Becker C, Porzig H
Br J Pharmacol. 1984 Jul;82(3):745-55. doi: 10.1111/j.1476-5381.1984.tb10814.x.
We have studied the recovery of receptor binding and of isoprenaline-stimulated cyclic AMP responses after chronic (2-5 days) exposure of tissue-cultured living rat heart cells to several beta-adrenoceptor antagonists. Most experiments were performed with [3H]- (+/-)-carazolol and [3H]-(+/-)-CGP 12177, as prototypes of high affinity lipophilic and hydrophilic ligands respectively. Chronic antagonist treatment did not alter the total number of receptors nor did it cause intracellular accumulation of the ligands. At the end of the treatment, radiolabelled antagonists were displaced either by 'infinite' dilution of the incubation medium or by competitive displacement with the non-labelled ligand (-)-timolol. In dilution assays dissociation of carazolol from specific sites was biphasic with t 1/2 values of 41 +/- 14 and 219 +/- 15 min. Dissociation of CGP 12177 was monophasic with t 1/2 of 102 +/- 2 min. Timolol enhanced the dissociation rates of both radioligands and suppressed the slow phase of carazolol dissociation. Isoprenaline-stimulated cyclic AMP formation did not recover in parallel with the release of the two antagonists from receptor binding sites. To reach about 80% of control values for receptor availability or cyclic AMP response required 3 h and 24 h washout periods, respectively, after carazolol (0.2 nM) treatment, or 1.5 and 12 h washout periods after CGP 12177 (4 nM) treatment. Such a 'decoupling' effect was not observed during recovery from chronic exposure to the antagonists, timolol and propranolol. We conclude that some antagonists cause a novel form of desensitization that is not linked to their partial agonistic potency. Moreover, carazolol-type drugs seem to induce an additional isomeric form of the beta-receptor that is not recognized by other antagonists. These observations could explain the well known discrepancy between long duration of action and rapid removal from the circulation of several antagonists in current therapeutic use.
我们研究了组织培养的活大鼠心脏细胞在长期（2 - 5天）暴露于几种β - 肾上腺素能拮抗剂后，受体结合以及异丙肾上腺素刺激的环磷酸腺苷（cAMP）反应的恢复情况。大多数实验分别使用[³H] - （±） - 咔唑洛尔和[³H] - （±） - CGP 12177进行，它们分别作为高亲和力亲脂性和亲水性配体的原型。长期拮抗剂处理既未改变受体总数，也未导致配体在细胞内积累。在处理结束时，放射性标记的拮抗剂通过孵育培养基的“无限”稀释或用未标记的配体（ - ） - 噻吗洛尔进行竞争性置换而被取代。在稀释试验中，咔唑洛尔从特定位点的解离是双相的，t₁/₂值分别为41±14分钟和219±15分钟。CGP 12177的解离是单相的，t₁/₂为102±2分钟。噻吗洛尔提高了两种放射性配体的解离速率，并抑制了咔唑洛尔解离的慢相。异丙肾上腺素刺激的环磷酸腺苷形成并未与两种拮抗剂从受体结合位点的释放平行恢复。在咔唑洛尔（0.2 nM）处理后，分别需要3小时和24小时的洗脱期才能使受体可用性或环磷酸腺苷反应达到对照值的约80％，而在CGP 12177（4 nM）处理后，洗脱期分别为1.5小时和12小时。从长期暴露于拮抗剂噻吗洛尔和普萘洛尔恢复过程中未观察到这种“解偶联”效应。我们得出结论：一些拮抗剂会引起一种新型脱敏形式，这种脱敏形式与其部分激动剂效力无关。此外，咔唑洛尔类药物似乎诱导了β受体的一种额外异构体形式，而其他拮抗剂无法识别这种形式。这些观察结果可以解释目前治疗中几种拮抗剂作用持续时间长与从循环中快速清除之间众所周知的差异。
