Abnormal Ca2+ homeostasis in platelets of patients with myeloproliferative disorders: low levels of Ca2+ influx and efflux across the plasma membrane and increased Ca2+ accumulation into the dense tubular system.

Ca2+ influx and efflux in unstimulated platelets and Ca2+ uptake by simultaneously isolated two membrane fractions of platelets from patients with myeloproliferative disorders (MPD) have been investigated. In MPD, Ca2+ influx and efflux across the plasma membrane in unstimulated platelets were in equilibrium at significantly lower levels than in normals. Ca2+ uptake by external membrane fraction isolated from MPD platelets was lower, whereas, uptake by internal membrane fraction enriched with dense tubular system (DTS) from MPD platelets was significantly higher than that from normal platelets. This corresponded with the membrane associated Ca2+-activated ATPase activity. These abnormalities of calcium ion movement in plasma membrane and dense tubular system illustrates one of the mechanisms of qualitative abnormalities of MPD platelets.