The properties of the specific binding of the muscarinic antagonist [125I]3-quinuclidinyl-4-iodobenzilate ([125I]4IQNB) to nervous tissue of Pleurobrancheae california were characterized. The specific binding of [125I]4IQNB to Pleurobranchaea nervous tissue was characterized by its high affinity (Kd = 0.61 ± 0.11 nM) and saturability (Bmax = 602 ± 46 fmol/mg protein). A comparison of the numbers of binding sites recognized by [125I]4IQNB and 1-[3H]QNB in nervous tissue of three invertebrate species indicated that in Aplysia and Cancer magister (crab) ganglia membranes the two radioligands labeled comparable numbers of binding sites; however, in Pleurobranchaea membranes l-[3H]QNB recognized only a subpopulation (8-10%) of the total number of [125I]4IQNB binding sites. The disparity in the numbers of binding sites labeled by these radioligands was consistent with our finding of a heterogeneity of muscarinic antagonist binding sites in l-QNB competition experiments in Pleurobranchaea. Computer-assisted analysis of l-QNB competition of [125I]4IQNB specific binding demonstrated that these data were best described by a two-site model with high- and low-affinity sites for l-QNB. The high-affinity site recognized by l-QNB possessed an IC50 value of 0.2 nM and comprised 18% of the total specific binding, while the lower affinity site had an IC50 value of 55.6 nM and comprised the remaining 83% of the total population of [125I]4IQNB recognition sites. The IC50 value for l-QNB at the high-affinity site in Pleurobranchaea membranes is in excellent agreement with Kd values for 1-[3H]QNB labeling of classical muscarinic receptors in a variety of invertebrate and vertebrate species.
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