NANOPHYSIOLOGY OF CELLS, CHANNELS AND NUCLEAR PORES - Life at the Nanoscale: Atomic Force Microscopy of Live Cells - page 127

Quantiication of the recognition events revealed values of six and two
CFTR molecules per μm 2 for non-CF and CF erythrocytes, respectively.
Extrapolating the results to the total erythrocyte surface area of 130 μm 2
results in about 800 CFTR/erythrocyte for non-CF and about 250 CFTR/
erythrocyte for CF samples. These values are slightly higher but still in
good correlation to the observation made with Qdot-labelled antibodies to
quantify CFTR on erythrocyte membranes. Qdots are several nanometres
in size, and therefore they could cause sterical hindering when individual
CFTR molecules are located in close vicinity. With the TREC technique,
sterical hindering does not occur, possibly explaining the slightly higher
number of CF recognition sites. Clearly, the TREC has an advantage over the
labelling method where direct visualization of the molecule is not possible
since Qdots lay on or very close to the target molecule.
In conclusion, these studies show that AFM experiments on isolated
plasma membranes not only allow quantiication and localization of
membrane proteins but also provide insight in their dynamics at a single-
molecule level.

6.2 ENDOTHELIAL CELLS

6.2.1 Mechanodynamics of Vascular Endothelial Cells

In physics, the term stiffness is clearly deined: “Stiffness is a measure of
the resistance offered by an elastic body to deformation”. Importing this
term into cellular physiology stiffness indicates a force (Newton) necessary
to compress a cell for a certain distance (metre). Application of force
happens to most tissues in real life, particularly to vascular endothelium.
Hemodynamic forces, born by the beating heart, generate shear stress at
the endothelial surface. It is inevitable that the apical cell surfaces undergo
reversible deformations. This mechanical stimulus triggers the activation of
the endothelial nitric oxide synthase (eNOS) and the release of NO. The latter
diffuses to the adjacent vascular smooth muscle cells, leading to vasodilation.
This regulatory mechanism distributes the blood in the organism according
to the metabolic demands and, at the same time, maintains systemic blood
pressure within physiological limits. Therefore, it is obvious that the same
shear force should cause a stiff (less deformable) cell to release less NO as
compared with a soft (more deformable) cell. This leads to the conclusion
that endothelial mechanical stiffness is a key parameter in the control of
local blood supply and arterial blood pressure.