ION CHANNELS IN HUMAN ERYTHROBLASTS - MODULATION BY ERYTHROPOIETIN

To investigate the mechanism of intracellular Ca2+ ([Ca(i]) increase in human burst-forming unit-erythroid-derived erythroblasts by erythropoietin, we measured [Ca(i)] with digital video imaging, cellular phosphoinositides with high performance liquid chromatography, and plasma membrane potential and currents with whole cell patch clamp. Chelation of extracellular free Ca2+ abolished [Ca(i)] increase induced by erythropoietin. In addition, the levels of inositol-1,4,5-trisphosphate did not increase in erythropoietin-treated erythroblasts. These results indicate that in erythropoietin-stimulated cells, Ca2+ influx rather than intracellular Ca2+ mobilization was responsible for [Ca(i)] rise. Both Ni2+ and moderately high doses of nifedipine blocked [Ca(i)] increase, suggesting involvement of ion channels. Resting membrane potential in human erythroblasts was -10.9+/-1.0 mV and was not affected by erythropoietin, suggesting erythropoietin modulated a voltage-independent ion channel permeable to Ca2+. No voltage-dependent ion channel but a Ca2+-activated K+ channel was detected in human erythroblasts. The magnitude of erythropoietin-induced [Ca(i)] increase, however, was insufficient to open Ca2+-activated K+ channels. Our data suggest erythropoietin modulated a voltage-independent ion channel permeable to Ca2+, resulting in sustained increases in [Ca(i)].