AN INCREASE IN PHOSPHATIDIC-ACID IN THE ABSENCE OF CHANGES IN DIACYLGLYCEROL IN HUMAN PLATELETS STIMULATED WITH ADP

1. ADP caused an increase in radioactivity of phosphatidic acid but not diacylglyderol in human platelets labelled with [H-3]arachidonic acid. 2. The radioactivity of phosphatidic acid was significantly increased 10 sec after adding 10 mu M ADP and this increase did not depend on production of thromboxane A2. 3. Thrombin (1 U/ml) caused an increase in both diacylglycerol and phosphatidic acid, the latter being much greater than that caused by ADP. 4. The results confirm that ADP stimulates phosphatidic acid production and suggest that a weak stimulus of the phosphatidyl inositol cycle, such as ADP, does not cause accumulation of diacylglycerol.