Basophil variants with impaired cromoglycate binding do not respond to an immunological degranulation stimulus.

Cromoglycate, which inhibits IgE-mediated degranulation of mast cells and a basophilic rat tumour cell line (RBL-2H3) (ref.3), is a drug widely used in the treatment of allergic asthma. We have demonstrated the presence of specific binding sites for that drug on the membranes of basophils and mast cells and more recently, we have succeeded in isolating the cromoglycate-binding protein from the membranes of RBL-2H3 cells. These findings together with the chelation by cromoglycate of alkaline-earth ions in low polarity medium, suggest that the binding site of the drug may either be part, or closely related to the calcium gate. To further investigate this, we have selected variants of the RBL-2H3 cell line that are defective in cromoglycate binding but bind IgE normally. In these variants, which have similar histamine content to the parental cells, IgE-mediated challenge did not lead to Ca2+ influx, degranulation and histamine release. In contrast, these cells were able to release histamine on exposure to the Ca2+ ionophore A23187, indicating that the degranulation mechanism distal to the Ca2+ gating step is unaffected. Taken together, these findings suggest that cromoglycate-binding protein has a role in the transmembrane calcium influx which induces degranulation.