pituitrin and ethylisopropylamiloride

We evaluated the effects of SMP-300 (N-(aminoiminomethyl)-11-chloro-5,6,7,8-tetrahydro-8-oxo-4H-pyrrolo[3,2,1-kl][1]benzazocine-2-carboxamide monomethanesulfonate monohydrate), a newly synthesized compound, on Na+/H+ exchange activity in rat cardiomyocytes and on other ion transporters, channels and receptors. We also investigated the protective effects of SMP-300 in isolated ischemic rat hearts and rat isoproterenol- or vasopressin-induced experimental angina models. SMP-300 concentration-dependently inhibited recovery from acidosis in rat myocytes, and its IC50 for Na+/H+ exchange was 6 nM. In comparison, its IC50s for Na+/Ca2+ exchange and for the Na+ channel were >1000 nM, and those for other channels or receptors tested were >10,000 nM. In rat isolated perfused hearts, SMP-300 (10(-8)-10(-7) M), administered only at preischemia and not during reperfusion, significantly improved the postischemic recovery of cardiac function. SMP-300 (0.03-0.3 mg/kg, i.v.) or 5-(N-ethyl-N-isopropyl)-amiloride (1 mg/kg, i.v.) prevented the isoproterenol-induced ST-segment depression in the ECG of anesthetized rats, in a dose-dependent manner. SMP-300 (0.1 mg/kg, i.v.) and 5-(N-ethyl-N-isopropyl)-amiloride (1 mg/kg, i.v.) also inhibited the vasopressin-induced ST-segment depression in the ECG of anesthetized rats. This is the first report presenting the protective effect of Na+/H+ exchange inhibitors on isoproterenol- or vasopressin-induced ECG changes in rats, providing the future perspective of SMP-300, a potent Na+/H+ exchange inhibitor, as an anti-anginal drug.

Topics: Amiloride; Angina Pectoris; Animals; Azocines; Blood Pressure; Disease Models, Animal; Electrocardiography; Heart; Heart Rate; Isoproterenol; Male; Myocardial Ischemia; Myocardium; Neuroprotective Agents; Pyrroles; Rats; Rats, Sprague-Dawley; Sodium-Hydrogen Exchangers; Vasopressins

The purpose of this study was to characterize the role of ions other than Ca2+ in hepatic responses to alpha 1-adrenergic stimulation. We report that the alpha 1-adrenoreceptor activation of hepatic functions is accompanied by extracellular acidification and an increase in intracellular pH. These effects are dependent on extracellular Na+ concentration and are inhibited by the Na+/H+ antiporter blocker 5-(N-ethyl-N-isopropyl) amiloride under conditions that preclude antagonistic effects on agonist binding. Thus, the activation of plasma membrane Na+/H+ exchange is an essential feature of the hepatic alpha-adrenoreceptor-coupled signaling pathway. The following observations indicate that the sustained hepatic alpha 1-adrenergic actions rely on a functional coupling between the plasma membrane Na+/H+ and Na+/Ca2+ exchangers, resulting in the stimulation of Ca2+ influx. 1) Inhibition of the Na+/K(+)-ATPase does not prevent the alpha 1-adrenergic effects. However, alpha 1-adrenoreceptor stimulation fails to induce intracellular alkalinization and to acidify the extracellular medium in the absence of extracellular Ca2+. 2) A non-receptor-induced increase in intracellular Na+ concentration, caused by the ionophore monensin, stimulates Ca2+ influx and increases vascular resistance. 3) Inhibition of Na+/Ca2+ exchange prevents, in a concentration-dependent manner, most of the alpha 1-agonist-induced responses. 4) The actions of Ca(2+)-mobilizing vasoactive peptide receptors or alpha 2-adrenoreceptors, which produce neither sustained extracellular acidification nor release of Ca2+, are insensitive to Na+/H+ exchange blockers.

Topics: Amiloride; Animals; Calcium; Carrier Proteins; Hydrogen-Ion Concentration; Liver; Membrane Potentials; Monensin; Ouabain; Rats; Rats, Wistar; Receptors, Adrenergic, alpha; Sodium; Sodium-Calcium Exchanger; Sodium-Hydrogen Exchangers; Vasopressins

To examine the mechanisms of cell volume regulation in response to hyperosmolality, segments of the inner stripe of rabbit outer medullary collecting duct (OMCDi) were perfused in vitro. The cross-sectional area of the tubule was monitored as an index of the relative cell volume. When luminal and basolateral osmolalities were increased from 290 to 390 mOsm simultaneously, the tubule cell shrank instantaneously and reswelled gradually, showing the so-called regulatory volume increase (RVI). Basolateral Na+ removal and addition of basolateral ethyl isopropyl amiloride (EIPA) decreased the RVI response by 76 and 66%, respectively. By contrast, apical Na+ removal had no effect on this response. RVI response was also inhibited by basolateral, but not luminal, Cl- removal (-63%), by total HCO3- removal (-74%), and by adding basolateral 4,4'-diisothiocyanostilbene-2,2'-disulfonic acid (DIDS) (-62%). Intracellular pH did not change significantly during RVI. Vasopressin increased RVI response by 56%. However, this increase was abolished in the absence of basolateral Na+ and Cl-, and in the presence of basolateral EIPA and DIDS. These results suggest that major mechanisms responsible for RVI are Na(+)-H+ and Cl(-)-HCO3- exchange systems in the basolateral membrane, and that these systems are stimulated by vasopressin in rabbit OMCDi.

Topics: 4,4'-Diisothiocyanostilbene-2,2'-Disulfonic Acid; Amiloride; Animals; Biological Transport; Cell Membrane; Chlorides; Female; Hydrogen-Ion Concentration; Kidney Tubules, Collecting; Osmolar Concentration; Rabbits; Sodium; Time Factors; Vasopressins