Membrane Stretch Affects Gating Modes of a Skeletal Muscle Sodium Channel

Abstract

The α subunit of the human skeletal muscle Na + channel recorded from cell-attached patches yielded, as expected for Xenopus oocytes, two current components that were stable for tens of minutes during 0.2 Hz stimulation. Within seconds of applying sustained stretch, however, the slower component began decreasing and, depending on stretch intensity, disappeared in 1–3 min. Simultaneously, the faster current increased. The resulting fast current kinetics and voltage sensitivity were indistinguishable from the fast components 1) left after 10 Hz depolarizations, and 2) that dominated when α subunit was co-expressed with human β1 subunit. Although high frequency depolarization-induced loss of slow current was reversible, the stretch-induced slow-to-fast conversion was irreversible. The conclusion that stretch converted a single population of α subunits from an abnormal slow to a bona fide fast gating mode was confirmed by using gigaohm seals formed without suction, in which fast gating was originally absent. For brain Na + channels, co-expressing G proteins with the channel α subunit yields slow gating. Because both stretch and β1 subunits induced the fast gating mode, perhaps they do so by minimizing α subunit interactions with G proteins or with other regulatory molecules available in oocyte membrane. Because of the possible involvement of oocyte molecules, it remains to be determined whether the Na + channel α subunit was directly or secondarily susceptible to bilayer tension.