Electrical and Morphological Changes of Human Erythrocytes under High Hydrostatic Pressure Followed by Dielectric Spectroscopy

The electrical and morphological properties of human erythrocytes under high hydrostatic pressure up to 500 MPa have been studied by dielectric spectroscopy. The pressure-induced changes in the dielectric behavior of erythrocyte suspensions indicate that hydrostatic pressure causes the change in cell shape from discoidal to spherical and hemolysis at 200–300 MPa, the formation of buds and spicular processes followed by vesiculation at 300–400 MPa, and the increase in the membrane capacitance at 400–500 MPa. © 1999 Biomedical Engineering Society. PAC99: 8716Dg, 8716Uv, 8380Lz, 8715La     

Effect of fluid shear stress on the permeability of the arterial endothelium

The localization of atherosclerotic lesions is due, in part, to regional variations in the permeability of arterial endothelium to macromolecules. In turn, endothelial permeability may be influenced by fluid shear stresses. The spatial variation in endothelial permeability is reviewed and evidence for shear stress dependence upon permeability is presented. These results are examined in light of various signaling mechanisms that increase permeability by increasing the transport of water and macromolecules through the junctions separating endothelial cells. Signaling pathways cause a change in the dense peripheral band of actin and actin stress fibers or alter the phosphorylation of junction proteins which affects their ability to localize in junctions. Future directions to clarify the effect of shear stress on permeability are considered. © 2002 Biomedical Engineering Society. PAC2002: 8716Dg, 8714Ee, 8719Tt, 8716Uv     

Solute transport to the endothelial intercellular cleft: the effect of wall shear stress

The interendothelial cleft is the major transport pathway across the endothelium for hydrophilic solutes including albumin and low density lipoprotein. Previous models of arterial wall transport have assumed that the entire endothelial cell surface is available for transport from the fluid (blood) phase. One of the consequences of a cleft-mediated solute uptake mechanism is the limited area available for mass transport. This effect, together with the influence of a predominantly longitudinal cleft orientation in relation to flow, dramatically alters the fluid–phase mass transport characteristics relative to what has been assumed previously in analyzing vascular solute uptake problems. We have used a finite element computational model to simulate fluid phase transport to a longitudinal endothelial cleft under realistic wall shear rate conditions. Our numerical results show reduced dependence of the mass transfer rate on the wall shear rate compared to the classical Leveque solution for mass transport in a cross-flow configuration and confirm the significance of the wall and not the fluid as the limiting resistance to transport of macromolecules. © 2002 Biomedical Engineering Society. PAC2002: 8719Rr, 8716Uv, 8716Dg, 8710+e, 8715Vv     

Role of subcellular shear-stress distributions in endothelial cell mechanotransduction

The endothelium of blood vessels presents a wavy surface to the flowing blood. The subcellular distribution of shear stress depends on the shape and orientation of the cells and on their spatial arrangement within the monolayer. By studying details of the distribution of stress at this scale and the morphological responses that serve to modify the distribution, we can gain insight into the physical mechanisms by which the cell senses its fluid mechanical environment. The rapidly growing body of evidence indicates that endothelial cells discriminate between subtle variations in the exact loading conditions including differences in temporal and spatial gradients of shear stress, steady and pulsatile laminar flow, and laminar and turbulent flows. While in a few studies the effects of these individual flow characteristics have been carefully isolated, it is difficult to assess the relative importance of any one parameter. To interpret the relationships between isolated flow characteristics or the integrated effects of combined loading conditions and the biochemical signaling events that mediate the cell response, a full stress analysis of the cell is needed. The microscopic distribution of shear stress acting upon the cell surface provides the boundary condition for such an analysis. Experimental and analytical tools are being developed to assess the stress distribution throughout the cellular structures that might be involved in mechanotransduction. © 2002 Biomedical Engineering Society. PAC2002: 8716Xa, 8719Uv, 8719Xx     

Review article: cyclic AMP sensors in living cells: what signals can they actually measure?

