Sodium-independent effect of aldosterone on initial rate of ouabain binding in A6 cells

Renin-aldosterone response, urinary Na/K ratio and growth in pseudohypoaldosteronism patients with mutations in epithelial sodium channel (ENaC) subunit genes

Multi-system pseudohypoaldosteronism (PHA) is a rare syndrome of aldosterone unresponsiveness characterized by symptoms of severe salt-losing caused by mutations in one of the genes that encode alpha, beta or gamma subunit of epithelial sodium channels (ENaC). We examined long-term changes in the renin-aldosterone response in patients with different mutations. Four PHA patients were followed-up for 7-22 years. Patient A with a heterozygous Gly327Cys missense mutation in alphaENaC is a mild case and patients B, C and D are severe cases. Two additional patients with renal PHA served as controls. In patient A, serum aldosterone and plasma renin activity (PRA) decreased with age, PRA reaching near normal values at age 11. In contrast, patients B-D showed a positive correlation between age and aldosterone (r>0.86 for all). In patient B with Arg508 stop mutation, aldosterone reached 166 nmol/L at age 19 (>300-fold higher than normal). Urinary Na/K ratios normalized gradually with age in all patients. Growth curves of the patients were reflective of the severity of PHA and compliance with salt therapy. Functional expression studies in oocytes showed that ENaC with alphaGly327Cys mutation, as observed in patient A, showed nearly 40% activity of the wild type ENaC. In contrast, stop mutation as in patient B reduces ENaC activity to less than 5% of the normal. Our results demonstrate distinct genotype-phenotype relationships in multi-system PHA patients. The degree of ENaC function impairment affects differently the renin-aldosterone system and urinary Na/K ratios. The differences observed are age-dependent and PHA form specific.

Regulation of Na,K-ATPase expression by endothelin-1 in transformed human ciliary non-pigmented epithelial (HNPE) cells

Endothelin-1 (ET-1) (1-100 nM) decreases the activity of Na,K-ATPase, a key enzyme responsible for aqueous humor formation, in transformed human non-pigmented ciliary epithelial (HNPE) cells. The present study sought to determine if ET-1 alters the expression of the catalytically active alpha subunit of Na,K-ATPase in HNPE cells and identify mechanisms underlying these effects. We report that acute (15 and 30 min) treatment with ET-1 results in an increase in mRNA expression of the alpha 1 subunit of Na,K-ATPase. Similar to ET-1's effects, ouabain (100 microM), a selective inhibitor of Na,K-ATPase, and monensin (10 microM), a sodium ionophore, also increased Na,K-ATPase expression in HNPE cells. The increase in Na,K-ATPase expression by short-term treatment with ouabain and monensin was dependent on their ability to elevate intracellular sodium concentrations. However, acute ET-1 treatment mediated increase in Na,K-ATPase expression was independent of changes in intracellular sodium. A prolonged (24 hr) ET-1 treatment results in an increase in both mRNA and protein levels of the alpha 1 subunit of Na,K-ATPase. These observations suggest that ET-1 could play a homeostatic role in modulating aqueous humor formation by having differential effects on the activity and expression of Na,K-ATPase by the ciliary epithelium in the eye.

Mechanisms of sodium pump regulation

The Na(+)-K(+)-ATPase, or sodium pump, is the membrane-bound enzyme that maintains the Na(+) and K(+) gradients across the plasma membrane of animal cells. Because of its importance in many basic and specialized cellular functions, this enzyme must be able to adapt to changing cellular and physiological stimuli. This review presents an overview of the many mechanisms in place to regulate sodium pump activity in a tissue-specific manner. These mechanisms include regulation by substrates, membrane-associated components such as cytoskeletal elements and the gamma-subunit, and circulating endogenous inhibitors as well as a variety of hormones, including corticosteroids, peptide hormones, and catecholamines. In addition, the review considers the effects of a range of specific intracellular signaling pathways involved in the regulation of pump activity and subcellular distribution, with particular consideration given to the effects of protein kinases and phosphatases.

Aldosterone regulates Na,K-ATPase and increases lung edema clearance in rats

Aldosterone increases the Na,K-ATPase function in renal cells involved in active Na(+) transport. Because the alveolar type 2 (AT2) cells participate in active Na(+) transport, we studied whether aldosterone regulates the Na,K-ATPase in rat AT2 cells and whether aldosterone delivered by aerosols to spontaneously breathing rats affects edema clearance in a model of isolated-perfused lungs. The AT2 cells treated with aldosterone had increased Na,K-ATPase beta1-subunit mRNA and protein, which was associated with a 4-fold increase in the Na,K-ATPase hydrolytic activity and the ouabain-sensitive (86)Rb(+) uptake. In physiologic experiments, 24 h after aldosterone was delivered by aerosols to the rat air spaces, the active Na(+) transport and lung edema clearance increased by approximately 53% as compared with control rats and rats in which saline aerosols were delivered. The data suggest that increased active Na(+) transport and lung edema clearance induced by aldosterone is probably due to Na,K-ATPase regulation in alveolar epithelial cells. Conceivably, aldosterone may be used as a strategy to increase lung edema clearance.

