We have investigated whether recombinant erythropoietin (r-Epo) elicits a change in intracellular free calcium (IFC) in purified Epo-responsive cells in spleens of mice treated with phenylhydrazine. Colony-forming units (CFU-E) were prepared by negative selection through immunologic panning. Anti-Forssman, Mac-1, Ia, and HSA antibodies were used to eliminate nonhematopoietic progenitors. After two pannings, 29 +/- 1.5% (mean +/- 1 SD) of the recovered cells were CFU-E. IFC was measured by labeling cells with the fluorescent dye Indo-1 and analyzing them on a flow cytometer from 15 seconds to 30 minutes after the addition of agonist. At each step of the panning procedure, there was no effect of r-Epo (0 to 10 U/mL) on IFC even in the larger cells that are predominantly CFU-E. As a positive control, calcium ionophore (A23187) significantly increased IFC in greater than 90% of the spleen cells enriched in CFU-E. During growth of CFU-E in methylcellulose, the calcium ionophore did not affect the r-Epo-dependent formation of erythroid colonies. EGTA inhibited the formation of erythroid colonies. This inhibition appeared to be the result of a toxic effect of the chelator because the colony growth could not be restored when Ca2+ was added to the cultures in the presence of the EGTA. We conclude that the biologic action of Epo on responsive erythroid cells does not depend on acute changes in IFC.

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