Cyclic AMP is a ubiquitous intracellular second messenger that transmits information to several proteins including cyclic nucleotide-gated ion channels and protein kinase A (PKA). In turn, these effectors regulate such diverse cellular functions as Ca²⁺ influx, excitability, and gene expression, as well as cell-specific processes such as glycogenolysis and lipolysis. The enzymes known to regulate cAMP levels, adenylyl cyclase and phosphodiesterase, have been studied in detail. Unfortunately, an understanding of how information is encoded within cAMP signals has been elusive, because, until recently, methods for measuring cAMP lacked both spatial and temporal resolution. In this paper, we describe two recently developed methods for detecting cAMP levels in living cells. The first method measures fluorescence energy transfer between labeled subunits of PKA. This method is particularly useful for monitoring cellular localization of PKA activity following increases in cAMP levels. However, the slow activation and deactivation rates, the necessarily high concentrations of labeled subunits, and the redistribution of labeled subunits throughout the cell, all intrinsic to this method, limit its utility as a cAMP sensor. The second method uses genetically modified cyclic nucleotide-gated channels to measure plasma membrane-localized cAMP levels in either cell populations or single cells. The rapid gating kinetics of these channels allow real-time measurement of cAMP concentrations. These methods have given us the first glimpses of cAMP signals within living cells. © 2002 Biomedical Engineering Society. PAC2002: 8716Uv, 8780-y, 8716Sr     

Theoretical prediction of low-density lipoproteins concentration at the luminal surface of an artery with a multiple bend

To elucidate the mechanisms of localization of atherosclerotic lesions in man, the effects of various physical and hemodynamic factors on transport of atherogenic low-density lipoproteins (LDL) from flowing blood to the wall of an artery with a multiple bend were studied theoretically by means of a computer simulation under the conditions of a steady flow. It was found that due to a semipermeable nature of an arterial wall to plasma, flow-dependent concentration polarization of LDL occurred at the luminal surface of the vessel, creating a region of high LDL concentration distal to the apex of the inner wall of each bend where the flow was locally disturbed by the formation of secondary and recirculation flows and where wall shear stresses were low. The highest surface concentration of LDL occurred distal to the acute second bend where atherosclerotic intimal thickening developed. At a Re₀=500, the values calculated using estimated diffusivities of LDL in whole blood and plasma were respectively 35.1% and 15.6% higher than that in the bulk flow. The results are consistent with our hypothesis that the localization of atherosclerotic lesions results from the flow-dependent concentration polarization of LDL which creates locally a hypercholesterolemic environment even in normocholesterolemic subjects, thus augmenting the uptake of LDL by vascular endothelial cells existing at such sites. © 2002 Biomedical Engineering Society. PAC2002: 8719Uv, 8719Xx, 8714Ee, 8716Uv     

Cyanide Increases Reduction but Decreases Sequestration of Methylene Blue by Endothelial Cells

The mechanisms of endothelial cell transplasma membrane electron transport (TMET) have not been completely identified. Redox probes such as methylene blue (MB) can be useful tools, but the complexity of their disposition upon exposure to the cells can hinder interpretation. For example, MB is reduced on the cell surface by TMET, but after entering the cell in reduced form, it is reoxidized and sequestered within the cell. We developed a method to separately quantify the reduction and reoxidation rates such that it can be determined whether a metabolic inhibitor such as cyanide affects the reduction or oxidation process. MB was introduced at the inlet to a column filled with endothelial cell covered beads either as a short 12 s injection (bolus) or a long 45 min infusion (pulse), and its effluent concentration was measured as a function of time. The cells extracted 56% of the MB from the bolus, but only 41% during the pulse steady state. In the presence of cyanide, these extractions increased to 70% and decreased to 4%, respectively. Mathematical model results support the interpretation that these paradoxical effects on bolus and pulse extractions reflect the differential effects of cyanide on extracellular reduction and intracellular oxidation, i.e., cyanide increased the reduction rate from 7.3 to 13.0 cm s^-1 x 10^-5 and decreased the oxidation rate from 1.09 to 0.02 cm s^-1 x 10^-3.Cyanide also increased intracellular NADH by almost eight times, suggesting that TMET is sensitive to the cell redox status, i.e., NADH is a direct or indirect electron source. The cyanide-induced decrease in sequestration indicates a cyanide-sensitive intracellular oxidation mechanism. The results also demonstrate the potential utility of this approach for further evaluation of these endothelial redox mechanisms. © 2000 Biomedical Engineering Society. PAC00: 8716Dg, 8716Uv, 8230-b     

