PROBENECID INHIBITS PLATELET RESPONSES TO AGGREGATING AGENTS IN-VITROAND HAS A SYNERGISTIC INHIBITORY EFFECT WITH PENICILLIN-G

Probenecid is an anion channel blocker and uricosuric agent, originall y developed to slow the rate of excretion of penicillin. It is now als o administered with many other drugs to reduce their required dosages. Recently, probenecid (2.5 mM) has been used to prevent leakage of fur a-2 or fluo-3 when these indicators of cytosolic Ca2+ levels have been introduced into cells. However, we found that probenecid markedly inh ibited the increases in cytosolic Ca2+ caused by ADP, thrombin, the th rombin receptor-activating peptide (SFLLRN, TRAP), ADP, sodium arachid onate, the thromboxane A(2) (TXA(2)) mimetic U46619, and platelet-acti vating factor (PAF). This finding precluded the use of probenecid with platelets in measurements of cytosolic Ca2+ with indicators such as f ura-2. We then investigated the effects of probenecid on aggregation a nd release of C-14-serotonin from prelabeled platelets. Responses to a ll the agonists were inhibited by 2.5 mM probenecid, but concentration s as low as 0.25-0.5 mM inhibited responses to agonists that act large ly via TXA(2) (collagen, sodium arachidonate and U46619). Collagen-ind uced TXA(2) formation was inhibited in a dose-dependent manner. Respon ses of aspirin-pretreated platelets to thrombin, SFLLRN, U46619 and PA F were also inhibited by probenecid, indicating that prevention of TXA (2) formation does not account for all the inhibitory effects, The com bination of probenecid with penicillin G produced additive or synergis tic inhibition of platelet responses; responses dependent on TXA(2) we re synergistically inhibited by concentrations of the drugs that are r eached in vivo. The synergistic inhibitory effect of probenecid on pla telet functions could further impair hemostasis if it has already been partially compromised by the administration of other drugs.