Uptake of 14C-5-hydroxytryptamine by human and rat platelets and its pharmacological inhibition

Abstract

In order to approach the uptake of ¹⁴C-5HT by platelets as a first-order process, experimental conditions were selected in which accumulation of the amine either by diffusion or by other passive nonsaturable processes could be excluded. These conditions included an incubation period of ¹⁴C-5HT with human or rat platelets of 4 min or 30 s, respectively and the use of substrate concentrations around the calculated apparent K _m values (0.25–2.0 μM). While the apparent K _m values were rather similar for human and rat platelets, V_max was about 5 times higher in rat than in human platelets. The kinetic model adopted in this study was used to evaluate the relative potency and the type of inhibition of ¹⁴C-5HT uptake exhibited by imipramine, chlorimipramine and (+)-fenfluramine. All 3 compounds inhibited ¹⁴C-5HT uptake by platelets. Chlorimipramine was about 10 times more effective than imipramine both in rat and in human platelets. Both drugs were more potent inhibitors on human than on rat platelets. (+)-Fenfluramine was almost as active as imipramine on rat but 30 times less potent than imipramine on human platelets. Both imipramine and chlorimipramine inhibited ¹⁴C-5HT uptake by an apparent non-competitive mechanism, whereas (+)-fenfluramine appeared to act as a competitive inhibitor. No differences were found in this respect between human and rat platelets. Pharmacological or therapeutic doses of these drugs usually result in plasma concentrations similar to those found in this study to effectively inhibit platelet ¹⁴C-5HT uptake.