Regulation of acetylcholine binding by ATP at the muscarinic M₁ receptor in intact CHO cells

ATP may have a modulatory effect on cholinergic transmission, as it is known that ATP is released as a co-transmitter with acetylcholine from nerve terminals. The ability of ATP to influence the binding of acetylcholine to the M₁ muscarinic acetylcholine receptor expressed in intact CHO cells was investigated. In competition binding experiments, acetylcholine completely inhibited the binding of [³H]N-methylscopolamine, but yielded a shallow competition isotherm that was best described in terms of two affinity states. When these experiments were repeated in the presence of 1 mM ATP, the acetylcholine competition curve was better described in terms of a single, low-affinity state with a Hill slope not significantly different from unity. This modulatory effect of ATP was completely reversed by the addition of the P₂ purinoceptor antagonist, suramin, to the assay medium. When the competition between the muscarinic receptor antagonist, atropine, and [³H]N- methylscopolamine was investigated, however, ATP was unable to modulate the binding of atropine, which was consistent with a one-site binding model in each instance. In contrast to the intact cell studies, ATP did not affect either affinity state of acetylcholine binding when studied in homogenate preparations. The results of the present study indicate that ATP, acting via endogenously expressed purinoceptors, is able to influence agonist binding to the M₁ muscarinic acetylcholine receptor via a cross-talk that requires the functional integrity of intact CHO cells.