Effects of mechanical loading on ion transport through lipid bilayer membranes

A variety of cells including neurons, endothelial cells, bone cells, and auditory hair cells exhibit physiologic responses to dynamic mechanical loading. This research is an investigation of the first stage of these responses, the alterations in the ionic conductance of a cell membrane caused by varying the mechanical tension imposed upon it. Cell membranes were modeled by lipid bilayers and gramicidin channels.

A system was developed which allows study of the effects of uniform hydrostatic tension on the conductance of lipid bilayer membranes under variable strain rate conditions. The lipid bilayer is formed on a patch-clamp pipette so that ionic channels can be incorporated into the membrane and single channels isolated. Tension is applied by a miniature pump and monitored by a pressure sensor, both of which are attached to the top of the pipette. The pump consists of a pressure source applied to a small compliant diaphragm. Voltage clamp circuitry is used to measure membrane conductance. By using a vacuum instead of a positive pressure source, this system can also be used to examine the effects of mechanical strain on patches of actual cell membranes.

The conductance of single gramicidin channels was not affected by membrane tension. However, increasing membrane tension resulted in increased conductance of the lipid bilayer in those cases in which the initial, unstressed, conductance exceeded 3 $\times$ 10$\sp6$ pS/cm$\sp2$. Bilayers which were less conductive did not exhibit this response.

Electrolyte filled pores in the bilayer are postulated to account for the strain dependent conductance observed. The presence of these pores results in an elevated initial membrane conductance which is further increased when the membrane is stretched and the pores widen. Comparison of the increases in conductance to increases in membrane area indicate that the pores resemble cracks more than round holes. Pore dimensions are estimated to be on the order of a few angstroms.