We investigated the action of piracetam on human polymorphonuclear leukocyte (PMN) responsiveness in vitro. We first studied phosphoinositide metabolism and calcium release with and without fMLP (formyl-methionyl-leucyl-phenylalanine) stimulation. Piracetam at concentrations from 10(-4) to 10(-2) M induced a slight increase in inositol 1,4,5-trisphosphate (IP3) release and phosphatidylinositol 4,5-bisphosphate (PIP2) breakdown. At concentrations above 10(-3) M, piracetam sensitized PMNs to subsequent stimulation by fMLP used at subliminal concentrations (10(-9) and 10(-8) M), inducing a significant increase in IP3 release and PIP2 breakdown similar to that obtained with cells stimulated by the highest effective concentrations of fMLP (10(-7) and 10(-6) M). In the same way, piracetam greatly enhanced calcium release induced by weak concentrations of fMLP. However, piracetam had no effect on oxidative metabolism. We then studied the binding of (3H)fMLP to the PMN membrane in the presence of various concentrations of piracetam. We were not able to demonstrate an obvious action of piracetam either on receptor recruitment or on receptor affinity to fMLP. The difference between the actions of piracetam on phosphoinositide metabolism and calcium release on the one hand and oxidative burst on the other could be explained by an uncoupling of the triggering and activating effects of piracetam on PMNs. The enhancement by piracetam of intracellular cyclic AMP levels rapidly induced termination of the PMN response and accounted for the lack of effect on superoxide production. Thus, piracetam was able to modulate human PMN reactivity and in particular to exert a "priming effect" (rather due to structural modifications of the membrane), which might be of importance in infectious episodes given the absence of deleterious actions such as oxygen free radical production leading to tissue injury.