Dynamic Regulation of β-Adrenergic Receptors by Endothelin-1 in Smooth-Muscle Cells

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Abstract
Elevated endothelin-1 (ET-1) levels are found in atherosclerosis, myocardial ischemia, and heart failure, and are correlated with increased mortality rates. Contrary to expectations, elevations of endogenous ET-1 levels in transgenic mice are not associated with increases in arterial blood pressure or with vasospasm, although these effects can be observed after i.v. ET-1 administration. The aim of this study was to determine the regulatory effects of ET-1 on the expression of vasodilator β-adrenergic receptors and their ability to activate adenylyl cyclase. Smooth-muscle cells were incubated with ET-1 (10−7 mol/L) for 3 days. The density of ET-1 or β-adrenergic receptor binding sites was determined using a radioligand binding procedure. Adenylyl cyclase activity was measured to assess any functional changes in the β-adrenergic receptor density. ET-1 incubation reduced ET-1 binding sites by 70%. In contrast, the β-adrenergic receptor density increased from 354 ± 35 to 538 ± 50 fmol/mg protein (p < 0.01; n = 7) after 3 days. ET-1 increased β-adrenergic receptors dose-dependently. Incubation with ET-1 for different periods of time showed an initial decrease of 30% after 6 h of ET-1 incubation. However, after 24 h ET-1 induced an increase of β-adrenergic receptors, reaching a maximal amount after 48 h. An increased stimulation of β-adrenergic receptor-activated adenylyl cyclase was observed after 3-day ET-1 incubation compared to controls. These data demonstrate that chronic ET receptor activation by ET-1 results in a functionally significant increase in β-adrenergic receptor density and adenylyl cyclase activity.