Clearance of Tissue Plasminogen Activator (TPA) and TPA/Plasminogen Activator Inhibitor Type 1 (PAI-1) Complex
Relationship to Elevated TPA Antigen in Patients With High PAI-1 Activity Levels
Background To evaluate the effect of plasminogen activator inhibitor type 1 (PAI-1) levels on the clearance of total tissue plasminogen activator (TPA) antigen, we studied the clearance of active TPA and TPA/PAI-1 complex in subjects with low (181±109 pmol/L; n=7) and high (1166±322 pmol/L; n=4) baseline active PAI-1.
Methods and Results A 5-μg/kg bolus of TPA was infused over a 15-second period followed by measurement of TPA activity, TPA antigen, TPA/PAI-1, TPA/C1 inhibitor, PAI-1 activity, and PAI-1 antigen over a 4-hour period. α-Phase clearance of total TPA antigen was faster in subjects with low PAI-1 (t1/2 of 3.5±0.7 minutes) versus high PAI-1 (t1/2 of 5.3±0.9 minutes) (P=.006). Clearance of all factors was best fit by a two-compartment pharmacokinetic model based on a computer-simulated human circulatory system. The average hepatic clearance fraction in the two-compartment model was greater for active TPA (89±10%, t1/2 of 2.4±0.3 minutes) than for TPA/PAI-1 complex (48±17%, t1/2 of 5.0±1.8 minutes) (P=.0006).
Conclusions Plasma clearance of active TPA was faster than clearance of TPA/PAI-1 complex. High levels of active PAI-1 converted more TPA into TPA/PAI-1 complex, effectively slowing the clearance of total TPA antigen and explaining in part why high levels of PAI-1 activity are associated with increases in total TPA antigen.
The fibrinolytic system plays an important role in regulating the removal of thrombi from the vascular system. Fibrinolysis in the vascular bed is initiated by TPA, an enzyme that converts plasminogen to plasmin on the fibrin surface. The concentration of active TPA in blood is regulated by the rate of secretion of TPA, the inhibition of TPA by PAI-1 and other inhibitors, and the hepatic clearance of TPA.1 2 3 Epidemiological studies have shown that reduced blood fibrinolytic activity4 and elevated plasma PAI-1 activity,5 6 PAI-1 antigen,5 and TPA antigen5 7 8 9 10 are associated with an increased risk of myocardial infarction. One consistent finding in many studies has been a strong association between elevated PAI-1 activity or antigen and elevated levels of total TPA antigen.11 12 13 14 TPA exists in several forms in blood, including free active TPA and TPA bound to PAI-1 and other inhibitors.15 16 17 18 19 20 In plasma, PAI-1 forms stable stoichiometric complexes with TPA.21 As PAI-1 levels rise in blood, the fraction of TPA in the active form falls and TPA/PAI-1 complex levels rise.13 22 If TPA/PAI-1 complex were cleared from the blood more slowly than active TPA, then total TPA levels would tend to rise when PAI-1 was increased. The purpose of the present study was to determine the rate of clearance of active TPA versus TPA/PAI-1 complex and to evaluate the effect of elevated PAI-1 levels on the relationship between total TPA antigen and PAI-1 activity.
The study was performed in the Center of Clinical Pharmacology and Therapeutic Evaluation. The local ethics committee approved the investigation. Eleven healthy volunteers (7 men and 4 women) aged 37.8±10.7 years (range, 21 to 51 years) and weighing 81.2±26.3 kg (range, 51 to 127 kg) were studied. Subjects were selected according to their plasma PAI-1 levels. Seven had low PAI-1 activity (181±109 pmol/L; 4 men, 3 women) and 4 had high PAI-1 activity (1166±322 pmol/L; 3 men, 1 woman). None of the subjects were taking medication.
TPA Infusion and Blood Sample Protocol
Before TPA infusion, subjects refrained from smoking or drinking caffeine-containing beverages for 48 hours, fasted for 12 hours, and rested (in a sitting or supine position) for 30 minutes. A 5-μg/kg bolus of recombinant TPA (Actilyse, Boehringer Ingelheim) was infused intravenously over a 15-second period at 8 am. A light breakfast was served 2 hours after the injection. Blood was collected from the arm oppo-site the infusion site via an antecubital catheter. The blood collections were taken just before TPA infusion and 0.5, 1, 2, 4, 8, 12, 15, 20, 30, 45, 60, 90, 120, 180, and 240 minutes after the bolus injection. After the first 2 mL of blood was discarded, 5 mL of blood was drawn into citrate supplemented with platelet-stabilizing agents (Diatube, Stago) to reduce PAI-1 antigen release from platelets, 5 mL was drawn into acidified citrate (Stabilyte, Biopool) to stabilize active TPA, and 5 mL was drawn into citrate containing PPACK at a final concentration of 100 μmol/L (Calbiochem) to block in vitro formation of complexes between TPA and its inhibitors.23 All samples were immediately centrifuged at 1800g for 30 minutes at 4°C, separated into aliquots, and frozen at −80°C until analyzed.
