Na transport stimulation by novobiocin: transepithelial parameters and evaluation of ENa

Microtubule disruption inhibits AVT-stimulated Cl- secretion but not Na+ reabsorption in A6 cells

The effects of microtubule disruption on arginine vasotocin (AVT)-stimulated Na+ and Cl- transport were studied in A6 cells by measuring short-circuit currents (Isc) across cell layers grown in tissue culture on permeable supports. Microtubule disruption inhibited an AVT-stimulated secretory Cl- current but did not prevent activation of amiloride-sensitive Na+ transport. This AVT-stimulated secretory Cl- current was significantly inhibited by glibenclamide, an inhibitor of the cystic fibrosis transmembrane conductance regulator (CFTR). Reverse transcription of RNA isolated from A6 cells followed by polymerase chain reaction (PCR) using primers designed to amplify a portion of the R-domain of CFTR cloned from Xenopus laevis skin and immunocytochemistry demonstrated the presence of CFTR in A6 cells and an apparent recruitment of cytoplasmic CFTR to the apical cell surface after AVT stimulation. In contrast, indirect immunofluorescent labeling of Na+ channels using a polyclonal antibody raised against a biochemically isolated Na+ channel complex from bovine renal medulla labeled the apical plasma membrane but failed to demonstrate intracellular labeling of Na+ channels (except in subconfluent cells) or recruitment of Na+ channels to the apical membrane region after AVT stimulation.

Uncoupling histogenesis from morphogenesis in the vertebrate embryo by collapse of the transneural tube potential

We have shown that unidirectional pumping of Na+ out of the neural tube's luminal fluids in amphibian embryos produces a large potential difference (40-90 mV, lumen negative to the abluminal surface). This transneural tube potential (TNTP) is analogous to the Na+ dependent transepithelial potential (TEP) that exists across surface ectoderm. This TEP is retained in ectoderm after it is internalized when the neural folds fuse to form the neural tube. The TNTP can be markedly reduced for several hours by injection of the Na+ channel blockers amiloride or benzamil into the lumen by iontophoresis through microelectrodes. Here we describe the effect of TNTP modification on developmental anatomy. Axolotl embryos possessing a fused and closed neural tube (stage 21-23) were injected with either amiloride or benzamil and allowed to continue development for 36-52 hr. These were compared to control embryos injected with vehicle alone, or to embryos in which amiloride or benzamil was iontophoresed just beneath surface ectoderm. All embryos in which the TNTP was reduced were grossly defective. These were characterized by a disaggregation of the cells comprising the structures that had already begun to form (otic primordia, brain, spinal cord, notochord) as well as a failure in the development of new structures. Remarkably, some of these embryos displayed continuing development of external form in the complete absence of concomitant internal histogenesis. We discuss the ways in which a large endogenous voltage gradient associated with an epithelial potential difference (the TNTP) may be required both for the structural integrity of the early neuroepithelium, and a prerequisite for normal morphogenesis.

The properties of ion channels formed by the coumarin antibiotic, novobiocin, in lipid bilayers

The coumarin antibiotic novobiocin forms ion channels of varying conductances in lipid bilayers. The conductances (about 20, 22, 14, 7 and 2 pS for 100 mM NH4Cl, CsCl, KCl, NaCl and LiCl, respectively) and selectivities (cation transference numbers in the range of 0.97-0.98) of one type of novobiocin-induced channel are similar to those found for channels formed by gramicidin A, an antibiotic of very different structure. The conductance of novobiocin channels of this type was independent of the species of the membrane lipid. This observation suggests that novobiocin molecules directly form these channels, and that channels are not formed through defects in lipid structure. The similarity in conductance and ion selectivity between channels induced by novobiocin and those formed by gramicidin A suggests that these structurally different molecules form channels with comparable internal diameter and internal surface charge distribution. Using HPLC purification we argue that the channel-forming activity of novobiocin is related to the activity of the novobiocin molecule itself, and not to a contaminant of the commercially available novobiocin sodium salt preparation.

Novobiocin forms cation-permeable ion channels in rat fetal distal lung epithelium

The antibiotic novobiocin has been previously reported to increase Na+ transport in frog skin, presumably by attenuation of Na+ self-inhibition of Na+ channels. To determine whether novobiocin had similar effects and utilized a similar mechanism in mammalian Na(+)-transporting tissues, we studied its effect on ion transport by primary cultures of fetal distal lung epithelium (FDLE) cultured from 20-day gestationally aged rats (term = 22 days). Novobiocin (10 mM) increased short-circuit current and markedly decreased the resistance in FDLE monolayers mounted in Ussing chambers. Fura-2 single-cell studies showed that novobiocin increased intracellular Ca2+ concentration and that this resulted from extracellular sources. Nystatin-perforated patch-clamp techniques demonstrated that novobiocin increased nonrectifying cation whole cell currents without inducing detectable anion currents. Novobiocin created nonrectifying monovalent cation-selective channels in lipid bilayers. These studies demonstrated that novobiocin affects the bioelectric properties of Na+ transporting lung epithelium and that this likely occurs by the formation of ion-permeant channels in their lipid membranes.

