Using dielectric spectroscopy to detect thermal hysteresis in frog muscle

Dielectric spectroscopy has been used to characterize a wide variety of physiological changes in biological systems. We show here how dielectric spectra taken during a temperature cycle can indicate the presence of subtle physiological changes in the tissue. We measured the impedance spectra of frog gastrocnemius muscle, in vivo, in the frequency range 1 Hz to 1 MHz for temperatures from about 10 °C up to a maximum value, Tmax, and back down to about 10 °C. The range of values for Tmax was from 20 °C to 42.5 °C. For frequencies below about 50 kHz we observed hysteresis for thermal cycling above Tmax = 35 °C. The impedance at 10 °C at the end of the run was significantly lower than the impedance at 10 °C at the start of the run, even after corrections had been made for the change in impedance with time of the exposed muscle. For Tmax = 42.5 °C thermal hysteresis was apparent at all frequencies. We speculate that the impedance decrease was due to subtle changes in the membrane glycocalyx above 35 °C and to membrane leakage above 40 °C. We conclude that thermal cycling of impedance spectra can provide a sensitive measure of structural change for any physical system, not just biological tissue.