TPA activity was measured in acidic citrate plasma by use of an amidolytic method (BIA TPA, Chromogenix). Total TPA antigen assays measure the total amount of TPA in blood, including free and complexed TPA. Total TPA antigen was measured in acidic citrate plasma, total PAI-1 antigen was measured in citrate plasma containing platelet inhibitors, and TPA/PAI-1 complex antigen was measured in PPACK plasma using enzyme immunoassays as previously described (Stago).22 TPA/C1-I antigen was measured in PPACK plasma by a modification of the method of Huisman et al24 with the use of microtiter plates coated with monoclonal anti–human TPA F(ab′)2 fragments and sheep polyclonal anti–human C1-I F(ab′)2 fragments conjugated to peroxidase (The Binding Site). PAI-1 activity was measured in acidic citrate plasma by use of a back-titration method (Spectrolyse/PL, Biopool). C1-I antigen was measured in PPACK plasma by use of an enzyme immunoassay (The Binding Site).
Three methods were used to evaluate the clearance of different forms of TPA: (1) determination of the half-life of total TPA from 2 to 12 minutes after TPA infusion in patients with low and high PAI-1 activity; (2) a single-compartment circulatory model with TPA infusion into the plasma compartment followed by first-order elimination of all factors by the liver; and (3) a two-compartment circulatory model with TPA infusion into plasma compartment, first-order elimination of all factors by the liver, and first-order transfer of active TPA and TPA/PAI-1 complex to and from separate second compartments (Fig 1⇓). Two-compartment models are used to evaluate clearance of factors that show an α- and β-phase of clearance as described previously for TPA.25 The circulatory model is described in more detail below.
Model Circulatory System
The model circulatory system consists of a cardiopulmonary circulation that distributes blood flow into coronary, cerebral, upper-extremity, splanchnic, renal, lower-extremity, and “other” circulations (Fig 2⇓).1 2 Estimates of resting total blood volume, plasma volume, and cardiac output were based on height, weight, and resting hematocrit as previously described.26 27 Estimated resting blood volumes and blood flows for different organs were determined as previously described.1 2 28 29
The fibrinolytic system of the model contains active TPA, active PAI-1, C1-I, TPA/PAI-1 complex, and TPA/C1-I complex. The molar concentrations of these proteins are regulated by four processes: (1) in vivo secretion of TPA and PAI-1, (2) infusion of exogenous TPA, (3) inhibition of TPA by PAI-1 and C1-I forming their respective complexes, and (4) hepatic clearance of all factors. Total TPA is assumed to be equal to active TPA plus TPA/PAI-1 plus TPA/C1-I. Total PAI-1 is equivalent to active PAI-1 plus TPA/PAI-1 complex. Total PAI-1 in the simulation underestimated total PAI-1 antigen in plasma by a small amount because latent PAI-1 was not accounted for. Two other proteins (α2-antiplasmin and α2-macroglobulin) that slowly inhibit TPA have been reported; these inhibitors were not included in the model because their role was felt to be negligible compared with PAI-1 and C1-I.
TPA is primarily secreted from vascular endothelial cells.30 The majority of the endothelium is located in small vessels and capillaries.29 TPA secretion in the simulation is simplified by assuming that (1) TPA is secreted only by vascular endothelial cells, (2) all vascular endothelium is located in the capillary beds, and (3) TPA secretion is the same in all capillary beds.
There may be several sources for PAI-1 in blood. Liver has been shown to secrete PAI-1 in vivo and in vitro.31 32 33 Endothelial cells in culture secrete PAI-1, but they do not appear to secrete PAI-1 in vivo except during the acute-phase response.11 13 22 28 34 35 Recent studies have indicated that adipose tissue, particularly in the abdominal region, may also be a source of plasma PAI-1.36 37 38 39 40 Separate simulations were run with secretion of PAI-1 either from the liver or from systemic capillary beds, which model endothelial secretion or secretion from adipose tissue, respectively. Because there were no significant changes in the measured concentration of C1-I during the infusion of TPA, the concentration of C1-I was held constant in the model at baseline levels.
In large arteries and veins, the ratio of endothelial cell surface area to blood volume is low. It was assumed that no secretion of TPA or other factors occurred in large vessels and that the only changes in concentration were due to the reaction between TPA and either PAI-1 or C1-I.1 A second-order rate constant of 3.7×107 mol/L−1·s−1 was used for the reaction between one-chain TPA and PAI-1.3 41 42 The rate constant for the reaction between one-chain TPA and C1-I was adjusted to produce the best fit between measured and simulated levels of TPA/C1-I complex.
