| Abstract: | The amounts of actin and myosin in rabbit neutrophils expressed as micrograms/10(6) cells are 5.6 +/- 0.75 and 0.56 +/- 0.08, respectively. The average value of the total actin in rabbit neutrophils under unstimulated conditions is distributed between Triton X-100 soluble fraction (74 +/- 7%) and Triton X-100 insoluble fraction (26 +/- 3%). The Triton X-100 soluble and insoluble fractions will be referred to as the cytoplasmic and the cytoskeletal components. When the cells are stimulated by the chemotactic factor formyl-Met-Leu-Phe the amount of actin associated with the cytoskeleton increases to 73.7 +/- 6% of the total cell actin. This increase is rapid, dose-dependent and mediated through fMet-Leu-Phe receptors. Neither the time course of the response nor the dose-response curve is affected by the removal of calcium from the suspending medium. Calcium ions at concentrations greater than 10(-7) M added after Triton X-100 extraction dissociate actin from the cytoskeleton. Calcium at 1.9 microM added after Triton X-100 extraction reduces the amount of cytoskeletal actin under control and stimulated conditions to 10.3 +/- 0.9 and 33 +/- 1.5% of the total cell actin, respectively. The average value of the total myosin in rabbit neutrophils under unstimulated conditions is distributed between the cytosol (32 +/- 10%) and the cytoskeleton (68 +/- 18%). When neutrophils are stimulated with the chemotactic factor fMet-Leu-Phe the amount of myosin associated with the cytoskeleton does not increase significantly. Cytochalasin B decreases cytoskeletal actin and myosin and causes a shift in the amount of actin and myosin from the cytoskeleton to the cytoplasm both under fMet-Leu-Phe-stimulated and control conditions. In the presence of 1.6 mM extracellular Ca2+ and cytochalasin B (5 micrograms/ml) the amount of actin associated with the cytoskeleton under control and stimulated conditions is reduced to 13 +/- 2.2 and 10.2 +/- 3.5% of total cell actin, and that of myosin is reduced to 50.2 +/- 14 and 2.3 +/- 0.8% of the total cell myosin. The effect of cytochalasin B on actin does not depend on the time of its addition relative to that of fMet-Leu-Phe and is more pronounced in the presence of Ca2+. These results are discussed in terms of the roles of cytochalasin B and calcium in the overall mechanism of neutrophil degranulation induced by chemotactic factors. |
