Brown R D, Berger K D, Taylor P
Mol Pharmacol. 1987 Jul;32(1):43-52.
The relationship between alpha 1-adrenergic receptor occupancy by agonists or antagonists and the regulation of intracellular Ca2+ was examined. Receptor occupancy was measured using the antagonist [3H]prazosin and correlated with agonist-elicited 45Ca2+ fluxes. The agonists epinephrine (E), norepinephrine (NE), and phenylephrine (PE) coordinately activated Ca2+ efflux, reflecting a substantial mobilization of intracellular Ca2+, as well as a smaller 45Ca2+ influx. The agonist concentration dependences for influx and efflux were similar, with the order of potency expected for alpha 1 receptors (E greater than or equal to NE greater than PE). To determine the relationship between receptor occupancy and response, the slowly dissociating antagonist prazosin was used to inactivate specified fractions of the receptor population. A linear relationship was observed between the remaining activatable receptors and residual 45Ca2+ efflux elicited by E or NE, except at saturating agonist concentrations where some curvature was observed. Moreover, the concentration dependence for agonist-elicited 45Ca2+ efflux was shifted toward slightly higher concentrations of E or NE following prazosin inactivation. These results suggest the presence of a modest receptor reserve which is revealed by E or NE, but not by PE. Agonist occupation was measured over the same interval as receptor activation by competition with the initial rate of [3H]prazosin association. All three agonists exhibited the major fraction of receptor occupation over the same concentration ranges required for the functional response. Exposure of receptors to specified agonist concentrations for 30 min had little effect on the number of receptors or their ligand affinities, whereas a 2.5-hr exposure to agonist decreased apparent agonist affinity as well as the number of receptors recognized by [3H]prazosin.
研究了激动剂或拮抗剂对α1 - 肾上腺素能受体的占据与细胞内Ca2+调节之间的关系。使用拮抗剂[3H]哌唑嗪测量受体占有率，并将其与激动剂引发的45Ca2+通量相关联。激动剂肾上腺素（E）、去甲肾上腺素（NE）和苯肾上腺素（PE）协同激活Ca2+外流，反映出细胞内Ca2+的大量动员，以及较小的45Ca2+内流。激动剂浓度对流入和流出的依赖性相似，其效力顺序符合α1受体的预期顺序（E≥NE＞PE）。为了确定受体占有率与反应之间的关系，使用解离缓慢的拮抗剂哌唑嗪使特定比例的受体群体失活。观察到剩余的可激活受体与E或NE引发的残余45Ca2+外流之间存在线性关系，但在饱和激动剂浓度下观察到一些曲率。此外，在哌唑嗪失活后，激动剂引发的45Ca2+外流的浓度依赖性向略高的E或NE浓度偏移。这些结果表明存在适度的受体储备，其通过E或NE揭示，但不通过PE揭示。通过与[3H]哌唑嗪结合的初始速率竞争，在与受体激活相同的时间间隔内测量激动剂占据情况。所有三种激动剂在功能反应所需的相同浓度范围内表现出大部分受体占据。将受体暴露于特定激动剂浓度30分钟对受体数量或其配体亲和力影响很小，而暴露于激动剂2.5小时会降低表观激动剂亲和力以及[3H]哌唑嗪识别的受体数量。
