Calmodulin activation of the Ca2+ pump revealed by fluorescent chelator dyes in human red blood cell ghosts

Ca2+ transport in red blood cell ghosts was monitored with fura2 or quin2 incorporated as the free acid during resealing. This is the first report of active transport monitored by the fluorescent intensity of the chelator dyes fura2 (5-50 microM) or quin2 (250 microM) in hemoglobin-depleted ghosts. Since there are no intracellular compartments in ghosts and the intracellular concentrations of all assay chelator substances including calmodulin (CaM), the dyes, and ATP could be set, the intracellular concentrations of free and total Ca [( Cafree]i and [Catotal]i) could be calculated during the transport. Ghosts prepared with or without CaM rapidly extruded Ca2+ to a steady-state concentration of 60-100 nM. A 10(4)-fold gradient for Ca2+ was routinely produced in medium containing 1 mM Ca2+. During active Ca2+ extrusion, d[Cafree]i/dt was a second order function of [Cafree]i and was independent of the dye concentration, whereas d[Catotal]i/dt increased as a first order function of both the [Cafree]i and the concentration of the Ca:dye complex. CaM (5 microM) increased d[Catotal]i/dt by 400% at 1 microM [Cafree]i, while d[Cafree]i/dt increased by only 25%. From a series of experiments we conclude that chelated forms of Ca2+ serve as substrates for the pump under permissive control of the [Cafree]i, and this dual effect may explain cooperativity. Free Ca2+ is extruded, and probably also Ca2+ bound to CaM or other chelators, while CaM and the chelators are retained in the cell.