Regulation of L-arginine uptake by Ca2+ in human platelets

L-Arginine uptake and Ca2+ changes in unstirred platelets activated by thrombin, collagen and Ca2+ 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 mu M. In FURA 2-loaded platelets collagen did not modify Ca2+ basal level, thrombin induced a late Ca2+ rise and A23187 dose-dependently increased cytosolic Ca2+, eliciting the highest increase at 5 mu M. It is likely that L-arginine uptake is inversely modulated by Ca2+ concentrations and is inhibited during platelet stimulation with agonists which induce cytosolic Ca2+ elevation