ion channel 1

The selectivity of acid-sensing ion channels to cations depends on interactions with binding sites both within the pore and in the outer vestibule.

The ability of acid-sensing ion channels (ASICs) to discriminate among cations was assessed based on changes in conductance and reversal potential with ion substitution. Human ASIC1a was expressed in Xenopus laevis oocytes, and acid-induced currents were measured using two-electrode voltage clamp. Replacement of extracellular Na⁺ with Li⁺, K⁺, Rb⁺, or Cs⁺ altered inward conductance and shifted the reversal potentials consistent with a selectivity sequence of Li ∼ Na > K > Rb > Cs. Permeability decreased more rapidly than conductance as a function of atomic size, with P_K/P_Na = 0.1 and G_K/G_Na = 0.7 and P_Rb/P_Na = 0.03 and G_Rb/G_Na = 0.3. Stimulation of Cl⁻ currents when Na⁺ was replaced with Ca²⁺, Sr²⁺, or Ba²⁺ indicated a finite permeability to divalent cations. Inward conductance increased with extracellular Na⁺ in a hyperbolic manner, consistent with an apparent affinity (K_m) for Na⁺ conduction of 25 mM. Nitrogen-containing cations, including NH₄⁺, NH₃OH⁺, and guanidinium, were also permeant. In addition to passing through the channels, guanidinium blocked Na⁺ currents, implying competition for a site within the pore. The role of negative charges in an external vestibule of the pore was evaluated using the point mutation D434N. The mutant channel had a decreased single-channel conductance, measured in excised outside-out patches, and a macroscopic slope conductance that increased with hyperpolarization. It had a weakened interaction with Na⁺ (K_m = 72 mM) and a selectivity that was shifted toward larger atomic sizes. We conclude that the selectivity of ASIC1 is based at least in part on interactions with binding sites both within and internal to the outer vestibule.

Sodium Gill Potential as a Tool to Monitor Valve Closure Behavior in Freshwater Clam Corbicula fluminea in Response to Copper

Valve closure behavior in freshwater clam Corbicula fluminea is a biologically sensitive endpoint. The purpose of this paper was to derive an electrophysiological response model of C. fluminea to assess copper (Cu)–sodium (Na) interactions in gill membrane, whereby valve closure behavior and Cu toxicity could be monitored. The proposed model was based on the integration of Cu bioavailability, Na and Cu internalizations, and electrochemically-based gill potentials. Based on Na active transport under non-equilibrium conditions, predicted gill potential of −8.2 mV agreed reasonably well with published the measured transepithelial potential of −7 mV in C. fluminea. Our proposed framework captured the general features observed in model applications including: (i) 50% inhibitory Cu²⁺ activities for Na membrane potential (E_Na) and uptake rate (J_Na) were estimated to be 0.072 and 0.043 μM, respectively, with a stoichiometry of 3Cu²⁺: 1E_Na and 1J_Na; (ii) the external Cu²⁺–dependent internal Na concentration could be parsimoniously estimated, and (iii) the site-specific clam gill potentials could be monitored. Here we provided a new approach to monitor waterborne metal toxicity to reduce the nationwide economic losses due to bans on harvesting of contaminated clam and the potential risks to the health of clams.

On the molecular basis of ion permeation in the epithelial Na+ channel

The epithelial Na+ channel (ENaC) is highly selective for Na+ and Li+ over K+ and is blocked by the diuretic amiloride. ENaC is a heterotetramer made of two alpha, one beta, and one gamma homologous subunits, each subunit comprising two transmembrane segments. Amino acid residues involved in binding of the pore blocker amiloride are located in the pre-M2 segment of beta and gamma subunits, which precedes the second putative transmembrane alpha helix (M2). A residue in the alpha subunit (alphaS589) at the NH2 terminus of M2 is critical for the molecular sieving properties of ENaC. ENaC is more permeable to Li+ than Na+ ions. The concentration of half-maximal unitary conductance is 38 mM for Na+ and 118 mM for Li+, a kinetic property that can account for the differences in Li+ and Na+ permeability. We show here that mutation of amino acid residues at homologous positions in the pre-M2 segment of alpha, beta, and gamma subunits (alphaG587, betaG529, gammaS541) decreases the Li+/Na+ selectivity by changing the apparent channel affinity for Li+ and Na+. Fitting single-channel data of the Li+ permeation to a discrete-state model including three barriers and two binding sites revealed that these mutations increased the energy needed for the translocation of Li+ from an outer ion binding site through the selectivity filter. Mutation of betaG529 to Ser, Cys, or Asp made ENaC partially permeable to K+ and larger ions, similar to the previously reported alphaS589 mutations. We conclude that the residues alphaG587 to alphaS589 and homologous residues in the beta and gamma subunits form the selectivity filter, which tightly accommodates Na+ and Li+ ions and excludes larger ions like K+.