Errors Caused by Combination of Di-4 ANEPPS and Fluo3/4 for Simultaneous Measurements of Transmembrane Potentials and Intracellular Calcium

Intracellular calcium concentration and transmembrane potentials are two important measurements used to study the mechanisms of cardiac activity. Fluorescent dyes have been used to measure these separately but not simultaneously in cardiac tissue. Fluo-3 and Fluo-4 (a recently improved version of Fluo-3) have been used to measure changes in intracellular calcium concentration and Di-4 ANEPPS has been used to measure transmembrane potentials. This paper addresses the feasibility of using these fluorescent dyes together in order to measure transmembrane potentials and intracellular calcium concentration simultaneously. For the dyes to be used simultaneously, their respective fluorescence spectra must be sufficiently separated in wavelength in order to allow them to be separated by optical filters or spectrographs. An apparatus was constructed to measure the dyes' spectra in a fluorescence imaging chamber as well as in an isolated perfused rabbit heart. The measured spectra were mathematically modeled in order to assess the spectral overlap error under different conditions. Error graphs were constructed which may help researchers select optical filters and dye concentrations that will result in an acceptable error. © 1999 Biomedical Engineering Society. PAC99: 8716Uv, 8764Ni, 0620Dk, 8716Yc, 8716Dg, 8719Nn     

Extracellular Measurement of Anisotropic Bidomain Myocardial Conductivities. I. Theoretical Analysis

The passive electrical properties of cardiac tissue, such as the intracellular and interstitial conductivities along the longitudinal and transverse axes, have not been often measured because intracellular electrodes are usually needed for these measurements. In this paper, we present a theoretical analysis of two myocardial models developed to estimate these properties by analyzing potentials recorded with a pair of extracellular electrodes while injecting alternating current between another pair of electrodes. First, the cardiac tissue is represented by a standard bidomain model which includes a membrane capacitance; second, this model is modified by adding an intracellular capacitance representing the intercalated disks. Numerical solutions are computed with a fast Fourier transform algorithm without constraining the anisotropy ratios of the interstitial and intracellular domains. We systematically investigate the effects of changes in the bidomain parameters on the voltage-to-current ratio curves. We also demonstrate how the bidomain parameters can be theoretically estimated by fitting, with a modified Shor's r algorithm, the simulated potentials along the longitudinal and transverse axes for different frequencies between 10 and 10000 Hz. An important finding is that the interelectrode distance must be similar to the myocardial space constant so as to obtain frequency dependent measurements. © 2001 Biomedical Engineering Society. PAC01: 8719Nn, 8719Hh, 8716Uv, 0230Uu, 8716Ac     

A Chamber to Permit Invasive Manipulation of Adherent Cells in Laminar Flow with Minimal Disturbance of the Flow Field

