Volume-sensitive chloride currents in primary cultures of human fetal vas deferens epithelial cells

Cell Volume-Activated and Volume-Correlated Anion Channels in Mammalian Cells: Their Biophysical, Molecular, and Pharmacological Properties

There are a number of mammalian anion channel types associated with cell volume changes. These channel types are classified into two groups: volume-activated anion channels (VAACs) and volume-correlated anion channels (VCACs). VAACs can be directly activated by cell swelling and include the volume-sensitive outwardly rectifying anion channel (VSOR), which is also called the volume-regulated anion channel; the maxi-anion channel (MAC or Maxi-Cl); and the voltage-gated anion channel, chloride channel (ClC)-2. VCACs can be facultatively implicated in, although not directly activated by, cell volume changes and include the cAMP-activated cystic fibrosis transmembrane conductance regulator (CFTR) anion channel, the Ca²⁺-activated Cl^- channel (CaCC), and the acid-sensitive (or acid-stimulated) outwardly rectifying anion channel. This article describes the phenotypical properties and activation mechanisms of both groups of anion channels, including accumulating pieces of information on the basis of recent molecular understanding. To that end, this review also highlights the molecular identities of both anion channel groups; in addition to the molecular identities of ClC-2 and CFTR, those of CaCC, VSOR, and Maxi-Cl were recently identified by applying genome-wide approaches. In the last section of this review, the most up-to-date information on the pharmacological properties of both anion channel groups, especially their half-maximal inhibitory concentrations (IC₅₀ values) and voltage-dependent blocking, is summarized particularly from the standpoint of pharmacological distinctions among them. Future physiologic and pharmacological studies are definitely warranted for therapeutic targeting of dysfunction of VAACs and VCACs.

Inhibition of the endogenous volume-regulated anion channel (VRAC) in HEK293 cells by acidic di-aryl-ureas

The endogenous volume-regulated anion channel (VRAC) from HEK293 cells was pharmacologically characterized using the whole-cell patch-clamp technique. Under isotonic conditions a small (1.3 nS), Ca(2+)-independent Cl conductance was measured. However, swelling at 75% tonicity activated a VRAC identified as an outward-rectifying anion current ( P(l) > P(Cl) > P(gluconate)), which was ATP-dependent and showed inactivation at positive potentials. Activation of this current followed a sigmoid time course, reaching a plateau conductance of 42.6 nS after 12-15 min ( t(1/2) = 7 min). The pharmacology of this VRAC was investigated using standard Cl(-)-channel blockers (NPPB, DIDS, and tamoxifen) as well as a new group (acidic di-aryl ureas) of Cl(-)-channel blockers (NS1652, NS3623, NS3749, and NS3728). The acidic di-aryl ureas were originally synthezised for inhibition of the human erythrocyte Cl(-) conductance in vivo. NS3728 was the most potent VRAC blocker in this series ( IC(50) = 0.40 micro M) and even more potent than tamoxifen (2.2 micro M). NS3728 accelerated channel inactivation at positive potentials. These results show that acidic di-aryl ureas constitute a promising starting point for the synthesis of potent inhibitors of VRAC.

Biophysical and pharmacological characterization of hypotonically activated chloride currents in cortical astrocytes

Rat cortical astrocytes regulate their cell volume in response to hypotonic challenge. This regulation is believed to depend largely on the release of chloride or organic osmolytes through anion channels. Using whole-cell recordings, we identified weakly outwardly rectifying chloride currents that could be activated in response to hypotonic challenge. These currents exhibited the following permeability sequence upon replacement of chloride in the bathing solution with various anions: I- > NO3- > Cl- > Gluc- > or = MeS- > Ise-. Interestingly, extracellular I-, albeit showing the greatest permeability, blocked the currents with an IC50 of approximately 50 mM. Currents were almost completely inhibited by 123 microM NPPB and partially inhibited by 200 microM niflumic acid or 200 microM DIDS. Additionally, the total number of Cl- ions effluxed through the hypotonically activated channels was markedly similar to the total solute efflux during volume regulation. We therefore propose the hypotonically activated chloride channel as a major contributor to volume regulation of astrocytes. To examine potential candidate chloride channel genes expressed by astrocytes, we employed RT-PCR to demonstrate the presence of transcripts for ClC-2, 3, 4, 5, and 7, as well as for VDAC and CFTR in cultured astrocytes. Moreover, we performed immunostaining with antibodies against each of these channels and showed the strongest expression of ClC-2 and ClC-3, strong expression of ClC-5 and VDAC, weak expression of ClC-7 and very weak expression of ClC-4 and CFTR. Intriguingly, although we found at least seven Cl- channel proteins from three different gene families in astrocytes, none appeared to be active in resting cells.

Cell volume regulation: osmolytes, osmolyte transport, and signal transduction

In recent years, it has become evident that the volume of a given cell is an important factor not only in defining its intracellular osmolality and its shape, but also in defining other cellular functions, such as transepithelial transport, cell migration, cell growth, cell death, and the regulation of intracellular metabolism. In addition, besides inorganic osmolytes, the existence of organic osmolytes in cells has been discovered. Osmolyte transport systems-channels and carriers alike-have been identified and characterized at a molecular level and also, to a certain extent, the intracellular signals regulating osmolyte movements across the plasma membrane. The current review reflects these developments and focuses on the contributions of inorganic and organic osmolytes and their transport systems in regulatory volume increase (RVI) and regulatory volume decrease (RVD) in a variety of cells. Furthermore, the current knowledge on signal transduction in volume regulation is compiled, revealing an astonishing diversity in transport systems, as well as of regulatory signals. The information available indicates the existence of intricate spatial and temporal networks that control cell volume and that we are just beginning to be able to investigate and to understand.

