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A single bolus of heparin administered to 8 normal volunteers resulted in a significant increase in levels in platelet poor plasma (PPP) of platelet factor-4 (PF4) but not low-affinity platelet factor-4/β-thromboglobulin (LA-PF4/βTG). However, the presence of heparin interfered with the binding of 125I-PF4 to antibody in radioimmunoassay (RIA). This effect was overcome by increasing the concentration of NaCl from 0.15 to 0.5 M in the buffer used for RIA. In order to establish that the increased amount of immunoreactive material present in PPP was indeed PF4, the protein was isolated from postheparin plasma. A bolus of 5000 units of porcine lung heparin (Upjohn) was administered intravenously to 2 volunteers and plasma samples obtained before and 5 minutes after the injection. The levels of PF4 in PPP rose from 18 and 10 ng/ml before to 185 and 454 ng/ml at 5 minutes after injection in the two volunteers, respectively. The 5 minute samples were adsorbed to heparin agarose columns and PF4 levels decreased to 16 and 10 ng/ml respectively. The immunoreactive material was eluted with 1.2 M NaCl from the heparin agarose columns, showing typical elution pattern for PF4. This material was applied to SDS-polyacrylamide gel electrophoresis in parallel with purified PF4 obtained from human platelets. RIA carried out on eluates from gel slices revealed a species of the same molecular weight as standard PF4. Thus, heparin injection results in appearance in the circulation of a material identical to PF4. LA-PF4/βTG and PF4 are located in same granules and released in parallel during platelet stimulation. Further, LA-PF4 is cleared from plasma 4 times slower than PF4. Therefore, the elevation of PF4alone suggests release from sites other than platelets.

Release Of Platelet Factor-4 In Vivo After Heparin Injection