Depending on the specific model used, changes in concentration in the capillary beds may occur due to secretion of TPA or PAI-1, the inhibition reactions with PAI-1 and C1-I, and transfer of active TPA and TPA/PAI-1 complex to a second compartment. In the liver sinusoids, changes in concentration occur due to the secretion of active PAI-1, the reaction between TPA and its inhibitors, and the clearance of the appropriate fraction of each factor as the blood flows into the hepatic veins. In the single-compartment clearance model, TPA is infused into the upper-extremity vein of the model, followed by clearance through the liver. The rate of clearance from plasma for all factors is directly proportional to liver blood flow.43 where E is the hepatic extraction fraction and Q is the hepatic plasma flow rate. Separate extraction fractions were used for each factor measured. In the two-compartment model, all forms of TPA and PAI-1 are cleared by the liver, and active TPA and TPA/PAI-1 complex move reversibly to separate second compartments (Fig 1⇑). It is assumed that transfer to these second compartments occurred primarily in the capillary beds.
Huisman et al24 reported a half-life of 4.2 minutes for the clearance of TPA/C1-I complex. The hepatic extraction fraction for TPA/C1-I was adjusted to produce a half-life of 4.2 minutes in the simulation. Due to the low levels of TPA/C1-I, only single-compartment hepatic clearance was modeled for this complex.
Cardiac output in the simulation was set to the estimated value for each subject. Baseline TPA and PAI-1 secretion rates were estimated as previously described.1 2 If a circadian decrease in TPA or PAI-1 was noted between the baseline and 4-hour sample points, TPA and PAI-1 secretion rates were adjusted to produce a linear decrease in TPA and PAI-1 to match the measured levels. The same dose of TPA given to the subject was infused into the upper-extremity vein of the model over a 15-second period. Hepatic extraction fractions and second-compartment microconstants were adjusted to minimize the sum of weighted squared errors between measured and simulated levels of active TPA, total TPA, TPA/PAI-1, and total PAI-1 from 2 minutes to 30 minutes after infusion of TPA.1 PAI-1 activity measurements were not used in the model because it was difficult to accurately measure PAI-1 activity during the TPA infusion.
The simulation was run on a Macintosh Quadra 700 (Apple Computers) using THINK Pascal 4.0 (Symantec). Complete details of the circulatory model, including a copy of the source code for the program, are available from the authors at no charge.
Group distributions are presented as the mean±SD. Comparisons within groups used repeated measures ANOVA and the Tukey honestly significant difference test; comparisons between groups used between-group ANOVA. Statistica/Mac software (Stat-Soft) was used for all statistical calculations.
Effects of Bolus Infusion of TPA
At baseline, before infusion of TPA, four subjects had high levels of active PAI-1 (1166±322 pmol/L) whereas the remaining seven subjects had low levels of active PAI-1 (181±109 pmol/L; P=.00002). Fig 3⇓ shows the effect of a bolus infusion of 5 μg/kg TPA in the low– and high–PAI-1 groups. Total TPA, active TPA, and TPA/PAI-1 complex levels peaked by 2 minutes, followed by a continuous decrease in the level of all three factors. TPA/C1-I levels increased 20% to 30% after bolus TPA infusion in both groups then fell at a rate similar to TPA/PAI-1 complex. Both low– and high–PAI-1 groups showed a drop in active PAI-1 and total PAI-1 antigen after infusion of TPA. In the high–PAI-1 group, PAI-1 antigen levels fell from a baseline of 1406±477 to 730±328 pmol/L 15 minutes after infusion of TPA (P=.0002); slowly rose again, peaking at 60 to 90 minutes; then progressively decreased, following the usual PAI-1 circadian rhythm.
Clearance of TPA
Total TPA antigen peaked by 2 minutes in all subjects at levels that were not significantly different in the low–PAI-1 group (1294±329 pmol/L) versus the high–PAI-1 group (1299±288 pmol/L), indicating that both groups started clearance with the same peak level of total TPA. The simple α-phase half-life of total TPA antigen from 2 to 12 minutes after TPA infusion was significantly longer in subjects with high PAI-1 (5.3±0.9 minutes) versus low PAI-1 (3.5±0.7 minutes; P=.006). At 30 minutes, total TPA antigen levels continued to be elevated in the high–PAI-1 group (258±72 pmol/L) versus the low–PAI-1 group (103±42 pmol/L; P=.009). Active TPA also peaked by 2 minutes in all subjects but fell faster in the high–PAI-1 group because of increased formation of TPA/PAI-1 complex.
As seen in Fig 3⇑, total TPA clearance was characterized by an initial rapid α-phase followed by a slower β-phase in both the low– and high–PAI-1 groups. Using a computer simulation of the human circulatory system, we studied two models of clearance: a single-compartment model with all clearance through the liver (α-phase only) and separate two-compartment models (Fig 1⇑) for active TPA and TPA/PAI-1 complex (α- and β-phases).
