Stochastic nature and red cell population distribution of the sickling-induced Ca2+ permeability

To explore basic properties of the sickling-induced cation permeability pathway, the Ca2+ component (Psickle-Ca) was studied in density-fractionated sickle cell anemia (SS) discocytes through its effects on the activity of the cells' Ca2+-sensitive K+-channels (K-Ca). The instant state of K-Ca channel activation was monitored during continuous or cyclic deoxygenation of the cells using a novel thiocyanate-dense-cell formation method, Each deoxy pulse caused a reversible, sustained Psickle-Ca which activated K-Ca channels in only 10-45% of cells at physiological [Ca2+](0) (''activated cells''), After removal of cells activated by each previous deoxy pulse, subsequent pulses generated similar activated cell fractions, indicating a random determination rather than the response of a specific vulnerable subpopulation, The fraction of activated cells rose monotonically with [Ca2+](0) along a curve reflecting the cells' distribution of Psickle-Ca, with values high enough in a small cell fraction to trigger near-maximal K-Ca channels, Consistent with the stochastic nature of Psickle-Ca, repeated deoxygenated-oxygenated pulsing led to progressive dense cell formation, whereas single long pulses caused one early density shift, Thus deoxygenation-induced Ca2+-permeabilization in SS cells is a probabilistic event with large cumulative dehydrating potential, The possible molecular nature of Psickle-Ca is discussed.