Objectives: 

Epithelial Na+ channels (ENaCs) belong to a highly conserved protein family whose members were originally identified to be part of a mechanosensor in C. elegans and ENaCs were found to be mechanosensitive as well. Five homologous subunits (alpha, beta, gamma, delta, epsilon) are identified and functional channels consist of a heterotrimer (abg, dbg or ebg). The present study characterizes the effect of shear force (SF) on abg and dbg ENaC in dependence of proteolytic cleavage and external pH.

Materials: 

Human abg and dbg ENaCs were heterologously expressed in Xenopus laevis oocytes. The activity of ENaCs was measured as transmembrane currents (IM) by the two-electrode voltage-clamp technique. For SF application a customized perfusion chamber in combination with a pressure controlled perfusion system was used. This setup enabled the generation of a laminar fluid stream that was used for shear flow application (0.001 – 1.3 dyn/cm2).

Results: 

SF increased amiloride-sensitive IM of oocytes expressing either abg or dbg. abg ENaC was more sensitive in response to shear force, compared with dbg. Proteolytic cleavage strengthened the SF-response of abg ENaC, whereas the SF-response of dbg ENaC was attenuated. Further, the mechanosensitive response of abg and dbg differed in dependence of external pH (pH 5-9). Acidic pH values (pH 5 and 6) decreased the SF effect of abg ENAC as well as dbg ENaC. In contrast to this, alkaline pH decreased the SF effect of abg ENaC but potentiated the SF-response of dbg ENaC.

Conclusions: 

abg and dbg ENaC are activated by SF. Mechanosensitivity of ENaC is modulated by external factors such as proteases and pH. Differences in the SF-response of abg and dbg after proteolytic cleavage and by different pH indicate that the alpha and delta subunit play a crucial role for mediating the mechanosensitive behavior of ENaCs.