Direct flow cytometric quantification of alkaline phosphatase activity in rat bone marrow stromal cells

Cell preparations in cytochemistry are conventionally analyzed with transmitted light after fixation and reaction with agents such as azo-coupling dyes. With cell suspensions stained with fluorescent cytochemical dyes, cells can also be analyzed and sorted by flow cytometry. We have exploited the intense red fluorescence of Fast Red Violet LB generated in cytochemical reactions to perform flow cytometric analyses of alkaline phosphatase (AP) expression in rat bone marrow stromal cells. By modifying staining protocols of single-cell suspensions, we demonstrate that in comparison to staining with Fast Red TR, the method is specific, can distinguish among various levels of enzyme expression within the whole population, and permits enzyme kinetic studies of heterogeneous cell populations. The method was applied to study the effect of the glucocorticoid dexamethasone (Dx) on cell proliferation and AP expression. In low AP-expressing cells, Dx treatment at 10(-8) M increased the [3H]-thymidine labeling index from 3.85% to 5.24% (p less than 0.01). In contrast, high AP-expressing cells were unlabeled by [3H]-thymidine. The staining and analytical methods reported here facilitate the detection, isolation, and quantification of subpopulations of bone marrow stromal cells that express alkaline phosphatase activity. These experiments demonstrate the value of flow cytometry as an adjunct to conventional cytochemical methods.