Competitive inhibition of tritium-labeled platelet-activating factor binding to rabbit platelet membranes by amiloride and amiloride analogs

Amiloride and its structural analogs, ethylisopropyl amiloride, benzamil, and dichlorobenzamil, inhibit both the specific [3H]C18-PAF binding to rabbit platelet membranes and PAF-induced aggregation of gel-filtered rabbit platelets. Detailed analysis of binding inhibitions demonstrate that ethylisopropyl amiloride is a competitive inhibitor with an equilibrium dissociation constant (KB) of 23 microM. The concentration of amiloride and its analogs needed to inhibit the PAF-induced aggregation is high and there exists no correlation between their inhibitory activities of platelet aggregation and those of Na+/H+ antiporter. However, the inhibitory effects on the PAF-induced aggregation are parallel to those on the specific [3H]C18-PAF binding. The inhibitory effects of amiloride and its analogs on the activation of platelets are at the PAF-receptor binding step, rather than at the Na+/H+ antiporter.