Effects of dexamethasone on (Na+ + K+)-ATPase and other parameters related to transepithelial Na+ transport by amphibian renal distal cells (A6) in culture

In the present report, the effects exerted by dexamethasone on transepithelial, electrogenic Na+ transport across A6 cell monolayers grown on permeable support were further characterized in terms of time course and relationship to the rate of Na+ transport; furthermore this agonist was compared to vasopressin and insulin. (Na+ + K+)-ATPase activity and density of ouabain binding sites were measured in cell homogenates and on dispersed cells, respectively, after documenting transepithelial electrical parameters of the preparations. Na+ transport, measured by short-circuit current (Isc), was increased almost five-fold (control: 6.7 +/- 0.1 microA/cm2) after incubation with 10(-7) M dexamethasone for 24 h. Stimulation of Na+ transport rate was associated with a 2.3-fold increase in (Na+ + K+)-ATPase activity (control: 5.5 +/- 0.3 micromol Pi/mg prot.h), and ouabain binding site density almost doubled (control: 236 +/- 10 fmol/10(6) cells). The steroid acted on the Na+ pump of A6 cells in the absence of transepithelial Na+ transport, with intracellular Na+ ion activity playing an additional role in terms of cell Na+ pump numbers. In the case of insulin and vasopressin, in contrast, there was no effect on Na+ pump activity in the absence of Na+ transport by A6 cell monolayers. The increase in (Na+ + K+)-ATPase activity observed in A6 cell monolayers treated with dexamethasone is therefore a result of the direct induction of Na+ pump biosynthesis, with an almost proportional insertion of operational Na+ pumps into the basolateral membrane. In contrast, increased Na+ entry at the apical cell pole appears to be essential for insulin and vasopressin action on A6 cell Na+ pump.

Ligand-receptor interaction rates in the presence of convective mass transport

The rate of binding of a ligand to receptors on the cell surface can be diffusion limited. We analyze the kinetics of binding, diffusion-limited in a stationary liquid, in the presence of convective mass transport. We derive a formula that expresses the reaction kinetics in terms of the mass transfer coefficient. A moderately transport-limited kinetics is not readily recognizable from the shape of the binding curve and may lead to erroneous estimates of the rate coefficients. We apply our results to practically important cases: a cell suspension in a stirred volume of liquid and a confluent cell colony under a laminar stream. Using typical numbers characterizing the ligand-receptor interactions, we show that stirring and perfusion can be important factors determining the reaction rates. With the confluent colony, the early reaction kinetics requires a different treatment, and we provide it for the case of low receptor occupancy. We show that, even with a fast perfusion, a cell monolayer can transiently generate a zone of depletion of the ligand, and that would affect the early stages of the reaction. Our results are expressed in a simple analytical form and can be used for the design and interpretation of experimental data.

Aldosterone modulates sodium kinetics of Na,K-ATPase containing an alpha 1 subunit in A6 kidney cell epithelia

Short-term aldosterone (10(-6) M, 2.5 h) induces in A6-C1 cell epithelia an increase in Na transport, which is due to the in situ activation of the apical Na channel and, presumably, the basolateral Na pump (Na,K-ATPase). We have now directly measured the effect of aldosterone on the transport activity of endogenous Na pumps and hybrid Na pumps containing an exogenous alpha 1 subunit by measuring the pump current (Ip) across epithelia apically permeabilized with amphotericin B. Aldosterone (2.5 h) had no significant early effect on the maximal Ip, nor on the Na concentration required for half-maximal activation. In contrast, it increased the Ip at physiological intracellular Na concentrations (1.7-fold at 5 mM Na). This effect was blocked by the protein synthesis inhibitor cycloheximide. Hybrid pumps containing the transfected cardiotonic steroid-resistant alpha 1 subunit of Bufo marinus were also stimulated by aldosterone (2.5 h). A long aldosterone treatment (4 days) increased the maximal Ip produced by the endogenous pumps 1.5 to 2.1-fold. In conclusion, aldosterone acts on Na pumps containing an alpha 1 subunit in two ways. During its early phase of action it stimulates their transport activity by increasing their apparent Na affinity at physiological intracellular Na concentrations. In the long term it produces an increase in the maximal transport capacity, which corresponds to the known increase in the number of Na pumps.

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

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