L-carnitine inhibits a subset of platelet activation responses in chronic uraemia

Abstract

Activated uraemic platelets expose the aminophospholipid phosphatidylserine (PS) at their outer surface, which generates a cell procoagulant phenotype and seems at least partly due to an increase in cell caspase-3 activity. L-carnitine (LC) may decrease surface-exposed PS in stored apheresis platelets and inhibit the activity of recombinant caspases, but its effects on platelet activation response with PS externalization have not been ascertained in chronic renal failure. In the present study, we investigated in vitro and in vivo the effects of LC on PS exposure in platelets from chronic uraemic patients. Platelet PS-exposure was assayed by flow cytometry using annexin V. Caspase activity in platelets was determined by the cleaving activity of the specific substrate DEVD-pNA and by a flow cytometric assay using rhodamine-fluorescence. The effects of LC in vivo were examined in a prospective cross-over trial including 10 patients on maintenance haemodialysis (HD) who were randomly allocated to two different treatment groups: LC (2 g i.v.) for 4 months followed by placebo (2 g i.v.) for another 4 months (group A), or placebo followed by LC (group B). PS-exposing platelets in blood samples obtained from HD patients were significantly higher than in healthy subjects (P<0.001) under both unstimulated and agonist-stimulated conditions. When uraemic platelets were pre-incubated with LC before agonist stimulation, platelet PS exposure proved to be significantly reduced (-13.7% for 0.5 mM LC and -25% for 5 mM LC). Pre-incubation of uraemic platelets with LC again significantly decreased the cells' caspase activity (P<0.05). In HD patients (Group A), LC supplementation was associated with a significant decrease (P<0.05) in platelet PS exposure followed by a progressive increase during treatment with placebo. In the other group of patients, while no change in platelet PS exposure was observed during the first 4 months of treatment with placebo, a significant reduction (P<0.05) in PS-positive platelets occurred after 2 and 4 months of LC therapy. Our data show that LC may reduce, possibly via inhibition of caspase activity, the exposure of PS in activated uraemic platelets. These findings may have implications for the thrombophilic tendency of uraemia.