guanylyl-imidodiphosphate and sulmazole

The effects of the new inotropic agents saterinone, sulmazole, UD-CG 212.Cl and milrinone at A1 adenosine receptors and m-cholinoceptors were evaluated in human myocardium from patients with heart failure. At A1 adenosine receptors, all compounds inhibited 3H-DPCPX-binding to ventricular membrane preparations at micromolar concentrations. As judged from the K1-values, the rank order of potency was saterinone greater than sulmazole greater than UD-CG 212.Cl greater than milrinone. The new inotropic agents also displaced the binding of 3H-QNB at m-cholinoceptors. Except for saterinone, the concentration ranges of mean Ki-values were considerably higher at m-cholinoceptors than at A1 adenosine receptors. The rank order of potency was saterinone greater than sulmazole greater than UD-CG 212.Cl greater than milrinone. Competition of the A1 adenosine receptor agonist R-PIA to 3H-DPCPX-binding showed a biphasic curve with a shallow slope (Hill coefficient nH = 0.63) and revealed two affinity states of the A1 adenosine receptor. In the presence of guanine nucleotides [Gpp(NH)p], the competition curve showed one low affinity class of binding sites and was shifted to the right. In contrast, the competition curves of the new inotropic agents were characterized by a monophasic, steeper slope (mean Hill coefficient nH = 0.98). Guanine nucleotides had no effect. Similar results were obtained with saterinone and carbachol at m-cholinoceptors. Competition with carbachol revealed three affinity states of the m-cholinoceptor, the super-high affinity binding was reversed by Gpp(NH)p. Competition with saterinone revealed one class of binding sites which was not influenced by Gpp(NH)p.(ABSTRACT TRUNCATED AT 250 WORDS)

Topics: Adult; Binding, Competitive; Cardiotonic Agents; Electric Stimulation; Guanylyl Imidodiphosphate; Heart; Heart Failure; Humans; Imidazoles; In Vitro Techniques; Membranes; Milrinone; Myocardium; Piperazines; Pyridazines; Pyridones; Quinuclidinyl Benzilate; Receptors, Cholinergic; Receptors, Purinergic; Xanthines

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.

Topics: Adenosine Diphosphate Ribose; Adenylate Cyclase Toxin; Adenylyl Cyclases; Adipose Tissue; Animals; Cardiotonic Agents; Cholera Toxin; Colforsin; Cyclic AMP; Fluorides; GTP-Binding Proteins; Guanosine Diphosphate; Guanosine Triphosphate; Guanylyl Imidodiphosphate; Imidazoles; In Vitro Techniques; Isoproterenol; Male; Pertussis Toxin; Rats; Rats, Inbred Strains; Virulence Factors, Bordetella