Question: 

Epithelial sodium channels (ENaC) consist of four homologous subunits (a, b, g, d) which form a heterotrimer (abg or dbg). The a-subunit appears in apical membranes of epithelia like lung, kidney or intestine. The d-subunit appears in two isoforms (d1 and d2) and was first identified in brain. ENaCs activity is regulated by pH-values and shear force (SF). A linkage between both factors is unknown. Therefore we investigated the effect of SF on ENaCs under different pH-values.

Methodology: 

Human abg, d1bg or d2bgENaCs were heterologously expressed in Xenopus oocytes. The activity of ENaCs was measured as transmembrane currents (I_M) of ENaC expressing oocytes by the two-electrode voltage-clamp technique. SF with integral pH-values from pH4 to pH10 were applied by a Pasteure pipette which was placed in front of the oocyte and perfused to generate a fluid stream.

Results: 

SF generally increased amiloride-sensitive I_M of oocytes expressing all subunit combinations by approx. 25% (pH 7.4). Basic pH-values decreased I_M, whereas acidic pH-values increased I_M an all channel combinations tested. Further, the SF-effect was decreased (compared with pH 7.4) in the presence of acidic pH. In contrast to this, the SF-effect was increased with basic pH. The EC₅₀ for pH stimulation of SF-effects was determined with pH-values of 7.9 for d1bgENaC and 8,3 for d2bgENaC.

Conclusion: 

SF increases the activity of abg, d1bg and d2bgENaC. This effect is depended on extracellular pH. Thus, protonation of protein residues located in the extracellular loops of ENaC, might be involved in mechanosensitve channel gating.