Pittman R N, Molinoff P B
Mol Pharmacol. 1983 Nov;24(3):398-408.
A nonfusing variant of L6 muscle cells was used to study the interactions of 16 agonists and 8 antagonists with beta-adrenergic receptors. Membranes prepared from L6 cells and intact cells in monolayer culture were used. Beta-adrenergic receptors on broken cells and on intact cells had the same affinities for all of the antagonists studied. Studies of the inhibition of the binding of [125I]iodohydroxybenzylpindolol by agonists indicated that two states of the receptor can exist on intact cells attached to the substratum. The form of the receptor normally present on intact cells appeared to have the same properties as receptors on membranes when assayed in the presence of GTP. Several full agonists converted this form of the receptor to a form which had a 40- to 50-fold lower affinity for agonists. This conversion appeared to occur during the first few minutes of exposure to an agonist. Four of the agonists tested did not convert any of the receptors on intact cells to a form with a low affinity for agonists. Included in this group of agents were two full agonists and two partial agonists. Therefore, interactions of these drugs with receptors on broken or intact cells were the same. Several other full and partial agonists converted some of the receptors on intact cells to a low-affinity form, and their interactions with receptors on intact cells were characterized by shallow inhibition curves. The conversion of beta-adrenergic receptors on intact cells to a low-affinity state did not appear to be a prerequisite for the decrease in the rate of agonist-stimulated cyclic AMP accumulation that occurs 1-2 min after exposure of L6 cells to agonists. Studies were also carried out on viable intact cells detached from plates following brief exposure to trypsin or EDTA. The properties of receptors on suspended cells were the same as those of receptors on broken cells when assayed in the presence of GTP, rather than being similar to the properties of receptors on attached cells. In summary, data are presented indicating that agonists with the same potency and intrinsic activity in membrane preparations (and intact cells in suspension) can interact very differently with beta-adrenergic receptors on intact cells attached to the substratum. Thus, certain agonists cause a rapid conversion of beta-adrenergic receptors to a form which has a low affinity for agonists. This effect is seen with some but not all agonists and is seen only in studies with attached cells.
一种L6肌细胞的非融合变体被用于研究16种激动剂和8种拮抗剂与β-肾上腺素能受体的相互作用。使用了从L6细胞制备的膜以及单层培养的完整细胞。破碎细胞和完整细胞上的β-肾上腺素能受体对所有研究的拮抗剂具有相同的亲和力。激动剂对[125I]碘羟基苄基吲哚洛尔结合的抑制研究表明，在附着于基质的完整细胞上，受体可存在两种状态。在完整细胞上正常存在的受体形式，在存在GTP的情况下进行检测时，其性质似乎与膜上的受体相同。几种完全激动剂将这种受体形式转变为对激动剂亲和力降低40至50倍的形式。这种转变似乎发生在暴露于激动剂的最初几分钟内。所测试的四种激动剂未将完整细胞上的任何受体转变为对激动剂亲和力低的形式。这组药物包括两种完全激动剂和两种部分激动剂。因此，这些药物与破碎或完整细胞上受体的相互作用是相同的。其他几种完全和部分激动剂将完整细胞上的一些受体转变为低亲和力形式，它们与完整细胞上受体的相互作用表现为浅抑制曲线。完整细胞上β-肾上腺素能受体向低亲和力状态的转变似乎不是L6细胞暴露于激动剂后1 - 2分钟发生的激动剂刺激的环磷酸腺苷积累速率降低的先决条件。还对短暂暴露于胰蛋白酶或乙二胺四乙酸（EDTA）后从平板上分离的活完整细胞进行了研究。在存在GTP的情况下进行检测时，悬浮细胞上受体的性质与破碎细胞上受体的性质相同，而不像附着细胞上受体的性质。总之，所呈现的数据表明，在膜制剂（以及悬浮的完整细胞）中具有相同效力和内在活性的激动剂，与附着于基质的完整细胞上的β-肾上腺素能受体的相互作用可能非常不同。因此，某些激动剂会使β-肾上腺素能受体迅速转变为对激动剂亲和力低的形式。这种效应在一些但不是所有激动剂中可见，并且仅在对附着细胞的研究中可见。
