Endothelial cells contain beta adrenoceptors

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Abstract
The direct identification of beta adrenoceptors in endothelial cell cultures has not been possible until the advent of a new beta-adrenergic radioligand, [125I]iodocyanopindolol ([125I]ICYP). Using [125I]ICYP, we report thes successful identification of a beta adrenoceptor in cultured bovine aortic endothelial cells. At 37°C, specific binding is saturable, stable and reversible. There is a single class of binding sites (21,500±2,900 sites/cell) with an equilibrium dissociation constant (K d) of 109±26 pM. The rate constant of association, k 1, is 1.22×109 M−1 min−1 and of dissociation, k −1, is 0.01 min−1. Binding studies on monolayers of endothelial cells grown in microtiter plates yield similar data (K d=53±9 pM, B max=20,000±1,900 sites/cell). Stereoselectivity of binding for the (−)-isomer is demonstrable for both agonists and antagonists. A series of adrenergic agonists competes with [125I]ICYP for binding with an order of potency suggesting beta2 subselectivity; isoproterenol (0.73 μM) > epinephrine (15 μM) > norepinephrine (71 μM). Furthermore, the beta2 inhibitor butoxamine is more potent than the beta1 inhibitor practolol (7.7 μM vs 22 μM respectively). The GTP analogue, Gpp(NH)p, reduces isoproterenol affinity to 1.9 μM and increases the Hill coefficient from 0.62–0.90.