Ramkumar V, Stiles G L
Department of Medicine (Cardiology), Duke University Medical Center, Durham, North Carolina 27710.
Mol Pharmacol. 1988 Dec;34(6):761-8.
The effect of the new cardiotonic agent sulmazole on the guanine nucleotide regulatory protein-adenylate cyclase system was studied in rat adipocyte membranes. The inotrope enhanced basal adenylate cyclase activity by 46%. This stimulation occurred only at GTP concentrations (5 microM) sufficient to activate Gi. This stimulatory effect of sulmazole was abolished after functional inactivation of Gi, either by pertussis toxin or by using 10 nM GTP in the assay mixture, suggesting an important role of an active Gi in this process. Similarly, sulmazole enhanced isoproterenol-, forskolin-, and fluoride-stimulated adenylate cyclase activity by 33, 34, and 45%, respectively. However, when these latter experiments were performed after inactivation of Gi, sulmazole actually inhibited by approximately 25% adenylate cyclase activity stimulated by 1 and 10 microM isoproterenol. Under similar treatment conditions, enhancement of forskolin- and fluoride-stimulated activity by sulmazole was abolished. Sulmazole inhibited in a dose-dependent manner pertussis toxin- and cholera toxin-catalyzed labeling of Gi and Gs, respectively, with the respective inhibition observed at 100 microM of the inotrope being 29% and 56% of control. In addition, sulmazole inhibited PGE1 and isoproterenol-stimulated [3H]GDP release from Gi and Gs to 32% and 64% of control, respectively. Finally, the inotrope completely abolished PGE1-stimulated [3H]Gpp(NH)p binding with IC50 in the low micromolar range. These findings suggest that, whereas sulmazole inhibits the functioning of Gi and (to a lesser extent) Gs at low micromolar concentrations, expression of these effects on adenylate cyclase activity requires high micromolar to low millimolar concentrations of the drug. Thus, it appears sulmazole inhibits the function of Gi by decreasing its activation process, i.e., GTP-GDP exchange. Effects on Gs are manifested (at least in terms of adenylate cyclase activity) only after inactivation of Gi.
在大鼠脂肪细胞膜中研究了新型强心剂舒马唑对鸟嘌呤核苷酸调节蛋白 - 腺苷酸环化酶系统的影响。该强心剂使基础腺苷酸环化酶活性增强了46%。这种刺激仅在足以激活Gi的GTP浓度（5 microM）下发生。在通过百日咳毒素或在测定混合物中使用10 nM GTP使Gi功能失活后，舒马唑的这种刺激作用被消除，这表明活性Gi在此过程中起重要作用。同样，舒马唑分别使异丙肾上腺素、福斯高林和氟化物刺激的腺苷酸环化酶活性增强了33%、34%和45%。然而，当在Gi失活后进行这些实验时，舒马唑实际上使1 microM和10 microM异丙肾上腺素刺激的腺苷酸环化酶活性抑制了约25%。在类似的处理条件下，舒马唑对福斯高林和氟化物刺激活性的增强作用被消除。舒马唑分别以剂量依赖性方式抑制百日咳毒素和霍乱毒素催化的Gi和Gs的标记，在100 microM强心剂时观察到的各自抑制率分别为对照的29%和56%。此外，舒马唑分别将PGE1和异丙肾上腺素刺激的[3H]GDP从Gi和Gs的释放抑制到对照的32%和64%。最后，该强心剂在低微摩尔范围内以IC50完全消除了PGE1刺激的[3H]Gpp(NH)p结合。这些发现表明，虽然舒马唑在低微摩尔浓度下抑制Gi和（在较小程度上）Gs的功能，但这些对腺苷酸环化酶活性的影响需要高微摩尔到低微摩尔浓度的药物。因此，舒马唑似乎通过减少其激活过程即GTP - GDP交换来抑制Gi的功能。对Gs的影响仅在Gi失活后才表现出来（至少就腺苷酸环化酶活性而言）。
