The study of the electrical properties of epithelial tissues involves measuring of the transepithelial resistance. A solution for the measurement system contains a Ussing chamber with the voltage and current electrodes connected to an EVC3 preamplifier followed by an EVC-4000 voltage clamp device (WPI).
The voltage and current signals are acquired by a Lab-Trax 8/16 device connected to our acquisition server WPI10a
WPI10a quick overview
Our server allows acquisition of analog input signals from Lab-Trax 8/16 at a maximum sampling rate of 20 kS/s per channel. The digital processing allows selecting for each channel the most appropriate configuration (sampling rate, high-pass, low-pass or notch 50/60 Hz filtering). Each input signal can be invert or an offset can be added and a quick feed-back is displayed about the range usage (see Newsletter #24).
The transepithelial voltage Vte and current Ite are acquired by WPI10a (figure 1). 8 analog inputs are available, so 4 chambers can be acquired in the same time. The high frequency noise is reduced by activating the low-pass filter; the power interference is reduced by activating the 50/60 Hz filter.
The transepithelial resistance Rte is calculated by the formula below:
where V2 – V1 is the voltage difference, I2 – I1 is the current difference induced by the clamping and kea is a coefficient that takes into account the exposure area.
The configuration to calculate and display the transepithelial resistance in real time is shown in figure 1.
The voltage and current signals are scaled by OPR10a1 and OPR10a2. THR10a detects the voltage transition between V1 and V2 due to the clamping. STA10a1 and STA10a2 parameters are adjusted to calculate mean values on appropriate intervals (figure 2) for the two levels of the voltage (V1, V2). STA10a3 and STA10a4 calculate mean values on appropriate intervals for the two levels of the current (I1, I2).
The levels are interpolated by ITP10a1…4 to produce continuous signals making possible to apply elementary arithmetic, like follows:
- OPR10a5 calculates the voltage difference,
- OPR6 calculates the current difference and
- OPR10a7 calculates the differences ratio.
The exposure area coefficient can also be taken into account in the parameter applied to the division in OPR10a7.
For now the control of EVC-4000 should be done manually, this will be implemented in the next commercial version of WPI10a server.