Cation permeability of a cloned rat epithelial amiloride-sensitive Na+ channel

1. Conductance of heterotrimeric rat epithelial Na+ channels (alpha, beta, gamma-rENaCs) for Li+ and Na+ in planar lipid bilayers was a non-linear function of ion concentration, with a maximum of 30.4 +/- 2.9 pS and 18.5 +/- 1.9 pS at 1 M Li+ and Na+, respectively. 2. The alpha, beta, gamma-rENaC conductance measured in symmetrical mixtures of Na(+)-Li+ (1 M) exhibited an anomalous mole fraction dependence, with a minimum at 4:1 Li+ to Na+ molar ratio. 3. Permeability ratios PK/PNa and PLi/PNa of the channel calculated from the bionic reversal potentials were dependent on ion concentration: PK/PNa was 0.11 +/- 0.01, and PLi/PNa was 1.6 +/- 0.3 at 50 mM; PK/PNa was 0.04 +/- 0.01 and PLi/PNa was 2.5 +/- 0.4 at 3 M, but differed from the ratios of single-channel conductances in symmetrical Li+, Na+ or K+ solutions. The permeability sequence determined by either method was Li+ > Na+ > K+ >> Rb+ Cs+. 4. Predictions of a model featuring two binding sites and three energy barriers (2S3B), and allowing double occupancy, developed on the basis of single ion current-voltage relationships, are in agreement with the observed conductance maximum in single ion experiments, conductance minimum in the mole fraction experiments, non-linearity of the current-voltage curves in bionic experiments, and the concentration dependence of permeability ratios. 5. Computer simulations using the 2S3B model recreate the ion concentration dependencies of single-channel conductance observed for the immunopurified bovine renal amiloride-sensitive Na+ channel, and short-circuit current in frog skin, thus supporting the hypothesis that ENaCs form a core conduction unit of epithelial Na+ channels.

Interactions of amiloride and small monovalent cations with the epithelial sodium channel. Inferences about the nature of the channel pore

The voltage dependence of amiloride-induced inhibition of current flow through apical membrane sodium channels in toad urinary bladder was studied at different ionic conditions. The "inert" salt N-methyl-D-glucamine HCl (NMDG HCl) affected neither the apparent inhibition constant (Kl) for the amiloride-induced current inhibition nor the apparent fraction of the transmembrane voltage that falls between the mucosal solution and the amiloride-binding site (delta). When NMDG+ was replaced with Na+, Kl increased, reflecting amiloride-Na+ competition, whereas delta was unchanged. Similar results were obtained with another permeant cation, Li+. When NMDG+ was replaced by K+, an impermeant but channel-blocking cation, Kl increased whereas delta decreased. Similar results were obtained using another impermeant, channel-blocking cation guanidinium. The results are interpreted on the premise that Na+ and K+ compete with amiloride by binding to cation binding sites within the channel lumen such that ion occupancy of these sites vary with voltage. Occupancy by K+, which cannot traverse the channel, will increase as the mucosal solution becomes positive, relative to the serosal solution. Occupancy by Na+, which can traverse the channel, is comparatively voltage independent. Ion movement through the channels was simulated using discrete-state kinetic models. Two types of models could describe the shape of the current-voltage relationship and the voltage dependence of the amiloride-induced channel block. One model had a single ion-binding site with a broad energy barrier at the inner (cytoplasmic) side of the site. The other model had two binding sites separated from each other and from the aqueous solutions by sharp energy barriers.

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.

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)

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.

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.

