Inhibition of thrombin on surfaces coated with immobilized heparin and heparin-like polysaccharides: a crucial non-thrombogenic principle

Blood-Compatible Materials: Vascular Endothelium-Mimetic Surfaces that Mitigate Multiple Cell-Material Interactions

When flowing whole blood contacts medical device surfaces, the most common blood-material interactions result in coagulation, inflammation, and infection. Many new blood-contacting biomaterials have been proposed based on strategies that address just one of these common modes of failure. This study proposes to mitigate unfavorable biological reactions that occur with blood-contacting medical devices by designing multifunctional surfaces, with features optimized to meet multiple performance criteria. These multifunctional surfaces incorporate the release of the small molecule hormone nitric oxide (NO) with surface chemistry and nanotopography that mimic features of the vascular endothelial glycocalyx. These multifunctional surfaces have features that interact with coagulation components, inflammatory cells, and bacterial cells. While a single surface feature alone may not be sufficient to achieve multiple functions, the release of NO from the surfaces along with their modification to mimic the endothelial glycocalyx synergistically improves platelet-, leukocyte-, and bacteria-surface interactions. This work demonstrates that new blood-compatible materials should be designed with multiple features, to better address the multiple modes of failure of blood-contacting medical devices.

Nanostructured Surfaces That Mimic the Vascular Endothelial Glycocalyx Reduce Blood Protein Adsorption and Prevent Fibrin Network Formation

Blood-contacting materials are critical in many applications where long-term performance is desired. However, there are currently no engineered materials used in cardiovascular implants and devices that completely prevent clotting when in long-term contact with whole blood. The most common approach to developing next-generation blood-compatible materials is to design surface chemistries and structures that reduce or eliminate protein adsorption to prevent blood clotting. This work proposes a new paradigm for controlling protein-surface interactions by strategically mimicking key features of the glycocalyx lining the interior surfaces of blood vessels: negatively charged glycosaminoglycans organized into a polymer brush with nanoscale domains. The interactions of two important proteins from blood (albumin and fibrinogen) with these new glycocalyx mimics are revealed in detail using surface plasmon resonance and single-molecule microscopy. Surface plasmon resonance shows that these blood proteins interact reversibly with the glycocalyx mimics, but have no irreversible adsorption above the limit of detection. Single-molecule microscopy is used to compare albumin and fibrinogen interactions on surfaces with and without glycocalyx-mimetic nanostructures. Microscopy videos reveal a new mechanism whereby the glycocalyx-mimetic nanostructures eliminate the formation of fibrin networks on the surfaces. This approach shows for the first time that the nanoscale structure and organization of glycosaminoglycans in the glycocalyx are essential to (i) reduce protein adsorption, (ii) reversibly bind fibrin(ogen), and (iii) inhibit fibrin network formation on surfaces. The insights gained from this work suggest new design principles for blood-compatible surfaces. New surfaces developed using these design principles could reduce risk of catastrophic failures of blood-contacting medical devices.

Glycocalyx-Inspired Nitric Oxide-Releasing Surfaces Reduce Platelet Adhesion and Activation on Titanium

The endothelial glycocalyx lining the inside surfaces of blood vessels has multiple features that prevent inflammation, blood clot formation, and infection. This surface represents the highest standard in blood compatibility for long-term contact with blood under physiological flow rates. Engineering materials used in blood-contacting biomedical devices, including metals and polymers, have undesirable interactions with blood that lead to failure modes associated with inflammation, blood clotting, and infection. Platelet adhesion and activation are key events governing these undesirable interactions. In this work, we propose a new surface modification to titanium with three features inspired by the endothelial glcyocalyx: First, titanium surfaces are anodized to produce titania nanotubes with high surface area. Second, the nanostructured surfaces are coated with heparin-chitosan polyelectrolyte multilayers to provide glycosaminoglycan functionalization. Third, chitosan is modified with a nitric oxide-donor chemistry to provide an important antithrombotic small-molecule signal. We show that these surfaces are nontoxic with respect to platelets and leukocytes. The combination of glycocalyx-inspired features results in a dramatic reduction of platelet and leukocyte adhesion and platelet activation.