Adenosine stimulates anion secretion across cultured and native adult human vas deferens epithelia

Experiments were conducted to determine the responsiveness of human vas deferens epithelial cell monolayers to adenosine and related agonists. Human abdominal vas deferens epithelial cells have been isolated from adult tissues and grown to confluence on permeable supports. All cells exhibit intense ZO-1 and cytokeratin immunoreactivity. Cultured cell monolayers exhibit high electrical resistance with a lumen-negative potential difference and short circuit current (I(sc)) indicative of anion secretion and/or cation absorption. A portion of the basal I(sc) is inhibited by amiloride. Amiloride-sensitive I(sc) is enhanced by exposure to glucocorticoids and is Na(+) dependent, indicating the presence of epithelial sodium channel-mediated Na(+) absorption. Epithelial anion secretion and intracellular generation of cAMP are acutely stimulated by adenosine and the adenosine receptor agonist 5'-(N-ethylcarboxamido)adenosine (NECA), with these effects being fully blocked by 8-phenyltheophylline. Adenosine receptors are localized to the apical membrane of the epithelial cells, as basolateral adenosine is without effect. Freshly excised human vas deferens recapitulate observations made on cultured epithelia when evaluated with the self-referencing vibrating probe: amiloride inhibition of basal ion transport, stimulation by adenosine, and inhibition by 8-phenyltheophyline. These results demonstrate that adult human vas deferens epithelium actively transports ions to generate the luminal environment of the deferent duct. Thus, vas deferens epithelium likely plays an active role in male fertility, and interventions that modulate epithelial function might be exploited to treat male-factor infertility or in contraception.

Ovine male genital duct epithelial cells differentiate in vitro and express functional CFTR and ENaC

To investigate the biology of the male genital duct epithelium, we have established cell cultures from the ovine vas deferens and epididymis epithelium. These cells develop tight junctions, high transepithelial electrical resistance, and a lumen-negative transepithelial potential difference as a sign of active transepithelial ion transport. In epididymis cultures the equivalent short-circuit current (I(sc)) averaged 20.8+/-0.7 microA/cm(2) (n = 150) and was partially inhibited by apical application of amiloride with an inhibitor concentration of 0.64 microM. In vas deferens cultures, I(sc) averaged 14.4+/-1.1 microA/cm(2) (n = 18) and was also inhibited by apical application of amiloride with a half-maximal inhibitor concentration (K(i)) of 0.68 microM. The remaining amiloride-insensitive I(sc) component in epididymis and vas deferens cells was partially inhibited by apical application of the Cl(-) channel blocker diphenylamine-2-carboxylic acid (1 mM). It was largely dependent on extracellular Cl(-) and, to a lesser extent, on extracellular HCO(-)(3). It was further stimulated by basolateral application of forskolin (10(-5) M), which increased I(sc) by 3.1+/-0.3 microA/cm(2) (n = 65) in epididymis and 0.9+/-0.1 microA/cm(2) (n = 11) in vas deferens. These findings suggest that cultured ovine vas deferens and epididymis cells absorb Na(+) via amiloride-sensitive epithelial Na(+) channels (ENaC) and secrete Cl(-) and HCO(-)(3) via apical cystic fibrosis transmembrane conductance regulator (CFTR) Cl(-) channels. This interpretation is supported by RT-PCR data showing that vas deferens and epididymis cells express CFTR and ENaC mRNA.

Osmotic pressure regulates alpha beta gamma-rENaC expressed in Xenopus oocytes

The hypothesis that amiloride-sensitive Na+ channels (ENaC) are involved in cell volume regulation was tested. Anisosmotic ND-20 media (ranging from 70 to 450 mosM) were used to superfuse Xenopus oocytes expressing alpha beta gamma-rat ENaC (alpha beta gamma-rENaC). Whole cell currents were reversibly dependent on external osmolarity. Under conditions of swelling (70 mosM) or shrinkage (450 mosM), current amplitude decreased and increased, respectively. In contrast, there was no change in current amplitude of H2O-injected oocytes to the above osmotic insults. Currents recorded from alpha beta gamma-rENaC-injected oocytes were not sensitive to external Cl- concentration or to the K+ channel inhibitor BaCl2. They were sensitive to amiloride. The concentration of amiloride necessary to inhibit one-half of the maximal rENaC current expressed in oocytes (Ki; apparent dissociation constant) decreased in swollen cells and increased in shrunken oocytes. The osmotic pressure-induced Na+ currents showed properties similar to those of stretch-activated channels, including inhibition by Gd3+ and La3+, and decreased selectivity for Na+. alpha beta gamma-rENaC-expressing oocytes maintained a nearly constant cell volume in hypertonic ND-20. The present study is the first demonstration that alpha beta gamma-rENaC heterologously expressed in Xenopus oocytes may contribute to oocyte volume regulation following shrinkage.