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1 CUNY Graduate School and Center for Biomedical Engineering, the City College of the City University of New York, New York 10031; and 2 Department of Physiology, Weill Medical College of Cornell University, New York, New York 10021
In the proximal tubule of the kidney, Na+ and
HCO3
reabsorption vary proportionally with changes in
axial flow rate. This feature is a critical component of
glomerulotubular balance, but the basic mechanism by which the tubule
epithelial cells sense axial flow remains unexplained. We propose that
the microvilli, which constitute the brush border, are physically
suitable to act as a mechanosensor of fluid flow. To examine this
hypothesis quantitatively, we have developed an elastohydrodynamic
model to predict the forces and torques along each microvillus and its resulting elastic bending deformation. This model indicates that: 1) the spacing of the microvilli is so dense that there is
virtually no axial velocity within the brush border and that drag
forces on the microvilli are at least 200 times greater than the shear force on the cell's apical membrane at the base of the microvilli; 2) of the total drag on a 2.5-µm microvillus, 74% appears
within 0.2 µm from the tip; and 3) assuming that the
structural strength of the microvillus derives from its axial actin
filaments, then a luminal fluid flow of 30 nl/min produces a deflection
of the microvillus tip which varies from about 1 to 5% of its 90-nm
diameter, depending on the microvilli length. The microvilli thus
appear as a set of stiff bristles, in a configuration in which changes in drag will produce maximal torque.
glomerulotubular balance; mechanosensory mechanism; actin cytoskeleton; microvilli force and torque
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