Engineering nanoassemblies of polysaccharides

Polysaccharides offer a wealth of biochemical and biomechanical functionality that can be used to develop new biomaterials. In mammalian tissues, polysaccharides often exhibit a hierarchy of structure, which includes assembly at the nanometer length scale. Furthermore, their biochemical function is determined by their nanoscale organization. These biological nanostructures provide the inspiration for developing techniques to tune the assembly of polysaccharides at the nanoscale. These new polysaccharide nanostructures are being used for the stabilization and delivery of drugs, proteins, and genes, the engineering of cells and tissues, and as new platforms on which to study biochemistry. In biological systems polysaccharide nanostructures are assembled via bottom-up processes. Many biologically derived polysaccharides behave as polyelectrolytes, and their polyelectrolyte nature can be used to tune their bottom-up assembly. New techniques designed to tune the structure and composition of polysaccharides at the nanoscale are enabling researchers to study in detail the emergent biological properties that arise from the nanoassembly of these important biological macromolecules.

The interactions between antithrombin III, thrombin and surface immobilized heparin

The interactions between antithrombin III (ATIII), thrombin, and surface immobilized heparin were investigated. Carboxylated polystyrene modified with covalently immobilized albumin-heparin conjugate contain sites which can bind ATIII from buffer and plasma solutions. Approximately 65% of the ATIII molecules present at the heparinized surface either adsorbed from a buffer or plasma solution, exchanged with ATIII in buffer solution. The exchange between surface bound ATIII and ATIII in solution was repeated several times on the same heparinized surface. The number of binding sites that could bind and release ATIII was much higher when the heparinized surface was exposed to an ATIII containing buffer solution than to a plasma solution. The reduction in binding sites available for ATIII using plasma solutions as compared to buffer solutions could be explained by the competition of other plasma proteins with ATIII for the heparinized surface. It was observed that heparin binding proteins were able to compete with ATIII for binding to the immobilized heparin. Furthermore adsorption of proteins on the heparinized surface significantly reduced the availability of binding sites for ATIII. Exposure of thrombin to the heparinized surface resulted in thrombin activity at the surface. The thrombin activity on the heparinized surfaces was lower on surfaces with a higher ATIII concentration. The activity of surface bound thrombin was not affected by the presence of other plasma proteins. Enzymatically active thrombin molecules present at the heparinized surface were completely inactivated when the surface was exposed to a solution containing ATIII. The inactivation rate of surface bound thrombin by ATIII was higher than the rate of the uncatalyzed inactivation of thrombin in solution. Part of the Thrombin-Antithrombin III (TAT) complexes (10-20%) that were formed upon inactivation of thrombin remained bound to the heparinized surface. In general it was concluded that only the surface immobilized heparin molecules that can bind ATIII in a reversible way determine the anticoagulant properties of the surface. The mechanism of inactivation of a protease on a heparinized surface depends either on the catalytic effect of heparin on the inactivation rate of proteases by ATIII or on an increased uncatalytic inactivation due to increased concentrations of ATIII near the surface as compared to the concentration of ATIII in the bulk phase.

Heparin-coated cardiopulmonary bypass circuits

The indications for heparin-coated extracorporeal circuits cannot be defined or limited at present. Clinical investigation remains at an early stage of development. In situations where the risk of systemic anticoagulation is high, this technology would seem to hold great promise. Examples include extracorporeal lung assist and resuscitation from accidental hypothermia. Some have also suggested the use of heparin-coated circuits for percutaneous bypass in cardiopulmonary resuscitation. A significant advantage might also accrue in noncardiac surgical procedures requiring cardiopulmonary bypass, such as complex cerebral aneurysm or arteriovenous malformation resections, resections of the tracheal carina, or bilateral lung transplantations. Its role in routine cardiac surgical procedures remains uncertain, but the work of von Segesser et al suggests a need for continued investigation in that setting using reduced levels of systemic anticoagulation. That endeavor will be greatly assisted by the recent development of heparin-coated cardiotomy reservoirs. Although heparin-coated circuits have been safely used for extracorporeal lung assist with little or no systemic anticoagulation, prospective studies are clearly needed to determine if this approach is advantageous, and it would seem appropriate to develop heparin coating for silicone-based membrane oxygenators.

