Heparin-decasaccharides impair the catabolism of chylomicrons.

On intravenous injection to rats, decasaccharides gave rise to a short-lived peak of lipoprotein lipase (LPL) activity, whereas octa- and hexasaccharides caused only marginal increases. In isolated hearts perfused by a single pass, decasaccharides released LPL more efficiently than conventional heparin on a mass basis. Octa- and hexasaccharides were much less efficient. Similar results were obtained for hepatic lipase, which was studied both in vivo and by liver perfusion. In the intact rat, the heparin fragments themselves disappeared rapidly from the circulating blood. The decay of hepatic lipase activity after the early peak roughly paralleled the decay of decasaccharide concentration, but for LPL the decay was faster, presumably because the liver extracted this lipase from plasma. To assess the lipase activities remaining in contact with blood a large dose of conventional heparin was injected at a series of times after the decasaccharides. LPL was decreased by 40% after 1 h. At that time, the LPL activity that could be released from isolated hearts by single-pass perfusion with heparin for 2 min ("functional LPL') was decreased by 75%. Chylomicrons labelled in vivo with [14C]oleic acid (primarily in triacylglycerols, providing a tracer for lipolysis) and [3H]retinol (primarily in ester form, providing a tracer for the particles) were injected intravenously to explore the effects of the LPL depletion on lipoprotein metabolism. Triacylglycerol lipolysis and particle clearance was markedly delayed from 30 min to 2 h after injection of decasaccharides. After 1 h the fractional catabolic rate was only one-third of the control value and the catabolism of chylomicron triacylglycerols by perfused hearts was delayed to a similar extent. Thus injection of decasaccharides leads to accelerated turnover of LPL with loss of functional LPL from extrahepatic tissues. This in turn leads to a period of delayed lipolysis and removal of chylomicron particles.

Experimental haemorrhagic effect of two-domain non-glycosylated tissue factor pathway inhibitor compared to low molecular weight heparin.

The glycosylated multivalent three-domain Kunitz inhibitor TFPI is a natural inhibitor of tissue factor-FVIIa complex in the presence of FXa. TFPI has an experimental antithrombotic capacity indistinguishable from LMWH in a prophylactic dose, regardless of glycosylation and of the third domain. An inherited equilibrium between antithrombosis and haemorrhage exists. The aim of the study was to evaluate whether a two-domain non-glycosylated TFPI (117QTFPI1-161) has a bleeding potential in a rat gastric mucosa model. Groups; placebo, LMWH (tinzaparin) 60 and 250 anti-Xa IU/kg and 117 QTFPI1-161 1.0 and 10.0 mg/kg, given i.v. (bolus injection), randomised double dummy design. All actively treated groups significantly prolonged both the bleeding volume (493-984 microliters) and the bleeding time (10-20 min) compared to placebo (41 microliters, 2 min). It was not possible to distinguish a difference between the lower dose of LMWH and 117QTFPI1-161 in either parameter (p = 0.23-0.71). The two doses of 117QTFPI1-161 caused elevation of plasma-TFPI, 18 and 150 times baseline value. Both LMWH doses (0.6-3.2 anti-Xa IU/ml) and both 117QTFPI1-161 doses (0.2-2.7 anti-Xa IU/ml), caused significant effect in the anti-Xa assay, however 117QTFPI1-161 significantly less. Only the largest dose of 117QTFPI1-161 caused significant prolongation in the APTT assay (34 s). Both doses of LMWH caused significant prolongation (60-300 s). LMWH was the only substance to prolong the dilute-PT assay. Non-glycosylated two-domain 1.0 mg/kg TFPI, yielding supraphysiological plasma concentration, has an experimental haemorrhagic potential indistinguishable from LMWH in a prophylactic dose. The effect mediated by this type of TFPI could primarily be due to an inhibition of FXa.

Does glycosylation influence the experimental antithrombotic effect of a two-domain tissue factor pathway inhibitor?

