Activation of the store-operated calcium current/CRAC can be dissociated from regulated exocytosis in rat basophilic leukaemia (RBL-1) cells

In many cell types, the emptying of intracellular Ca(2+) stores results in the opening of store-operated Ca(2+) channels in the plasma membrane. However, the nature of the signal that couples store content to the opening of these Ca(2+) channels is unclear. One model proposes that the Ca(2+) channels are initially stored in cytoplasmic vesicles but inserted into the plasma membrane upon store depletion via a regulated exocytoytic mechanism (vesicular fusion model). Using the whole-cell patch-clamp technique to measure the store-operated Ca(2+) current I(CRAC) and the capacitance method to monitor vesicular fusion, an indicator of exocytosis, we have investigated the effects of interfering with regulated exocytosis on the ability of I(CRAC) to activate. We find that the recombinant protein alpha-SNAP(1-285), an inhibitor of exocytosis in many systems, suppresses such fusion but has no impact on the activation of I(CRAC). A variety of other manoeuvres that interfere with vesicle trafficking and exocytosis were also without effect on I(CRAC). Impairing constitutive exocytosis with brefeldin A reduced the extent of I(CRAC), but this effect was less pronounced when current density was considered instead. Activation of I(CRAC) can therefore be clearly dissociated from an exocytotic mechanism, a finding that is not easily reconcilable with the vesicular fusion model.