Nonspecific adsorption and covalent coupling of heparin on polyacrylate based microbeads

Polyacrylate based microbeads were prepared by copolymerization of four different acrylate monomers, namely 2-hydroxyethylmethacrylate (HEMA), ethyleneglycoldimethacrylate (EGDMA), methylmethacrylate (MMA) and dimethylaminoethylmethacrylate (DMEAMA). These beads were further activated with CNBr at alkaline pH. The extend of nonspecific adsorption and covalent coupling of heparin on these beads were investigated in a batch reactors at different temperatures. The effects of initial concentrations of activation agent and heparin were also studied. Nonspecific heparin adsorption on the microbeads containing DMAEMA was significantly higher than the others. Nonspecific adsorption decreased with increasing temperature. Heparin was covalently coupled on CNBr activated microbeads. The amount of coupled heparin increased by increasing concentration of CNBr.

Surface modification of polymeric biomaterials with poly(ethylene oxide), albumin, and heparin for reduced thrombogenicity

Appropriate surface modification has significantly improved the blood compatibility of polymeric biomaterials. This article reviews methods of surface modification with water-soluble polymers, such as polyethylene oxide (PEO), albumin, and heparin. PEO is a synthetic, neutral, water-soluble polymer, while albumin and heparin are a natural globular protein and an anionic polysaccharide, respectively. When grafted onto the surface, all three macromolecules share a common feature to reduce thrombogenicity of biomaterials. The reduced thrombogenicity is due to the unique hydrodynamic properties of the grafted macromolecules. In aqueous medium, surface-bound water-soluble polymers are expected to be highly flexible and extend into the bulk solution. Biomaterials grafted with either PEO, albumin, or heparin are able to resist plasma protein adsorption and platelet adhesion predominantly by a steric repulsion mechanism.

Biologically modified PHEMA beads for hemoperfusion: preliminary studies

Polyhydroxyethylmethacrylate (PHEMA) beads were prepared by phase separation polymerization. Hydroxyl groups on PHEMA beads were activated with CNBr at alkaline pH. Adsorption of heparin, blood proteins (i.e. albumin, fibrinogen and gamma-globulin), protein A, HIgG and DNA on these beads were studied. Preparation and activation procedures are given here. The preliminary results of these studies are also reported.

Periodontal repair in dogs. Effect of heparin treatment of the root surface

Studies on periodontal repair to denuded root surfaces have suggested that initial clot adhesion to the root surface may be important for the nature of subsequent healing. To study this hypothesis, circumferential periodontal defects, approximately 5 mm in vertical dimensions, were surgically created and immediately treated around the mandibular premolars in 4 beagle dogs. Prior to wound closure, the root surfaces were treated with either the anticoagulant heparin or with saline. Tissue blocks were obtained at sacrifice 4 weeks after surgery. Histometric analysis showed that connective tissue repair to the root surface averaged 50% of the defect height for heparin-treated teeth as compared to 95% for saline-treated teeth. Junctional epithelium amounted to an average of 33% of the defect height in heparin-treated teeth in contrast to 5% following saline treatment. It can be concluded that heparin treatment of the root surface compromises connective tissue repair, confirming clot adhesion as one prerequisite for connective tissue repair of periodontal defects.

Influence of high and low wall shear rates on the inhibition of factor Xa and thrombin at surfaces coated with immobilized heparin

The thromboresistant function of a surface with end-point attached heparin is based upon interaction among the immobilized heparin, antithrombin, and at least factor Xa or thrombin. Heparinized arteriovenous shunts were implanted in dogs. By compressing a segment of the shunt, high and low wall shear rate regions were obtained in each shunt. After removal, the tubings were tested for their factor Xa and thrombin inhibitory capacity. It was found that on a molar basis, the factor Xa and thrombin inhibitory capacity were similar in low wall shear rate segments. In high wall shear rate segments, the thrombin inhibitory capacity was decreased, thus indicating that the AT-mediated inhibition of the serine protease is dependent on the wall shear rate.

