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We administered a dose of unfractionated heparin (UFH) and two doses of a low molecular weight heparin (LMWH) to healthy volunteers by SC injection. The doses given were: a) UFH, 5000 IU, which represents 8.7 mg of5,400 MW active heparin (ACLM) and no5,400 active heparin (BCLM), b) enoxaparin 40 mg (3.4 mg ACLM, 2.2 mg BCLM) and c) enoxaparin 1 mg/kg body weight (on the mean 75 mg, containing 6.4 mg ACLM and 4.1 mg BCLM). We determined the effect on thrombin generation in platelet rich plasma (PRP) between 1 and 8 h after injection. UFH administration caused only a 5-8% inhibition of the thrombin potential (i.e. the area under the thrombin generation curve). Significantly higher inhibition of the thrombin potential was seen after administration of both doses of enoxaparin. To wit 9-26% at the low dose and 29-46% at the high dose. UFH injection caused a prolongation of the lag-time before the thrombin burst. Only with the high dose of enoxaparin the lag-times were significantly more prolonged with enoxaparin than with UFH. Excess amounts of platelet factor 4 (PF4) were able to neutralize completely the anti-thrombin activity in normal plasma spiked with enoxaparin as well as in plasma samples obtained after SC enoxaparin injection. With a large excess of PF4 the anti-factor Xa activity could be inhibited to a maximum of 50%. This indicates that ACLM (above critical length material, MW5400) is neutralized completely by PF4 whereas BCLM (below critical length material, MW5400) is not.(ABSTRACT TRUNCATED AT 250 WORDS)

We define a standard independent unit (SIU) of heparin as that amount that, in plasma containing 1 mumol of ATIII, raises the (pseudo-)first-order breakdown constant of factor Xa by 1 min-1. These units measure all material with a high affinity for ATIII (HAM); only material above the critical chain length of 17 monosaccharide units (above critical chain length material; ACLM) catalyzes the inactivation of thrombin. An SIU of ACLM is therefore analogously defined as the amount that, in plasma containing 1 mumol of ATIII, will raise the (pseudo-)first-order breakdown constant of thrombin by 1 min-1. Of any given heparin preparation one can determine the specific HAM and ACLM activities in terms of SIU/mg. On the basis of the factor Xa and thrombin breakdown constants found in a plasma sample one can then determine the levels of HAM and ACLM. Preliminary experiments were carried out in plasma samples obtained after subcutaneous injection of unfractionated heparin (UFH) and of two types of low-molecular-weight heparin (LMWH). About three times more of UFH activity than of LMWH activity has to be injected to obtain the same levels of ACLM in the plasma. Only with the LMWHs significant amounts of BCLM are found, which rises higher and persists longer than the ACLM. We determined the course of thrombin generation in platelet-rich plasma (PRP) and in platelet-poor plasma (PPP), as well as in the PPP factor Xa generation curve and the course of prothrombin conversion. The observed inhibitions correlated much better with the levels of ACLM than with those of below critical chain length material. The difference between UFH and LMWHs can therefore not be explained in terms of antithrombin and anti-factor-Xa activity. The essential difference between UFH and LMWH appears in the feedback effect of thrombin in PRP, where thrombin generation is both inhibited and retarded by LMWH, while it is only retarded but hardly inhibited by UFH.