The interaction of Na+ and K+ in the pore of cyclic nucleotide-gated channels

The permeability ratio between K+ and Na+ ions in cyclic nucleotide-gated c hannels is close to 1, and the single channel conductance has almost the sa me value in the presence of K+ or Na+. Therefore, K+ and Na+ ions are thoug ht to permeate with identical properties. In the alpha -subunit from bovine rods there is a loop of three prolines at positions 365 to 367. When proli ne 365 is mutated to a threonine, a cysteine, or an alanine, mutant channel s exhibit a complex interaction between K+ and Na+ ions. Indeed K+, Rb+ and Cs+ ions do not carry any significant macroscopic current through mutant c hannels P365T, P365C and P365A and block the current carried by Na+ ions. M oreover in mutant P365T the presence of K+ in the intracellular (or extrace llular) medium caused the appearance of a large transient inward (or outwar d) current carried by Na+ when the voltage command was quickly stepped to l arge negative (or positive) membrane voltages. This transient current is ca used by a transient potentiation, i.e., an increase of the open probability . The permeation of organic cations through these mutant channels is almost identical to that through the wild type (w.t.) channel. Also in the w.t. c hannel a similar but smaller transient current is observed, associated to a slowing down of the channel gating evident when intracellular Na+ is repla ced with K+. As a consequence, a rather simple mechanism can explain the co mplex behavior here described: when a K+ ion is occupying the pore there is a profound blockage of the channel and a potentiation of gating immediatel y after the K+ ion is driven out. Potentiation occurs because K+ ions slow down the rate constant K-off controlling channel closure. These results ind icate that K+ and Na+ ions do not permeate through CNG channels in the same way and that K+ ions influence the channel gating.