Potassium channel activity recorded from the apical membrane of freshly isolated epithelial cells in rat caudal epididymis

Picrotoxin Delineates Different Transport Configurations for Malate and γ Aminobutyric Acid through TaALMT1

Plant-derived pharmacological agents have been used extensively to dissect the structure–function relationships of mammalian GABA receptors and ion channels. Picrotoxin is a non-competitive antagonist of mammalian GABAA receptors. Here, we report that picrotoxin inhibits the anion (malate) efflux mediated by wheat (Triticum aestivum) ALMT1 but has no effect on GABA transport. The EC50 for inhibition was 0.14 nM and 0.18 nM when the ALMTs were expressed in tobacco BY2 cells and in Xenopus oocytes, respectively. Patch clamping of the oocyte plasma membrane expressing wheat ALMT1 showed that picrotoxin inhibited malate currents from both sides of the membrane. These results demonstrate that picrotoxin inhibits anion efflux effectively and can be used as a new inhibitor to study the ion fluxes mediated by ALMT proteins that allow either GABA or anion transport.

Cytosolic GABA inhibits anion transport by wheat ALMT1

Anion transport by aluminium-activated malate transporter (ALMT) proteins is negatively regulated by gamma-aminobutyric acid (GABA), which increases in concentration during stress. Here, the interaction between GABA and wheat (Triticum aestivum, Ta) TaALMT1 heterologously-expressed in Xenopus laevis oocytes was investigated. GABA inhibited anion transport by TaALMT1 in membrane patches from the cytosolic, not extracellular membrane face, via a reduction in open probability (NP_open ), not an inhibition of channel current magnitude. TaALMT1 currents in patches frequently exhibited rundown with complete removal of cytosolic factors, but were partially sustained by protein kinase C dependent phosphorylation. When applied to whole oocytes a GABA-analogue-BODIPY conjugate inhibited TaALMT1 anion currents from the cytoplasmic face only, whereas free GABA inhibited from both the inside and outside consistent with GABA traversing the TaALMT1 pore then acting from the inside. We propose GABA does not competitively inhibit ALMT conductance through the same pore but rather leads to an allosteric effect, reducing anion channel opening frequency. Across plants GABA is a conserved regulator of anion transport via ALMTs - a family with numerous physiological roles beyond Al³⁺ tolerance. Our data suggests that a GABA-ALMT interaction from the cytosolic face has the potential to form part of a novel plant signalling pathway.

Cellular Mechanism Underlying Hydrogen Sulfide Mediated Epithelial K+ Secretion in Rat Epididymis

As a novel gasotransmitter, hydrogen sulfide (H₂S) elicits various physiological actions including smooth muscle relaxation and promotion of transepithelial ion transport. However, the pro-secretory function of H₂S in the male reproductive system remains largely unclear. The aim of this study is to elucidate the possible roles of H₂S in modulating rat epididymal intraluminal ionic microenvironment essential for sperm storage. The results revealed that endogenous H₂S-generating enzymes cystathionine β-synthetase (CBS) and cystathionine γ-lyase (CSE) were both expressed in rat epididymis. CBS located predominantly in epithelial cells whilst CSE expressed primarily in smooth muscle cells. The relative expression level of CBS and CSE escalated from caput to cauda regions of epididymis, which was paralleled to the progressively increasing production of endogenous H₂S. The effect of H₂S on epididymal epithelial ion transportation was investigated using short-circuit current (I_SC), measurement of intracellular ion concentration and in vivo rat epididymal microperfusion. Our data showed that H₂S induced transepithelial K⁺ secretion via adenosine triphosphate-sensitive K⁺ (K_ATP) channel and large conductance Ca²⁺-activated K⁺ (BK_Ca) channel. Transient receptor potential vanilloid 4 (TRPV4) channel-mediated Ca²⁺ influx was implicated in the activation of BK_Ca channel. In vivo studies further demonstrated that H₂S promoted K⁺ secretion in rat epididymal epithelium. Inhibition of endogenous H₂S synthesis caused a significant decrease in K⁺ concentration of cauda epididymal intraluminal fluid. Moreover, our data demonstrated that high extracellular K⁺ concentration actively depressed the motility of cauda epididymal sperm in a pH-independent manner. Collectively, the present study demonstrated that H₂S was vital to the formation of high K⁺ concentration in epididymal intraluminal fluid by promoting the transepithelial K⁺ secretion, which might contribute to the maintenance of the cauda epididymal sperm in quiescent dormant state before ejaculation.

