32] Impedance analysis in tight epithelia

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Publisher Summary This chapter discusses the measurement of transepithelial impedance, as well as the subsequent quantitative analysis using equivalent epithelial circuits. The techniques described is used to investigate ionic transport mechanisms in a number of different preparations, notably sodium transport in amphibian and mammalian urinary bladder, gastric acid secretion in amphibian gastric mucosa, sodium and potassium transport in mammalian descending colon, chloride secretion in amphibian cornea, and proton secretion in renal epithelia. Measurements of impedance are generally performed at a range of different frequencies, typically between 0.001 and 10 kHz, logarithmically spaced in frequency. The analyses involve the extraction of a number of membrane parameters using curve-fitting techniques, the number of frequencies must be sufficient to obtain statistical significance. Two important points must be stressed regarding the measurement of impedance. As impedance describes the steady-state voltage response to an applied sinusoidal current, a period of time must elapse between changing the applied current and measuring the resulting voltage, otherwise the resulting voltage will be contaminated by the transient response to the change in current. Also, as the analysis of impedance is done using an equivalent circuit composed of linear circuit elements, one must ensure that the voltage response is independent of applied-current amplitude.