Interactions of biospecific functional polymers with blood proteins and cells

Biospecific functional polymers, i.e. synthetic or artificial polymers substituted with specific chemical functional groups carried by the macromolecular chain are designed to interact with living systems. These polymers are either insoluble or soluble, derived from polystyrene and dextran. Polymers substituted with aryl sulfonate and carboxyl groups specifically interact with antithrombin III and serine-proteases involved in the coagulation of blood. As a consequence, these polymers possess heparin-like activity and are therefore of low thrombogenicity when exposed to flowing blood. Other functional polymers have been prepared in order to interact with various components of the immune system. Soluble and insoluble functional polymers in contact with cells can affect both cell proliferation and metabolism. Some functional polymers have the ability to inhibit or to stimulate cell growth while others can alter cell function without a change in growth characteristics. The functional polymers described have possible applications as plasma expanders, non-thrombogenic catheters, non-complement activating surfaces and other applications in oncology, biotechnology and immunochemistry.

Trigger action of dextran sulfate on inactivation of thrombin

Inactivation of thrombin was enhanced by dextran sulfate (DS) coated on polypropylene tubes with plastic adhesive. Long interaction of thrombin with DS was not essential to enhance the enzyme inactivation. Albumin inhibited the inactivation of thrombin by DS-coated tubes, but had no effect after interaction of the enzyme with DS. NaCl, especially, sodium ions, inhibited the inactivation by DS-coated tubes. Enzyme inactivation occurred at 37 degrees C, but not at 4 degrees C. Thus, the inactivation was due not only to ionic interaction with DS, but also to intramolecular perturbation of thrombin.

Antithrombogenic heparin-bound polyurethanes

Many kinds of heparin-bound polyurethanes have been developed. Polyurethanes are a family of elastomers displaying better blood-compatibility than other polymeric materials. It is useful to modify this material by heparinization. Several approaches to heparinization have been devised: 1) a general method of heparinization, applicable to all polymeric materials, 2) a heparinization method specific to polyurethanes, and 3) the design of heparinizable polyurethane derivatives. These three approaches are first explained in detail. Then, the antithrombogenic mechanism of the heparinized polymers is discussed. Finally, the interactions of the heparinized polymers with blood coagulation factors, plasma proteins, and platelets are discussed.

Uptake and inactivation of thrombin on the subendothelium: comparisons with endothelium

One aspect of the non-thrombogenicity of endothelium is the property to bind and inhibit thrombin. Loss of the endothelial layer gives rise to a thrombogenic surface. The present study concerns uptake and inactivation of thrombin on endothelium as well as subendothelium. Both the immediate subendothelial layer and the media were investigated. Binding and inhibition of thrombin was assayed using both a synthetic chromogenic substrate and fibrinogen. In the latter case thrombin enzymatic activity was assessed by measuring liberation of fibrinopeptide A. There was a linear relationship between thrombin in the solution and thrombin enzymatic activity recovered on the surface on both endothelium and media. 30-50 per cent of the surface bound thrombin measured with the chromogenic substrate was also active towards fibrinogen. Preincubation of endothelium with plasma gave rise to an increased loss of thrombin from the solution at the same time as less thrombin was recovered on the surface. This indicates increased inactivation. Increased inactivation could also be obtained by pretreatment with antithrombin (AT) but not with AT-depleted plasma. The same general pattern was seen on the subendothelium but not on the media. It is concluded that inhibition of thrombin occurs on the endothelium and can be augmented by prior binding of AT to the surface. This effect although less pronounced can also be obtained on the subendothelium. The deeply injured vessel is much less capable of inhibiting thrombin.

Stimulation by dextran sulfate of denaturation of bovine thrombin

At high thrombin concentration, loss of enzyme activity was proportional to the initial enzyme concentration. At low thrombin concentration, loss of activity followed first order kinetics, indicating mainly denaturation (inactivation) of the enzyme. Dextran sulfate stimulated thrombin denaturation, while serum albumin and polyethylene glycol 6000 (PEG) protected the enzyme from denaturation. These results indicate that the enzyme molecule is stabilized by weak interactions with albumin and PEG, but destabilized by strong interaction with dextran sulfate.

Thromboresistance of bulk heparinized catheters in human

Bulk heparinized catheters (1 mm internal diameter) containing 10% heparin ionically bound, were tested in four human volunteers. Catheters containing 0% and 10% heparin were compared in each individual using ultrasound microflow velocimetry, permeability test, sequential determinations of activated partial thromboplastin time, heparin levels and generation of Fibrinopeptide A, beta thromboglobulin and Platelet factor 4. Although the release of heparin expressed by its anti-IIa activity is of similar range in the four individuals the release of anti-Xa activity is variable and generally of greater magnitude, suggesting a privileged migration of low molecular weight components of heparin. These antiproteasic activities of heparin are sufficient to inhibit fibrin formation and blood coagulation despite their relative inability to prevent platelet activation.

