Ionophore activation of RBL-CR1 results in increased cell surface expression of CR1, but not in the release of the granule constituent β-hexosaminidase. (A) Resting cells in HBSS/g; (B) cells activated with A23187 and Ca⁺⁺ as described; (C) cells pretreated for 5 minutes with 50 nmol/L PMA, then activated with A23187 and Ca⁺⁺ as in (B). CR1 expression was examined by FACS analysis (left panel). Degranulation was assessed by determining the β-hexosaminidase content of both the cell pellet and the supernatant and calculating the percent β-hexosaminidase that was released (right panel). Results are given as the mean ± SEM for four experiments.