Optical force measurements on pulmonary surfactant:

We have used optical tweezers to study mechanical aspects ion the exocytotic release of pulminary surfactant of the specialized alveolar type II cells.

Movie (3.5MB)
Fig. 1 Using optical tweezers to measure the spring constant of surfactant material still attached to the fusion pore (cytosol stained in green with calcein, surfactant in orange with FM 1-43). The movie shows the amazing elasticity of the thread of surfactant when pulled out more than 2 cell diameters.

At constant trapping force, the probability to expand contents of the surfactant-vesicles (the "lamellar bodies"), i.e., to ''pull'' surfactant into the extracellular fluid, increased with time after fusion of the vesicle with the plasma membrane. This represents some evidence that the fusion pore expansion in these cells is slow. Since the hydrophobic surfactant did not disintegrate in the extracellular space, this method permitted for the first time the determination of elastic and recoil properties of the macromolecular complex, yielding a spring constant of ;12.5 pN/mm. This summary, this is the first functional evidence that release of the surfactant material is mechanically impeded and occurs in an ''all-or-none'' fashion.


W. Singer, S. Bernet, N. Hecker, M. Ritsch-Marte: 3-dimensional force calibration of optical tweezers, J. Mod. Opt. 47, 2921-2931 (2000).

W. Singer, M. Frick, Th. Haller, S. Bernet, P. Dietl, M. Ritsch-Marte: Mechanical forces impeding exocytotic surfactant release revealed by optical tweezers, Biophys. J. 84, 1344-1351 (2003)