Biocompatible surfaces prepared by immobilized heparin or hyaluronate

Artificial surfaces modified with immobilized heparin or hyaluronate were evaluated as possible approaches to improve biocompatibility. The heparin surface was highly thromboresistant in contact with blood, as manifested by minimal cell adhesion and a pronounced capacity to inactivate coagulation enzymes such as thrombin and factor Xa. The hyaluronate coating also reduced cell adhesion, but this coating seemed to be less effective than the heparin surface. Outgrowth of human macrophages was not affected by the hyaluronate coating, nor did it display any capacity to inactivate coagulation enzymes. It is concluded that the heparin surface exhibits a high degree of biocompatibility in contact with blood and cell culture, which should make it an interesting candidate for use in the ear compartment.

Thrombin inactivation on surfaces with covalently bonded heparin

About 8000 Daltons porcine mucosa heparin fragments were covalently bonded by end-point attachment to polyethylene. The interaction between the immobilized heparin, added thrombin, and antithrombin III [AT] was investigated. The heparin surface was adsorbed with either albumin, AT dissolved in albumin or Tyrode, or platelet free plasma. Irrespective of the pre-treatment procedure, exposure of the surface to thrombin resulted in the same substantial decrease of thrombin in solution and the same degree of surface-confined thrombin activity. It was concluded that the heparin surface has a large capacity to bind thrombin and that the thrombin inhibitory capacity of high affinity heparin fragments is limited. On exposure of the thrombin-loaded surfaces to defibrinogenated plasma or AT, the surface-confined thrombin was inhibited within 30 seconds. Successive dilutions of plasma or AT decreased the inhibition rate but not the inhibition capacity. It is concluded that inhibition of thrombin adsorbed on the heparin surface occurs as follows: Added AT adheres to high affinity heparin fragments on the surface whereupon adsorbed thrombin migrates in the hydrophilic heparin coating towards the reaction site of AT and becomes inhibited. The inactivated thrombin-AT complex leaves then the surface, thus enabling the process to be repeated.

Endothelial binding sites for heparin. Specificity and role in heparin neutralization

The specificity of endothelial binding sites for heparin was investigated with heparin fractions and fragments differing in their Mr, charge density and affinity for antithrombin III, as well as with heparinoids and other anionic polyelectrolytes (polystyrene sulphonates). The affinity for endothelial cells was estimated by determining I50 values in competition experiments with 125I-heparin. We found that affinity for endothelial cells increases as a function of Mr and charge density (degree of sulphation). Binding sites are not specific receptors for heparin. Other anionic polyelectrolytes, such as pentosan polysulphates and polystyrene sulphonates, competed with heparin for binding to endothelial cells. Fractions of standard heparin with high affinity for antithrombin III also had greater affinity for endothelium. However, these two properties of heparin (affinity for antithrombin III and affinity for endothelial cells) could be dissociated. Oversulphated heparins and oversulphated low-Mr heparin fragments had lower anticoagulant activity and higher affinity for endothelial cells than did their parent compounds. Synthetic pentasaccharides, bearing the minimal sequence for binding to antithrombin III, did not bind to endothelial cells. Binding to endothelial cells involved partial neutralization of heparin. Bound heparin exhibited only 5% and 7% of antifactor IIa and antifactor Xa specific activity, respectively. In the presence of 200 nM-antithrombin III, and in the absence of free heparin, a limited fraction (approx. 30%) of bound heparin was displaced from endothelial cells during a 1 h incubation period. These data suggested that a fraction of surface-bound heparin could represent a pool of anticoagulant.

Why small caliber vascular grafts fail: a review of clinical and experimental experience and the significance of the interaction of blood at the interface

The results using polytetrafluoroethylene (PTFE) and tanned human umbilical cord vein (HUV) grafts are discussed. These two grafts perform well for reconstructions above the knee and the results are similar to the autogenous saphenous vein (ASV) grafts in several series; however, in more distal reconstructions the PTFE and HUV are far inferior to ASV grafts. The factors responsible for graft failure including the events which take place at the blood interface are reviewed. New material surfaces under investigation such as degradable grafts, heparin-bonded surfaces, and endothelial-seeded fabrics are also addressed.

