Toborinone and olprinone,phosphodiesterase III inhibitors,inhibit human platelet aggregation due to the inhibition of both calcium release from intracellular stores and calcium entry

Purpose We investigated the inhibitory effects of toborinone and olprinone on human platelet aggregation and calcium mobilization.Methods Washed human platelets were preincubated with toborinone or olprinone, then exposed to 0.015U·ml^–1 of thrombin. Aggregation curves were measured using an aggregometer. Effects of toborinone or olprinone on changes in intracellular calcium concentration ([Ca²⁺]_i) were measured fluorometrically using fura-2 acetoxymethyl ester (fura-2). Levels of intracellular cyclic 3,5-adenosine monophosphate concentration ([cAMP]_i) were also measured, using enzyme-linked immunosorbent assay (ELISA) techniques.Results The concentrations required to cause 50% inhibition of aggregation (IC₅₀) induced by thrombin were 9.7 ± 0.9µM for toborinone and 3.6 ± 0.2µM for olprinone. Both drugs at IC₅₀ significantly elevated [cAMP]_i levels and significantly inhibited Ca²⁺ release from intracellular stores. Release of [Ca²⁺]_i induced by thrombin was 272.9 ± 87.1nM, 153.3 ± 28.7nM, and 138.9 ± 58.2nM in the control, toborinone, and olprinone groups, respectively (P 0.02). Calcium influx through calcium channels in the plasma membrane was also suppressed by toborinone and olprinone.Conclusion Toborinone (9.7µM) and olprinone (3.6µM) inhibit human platelet aggregation, though these concentrations are higher than their therapeutic plasma concentrations. The inhibitory effects of both drugs are related to the inhibition of both Ca²⁺ release and Ca²⁺ entry through [cAMP]_i elevation.