Regulation of paracellular ion conductances by NaCl gradients in renal epithelial cells

Simulation of Cl(-) Secretion in Epithelial Tissues: New Methodology Estimating Activity of Electro-Neutral Cl(-) Transporter

Transcellular Cl⁻ secretion is, in general, mediated by two steps; (1) the entry step of Cl⁻ into the cytosolic space from the basolateral space across the basolateral membrane by Cl⁻ transporters, such as Na⁺-K⁺-2Cl⁻ cotransporter (NKCC1, an isoform of NKCC), and (2) the releasing step of Cl⁻ from the cytosolic space into the luminal (air) space across the apical membrane via Cl⁻ channels, such as cystic fibrosis transmembrane conductance regulator (CFTR) Cl⁻ channel. Transcellular Cl⁻ secretion has been characterized by using various experimental techniques. For example, measurements of short-circuit currents in the Ussing chamber and patch clamp techniques provide us information on transepithelial ion movements via transcellular pathway, transepithelial conductance, activity (open probability) of single channel, and whole cell currents. Although many investigators have tried to clarify roles of Cl⁻ channels and transporters located at the apical and basolateral membranes in transcellular Cl⁻ secretion, it is still unclear how Cl⁻ channels/transporters contribute to transcellular Cl⁻ secretion and are regulated by various stimuli such as Ca²⁺ and cAMP. In the present study, we simulate transcellular Cl⁻ secretion using mathematical models combined with electrophysiological measurements, providing information on contribution of Cl⁻ channels/transporters to transcellular Cl⁻ secretion, activity of electro-neutral ion transporters and how Cl⁻ channels/transporters are regulated.

NaCl flux between apical and basolateral side recruits claudin-1 to tight junction strands and regulates paracellular transport

In multicellular organisms, epithelia separate and divide the internal environment maintaining appropriate conditions in each compartment. To maintain homeostasis in these compartments, claudins, major cell adhesion molecules in tight junctions (TJs), regulate movements of several substances through the paracellular pathway (barrier function). In this study, we investigated effects of the flux of several substances between apical and basolateral side on paracellular transport and TJ protein localization. NaCl flux from apical to basolateral side increased paracellular conductance (Gp) and recruited claudin-1 from lateral cell membrane to the apical end with the colocalization with occludin, one of the TJ proteins concentrated at TJ strands. Oppositely-directed flux of sucrose against NaCl flux inhibited these reactions and same directional flux of sucrose with NaCl enhanced the increase of Gp, whereas 10-kDa dextran inhibited these reactions regardless of the side of administration. Our present findings indicated that TJ protein localization and barrier function are regulated depending on the environmental differences between apical and basolateral side.

Hydrostatic pressure regulates tight junctions, actin cytoskeleton and transcellular ion transport

In the epithelia and endothelia, tight junctions regulate the movement of several substances through the paracellular pathway, maintaining several gradients between apical and basal compartments including osmolality and hydrostatic pressure. In this study, we show that the change of hydrostatic pressure gradient affected tight junctions as well as actin cytoskeleton, cell height and transcellular ion transport. Hydrostatic pressure gradient from basolateral to apical side increased transepithelial conductance and altered claudin-1 localization within several tens of minutes. These changes were promptly restored by the elimination of hydrostatic pressure gradient. Hydrostatic pressure gradient also induced dynamic changes in the actin structure and cell height. We further found that hydrostatic pressure gradient from basolateral to apical side stimulates transcellular Cl(-) transport. Our present findings indicate that the epithelial cell structures and functions are regulated by the hydrostatic pressure gradient which is generated and maintained by the epithelia themselves.

Effects of extracellular chloride ion on epithelial sodium channel (ENaC) in arginine vasotocin (AVT)-stimulated renal epithelial cells

The epithelial Na(+) channel (ENaC) contributes to control of blood pressure by reabsorbing Na(+) in the cortical collecting duct of the kidney. The luminal Cl(-) concentration in the duct varies under physiological conditions. As the body Na(+) content is lower, the luminal Cl(-) concentration in the duct becomes lower. Thus, we hypothesized that the extracellular Cl(-) elevates ENaC activity in AVT-stimulated renal epithelial A6 cells (a model cell line of the cortical collecting duct) leading to recovery from a low body Na(+) content. To clarify this point, we studied effects of extracellular Cl(-) concentration on ENaC activity using cell-attached patch clamp technique. We found that ENaC had a single-channel conductance of 4.6 +/- 0.1 pS (mean +/- SE) and channel activity (open probability, Po) of 0.30 +/- 0.02 at a pipette potential of 60 mV. Lowering pipette Cl(-) concentration diminished Po to 0.23 +/- 0.02 associated with a significant decrease in open time from 0.78 +/- 0.03 to 0.61 +/- 0.02 s with no significant change in closed time, and shifted the current-voltage relationship leftward. These results suggest that the extracellular Cl(-) regulates the ENaC-mediated Na(+) reabsorption by affecting ENaC properties in AVT-stimulated renal epithelial cells.

