Involvement of cyclic AMP in the effects of phosphodiesterase IV inhibitors on arachidonate release from mononuclear cells

The effects of selective phosphodiesterase inhibitors, cyclic AMP (cAMP) elevating agents and stable analogues of cyclic nucleotides, on the release of arachidonate induced by N-formyl-Met-Leu-Phe (fMLP) were investigated on human peripheral blood mononuclear cells. The selective phosphodiesterase IV inhibitors, rolipram and Ro 20–1724, and the non-selective phosphodiesterase inhibitor, theophylline, elicited a concentration-dependent inhibition of arachidonate release (EC₅₀ = 1.3 × 10⁻⁶ M, 3.2 × 10⁶⁻ M and 3.7 × 10⁻⁴ M respectively). The selective phosphodiesterase III inhibitor, milrinone (10⁻⁵ M), only caused a slight effect while the phosphodisterese V inhibitor, zaprinast (10⁻⁵ M), the β₂-adrenoceptor agonists, salbutamool and fenoterol (10⁻⁵ M), failed to inhibit arachidonate release. Forskolin (10⁻⁵ M) and N⁶,2′-O-dibutyryladenosien 3′:5′cyclic monophosphate (db-cAMP, 10⁻³ M) elicited a moderate inhibition. Forskolin increased the effects of rolipram and Ro 20–1724 (EC₅₀ = 4.5 × 10⁻⁷ M and 4 × 10⁻⁷M respectively). Incubation of the cells with rolipram (10⁻⁸ to 10⁻⁵ M), Ro 20–1724 (10⁻⁸ to 10⁻⁵ M), forskolin (10⁻⁵ M) or salbutamol (10⁻⁵ M) alone, induced a moderate increase or no increase at all in intrecellular cAMP. However, in the presence of forskolin, rolipram (10⁻⁸ to 10⁻⁶M) and Ro 20–1724 (10⁻⁸ to 10⁻⁶M) induced a significant and concentration-dependent increase in intracellular levels of cAMP. These results suggest that the potent inhibition of arachidonate release from mononuclear cells by selective phosphodiesterase IV inhibitors may be due to increases in discrete pools of intracellular cAMP.