The inhibition of human platelet function by ganodermic acids

Human gel-filtered platelets aggregate at > 20 μM-ganodermic acid S [lanosta-7,9(11),24-triene-3β,15α-diacetoxy-26-oic acid] [Wang, Chen, Shiao and Wang (1989) Biochim. Biophys. Acta 986, 151-160]. This study showed that platelets at < 20 μM-ganodermic acid S displayed both concentration- and time-dependent inhibition of function, in which the agent potency in response to inducers was ADP-fibrinogen > collagen > thrombin. The agent caused a biphasic time-dependent effect on platelet phosphoinositide metabolism. The first phase involved the decrease in the pool size of phosphoinositide by 10-20%. The second phase, in which both the resynthesis of phosphatidylinositol 4,5-bisphosphate (PIP₂) and the decrease of [³²P]phosphatidic acid occurred, took place after 30 min. Scanning electron microscopy also revealed a time-dependent morphological change in platelets in the presence of the agent. The cells initially became spiculate discs, then swelled to a 'potato-like' morphology at 60 min. Further studies on the time-dependent inhibition of thrombin response revealed that: (1) the percentage inhibition of cell aggregation was comparable with that occurring with an increase of cytosolic free Ca²⁺ concentration ([Ca²⁺](i)) or the phosphorylation of marker proteins; (2) [³²P]P(i)-labelled platelets showed the time-dependent inhibition of thrombin-stimulated PIP₂ resynthesis as indicated by first-2-min time-course studies of phosphoinositide interconversion; (3) scanning electron microscopy revealed that the aged platelet population showed an increase in the percentage of non-responding cells on prolonged incubation. The results, taken together, enabled one to discuss a possible mechanism for the time-dependent inhibition by ganodermic acid S of platelet response to thrombin.