Heterogeneity of rabbit platelets. V. Preferential splenic sequestration of megathrombocytes.

Rabbits were either splenectomized or subjected to splenic blockade with phenylhydrazine in order to determine the splenic platelet and megathrombocyte (large-platelet) pools. The average splenic platelet pool calculated from both methods was 35% of total platelets. The average splenic megathrombocyte pool was 54% of total megathrombocytes. Adrenaline injection into rabbits or dogs revealed a rapid increase in both platelet count and megathrombocyte number which peaked at 2-6 min and returned toward normal in 5-10 min. The platelet volume distribution curve was shifted to the right, indicating the release of large platelets (megathrombocytes) into the circulation. The peak rise in megathrombocyte number was significantly greater than the peak rise in platelet count. Rabbits subjected to splenic massage shifted their platelet volume distribution curve to the right in the absence of a rise in platelet count. It is concluded that the spleen preferentially sequesters megathrombocytes.