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August Krogh Institute, University of Copenhagen, Denmark.

1. Mitochondria-rich (MR) cells in free suspension were obtained by collagenase and trypsin treatments of toad skin epithelium and studied by whole-cell voltage clamp and membrane current fluctuation analysis. 2. Cells studied with a 100 microM amiloride and 5 mM Ba2+ Ringer solution on the outside and 10 mM Cs+ in the pipette generated large membrane currents with reversal potentials varying in a Nernstian way with pipette [Cl-]. 3. The membrane chloride currents were activated in excess of Goldman-Hodgkin-Katz rectification by cell depolarization and clamping to positive cell potentials (VC). The resulting Cl- permeability was presented as an S-shaped function of membrane potential with half-maximal activation in the range 0 mV < VC < 50 mV. 4. The power density spectrum of Cl- current fluctuations could be fitted with a single Lorentzian component with a corner frequency, fc, of 34.9 +/- 2.6 Hz, and a low frequency asymptote, S(o), or 14.6 +/- 1.3 pA2 s per cell (mean +/- S.E.M.; VC = 25 mV, ECl = 0 mV, n = 6). 5. The lower-limit single channel conductance, gamma Cl(1 - Po), was 128 +/- 9 pS (VC = 25 mV, n = 6), where Po is the open channel probability. With Po = 0.5, this result indicates that Cl- channels of large unitary conductance (200-300 pS) are present in the mitochondria-rich cell membrane.