© 2003 Robert A. Freitas Jr. All Rights Reserved.

Robert A. Freitas Jr., Nanomedicine, Volume IIA: Biocompatibility, Landes Bioscience, Georgetown, TX, 2003

15.5.3.1.1 Fluid Shear Stress

Endothelial cells (EC) are randomly oriented in areas of low shear stress but elongated and aligned in the direction of fluid flow in regions of high shear stress [3795-3797]. In vitro endothelial cells previously acclimatized to physiological fluid shear stresses respond to artificial changes in local fluid shear stress only very slowly, and in three stages [3797]. In the first stage, EC initially respond to the imposition of stress within 3 hours by enhancing their attachments to the substrate and to neighboring cells. The cells elongate and have more stress fibers, thicker intercellular junctions, and more apical microfilaments. In the second stage, after 6 hours the EC show constrained motility as they realign, losing their dense peripheral bands and relocating more of their microtubule organizing centers and nuclei to the upstream region of the cell. In the third stage, after 12 hours the EC become elongated cells oriented in the new apparent direction of fluid flow. Stress fibers are thicker and longer, the height and thickness of intercellular junctions are higher, and the number and height of apical microfilaments are increased. This produces a new cytoskeletal organization that alters how the forces produced by fluid flow act on the cell and how the forces are transmitted to the cell interior and substrate [3797].

Physiological fluid mechanical stimuli (e.g., fluid shear stresses*) are important modulators of regional endothelial phenotype and function [3798-3802]. For example, endothelium exposed to fluid shear stress undergoes cell shape change, alignment, and microfilament network remodeling in the direction of flow (though nanorobots could block this remodeling, as illustrated crudely via microtubule disruption using nocodazole) [3803]. Interestingly, the application of a steady laminar shear stress (a physiological stimulus) upregulates the human prostaglandin transporter (hPGT) gene at the level of transcriptional activation, whereas a comparable level of turbulent shear stress (a nonphysiological stimulus) or low stress (such as a vascular surface coated with sessile nanorobots) does not [3802]. The precise molecular mechanisms that mediate shear stress response were unknown in 2002, although the cell-cell adhesion site is a likely location of flow sensing and PECAM-1, a cell adhesion molecule found at that site, has been suggested [5768] as one possible mechanoresponsive mechanism. Fourfold-elevated hemodynamic wall shear stress also produces elevated neointimal SMC apoptosis in baboon aortoiliac grafts [5949], and an increase in blood flow and velocity in canine vein grafts produced elevated apoptosis within the adventitia and media of the vein during the first week following grafting [5952].

* For laminar fluid flow in cylindrical tubes of radius R and length L through a pressure differential of DeltaP, the fluid shear stress [3814] is R (DeltaP) / 2 L.

Endothelial cells thus respond to sustained physiological fluid shear stresses* from 0.02-100 N/m², spanning the range of normal arterial wall fluid shear stresses of 1.0-2.6 N/m² from the aorta through the capillaries [3813, 3814] and 0.14-0.63 N/m² for the venous circulation [3814, 3815]. By contrast, legged vasculomobile medical nanorobots may apply shear stresses during luminal anchorage or cytoambulation at velocities up to 1 cm/sec of at least 40-200 N/m² or higher (Section 9.4.3.5). (Self-expanding aortic stents forcibly pulled from the vessel require an extraction force of ~400 N/m² assuming a 10-cm length, rising to ~1200-3600 N/m² for stents anchored with hooks and barbs [3816]. Varying the radial force applied by stents against the vascular wall has little impact on the required extraction force.) Such shear forces, if imposed unidirectionally by large numbers of closely-packed co-ambulating nanorobots for time periods of >10³ sec, may induce significant changes in shape, orientation, and physiological function in the underlying endothelial cell population. If instead these forces are applied in randomized directions by opportunistic individual nanorobots cytoambulating across the local endothelium for very short durations, then mechanically-induced modulation of endothelial phenotype and function would be greatly diminished or possibly eliminated.

