The exchangeable splenic platelet pool in response to selective adrenergic beta-i-receptor blockade.

The aim of the present work was to investigate the effect of selective beta-I-blockade on the exchangeable splenic platelet pool (ESPP). Therefore, 50 mg of metoprolol (a selective beta-I-receptor blocking agent) was given by mouth to three groups of subjects: (1) 15 healthy volunteers, (2) five asplenic subjects, and (3) 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 less than 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 greater than P greater than 0.05). In the group of asplenic subjects no change in the platelet count occurred. It is concluded that acute oral administration of metoprolol causes a significant release of platelets from the ESPP. This platelet release is, however, no enhanced if the ESPP is enlarged.