Biorheology - Volume 36, issue 3

Authors: Wada, Shigeo | Karino, Takeshi

Article Type: Research Article

Abstract: It is suspected that physical and fluid mechanical factors play important roles in the localization of atherosclerotic lesions and intimal hyperplasia in man by affecting the transport of cholesterol in flowing blood to arterial walls. Hence, we have studied theoretically the effects of various physical and fluid mechanical factors such as wall shear rate, diffusivity of low density lipoproteins (LDL), and filtration velocity of water at the vessel wall on surface concentration of LDL at an arterial wall by means of a computer simulation of convective and diffusive transport of LDL in flowing blood to the wall of a straight …artery under conditions of a steady flow. It was found that under normal physiologic conditions prevailing in the human arterial system, due to the presence of a filtration flow of water at the vessel wall, flow‐dependent concentration polarization (accumulation or depletion) of LDL occurs at a blood/endothelium boundary. The surface concentration of LDL at an arterial wall takes higher values than that in the bulk flow in that vessel, and it is affected by three major factors, that is, wall shear rate, \gamma_\mathrm{w} , filtration velocity of water at the vessel wall, {V}_\mathrm{w} , and the distance from the entrance of the artery, { L} . It increases with increasing {V}_\mathrm{w} and { L} , and decreasing \gamma_\mathrm{w} hence the flow rate. Thus, under certain circumstances, the surface concentration of LDL could rise locally to a value which is several times higher than that in the bulk flow, or drop locally to a value even lower than a critical concentration for the maintenance of normal functions and survival of cells forming the vessel wall. These results suggest the possibility that all the vascular phenomena such as the localization of atherosclerotic lesions and intimal hyperplasia, formation of cerebral aneurysms, and adaptive changes of lumen diameter and wall structure of arteries and veins to certain changes in hemodynamic conditions in the circulation are governed by this flow‐dependent concentration polarization of LDL which carry cholesterol. Show more

Keywords: Atherosclerosis, lipoproteins, concentration polarization, blood flow, filtration, mass transfer

Citation: Biorheology, vol. 36, no. 3, pp. 207-223, 1999

Authors: Naiki, Takeru | Sugiyama, Hironobu | Tashiro, Rumi | Karino, Takeshi

Article Type: Research Article

Abstract: Flow‐dependent concentration or depletion of atherogenic low density lipoproteins which has been theoretically predicted to occur at a blood/endothelium boundary may play an important role in the genesis, progression, and regression of atherosclerosis in man and intimal hyperplasia in vascular grafts implanted in the arterial system in man and experimental animals. Hence to explore such a possibility, we have studied the effect of a steady shear flow on concentration polarization of plasma proteins and lipoproteins at the luminal surface of a cultured bovine aortic endothelial cell (BAEC) monolayer which served as a model of the vessel wall of an artery …or an implanted vascular graft. The study was carried out by circulating a cell culture medium containing fetal calf serum or bovine plasma lipoproteins in steady flow through a parallel‐plate flow cell in which a cultured BAEC monolayer was installed, over the physiologic ranges of wall shear rate and water filtration velocity at the BAEC monolayer. The water (cell culture medium) filtration velocity at the BAEC monolayer was determined to provide a measure of the change in concentration of plasma protein particles at the luminal surface of the BAEC monolayer. It was found that for perfusates containing plasma proteins and/or lipoproteins, water filtration velocity varied as a function of flow rate, being lowest in the absence of flow. Water filtration velocity increased or decreased as flow rate increased or decreased from an arbitrarily set non‐zero value, indicating that surface concentration of protein particles varied as a direct function of flow rate, and the process was reversible. It was also found that at particle concentrations equivalent to those found in a culture medium containing serum at 20% by volume, plasma lipoproteins which were much smaller in number and lower in concentration but larger in size than albumin, showed almost the same effect as observed with serum which contained both lipoproteins and albumin, indicating that the substance responsible for this phenomenon is not albumin but lipoprotein whose diffusivity is much smaller than that of albumin. The results strongly support our hypothesis that flow‐dependent concentration polarization of lipoproteins occurs at a blood endothelium boundary, and this in turn promote the localization of various vascular diseases which develop in our arterial system. Show more

Keywords: Atherosclerosis, lipoprotein, concentration polarization, permeability, endothelial cell, molecular seaving

