Ion channel classes in purified olfactory cilia membranes: planar lipid bilayer studies

Ciliary membrane fragment fusion to planar lipid bilayers resulted in the insertion of four ion channel types. cAMP-activated, cation-selective channels could be detected only in the absence of Ca2+ and had a conductance of 23 pS. They exhibited an apparent dissociation constant (Kd) for the cyclic nucleotide of approximately 30 microM and an estimated permeability ratio (PNa/PK) of 2.4. The cAMP cation-selective channel coinserted with a K(+)-selective channel refractory to cAMP, Ca2+, and D-myo-inositol 1,4,5-trisphosphate. This K+ channel was voltage independent and exhibited open-conductance substates of 60 and 112 pS. cAMP was also found to modulate a novel K+ channel with a Kd = 140 microM. It displayed three nearly equally spaced open substates with conductances of 34, 80, and 130 pS. In the absence and in the presence of cAMP the probability of occurrence of the open substates was binomially distributed. A fourth channel type was a Ca(2+)-activated K+ channel with a conductance of 240 pS. It was blocked by charybdotoxin at nanomolar concentrations (Kd = 3 nM). These results add support to the idea that, besides cAMP-activated cation-selective channels, vertebrate chemosensory olfactory membranes possess an arrangement of ion channels.