An obstacle to real-time in vitro measurements of endothelial cell responses to hemodynamic forces is the inaccessibility of the cells to instruments of measurement and manipulation. We have designed a parallel plate laminar flow chamber that permits access to adherent cells during exposure to flow. The minimally invasive flow device (MIF device) has longitudinal slits (1 mm wide) cut in the top plate of the chamber to allow insertion of a recording, measurement, or stimulating instrument (e.g., micropipette) into the flow field. Surface tension forces at the slit openings are sufficient to counteract the hydrostatic pressure generated in the chamber and thus prevent overflow. The invasive probe is brought near to the cell surface, makes direct contact with the cell membrane, or enters the cell. The slits provide access to a large number (and choice) of cells. The MIF device can maintain physiological levels of shear stress (<1–15 dyn/cm²) without overflow in the absence and presence of fine instruments such as micropipettes used in electrophysiology, membrane aspiration, and microinjection. Microbead trajectory profiles demonstrated negligible deviations in laminar flow near the surface of target cells in the presence of microscale instruments. Patch-clamp electrophysiological recordings of flow-induced changes in membrane potential were demonstrated. The MIF device offers numerous possibilities to investigate real-time endothelial responses to well-defined flow conditions in vitro including electrophysiology, cell surface mechanical probing, local controlled chemical release, biosensing, microinjection, and amperometric techniques. © 2000 Biomedical Engineering Society. PAC00: 8780Fe, 8717Jj, 8719Uv, 8716Uv, 8719Nn     

PET Contributions to Understanding Normal and Abnormal Cardiac Perfusion and Metabolism

Noninvasive positron emission tomography (PET)-based studies of myocardial blood flow and substrate metabolism characterized the human heart as an organ fully integrated with the general function of the human body. Cardiac energy demands are tightly coupled to peripheral needs in oxygen and, in turn, govern changes in myocardial blood flow and substrate supply. Substrate selection and utilization depend largely on substrate availability and, hence, on concentrations of fuel substrate in blood. Endocrine and neuronal factors together with regional transport processes modulate and fine tune regional rates of substrate utilization. Manipulation of substrate availability as for example through dietary or pharmacologic maneuvers offer a means to probe regional substrate interactions, to demonstrate shifts in substrate selection between free fatty acid and glucose and, hence, to confirm the operation of regulatory mechanisms established previously in animal experiments. In abnormal states, local factors modulate the generally integrated responses and synchronize regional substrate utilization and metabolism with regional needs. Diminished substrate delivery in chronic low flow conditions is matched by a down regulation in regional contractile function possibly as an energy saving measure, together with a decline in oxidative metabolism as evidenced by reduced oxidation of ¹¹C-palmitate and delayed turnover of 11C-acetate. Activation of rate controlling enzymes together with enhanced transmembraneous transport systems represent flux generating steps for enhanced regional glucose consumption possibly as a means for reducing oxygen needs and at the same time, preserving cellular homeostasis. PET identifies such regional metabolic adjustments as regional increases in 18F-deoxyglucose uptake as a clinically useful hallmark of myocardial viability. Regional glucose utilization in this case no longer fully responds to general control mechanisms of substrate selection but is modified by local factors or, ultimately may become part of a local microsystem as a means of protection against potentially deleterious consequences of disease. © 2000 Biomedical Engineering Society. PAC00: 8758Fg, 8719Hh, 8719Uv, 8716Uv, 8715Rn     

Deterministic Model of Ion Channel Flipping with Fractal Scaling of Kinetic Rates

The flipping of ion channels in biological membranes has usually been modeled in terms of Markov transitions between open and closed states. The basic assumption of this approach is that channel flipping between open and closed states is an inherent stochastic process, due to random thermal fluctuations of units forming the channel protein. In this paper, we propose a different view of channel flipping, one not involving external stochastic causes. We consider the channel as a physical dynamic system, the unpredictable flipping of which is due to a deterministic mechanism which sustains a chaotic dynamics. In particular, we presume the changes in the channel conformation are due to delayed interaction between the ionic flow through the channel and the protein forming the channel. The model proposed here describes the channel by means of macroscopic physical quantities such as conductance, current, membrane, and reversal potentials and predicts open and closed dwell time distributions consisting of multiple exponential components and exhibiting power-law scaling over a wide range of time scales. The effective kinetic rate computed through use of simulation data shows fractal properties in good agreement with those seen experimentally. This mathematical model of the ion channel is physically consistent in terms of a plausible real system and may provide a novel key to understanding the complex behavior of the flipping process. © 1999 Biomedical Engineering Society. PAC99: 8716Uv, 8710+e, 8717Aa, 0545Df, 8716Dg, 8719Nn, 8714Ee     

