Preparation and characterization of muscarinic-acetylcholine-receptor-enriched membranes from pig atria.

A procedure was developed for the large scale preparation of membranes from pig atria which are enriched 10-13 fold in the muscarinic acetylcholine receptor. The procedure involved differential centrifugation and sucrose-gradient centrifugation in solutions containing 150 mM-NaClO4 and 5 mM-EDTA to minimize membrane aggregation. The final membrane preparation bound about 1.1 pmol of L-quinuclidinyl benzilate/mg of protein. Comparable results were obtained with either fresh or frozen tissue. About the same yield (120 pmol of L-quinuclidinyl benzilate sites/100 g of tissue) and specific activity of membranes were obtained from different regions of the atria. The final preparation was stable at -80 degrees C in buffered sucrose solutions. The membranes appeared mostly as sheets or fragments and partly as closed vesicles in the electron microscope and were heterogeneous in isopycnic Percoll gradients. Marker enzyme studies showed that the receptor was enriched in parallel with the plasma membrane markers guanylate cyclase (particulate form) and (Na+ + K+)-activated ATPase. Some contamination by mitochondrial outer and endoplasmic reticulum membranes was evident from the distribution of monoamine oxidase and glucose-6-phosphatase activity, but the preparation was largely free of sarcoplasmic reticulum, mitochondrial inner, and lysosomal membranes.