Thrombin uptake and inhibition on heparinized polytetrafluoroethylene (PTFE) grafts and native sheep vessels

Non-heparinized and covalently-heparinized PTFE grafts, sheep aorta, carotid artery and jugular vein were evaluated according to their ability to adsorb and inactivate thrombin. All surfaces, except non-heparinized PTFE caused considerable losses of enzymatically active thrombin from the solution. Covalently-heparinized PTFE and jugular veins adsorbed thrombin, which was inactivated on subsequent contact with antithrombin III. Despite the same loss from the solution as was encountered with veins arteries adsorbed significantly smaller amounts of thrombin. Protamine treatment of covalently-heparinized PTFE almost totally abolished the ability to adsorb thrombin. It is concluded that 1) arteries possess a stronger thrombin inhibitory capacity than veins, possible explanations to this being a different glucosaminoglycan or the presence of antithrombin III in the arterial wall and 2) that heparinizing PTFE results in a pronounced ability to inactivate thrombin.

Thrombin uptake and inhibition on endothelium and surfaces with a stable heparin coating: a comparative in vitro study

The endothelial lining and two differently heparin-coated surfaces were compared in vitro regarding thrombin uptake and inhibition. One heparin surface was based on stabilized ionic binding of heparin, the other on covalent binding of partially degraded heparin. Both heparin surfaces have previously been shown to have pronounced thromboresistant properties. The two heparinized polyethylene surfaces and the endothelial surface of segments of the porcine aorta were studied. After exposure to the surfaces, thrombin disappeared from the solution and appeared bound to the surfaces. The disappearance rate of thrombin from the solution was the same on exposure to the endothelium and the covalently bonded heparin surface, but less following exposure to the ionically bonded heparin surface. The thrombin activity appearing on the endothelium was lower than on the heparin surfaces, indicating that the endothelium exerted a slow thrombin inhibiting capacity. On exposure of the thrombin-loaded endothelium to plasma, thrombin was rapidly inhibited. Thrombin bound to the covalently bonded heparin surface was inhibited at a slower rate than on the ionically bonded surface, but still faster than the rate at which free thrombin was inhibited in nonheparinized plasma. It is concluded that the endothelium and stabilized heparin coatings bind thrombin and accelerate its inhibition by plasma.

Heparinized polyurethanes: in vitro and in vivo studies

Heparin immobilization chemistry using alkyl spacer arms was adapted to optimize yield on polyurethane (PU) surfaces. The resultant biological activity of immobilized heparin (HI) was examined in vitro and in vivo, and compared with a heparin releasing (HR) system. Immobilized heparin retained its ability to bind and inactivate thrombin and Factor Xa; nonspecific coagulation factor binding was insignificant. Such activity cannot be attributed to the leakage of improperly bound heparin. Immobilized heparin-polyurethane catheters implanted in canine femoral and jugular veins for 1 h periods exhibited significant reduction in thrombus formation compared with untreated PU contralateral controls. Polyurethane catheters coated with a 9% heparin dispersion in PU (HR) system provided even greater improvement in antithrombogenicity.

Inhibition of surface induced coagulation by preadsorption of albumin-heparin conjugates

Surface coatings of the albumin-heparin conjugates were developed to improve the blood compatibility of polymeric materials. Glass, PVC, Biomer and cellulose acetate were coated with albumin-heparin conjugate and its adsorption and desorption behavior on glass in particular was studied using 3H and 51Cr radiolabeled conjugates. Precoated materials showed a significant prolongation of the Lee-White clotting time as compared with noncoated ones. It was demonstrated that the prolonged clotting time for pretreated glass was due to surface bound conjugate. Prolonged recalcification times of plasma exposed to glass, Biomer, and PVC were obtained using albumin-heparin conjugate precoated surfaces. Albumin-heparin conjugates with high affinity for antithrombin III gave more prolonged clotting times as low affinity conjugates when used as coatings for glass. This indicates that the behaviour of heparin in preadsorbed conjugates resembles that of heparin in solution.

