Identification of α1-adrenergic receptors in cultured rat myocardial cells with a new iodinated α1-adrenergic antagonist, [125I]IBE 2254

Abstract

The presence of α-adrenergic receptors in purified cultures of neonatal rat ventricular muscle cells first was confirmed pharmacologically and then demonstrated directly with a new iodinated α1-selective radioligand [125I]-I-2-[β-(4-hydroxyphenyl)ethylaminomethyl]tetralone ([125I] IBE 2254). These cells respond to the α-adrenergic agonist phenylephrine with a positive chronotropic response that is markedly inhibited by the α1-selective antagonist, prazosin. The radioligand [125I]IBE 2254 binds to suspensions of intact cells rapidly (15 minutes), reversibly and with high specificity (80-85% inhibited by excess unlabeled BE 2254). as identified by [125I]IBE 2254, cardiac cells contain a homogeneous class of binding sites of high affinity [K(D) = 324 ± 42 pM] and limited capacity (33,000 ± 4,000 sites/cell). Binding is stereoselective, as determined by the greater potency of l- than d-norepinephrine in competing for specific binding sites. Adrenergic antagonists compete with [125I]IBE 2254 in the order expected for binding to an α1-receptor (prazosin ≃ BE 2254 > phentolamine > yohimbine). Therefore, the high specific binding observed with [125I]IBE 2254, together with the inherently high specific activity of iodinated radioligands, suggest that [125I]IBE 2254 will be a useful probe for the α1-adrenergic receptor in a variety of cell systems.