Alpha-adrenoceptor antagonistic action of amiloride

1. In isolated perfused rat liver, the effects of alpha-adrenergic stimulation by phenylephrine (2 microM), such as an increase of portal pressure, glucose output, Ca2+ release into the perfusate and the characteristic K+ flux changes across the hepatocyte plasma membrane were almost completely abolished in the presence of amiloride (0.5 mM). 2. When the phenylephrine concentration was raised to about 100 microM, the effects of the alpha-adrenergic agonist on hepatic metabolism, Ca2+ and K+ fluxes, but not on the portal venous pressure, were restored, suggesting a competitive antagonism by amiloride. 3. Amiloride antagonized in a concentration-dependent manner noradrenaline-induced isometric contractions of strips of the rabbit pulmonary artery. The concentration-response curve of noradrenaline was shifted to the right, and the maximal response obtained was also depressed, suggesting a mixed competitive and non-competitive antagonism. The estimated amiloride-adrenoceptor-dissociation constant was 8 microM. 4. The affinity of amiloride to the alpha- and beta-adrenoceptor subtypes was determined by radioligand binding assays using [125I]BE 2254 binding to rat liver plasma membranes (alpha 1-subtype), [3H]yohimbine binding to human platelet membranes (alpha 2-subtype), (-)-[125I]iodocyanopindolol (ICYP) binding to rabbit lung membranes in presence of the beta 2-adrenoceptor antagonist ICI 118,551 (beta 1-subtype) and ICYP binding to rat lung membranes in presence of the beta 1-blocker atenolol (beta 2-subtype). In all systems, amiloride inhibited specific ligand binding concentration-dependently, the Ki values for amiloride were about 25, 52, 148 and 161 microM for alpha 1- alpha 2-, beta 1- and beta 2-adrenoceptor subtypes, respectively. 5. It is concluded that amiloride in concentrations below those required for inhibition of the Na+/H+ exchanger is a potent antagonist of alpha- and beta-adrenoceptors in a variety of experimental systems. Whether the adrenergic antagonism of amiloride is important for antihypertensive therapy, remains to be elucidated.