The stimulatory effects of cationic amphiphilic drugs on human platelets treated with thrombin

The actions of eight cationic amphiphilic drugs on human platelets displaye d three different effects according to drug concentration ranges. At lower concentrations (below similar to 25 mu M), the drugs stimulated secretory r esponses induced by 0.2 U/mL of thrombin, while at concentrations in the 25 -50 mu M range they inhibited these responses. Above 50-100 mu M, the drugs caused permeabilization of the platelet plasma membrane as measured by lea kage of cytoplasmic adenine nucleotides. The effects of these agents on pho sphoinositide metabolism were monitored in platelets prelabeled with P-32-i norganic phosphate, such that phosphatidic acid (PA), phosphatidylinositol 4-phosphate (PIP), and phosphatidylinositol 4,5-bisphosphate (PIP2), but no t phosphatidylinositol (PI), were labeled to equilibrium. In unstimulated p latelets, the level of labeled PA decreased slightly labour. 25%), with cor responding increases in PIP2 labeling up to drug concentrations of about 50 mu M In contrast to the relatively small changes in PI and PIP2, the level s of labeled PIP, precursor to PIP2, increased 2- to 4-fold in both resting and thrombin treated platelets from 5 mu M up to about 50-100 mu M of drug s and remained elevated throughout the permeabilization concentrations. [P- 32]PA increased 20-fold over control upon thrombin activation and 5-30 mu M of drugs caused [P-32]PA to increase 30-37 times over that seen in control , resting platelets; the concentration of drugs that potentiated thrombin-i nduced [P-32]PA elevation corresponded to that causing the potentiation of platelet secretion. Higher drug concentrations decreased [P-32]PA elevation . [P-32]PIP2 levels increased about 25% in response to thrombin treatment a lone; low concentrations of drugs led to another 25% elevation. A significa nt decrease in [P-32]PIP2 was seen above 30 mu M, corresponding to inhibiti on of platelet secretion, Concentrations of 5-30 mu M of several psychoacti ve agents, both neuroleptics and antidepressants, potentiated the thrombin- induced activation of platelets as measured by dense granule secretion and increased turnover of phosphoinositides. Remarkably, all of the drugs incre ased the levels of PIP even in resting platelets, indicating that they have common effects apart from the specific receptor interactions currently att ributed to them. These common effects, e.g. an increase in membrane fluidit y such as is known to be caused by amphipathic agents, may be in part respo nsible for the observed overlapping psychotrophic effects of tricyclic anti depressants and phenothiazines. (C) 2000 Elsevier Science Inc.