Mast cell protease expression, histamine content, and uptake of ³H-5HT in different populations of mast cells in vitro. RT-PCR analysis of mast cell protease expression was performed on BMCMC in CM and BMCMC in SCF for the murine mast cell proteases (mMCP)-2 and -4, and the exopeptidase carboxypeptidase A (mMCCPA) (A). Total histamine content of Cl.MC/C57.1, BMCMC in CM, and BMCMC in SCF was determined by fluorometric assay and expressed as pg of histamine per cell (B). The ability of the three mast cell populations to take up³H-5HT was assessed. Mast cells were incubated with³H-5HT for 2 hours, then washed. Total radioactive counts in the cell pellets were measured and expressed as cpm (C). Data in (B and C) are shown as mean ± SEM (n = 2 to 4/group). Similar results were obtained in eight repeat experiments.