Inhibition of blood platelet function by cytochalasins: effects on thrombosthenin and on glucose metabolism

Abstract

in the inhibitor molecule was found to have a profound effect on the inhibition constant of the acid. Saturated dicarboxylic acids, with the exception of oxalate, were less inhibitory as the molecular weight increased. A cis effect was observed in the inhibition by unsaturated organic acids; compounds with two cis-carboxyl groups were much more inhibitory than the corresponding trans-isomers. This position effect was investigated quantitatively by measurement of the distances between carboxyl groups in space-filling models of the inhibitors. When these distances were plotted against the inhibition constant of the inhibitor, the degree of inhibition was observed to increase linearly with the increasing proximity of the carboxyl groups. The cytochalasins are mould metabolites that inhibit cytokinesis and cell movement (Carter, 1967). Cytochalasin B can also inhibit platelet aggregation (White, 1971) and clot retraction (Shepro et al., 1970;White, 1971). An attempt has been made to correlate the effects of cytochalasins A and B on platelet function with biochemical changes.Citrated pig platelet-rich plasma at 37°C was used. Platelet aggregation was initiated with ADP (1O0UM) and clotting with bovine thrombin (10N.I.H. units/ ml) in both cases 5min after addition of cytochalasin. Cytochalasin A inhibited aggregation completely at 20,ug/ml (42&M), but cytochalasin B only partially at 100lg/ml (209jzM). Clot retraction was inhibited completely by 4pg/ml of cytochalasin A and by 20,g/ml of cytochalasin B. Cytochalasin A (but not cytochalasin B) reacted with added cysteine, which blocked its effects.Thrombosthenin (platelet actomyosin) was extracted from pig platelets (Grette, 1962; Haslam & Mustard, 1969). The effects of 0.lmM-ATP on the turbidity of stirred suspensions of thrombosthenin in 60mM-KCI at pH7.0 were recorded. In the presence of 4mM-MgC12, ATP cleared the suspension. This effect was inhibited by cyotchalasin A (5-50,ug/ml) but not by cytochalasin B (100lg/ml). With 15mM-MgCl2, ATP caused a rapid superprecipitation which was similarly inhibited by cytochalasin A but not by cytochalasin B. Thrombosthenin was also isolated from platelets that had been incubated with cytochalasin A (4 or 20,ug/ml) for 5 min and then washed. Only with 20,ug/ml were the clearing and superprecipitation phenomena markedly inhibited. Thus only the inhibition of aggregation by cytochalasin A was associated with marked changes in thrombosthenin.Washed rabbit platelets depleted of endogenous substrates rapidly form 14CO2 from [6-14C]glucose (McElroy et al., 1971). In this system cytochalasin A at 4,ug/ml and at 20,tg/ml inhibited "4CO2 formation by about 30 and 70% respectively, whereas these concentrations of cytochalasin B inhibited by 91 and 98 %. Thus the relative effectiveness of cytochalasins A and B was reversed in these experiments.The biological activities of cytochalasin B have been attributed to disruption of contractile microfilaments (Schroeder, 1970; Wessells etal., 1971). The effect ofcytochalasin B on glucose metaboli...

On the molecular basis of action of cytochalasin B

“…Platelets were used because in these cells cytochalasin B inhibits utilization of exogenous glucose (23) and affects motility in the form of blood clot retraction (1,17,23,24) and pseudopod formation (1).…”

Platelets, Thrombosis and Drugs

“…Other compounds which have also been reported to inhibit platelet function include the methylxanthines caffeine and theophylline (Ardlie et al, 1967;Brinson, 1972;Mustard and Packham, 1970), papaverine (Markwardt et al, 1967;Born and Mills, 1969), adenine compounds such as adenosine, AMP and ATP (Born and Cross, 1963a;Mustard and Packham, 1970), colchicine (White, 1968b), cytochalasins A and B (Shepro et al, 1970;White, 1971a ;Haslam, 1972), local anaesthetics such as lignocaine and cinchocaine (0 'Brien, 1961(0 'Brien, , 1962Aledort and Niemetz, 1968) , and ketolactones (Bicher and Beemer, 1971).…”

Cytochalasin B binding by human platelets

“…Platelets are known to transport glucose actively by a carrier mechanism (Solomon and Gaut, 1970), and CB inhibits this transport (Haslam, 1972). Thus, it is very likely that the low affinity (KD o 1.4 x l M) glucose sensitive CBbinding observed here also is intimately related to the glucose carrier.…”