IgE receptor-activated calcium permeability pathway in rat basophilic leukemia cells: measurement of the unidirectional influx of calcium using quin2-buffered cells

The intracellular calcium indicator and buffer quin2 has been used to generate a large calcium buffering capacity in the cytoplasm of rat basophilic leukemia cells. Above 3 mM intracellular quin2, there is no further increase in the initial rate of antigen-induced 45Ca uptake, suggesting that 45Ca buffering by quin2 is now sufficient to prevent the immediate efflux of 45Ca from the cells. Thus, the initial rate of 45Ca uptake should reflect the true unidirectional influx of calcium that occurs when immunoglobulin E (IgE) receptors are aggregated by antigen. The antigen-induced calcium permeability pathway appears to be saturable, with a Km of about 0.7 mM and a Vmax of 0.9 nmol of calcium (10(6) cells)-1 min-1. Although net 45Ca uptake reaches a plateau a few minutes after antigen stimulation, the increase in plasma membrane permeability is maintained for at least an hour, provided that receptors for IgE remain aggregated. The initial rate of 45Ca influx correlates well with the subsequent secretion of 3H]serotonin in response to different concentrations of antigen. Both 45Ca uptake and 3H]serotonin secretion are maximal when only 10% of the receptors are occupied with antigen-specific IgE. Thus, 45Ca influx correlates more closely with secretion than with the number of IgE receptors aggregated by antigen.