Fig 4⇓ shows the effect of single-compartment versus two-compartment clearance models on the fit between measured and simulated levels of active TPA, TPA/PAI-1 complex, total TPA, and total PAI-1 in one subject with high PAI-1 activity. As can be seen, the single-compartment model based on hepatic clearance tended to overestimate TPA/PAI-1 complex levels soon after the bolus infusion was completed while underestimating total TPA and TPA/PAI-1 complex at 30 minutes. Addition of the second compartment had the effect of rapid transport or binding of TPA, lowering the concentration soon after infusion, followed by slow release later. The model predicted that the β-phase of clearance noted in the original data above was essentially due to this slow release from the second compartment. The Table⇓ gives the average hepatic extraction fractions, second-compartment microconstants, and total squared errors for the one- and two-compartment models. The two-compartment model produced a better fit between simulated and measured levels of active and complexed TPA in both the high– and low–PAI-1 groups. Overall, the average total squared error was 64% lower in the two-compartment versus the one-compartment model (P=.016). The best fit in both the one- and two-compartment models occurred when active TPA was cleared faster by the liver than was TPA/PAI-1 complex. In the two-compartment model, the estimated hepatic-clearance half-lives were 2.4±0.3 minutes for active TPA versus 5.0±1.8 minutes for TPA/PAI-1 complex (P=.0036).
Two subjects had high transfer rates for essentially all of the second-compartment constants. Their data tended to skew the averages shown in the Table⇑ toward higher values. The predicted rate of transfer from the blood to the second compartment for active TPA (K2) ranged from 3 to 28 h−1, with a median rate of 6 h−1, whereas the transfer rate for TPA/PAI-1 complex (K4) ranged from 4 to 22 h−1, with a median rate of 12 h−1. The predicted rate of transfer from the second compartment to the blood for active TPA (K−2) ranged from 4 to 72 h−1, with a median rate of 16 h−1, whereas the transfer rate for TPA/PAI-1 complex (K−4) also ranged from 4 to 72 h−1, with a median rate of 7 h−1. There were no significant differences in the second-compartment microconstants for active TPA versus TPA/PAI-1 complex.
The source of PAI-1 secretion is still controversial; endothelium, hepatocytes, and adipocytes have all been suggested as sources of PAI-1 in plasma. To determine whether different sites for PAI-1 secretion might affect the clearance model, we compared (1) PAI-1 secretion from the liver versus (2) secretion from the systemic capillary beds, modeling a mixture of endothelial and adipose tissue secretion. The overall errors, hepatic extraction fractions, and second-compartment microconstants were not significantly different in the liver versus capillary models of PAI-1 secretion.
The model also provided a rough indication of the rate of active PAI-1 clearance by the liver. The model predicted hepatic extraction fractions for active PAI-1 ranging from 5% to ≈20%, with a mean of ≈10%. Finally, the model predicted two different second-order rate constants for the reaction between TPA and C1-I. The rate constant for TPA secreted by endothelium in vivo (based on baseline levels of TPA/C1-I) was predicted to be 519 mol/L−1·s−1, but the rate constant for C1-I reacting with infused TPA was 23 mol/L−1·s−1.
Effect on Total TPA Levels
To evaluate the effect of slower TPA/PAI-1 clearance on total TPA levels, we ran steady-state clearance simulations using the circulatory model with hepatic extraction fractions set to the average values for the two-compartment model in the Table⇑: 89% for active TPA and 48% for TPA/PAI-1 complex. The simulation was allowed to run with TPA and PAI-1 secretion held constant until steady-state levels of all factors were reached. This simulated baseline levels of these factors in vivo. Fig 5⇓ shows the effect of using progressively higher baseline PAI-1 secretion rates while holding the TPA secretion rate the same for all simulations. As baseline PAI-1 increased, more secreted TPA was converted into TPA/PAI-1, which in turn was cleared more slowly, resulting in an increase in the total TPA level. The model predicted that an increase in active plasma PAI-1 from 6 to 50 ng/mL would increase total TPA antigen from 10 to 16 ng/mL with no change in TPA secretion.
