[H-3]PIRENZEPINE AND [H-3] QUINUCLIDINYL BENZILATE BINDING TO BRAIN MUSCARINIC CHOLINERGIC RECEPTORS - DIFFERENCES IN MEASURED RECEPTOR DENSITY ARE NOT EXPLAINED BY DIFFERENCES IN RECEPTOR ISOMERIZATION

  • 1 January 1984
    • journal article
    • research article
    • Vol. 26  (2) , 164-169
Abstract
Muscarinic receptor densities were measured in membranes prepared from rat cerebral cortex using [3H]pirenzepine and [3H]quinuclidinyl benzilate. Isotherms of equilibrium binding data modeled to a single apparent binding site for both ligands. [3H]Pirenzepine labeled only a small fraction of the binding sites that were labeled by [3H]quinuclidinyl benzilate. Evidently, subtypes of muscarinic receptors exist. Antagonist binding to muscarinic receptors may involve an isomerization of the receptor-antagonist complex, and it is only the isomerized form of the receptor that is identified by radioligad binding studies. To examine the possibility that the difference in the density of binding sites identified by [3H]pirenzepine and [3H]quinuclidinyl benzilate is due to differences in the degree of isomerization of the receptor associated with the binding of each ligand, the kinetics of the binding of [3H]pirenzepine and [3H]quinuclidinyl benzilate to membranes prepared from rat cerebral cortex were examined. The pseudo-1st-order rate constant of association for both ligands showed a nonlinear (hyperbolic) dependence on ligand concentration. Apparently, a rapidly equilibrating initial binding step was followed by a more slowly equilibrating isomerization of the initially formed ligand-receptor complex. The kinetic data were computer-modeled to obtain estimates of the equilibrium constants for both reaction steps. The equilibrium constants for the isomerization step were 0.1 and 0.004 for [3H]pirenzepine and [3H]quinuclidinyl benzilate, respectively. These measurements, in agreement with others, suggested that only the fraction of receptors which isomerized were measurable using filtration binding assays. Although essentially all (99.6%) of the [3H]quinuclidinyl benzilate binding sites appeared to isomerize, only 90% of the [3H]pirenzepine binding sites isomerized, and thus, only 90% were measured in this assay. Evidently, differences in receptor isomerization can partially, but not wholly, account for the differences between [3H]pirenzepine and (3H]quinuclidinyl benzilate binding in rat cerebral cortex.