Actin modifies Ca2+ block of epithelial Na+ channels in planar lipid bilayers

The mechanism by which the cytoskeletal protein actin affects the conductan ce of amiloride-sensitive epithelial sodium channels (ENaC) was studied in planar lipid bilayers. In the presence of monomeric actin, we found a decre ase in the single-channel conductance of alpha -ENaC that did not occur whe n the internal [Ca2+](free) as buffered to <10 nM. An analysis of single-ch annel kinetics demonstrated that Ca2+ induced the appearance of long-lived closed intervals separating bursts of channel activity, both in the presenc e and in the absence of actin. In the absence of actin, the duration of the se bursts and the time spent by the channel in its open, but not in its sho rt-lived closed state, were inversely proportional to [Ca2+]. This, togethe r with a lengthening of the interburst intervals. translated into a dose-de pendent decrease in the single-channel open probability. In contrast, a [Ca 2+]-dependent decrease in a-ENaC conductance in the presence of actin was a ccompanied by lengthening of the burst intervals with no significant change s in the open or closed (both short- and long-lived) times. We conclude tha t Ca2+ acts as a "fast-to-intermediate" blocker when monomeric actin is pre sent, producing a subsequent attenuation of the apparent unitary conductanc e of the channel.