AbstractWe have previously demonstrated that the chlorphentermine (CP)1-induced impairment in lymphocyte blastogenesis involves drug-induced inhibition of an event which occurs very early during lymphocyte activation. An early event, which is associated with mitogen-induced lymphocyte activation, involves the hydrolysis of phosphatidylinositol by phospholipase C to yield inositol phosphates and diacylglycerol as products. Inositol phosphates and diacylglycerol then function as mediators of a trans-membrane signal for the continuation of the cellular response. It was the purpose of the present study to determine the effects of CP on this phosphatidylinositol pathway. We demonstrated that formation of inositol phosphates in lymphocytes increases progressively above control over a 2 hour period following concanavalin A (Con A)-stimulation. In contrast, lymphocytes pre-incubated with 10−5M CP for 60 min, then stimulated with Con A for 2 hours in the presence of 10−5M CP, exhibit a significantly depressed inositol phosphate formation. In addition, CP also inhibited the activity of phospholipase C (IC50= 0.58 mM), the enzyme responsible for the formation of inositol phosphates during lymphocyte activation. Further, lymphocytes activated in a manner that bypasses the phosphatidylinositol pathway are not inhibited by 10−7M or 10−9M CP as are cells activated with Con A. These results suggest that the suppression of the phosphatidylinositol pathway may be involved in the inhibition by CP of lymphocyte blastogenesis induced by Con A.