Inhibition of canine platelet aggregation by barbiturates

The ability of barbiturates to affect in vitro platelet aggregation of canine platelets was examined. Platelet-rich plasma was incubated with thiamylal, pentobarbital, and barbital for 10 minutes before the addition of the aggregating stimulus. All three barbiturates produced a concentration-related inhibition of platelet aggregation induced by adenosine diphosphate (ADP) and collagen. The inhibitory effect of the barbiturates could not be overcome by increasing the concentration of extracellular calcium. In contrast to ADP- and collagen-induced aggregation, no inhibitory effect was observed on aggregation initiated by A23187, 12-O-tetradecanoylphorbol-13-acetate, or phospholipase C. In further studies, the ADP-induced rise in free cytosolic calcium was blocked by the barbiturates. These findings suggest that barbiturates may interfere with the rise in internal calcium associated with agonist-receptor stimulation of platelets.