The Exchangeable Splenic Platelet Pool in Response to Selective Adrenergic Beta-1-Receptor Blockade

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Abstract
Metoprolol [50 mg] (a selective .beta.-I-receptor blocking agent) was given by mouth to 3 groups of subjects: 15 healthy volunteers, 5 asplenic subjects, and 10 patients with different myeloproliferative disorders (MPD) with and without splenomegaly. The platelet count was determined 90, 150, 210, 270 and 330 min after the ingestion of the drug. In the group of healthy controls a significant increase (P < 0.005) in the platelet count occurred and amounted to about 10% over baseline. The average peak value was reached at 150 min and the elevation in platelet count was maintained more than 4 h. An increase in the platelet count was also induced in patients with MPD, but the mean for the peak value only reached borderline significance (0.10 > P > 0.05). In the group of asplenic subjects no change in the platelet count occurred. Acute oral administration of metoprolol apparently causes a significant release of platelets from the ESPP [exchangeable splenic platelet pool]. This platelet release is not enhanced if the ESPP is enlarged.
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