Activation of neutrophil superoxide production by concanavalin A can occur at low levels of intracellular ionized calcium

The effect of concanavalin A (Con A) on the concentration of ionized intracellular calcium [( Cai++]) in human granulocytes (PMN) was monitored using the fluorescent calcium indicator and chelator, Quin2. The addition of Con A to PMN resulted in a rise in [Cai++] that was markedly enhanced by the presence of Ca++ in the external buffer. The onset of the increment in [Cai++] preceded the onset of O2- production. The rise in [Cai++] induced by Con A is not transient, with a new, higher steady-state level of [Cai++] being attained within five minutes. The addition of alpha-methylmannoside (alpha-MM) one minute after Con A resulted in the return of [Cai++] to the original baseline level and the cessation of O2- production. The addition of a second stimulus (such as arachidonic acid) to these cells resulted in a second increment in [Cai++] and the return of O2- production. Thus the rise in [Cai++] induced by Con A is tightly coupled to the activation, inactivation, and reactivation of the O2- generating system by Con A. Further experiments were undertaken to assess the possible requirement for the rise in [Cai++] in the activation of PMN by Con A. PMN could be depleted of Cai++ by loading with Quin2 in the absence of extracellular Ca++. These Ca++- depleted PMN can be induced to produce O2- after treatment with PMA but not with Con A. The addition of Ca++ to Ca++ - depleted PMN results in a return of [Cai++] to the normal resting level of Ca++ -replete PMN. The time required to return to baseline is a function of the concentration of intracellular Quin2. The addition of Ca++ to Ca++ -depleted, Con A-treated PMN results in the elevation of [Cai++] and the production of O2-. Over a tenfold range of intracellular Quin2 concentration, the onset of O2- production always occurred at [Cai++] that were less than the normal resting level. Thus, activation of the O2- generating system by Con A can occur at [Cai++] which are much lower than the incremental level induced by Con A in Ca++ -replete PMN. Supporting this is the observation that only a very small increment in [Cai++] is induced by Con A in PMN cytoplasts, even though Con A could induce O2- production in the Quin2-loaded cytoplasts.