Relationship between strain levels and permeability of the plasma membrane in statically stretched myoblasts

Deep tissue injury (DTI) is a life-threatening type of pressure ulcer which initiates subdermally with muscle necrosis at weight-bearing anatomical locations, where localized elevated tissue strains exist. Though it has been suggested that excessive sustained soft tissue strains might compromise cell viability, which then initiates the DTI, there is no experimental evidence to describe how specifically such a process might take place. Here, we experimentally test the hypothesis that macroscopic tissue deformations translated to cell-level deformations and in particular, to localized tensile strains in the plasma membrane (PM) of cells, increase the permeability of the PM which could disrupt vital transport processes. In order to determine whether PM permeability changes can occur due to static stretching of cells we measured the uptake of fluorescein isothiocyanate (FITC)-labeled Dextran (molecular weight = 4 kDa) by deformed vs. undeformed myoblasts, using a fluorescence-activated cell sorting (FACS) method. These PM permeability changes were then correlated with tensile strains in the PM which correspond to the levels of substrate tensile strain (STS) that were applied in the experiments. The PM strains were evaluated by means of confocal-microscopy-based cell-specific finite element (FE) modeling. The FACS studies demonstrated a statistically significant rise in the uptake of the FITC-labeled Dextran with increasing STS levels in the STS ≤ 12% domain, which thereby indicates a rise in the permeability of the PM of the myoblasts with the extent of the applied cellular deformation. The cell-specific FE modeling simulating the experiments further demonstrated that applying average PM tensile strains which exceed 3%, or, applying peak PM tensile strains over 9%, substantially increases the permeability of the PM of myoblasts to the Dextran. Moreover, the permeability of the PM grew rapidly with any further increase in PM strains, though there were no significant changes in the uptake above average and peak PM tensile strain values of 9 and 26%, respectively. These results provide an experimental basis for studying the theory that cell-level deformation-diffusion relationships may be involved in determining the tolerance of soft tissues to sustained mechanical loading, as relevant to the etiology of DTI.