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

In many cell types, the emptying of intracellular Ca super(2+) stores results in the opening of store-operated Ca super(2+) channels in the plasma membrane. However, the nature of the signal that couples store content to the opening of these Ca super(2+) channels is unclear. One model proposes that the Ca super(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 super(2+) current I sub(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 sub(CRAC) to activate. We find that the recombinant protein alpha -SNAP super(1-285), an inhibitor of exocytosis in many systems, suppresses such fusion but has no impact on the activation of I sub(CRAC). A variety of other manoeuvres that interfere with vesicle trafficking and exocytosis were also without effect on I sub(CRAC). Impairing constitutive exocytosis with brefeldin A reduced the extent of I sub(CRAC), but this effect was less pronounced when current density was considered instead. Activation of I sub(CRAC) can therefore be clearly dissociated from an exocytotic mechanism, a finding that is not easily reconcilable with the vesicular fusion model.