We have earlier shown that both full-length and truncated glycosylated tissue factor pathway inhibitor (TFPI) lacking the third Kunitz domain and the c-terminal region has an antithrombotic effect comparable to lowmolecular-weight heparin (LMWH) in an experimental venous thrombosis model. The aim of this study was to investigate whether a recombinant truncated non-glycosylated TFPI (117QTFPI1-161) had an antithrombotic effect similar to the glycosylated TFPI1-161 and LMWH. We also followed the coagulation parameters. The thrombi were induced in rabbit jugular veins with a combination of endothelium destruction and restricted blood flow. Group 1: placebo; group 2: LMWH 60 anti-Xa IU/kg, i.v.; groups 3 and 4: TFPI1-161 0.8 and 0.2 mg/kg, i.v., respectively; groups 5 and 6: 0.8 and 0.2 mg/kg 117QTFPI1-161, i.v., respectively, in a randomized double-dummy fashion. Twelve animals were included in the placebo group and 6 in each of the other groups. The frequency of thrombosis and also of occlusive thrombosis was reduced in all groups compared to placebo. The thrombus weight was reduced (0-9.9 mg) in all groups, significantly in groups 2, 4 and 5 (p = 0.004-0.02) compared to placebo (21.1 mg). In group 3, a borderline p value was achieved (0.06 likely a beta-error). The two forms of TFPI1-161 given in the higher doses showed a significantly greater increase of anti-Xa activity, but with a shorter duration compared to LMWH (1.7-1.9 vs. 0.9 anti-Xa IU/ml). Activated partial thromboplastin time (aPTT)-analysis revealed that only LMWH (52 s) caused a significant transient elevation 2 min after injection. In the other groups, a temporary but insignificant elevation of aPTT (27-37 s) was seen. No detectable effect on anti-IIa activity and prothrombin time (PT) was seen in any TFPI group. The glycosylation of the second domain on TFPI does not substantially contribute to the antithrombotic effect of TFPI. Regardless of the glycosylation of TFPI1-161, it has a dose-dependent effect on anti-Xa, a small effect on the aPTT, but no effect on anti-Ila and PT. LMWH has a more pronounced and sustained impact on these parameters.

Biphasic effects of low-molecular-weight and conventional heparins on chylomicron clearance in rats.

Chylomicrons labeled in vivo with [14C]triglycerides and [3H]retinyl esters were injected in rats at a series of times after administration of conventional unfractionated heparin (UFH), low-molecular-weight heparin (LMWH), or saline. In saline controls the clearance of both chylomicron triglycerides and retinyl esters seemed to follow exponential courses, with half-lives of about 5 and 10 minutes, respectively. Five minutes after administration of LMWH or UFH, the triglyceride clearance rates were dramatically increased and were associated with an increased appearance of the radiolabel in circulating free fatty acids (FFAs). The clearance of [3H]retinol radioactivity, ie, chylomicron particles, was also enhanced 5 minutes after heparin injection. From 75% to 90% disappeared from the circulation within the first 5 minutes. Their continued disappearance was much slower, with a slope similar to that of the saline-treated rats. Hence, it was as if a new, rapid exponent had been added to the disappearance curve that accounted for most of the particle clearance. Injection of chylomicrons 1 hour after the heparins resulted in substantially slower clearance compared with saline-treated controls of both triglyceride and retinol radioactivity in rats given a high dose of LMWH or a low dose of either heparin. Appearance of label in plasma FFAs was also decreased, suggesting that impeded lipolysis was responsible, at least in part, for the impeded chylomicron clearance. Four and 24 hours after heparin injection all studied parameters of chylomicron clearance had returned to normal.(ABSTRACT TRUNCATED AT 250 WORDS)

Antithrombotic properties of a truncated recombinant tissue factor pathway inhibitor in an experimental venous thrombosis model.

The aim of this study was to investigate whether a truncated recombinant tissue factor pathway inhibitor (rTFPI1-161) had an antithrombotic effect comparable to low-molecular-weight (LMW) heparin. A randomized double-dummy study was conducted with 40 rabbits in 6 groups. An experimental thrombosis was induced in the jugular veins by a combination of destroyed endothelium and restricted blood flow. Group 1 was given placebo; group 2, LMW heparin 60 anti-factor Xa units/kg; group 3, rTFPI1-161 0.1 mg/kg; group 4, rTFPI1-161 1.0 mg/kg and group 5, rTFPI1-161 10.0 mg/kg. rTFPI1-161 reduced the thrombus weights in all treated groups, with a significant effect in doses between 1.0 and 10.0 mg/kg compared to placebo. The frequency of thrombosis was significantly reduced in all treated groups. No hemorrhagic side effects were noted. In conclusion, rTFPI1-161 (1.0-10.0 mg/kg) has an antithrombotic effect comparable to that of LMW heparin.