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J D Hellums

Publications and source records attributed to J D Hellums.

At least 37 records · Page 2Linked to original sources
PubMed1994

A theoretical model for gas transport and acid/base regulation by blood flowing in microvessels.

An investigation was made of the coupling between O2 and CO2 transport by blood flowing in microvessels. The blood was treated as two continuous coexisting phases: a red blood cell (RBC) phase and a plasma phase. The microvessel was divided into two regions: the central, RBC-rich and the outer, cell-free region. The radial distribution of RBCs and transport of various species due to bulk convection and radial diffusion were taken into account. Chemical and transport processes which were included in the model are (1) interactions of hemoglobin with O2 and CO2, (2) the Bohr and Haldane effects (the inter-dependence of O2/CO2 transport), (3) CO2 hydration-dehydration reactions, (4) buffering actions of hemoglobin and plasma proteins, and (5) anion exchange across the red cell membrane. The governing equations of the model subjected to the imposed inlet and boundary conditions were solved numerically to provide the concentration distributions of various species in blood that are important in the simultaneous gas exchange and pH regulation process. Predictions of the new model of simultaneous O2/CO2 transport by flowing blood were shown to be in excellent agreement with prior workers' experimental results from large artificial membrane tubes. A previous mathematical model which treats blood as a homogeneous continuum and uses a local chemical equilibrium approximation to describe the gas transport was shown to satisfactorily predict the amount of O2 transport for blood oxygenation accompanied by CO2 elimination. However, the previous model significantly underpredicts O2 transfer for blood deoxygenation accompanied by CO2 uptake. Furthermore, the previous model disagrees substantially with the CO2 transport results under both oxygenation and deoxygenation conditions.

Acid-Base Equilibrium↗
PubMed1993

Protein kinase C is activated in platelets subjected to pathological shear stress.

High levels of fluid shear stress at the blood vessel wall directly stimulate von Willebrand factor (vWF)-mediated platelet adhesion and aggregation and thereby contribute to the pathogenesis of arterial thrombosis. We have found that a pathological level of arterial wall shear stress (90 dynes/cm2) induces platelet aggregation that is associated with the phosphorylation of pleckstrin, a M(r) 47,000 protein kinase C substrate (p47). Shear-induced p47 phosphorylation depends entirely on vWF binding to platelet glycoprotein (Gp) Ib and GpIIb-IIIa, and the specific inhibition of protein kinase C with the staurosporine analogue Ro 31-7549 inhibits the full aggregation response to shear. Shear stress-induced platelet p47 phosphorylation occurs independent of any measurable change in diacylglycerol mass or hydrolysis of phosphatidylinositol 4,5-bisphosphate. These results indicate that mechanical shear stress induces vWF to bind to platelet GpIb and GpIIb-IIIa, stimulating a diacylglycerol-independent pathway of protein kinase C activation that contributes to platelet aggregation in response to shear.

Blood Platelets↗
PubMed1993

Aggregation and disaggregation kinetics of human blood platelets: Part I. Development and validation of a population balance method.

Hydrodynamic shear stress of sufficient intensity is known to cause platelet activation and aggregation and to alter the effects of biochemical platelet agonists and antagonists. In this work, a population balance equation (PBE) model is developed for analysis of platelet aggregation and disaggregation kinetics under the influence of a shear field. The model incorporates both aggregation and disaggregation by splitting and/or erosion mechanisms. This paper, the first of a series of three, deals with the formulation, simplification, and validation of the PBE and with the estimation of parameters involved in the PBE. These population parameters include collision efficiency, void fraction (related to the particle collision diameter), and the breakage rate coefficient. The platelet particle size distribution is determined experimentally, both initially and at some later times. The PBE can then be used to match satisfactorily the observed particle histograms, by appropriate choice of parameters of the model as functions of time, platelet size, and magnitude of physical or chemical stimuli. Besides providing information on adhesive forces and on the rates of aggregation and disaggregation, these parameters infer the physical properties of platelets and platelet aggregates. These properties are of potential value in increasing our understanding of the processes involved in thrombotic disease and/or therapy. A numerical procedure for solving the PBE is validated by application to simple cases for which analytical solutions are available. The model is applied to analysis of experiments, and parameter sensitivity studies are used to order the importance of the parameters and to reduce the complexity of the model. The simplified model is shown to give good agreement with experimental observations.

Adenosine Diphosphate↗
PubMed1993

Aggregation and disaggregation kinetics of human blood platelets: Part II. Shear-induced platelet aggregation.

