The series resistance, Rs, is a source of error while measuring large currents in oocytes. We have designed an electronic instrument that can be used in conjunction with a standard TEVC amplifier TURBO TEC series to measure and display Rs automatically.

The instrument is based on the injection of symmetrical current pulses of 10 µA and a few kHz around the holding or resting potential of the cell. The membrane potential deviation that appears on the positive slope of the pulse is proportional to Rs and is measured using high precision sample-and-hold circuits controlled by a timing unit that is synchronized from the injection pulses applied.

Rs is displayed on a digital meter with a resolution of 10 Ω and can be also stored on a computer. Rs measurement can be started manually or through a TTL input. After the measurement the stored Rs value is displayed continuously until the next measurement is started. Therefore, measurement can be automated easily using standard lab software.

The high frequency, as well as the symmetry of the amplitude and duration of the applied pulses is very important in order to avoid DC changes of membrane potential that can lead to channel openings.

This instrument is a useful tool that helps to avoid erroneous TEVC recordings in Xenopus oocytes.