Different calcium and oxidative metabolic responses in human blood monocytes during exposure to various agonists

It has become increasingly apparent that certain agonists of phagocytic cells exert their effects by using calcium as an intracellular messenger. Most studies have employed neutrophils for such analyses. We have investigated the requirement of calcium for respiratory burst activation in human blood monocytes by using three triggering agents, concanavalin A (Con A), N-formyl-methionyl-leucil-phenylalanine (FMLP), and phorbol myristate acetate (PMA). There was no overall correlation between the maximal extent of transient rise in cytosolic-free calcium (FMLP greater than Con A greater than PMA) and superoxide (O2-) release (PMA greater than Con A greater than FMLP). PMA, as the most effective agonist of respiratory burst activity, neither increased ionized cytosolic calcium [Ca2+]i nor plasma membrane 45Ca2+ permeability. FMLP evoked increments in [Ca2+]i occurred at lower concentrations than did respiratory burst activation, whereas with Con A a rise in [Ca2+]i and O2- release were parallel. Intracellular Ca2+ release into cytosol was blocked by 8-(N,N-diethylamino)octyl-3,4,5-trimethoxy benzoate, HCl (TMB-8). This caused a parallel inhibition of Con A or FMLP elicited [Ca2+]i transients and O2- release, while burst activity stimulated by PMA was unaffected. Our data thus suggest that a transient rise in [Ca2+]i is essential for Con A and FMLP activation in human blood monocytes. In addition to the calcium signal, other intracellular messengers are required for FMLP stimulation. Respiratory burst activation by PMA appears to act independently of [Ca2+]i transients.