Rapid alterations in lateral mobility of lipids in the plasma membrane of activated human megakaryocytes.

In the present study we measured membrane fluidity as the lateral mobility of the lipid probe 1,1'-ditetradecyl-3,3,3',3'-tetramethylindocarbocyanine perchlorate by fluorescence recovery after photobleaching in the plasma membrane of a single megakaryocyte, the progenitor cell of platelets. Megakaryocytes after 13 days in culture (maturation stage III) had a lateral diffusion coefficient (D) of (4.56 +/- 0.10) x 10(-9) cm2/s and a mobile fraction of 65 +/- 2% (means +/- SEM, n = 140). Megakaryocytes isolated from rib had a similar D and mobile fraction. Stimulation with alpha-thrombin (1-10 U/ml) induced a dose-dependent decrease in D to (3.40 +/- 0.22) x 10(-9) cm2/s between 1-5 min after stimulation (P < 0.001). The mobile fraction did not change. A similar decrease in D was found following stimulation with ADP (20 microM) and ionomycin (100 nM). Modulation of calpain I activity with calpain I inhibitor or tetracain had no effect. Pretreatment with cytochalasin B or colchicine decreased D to (3.64 +/- 0.29) x 10(-9) cm2/s (P < 0.003) and (3.96 +/- 0.18) x 10(-9) cm2/s (P < 0.013) respectively. After stimulation D decreased further in cytochalasin-treated cells (3.37 +/- 0.16) x 10(-9) cm2/s (P < 0.020) but remained at the same level in colchicine-treated cells. Both treatments increased the mobile fraction to 73-75% in stimulated megakaryocytes (P < 0.03). These data indicate that the diffusion velocity of lipids in megakaryocytes is low and decreases further after stimulation. These changes are independent of calpain I. Treatments that decrease the cytoskeletal mass and thereby increase the mobility of proteins in the plasma membrane increase the number of lipids that participate in this process.