Muscarinic acetylcholine receptors in albino rabbit iris-ciliary body

The binding of tritium-labeled N-methyl-4-piperidyl benzilate to homogenates of iris-ciliary body complex of albino rabbits was investigated. The binding exhibited all the characteristics typical of binding to the muscarinic acetylcholine receptor: it was saturable, of high affinity, and could be reduced by muscarinic non-labeled ligands whereas non-muscarinic drugs had no such effect. Hill coefficients were approximately 1 for the antagonists and less than 1 for the agonists. Equilibrium experiments indicated the existence of a single population of binding sites (0.44 pmol/iris-ciliary body). The formation of at least two types of ligand receptor complexes had to be assumed, however, to explain the kinetics of [ ³H]-N-methyl-4-piperidyl benzilate binding. The simplest model which fits the experimental findings consists of a fast binding step followed by a slow isomerization of the receptor-ligand complex, as previously suggested by us for the muscarinic receptor in the mouse brain.