Antimotility Effect of Cytochalasin B Observed in Mammalian Clot Retraction

A variety of agents, e .g. colchicine, that are known to affect mitosis are also of value in providing information about motility of and within the cell . Cytochalasins isolated from yeast by W . B . Turner, and shown by Carter and others to inhibit motility and to block cell division, may be classified as another antimotility group (1) . One unique feature of this compound is that it may be specific for microfibers since cytoplasmic cleavage of mouse fibroblasts (1) and human lymphocytes (2) is inhibited, but not nuclear division. Direct evidence on the effect of cytochalasins on microtubules and microfibers or its way of action is not present . We demonstrated that the mammalian platelet and nonmammalian thrombocyte contractile mechanism as seen in clot retraction is remarkably similar to the contractile systems controlling biological movement described for other cells (3), and a theoretical model for contractile microfiber formation in platelets was postulated (4) . Recently, we described in glycerinated platelets the complexing of a 60 A diameter microfiber with heavy meromyosin that produced a distinct arrowhead pattern similar to that described for muscle actin or with microfibers from nonmuscle cells (5) . We now wish to report the effect of cytochalasin B on the contractile system of human and other mammalian platelets as seen in retraction . The results of incubating for 45 min 10 .tg of cytochalasin-0 .01 % DMSO/ml PRP are illustrated in Fig . 1, and they show that cytochalasin B inhibits contraction of human PRP clots and the onset and duration of retraction of calf PRP clots . We assume that the difference in rates and degree of contraction between the two species is a reflection of the reduced amount of cytochalasin per cell in the calf experiment, and permeability specificity . An equivalent inhibition of clot retraction was obtained with 5 ug of cytochalasin per ml PRP, but no alteration in clot retraction was observed with the 1 ,ag of cytochalasin per ml of PRP. (Experiments with cow and rabbit PRP and with amphibian thrombocyte-rich plasma produced parallel inhibition of clot retraction.) Varying the incubation time from 15 min to 1 hr had a negligible effect on the rate and degree of clot retraction, and cytochalasinDMSO does not appear to affect protein synthesis as measured by leucine-"C uptake . These data demonstrate that cytochalasin B produces an effect on platelet contractile system that is similar to that observed when platelets are incubated with 10-2 M colchicine or when platelets are suspended in physiological saline made with heavy water . Because of the temporary inhibition of clot retraction by cytochalaI Several cytochalasins differ in potency but have sin B, one assumption is that the cells are prosimilar antimotility activity . ducing sufficient contractile proteins to overcome clot