SUBCELLULAR DISTRIBUTION OF RESERPINE IN BLOOD PLATELETS: EVIDENCE FOR MULTIPLE POOLS

¹ Research Department, F. Hoffmann-La Roche & Co. Ltd., Basel, Switzerland

After i.v. administration, ³H-reserpine accumulated in blood platelets showed an initial rapid decrease between and 6 hours. Thereafter, the ³H-reserpine content remained virtually constant through day 5 followed by another marked diminution which was accompanied by a gradual recovery of the 5-hydroxytryptamine (5-HT). The 5-HT organelles isolated from the platelets showed a similar time course of the ³H-reserpine disappearance except that no initial rapid decrease occurred. Pretreatment with unlabeled reserpine interfered with the accumulation of ²H-reserpine in whole platelets and 5-HT organelles, whereas administration of the unlabeled after the labeled drug did not diminsh the level of radioactivity. Imipramine, given shortly before ³H-reserpine, inhibited the accumulation of the labeled drug in the whole platelets during the first hour, but not at later times. The content of ³H-reserpine in the 5-HT organelles was not changed by imipramine. Subcellular distribution experiments indicated a localization of the ³H-reserpine in the membranes of the 5-HT storage granules as well as in other subcellular organelles. Furthermore, platelet survival as measured by ⁵¹Cr was identical to that determined by in vitro or in vivo labeling of the platelets with ³H-reserpine. It is concluded that some reserpine initially accumulates reversibly outside the 5-HT granules, but that the bulk of the drug is permanently and irreversibly bound to 5-HT granules as well as to other sites in platelets and probably megakaryocytes. Owing to the permanence of this binding, reserpine might be useful as a tool for measuring platelet survival and for investigations on production of platelets from megakaryocytes.