Cardiac ventricular ?2-adrenoceptors in guinea-pigs and rats are localized on the coronary endothelium

Abstract

In mammalian heart tissue β₂ are known to coexist with β₁. In the present study, evidence that β₂ in guinea-pig and rat ventricles are primarily localized on the coronary endothelium is provided by competition binding studies with the subtype-selective β-adrenoceptor antagonists ICI 89.406 (β₁) and ICI 118.551 (β₂) on four different plasma membrane preparations. (1) Following density gradient centrifugation of cardiac ventricular microsomes from rats or guinea-pigs, endothelial plasma membranes migrated at slightly higher density than the sarcolemmal membranes, as verified by endothelial (angiotensin converting enzyme) and sarcolemmal markers (adenylate cyclase, [³H] ouabain binding). At the activity peak of angiotensin converting enzyme, the relative amount of β₂ in guinea-pigs and rats was 25% and 65%, respectively. (2) On sarcolemmal membranes corresponding to the activity peak of adenylate, cyclase, β-adrenoceptors consisted of the β₁ exclusively (guinea-pig), or to at least 90% (rat). (3) Cultures of coronary endothelial cells derived from guinea-pigs revealed only β₂. (4) Isolated guinea-pig cardiomyocytes contained only β₁, a finding recently established in rat myocytes as well.