Accumulation of debrisoquin-14C by the human platelet

A modified assay method for determining serotonin uptake in human platelets

Effects of various experimental conditions on serotonin (5-HT) uptake in human platelets were examined. The experimental design allowed the evaluation of the effect of diffusion and other non-saturable processes on the affinity and maximum activity of the membrane pump for 5-HT uptake. Total 5-HT uptake was determined by incubating platelet-rich plasma (PRP) with increasing concentrations of serotonin at 37 degrees C for 4 min. The passive uptake was measured by the addition of various 5-HT concentrations to PRP in buffer at 37 degrees C, followed by immediate transfer to an ice-cold water bath. The difference between the total and passive uptake was linear for 6 min. The affinity (Km) for active platelet serotonin uptake was 0.45 +/- 0.09 mumol/l and maximal rate of uptake (V) was 10.7 +/- 2.1 pmol/10(7) platelets/min. The described method provides a convenient and reliable measure of active 5-HT uptake suitable for clinical investigation. The effect of passive diffusion on kinetic parameters is discussed.

Differential adriamycin distribution to blood components

The differential distribution of doxorubicin (Adriamycin = AM) and daunorubicin (Daunomycin = DM) within the blood components, after an i.v. injection of 10 or 15 mg/kg of body weight, was investigated from its metabolites and quantified by means of the TLC scanning fluorescence technique. AM accumulated in the following order (of decreasing percentages): plasma and red cells (RBC), white cells (WBC), and platelets (PT), but the absolute amount of drug that reached each cell type was related to its relative volume. In the presence of higher blood concentrations (after injection of 15 mg/kg of body weight) the RBCs accumulated much more AM than the plasma, WBC, and PT; suggesting that the RBC fraction has a greater capacity to concentrate the drug. However, if the concentration of AM is expressed per unit volume of each component, a markedly higher value was observed for PT, and this was confirmed by in vitro results obtained by incubating blood in the presence of AM. DM seemed to be distributed on a percent basis to a greater extent than AM in the RBC fraction. Both compounds were taken up by blood cells, particularly platelets, to levels in excess of the extracellular concentration.

Accumulation of debrisoquine by platelets in vivo: a model of events at the peripheral adrenergic neurone

1 The possibility that in vivo uptake of D by human platelets might reflect its accumulation in the post-ganglionic adrenergic neurone, and hence be a useful predictor of hypotensive response, was investigated.

2 During chronic oral dosage of D in three hypertensive patients average concentrations of the drug in blood fractions relative to plasma were: platelet rich plasma, 2.93; whole blood, 2.23; red cells plus granulocytes, 0.75. These findings indicate extensive uptake of D by platelets but not by other cells.

3 After single oral doses of 30 mg debrisoquine to four healthy volunteers platelets continued to accumulate the drug over at least 24 h. Although platelet D concentrations varied between subjects their platelet/plasma drug concentration ratios were similar.

4 Amitriptyline (75 mg p.o.) given 2 h before a single 30 mg oral dose of D inhibited platelet uptake of the latter by 40 ± 14 s.d.% over a 24 h period.

5 Platelets accumulated D but not its inactive 4-hydroxy metabolite. During chronic dosage of D in 10 patients the mean pre-dose platelet/plasma D concentration ratio was 8.52 ± 429 s.d. Within a 12 h dosing interval the concentration of D was constant in platelets but varied two-fold in plasma.

6 Uptake of D by platelets approached saturation with increasing plasma D concentrations.

7 After chronic D therapy in patients the fall in standing diastolic bp was more closely correlated with plasma D concentration (r = -0.88; P < 0.001) than with platelet D concentration (r = -0.65; P < 0.05).

8 In relation to the therapeutic response to D, observations 3-5 but not 7 are consistent with a view of the platelet as a useful model of the peripheral adrenergic neurone.

Uptake of 14C-5-hydroxytryptamine by human and rat platelets and its pharmacological inhibition. A comparative kinetic analysis

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 Km values (0.25 - 2.0 muM). While the apparent Km values were rather similar for human and rat platelets, Vmax 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 inhibiton 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.