Inward rectifier current noise in frog skeletal muscle

Inwardly rectifying K+ currents were studied in cut muscle fibres from frogs using the Vaseline-gap voltage-clamp method. Both faces of the membrane were exposed to 120 mM-K+ methylsulphate solution. At small negative potentials, -10 and -21 mV, the current noise spectrum, after subtraction of a control spectrum at the zero current potential, could be fitted by a Lorentzian spectral component, usually with an additional 1/f component, where f is the frequency. At more negative potentials the 1/f component predominated. The zero frequency amplitude of the Lorentzian averaged 2.6 X 10(-24) A2 Hz-1 at -10 mV and 4.6 X 10(-24) A2 Hz-1 at -21 mV, with a mean half-power frequency, fc, of 34 Hz and 45 Hz, respectively. The time constant of the K+ current activation upon hyperpolarization agrees with that calculated from fc, and the Lorentzian disappears upon replacement of external K+ by tetraethylammonium (TEA+) or Rb+. Thus, the Lorentzian component appears to be ascribable to fluctuations originating in the inwardly rectifying mechanism. The noise spectra and macroscopic currents were interpreted by assuming that the inwardly rectifying K+ conductance is proportional to the product of two parameters: ps representing the state of the mechanism that gives rise to the observable macroscopic current relaxations and to the current fluctuations resulting in the observed Lorentzian spectra, and pf describing the instantaneous rectification of the single-channel conductance. Alternatively, pf may represent another mechanism in series with ps, but which fluctuates too rapidly to measure. Using this model the limiting single-channel conductance, gamma, was found to be approximately 9 pS. The corresponding specific density of channels is about 1 micron-2, assuming uniform distribution over all regions of the membrane. A preliminary value for gamma ( DeCoursey & Hutter , 1982) was derived without consideration of instantaneous rectification. Systematic errors in these results due to voltage decrement in the T-tubules are evaluated in an Appendix, and are found to be tolerably small in the voltage range studied.