* A few of the many quantitative experimental observations include:

(1) shear stresses from 0.02-1.70 N/m² produce flow-induced membrane K⁺ currents [3798];

(2) cultured subconfluent bovine aortic endothelial cells subjected to uniform fluid shear stress of 0.1-0.5 N/m² proliferate at the similar rates and achieve similar saturation density as static cultures, but confluent monolayers exposed to 0.5-1.0 N/m² laminar shear stress undergo a time-dependent change in cell shape from polygonal to ellipsoidal, becoming uniformly oriented with flow [5960];

(3) physiological shear stresses of 0.35-11.7 N/m² stimulate mitogen-activated protein kinase in a 5-min peak response time [3804];

(4) 0.04-6 N/m² shear stresses increase inositol trisphosphate levels in human endothelial cells, with a 10-30 sec peak response time [3805, 3806];

(5) shear stresses from 0.5-1.8 N/m² regulate (in frequency and amplitude) oscillating K⁺ currents known as spontaneous transient outward currents or STOC which are observed both in isolated bovine aortic endothelial cells and in intact endothelium; activation of STOC depends on the existence of a Ca⁺⁺ influx and is blocked by 50 µM of Gd⁺⁺⁺ or is significantly reduced by 20 µM of ryanodine [3807];

(6) shear stress of 1.2 N/m² induces transcription factor activation over response times ranging from 0.3-2 hours [3779];

(7) shear stresses of 1.0-2.5 N/m² induce increased ATP release from endothelial cells [5715];

(8) arterial shear stresses of 1.5-2.5 N/m² induce endothelial fibrinolytic protein secretion [3800] (though a venous shear stress of 0.4 N/m² does not);

(9) shear stress of 2 N/m² induces TGF-beta1 transcription and production in a ~60 sec initial response time, with a sustaining increase in expression after 2 hours [3808];

(10) a shear stress of 2 N/m² suppresses ET-1 mRNA on confluent bovine aortic endothelial cell monolayers [3809]; these effects of shear may be completely blocked (thus allowing ET-1 to be expressed) using 875 nM of herbimycin to inhibit tyrosine kinases or 10 µM of quin 2-AM to chelate intracellular Ca⁺⁺, partially inhibited using 3mM of tetraethylammonium (TEA), or attenuated by elevated extracellular K⁺ at 70 mM or completely inhibited by K⁺ at 135 mM [3809];

(11) shear stress of 3 N/m² induces Ca⁺⁺ membrane currents in a 30 sec peak response time [3810];

(12) shear stress of 6 N/m² applied for 12 hours causes endothelial cells to align with their longitudinal axes parallel to flow [3795];

(13) membrane hyperpolarization occurs as a function of local shear stress up to 12.0 N/m², with an exponential approach to steady state in ~1 minute; the process is fully reversed once the artificial fluid flow stress is removed [3799];

(14) critical shear stress of 42 N/m² is the disruptive threshold for endothelial cells, inducing cell mobility [3811]; and

(15) shearing stresses of 5-100 N/m² occur at the contact interface when a leukocyte is adhering to or rolling on the endothelium of a venule [3812].

A nanorobot aggregate that shields vascular cells from fluid shear for an extended time may induce those cells to revert to their flow-unstressed phenotype or to undergo apoptosis. Analogously, endothelial cells cultured in the absence of shear stress rapidly lose many of their differentiated features and become insufficiently adherent to artificial surfaces to resist physiological shear stress [3817]. In one study [3818], after blood shear was artificially reduced near a wound lesion for 24 hours the local endothelial cells became less elongated, contained fewer central microfilament bundles, and exhibited a slower repair process. Endothelial cell apoptosis was observed for a week after a decrease in carotid arterial flow by closure of an arteriovenous fistula in rabbits [5951]. In another study with rabbits, vein grafts removed from the higher-shear arterial circulation and reimplanted in the lower-shear venous circulation of the same animal showed regression of intimal hyperplasia and medial rethickening in 14 days, apparently due to induction of smooth muscle cell apoptosis by a reduction in pressure or flow forces [3819]. Stent implantation in the canine portal vein also has induced a prolonged apoptotic response in intimal and medial smooth muscle cells [5950].

Last updated on 30 April 2004