The purpose of this study was to determine the rate of clearance of active TPA versus TPA/PAI-1 complex and to evaluate the effect of increased PAI-1 levels on total TPA antigen in plasma. First, we evaluated the simple α-phase clearance of total TPA in seven subjects with low PAI-1 and four with high PAI-1 activity. We found that total TPA was cleared more slowly in subjects with high PAI-1. Most prior studies used radiolabeled TPA or measurements of TPA antigen to determine total TPA clearance.25 43 44 Using these methods, determination of clearance rates for individual forms of TPA was not possible. Where clearances of both active and total TPA were measured, active TPA was cleared faster than total TPA, but the contribution of hepatic clearance versus inhibitors to the overall clearance of active TPA was not determined.45 46
The use of a circulatory model of TPA regulation allowed us to track the bolus infusion of TPA, the reaction between TPA and PAI-1 forming TPA/PAI-1 complex, and the hepatic clearance of active and complexed TPA. By fitting the computer model to measured levels of these factors, we estimated that ≈89% of active TPA was removed from the plasma as it flowed through the liver versus only 48% for TPA/PAI-1 complex. These clearance fractions were equivalent to half-lives of 2.4 minutes for active TPA and 5.0 minutes for TPA/PAI-1 complex. Brommer et al32 reported similar findings using direct measurements of TPA activity and total TPA antigen in the hepatic artery versus the hepatic vein. They reported that the liver clears ≈71% of TPA activity (half-life of 2.9 minutes) while clearing 39% of total TPA antigen (half-life of 5.3 minutes). Because most TPA in resting samples is in the form of TPA/PAI-1 complex, Brommer et al suggested that complexed TPA was cleared more slowly than free TPA.
Other studies using perfused rat liver systems reported faster clearance of purified TPA/PAI-1 complex (α-phase half-life of 9.7 minutes) versus free TPA (α-phase half-life of 20.1 minutes).47 Perfused rat liver systems show half-lives for TPA clearance that are ≈20-fold longer than in the intact rat (α-phase half-life of 1 minute), even though the liver is the major clearance organ.48 It is possible either that rat clearance is different from human clearance or that clearance in isolated rat livers differs from clearance in the intact animal.
This difference in clearance rates for active versus complexed TPA in humans may have important clinical implications. Recent work has shown that increased levels of total TPA antigen and elevated levels of PAI-1 activity or PAI-1 antigen are associated with an increased risk of arterial thrombosis leading to myocardial infarction.5 6 7 8 9 10 High total TPA antigen as a risk factor seems counterintuitive, and a number of explanations have been suggested, including increased secretion and endothelial injury.9 10 Differences in clearance may provide another explanation. Higher levels of PAI-1 in blood convert a greater fraction of the secreted TPA in TPA/PAI-1 complex (Fig 5⇑). Because TPA/PAI-1 is cleared more slowly than active TPA, the total level of TPA rises even though TPA activity is falling. Thus, elevated total TPA antigen is in part a surrogate measure of high PAI-1 activity.
Prior studies25 44 have reported that clearance of total TPA in humans is biphasic, with α- and β-phase half-lives of ≈4 and 30 minutes, respectively. This same biphasic pattern was seen in our data (Fig 3⇑). We found that active TPA, TPA/PAI-1, and total TPA clearance were best fit using a two-compartment circulatory model with elimination from the central or plasma compartment. The β-phase of clearance could be explained by the slow release of active TPA and TPA/PAI-1 complex from the second compartment into the plasma. The circulatory model directly estimated microconstants for the transfer of active TPA and TPA/PAI-1 complex to and from the second compartment (Table⇑). In two subjects, the model predicted much higher second-compartment transfer rates than in the remaining subjects; this skewed the average results shown in the Table⇑ toward higher values. Whereas the hepatic extraction fraction for active TPA was greater than that for TPA/PAI-1, there were no significant differences in the second-compartment microconstants for active versus complexed TPA. Although this may indicate that a single second-compartment mechanism operates for both active and complexed TPA, it is more likely that the current data set and second-compartment model were not adequate to accurately simulate the true mechanism of the second compartment.
TPA clearance by the liver involves at least two pathways: the mannose receptor on liver endothelial cells and Kupffer cells and the LRP receptor on parenchymal cells.49 Recent work suggests that the mannose receptor primarily binds free TPA, whereas the LRP receptor binds free and complexed TPA but clears complexed TPA more quickly. Clearance by the LRP receptor alone would predict faster clearance of complex compared with active TPA. It may be the combination of the two clearance mechanisms, mannose and LRP receptors, in vivo that results in faster active TPA clearance.
It is interesting to speculate on the composition of the second compartment for active and complexed TPA. One possibility is receptors such as annexin II, an endothelial cell receptor that binds active TPA or endothelial cell TPA-binding proteoglycans.50 51 52 Clearance receptors might also act as a second compartment, but the rate of endocytosis is so fast it is unlikely they would release a significant amount of TPA once bound. For a receptor to act as a second compartment, it must have a high-enough density, an appropriate binding constant, and slow internalization, allowing the release of bound TPA.
