Renal tissue impedance: responses of the real and imaginary component to experimental variations in medullary...

In order to assess the renal corticomedullary electrolyte gradient, electrical impedance (zeta) and phase angle (phi) were measured in the in-situ kidney of anaesthetized rats. A set of platinum/iridium needle electrodes was used, and the frequency (f) of the measuring current was varied between 0.5 and 50 kHz. zeta and its imaginary component (chi c) fell sharply as f increased from 0.5 to 3.5 kHz, and then decreased slowly or stabilized, whereas the real component (R) decreased progressively over the entire f range. Ion concentration in the renal medulla was experimentally lowered by administration of furosemide or hypertonic mannitol or by inducing a haemorrhage, and raised by infusing hypertonic NaCl solution. Both R and chi c varied inversely with tissue electrolyte concentration. Raising f from 3.5 to 10 and further to 20 kHz did not significantly amplify the changes in zeta. Within this frequency range R was not a more sensitive index of tissue electrolyte changes compared with overall zeta-value. The results verify previous empirical data indicating that changes in medullary tissue electrolytes can be dynamically estimated by monitoring tissue impedance.