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The effects of cilostazol (OPC-13013, 6-[4-(1-cyclohexyl-1H-tetrazol-5-yl)butoxy]-3,4-dihydro-2(1H)-quinolinone) on cyclic nucleotide metabolism and Ca²⁺-induced contraction of intact and skinned rabbit arterial smooth muscles were investigated. The concentrations of cilostazol producing 50% inhibition of cyclic adenosine monophosphate phosphodiesterase and Ca²⁺-dependent cyclic nucleotide phosphodiesterase were 0.4 μM and above 100 μM, respectively. This compound has no significant effect on adenylate cyclase in concentrations of up to 100 μM. Addition of cilostazol increased significantly the cAMP content without significant effect on cyclic guanosine monophosphate level of rabbit thoracic aorta in the presence of forskolin. Moreover, the ED₅₀ value of cilostazol in relaxation of rabbit mesenteric arterial strips was decreased selectively by addition of 0.01 μM forskolin, which alone at this concentration has no effect on vascular contraction. Cilostazol of up to 30 μM did not suppress the Ca²⁺-induced contraction of the chemically skinned rabbit mesenteric artery. Therefore, cilostazol may produce the relaxation of intact vascular smooth muscle by its inhibition of cyclic adenosine monophosphate hydrolysis.

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