Active TPA is also removed from the blood by reaction with inhibitors. As expected, all subjects showed decreases in PAI-1 activity and increases in TPA/PAI-1 complex immediately after the infusion of TPA. What was not expected was that total PAI-1 antigen was also decreased by infusion of TPA. This suggests that TPA/PAI-1 complex is cleared more quickly by the liver than is active PAI-1. Wing et al47 report similar findings for a isolated perfused rat liver model. The circulatory model indicated hepatic extraction fractions for active PAI-1 ranging from 5% to ≈20%, with a mean of ≈10%. This was similar to the 20±10% hepatic extraction fraction reported by Brommer et al for active PAI-1.32
The principal inhibitor of TPA is PAI-1, but C1-I also plays a role.1 18 24 Subjects in the present study had on average 27±12 pmol/L TPA/C1-I complex at baseline. TPA/C1-I levels rose 20% to 30% during TPA infusion, followed by rapid clearance. The average half-life for TPA/PAI-1 complex of 5.0±1.8 minutes was similar to the half-life of 4.2 minutes reported by Huisman et al24 for TPA/C1-I complex. This may indicate a similar clearance mechanism for all TPA/inhibitor complexes, possibly through the LRP receptor.
As in our previous study,1 the circulatory model indicated that the in vivo reaction rate between TPA and C1-I was faster than the rate measured in vitro. The model predicted that C1-I reacts with TPA secreted from endothelium with a rate constant >500 mol/L−1·s−1. The predicted rate for the in vivo reaction between C1-I and infused TPA was ≈23 mol/L−1·s−1. Huisman et al24 reported an in vitro rate constant for the reaction between TPA and C1-I of 5 mol/L−1·s−1. This suggests that endothelium or some other cell surface may in some way accelerate the reaction between TPA and C1-I in vivo.
In summary, we found that active TPA was cleared from the blood more quickly than TPA/PAI-1 complex and that the clearance profile for active TPA, TPA/PAI-1 complex, and total TPA antigen was best fit by a two-compartment mechanism. The slower clearance of TPA/PAI-1 complex may explain in part the higher levels of total TPA antigen seen in patients with high levels of PAI-1 activity.
Selected Abbreviations and Acronyms
|PAI-1||=||plasminogen activator inhibitor type 1|
|TPA||=||tissue plasminogen activator|
This study was supported by grants from the Institut National de la Santé et de la Recherche Médicale, from the Ministère de l’Education Nationale, de l’Enseignement Supérieur et de la Recherche, and from Boehringer Ingelheim. The authors are very grateful to Prof D. Collen and Prof R. Lijnen (Center for Molecular and Vascular Biology, Leuven, Belgium) for helpful advice. The authors also wish to thank Dr O. Blin (Center of Clinical Pharmacology and Therapeutic Evaluation, CHU Timone, Marseille, France) for his skillful assistance and J. Ansaldi for excellent technical work.
- Received October 28, 1996.
- Revision received February 3, 1997.
- Accepted February 16, 1997.
- Copyright © 1997 by American Heart Association
Chandler WL, Levy WC, Stratton JR. The circulatory regulation of TPA and UPA secretion, clearance, and inhibition during exercise and during the infusion of isoproterenol and phenylephrine. Circulation. 1995;92:2984-2994.
Chandler WL, Levy WC, Veith RC, Stratton JR. A kinetic model of the circulatory regulation of tissue plasminogen activator during exercise, epinephrine infusion, and endurance training. Blood. 1993;81:3293-3302.
Chandler WL. A kinetic model of the circulatory regulation of tissue plasminogen activator. Thromb Haemost. 1991;66:321-322.
Meade TW, Ruddock V, Stirling Y, Chakrabarti T, Miller GJ. Fibrinolytic activity, clotting factors and long-term incidence of ischaemic heart disease in the Northwick Park Heart Study. Lancet. 1993;342:1076-1079.
Juhan-Vague I, Pyke SDM, Alessi MC, Jespersen J, Haverkate F, Thompson SG. Fibrinolytic factors and risk of myocardial infarction or sudden death in patients with angina pectoris. Circulation. 1996;94:2057-2063.
Hamsten A, Walldius G, Szamosi A, Blomback M, De Faire U, Dahlen G, Landou C, Wiman B. Plasminogen activator inhibitor in plasma: risk factor for recurrent myocardial infarction. Lancet. 1987;2:3-9.
Jansson JH, Nilsson TK, Olofsson BO. Tissue plasminogen activator and other risk factors as predictors of cardiovascular events in patients with severe angina pectoris. Eur Heart J. 1991;12:157-161.
Jansson JH, Olofsson BO, Nilsson TK. Predictive value of tissue plasminogen activator mass concentration on long-term mortality in patients with coronary artery disease. Circulation. 1993;88:2030-2034.
Thompson S, Kienast J, Pyke SDM, Haverkate F, van de Loo JCW. Hemostatic factors and the risk of myocardial infarction and death. N Engl J Med. 1995;332:635-641.
Ridker PM, Vaughan DE, Stampfer MJ, Manson JE, Hennekens CH. Endogenous tissue-type plasminogen activator and risk of myocardial infarction. Lancet. 1993;341:1165-1168.
Juhan-Vague I, Alessi MC, Joly P, Thirion X, Vague P, Declerck PJ, Serradimigni A. Plasma plasminogen activator inhibitor-1 in angina pectoris: influence of plasma insulin and acute-phase response. Arteriosclerosis. 1989;9:362-367.
