NEONATAL PLATELETS ARE LESS REACTIVE THAN ADULT PLATELETS TO PHYSIOLOGICAL AGONISTS IN WHOLE-BLOOD

Previous studies have reported that the platelets of healthy term neon ates have either diminished or normal reactivity compared to the plate lets of adults. To circumvent the methodologic problems of previous st udies, we used a whole blood flow cytometric method to study neonatal platelet reactivity to thrombin, a combination of ADP and epinephrine, and U46619 (a stable thromboxane A(2) analogue). Inclusion in the ass ay of the peptide GPRP (an inhibitor of fibrin polymerization) enabled us to study platelet reactivity to human alpha-thrombin in whole bloo d. Umbilical cord blood and day 1 peripheral blood were collected from 30 healthy term neonates and compared to peripheral blood from 20 nor mal adults, In whole blood samples without added agonist, there were n o significant differences between neonates and adults in the platelet binding of monoclonal antibodies 6D1 (GPIb-specific) or 7E3 (GPIIb-III a complex-specific). As determined by S12 (a P-selectin-specific monoc lonal antibody), neither neonates nor adults had circulating degranula ted platelets. However, in both cord and peripheral whole brood sample s, neonatal platelets were significantly less reactive than adult plat elets to thrombin, ADP/epinephrine, and U46619, as determined by the e xtent of increase in the platelet surface expression of P-selectin and the GPIIb-IIIa complex, and the extent of decrease in the platelet su rface expression of the GPIb-IX complex, For example, as compared to m aximal platelet surface P-selectin in adults (with thrombin 10 U/ml), thrombin 1 U/ml resulted in platelet surface P-selectin of 95 +/- 2% ( mean +/- S.E.M.) in adult peripheral blood, but only 70 +/- 4% in cord blood and 70 +/- 3% in neonatal peripheral blood (p <0.0001), Thrombi n 0.1 U/ml resulted in platelet surface P-selectin of 49 +/- 4% in adu lt peripheral blood, but only 10 +/- 2% in cord blood and 17 +/- 2% in neonatal peripheral blood (p <0.0001). Similar results were obtained in a washed platelet system. In summary: 1) Compared to adult controls , neonatal platelets are hyporeactive to thrombin, a combination of AD P and epinephrine, and a thromboxane A(2) analogue in the physiologic milieu of whole blood. 2) The hyporeactivity of neonatal platelets com pared to adult platelets is the result of a defect intrinsic to neonat al platelets. 3) Whole blood flow cytometry is particularly advantageo us for neonatal studies because only 5 mu l of blood per assay is requ ired.