Differential effects of propranolol on responses to receptor-dependent and receptor-independent stimuli in human neutrophils.

Recent evidence suggests that the hydrolysis of phosphatidylcholine (PC) by phospholipase D (PLD) may mediate superoxide anion (O2-) production in human neutrophils. To define the role of the PC-specific PLD products phosphatidic acid (PA) and diacylglycerol (DAG) in O2- production in response to agonists which activate the PLD pathway, we blocked the metabolism of PA to DAG with propranolol, an inhibitor of PA phosphohydrolase. Propranolol (150 microM) enhanced the production of O2- in response to the receptor agonists n-formyl-methionyl-leucyl-phenylalanine (FMLP, 292 +/- 94% of controls), platelet-activating factor (PAF, 932 +/- 215%) and leukotriene B4 (LTB4, 1305 +/- 475%). In the presence of propranolol, total O2- production in response to PAF and LTB4, which are potent priming stimuli but very weak direct agonists, was similar to that obtained with FMLP. IN contrast, responses to receptor-independent agonists phorbol myristate acetate (PMA) and ionomycin were inhibited (81 +/- 8% and 87 +/- 5% inhibition, respectively). The effects of propranolol were demonstrable in the absence of cellular calcium and were shared by both stereoisomers of the drug. These data are consistent with the hypothesis that PA produced through the hydrolysis of PC by PLD is an important mediator of O2- production in response to receptor-dependent agonists. However, the inhibitory effects of propranolol on receptor-independent stimuli suggest that PA generated through the PLD pathway plays a different role in the signal transduction mechanisms of these agonists or that propranolol may have additional effects beyond inhibition of PA phosphohydrolase.