SQUID-Based Biosensor for Probing Ion Transporters in Cell Suspensions and Tissue

Abstract

We have developed a SQUID-based technique to probe membrane-bound proteins, such as cation transporters. The method involves excitation of a biological sample with a sinusoidal electric field (0-5 V/cm, 10-300 Hz), detection, with a SQUID magnetometer, of the magnetic field produced by ac currents in the tissue or cell suspension, and calculation of the Fast Fourier Transform (FFT) of the measured response to obtain a spectrum. We find that the harmonic response of budding yeast cells (S. cerevisiae, 10 8 cells/ml) is sensitive to sodium metavanadate, an inhibitor of a proton pump known as H + -ATPase, glucose, a substrate of H + -ATPase, and gramicidin, a membrane depolarizer. SQUID-based field and frequency dependent harmonic response measurements are also made of a live earthworm (Lumbricus terrestris). Use of the SQUID significantly reduces spurious harmonics and distortion of the excitation field by the measurement electrodes.

Harmonic Analysis of Neuronal Membranes and Tissue Using SQUIDs

“…The third harmonic is maximal near 1.16 V/cm. Similar field-dependent third harmonic behavior was observed in yeast suspensions [1], [8], [9]. The first harmonic response of the earthworm increased linearly with amplitude (not shown).…”

“…We have previously demonstrated that remote harmonic detection using SQUIDs reduces polarization impedance and field distortion at pickup electrodes [8], [9]. Electrode polarization can be modeled as equivalent complex impedance formed by an electrical double layer containing polar molecules at the interface between the electrodes and biological material resulting in spurious signals especially at low frequencies [13].…”

“…We have previously reported the use of SQUIDs in harmonic response measurements of yeast in suspension [8] and tissue in the earthworm Lumbricus terrestris [9]. Candidates for the earthworm's harmonic response to electrical excitation include gap junctions between tissue cells, ion channels in muscle, median giant, and lateral giant nerve fibers.…”