A population balance equation (PBE) mathematical model for analyzing platelet aggregation kinetics was developed in Part I (Huang, P. Y., and J. D. Hellums. 1993. Biophys. J. 65: 334-343) of a set of three papers. In this paper, Part II, platelet aggregation and related reactions are studied in the uniform, known shear stress field of a rotational viscometer, and interpreted by means of the model. Experimental determinations are made of the platelet-aggregate particle size distributions as they evolve in time under the aggregating influence of shear stress. The PBE model is shown to give good agreement with experimental determinations when either a reversible (aggregation and disaggregation) or an irreversible (no disaggregation) form of the model is used. This finding suggests that for the experimental conditions studied disaggregation processes are of only secondary importance. During shear-induced platelet aggregation, only a small fraction of platelet collisions result in the binding together of the involved platelets. The modified collision efficiency is approximately zero for shear rates below 3000 s-1. It increases with shear rates above 3000 s-1 to about 0.01 for a shear rate of 8000 s-1. Addition of platelet chemical agonists yields order of magnitude increases in collision efficiency. The collision efficiency for shear-induced platelet aggregation is about an order of magnitude less at 37 degrees C than at 24 degrees C. The PBE model gives a much more accurate representation of aggregation kinetics than an earlier model based on a monodispersed particle size distribution.

Adult↗
PubMed1993

Aggregation and disaggregation kinetics of human blood platelets: Part III. The disaggregation under shear stress of platelet aggregates.

In the preceding two papers (1, 2), a population balance equation (PBE) mathematical model was developed, validated, and applied to the analysis of platelet aggregation kinetics under the influence of hydrodynamic shear stress. The present work involves the application of the model to the analysis of platelet reactions under shear stress in circumstances where disaggregation processes are of dominant importance: the disaggregation of aggregates formed in response to added agonists. Aggregation-disaggregation experiments were performed in the constant shear field of a rotational viscometer, and the evolution of the particle size distribution was determined by use of an electronic particle counter. The PBE model was used to simulate the experimental results. Exploratory calculations made it possible to reduce a rather complete, complex model to a more tractable form which retains the capability of simulating the experimental observations. For the experimental conditions studied, disaggregation by a splitting mechanism was found to be of dominant importance. The surface erosion mechanism can be neglected without significant impact on results. Physical reasoning confirmed by exploratory calculations showed that a discontinuous form of the breakage rate expression which incorporates a minimum friable particle size, gives significantly better results than a continuous expression. A simple step function void fraction parameter was found to be at least as successful as a more complicated, continuous function. The resulting simplified model has the potential of increasing our understanding of kinetics and mechanisms of platelet reactions, and of characterizing the state of platelet activity. Hence, it may be useful in efforts to understand thrombotic and hemostatic processes.

Adenosine Diphosphate↗
PubMed1992

Shear stress-induced von Willebrand factor binding to platelet glycoprotein Ib initiates calcium influx associated with aggregation.

Platelets subjected to elevated levels of fluid shear stress in the absence of exogenous agonists will aggregate. Shear stress-induced aggregation requires von Willebrand factor (vWF) multimers, extracellular calcium (Ca2+), adenosine diphosphate (ADP), and platelet membrane glycoprotein (GP)Ib and GPIIb-IIIa. The sequence of interaction of vWF multimers with platelet surface receptors and the effect of these interactions on platelet activation have not been determined. To elucidate the mechanism of shear stress-induced platelet aggregation, suspensions of washed platelets were subjected to different levels of uniform shear stress (15 to 120 dyne/cm2) in an optically modified cone and plate viscometer. Cytoplasmic ionized calcium ([Ca2+]i) and aggregation of platelets were monitored simultaneously during the application of shear stress; [Ca2+]i was measured using indo-1 loaded platelets and aggregation was measured as changes in light transmission. Basal [Ca2+]i was approximately 60 to 100 nmol/L. An increase of [Ca2+]i (up to greater than 1,000 nmol/L) was accompanied by synchronous aggregation, and both responses were dependent on the shear force and the presence of vWF multimers. EGTA chelation of extracellular Ca2+ completely inhibited vWF-mediated [Ca2+]i and aggregation responses to shear stress. Aurin tricarboxylic acid, which blocks the GPIb recognition site on the vWF monomer, and 6D1, a monoclonal antibody to GPIb, also completely inhibited platelet responses to shear stress. The tetrapeptide RGDS and the monoclonal antibody 10E5, which inhibit vWF binding to GPIIb-IIIa, partially inhibited shear stress-induced [Ca2+]i and aggregation responses. The combination of creatine phosphate/creatine phosphokinase, which converts ADP to adenosine triphosphate and blocks the effect of ADP released from stimulated platelets, inhibited shear stress-induced platelet aggregation without affecting the increase of [Ca2+]i. Neither the [Ca2+]i nor aggregation response to shear stress was inhibited by blocking platelet cyclooxygenase metabolism with acetylsalicylic acid. These results indicate that GPIb and extracellular Ca2+ are absolutely required for vWF-mediated [Ca2+]i and aggregation responses to imposed shear stress, and that the interaction of vWF multimers with GPIIb-IIIa potentiates these responses. Shear stress-induced elevation of platelet [Ca2+]i, but not aggregation, is independent of the effects of release ADP, and both responses occur independently of platelet cyclooxygenase metabolism. These results suggest that shear stress induces the binding of vWF multimers to platelet GPIb and this vWF-GPIb interaction causes an increase of [Ca2+]i and platelet aggregation, both of which are potentiated by vWF binding to the platelet GPIIb-IIIa complex.

