Ionic selectivity of Ih channels of tiger salamander rod photoreceptors was investigated using whole-cell voltage clamp. Measured reversal potentials and the Goldman-Hodgkin-Katz voltage equation were used to calculate permeability ratios with 20 mM K+ as a reference. In the absence of external K+, Ih is small and hard to discern. Hence, we defined Ih as the current blocked by 2 mM external Cs+. Some small amines permeate Ih channels, with the following permeability ratios (PX/PK):NH4+, 0.17; methylammonium, 0.06; and hydrazine, 0.04. Other amines are tially impermeant: dimethylammonium (< 0.02), ethylammonium (< 0.01), and tetramethylammonium (< 0.01). When K+ is the only external permeant ion and its concentration is varied, the reversal potential of Ih follows the Nernst potential for a K+ electrode. Ih channels are also permeable to other alkali metal cations (PX/PK): T1+, > 1.55; K+, 1; Rb+, > 0.55; Na+, 0.33; Li+, 0.02. Except for Na+, the relative slope conductance had a similar sequence (GX/GK): T1+, 1.07; K+, 1; Rb+, 0.37; NH4+, 0.07; Na+, 0.02. Based on permeabilities to organic cations, the narrowest part of the pore has a diameter between 4.0 and 4.6 A. Some permeant cations have large effects on the gating kinetics of Ih channels; however, permeant cations appear to have little effect on the steady-state activation curve of Ih channels. Lowering K+ or replacing K+ with Na+ reduces the maximal conductance of Ih but does not shift or change the steepness of its voltage dependence. With ammonium or methylammonium replacing K+ a similar pattern is seen, except that there is a small positive shift of approximately 10 mV in the voltage dependence.

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