Evidence for Heterogeneity of Muscarinic Receptors in the Mollusc Pleurobranchaea

摘要

The properties of the specific binding of the muscarinic antagonist (Iodine 125)3-quinuclidinyl-4-iodobenzilate ((125I)4IQNB) to nervous tissue of Pleurobranchaea californica were characterized. The specific binding of (125)41QNB to Pleurobranchaea nervous tissue was characterized by its high affinity and saturability. A comparison of the numbers of binding sites recognized by (125))4IQNB and 1-(Tritium)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 1-(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 1-QNB competition experiments in Pleurobranchaea. Computer-assisted analysis of 1-QNB competition of (125I)41IQNB specific binding demonstrated that these data were best described by a two-site model with high- and low-affinity sites for I-QNB. Reprints. (AW)