Bellorin-Font E, Martin K J
Am J Physiol. 1981 Oct;241(4):F364-73. doi: 10.1152/ajprenal.1981.241.4.F364.
These studies examine the interactions of Ca2+, Mg2+, and guanine nucleotides with PTH binding to renal receptors and activation of adenylate cyclase in a purified preparation of basolateral renal cortical membranes. Inhibition of adenylate cyclase activity by increasing concentrations of CA2+ was more pronounced in the presence of PTH than on basal activity. This effect was not explained by an effect of Ca2+ on equilibrium PTH binding or binding affinity. The presence of PTH also decreased the requirements of the enzyme for Mg2+, consistent with the competitive nature of Ca2+ and Mg2+ interaction. Guanine nucleotides produced similar changes in the metal requirements for enzyme activity. PTH and guanine nucleotides, however, were not additive in this regard in spite of marked synergism in total enzyme activity. Additional studies were performed to further define the effects of guanine nucleotides on the receptor-cyclase system. GTP and Gpp(NH)p caused a dose-dependent decline in equilibrium PTH binding and a small increase in half-maximal displacement. PTH-stimulated adenylate cyclase activity was markedly increased by either nucleotide, and the half-maximal activity was decreased (Gpp(NH)p greater than GTP). This was partly explained by a marked effect of guanine nucleotides on the dissociation rate of receptor-bound PTH. These data suggest that Ca2+ inhibition of PTH-stimulated adenylate cyclase activity is not due to a decreased binding of PTH but to an effect of PTH on the metal requirements for adenylate cyclase activation at the nucleotide regulatory site.
这些研究在纯化的肾皮质基底外侧膜制剂中，检测了钙离子、镁离子和鸟嘌呤核苷酸与甲状旁腺激素（PTH）结合至肾受体以及激活腺苷酸环化酶之间的相互作用。在PTH存在的情况下，随着钙离子浓度升高对腺苷酸环化酶活性的抑制作用，比对基础活性的抑制作用更为明显。钙离子对PTH平衡结合或结合亲和力的影响，无法解释这种效应。PTH的存在也降低了该酶对镁离子的需求，这与钙离子和镁离子相互作用的竞争性本质一致。鸟嘌呤核苷酸对酶活性的金属需求产生了类似的变化。然而，尽管在总酶活性方面存在明显的协同作用，但在这方面PTH和鸟嘌呤核苷酸并非相加作用。进行了额外的研究以进一步确定鸟嘌呤核苷酸对受体 - 环化酶系统的影响。鸟苷三磷酸（GTP）和鸟苷 - 5'-三磷酸（Gpp(NH)p）导致PTH平衡结合呈剂量依赖性下降，并且半数最大置换略有增加。两种核苷酸均显著增加了PTH刺激的腺苷酸环化酶活性，且半数最大活性降低（Gpp(NH)p大于GTP）。这部分是由于鸟嘌呤核苷酸对受体结合的PTH解离速率有显著影响。这些数据表明，钙离子对PTH刺激的腺苷酸环化酶活性的抑制作用，并非由于PTH结合减少，而是由于PTH对核苷酸调节位点处腺苷酸环化酶激活的金属需求产生了影响。
