Ion permeation and selectivity of wild-type recombinant rat CNG (rOCNC1) channels expressed in HEK293 cells

The permeation properties of adenosine 3', 5'-cyclic monophosphate (cAMP)-a ctivated recombinant rat olfactory cyclic nucleotide-gated channels (rOCNC1 ) in human embryonic kidney (HEK 293) cells were investigated using inside- out excised membrane parches. The relative permeability of these rOCNC1 cha nnels to monovalent alkali cations and organic cations was determined from measurements of the changes in reversal potential upon replacing sodium in the bathing solution with different test cations. The permeability ratio of Cl- relative to Na+ (P-Cl/P-Na) was about 0.14, confirming that these chan nels are mainly permeable to cations. The se sequence of relative permeabil ities of monovalent alkali metal ions in these channels was P-Na greater th an or equal to P-K > P-Li > P-Cs greater than or equal to P-Rb, which close ly corresponds to a high-strength field sequence as previously determined f or native rat olfactory receptor neurons (ORNs). The permeability sequence for organic cations relative to sodium was P-NH3OH > P-NH4 > P-Na > P-Tris > P-Choline > P-TEA again in good agreement with previous permeability rati os obtained in native rat ORNs. Single-channel conductance sequences agreed surprisingly well with permeability sequences. These conductance measureme nts also indicated that, even in asymmetric bi-ionic cation solutions, the conductance was somewhat independent of current direction and dependent on the composition of both solutions. These results indicate that the permeabi lity properties of rOCNC1 channels are similar to those of native rat CNG c hannels, and provide a suitable reference point for exploring the molecular basis of ion selectivity in recombinant rOCNC1 channels using site-directe d mutagenesis.