Structure-activity relationship of amiloride analogs as blockers of epithelial Na channels: I. Pyrazine-ring modifications

The overall on- and off-rate constants for blocking epithelial Na channels by amiloride analogs were estimated by noise analysis of frog skin epithelium. The substituents at position-5 and -6 of the pyrazine ring of amiloride were varied in order to obtain the structure/rate constant relationship. (1) The off-rate constant increases with halo-substitutions at position-6 in the order Cl less than Br less than l less than F less than H. Substitution of Cl by H lowers the standard free energy of activation of the off-step by 2.3 kcal mol-1. The on-rate constant is not affected. Apparently the substituent at ring position-6 controls the duration of attachment in the blocking position. pKa considerations show that the duration is longer when the 6-substituent is more negatively polarized. We suggest that this substituent binds to the receptor by virtue of its electronegativity. (2) In contrast, replacement of the adjacent 5-amino group (electron donor) by H or Cl affects both the on-rate and the off-rate. The dual effect may be explained by a decrease of the electronic charge at more remote parts of the molecule (on-rate decrease), as well as at the 6-position (off-rate increase). Apparently the 5-amino group stabilizes the blocking position by increasing the electron density on the 6-ligand.

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.

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.

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.

Apical sodium uptake in toad kidney epithelial cell line A6

The characteristics of the apical entry pathway for sodium into the cultured toad kidney epithelial cell line A6 are studied. Unidirectional apical sodium fluxes were determined by measuring the uptake of 22Na into confluent A6 epithelia growing in filter-bottomed cups. Apical sodium uptake was found to be a saturable function of sodium concentration with a Michaelis constant of 18 mM and a maximum velocity of 2.5 nmol X min-1 X cm-2. Amiloride competitively inhibits this sodium entry pathway with an inhibitor dissociation constant of 5 X 10(-8) M. Incubation of the epithelium with 10(-7) M aldosterone leads to a threefold increase in apical sodium uptake after 4 h. Both the aldosterone-stimulated and base-line sodium fluxes are completely inhibited by 10(-4) M amiloride. The similarity of these results to those from other tissues such as toad bladder and frog skin indicate that the A6 cells provide a useful model system for studying the apical entry pathway for sodium in tight epithelia.

Sodium flux ratio through the amiloride-sensitive entry pathway in frog skin

The sodium flux ratio of the amiloride-sensitive Na+ channel in the apical membrane of in vitro Rana catesbeiana skin has been evaluated at different sodium concentrations and membrane potentials in sulfate Ringer solution. Amiloride-sensitive unidirectional influxes and effluxes were determined as the difference between bidirectional 22Na and 24Na fluxes simultaneously measured in the absence and presence of 10(-4) M amiloride in the external bathing solution. Amiloride-sensitive Na+ effluxes were induced by incorporation of cation-selective ionophores (amphotericin B or nystatin) into the normally Na+-impermeable basolateral membrane. Apical membrane potentials (Va) were measured with intracellular microelectrodes. We conclude that since the flux ratio exponent, n', is very close to 1, sodium movement through this channel can be explained by a free-diffusion model in which ions move independently. This result, however, does not necessarily preclude the possibility that this transport channel may contain one or more ion binding sites.

Modelling the control mechanism of the sodium channel in the apical membrane of tight epithelia

The unidirectional inward Na + flux, J Na IN , passing through the sodium channel in the apical membrane of some tight epithelia depends approximately linearly on [Na] OUT as expected, but is also known to depend sensitively and inversely upon [Na] IN while the ratio of the fluxes obeys the relation J Na IN / J Na OUT = [Na] OUT / [Na] IN In this paper it is shown that such behaviour may be simply modelled based on the assumption of only single occupancy of the channel. With use of linear flux equations the calculated flux ratio is as measured. The additional assumption of single occupancy of the channel results in the calculated values of J Na IN and J Na OUT both being multiplied by the same factor, 1— P , where P is the probability that the channel is occupied by an ion. The flux ratio thus remains unchanged but J Na IN and J Na OUT may now depend inversely upon [Na + ] IN or [Na + ] OUT or both, owing to the dependence of P upon these parameters. To illustrate these general conclusions, detailed calculations are made on a particular assymetrical voltage-controlled model ion channel, based upon an ordered water structure. The results obtained are shown to simulate those found experimentally for tight epithelia.