Nicoloso G, Hauert J, Kruithof EKO, Melle GV, Bachmann F. Fibrinolysis in normal subjects: comparison between plasminogen activator inhibitor and other components of the fibrinolytic system. Thromb Haemost. 1988;59:299-303.
Chandler WL, Trimble SL, Loo SC, Mornin D. Effect of PAI-1 levels on the molar concentrations of active tissue plasminogen activator (t-PA) and t-PA/PAI-1 complex in plasma. Blood. 1990;76:930-937.
Haverkate F, Thompson SG, Duckert F. Haemostasis factors in angina pectoris: relation to gender, age and acute-phase reaction. Thromb Haemost. 1995;73:561-567.
Rijken DC, Juhan-Vague I, Collen D. Complexes between tissue-type plasminogen activator and proteinase inhibitors in human plasma, identified with an immunoradiometric assay. J Lab Clin Med. 1983;101:285-294.
Alessi M, Juhan-Vague I, Declerck P, Collen D. Molecular forms of plasminogen activator inhibitor-1 (PAI-1) and tissue-type plasminogen activator (t-PA) in human plasma. Thromb Res. 1991;62:275-285.
Lucore CL, Sobel BE. Interactions of tissue-type plasminogen activator with plasma inhibitors and their pharmacologic implications. Circulation. 1988;77:660-669.
Booth NA, Walker E, Maughan R, Bennett B. Plasminogen activator in normal subjects after exercise and venous occlusion: t-PA circulates as complexes with C1-inhibitor and PAI-1. Blood. 1987;69:1600-1604.
Thorsen S, Philips M. Isolation of tissue-type plasminogen activator-inhibitor complexes from human plasma: evidence for a rapid plasminogen activator inhibitor. Biochim Biophys Acta. 1984;802:111-118.
Rånby M, Nguyen G, Scarabin PY, Samama M. Immunoreactivity of tissue plasminogen activator and of its inhibitor complexes: biochemical and multicenter validation of a two site immunosorbent assay. Thromb Haemost. 1989;61:409-414.
Kruithof EKO, Tran-Thang C, Ransijn A, Bachmann F. Demonstration of a fast acting inhibitor of plasminogen activators in human plasma. Blood. 1984;64:907-913.
Alessi MC, Juhan-Vague I, Declerck PJ, Anfosso F, Gueunoun E, Collen D. Correlations between t-PA and PAI-1 antigen and activity and t-PA/PAI-1 complexes in plasma of control subjects and of patients with increased t-PA or PAI-1 levels. Thromb Res. 1990;60:509-516.
Rånby M, Sundell B, Nilsson TK. Blood collection in strong acidic citrate anticoagulant used in a study of dietary influence on basal tPA activity. Thromb Haemost. 1989;62:917-922.
Huisman LGM, van Griensven JMT, Kluft C. On the role of C1-inhibitor as inhibitor of tissue-type plasminogen activator in human plasma. Thromb Haemost. 1995;73:466-471.
Baughman RA. Pharmacokinetics of tissue plasminogen activator. In: Sobel BE, Collen D, Grossbard EB, eds. Tissue Plasminogen Activator in Thrombolytic Therapy. New York, NY: Marcel Dekker Inc; 1987:41-53.
Faulkner JA, Heigenhauser GF, Schork MA. The cardiac output– oxygen uptake relationship of men during graded bicycle ergometry. Med Sci Sports Exerc. 1976;9:148-154.
Mollison PL. Blood Transfusion in Clinical Medicine. Boston, Mass: Blackwell Scientific Publications Inc; 1983.
Keber D, Blinc A, Fettich J. Increase of tissue plasminogen activator in limbs during venous occlusion: a simple haemodynamic model. Thromb Haemost. 1990;64:433-437.
Witzleb E. Functions of the vascular system. In: Schmidt RF, Thews G, eds. Human Physiology. New York, NY: Springer-Verlag; 1983:397-455.
van Hinsbergh VWM. Synthesis and secretion of plasminogen activators and plasminogen activator inhibitor by endothelial cells. In: Kluft C, ed. Tissue-Type Plasminogen Activator (t-PA): Physiological and Clinical Aspects. Boca Raton, Fla: CRC Press; 1988:3-20.
Kooistra T, Bosma PJ, Tons HAM, van den Berg AP, Meyer P, Princen HMG. Plasminogen activator inhibitor 1: biosynthesis and mRNA level are increased by insulin in cultured human hepatocytes. Thromb Haemost. 1989;62:723-728.
Brommer EJ, Derkx FH, Schalekamp MA, Dooijewaard G, v d Klaauw MM. Renal and hepatic handling of endogenous tissue-type plasminogen activator and its inhibitor in man. Thromb Haemost. 1988;59:404-411.
