Transepithelial Electrical Resistance (TER) Measurements

Transepithelial Electrical Resistance (TER) Measurements

Important notes:

• All work is performed under sterile conditions in a biosafety cabinet.
• It is ideal to record TER measurements 1-2 days after media change. Be consistent when taking measurements- for different lines, compare measurements that were taken after same time period following media change.
• Measurements are done with voltohmmeter set to Ohms (Ω).
• The electrodes are kept dipped in ethanol when not in use between measurements.

1. Prepare one 50 mL tube containing 100% ethanol, one 50 mL tube containing sterile Milli-Q water, and one 50 mL tube containing 1X HBSS. Maintain tubes and its contents under sterile conditions.
2. Open box containing chopstick electrodes (STX2, World Precision Instruments) in the biosafety cabinet. Remove the covers and rub the tip of each electrode with the provided sandpaper. Connect electrodes to the epithelial voltohmmeter (EVOM2, World Precision Instruments) by plugging in the attached wire, and then turn the unit ‘ON’.
3. Set the Voltohmmeter switch to measure “Ohms”.
4. Prior to performing readings, touch electrodes together and ensure that the instrument is reading 0 Ω.
5. Insert the electrodes into the solutions in order as follows:
   1) Ethanol
   2) Sterile Water
   3) 1X HBSS
   Make sure there is no HBSS dripping from the electrode prior to placing the electrodes in transwells.
6. Measure the transepithelial resistance of each transwell culture by placing the shorter prong in the transwell and the longer prong in the bottom well. The electrode tips should be completely submerged in media. Once stabilized, record the value. Takecare not to touch the surface of the RPE monolayer.
   Notes:
   - If electrodes are not measuring resistance properly, it is okay to slightly tilt the plate in order to better immerse the electrodes in media.
   - To avoid scraping cells off of monolayer, make sure to not touch the RPE monolayer with the electrode. Any gap in monolayer leads to loss of good TER measurements.
7. Continue with measurements, cleaning the electrodes by placing them in the same order of solutions, as listed above in step 5.
8. A cell-free transwell insert with media only is used to measure the background resistance for the transwell membrane.
9. When finished, rinse off probes with ethanol and replace the covers.
10. The following equation is used obtain net TER values:
    Net TER value (Ω cm2) =       (Recorded TER value of RPE culture – background resistance of transwell insert) x surface area of the transwell insert

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