  We propose a mechanism for mechanical regulation at the membrane of living
cells, based on the exchange of membrane area between the cell membrane and a
membrane reservoir. The reservoir is composed of invaginated membrane
microdomains which are liable to flatten upon increase of membrane strain,
effectively controlling membrane tension. We show that the domain shape
transition is first order, allowing for coexistence between flat and
invaginated domains. During coexistence, the membrane tension is controlled by
the domains elasticity and by the kinetics of the shape transition. We show
that the tension of the plasma membrane of living cells is inherently transient
and dynamical, and that valuable insights into the organization of the cell
membrane can be obtained by studying the variation of the cell membrane tension
upon mechanical perturbation.
