Sarcolemmal beta-Adrenoceptors Determined in Rat Ventricular Heart Biopsies with (-)-[3H]CGP-12177

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Abstract
Binding of the hydrophilic beta-adrenoceptor ligand (-)-[H]CGP-12177 was investigated by incubating biopsies from rat hearts (left ventricle/interventricular septum) to elucidate the applicability of this approach in determining the content of cell membrane beta-adrenoceptors in heart biopsies. Binding of (-)-[3H]CGP-12177 at 1 nM and 37 degrees reached maximum after 4-10 hr and declined after 10 hr. Binding of (-)-[3H]CGP-12177 at nM and 4 degrees reached equilibrium at 24 hr and was stable up to 96 hr. Total and specific binding was independent of biopsy size in the weight range of 4-35 mg. Competition binding studies with (+)- and (-)-isoprenaline showed that binding was stereospecific. Non-specific binding, determined in the presence of 5 microM (-)-timolol, was 6-8% of total binding at 0.1 nM (at Kd) and 15% of total binding at 1 nM (-)-[3H]CGP-12177. The coefficient of variation for total binding was 5.1%. Dissociation initiated at equilibrium showed complete reversibility of specific binding and was monoexponential with half-life of 0.6 hr at 37 degrees and 30.1 hr at 4 degrees. Binding-saturation experiments at 4 degrees showed beta-adrenoceptor density of 7 fmol/mg wet weight and equilibrium dissociation constant of 0.1 nM. Kd calculated from the rate constants of association and dissociation was 0.15 nM. Rapid freezing of tissue in liquid nitrogen with subsequent thawing and binding at 4 degrees C reduced receptor density by 21%. Density of beta-adrenoceptors did not differ in hearts from lean and obese insulin resistant Zucker rats. The results show that the method allows direct determination of sarcolemmal beta-adrenoceptors in small myocardial biopsies at 4 degrees with a minimum of preparation and equipment, using (-)-[3H]CGP-12177 . The method may be useful for other hydrophilic ligands.

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