Regulation of L-arginine uptake by Ca(2+) in human platelets

L-Arginine uptake and Ca(2+) changes in unstirred platelets activated by thrombin, collagen and Ca(2+) ionophore A23187 were evaluated. Thrombin did not affect L-arginine uptake at short incubation times (2-15 min), but at prolonged times slowed down the amino acid transport. Collagen was ineffective. A23187 decreased the L-arginine uptake in a dose-dependent manner, producing the maximal inhibition at 5 microM. In FURA 2-loaded platelets collagen did not modify Ca(2+) basal level, thrombin induced a late Ca(2+) rise and A23187 dose-dependently increased cytosolic Ca(2+), eliciting the highest increase at 5 microM. It is likely that L-arginine uptake is inversely modulated by Ca(2+) concentrations and is inhibited during platelet stimulation with agonists which induce cytosolic Ca(2+) elevation.