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1.  Electrolyte and Fluid Transport in Mesothelial Cells 
Mesothelial cells are specialized epithelial cells, which line the pleural, pericardial, and peritoneal cavities. Accumulating evidence suggests that the monolayer of mesothelial cells is permeable to electrolyte and fluid, and thereby govern both fluid secretion and re-absorption in the serosal cavities. Disorders in these salt and fluid transport systems may be fundamental in the pathogenesis of pleural effusion, pericardial effusion, and ascites. In this review, we discuss the location, physiological function, and regulation of active transport (Na+-K+-ATPase) systems, cation and anion channels (Na+, K+, Cl−, and Ca2+ channels), antiport (exchangers) systems, and symport (co-transporters) systems, and water channels (aquaporins). These secretive and absorptive pathways across mesothelial monolayer cells for electrolytes and fluid may provide pivotal therapeutical targets for novel clinical intervention in edematous diseases of serous cavities.
doi:10.2174/1875044300801010001
PMCID: PMC2629602  PMID: 19169368
mesothelioma; ion channel; permeability; effusion; filtration; ENaC
2.  8-(4-Chlorophenylthio)-Guanosine-3′,5′-Cyclic Monophosphate-Na Stimulates Human Alveolar Fluid Clearance by Releasing External Na+ Self-Inhibition of Epithelial Na+ Channels 
Salt absorption via alveolar epithelial Na+ channels (ENaC) is a critical step for maintaining an airspace free of flooding. Previously, we found that 8-(4-chlorophenylthio)-guanosine-3′,5′-cyclic monophosphate-Na (CPT-cGMP) activated native and heterologous ENaC. To investigate the potential pharmacological relevance, we applied this compound intratracheally to human lungs and found that ex vivo alveolar fluid clearance was increased significantly. Furthermore, this compound eliminated self-inhibition in human lung H441 cells and in oocytes expressing human αβγ but not δβγ channels. To further elucidate this novel mechanism, we constructed mutants abolishing (βΔV348 and γH233R) or augmenting (αY458A and γM432G) self-inhibition. The mutants eliminating self-inhibition lost their responses to CPT-cGMP, whereas those enhancing self-inhibition facilitated the stimulatory effects of this compound. CPT-cGMP was unable to activate a high Po mutant (βS520C) and plasmin proteolytically cleaved channels. Our data suggest that elimination of self-inhibition of αβγ ENaC may be a novel mechanism for CPT-cGMP to stimulate salt reabsorption in human lungs.
doi:10.1165/rcmb.2011-0004OC
PMCID: PMC3262684  PMID: 21562313
lung fluid reabsorption; amiloride-sensitive sodium channel; CPT-cGMP; ENaC self-inhibition
3.  K+ channel openers restore verapamil-inhibited lung fluid resolution and transepithelial ion transport 
Respiratory Research  2010;11(1):65.
Background
Lung epithelial Na+ channels (ENaC) are regulated by cell Ca2+ signal, which may contribute to calcium antagonist-induced noncardiogenic lung edema. Although K+ channel modulators regulate ENaC activity in normal lungs, the therapeutical relevance and the underlying mechanisms have not been completely explored. We hypothesized that K+ channel openers may restore calcium channel blocker-inhibited alveolar fluid clearance (AFC) by up-regulating both apical and basolateral ion transport.
Methods
Verapamil-induced depression of heterologously expressed human αβγ ENaC in Xenopus oocytes, apical and basolateral ion transport in monolayers of human lung epithelial cells (H441), and in vivo alveolar fluid clearance were measured, respectively, using the two-electrode voltage clamp, Ussing chamber, and BSA protein assays. Ca2+ signal in H441 cells was analyzed using Fluo 4AM.
Results
The rate of in vivo AFC was reduced significantly (40.6 ± 6.3% of control, P < 0.05, n = 12) in mice intratracheally administrated verapamil. KCa3.1 (1-EBIO) and KATP (minoxidil) channel openers significantly recovered AFC. In addition to short-circuit current (Isc) in intact H441 monolayers, both apical and basolateral Isc levels were reduced by verapamil in permeabilized monolayers. Moreover, verapamil significantly altered Ca2+ signal evoked by ionomycin in H441 cells. Depletion of cytosolic Ca2+ in αβγ ENaC-expressing oocytes completely abolished verapamil-induced inhibition. Intriguingly, KV (pyrithione-Na), K Ca3.1 (1-EBIO), and KATP (minoxidil) channel openers almost completely restored the verapamil-induced decrease in Isc levels by diversely up-regulating apical and basolateral Na+ and K+ transport pathways.
Conclusions
Our observations demonstrate that K+ channel openers are capable of rescuing reduced vectorial Na+ transport across lung epithelial cells with impaired Ca2+ signal.
doi:10.1186/1465-9921-11-65
PMCID: PMC2889873  PMID: 20507598
4.  Expression and Regulation of Epithelial Na+ Channels by Nucleotides in Pleural Mesothelial Cells 
Pleural effusions are commonly clinical disorders, resulting from the imbalance between pleural fluid turnover and reabsorption. The mechanisms underlying pleural fluid clearance across the mesothelium remain to be elucidated. We hypothesized that epithelial Na+ channel (ENaC) is expressed and forms the molecular basis of the amiloride-sensitive resistance in human mesothelial cells. Our RT-PCR results showed that three ENaC subunits, namely, α, β, γ, and two δ ENaC subunits, are expressed in human primary pleural mesothelial cells, a human mesothelioma cell line (M9K), and mouse pleural tissue. In addition, Western blotting and immunofluorescence microscopy studies revealed that α, β, γ, and δ ENaC subunits are expressed in primary human mesothelial cells and M9K cells at the protein level. An amiloride-inhibitable short-circuit current was detected in M9K monolayers and mouse pleural tissues when mounted in Ussing chambers. Whole-cell patch clamp recordings showed an ENaC-like channel with an amiloride concentration producing 50% inhibition of 12 μM in M9K cells. This cation channel has a high affinity for extracellular Na+ ions (Km: 53 mM). The ion selectivity of this channel to cations follows the same order as ENaC: Li+ > Na+ > K+. The unitary Li+ conductance was 15 pS in on-cell patches. Four ENaC subunits form a functional Na+ channel when coinjected into Xenopus oocytes. Furthermore, we found that both forskolin and cGMP increased the short-circuit currents in mouse pleural tissues. Taken together, our data demonstrate that the ENaC channels are biochemically and functionally expressed in human pleural mesothelial cells, and can be up-regulated by cyclic AMP and cyclic GMP.
doi:10.1165/rcmb.2008-0166OC
PMCID: PMC2677435  PMID: 18927349
M9K mesothelioma cells; Ussing chamber; protein kinase A; protein kinase G; human primary mesothelial cells

Results 1-4 (4)