Regulation of paracellular Na+ and Cl(-) conductances by hydrostatic pressure

The effect of hydrostatic pressure on the paracellular ion conductance (Gp) composed of the Na(+) conductance (G(Na)) and the Cl(-) conductance (G(Cl)) has been Investigated. Gp, G(Na) and G(Cl) were time-dependently increased after applying an osmotic gradient generated by NaCl with basolateral hypotonicity. Hydrostatic pressure (1-4cm H2O) applied from the basolateral side enhanced the osmotic gradient-induced increase in Gp, G(Na) and G(Cl) in a magnitude-dependent manner, while the hydrostatic pressure applied from the apical side diminished the osmotic gradient-induced increase in Gp, G(Na) and G(Cl). How the hydrostatic pressure influences Gp, G(Na) and G(Cl) under an isosmotic condition was also investigated. Gp, G(Na) and G(Cl) were stably constant under a condition with basolateral application of sucrose canceling the NaCl-generated osmotic gradient (an isotonic condition). Even under this stable condition, the basolaterally applied hydrostatic pressure drastically elevated Gp, G(Na) and G(Cl), while apically applied hydrostatic pressure had little effect on Gp, G(Na) or G(Cl). Taken together, these observations suggest that certain factors controlled by the basolateral osmolality and the basolaterally applied hydrostatic pressure mainly regulate the Gp, G(Na) and G(Cl).

Quercetin stimulates Na+/K+/2Cl- cotransport via PTK-dependent mechanisms in human airway epithelium

We investigated regulatory mechanisms of Cl(-) secretion playing an essential role in the maintenance of surface fluid in human airway epithelial Calu-3 cells. The present study reports that quercetin (a flavonoid) stimulated bumetanide-sensitive Cl(-) secretion with reduction of apical Cl(-) conductance, suggesting that quercetin stimulates Cl(-) secretion by activating an entry step of Cl(-) across the basolateral membrane through Na(+)/K(+)/2Cl(-) cotransporter (NKCC1). To clarify the mechanism stimulating NKCC1 by quercetin, we verified involvement of protein kinase (PK)A, PKC, protein tyrosine kinase (PTK), and cytosolic Ca(2+)-dependent pathways. A PKA inhibitor (PKI-14-22 amide), a PKC inhibitor (Gö 6983) or a Ca(2+) chelating agent did not affect the quercetin-stimulated Cl(-) secretion. On the other hand, a PTK inhibitor (AG18) significantly diminished the stimulatory action of quercetin on Cl(-) secretion without inhibitory effects on apical Cl(-) conductance, suggesting that a PTK-mediated pathway is involved in the stimulatory action of quercetin. The quercetin action on Cl(-) secretion was suppressed with brefeldin A (BFA, an inhibitor of vesicular transport from ER to Golgi), and the BFA-sensitive Cl(-) secretion was not observed in the presence of an epidermal growth factor receptor (EGFR) kinase inhibitor (AG1478), suggesting that quercetin stimulates Cl(-) secretion by causing the EGFR kinase-mediated translocation of NKCC1 or an NKC1-activating factor to the basolateral membrane in human airway epithelial Calu-3 cells. However, the surface density of NKCC1 was not increased by quercetin, but quercetin elevated the activity of NKCC1. These observations indicate that quercetin stimulates Cl(-) secretion by activating NKCC1 via translocation of an NKCC1-activating factor through an EGFR kinase-dependent pathway.

Action of N-acylated ambroxol derivatives on secretion of chloride ions in human airway epithelia

We report the effects of new N-acylated ambroxol derivatives (TEI-588a, TEI-588b, TEI-589a, TEI-589b, TEI-602a and TEI-602b: a, aromatic amine-acylated derivative; b, aliphatic amine-acylated derivative) induced from ambroxol (a mucolytic agent to treat human lung diseases) on Cl(-) secretion in human submucosal serous Calu-3 cells under a Na(+)/K(+)/2Cl(-) cotransporter-1 (NKCC1)-mediated hyper-secreting condition. TEI-589a, TEI-589b and TEI-602a diminished hyper-secretion of Cl(-) by diminishing the activity of NKCC1 without blockade of apical Cl(-) channel (TEI-589a>TEI-602a>TEI-589b), while any other tested compounds including ambroxol had no effects on Cl(-) secretion. These indicate that the inhibitory action of an aromatic amine-acylated derivative on Cl(-) secretion is stronger that that of an aliphatic amine-acylated derivative, and that 3-(2,5-dimethyl)furoyl group has a strong action in inhibition of Cl(-) secretion than cyclopropanoyl group. We here indicate that TEI-589a, TEI-589b and TEI-602a reduce hyper-secretion to an appropriate level in the airway, providing a possibility that the compound can be an effective drug in airway obstructive diseases including COPD by reducing the airway resistance under a hyper-secreting condition.