Binding sites in guinea pig myocardial tissue labelled by (-)-[125I] cyanopindolol (CYP) were investigated using differential centrifugation and autoradiographic techniques. Autoradiographs of myocardial sections (0.1 microns) indicated (-)-[125I]CYP binding to sarcolemmal membrane. A low density of binding sites was observed to mitochondria. 2. Binding studies were performed in subcellular fractions. The density of binding sites in the mitochondrial fraction (36.1 +/- 9.4 fmol/mg protein) was less than 10% that in the sarcolemmal membrane (371.7 +/- 38.2 fmol/mg protein). The beta 1/beta 2-adrenoceptor subtype ratio in the mitochondrial fraction (83.3/16.7) was similar to that in the sarcolemmal fraction (87.1/12.9). 3. Ouabain (100 microM), in the presence of sodium azide (0.4 mM), inhibited a Na+K+ stimulated ATPase activity (1.0 +/- 0.2 mumol Pi/mg protein/hr reduction), indicating a low but significant level of sarcolemmal contamination of the mitochondrial fraction. 4. The study showed beta-adrenoceptors in guinea pig heart are located primarily on the sarcolemmal membrane of myocardium. No evidence was obtained for beta-adrenoceptors over mitochondria, as has been suggested in other tissues and species, but that this binding was to sarcolemmal inclusions in the mitochondrial fraction.