Adenosine Diphosphate↗
PubMed1990

A simple model for prediction of oxygen transport rates by flowing blood in large capillaries.

A simple model has been developed for simulation of oxygen transport to and from blood flowing in conduits of the diameter of arterioles and larger (greater than or equal to 20 microns). The basis is the large capillary model (LCAP) of P.K. Nair, et al. 1989 which has been validated experimentally. Detailed calculations of the oxygen concentration distribution reveal that the dominant resistance to oxygen transport is distributed in the plasma. Relatively little resistance is present within or in the immediate vicinity of the red cells. On the basis of these findings, LCAP was simplified from four simultaneous nonlinear partial differential equations (PDEs) to one PDE by (1) assuming chemical equilibrium within the red blood cells, (2) neglecting intracellular and extracellular boundary layer resistances, and (3) incorporating transport in the cell-free region adjacent to the capillary wall into the boundary conditions. The simplified model is much easier to apply mathematically to new situations. A comparison between LCAP and the simpler model shows that they give virtually the same predictions, and the predictions agree well with experimental measurements. The model is predictive in that all the parameters are determined from the literature or from independent measurements. Thus it should be useful in studies of physiological significance, as well as in design and analysis of extracorporeal blood oxygenators.

Biological Transport, Active↗
PubMed1989

Prediction of oxygen transport rates in blood flowing in large capillaries.

A mathematical model has been developed to predict oxygen transport to and from blood flowing in tubes of the diameter of arterioles and larger (approximately 20 microns and larger). The resistance to oxygen transport in red cell suspensions is much higher than that of a comparable homogeneous hemoglobin solution. The increased resistance is associated with encapsulation of the hemoglobin in the red cells. Yet, somewhat paradoxically, for large capillaries relatively little resistance is within or in the immediate vicinity of the red cells. The great majority of the resistance is shown to be distributed in the plasma. Predictions of oxygen uptake and release are shown to be in excellent agreement with results of measurements taken on red cell suspensions flowing in capillaries of 27- and 100-microns diameter. The model seems to be the first for oxygen transport in flowing blood that is validated by detailed comparison with experimental results. It is a predictive model in that all parameters in the model are determined from independent measurements or from the literature.

Arterioles↗
PubMed1988

A cone and plate viscometer for the continuous measurement of blood platelet activation.

A cone and plate viscometer was modified to permit the continuous study of platelet response during shear stress exposure times on the order of one second to 180 seconds. Platelets may be stimulated by uniform, controlled shear stress alone or with the addition of chemical platelet agonists. The time course of platelet aggregation is interpreted from alterations in the apparent optical density of the platelet suspension. The rate and extent of platelet dense granule release is estimated from the intensity of the luminescent reaction of platelet released ATP with firefly luciferase and luciferin. Intracellular calcium ion concentration is determined as a function of shear exposure time through the fluorescence intensity of indo-1(5-), a membrane-permeant pentacarboxylate calcium ion chelator.

Animals↗
PubMed1988

Shear-induced platelet aggregation can be mediated by vWF released from platelets, as well as by exogenous large or unusually large vWF multimers, requires adenosine diphosphate, and is resistant to aspirin.

