Ion channels in human erythroblasts: Modulation by erythropoietin

To investigate the mechanism of intracellular Ca²⁺ ([Ca_il) 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 Chromatograph), 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].