PROFOUNDLY DIFFERENT CALCIUM PERMEATION AND BLOCKAGE DETERMINE THE SPECIFIC FUNCTION OF DISTINCT CYCLIC NUCLEOTIDE-GATED CHANNELS

Sensory transduction in vertebrate photoreceptors and olfactory sensor y neurons is mediated by cyclic nucleotide-gated (CNG) channels that c onduct mono- and divalent cations. Ca2+ entering the cell through CNG channels intimately controls signaling pathways by regulating several key enzymes. Cloned CNG channels from photoreceptors and olfactory sen sory neurons profoundly differ in their relative Ca2+ permeability, th eir blockage by external divalent cations, and the fraction of current carried by Ca2+. In particular, CNG channels from cone photoreceptors conduct significantly more Ca2+ than those from rod photoreceptors. F urthermore, the current through the olfactory CNG channel is entirely carried by Ca2+ at similar to 3 mM extracellular Ca2+. These results s uggest that a major function of CNG channels is to provide a pathway f or Ca2+ entry.