Blood biocompatibility analysis in the setting of ventricular assist devices.

Ventricular assist devices (VADs) are increasingly applied to support patients with advanced cardiac failure. While the benefit of VADs in supporting this patient group is clear, substantial morbidity and mortality occur during the VAD implant period due to thromboembolic and infective complications. Efforts at the University of Pittsburgh aimed at evaluating the blood biocompatibility of VADs in the clinical, animal, and in vitro setting over the past decade are summarized. Emphasis is placed on understanding the mechanisms of thrombosis and thromboembolism associated with these devices.

Computational simulation of platelet deposition and activation: II. Results for Poiseuille flow over collagen.

We have previously described the development of a two-dimensional computational model of platelet deposition onto biomaterials from flowing blood (Sorensen et al., Ann. Biomed. Eng. 27:436-448, 1999). The model requires estimation of four parameters to fit it to experimental data: shear-dependent platelet diffusivity and three platelet-deposition-related reaction rate constants. These parameters are estimated for platelet deposition onto a collagen substrate for simple parallel-plate flow of whole blood in both the presence and absence of thrombin. One set of experimental results is used as a benchmark for model-fitting purposes. The "trained" model is then validated by applying it to additional test cases from the literature for parallel-plate Poiseuille flow over collagen at both higher and lower wall shear rates, and in the presence of various anticoagulants. The predicted values agree very well with the experimental results for the training cases, and good reproduction of deposition trends and magnitudes is obtained for the heparin, but not the citrate, validation cases. The model is formulated to be easily extended to synthetic biomaterials, as well as to more complex flows.

Computational simulation of platelet deposition and activation: I. Model development and properties.

To better understand the mechanisms leading to the formation and growth of mural thrombi on biomaterials, we have developed a two-dimensional computational model of platelet deposition and activation in flowing blood. The basic formulation is derived from prior work by others, with additional levels of complexity added where appropriate. It is comprised of a series of convection-diffusion-reaction equations which simulate platelet-surface and platelet-platelet adhesion, platelet activation by a weighted linear combination of agonist concentrations, agonist release and synthesis by activated platelets, platelet-phospholipid-dependent thrombin generation, and thrombin inhibition by heparin. The model requires estimation of four parameters to fit it to experimental data: shear-dependent platelet diffusivity and resting and activated platelet-surface and platelet-platelet reaction rate constants. The model is formulated to simulate a wide range of biomaterials and complex flows. In this article we present the basic model and its properties; in Part II (Sorensen et al., Ann. Biomed. Eng. 27:449-458, 1999) we apply the model to experimental results for platelet deposition onto collagen.

Effects of sodium and lithium ions on the potassium ion transport systems of Escherichia coli.

The effects of the cations choline, Li+, and the Na+ on the TrkA and Kdp K+ transport systems in Escherichia coli were studied by observing the accumulation of 204Tl+ and K+. Tl+ uptake via the TrkA system was stimulated by Na+ but not Li+ when compared to choline. A similar effect was observed for K+ transport via the TrkA system. On the other hand, Tl+ uptake via the Kdp system was stimulated more by Li+ than by Na+ when compared to choline. In addition, Li+ enhanced the effectiveness of Rb+ as an inhibitor of Tl+ uptake via the Kdp system. Na+, however, was a more effective stimulator of K+ transport via the Kdp system than Li+. We suggest that Na+ may be involved in the mechanisms of K+ transport via the TrkA and Kdp systems in E. coli.

Cation/proton antiport systems in Escherichia coli. Properties of the potassium/proton antiporter.

The potassium/proton antiport system of Escherichia coli has been characterized by the effect of monovalent cations on the pH gradient formed by oxidation of lactate in everted membrane vesicles. Substrates of the system include K+, Na+, Li+, Rb+, and Tl+. The antiporter could also be assayed by uptake of 204Tl+ into everted vesicles. The antiporter exhibits a basic pH optimum and catalyzes electroneutral proton/cation exchange. Antiporter activity is trypsin-sensitive, but trypsin inactivation is prevented by prior formation of an electrochemical proton gradient. Two other proton/cation exchangers, the Na+/H+ and Ca2+/H+ antiporters, were unaffected by the trypsin treatment. Regulation of cytosolic pH by K+/H+ exchange is postulated, where proton return to the cytosol by the K+/H+ antiporter prevents alkalinization of the cytosol during proton extrusion associated with the formation of a protonmotive force or during growth at alkaline pH.

Thallous ion is accumulated by potassium transport systems in Escherichia coli.

The accumulation of 204T1+ by Escherichia coli occurs primarily via either of two K+ transport systems called Kdp and TrkA. T1+ influx is inhibited and T1+ efflux is stimulated by the addition of K+ to the assay medium. Mutants defective in both the Kdp and TrkA systems accumulate little T1+. Uptake of triphenylmethylphosphonium, a lipid-soluble cation whose distribution is widely used to estimate the membrane electrical potential in bacteria, occurs to about the same extent in mutants that accumulate little T1+ as in strains that accumulate T1+ to high levels. These findings indicate that T1+ may be useful as a probe of bacterial K+ transport systems but is not a reliable indicator of the membrane electrical potential in E. coli.

Hybridization by cosonication of pigeon erythrocyte membrane with exogenous lipid vesicles.

Concentrated mixtures of lipid vesicles and pigeon erythrocyte membrane were cosonicated in order to produce functional hybrid vesicles. From the properties of the resulting material, we conclude that hybrids were very probably formed. These properties were as follows: (i) The presence of membrane increased the sonic fragmentability of lipid vesicles. Sonic fragmentability was assessed by measuring sonication-induced release of previously trapped [14C]-choline and trapping of external [3H]-choline. (ii) Space enclosed by lipid was served by the membrane-like properties of 36Cl- permeability and ATP-dependent 45Ca++ uptake activity. (iii) 36Cl-permeability was more readily and fully induced into the more easily fragmented lipid vesicles. Further sonication caused loss of the induced 36Cl--permeability. This loss was less rapid with the less easily fragmented lipid vesicles; i.e., less easily fragmented lipids protected 36Cl--permeability better. (iv) Glycine uptake activity was partially protected from sonic damage by the presence of lipid vesicles. (v) On centrifugation in bovine serum albumin density gradients, cosonicated material showed lipid properties (enclosed choline and 32Pi space and [3H]-cholesterol) and membrane properties (36Cl--permeability and ATP-dependent 45Ca2+ uptake) coinciding at a density intermediate between those reached by separately sonicated membrane and lipid vesicles. (vi) Electron micrographs showed the disappearance of pure membrane-like structures and the appearance of large amounts of new vesicles whose appearance is consistent with a hybrid structure.