Figure 3.

Nyquist plot of impedance spectra recorded from a HT-29/B6 cell layer in the absence and presence of EGTA (A and C) and forskolin (B and D). (A) Parameters (mean ± SE) obtained from the three control spectra are R^sub, 5.80 ± 0.13 Ω × cm²; R^epi, 615.5 ± 9.4 Ω × cm²; C^epi, 4.63 ± 0.03 μF/cm². Simultaneously measured fluorescein flux increased from 0.10 ± 0.01 nmol/cm²/h (n = 3) under control conditions by a factor of 20 at an R^epi_−EGTA of 56 Ω × cm². This allowed to calculate R^trans to 1300 Ω × cm², $R_{\mathrm{control}}^{\text{para}}$ to 1170 Ω × cm² and $R_{\mathrm{EGTA}}^{\text{para}}$ to 58.5 Ω × cm² (Eqs. 1 and 2). (B) Parameters (mean ± SE) obtained from the three control spectra: R^sub, 6.90 ± 0.11 Ω × cm²; R^epi, 618.5 ± 12.8 Ω × cm²; C^epi, 3.93 ± 0.04 μF/cm². The simultaneously measured fluorescein flux remained virtually unchanged (control, 0.31 ± 0.05 nmol/cm²/h; forskolin 0.26 ± 0.03 nmol/cm²/h). Without further information, R^trans and R^para cannot be determined from these data. (C and D) Same as A and B, but normalized with respect to R^epi to illustrate, that changes in the paracellular pathway (EGTA application, C) did not alter the general shape of the curves. In contrast, changing the transcellular pathway by forskolin-induced Cl⁻ secretion, greatly alters the general shape of the curves (D).