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To maintain cellular and bodily homeostasis, cells respond to extracellular stimuli including osmotic stress by activating various ion channels, which have been implicated in many physiological and pathophysiological conditions. However, cellular osmosensory mechanisms remain elusive. Here, we report a novel voltage-dependent current in N2a cells activated by exposure to hypotonic stress. After a hypotonic challenge, N2a cells sequentially develop two distinct currents. The volume-regulated anion channel (VRAC) current emerges first and, after a delay, activation of a previously uncharacterized strongly outwardly rectifying current follows. The latter, delayed current ( I d ) is insensitive to NPPB, a nonspecific blocker of Cl − channels, and intracellular Mg 2+ , which inhibits VRAC and swelling-activated TRPM3 and TRPM7 channels. Replacement of extracellular Na + with NMDG + reduces inward tail currents, suggesting that I d is mediated by cations. Finally, I d shows voltage-dependent activation with slow activation kinetics and half-maximal activation at +76 mV. These pharmacological and biophysical characteristics of I d are distinct from those of known osmotic cell swelling-activated ion channels. In conclusion, our data identify and characterize a novel osmotically-activated, voltage-dependent ion channel in N2a cells. [ABSTRACT FROM AUTHOR]

Abstract: Aldosterone and osmotic stress are well known to regulate the epithelial Na+ channel (ENaC)-mediated Na+ transport in renal epithelial cells. However, we have no information on how aldosterone and osmotic stress interact on stimulation of ENaC-mediated Na+ transport in renal epithelium. In the present report, we studied how application of aldosterone (1μM for 1day) modifies the action of hypotonic stress on the ENaC-mediated Na+ transport in renal A6 epithelial cells by measuring the benzamil (a specific inhibitor for ENaC)-sensitive short-circuit current. The present study suggests that: (1) most ENaCs in cells without aldosterone treatment are translocated to Golgi apparatus, (2) major parts of aldosterone-generated ENaCs are located at the endoplasmic reticulum, (3) aldosterone diminishes the endocytosis rate of ENaCs from the apical membrane without any significant changes in the insertion rate of ENaCs into the apical membrane, and (4) application of sucrose after hypotonic stress stimulates the endocytosis of ENaCs, and elongates the functional life time of ENaCs by enhancing recycle of ENaCs into the endoplasmic reticulum in a retrograde manner. [Copyright &y& Elsevier]

Abstract: We investigated a role of p38 MAPK in the regulation of transepithelial Na+ reabsorption by chronic application (20–24h) of hypotonicity (hypotonic stress) in renal epithelial A6 cells. Pretreatment with a specific p38 MAPK inhibitor (SB202190) significantly reduced the chronic hypotonicity-stimulated transepithelial Na+ reabsorption by diminishing the Na+ entry through epithelial Na+ channel (ENaC) in the apical membrane and the Na+ extrusion via the Na+/K+ ATPase (pump), although the rate limiting step was still the Na+ entry step. We further examined whether the inhibitory effects of SB202190 on the transepithelial Na+ reabsorption is caused through suppression of mRNA expression of ENaC participating in the transepithelial Na+ reabsorption as the Na+ entry pathway. The chronic hypotonicity increased the mRNA expression of α-, β-, and γ-subunits of ENaC. Moreover, we found that inhibition of p38 MAPK by SB202190 diminished the mRNA expression of β- and γ-ENaC but not α-ENaC. Based on these observations, it is suggested that the chronic hypotonicity stimulates the renal transepithelial Na+ reabsorption by upregulating the mRNA expression of β- and γ-ENaC via a p38 MAPK-dependent pathway. [Copyright &y& Elsevier]

Abstract: Neltenexine has been applied to human lung diseases such as chronic obstructive pulmonary disease (COPD) as a mucolytic agent. However, we have no information on the neltenexine action in bronchial epithelial cells. We studied the neltenexine action on the ion transport in human submucosal serous Calu-3 cells. Under a hyper-secreting condition caused by terbutaline (a β2-adrenergic agonist), neltenexine diminished anion secretion by inhibiting the Cl− and uptake via Na+/K+/2Cl− cotransporter and cotransporter without blockade of the cystic fibrosis transmembrane conductance regulator (CFTR) channel, and also diminished anion secretion via stimulation of exchanger, which facilitates the extrusion of more CFTR-permeant anion, Cl−, with the uptake of less CFTR-permeant anion, . Thus, neltenexine reduced the hyper-secretion to keep an appropriate fluid level in the airway, providing a possibility that neltenexine can be an effective drug in airway obstructive diseases by decreasing the airway resistance under a hyper-secreting condition. [Copyright &y& Elsevier]