The accumulation of guanethidine by human blood platelets

Abstract

1. When human blood platelets were incubated aerobically in plasma containing 2 x 10-7 to 10-3M radioactive guanethidine for 10 min to 6 hr, the drug was accumulated against a concentration gradient until concentration ratios (platelet/plasma) of up to 80: 1 were obtained. 2. The decline in rate of uptake after 3 hr appeared to result from a decrease in platelet viability, because accumulation was reduced by prolonged incubation before addition of guanethidine. 3. Uptake was energy-dependent because it was inhibited by cold and ouabain. 4. Sodium ions were essential for guanethidine uptake and retention of 5-hydroxytryptamine (5-HT). 5. Accumulation was inhibited by 5-HT, desipramine, cocaine, dexamphetamine, bretylium, tyramine and noradrenaline; bethanidine, p-chlorophenylalanine and (-)-a-methyldopa were inactive. 6. Guanethidine was tightly bound to platelets, only 10% being lost from labelled cells during 60 min incubation in drug-free plasma; but efflux was increased by addition of amphetamine. 7. The binding sites for guanethidine seemed to be different from those for 5-HT since guanethidine accumulation was independent of 5-HT levels, and neither guanethidine uptake or release were affected by reserpine. 8. Guanethidine was not metabolized by platelets or plasma in vitro. 9. We consider that, if our results regarding uptake, binding and release of guanethidine are confirmed in vivo, and also found to apply to other pharmacologically active agents, then the eventual loss of a platelet-bound substance may increase pharmacological action by raising plasma levels.Although there is considerable information concerning the mechanisms of uptake and sites of storage of 5-hydroxytryptamine (5-HT) and noradrenaline (NA) by blood platelets, the fact that platelets can also accumulate a wide variety of other substances such as procaine, histamine, reserpine, quinidine, amino-acids, sugars Guanethidine uptake by platelets and diuretic drugs has only recently been demonstrated (Solomon & Zieve, 1967;Zieve & Solomon, 1967.Recent work indicates that in several respects the blood platelet resembles the sympathetic nerve ending. Although 5-HT is the'predominant amine in platelets and sympathetic nerves contain only NA, both structures preferentially accumulate the respective amine by an energy-dependent mechanism and store it in sub-cellular vesicles which show a dense core under the electron microscope (for references see Potter, 1967;Pletscher, 1968). As the uptake of guanethidine by sympathetic nerves is well established (for references see Boullin, 1968), it was thought that blood platelets might also take up guanethidine. This paper shows that this is the case: guanethidine is concentrated to a considerable degree by an energy-dependent process. As the ratio of the concentration of guanethidine in platelets to that in the incubation medium may exceed 50: 1, the possibility that platelet-bound guanethidine may be of clinical significance is discussed.Some of these experiments have been reported in preliminary communica...

Transport of Taurine by Normal Human Blood Platelets

“…When the plasma concentration of taurine was low, I nmol/ml, accumulation was approximately linear, but at the higher concentrations of 10 and 100 nmol/ml the platelet content of [ 14C] -taurine approached equilibrium after 2 h incubations ( Figure 1). (Boullin & O'Brien, 1969; Table 3. The ability of platelets to accumulate sufficient exogenous radio-labelled taurine in vitro to result in a highly significant decrease in medium concentrations, suggests that The results are from experiments in which platelets were incubated in autologous plasma or Krebs solution for 2 hours.…”

“…Platelets were counted visually by the method of Feissley (1961) and the volume of cells per ml of PRP was determined with thrombocytocrits as described by Boullin & O'Brien (1969 …”

Accumulation of quinidine by human blood platelets: effects on platelet ultrastructure and 5‐hydroxytryptamine

“…The uptake and efflux of quinidine, 5-HT and 14C-5-HT were measured by the procedures given earlier (Boullin & O'Brien, 1969) except that before assay the platelets were disrupted by sonification with a 'Biosonik 2' sonifier (Bronwill Scientific, Rochester, N.Y.), at probe setting 50 in either distilled water or 0-1 N H2SO4. The volume of fluid trapped between the packed platelets after centrifugation was determined by estimation of the radioactivity recovered from the platelet pellet after incubation with 14C-inulin for 10-90 minutes.…”

The mechanism of adenosine diphosphate induced platelet aggregation: binding to platelet receptors and inhibition of binding and aggregation by prostaglandin E1

“…Also, we separated platelets from plasma within 25 see of the end of each experiment in order to reduce to the minimum metabolic or other changes that occurred in platelet bound nucleotides which were unrelated to the aggregation response and took place in the interim between the end of aggregation and the separation of the various nucleotide components. METHODS Platelet-rich plasma was obtained from seventeen normal subjects as previously described (Boullin & O'Brien, 1969) except that 9 ml. venous blood was mixed with 1 ml.…”