Involvement of Naplus-Hplus exchanger in desmopressin-induced platelet procoagulant response

Desmopressin (DDAVP) action on platelets is associated with the development of procoagulant response but the underlying mechanism of this phenomenon is not known. We investigated whether this effect of DDAVP might be due to activation of plasma membrane Na+ /H+ exchanger. The DDAVP-induced platelet procoagulant re­sponse, measured as phospholipid-dependent thrombin generation, was dose de­pendent and significantly weaker than that produced by collagen or monensin (mim­ics Na+ /H+ antiport). Both the DDAVP- and collagen-produced procoagulant re­sponses were less pronounced in the presence of EIPA, an Na+/H+ exchanger inhibi­tor. Flow cytometry studies revealed that in vitro treatment of platelets with DDAVP or collagen was associated with the appearance of both degranulated (and frag­mented) and swollen cells. The DDAVP-evoked rise in size and granularity heteroge­neity was similar to that produced by collagen or monensin and was not observed in the presence of EIPA. Using flow cytometry and annexin V-FITC as a probe for phosphatidylserine (PS) we demonstrated increased and uniform binding of this marker to all subsets of DDAVP-treated platelet population. The DDAVP-evoked PS expression was dose dependent, strongly reduced by EIPA and weaker than that caused by monensin or collagen. As judged by optical swelling assay, DDAVP in a dose dependent manner produced a rise in platelet volume. The swelling was inhib­ited by EIPA and its kinetics was similar to that observed in the presence of monensin. Electronic cell-sizing measurements showed an increase in mean platelet volume and a decrease in platelet count and platelet crit upon treatment with DDAVP. DDAVP elicited a slow (much slower than collagen) alkalinization of platelet cytosol. Altogether the data indicate an involvement of Na+/H+ exchanger in the generation of procoagulant activity in DDAVP-treated platelets.