Sprengers ED, Princen HMG, Kooistra T, Van Hinsbergh VWM. Inhibition of plasminogen activators by conditioned medium of human hepatocytes and hepatoma cell line Hep G2. J Lab Clin Med. 1985;105:751-758.
Wieczorek I, Ludlam CA, MacGregor I. Venous occlusion does not release von Willebrand factor, factor VIII, or PAI-1 from endothelial cells: the importance of consensus on the use of correction factors for haemoconcentration. Thromb Haemost. 1993;69:91-93.
van Hinsbergh VWM, Bauer KA, Kooistra T, Kluft C, Dooijewaard G, Sherman ML, Nieuwenhuizen W. Progress of fibrinolysis during tumor necrosis factor infusions in humans: concomitant increase in tissue-type plasminogen activator, plasminogen activator inhibitor type-1, and fibrin(ogen) degradation products. Blood. 1990;76:2284-2289.
Samad F, Yamamoto K, Loskutoff DJ. Distribution and regulation of plasminogen activator inhibitor-1 in murine adipose tissue in vivo. J Clin Invest. 1996;97:37-46.
Shimomura I, Funahashi T, Takahashi M, Maeda K, Kotani K, Nakamura T, Yamashita S, Miura M, Fukuda K, Takemura K, Tokunaga K, Matsuzawa Y. Enhanced expression of PAI-1 in visceral fat: possible contributor to vascular disease in obesity. Nat Med. 1996;2:800-803.
Alessi MC, Parrot G, Guenoun E, Scelles V, Vague P, Juhan-Vague I. Relation between plasma PAI activity and adipsin levels. Thromb Haemost. 1995;74:1200-1202. Letter.
Lundgren CH, Brown NA, Nordt TK, Sobel BE, Fujii S. Elaboration of type 1 plasminogen activator inhibitor from adipocytes: a potential pathogenic link between obesity and cardiovascular disease. Circulation. 1996;97:106-110.
Alessi MC, Peiretti F, Morange P, Henry M, Nalbone G, Juhan-Vague I. Production of plasminogen activator inhibitor 1 by human adipose tissue: possible link between visceral fat accumulation and vascular disease. Diabetes. In press.
Coleman PL, Patel PD, Cwikel BJ, Rafferty UM, Sznycer R, Gelehrter TC. Characterization of the dexamethasone-induced inhibitor of plasminogen activator in HTC hepatoma cells. J Biol Chem. 1986;261:4352-4357.
Hekman CM, Loskutoff DJ. Kinetic analysis of the interactions between plasminogen activator inhibitor 1 and both urokinase and tissue plasminogen activator. Arch Biochem Biophys. 1988;262:199-210.
de Boer A, Kluft C, Kroon JM, Kasper FJ, Schoemaker HC, Pruis J, Breimer DD, Soons PA, Emeis JJ, Cohen AF. Liver blood flow as a major determinant of the clearance of recombinant human tissue-type plasminogen activator. Thromb Haemost. 1992;67:83-87.
Krause J. Catabolism of tissue-type plasminogen activator (t-PA), its variants, mutants and hybrids. Fibrinolysis. 1988;2:133-142.
Tanswell P, Seifried E, Su PCAF, Feurer W, Rijken DC. Pharmacokinetics and systemic effects of tissue-type plasminogen activator in normal subjects. Clin Pharmacol Ther. 1989;46:155-162.
Koster RW, Cohen AF, Kluft C, Kasper FJ, van der Wouw PA, Weatherley BC. The pharmacokinetics of recombinant double-chain t-PA (duteplase): effects of bolus injection, infusions, and administration by weight in patients with myocardial infarction. Clin Pharmacol Ther. 1991;50:267-277.
Wing LR, Hawksworth GM, Bennett B, Booth NA. Clearance of t-PA, PAI-1, and t-PA-PAI-1 complex in an isolated perfused rat liver system. J Lab Clin Med. 1991;117:109-114.
Emeis JJ, van den Hoogen CM, Jense D. Hepatic clearance of tissue-type plasminogen activator inhibitor in rats. Thromb Haemost. 1985;54:661-664.
Camani C, Kruithof EKO. Clearance receptors for tissue-type plasminogen activator. Int J Hematol. 1994;60:97-109.
Strickland DK, Kounnas MZ, Williams SE, Argraves WS. LDL receptor-related protein (LRP): a multiligand receptor. Fibrinolysis. 1994;8(suppl 1):204-215.
Cesarman GM, Guevara CA, Hajjar KA. An endothelial cell receptor for plasminogen/tissue plasminogen activator (t-PA). J Biol Chem. 1994;269:21198-21203.
Bohm T, Geiger M, Binder BR. Isolation and characterization of tissue-type plasminogen activator-binding proteoglycans from human umbilical vein endothelial cells. Arterioscler Thromb Vasc Biol. 1996;16:665-672.