Defective calcium signalling in uraemic platelets and its amelioration with long-term erythropoietin therapy.

BACKGROUND: Chronic renal failure (CRF) is associated with prolonged bleeding time and impaired platelet adhesion and aggregation. Erythropoietin (Epo) administration improves platelet adhesion/aggregation and ameliorates prolongation of bleeding time in CRF. However, the mechanisms of improved platelet function after Epo therapy have not been fully elucidated. The present study examined the hypothesis that the improved uraemic platelet function after Epo therapy is, in part, due to correction of the platelet calcium signalling. METHODS: Rats were randomized into four groups after 5/6 nephrectomies to produce CRF. The Epo-treated CRF group received Epo, 150 U/kg, twice weekly for 6 weeks to prevent anaemia; the felodipine and Epo-treated CRF group received Epo but was kept normotensive by felodipine treatment; the placebo-treated CRF group received placebo injections and became anaemic; and the iron-deficient CRF group received Epo but was kept anaemic by dietary iron-deficiency. A group of sham-operated rats was included as normal control. Basal and thrombin-stimulated platelet cytosolic calcium (Ca(2+)](i)) were determined using a Ca(2+)-sensitive dye (fura-2). RESULTS: Platelets from placebo-treated CRF group exhibited a profound attenuation of thrombin-stimulated surge in Ca(2+)](i), which is the final pathway of platelet activation. Long-term Epo administration led to a normalization of the thrombin-induced rise in platelet Ca(2+)](i) in the CRF animals, independent of either haematocrit or blood pressure values. Further studies revealed that improved Ca(2+) signalling with Epo is associated with increased Ca(2+) uptake and expanded Ca(2+) stores in the platelets. CONCLUSIONS: The defective Ca(2+) signalling in uraemic animals and its improvement with chronic Epo therapy provides the biochemical basis of the previously reported platelet dysfunction and prolonged bleeding time in uraemic patients and animals, and their amelioration with chronic Epo therapy.