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Selectivity for cations in potassium and gramicidin channels of the muscle fibre membrane.

The sequences of conductance ratios (GX/GK) and permeability ratios (PX/PK) for monovalent cations (X = Rb+, Cs+, Na+, Li+, NH4+) have been studied in frog skeletal muscle before and after addition of the channel-forming antibiotic gramicidin A. The experiments were carried out under current clamp conditions using a double sucrose gap technique. For inwardly rectifying potassium channels the selectivity measured by membrane conductance ratios was K+ greater than Rb+ greater than Cs+ greater than NH4+ greater than Na+ greater than Li+. In gramicidin channels of the same muscle fibre both the permeability and the conductance ratios had the identical sequence NH4+ greater than Cs+ greater than Rb+ greater than K+ greater than Na+ greater than Li+. However, after external addition of Tl+ (2.5 x 10(-3) M to 20 x 10(-3) M) conductance and permeability ratios were different. The effect may result from specific binding of Tl+ in gramicidin channels.