Citation: Biorheology, vol. 36, no. 3, pp. 225-241, 1999

Authors: Naiki, Takeru | Karino, Takeshi

Article Type: Research Article

Abstract: The effect of steady shear flow on concentration polarization of plasma proteins and lipoproteins at the luminal surface of a semipermeable vessel wall was studied experimentally using suspensions of these molecules in a cell culture medium and a semipermeable membrane dialysis tube which served as a model of an implanted vascular graft or an artery. The study was carried out by flowing a cell culture medium containing fetal calf serum or bovine plasma lipoproteins or bovine albumin through a 7.5 mm diameter, 60 mm‐long dialysis tube in steady flow under a physiologic mean arterial perfusion pressure of 100 mmHg, and …measuring the filtration velocity of water (cell culture medium) at the vessel wall which varied as a consequence of the change in concentration of plasma protein particles at the luminal surface of the semipermeable membrane dialysis tube. It was found that for perfusates containing plasma proteins and/or lipoproteins, filtration velocity of water was the lowest in the absence of flow, and it increased or decreased as the flow rate (hence wall shear rate) increased or decreased from a certain non‐zero value, indicating that surface concentration of protein particles varied reversibly as a direct function of flow rate. It was also found that at particle concentrations equivalent to those found in a culture medium containing serum at 5% by volume, plasma lipoproteins which were much smaller in number and lower in concentration but larger in size than albumin, had a much larger effect on the filtration velocity of water than albumin. These findings were very much the same as those previously obtained with a cultured endothelial cell monolayer, strongly suggesting that the flow‐dependent variation in filtration velocity of water at a vessel wall results from a physical phenomenon, that is, flow‐dependent concentration polarization of low density lipoproteins at the luminal surface of the endothelial cell monolayer. Show more

Keywords: Atherosclerosis, intimal hyperplasia, lipoprotein, permeability, molecular seaving, concentration polarization

Citation: Biorheology, vol. 36, no. 3, pp. 243-256, 1999

Authors: Henderson, J.M. | Aukerman, J.A. | Clingan, P.A. | Friedman, M.H.

Article Type: Research Article

Abstract: In support of an in vivo investigation in swine of the influence of changes in fluid dynamic wall shear on arterial macromolecular permeability, a procedure has been developed to alter the flows in the porcine posterior arterial vasculature by opening and closing a reversible arteriovenous shunt placed on one of the femoral arteries. Laparoscopic techniques were used to place appropriately modified Transonic Systems ultrasonic flow probes on both external and circumflex iliac arteries, and on the terminal aorta. Flow measurements were made prior to shunt placement, and with the shunt open and closed, to measure the influence of altered external …iliac artery flow on the distribution to the infrarenal abdominal vessels. Similar experiments were carried out to relate the flow rates in the external iliac arteries to those in the femoral arteries, which are more accessible. Based on the relationships among the measured flow rates, rules have been developed to estimate the major infrarenal flows in the pig, at baseline and with the shunt opened and closed, from only the flow rates measured at the two femoral arteries. Show more

Keywords: Arteriovenous shunt, laparoscopic techniques, fistula, ultrasonic flow measurement

Citation: Biorheology, vol. 36, no. 3, pp. 257-266, 1999

Authors: Braidot, Ariel A.A. | Deiber, Julio A.

Article Type: Research Article

Abstract: The linear viscoelastic model proposed in this work considers the viscoelastic nature of maturing gelatin solutions through a relaxation modulus that depends on temperature and maturation. This modulus is defined in the conceptual contexts of the classical rubber elasticity theory and the rheometric gel theory. An analysis of the relationship between the equilibrium elastic modulus and the percolation variable around the gel point is also included yielding a percolation exponent close to 1.7 as expected from previous theoretical predictions. Additionally, a simple kinetic model is proposed to follow the microstructural changes obtained as a consequence of the generation of junction …zones, the number of which vary with time during the dynamic rheometric tests used in this work. Thus, the storage and loss moduli are measured at different temperatures and frequencies, during the period of gelatin maturation. The theoretical aspects of the rheological model are presented emphasizing the quantitative changes of rheological parameters with the maturation. Show more

Keywords: Gel linear viscoelasticity, gelation kinetics, gelatin maturation, percolation parameters

Citation: Biorheology, vol. 36, no. 3, pp. 267-284, 1999

Authors: Pawłowski, P. | Ficus, M.

Article Type: Correction

Citation: Biorheology, vol. 36, no. 3, pp. 285-285, 1999