FIGURE 4.

(A) Diffusion coefficients for 524-Da HPTS, and for 3- and 10-kDa dextran molecules measured by Kovbasnjuk et al. (6) in the LIS of MDCK cells (squares connected by dotted line), and in free solution (circles). For a 10-kDa dextran with D_out = D_FREE = 98 μm²/s, hindered diffusion in the LIS was observed (D_LIS = 18 μm²/s) (6). For HB-EGF of size 22 kDa, we estimated a LIS diffusion coefficient of D_LIS = 1.8 μm²/s (down-triangle), an order-of-magnitude less than that measured for a 10-kDa dextran; outside the LIS the HB-EGF free solution value was estimated as D_out = D_FREE = 75 μm²/s (up-triangle), based on extrapolation of the measured free solution values, assuming D_FREE ∝ (mol. wt.)^−1/3. (B) Incorporating the estimated HB-EGF diffusion coefficients, we examined three cases of LIS collapse when the LIS width decreased linearly to 15% of its initial, precollapse value (from 1.5 to 0.225 μm) over t_collapse = 60, 600, and 1200 s. (C) Additional cases, where the same LIS collapse (to 15% of the initial width) occurred in t_collapse = 1 and 10 s (solid lines). The 60-s collapse case from panel B (dashed line) is shown for comparison. For all cases we assumed that the shedding rate is constant and equal to q = 10 molecules/cell/min.