Sodium coupled bicarbonate influx regulates intracellular and apical pH in cultured rat caput epididymal epithelium

Background

The epithelium lining the epididymis provides an optimal acidic fluid microenvironment in the epididymal tract that enable spermatozoa to complete the maturation process. The present study aims to investigate the functional role of Na⁺/HCO₃⁻ cotransporter in the pH regulation in rat epididymis.

Method/Principal Findings

Immunofluorescence staining of pan cytokeratin in the primary culture of rat caput epididymal epithelium showed that the system was a suitable model for investigating the function of epididymal epithelium. Intracellular and apical pH were measured using the fluorescent pH sensitive probe carboxy-seminaphthorhodafluor-4F acetoxymethyl ester (SNARF-4F) and sparklet pH electrode respectively to explore the functional role of rat epididymal epithelium. In the HEPES buffered Krebs-Henseleit(KH) solution, the intracellular pH (pHi) recovery from NH₄Cl induced acidification in the cultured caput epididymal epithelium was completely inhibited by amiloride, the inhibitor of Na⁺/H⁺ exchanger (NHE). Immediately changing of the KH solution from HEPES buffered to HCO₃⁻ buffered would cause another pHi recovery. The pHi recovery in HCO₃⁻ buffered KH solution was inhibited by 4, 4diisothiocyanatostilbene-2, 2-disulfonic acid (DIDS), the inhibitor of HCO₃⁻ transporter or by removal of extracellular Na⁺. The extracellular pH measurement showed that the apical pH would increase when adding DIDS to the apical side of epididymal epithelial monolayer, however adding DIDS to the basolateral side had no effect on apical pH.

Conclusions

The present study shows that sodium coupled bicarbonate influx regulates intracellular and apical pH in cultured caput epididymal epithelium.

α-Catulin CTN-1 is required for BK channel subcellular localization in C. elegans body-wall muscle cells

The BK channel, a voltage- and Ca(2+)-gated large-conductance potassium channel with many important functions, is often localized at specific subcellular domains. Although proper subcellular localization is likely a prerequisite for the channel to perform its physiological functions, little is known about the molecular basis of localization. Here, we show that CTN-1, a homologue of mammalian α-catulin, is required for subcellular localization of SLO-1, the Caenorhabditis elegans BK channel α-subunit, in body-wall muscle cells. CTN-1 was identified in a genetic screen for mutants that suppressed a lethargic phenotype caused by expressing a gain-of-function (gf) isoform of SLO-1. In body-wall muscle cells, CTN-1 coclusters with SLO-1 at regions of dense bodies, which are Z-disk analogs of mammalian skeletal muscle. In ctn-1 loss-of-function (lf) mutants, SLO-1 was mislocalized in body-wall muscle but its transcription and protein level were unchanged. Targeted rescue of ctn-1(lf) in muscle was sufficient to reinstate the lethargic phenotype in slo-1(gf);ctn-1(lf). These results suggest that CTN-1 plays an important role in BK channel function by mediating channel subcellular localization.

PVD9902, a porcine vas deferens epithelial cell line that exhibits neurotransmitter-stimulated anion secretion and expresses numerous HCO3(-) transporters

