The presence of a-adrenergic receptors in purified cultures of neonatal rat ventricular muscle cells first was confirmed pharmacologically and then demonstrated directly with a new iodinated ai-selective radioligand [ 125 I]-I-2-[/6-(4-hydroxyphenyl)ethylaminomethyI]tetralone ([ I25 I] IBE 2254). These cells respond to the a-adrenergic agonist phenylephrine with a positive chronotropic response that is markedly inhibited by the ai-selective antagonist, prazosin. The radioligand [' 5 I] 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 [ 125 I]IBE 2254, cardiac cells contain a homogeneous class of binding sites of high affinity (K D = 324 42 pin) and limited capacity (33,000 4,000 sites/cell). Binding is stereoselective, as determined by the greater potency of I-than d-norepinephrine in competing for specific binding sites. Adrenergic antagonists compete with [ 125 I] IBE 2254 in the order expected for binding to an ai-receptor (prazosin = BE 2254 > phentolamine > yohimbine). Therefore, the high specific binding observed with [ 125 I]IBE 2254, together with the inherently high specific activity of iodinated radioligands, suggest that [ 125 I|IBE 2254 will be a useful probe for the ai-adrenergic receptor in a variety of cell systems.
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Kupfer et al. (1982) studied this question.