Sill, J. C.

Experiments were designed to determine the role of the L-arginine pathway in endothelium-dependent relaxations to vasopressin. The effects of L-arginine analogues NG-nitro-L-arginine (L-NNA), NG-nitro-L-arginine methyl ester (L-NAME), and NG-monomethyl-L-arginine (L-NMMA) on basal and vasopressin-induced activity of nitric oxide synthase were studied in isolated canine basilar arteries. Rings with and without endothelium were suspended for isometric tension recording in Krebs-Ringer bicarbonate solution bubbled with 94% O2-6% CO2 (37 degrees C, pH 7.4). Radioimmunoassay was used to determine the level of guanosine 3',5'-cyclic monophosphate (cGMP). All experiments were performed in the presence of indomethacin, a cyclooxygenase inhibitor. L-NAME and L-NMMA caused endothelium-dependent contractions and inhibited basal production of cGMP. In contrast, L-NNA did not affect basal tone or basal production of cGMP. L-Arginine analogues inhibited relaxations to vasopressin but did not affect relaxations to a nitric oxide donor, molsidomine (SIN-1). The effects of L-NNA, L-NAME, and L-NMMA were reversed in the presence of L-arginine. The relaxations to vasopressin were associated with an increase of cGMP levels in the arterial wall. This effect of vasopressin was inhibited in the presence of L-NNA. These studies suggest that the relaxations to vasopressin are mediated by activation of the endothelial L-arginine pathway, leading to increased production of nitric oxide, with subsequent activation of guanylate cyclase in smooth muscle cells. In canine basilar artery, L-NAME and L-NMMA are nonselective inhibitors of both basal and stimulated production of nitric oxide, whereas L-NNA selectively inhibits vasopressin-induced activation of the L-arginine pathway.

BackgroundInotropic drugs are often used to treat acute, severe heart failure resulting from acute myocardial infarction and other unstable coronary artery syndromes. However, catecholamine inotropic agents may potentiate coronary thrombosis via a platelet α2-adrenergic mechanism, thus exacerbating the original problem. The present studies were designed to determine whether the nonadrenergic inotropic and vasodilator drug amrinone, which elevates platelet cAMP levels, would both inhibit human platelet Ca2+mobilization and adhesion molecule expression ex vivo and protect against experimental coronary thrombosis in vivo in dogs.Methods and ResultsHuman platelets in suspension were preincubated with amrinone 2.5 to 15 μg/mL; stimulated with the agonists thrombin 0.1 U/mL, ADP 10−6mol/L, or arginine vasopressin 10−7mol/L; and studied for Ca2+mobilization, glycoprotein IIb/IIIa activation, and P-selectin expression by fluorescent flow cytometry methods. Experimental coronary thrombosis in vivo was studied in an open-chest dog model with critical coronary artery stenosis and deep vessel wall injury. Results showed that at the cellular level, amrinone inhibited agonist-induced Ca2+mobilization and had modest inhibitory effects on adhesion molecule expression. In vivo in dogs, intravenous amrinone 2 mg/kg plus infusion at 20 μg·kg−1·min−1completely abolished coronary thrombosis.ConclusionsThe fact that amrinone inhibited human platelet activation at the cellular level and protected against experimental coronary thrombosis in vivo in dogs suggests a potentially advantageous antithrombotic action for this inotropic and vasodilator drug.

We designed experiments to characterize the role of superoxide anions in the mediation of endothelium-dependent contractions in isolated canine basilar arteries. Rings with and without endothelium were suspended for isometric tension recording in Krebs-Ringer bicarbonate solution bubbled with 94% O2-6% CO2 (37 degrees C, pH 7.4). Radioimmunoassay was used to determine the levels of cyclic GMP and cyclic AMP. Calcium ionophore A23187 (10(-9) to 10(-6) mol/L) caused concentration-dependent contractions. The removal of endothelium abolished the effect of A23187. Contractions to A23187 were reversed into relaxations in the presence of superoxide dismutase (150 U/mL) or the prostaglandin H2/thromboxane A2 receptor antagonist SQ29548 (10(-6) mol/L). NG-nitro-L-arginine methyl ester (3 x 10(-4) mol/L) augmented contractions to A23187. In rings with endothelium, A23187 (3 x 10(-7) mol/L) significantly increased levels of both cyclic AMP and cyclic GMP. Indomethacin (10(-5) mol/L) inhibited stimulatory effects of A23187 on cyclic AMP production. In contrast, indomethacin augmented A23187-induced production of cyclic GMP. Selective augmentation of cyclic GMP production by indomethacin appears to be due to protection of nitric oxide or a closely related molecule released following translocation of calcium into endothelial cells. Our findings suggest that (1) an increased concentration of calcium in endothelial cells may activate both cyclooxygenase and the L-arginine/nitric oxide pathway, (2) arachidonic acid metabolism via cyclooxygenase is a source of superoxide anions, and (3) superoxide anions may be responsible for impairment of balance between relaxing and contracting factors leading to contraction of underlying smooth muscle cells.