Properties of cyclic nucleotide-gated channels mediating olfactory transduction. Activation, selectivity, and blockage.
Cyclic nucleotide-gated channels (cng channels) in the sensory membrane of olfactory receptor cells, activated after the odorant-induced increase of cytosolic cAMP concentration, conduct the receptor current that elicits electrical excitation of the receptor neurons. We investigated properties of cng channels from frog and rat using inside-out and outside-out membrane patches excised from isolated olfactory receptor cells. Channels were activated by cAMP and cGMP with activation constants of 2.5-4.0 microM for cAMP and 1.0-1.8 for cGMP. Hill coefficients of dose-response curves were 1.4-1.8, indicating cooperativity of ligand binding. Selectivity for monovalent alkali cations and the Na/Li mole-fraction behavior identified the channel as a nonselective cation channel, having a cation-binding site of high field strength in the pore. Cytosolic pH effects suggest the presence of an additional titratable group which, when protonated, inhibits the cAMP-induced current with an apparent pK of 5.0-5.2. The pH effects were not voltage dependent. Several blockers of Ca2+ channels also blocked olfactory cng channels. Amiloride, D 600, and diltiazem inhibited the cAMP-induced current from the cytosolic side. Inhibition constants were voltage dependent with values of, respectively, 0.1, 0.3, and 1 mM at -60 mV, and 0.03, 0.02, and 0.2 mM at +60 mV. Our results suggest functional similarity between frog and rat cng channels, as well as marked differences to cng channels from photoreceptors and other tissues.
Frings
S
S
Department of Physiology, Universität des Saarlandes, Homburg/Saar, Germany.
Lynch
J W
JW
Lindemann
B
B
D000818:Animals; D002628:Chemoreceptor Cells / Q000502:physiology; D000242:Cyclic AMP / Q000502:physiology*; D006152:Cyclic GMP / Q000502:physiology*; D003600:Cytosol / Q000378:metabolism / Q000502:physiology; D004847:Epithelial Cells; D004848:Epithelium / Q000502:physiology; D006863:Hydrogen-Ion Concentration; D066298:In Vitro Techniques; D015640:Ion Channel Gating / Q000502:physiology; D009297:Nasal Mucosa / Q000166:cytology / Q000502:physiology; D011893:Rana esculenta; D011895:Rana ridibunda; D051381:Rats; D015398:Signal Transduction / Q000502:physiology*; D012903:Smell / Q000502:physiology*
D000242:Cyclic AMP; D006152:Cyclic GMP