Modulation of the Cationic Amino Acid Transport System y+L by Surface Potential, Ouabain and Thrombin in Human Platelets: Effects of Uremia

Abstract

Background: Nitric oxide (NO), a key endogenous mediator involved in the maintenance of platelet function, is synthesized from the amino acid L-arginine. We have shown that L-arginine transport in platelets is rate-limiting for NO synthesis. A disturbance in the L-arginine-NO pathway in platelets was previously described in chronic renal failure (CRF) patients. Methods: Detailed kinetic studies were performed in platelets from controls (n = 60) and hemodialysis patients (n = 26). Results: The transport of L-arginine in platelets is mediated via system y⁺L, which is competitively inhibited by L-leucine in the presence of Na⁺ and by the irreversible inhibitor pCMB. In platelets, system y⁺L is markedly stimulated by an Na⁺/K⁺-ATPase inhibitor, ouabain, and by changes in surface potential, while it is downregulated by intraplatelet amino acid depletion (zero-trans) and by thrombin. In CRF patients, activation of L-arginine transport was limited to well-nourished patients compared to malnourished patients and controls, where it was reduced and did not differ significantly among the groups under zero-trans conditions. Conclusion: Our results provide the first evidence that system y⁺L in platelets is modulated by zero-trans conditions, surface potential, thrombin and intraplatelet Na⁺ concentration. Our findings suggest that enhanced transport in CRF involves increased L-arginine exchange with intraplatelet neutral amino acids.