The in vitro plasma clot technique was employed to examine the role of calcium during the interaction of erythropoietin and mouse erythroid progenitor cells. Erythropoietin-induced erythroid colony formation was increased 24% by the carboxylic ionophore A23187 (10 nM), whereas a 35% increase was produced by the carboxylic ionophore Ro 2-2985/1 (1 nM). EGTA (3 mM) inhibited erythropoietin-induced erythroid colony formation. Inhibition of erythroid colony formation by EGTA could be reversed by Ca2+, but not by Mn2+, Mg2+, Zn2+, or Fe2+. At least 30 min exposure of marrow cells to erythropoietin in vitro was required for production of erythroid colonies. EGTA substantially inhibited erythropoietin-induced erythroid colony formation even when the marrow cells were exposed to the hormone for up to 2 h before addition of the chelator. Marrow cells incubated first in calcium-free medium with erythropoietin and then cultured in the presence of calcium but not erythropoietin, failed to form erythroid colonies although colony formation occurred when erythropoietin was provided. Taken together, the data indicate that calcium is required for both extracellular and intracellular events during the interaction of erythropoietin with its target cells.
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