Muscarinic receptor extracted from porcine atria in digitonin-cholate copurified with G alpha(o), G alpha(il-3), and caveolins. The presence of complexes was confirmed by coimmunoprecipitation of the receptor, alpha-subunits, and caveolins in various combinations. Homooligomers of alpha(i2) were detected on Western blots, and heterooligomers of alpha(i2) and a. were identified by coimmunoprecipitation; thus, a complex may contain at least two a-subunits. Other combinations of alpha-subunit were not detected. The ratio of total a-subunit to receptor was near 1, as measured by [S-35]GTP gamma S and the antagonist [H-3]quinudidinylbenzilate, and the binding of [S-35]GTP gamma S was manifestly biphasic. The ratio of alpha(o), to alpha(i1,2) also was near 1, as determined from the intensity of Western blots. Cardiac muscarinic receptors therefore can be purified as a mixture of complexes that contain caveolins and oligomers of alpha-subunit, some of which are heteromeric. Each complex would appear to contain equal numbers of alpha-subunit and the receptor. (C) 2008 Elsevier Inc. All rights reserved.