Fluid shear stress in arteries and arterioles partially obstructed by atherosclerosis or spasm may exceed the normal time-average level of 20 dyne/cm2. In vitro, at fluid shear stresses of 30 to 60 dyne/cm2 applied for 30 seconds, platelet aggregation occurs. At these shear stresses, either large or unusually large von Willebrand factor (vWF) multimers in the suspending fluid exogenous to the platelets mediates aggregation. Adenosine diphosphate (ADP) is also required and, in these experiments, was released from the platelets subjected to shear stress. At 120 dyne/cm2, the release of endogenous platelet vWF multimers can substitute for exogenous large or unusually large vWF forms in mediating aggregation. Endogenous released platelet vWF forms, as well as exogenous large or unusually large vWF multimers, must bind to both glycoproteins Ib and the IIb/IIIa complex to produce aggregation. Shear-induced aggregation is the result of shear stress alteration of platelet surfaces, rather than of shear effects on vWF multimers. It is mediated by either large plasma-type vWF multimers, endogenous released platelet vWF forms, or unusually large vWF multimers derived from endothelial cells, requires ADP, and is not inhibited significantly by aspirin. This type of aggregation may be important in platelet thrombus formation within narrowed arterial vessels, and may explain the limited therapeutic utility of aspirin in arterial thrombosis.

Adenosine Diphosphate↗
PubMed1987

Use of Adair four-step kinetics in mathematical simulation of oxygen transport in the microcirculation.

The Adair four-step kinetic model for the reactions of haemoglobin and oxygen recognizes five haemoglobin species, corresponding to deoxyhaemoglobin and one species for each level of oxygenation of the four haem groups. Thus, an oxygen transport problem involves a system of five simultaneous non-linear partial differential equations for diffusion with chemical reaction. This mathematical complexity has impeded application of the Adair model despite its theoretical advantages over the one-step model often used in practice. The Adair kinetic model has been incorporated into a simulation of microcirculatory oxygen transport. The results show that the usual one-step kinetic model is inaccurate in comparison with the Adair model. However, an empirical modification can be made to the one-step model to ensure compatibility with the equilibrium curve. This modified one-step kinetic model (the VRC model) is much more tractable mathematically than the Adair model. In the physiological range of fluxes, the VRC kinetic model appears to be of sufficient accuracy for most purposes, and the mathematical complexity of the Adair model is not required.

Biological Transport, Active↗
PubMed1987

Oxygen transport studies of normal and sickle red cell suspensions in artificial capillaries.

Oxygen transport from normal and sickle red cells was studied under known and carefully controlled conditions simulating the microcirculation. Oxygenated red cell suspensions became deoxygenated as they traversed silicone rubber artificial capillaries of 27 microns diameter. Oxygen saturation values of the flowing red cell suspensions were measured at several axial positions along the artificial capillary by use of a microspectrophotometric technique. Oxygen saturation decreased with increasing distance from the entrance of the artificial capillary and was influenced strongly by the flow rate. Under the same hematocrit and flow conditions, the rate of oxygen saturation decrease was significantly higher for the sickle red cells than that for the normal red cells. Similar results were obtained by use of a mathematical simulation of oxygen transport in the microcirculation for both normal and sickle red cells. Sickle red cells would be expected to have a higher diffusional resistance to oxygen transport than would normal red cells. However, the higher diffusional resistance is more than offset by the lower oxygen affinity of the sickle cells. The difference in oxygen affinity appears to account for the difference in oxygen transport rates between normal and sickle red cells.

Adult↗
PubMed1987

An in vitro capillary system for studies on microcirculatory O2 transport.

An in vitro artificial capillary system has been developed for use in examining the O2 transport properties of free hemoglobin and erythrocytes. The artificial capillary was constructed by casting a thin film of transparent silicone rubber around a strand of tungsten wire that was 24 micron in diameter. After the rubber had polymerized, the wire was removed. Typical dimensions of the silicone rubber film were 170 micron thick, 1 cm wide, 5 mm long in the direction of flow, and a 27-micron lumen diameter. The artificial capillary bed was mounted on a microscope and perfused by either hemoglobin solutions or cell suspensions. Fractional saturation was measured as a function of axial position by a dual-wave-length microspectrophotometer, and the flow rate was regulated precisely by a syringe pump. O2 release experiments were carried out by suffusing the gas space surrounding the artificial capillary film with 100% N2 and perfusing with an oxygenated sample. O2 uptake experiments were carried out by suffusing the gas space with O2-N2 mixtures and perfusing with deoxygenated samples. The axial velocities were varied from 3 to 15 mm/s. The residence time (the time a particular red cell or hemoglobin molecule has spent in the capillary) for 50% oxygenation of a 4 mM (heme) deoxyhemoglobin solution was approximately 0.05 s at 37 degrees C when the gas space surrounding the capillary contained air. The corresponding time for 50% oxygenation of an equivalent red cell suspension was approximately 0.25 s.(ABSTRACT TRUNCATED AT 250 WORDS)

Biological Transport↗