Inhibitors of Na+/H+ exchange block epinephrine- and ADP-induced stimulation of human platelet phospholipase C by blockade of arachidonic acid release at a prior step.

The ability of epinephrine or ADP to cause an increase in the production of phospholipase C products (diacylglycerol and inositol phosphates) in human platelets is blocked by perturbants of Na+/H+ exchange, i.e. ethylisopropylamiloride, decreased extraplatelet pH, or removal of extraplatelet Na+. These perturbants do not, however, block inositol phosphate production in response to 0.2 unit/ml thrombin, indicating that inhibition of Na+/H+ exchange does not inhibit the phospholipase C enzyme directly. Since the cyclooxygenase inhibitor indomethacin and the endoperoxide/thromboxane antagonist SQ29548 block epinephrine- and ADP-induced inositol phosphate production, it can be concluded that these agonists activate phospholipase C secondary to mobilization of arachidonic acid and production of cyclooxygenase products. This conclusion is consistent with the observation that the endoperoxide analogue U46619 causes inositol phosphate production. Furthermore, the effect of U46619 is not blocked by inhibitors of Na+/H+ exchange. The initial pool of arachidonic acid mobilized by epinephrine can be measured using negative ion gas chromatography/mass spectrometry and is sensitive to inhibition of Na+/H+ exchange. The present data suggest that epinephrine and ADP cause mobilization of a small pool of arachidonic acid by a pathway involving Na+/H+ exchange. The cyclooxygenase products derived from this pool subsequently activate phospholipase C. Since the same treatments that block epinephrine- and ADP-induced diacylglycerol and inositol phosphate production also block epinephrine- and ADP-induced dense granule secretion, it appears that activation of phospholipase C, albeit indirectly via cyclooxygenase products, may be required for epinephrine and ADP to evoke platelet secretion.