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 pole 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 KM of 4.0 microM at -50 mV and of 2.5 microM at +50 mV. However, addition of 0.2 or 0.5 mM Ca2+ shifted the KM of cAMP reversibly to the higher cAMP concentrations of 33 or 90 microM, 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 KM at all. This potency sequence corresponds closely to that required for the activation of calmodulin. However, the Ca(2+)-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.
Lynch
J W
JW
Department of Physiology, Universität des Saarlandes, Homburg, Germany.
Lindemann
B
B
D000818:Animals; D002118:Calcium / Q000494:pharmacology / Q000502:physiology; D002147:Calmodulin / Q000502:physiology; D002413:Cations, Divalent / Q000494:pharmacology*; D000242:Cyclic AMP / Q000494:pharmacology / Q000502:physiology*; D006152:Cyclic GMP / Q000502:physiology*; D003600:Cytosol / Q000187:drug effects / Q000378:metabolism; D066298:In Vitro Techniques; D015640:Ion Channel Gating / Q000502:physiology*; D007473:Ion Channels / Q000187:drug effects / Q000378:metabolism / Q000502:physiology; D008274:Magnesium / Q000494:pharmacology; D051381:Rats; D018035:Receptors, Odorant / Q000502:physiology*
D002147:Calmodulin; D002413:Cations, Divalent; D007473:Ion Channels; D018035:Receptors, Odorant; D000242:Cyclic AMP; D006152:Cyclic GMP; D008274:Magnesium; D002118:Calcium