Distinction between high-affinity [³H]phencyclidine binding sites and muscarinic receptors in guinea-pig ileum muscle

[³H]phencyclidine ([³H]PCP) binding was studied in guinea-pig ileum muscle membranes. Specific binding of [³H]PCP was time dependent, reversible and saturable, with an equilibrium dissociation constant of 154 nM and maximum binding of 12.9 pmol/mg of protein at pH 9. Its pH dependency suggests that the unionized PCP is the pharmacologically active form. The binding site was on a protein which was sensitive to heat, proteolytic enzymes and the carboxylic group reagent dicyclohexylcarbodiimide, but insensitive to phospholipase A and C, concanavalin A, dithiothreitol and N-ethylmaleimide. Specific [³H]PCP binding was displaced effectively by several PCP analogs and Ca⁺⁺ channel antagonists including verapamil, to which these sites had a high affinity. These high-affinity PCP-binding sites were found at a much higher concentration in the same membrane preparation than muscarinic receptor sites identified by their specific binding of [³H]quinuclidinyl benzilate. PCP bound to both sites, but with a lower affinity to the muscarinic receptor sites. The PCP and muscarinic receptor sites differed in their sensitivities to pH and drug specifities.