K+ Conduction description from the low frequency impedance and admittance of squid axon

Abstract

The form of power spectra of K⁺ conduction fluctuations in patches of squid axon suggested that K⁺ conduction kinetics are higher than first order (Fishman, Moore & Poussart, 1975,J. Membrane Biol. 24:305). To obtain an alternative description of ion conduction kinetics consistent with spontaneous fluctuations, the complex impedance and admittance of squid (Loligo pealei) axon were measured at low frequencies (1–1000 Hz) with a four electrode system using white Gaussian noise as a stochastic perturbation. As predicted from the spontaneous noise measurements, a low frequency impedance feature is observed between 1 and 30 Hz which is voltage and temperature dependent, disappears after substantial reduction in [K _i ⁺ ], and is unaffected by the state of Na⁺ conduction or active transport. These measurements confirm and constitute strong support for the patch noise measurements and interpretations. The linearized Hodgkin-Huxley (HH) equations do not produce the low frequency feature since first order ion conduction kinetics are assumed. Computation of diffusion polarization effects associated with the axon sheath gives a qualitative account of the low frequency feature, but the potential dependence is opposite to that of the data. Thus, K⁺ conduction kinetics in the axon are not adequately described by a single first order process. In addition, significant changes inHH parameter values were required to describe the usual impedance (resonance) feature inLoligo pealei axon data.