SELECTIVITY OF MAXI CHLORIDE CHANNELS IN THE L6 RAT MUSCLE-CELL LINE

A method is described for the determination of reversal potentials (E( rev)) from variance of single-channel currents vs. membrane potential in ramp pulse mode. The variance-voltage relationship is represented b y a parabola with a minimum of the best fit curve corresponding to the value of E(rev). The reversal potential of the maxi-Cl channels chang es according to the activity of Cl ions at the internal side of the ex cised (inside-out) patch membrane as expressed by the Goldman-Hodgkin- Katz equation with P-A/P-Cl = 0.15, indicating the anion nature of the channel. The relative permeabilities (P-A/P-Cl) found for halide anio ns, were 1.15 for iodide and 1.18 for bromide. The relative permeabili ties measured (P-A/P(C)l) for other anions were 1.13 for nitrate, 0.59 for bicarbonate, 0.60 for methanesulfonate, 0.40 for SO42-, 0.44 for propionate, and 0.10 for glutamate. No significant differences in P-A/ P(C)l of the investigated anions were observed between proliferating m yoballs and quiescent myoblasts. This may mean that the newly formed c hannels possess full-grown selective filters. A close correlation (r = 0.89) was found between the calculated Stokes diameters of the anions under investigation and their relative permeabilities (P-A/P-Cl) The intercept of the best fit line with the abscissa is 7.2 Angstrom (7.2 x 10(-10) m), which may correspond to the diameter of the selectivity filter of the maxi-chloride channel in L6 myoblasts. A similar value o f the channel size was obtained from the relationship between the mini mum cross-sectional areas of the anions and their relative permeabilit ies, P-A/P-Cl. The best fit line intercepts the abscissa at 27.5 Angst rom(2), indicating a pore size similar or equal to 6 Angstrom. The min imum areas were obtained by computer from molecular models of the anio ns.