Verlag des Forschungszentrums Jülich
JUEL-3778
Reuter, Dirk
Untersuchungen zur Funktion von Ca2+ -aktivierten Ci-Kanälen und cAMP-gesteuerten Kationenkanälen bei der Erzeugung des Rezeptorsstroms in Riechzellen der Ratte
87 S., 2000
cAMP-gated ion channels in olfactory sensory neurons are composed of three distinct subunits, denominated CNC[alpha]3, CNC[alpha]4 and CNC[beta]1b. The channels are cooperatively activated by cAMP, and the cAMP-sensitivity is modulated by calmodulin in a Ca2+-dependent manner. cAMP-sensitivity, modulation by calmodulin, gating behavior and ionic selectivity of the channels are determined by the quarternary structure.
In this study, the kinetics of the modulation of the cAMP-sensitivity of these channels by calmodulin were determined by performing patch-clamp recordings in the inside-out configuration, revealing a bi-phasic time-course of the modulation. By comparing the kinetics of different heterologously coexpressed channels and of native channels from rat olfactory sensory neurons, the function of the modulatory subunits CNC[alpha]4 and CNC[beta]1b could be determined. This comparison yielded the highest congruency between the properties of CNC([alpha]3[alpha]4[beta]1b) channels and of the native channel, strongly suggesting that the native channel must contain all three different subunits, in order to properly perform its physiological function in the olfactory transduction cascade. The molecular mechanism, by which Calmodulin modulates the ligand-sensitivity of CNG-channels, has yet to be determined.
CNG-channels conduct Na+, K+ and Ca2+-Ions. Under physiological conditions, the channels primarily conduct Ca2+-Ions, thus serving as functional Ca2+-channels in the olfactory transduction cascade. Ca2+-ions entering the cell through CNG-channels activate Ca2+-activated Cl- channels, which conduct a substantial fraction of the receptor current. In this study, activation of these channels by Ca2+ and time-dependent inactivation were observed in dissociated olfactory receptor neurons from rats using electrophysiological recordings. Furthermore, the concentrations of chloride inside the cellular lumen and the olfactory mucus were determined using analytical electron microscopical measurements in order to determine the electrochemical potential relevant for the polarity of the chloride current in vivo. These measurements validate the notion that chloride ions carry an excitatory current in the olfactory transduction cascade to further amplify the primary cationic current carried by CNG-channels.
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