The aggregation of human platelet induced by ganodermic acid S

Incubation of gel-filtered human platelets in ganodermic acid S (lanosta-7,9(11),24-trien-3β,15α-diacetoxy-26-oic acid) showed that within a min 80% of the agent was taken up by the cells. The process of uptake was a simple diffusion, and the partition coefficient was about 10⁵. The agent caused platelet aggregation at a concentration above 20 μM. Above the threshold, the extent of cell aggregation was in a linear relationship to the agent concentration. Also, the % of cell aggregation was comparable to the elevation of: (1) cytosolic free Ca²⁺]_i); (2) protein phosphorylation; and (3) serotonin release. Also, it was correlated with the change in the interconversion of phosphoinositides. Moreover, platelets in various concentrations of ganodermic acid S appeared to show different time-course profiles in the changes of [³²P]phosphoinositides and [³²P]phosphatidic acid (PA). Upon addition of the agent, platelets showed an initial increase in all of the [³²P]phosphoinositides, and then the level of each kind of phosphoinositide decreased sequentially in phosphatidylinositol 4,5-bisphosphate (PIP₂), phosphatidylinositol 4-phosphate (PIP) and phosphatidylinositol (PI). Below the aggregation threshold, platelets showed neither the resynthesis of [³²P]PIP_z and [³²P]PIP nor the accumulation of [³²P]PA. However, at 25 and 50 μM, platelets showed not only the resynthesis of [³²P]PIP₂ and [³²P]PIP but also the accumulation of [³²P]PA. Interestingly, at 100 μM ganodermic acid S, platelets did not show the resynthesis of [³²P]PIP₂ and [³²P]PIP. In this case, the level of [³²P]PA accumulation and that of [³²P]PI decrease were less than those found in platelets at 50 μM ganodermic acid S. The results suggested that ganodermic acid S caused the activation of PIP₂ hydrolysis. Scanning electron microscopy (scanning EM) revealed that the morphology of platelets below the aggregation threshold appeared to be spiculate discoid shape. Above the threshold, the cells rounded up to spiculate irregular forms, which showed an elongation of filopodia after prolonged 30-s incubation. In addition, platelets at ≥50 μM ganodermic acid S showed the occurrence of membrane vesiculation. Hence, the incorporation of ganodermic acid S into platelet membrane resulted in the change of membrane morphology.