Owen, Whyte G.

Abstract Immunohistochemical analysis of surgically obtained porcine tissue samples reveals ubiquitous staining for prothrombin in organs rich in smooth muscle content and universal staining of smooth muscle in tissue vasculature. The native character of tissue prothrombin is verified first by chromogenic substrate hydrolysis and hirudin inhibition after incubation of tissue extracts with taipan snake venom and phospholipid. Western analysis of tissue extracts confirms the native zymogen molecular weight. In addition, prothrombin purified in good yield from porcine uterus is activated by Echis carinatus venom which, like taipan venom, is 4-carboxyglutamic acid-sensitive. After correction for blood (gross heme) and interstitial fluid (albumin), excess functional prothrombin is observed in extracts of tissues having abundant smooth muscle. In contrast with factor X, the yield of prothrombin purified from porcine uterus greatly exceeds that attributable to contamination by whole blood. Northern blot analysis from selected bovine tissues extracted for polyadenylated messenger RNA is equivocal for prothrombin mRNA with the exception of liver, which is positive. It is concluded that functionally intact prothrombin is widely distributed among tissues owing to smooth muscle content, although the mechanism of emplacement and physiologic significance of prothrombin in these tissues remains unclear.

Addition of exogenous plasminogen activator inhibitor-1 (PAI-1) to fibrin clots inhibits fibrinolysis in vivo. However, it is unknown whether the localized concentrations of active PAI-1 necessary to produce this antifibrinolytic effect can be recruited to acute arterial thrombi by endogenous mechanisms. We measured PAI-1 activity and antigen in porcine coronary artery thrombi that formed in response to acute vascular injury. Mean PAI-1 activity in thrombi (n=5) was 36±5.1 μg/mL, which is >2000 times its concentration in normal porcine plasma. The presence of markedly elevated concentrations of active PAI-1 in thrombi was confirmed by an immunoactivity assay and by demonstrating formation of sodium dodecyl sulfate–stable complexes after addition of 125 I-urokinase to thrombus extracts. Comparative analysis of PAI-1 antigen by Western blotting and urokinase inhibition assay suggested that approximately one third of thrombus-associated PAI-1 was active. Histological examination of coronary thrombi revealed that they consisted predominantly of dense aggregates of platelets with interspersed islands of fibrin, which closely resemble the histological appearance of thrombi in patients with myocardial infarction and unstable angina pectoris. Washed porcine platelets prepared from peripheral blood contained sufficient PAI-1 antigen and activity to account for the concentrations observed in coronary artery thrombi. However, the specific activity of human platelet PAI-1 was lower than that of porcine platelet PAI-1 (2% versus 50% active, respectively), and human platelets inhibited in vitro fibrinolysis to a lesser extent than did porcine platelets. These results indicate that active PAI-1 accumulates in porcine coronary artery thrombi in concentrations markedly higher than those present in plasma and that PAI-1 may be an important determinant of the known resistance of platelet-rich thrombi to lysis by tissue-type plasminogen activator. These studies also underscore the importance of considering possible species differences in protein function when comparing animal models of thrombosis to acute coronary thrombosis in humans.

Objective— To test the hypothesis that statins will acutely inhibit platelet thrombus formation, intravenous lovastatin was assessed in our well-characterized porcine carotid injury model. Methods and Results— The first carotid artery was crush-injured and harvested after 30 minutes. Pigs then received intravenous lovastatin (100 μg/kg bolus+100 μg/kg/h infusion, n=6) or saline (n=11) before injury of the second carotid artery. Thrombus size was quantified by scintillation detection of autologous 111 In-platelets. Sequential carotid injury produced a thrombus more than 50% greater in volume in the second (3149±2053×10 6 /cm 2 ) relative to the first injured artery (2081±1552×10 6 /cm 2 ; P =0.04) in control pigs. This augmentation was inhibited by intravenous lovastatin which acutely reduced platelet deposition (944±246×10 6 /cm 2 ) relative to saline control ( P =0.02). Flow chamber closure times increased on average by 2.45-fold in response to whole blood lovastatin incubation. Lovastatin ( P <0.05) and simvastatin ( P <0.05) reduced platelet dense granule secretion in vitro. Conclusions— Sequential arterial injury augments the thrombotic response suggesting that the propensity for arterial thrombosis is at least partially acquired. This thrombotic augmentation can be acutely attenuated by intravenous lovastatin which may result from a pleiotropic impact on platelet function. These results appear to be a class effect of 3-hydroxy-3-methyl-glutaryl-CoA reductase inhibitors.

The amount of thrombin active in vivo in the intravascular space (blood and endothelial surface), both basally and in experimental intravascular coagulation, is measured by way of the accessibility of thrombin to intravascular hirudin. Blood samples from pigs given intravenous 125I-labeled hirudin contain 125I-labeled hirudin-thrombin complex in concentrations indicative of a basal thrombin concentration in vivo of 0.5 nmol/liter. Intravenous infusion of Salmonella endotoxin elicits an increase in the circulating concentration of hirudin-thrombin complex that begins within 15 min and is 20-30 times basal after 4 hr. Induction of mild intravascular coagulation is evidenced by a modest reduction in plasma fibrinogen concentrations. It is concluded that there is a basal pool of hirudin-accessible thrombin in the intravascular space that, were it free in the plasma phase, would be sufficient in principle to sustain intravascular coagulation.

Objective— Occurrence of arterial thrombosis secondary to vascular disease in an individual is not easily predicted. After establishing that this poor predictability arises at least in part from an intrinsic thrombosis propensity of the individual, we sought to determine whether the propensity for arterial thrombosis is governed by blood or arterial wall factors. Methods and Results— To evaluate the variability arising from the blood, autologous 111 In-labeled platelet deposition was measured after high-shear perfusion of compressed aortic strips, prepared from a single pig, with heparinized blood from 25 pigs. To evaluate the variability arising from the vessel wall, aortic strips from 8 pigs were superfused with blood from a single animal. Blood samples from 25 animals superfused over aortic substrate from a single source yielded a 24-fold range of platelet deposition. In contrast, when aortic substrates from 8 different animals were superfused with blood from a single animal, platelet deposition spanned a 3-fold range. Platelet deposition was significantly correlated with whole-blood lymphocyte counts and with platelet counts . Conclusions— Individual propensity for arterial thrombosis in pigs is more greatly influenced by blood components than by elements within the arterial wall.

Atherosclerosis manifests as a systemic disease with near global involvement of the named segments of the arterial tree. Acute thrombotic arterial occlusion, however, is not equally distributed. To evaluate intra-individual regional differences in arterial thrombogenicity, we compared 111 In-platelet deposition in porcine carotid and femoral arteries after a standardized crush injury. Within the unidirectional flow conditions of elastic carotid arteries, platelet deposition was more than 3-fold higher compared with predominantly muscular femoral arteries with triphasic arterial flow. To determine the influence of rheology on platelet deposition after crush injury, carotid arteries were transplanted into the femoral position and compared with the paired native carotid and femoral arteries. Similarly, femoral arteries transposed to the carotid position were compared with the paired native carotid artery. In each of these experiments, arterial transposition to a new anatomic location imparts a predilection for platelet deposition indigenous to the new location. In the controlled environment of two high-shear thrombin–independent and –dependent flow chambers, porcine carotid and femoral arterial substrates were indistinguishable from one another with respect to platelet deposition. Regional differences in arterial hemodynamics may account for substantial differences in thrombosis arising from deep arterial injury.