CYCLIC NUCLEOTIDE-GATED CHANNELS OF RAT OLFACTORY RECEPTOR-CELLS - DIVALENT-CATIONS CONTROL THE SENSITIVITY TO CAMP

cAMP-gated channels were studied in inside-out membrane patches excised from the apical cellular pore of isolated olfactory receptor cells of the rat. in the absence of divalent cations the dose-response curve of activation of patch current by cAMP had a K-M Of 4.0 mu M at -50 mV and of 2.5 mu M at +50 mV. However, addition of 0.2 or 0.5 mM Ca2+ shifted the K-M of cAMP reversibly to the higher cAMP concentrations of 33 or 90 mu M, respectively, at -50 mV. Among divalent cations, the relative potency for inducing cAMP affinity shifts was: Ca2+ > Sr2+ > Mn2+ > Ba2+ > Mg2+, of which Mg2+ (up to 3 mM) did not shift the K-M at all. This potency sequence corresponds closely to that required for the activation of calmodulin. However, the Ca2+-sensitivity is lower than expected for a calmodulin-mediated action. Brief(60 s) transient exposure to 3 mM Mg2+, in the absence of other divalent cations, had a protective effect in that following washout of Mg2+, subsequent exposure to 0.2 mM Ca2+ no longer caused affinity shifts. This protection effect did not occur in intact cells and was probably a consequence of patch excision, possibly representing ablation of a regulatory protein from the channel cyclic nucleotide binding site. Thus, the binding of divalent cations, probably via a regulatory protein, controls the sensitivity of the cAMP-gated channels to cAMP. The influx of Ca2+ through these channels during the odorant response may rise to a sufficiently high concentration at the intracellular membrane surface to contribute to the desensitization of the odorant-induced response. The results also indicate that divalent cation effects on cyclic nucleotide-gated channels may depend on the sequence of pre-exposure to other divalent cations.