Epithelial ion transport disorders, including cystic fibrosis, adversely affect male reproductive function by nonobstructive mechanisms and by obstruction of the distal duct. Continuous cell lines that could be used to define ion transport mechanisms in this tissue are not readily available. In the present study, porcine vas deferens epithelial cells were isolated by standard techniques, and the cells spontaneously immortalized to form a porcine vas deferens epithelial cell line that we have titled PVD9902. Cells were maintained in continuous culture for >4 yr and 200 passages in a typical growth medium. Frozen stocks were generated, and thawed cells exhibited growth characteristics indistinguishable from their nonfrozen counterparts. Molecular and immunocytochemical studies confirmed the origin and epithelial nature of these cells. When seeded on permeable supports, PVD9902 cells grew as electrically tight (>6,000 ohms x cm2), confluent monolayers that responded to forskolin with an increase in short-circuit current (I(sc); 8 +/- 1 microA/cm2) that required Cl-, HCO3(-), and Na+, and was partially sensitive to bumetanide. mRNA was expressed for a number of anion transporters, including CFTR, electrogenic Na+-HCO3(-) cotransporter 1b (NBCe1b), downregulated in adenoma, pendrin, and Cl-/formate exchanger. Both forskolin and isoproterenol caused an increase in cellular cAMP levels. In addition, PVD9902 cell monolayers responded to physiological (i.e., adenosine, norepinephrine) and pharmacological [i.e., 5'-(N-ethylcarboxamido)adenosine, isoproterenol] agonists with increases in I(sc). Unlike their freshly isolated counterparts, however, PVD9902 cells did not respond to glucocorticoid exposure with an increase in amiloride-sensitive I(sc). RT-PCR analysis revealed the presence of both glucocorticoid and mineralocorticoid receptor mRNA as well as mRNA for the alpha- and gamma-subunits of the epithelia Na+ channels (alpha- and gamma-ENaC), but not beta-ENaC. Nonetheless, PVD9902 cells recapitulated most observations in freshly isolated cells and thus represent a powerful new tool to characterize mechanisms that contribute to male reproductive function.

Localization of Ca2+ -activated big-conductance K+ channels in rabbit distal colon

Big-conductance Ca(2+)-activated K(+) channels (BK channels) may play an important role in the regulation of epithelial salt and water transport, but little is known about the expression level and the precise localization of BK channels in epithelia. The aim of the present study was to quantify and localize the BK channels in the distal colon epithelium by iberiotoxin (IbTX) binding using the radiolabeled iberiotoxin analogue (125)I-IbTX-D19Y/Y36F, by autoradiography and by immunohistochemical studies. The results showed that the surface cells, responsible for Na(+) absorption, contained a high number of BK channels, whereas the abundance of the channels in the Cl(-)-secreting crypt cells was very low or absent. Surprisingly, the (125)I-IbTX-D19Y/Y36F binding and immunohistochemical studies showed expression of BK channels in the apical as well as in the basolateral membranes of the surface cells. In conclusion, the significant and distinct expression of BK channels in epithelia, combined with their strict regulation, indicate that these channels may play an important role in the overall regulation of salt and water transport.

The cytoplasmic tail of large conductance, voltage- and Ca2+-activated K+ (MaxiK) channel is necessary for its cell surface expression

The large conductance, voltage- and Ca(2+)-activated K(+) channel (MaxiK) is expressed in several renal segments and functions in cell volume regulation and flow-mediated K(+) secretion. Previously, we cloned two MaxiK channel isoforms, named rbslo1 and rbslo2, from rabbit renal cells. rbslo1 has a 58-amino acid insertion after the S8 hydrophobic domain, whereas rbslo2 is truncated and cannot be activated. Here we use the sequence differences between the two variants to examine their plasma membrane processing. Plasma membrane localization of rbslo1 and 2 expressed in HEK293 cells was assayed by electrophysiology, immunocytochemistry, and biochemistry studies. Consistent with its functional silence, rbslo2 localized primarily within the cytoplasm, presumably in the endoplasmic reticulum and Golgi region. Coexpression with MaxiK beta subunits did not alter the cellular localization of either rbslo1 or rbslo2. When rbslo1 and 2 are cotransfected in non-polarized cells, they colocalized primarily within the cell with only rbslo1 detected at the plasma membrane. When transfected into polarized, medullary-thick ascending limb (mTAL) cells, rbslo1 is expressed at the apical membrane whereas the majority of rbslo2 localized throughout the cytoplasm. Given the high degree of similarity between the two isoforms, we conclude that the cytoplasmic tail of rbslo1 is important for the cell surface expression of MaxiK channels.