Evidence for an endogenous factor involved in maintenance of pirenzepine high-affinity binding in rat brain stem

Sucrose gradient centrifugation was used to isolate membranes enriched in muscarinic receptors from bovine brain stem. Unlike the receptors in crude synaptosomal preparations of this tissue, the enriched preparations displayed only low-affinity pirenzepine binding. Similar results were obtained when purified preparations were preincubated for 1 hr in pH 7.0 buffer at 37 degrees C; however, preincubation in a pH 5.0 buffer partially restored the high-affinity pirenzepine binding. These results suggest that an endogenous factor, which is present in the crude synaptosomes of the brain stem and is removed by sucrose gradient centrifugation, is involved in maintenance of the high-affinity pirenzepine binding of the muscarinic receptors.