Cyclic AMP counteracts mitogen‐induced inositol phosphate generation and increases in intracellular Ca2+ concentrations in human lymphocytes

Abstract

The effects of increases in intracellular adenosine 3′:5′‐cyclic monophosphate (cyclic AMP) on mitogen‐induced generation of inositol phosphates and increases in intracellular Ca2+ concentration were investigated in human peripheral blood mononuclear leukocytes (MNL). The mitogens concanavalin A (Con A), pokeweed mitogen (PWM) and phytohaemagglutinin (PHA) concentration‐dependently stimulated generation of inositol phosphates. Catecholamines inhibited this process with an order of potency: isoprenaline > adrenaline > noradrenaline indicating involvement of β2‐adrenoceptors. This order of potency was also consistent with the catecholamine potencies for stimulating the generation of cyclic AMP. In addition to catecholamines, the cyclic AMP formation‐stimulating agents prostaglandin E1 (PGE1) and forskolin concentration‐dependently inhibited mitogen‐induced inositol phosphate generation, too. Moreover, the inhibitory effect of isoprenaline was potentiated by co‐incubation with the phosphodiesterase inhibitor isobutylmethylxanthine demonstrating that these inhibitory effects were mediated by cyclic AMP. Con A and PHA concentration‐dependently increased the intracellular Ca2+ concentration in human MNL (assessed by the fluorescent indicator dye Fura‐2). This increase was almost completely blocked by chelation of extracellular Ca2+, demonstrating influx rather than mobilization from intracellular stores. The elevation of intracellular Ca2+ was not blocked by pretreatment with pertussis toxin, 100 ng ml−1, for 16 h. Isoprenaline, PGE1, and forskolin, however, inhibited the mitogen‐stimulated elevation of intracellular Ca2+. This inhibition was enhanced by the phosphodiesterase inhibitors isobutylmethylxanthine and Ro 20–1724, demonstrating mediation by cyclic AMP. We conclude that catecholamines and other cyclic AMP increasing agents can inhibit mitogen‐stimulated generation of inositol phosphates and elevation of intracellular Ca2+ in resting human MNL. Since generation of inositol phosphates and increases in intracellular Ca2+ are very early events in activation of MNL, attenuation of these events might well contribute to the inhibitory effect of cyclic AMP on MNL function. 1991 British Pharmacological Society