Sodium dependence of the epithelial sodium conductance expressed in Xenopus laevis oocytes

The epithelial Na+ conductance was expressed in Xenopus laevis oocytes by injection of size-fractionated mRNA of bovine tracheal epithelium. Fractionation was achieved by sucrose density gradient centrifugation. Successful expression was analysed by recording current/voltage (I/V) curves in the presence and absence of amiloride (10 mumol/l). The newly expressed conductance was half-maximally inhibited by 44 nmol/l amiloride and exhibited a selectivity for Na+ over K+ of 140:1. I/V curves obtained at different extracellular Na+ concentrations ([Na+]o) were subjected to a Goldman-fit analysis to obtain the relation between Na+ permeability (PNa) and [Na+]o. The data show that decreasing [Na+]o from 85 mmol/l to 0.85 mmol/l increased PNa by more than threefold, which is thought to reflect Na+ channel inhibition by increasing [Na+]o. This effect clearly exceeded what can be attributed to concentration saturation of single Na+ channel conductance (Palmer and Frindt (1986) Proc Natl Acad Sci USA 83:2767). No correlation of inhibition with intracellular Na+ concentration was observed. Preservation of the [Na+]o-dependent self-inhibition by the newly expressed Na+ conductance suggests that it is an intrinsic property of the Na+ channel protein, probably mediated by an extracellular Na+ binding site.

Stimulation of sodium transport by aldosterone and arginine vasotocin in A6 cells

The effects of aldosterone and arginine vasotocin (AVT) on transepithelial Na+ transport of cultured A6 cells were investigated. All experiments were performed with cells grown on Millicell TM culture-plate inserts for a period of 2-4 weeks in defined, serum-free medium. Omitting fetal bovine serum 2 days after seeding the cells on filters did not influence potential difference (PD) development or the hormonal responses tested. The cell layers were placed in an Ussing chamber for short-circuit current (ISC) and transepithelial conductance (G) measurements. Base-line values were (n = 93): PD, 51.0 +/- 0.2 mV (apical side negative); ISC, 14.55 +/- 0.06 microA/cm2; G, 0.306 +/- 0.001 mS/cm2. ISC and G were higher in cells pretreated with 10(-7) M aldosterone for 24 h in the incubator, when compared to controls (ISC, 28 +/- 2 vs. 16 +/- 2 microA/cm2, G, 0.41 +/- 0.04 vs. 0.26 +/- 0.01 mS/cm2, n = 5) and both remained stable for at least 6 h. In cells not treated with aldosterone, 10(-7) M AVT increased ISC within 1 min after addition, producing a maximum ISC within 15 min which then declined to baseline levels over the next 5 h. Addition of AVT to aldosterone-pretreated cells resulted in a significantly greater peak increase in ISC than in non-pretreated cells (change in ISC compared to controls: 8.1 +/- 0.4 vs. 4.9 +/- 0.4 microA/cm2, n = 5, P less than 0.001), indicating a synergistic effect. A dose-response curve for amiloride obtained in the presence of AVT showed that amiloride completely inhibits ISC. Pretreatment of the A6 cells with aldosterone for 24 h shifted the amiloride dose-response curve to the right, as expressed in a doubling of the apparent Ki value (from 0.17 +/- 0.02 to 0.33 +/- 0.04 microM). In conclusion, A6 cells grown in defined, serum-free medium express a greater than additive synergism between aldosterone and AVT in stimulating transepithelial Na+ transport.

Na transport stimulation by novobiocin: intracellular ion concentrations and membrane potential

Microelectrodes and electron microprobe analysis were employed to study the effect of novobiocin on membrane potential and intracellular electrolyte concentrations in the frog skin epithelium. In both species investigated (Rana esculenta and Rana temporaria), novobiocin (1 mM, outer bath) caused a stimulation of transepithelial Na transport, a depolarization of apical membrane potential, a fall in the apical fractional resistance, and an increase in the intracellular Na concentration. The rise in the Na concentration was accompanied by an equivalent fall in the K concentration. All effects of novobiocin were fully reversible by subsequent application of amiloride. The depolarization as well as the Na increase suggests that the natriferic effect of novobiocin is due to a stimulation of the apical Na influx. Combining both measurements it was possible to calculate the effect of novobiocin on the Na permeability of the apical membrane directly. In Rana esculenta novobiocin increased the permeability from 4.5 to 23.2 nm/s. In Rana temporaria the increase was significantly smaller, from 8.7 to 16.9 nm/s. The transport rate as measured by the short-circuit current showed a non-linear dependence on the apical Na permeability. In the range of transport rates normally encountered, however, the current was a linear function of the Na permeability consistent with the view that the apical membrane is rate-limiting in transepithelial Na transport.