ANALYSIS AND MODELING OF THE PRIMARY CILIUM BENDING RESPONSE TO FLUIDSHEAR

Since a nonmotile, primary(9 + 0) cilium projects from most mammalian kidney epithelial cells into the tubule lumen, where it is exposed to fluid motion, the present study examined primary cilium response to fl uid shear stress. The reversible, large-angle bending of the primary c ilium upon exposure to fluid shear forces (10(-11)-10(-10) N . m(2) = 10(-8)-10(-7) dyn/cm) was characterized in vitro using videomicroscopi c side views of PtK1 cells, and the cilium was then mathematically mod eled as a cantilevered beam. The flexural rigidity of the primary cili um was calculated to be 3.1 +/- 0.8 x 10(-23) N . m(2) with a correcte d quadruple integration approach and 1.4-1.6 x 10(-23) N . m(2) with t he ''heavy elastica'' theory. Comparison of theoretical profiles to th e experimental bending responses of cilia established the validity of the ''heavy elastica'' model; this model, in turn, was used to predict primary cilium bending behavior under representative conditions in th e rat nephron. The results of the study are consistent with the hypoth esis that primary cilia serve a mechanosensory function in kidney epit helial cells.