Isoprenaline induction of cAMP-phosphodiesterase in guinea-pig macrophages occurs in the presence, but not in the absence, of the phosphodiesterase type IV inhibitor rolipram

The long-term effects of incubating freshly isolated, elicited guinea-pig peritoneal macrophages with the beta-adrenoceptor agonist isoprenaline and the selective inhibitor of phosphodiesterase (PDE) IV rolipram, on adenosine-3',5'-cyclic phosphate (cAMP)-specific PDE IV activity have been investigated. The level of cAMP PDE activity in macrophages was unaffected by long-term exposure of cells to rolipram alone. In contrast, in the presence of isoprenaline (10 mu M), a concentration-related (0.05-50 mu M) increase in cAMP PDE activity was observed in the cytosolic and particulate fractions. Incubation with isoprenaline alone did not affect macrophage cAMP PDE. cAMP PDE activities in homogenates of control cells and macrophages treated with isoprenaline (10 mu M) and rolipram (5 mu M) (3-fold activation) were inhibited by the selective PDE IV inhibitor, rolipram, with similar potencies (IC50: 2-3 mu M). The increase in cAMP PDE activity in response to rolipram and isoprenaline was completely blocked by cyclohexamide (10 mu g/ml). Incubating macrophages for 10 min with rolipram increased cAMP accumulation in the presence, but not in the absence, of isoprenaline (10 mu M) over the same concentration range that induction of cAMP PDE activity was observed. cAMP levels remained elevated for at least 1 hr. Isoprenaline (10 mu M) alone induced a transient elevation in cAMP levels that peaked at 2 min and had returned to basal levels by 10 min. Protein kinase A activity (PKA) was increased almost 10-fold (at 10 min) by exposing cells to rolipram plus isoprenaline and remained elevated for at least 4 hr. Isoprenaline alone induced a small (2-fold) increase in PKA activity and rolipram alone was without effect.