Modulation of ATP/ADP concentration at the endothelial surface by shear stress: effect of flow-induced ATP release

The adenine nucleotides ATP and ADP induce the production of vasoactive compounds in vascular endothelial cells (ECs). Therefore, knowledge of how flow affects the concentration of ATP and ADP at the EC surface may be important for understanding shear stress-mediated vasoregulation. The concentration of ATP and ADP is determined by convective and diffusive transport as well as by hydrolysis of these nucleotides by ectonucleotidases at the EC surface. Previous mathematical modeling has demonstrated that for steady flow in a parallel plate flow chamber, the combined ATP+ADP concentration does not change considerably over a wide range of shear stress. This finding has been used to argue that the effect of flow on adenine nucleotide transport could not account for the dependence of endothelial responses to ATP on the magnitude of applied shear stress. The present study extends the previous modeling to include pulsatile flow as well as flow-induced endothelial ATP release. Our results demonstrate that flow-induced ATP release has a pronounced effect on nucleotide concentration under both steady and pulsatile flow conditions. While the combined ATP+ADP concentration at the EC surface in the absence of flow-induced ATP release changes by only 10% over the wall shear stress range 0.1-10 dyne/cm ^-2, inclusion of this release leads to a concentration change of 34% –106% over the same shear stress range, depending on how ATP release is modeled. These results suggest that the dependence of various endothelial responses to shear stress on the magnitude of the applied shear stress may be partially attributable to flow-induced changes in cell-surface adenine nucleotide concentration. © 2001 Biomedical Engineering Society. PAC01: 8716Ac, 8716Uv, 8719Uv, 8715Vv, 8710+e     

Intracellular calcium changes in rat aortic smooth muscle cells in response to fluid flow

Vascular smooth muscle cells (VSM) are normally exposed to transmural fluid flow shear stresses, and after vascular injury, blood flow shear stresses are imposed upon them. Since Ca²⁺ is a ubiquitous intracellular signaling molecule, we examined the effects of fluid flow on intracellular Ca²⁺ concentration in rat aortic smooth muscle cells to assess VSM responsiveness to shear stress. Cells loaded with fura 2 were exposed to steady flow shear stress levels of 0.5–10.0 dyn/cm² in a parallel-plate flow chamber. The percentage of cells displaying a rise in cytosolic Ca²⁺ ion concentration ([Ca²⁺]_i) increased in response to increasing flow, but there was no effect of flow on the ([Ca²⁺]_i) amplitude of responding cells. Addition of Gd³⁺ (10 M) or thapsigargin (50 nM) significantly reduced the percentage of cells responding and the response amplitude, suggesting that influx of Ca²⁺ through ion channels and release from intracellular stores contribute to the rise in ([Ca²⁺]_i) in response to flow. The addition of nifedipine (1 or 10 M) or ryanodine (10 M) also significantly reduced the response amplitude, further defining the role of ion channels and intracellular stores in the Ca²⁺ response. © 2002 Biomedical Engineering Society. PAC2002: 8716Uv, 8719Uv, 8716Dg, 8719Ff     

Elastodynamics and arterial wall stress

Recent advances in molecular and cell biology have emphasized the fundamental importance of mechanical factors in regulating the structure and function of cells and extracellular matrix in the vasculature. Consequently, there is an ever-greater motivation to calculate accurately the stress and strain fields in the arterial wall and how they change with disease, injury, and clinical treatment. Although there is an extensive literature on arterial mechanics, our understanding is still far from complete. In this paper, we review some of the salient findings with regard to wall properties, suggest some possible improvements in the calculation of wall stress, and identify some unresolved problems for further research. © 2002 Biomedical Engineering Society. PAC2002: 8719Rr, 8719Uv, 8719Xx, 8716Xa     