In vitro evaluation of a biologic graft surface. Effect of treatment with conventional and low molecular weight (LMW) heparin

Human umbilical vein grafts were treated with either conventional or LMW heparin, followed by exposure to alcohol. The grafts were investigated for their ability to adsorb and inactivate thrombin, and comparison was made with non-heparinized and saline-alcohol treated grafts and grafts supplied with a covalently bonded layer of conventional heparin. In addition, the effect of protamine exposure to heparin-alcohol and LMW heparin-alcohol treated grafts as well as native human umbilical veins (HUV) was studied. Native HUV and heparin treated graft surfaces adsorbed and inactivated thrombin, whereas non-heparinized and saline-alcohol treated grafts inactivated surface-bound thrombin to only a small degree. Surface-bound LMW heparin exhibited a significantly lower ability to inactivate thrombin as compared with conventional heparin, but LMW heparin-alcohol surfaces were better than non-heparinized ones. Protamine treatment of "heparinized" surfaces impaired the thrombin inhibiting ability of the heparin-alcohol surface, whereas this property was totally abolished for the LMW heparin-alcohol surface. The findings indicate that LMW heparin, despite its weaker thrombin inhibiting capacity, may be an alternative to conventional heparin, for "heparinizing" the human umbilical vein graft. Protamine exposure may be potentially harmful for a heparin treated surface, although protamine concentrations used in the present in vitro study may not be reached in vivo. The native HUV was not at all affected by protamine exposure regarding its ability to inactivate thrombin.

Effect of glycosaminoglycans and antithrombin III on uptake and inhibition of thrombin by the vascular wall

Thrombin binds to and is inactivated by the endothelium. The inactivation is potentiated by plasma. The present investigation was designed to clarify the role of vessel wall glycosaminoglycans (GAG) and plasma antithrombin III (AT III) in the inactivation and binding of thrombin by endothelium. Thrombin was shown to bind to vascular endothelium and artificial surfaces containing GAG:s. The binding could be inhibited on both types of surfaces by pretreating them with protamine. Thrombin bound to endothelium was rapidly inactivated in the presence of plasma but only slowly if the plasma was replaced by AT III, AT III-depleted plasma or a balanced salt solution. It is concluded that thrombin binds to vessel wall GAG:s and is inactivated by the endothelium. Potentiation of the inhibition of the endothelially bound thrombin by plasma is dependent upon presence of AT III but an additional plasma factor is also required.

Extracorporeal left ventricular assistance with prostacyclin and heparinized centrifugal pump

A temporary left ventricular assist system utilizing an improved left atrial catheter and centrifugal pump was developed and tried in 22 anesthetized sheep to test the hypothesis that prostacyclin (PGI2) and ionically bound heparin-coated surfaces could prevent thrombosis in this perfusion system without inhibiting soluble, circulating coagulation factors. Sheep were perfused at 53.6 ml/kg/min (standard error, 1.5) from left atrium to aorta for two hours. The pump maintained flow at left atrial pressures of 3 to 4 mm Hg. As expected, systemic heparin inhibited coagulation, mildly activated platelets, and prevented thrombosis in uncoated perfusion circuits and wounds. Infusion of PGI2 (30 ng/kg)min) without systemic heparin or heparin-bonded extracorporeal surfaces prevented platelet activation and did not alter coagulation measurements; however, bleeding times increased and thrombi formed in every circuit. When heparin was ionically bound to all extracorporeal surfaces, infusion of PGI2 completely prevented thrombosis within the circuit, inhibited platelet activation, and did not alter coagulation measurements. Blood spontaneously clotted in three of six thoracotomy wounds during perfusion. When PGI2 infusion was stopped, heparin-coated surfaces alone failed to prevent thrombosis within the system in 3 of 5 animals. The combination of PGI2 infusion and heparin-coated extracorporeal surfaces prevents thrombosis within the perfusion circuit without inhibiting soluble coagulation factors. This method, using the assist system, overcomes many of the implementation and bleeding problems associated with other temporary left ventricular assist devices.

