Sodium fluoride mimics effects of both agonists and antagonists on intact human platelets by simultaneous modulation of phospholipase C and adenylate cyclase activity

Using intact human platelets, we studied the effect of sodium fluoride (NaF) on platelet aggregation and release reaction and correlated the functional changes to intracellular events specific for either agonist- induced or antagonist-induced platelet responses. At lower concentrations, with a peak activity between 30 and 40 mmol/L, NaF induced aggregation and release of adenosine 5′-triphosphate (ATP) that was associated with increased formation of inositol phosphates, a rise in cytosolic free Ca2+, and phosphorylation of 20-kd and 40-kd proteins. At NaF concentrations greater than 40 mmol/L, aggregation and ATP release decreased dose-dependently in parallel with a decrease in Ca2+ mobilization, whereas neither inositol phosphate formation nor 40- kd protein phosphorylation was reduced. At these concentrations, NaF caused a dose-dependent transient rise in platelet cyclic adenosine 3′,5′-monophosphate (cAMP) levels that was sufficient to account for the observed reduction in Ca2+ mobilization, aggregation, and ATP release. Stimulated cAMP levels started declining rapidly within 30 seconds of addition of NaF, however. Similarly, prostacyclin (PGI2)- induced cAMP accumulation was temporarily enhanced but subsequently suppressed by NaF, suggesting either stimulation of a cAMP phosphodiesterase or delayed inhibition of adenylate cyclase. Evidence for the latter was provided by the finding that NaF pretreatment of platelets resulted in partial inhibition of PGI2-stimulated cAMP formation in the presence of the cAMP phosphodiesterase inhibitor 3- isobutyl-1-methyl-xanthine (MIX). We conclude that NaF exerts a dual (stimulatory and inhibitory) effect on adenylate cyclase in intact platelets that is accompanied by simultaneous activation of a phosphoinositide-specific phospholipase C; in addition, a cAMP phosphodiesterase may be activated.