  Protein mechanosensitive channels (MS) are activated by tension transmitted
through the lipid bilayer. We propose a theory of lateral stress relaxation in
a bilayer lipid membrane exposed to external pressure pulse in the patch-clamp
experimental setting. It is shown that transfer of lipid molecules into a
strained region is thermodynamically advantageous due to local decrease of the
stress. Considered stress relaxation mechanism may explain recent experimental
observations (Davidson and Martinac 2003) of adaptation of MscL, bacterial
mechanosensitive channel of large conductance, to sustained membrane stretch.
Lateral stress relaxation in the monolayer, which controls the gating of MscL,
triggers thermally activated transition of the open channels back to the closed
state ("adaptation"). We evaluate the contribution of the hydrophobic mismatch
between MS channel and lipid bilayer to the energy barrier separating open and
closed states. Then, using the MscL thermodynamic model (Sukharev et. al,
1999), we estimate characteristic adaptation times at room temperature to be of
the order of seconds, well in the range of the experimental data (Davidson and
Martinac 2003). Estimated propagation time of the initial channel-opening
stress over the whole membrane is 4-5 orders of magnitude shorter.