Thrombogenicity and the interaction of proteins, platelets and white cells

Radioactive labeled protein and platelet studies suggest that an ideal vascular surface absorbs albumin and prevents platelet adherence and aggregation. This study examines the interaction of serum and plasma proteins, white cells, and platelets in the thrombotic response to small internal diameter vascular prostheses. Thirty-seven replamineform silicone rubber vascular prostheses, 4 mm internal diameter and 6 cm length, had 5% aspirin and 5% theophylline by weight dispersed in a luminal coating (D); the remaining 37 control (C) prostheses had an identical silicone rubber coating without drugs. Scanning electron and darkfield microscopy confirmed similar surface topography in C and D grafts. One C and one D graft were studied in parallel using a modified canine arteriovenous fistula. Consistently, D prosthesis had increased bleeding through the graft wall. The 7 serum and 8 plasma experiments demonstrated greater accumulation on the D prostheses, the greatest deposition occurring within the first 30 minutes. In 8 platelet experiments, D grafts accumulated more radioactivity at both the 5 and 30 minute intervals. Six mononuclear and 8 PMN experiments had less accumulation on the D grafts at 15 minutes and no consistent difference at 60 minutes. Elution of C and D Grafts demonstrated significant drug levels in only the D grafts at the beginning of the experiments and after 15 minutes of exposure of blood. We conclude: (1) aspirin and theophylline have an effect on the initial interaction of plasma proteins, platelets and white cells in the thrombogenic response, (2) the effect diminishes as time released drug levels decrease.

A new non-thrombogenic surface prepared by selective covalent binding of heparin via a modified reducing terminal residue

A new method for the covalent binding of heparin to artificial surfaces has been developed. The heparinized surface releases insignificant amounts of heparin and can be regarded as stable. The blood contact properties as studied in vitro revealed that the surface was highly thromboresistant in terms of reduced platelet adhesion, surface catalyzed adsorption and inhibition of thrombin and capacity to prevent clotting of nonanticoagulated blood.

Uptake and inhibition of thrombin by the vascular wall

Thrombin activity and inhibition were assayed on the aortic surface of dogs and pigs. After sacrifice, the aortae were excised and thrombin activity was measured with an amidolytic assay. A small thrombin activity was found on the endothelium. Heparinization of the animals lowered endothelial thrombin activity. After exposure of the aortic endothelium to a thrombin/albumin solution in vitro, thrombin activity disappeared from the solution and was recovered on the surface. De-endothelialized vessels took up more thrombin than those with intact endothelium. Endothelium confined thrombin was rapidly inactivated when exposed to plasma. A slow inactivation was seen upon exposure to a modified Ringer's solution. Thrombin confined to de-endothelialized aortae, i.e. to medial structures, was always inactivated at a slow rate irrespective as to whether it was exposed to plasma or Ringer's solution. It is concluded that endothelium and subendothelium bind thrombin and subsequently inactivate it. The inactivation proceeds faster on endothelium when exposed to plasma. The possible role of glycosaminoglycans is discussed.

Polarographic pO2 sensors with heparinized membranes for in vitro and continuous in vivo registration

The membranes of the pO2 sensor in a conventional blood gas analyser and a commercially available catheter pO2 electrode were coated with a glutardialdehyde stabilized heparin layer. It was analysed whether or not the "non-thrombogenic" heparin surface could improve the accuracy of in vitro pO2 registration and continuous in vivo paO2 monitoring. The in vivo experiments were performed on anaesthetized and non-anticoagulated dogs. Two identical blood gas analysers were used, one furnished with a heparin coated pO2 membrane and the other with a standard pO2 membrane. In vitro pO2 determinations exhibited--when simultaneously analysing the same blood sample--identical mean values. For repeated pO2 determinations, however, the standard deviation was significantly lower on the analyser furnished with a heparinized membrane. The non-heparinized catheter pO2 electrode showed a marked deterioration in accuracy with time as compared with blood-gas analysis on reference blood samples. Falsely low paO2 values were presented. The heparinized catheter electrode monitored values in full agreement with those of the conventional blood gas analyser using standard membranes. It is concluded that the heparin surface on the oxygen diffusible membranes diminishes the variability of the diffusion characteristics both at contact with anticoagulated blood in vitro and during in vivo paO2 monitoring. The "non-thrombogenic" heparin surface prevents diffusion impeding clot formations when an arterial catheter pO2 sensor is used.