Modeling Tracer Transport in an Osteon under Cyclic Loading

A mathematical model is developed to explain the fundamental conundrum as to how during cyclic mechanical loading there can be net solute (e.g., nutrient, tracer) transport in bone via the lacunar-canalicular porosity when there is no net fluid movement in the canaliculi over a loading cycle. Our hypothesis is that the fluid space in an osteocytic lacuna facilitates a nearly instantaneous mixing process of bone fluid that creates a difference in tracer concentration between the inward and outward canalicular flow and thus ensures net tracer transport to the osteocytes during cyclic loading, as has been shown experimentally. The sequential spread of the tracer from the osteonal canal to the lacunae is investigated for an osteon experiencing sinusoidal loading. The fluid pressure in the canaliculi is calculated using poroelasticity theory and the mixing process in the lacunae is then simulated computationally. The tracer concentration in lacunae extending radially from the osteonal canal to the cement line is calculated as a function of the loading frequency, loading magnitude, and number of loading cycles as well as the permeability of the lacunar-canalicular porosity. Our results show that net tracer transport to the lacunae does occur for cyclic loading. Tracer transport is found to increase with higher loading magnitude and higher permeability and to decrease with increasing loading frequency. This work will be helpful in designing experimental studies of tracer movement and bone fluid flow, which will enhance our understanding of bone metabolism as well as bone adaptation. © 2000 Biomedical Engineering Society. PAC00: 8716Uv, 8719Rr, 8716Ac     

Origins of Spiral Wave Meander and Breakup in a Two-Dimensional Cardiac Tissue Model

We studied the stability of spiral waves in homogeneous two-dimensional cardiac tissue using phase I of the Luo–Rudy ventricular action potential model. By changing the conductance and the relaxation time constants of the ion channels, various spiral wave phenotypes, including stable, quasiperiodically meandering, chaotically meandering, and breakup were observed. Stable and quasiperiodically meandering spiral waves occurred when the slope of action potential duration (APD) restitution was <1 over all diastolic intervals visited during reentry; chaotic meander and spiral wave breakup occurred when the slope of APD restitution exceeded 1. Curvature of the wave changes both conduction velocity and APD, and their restitution properties, thereby modulating local stability in a spiral wave, resulting in distinct spiral wave phenotypes. In the LR1 model, quasiperiodic meander is most sensitive to the Na⁺ current, whereas chaotic meander and breakup are more dependent on the Ca²⁺ and K⁺ currents. © 2000 Biomedical Engineering Society. PAC00: 8719Hh, 8717Nn, 8717Aa, 8716Uv     

Comparison of algorithms for the simulation of action potentials with stochastic sodium channels

This paper presents a comparison of computational algorithms to simulate action potentials using stochastic sodium channels. Four algorithms are compared in single-node models: Strassberg and DeFelice (1993) (SD), Rubinstein (1995) (R), Chow and White (1996) (CW), and Fox (1997) (F). Neural responses are simulated to a simple and a preconditioned monophasic current pulse. Three exact algorithms implementing Markov jumping processes (SD, R, CW) resulted in similar responses, while the approximation algorithm using Langevins equation (F) showed quite different responses from those in the exact algorithms. The computational time was measured as well: 1(F), 7(CW), 32(SD), 39(R) relative to that of the F algorithm. Furthermore, it is shown that as the sampling step for integration of the transmembrane potential increases, neural responses in all algorithms tended to be different from those in dense sampling steps, however, the CW algorithm was robust even at a sparse sampling step. It is concluded that the most computationally efficient algorithm having appropriate properties of neural excitability is the CW algorithm. © 2002 Biomedical Engineering Society. PAC2002: 8716Uv, 8716Ac, 0250Ga, 0250Ey, 0260Jh     

Principal Dynamic Mode Analysis of Nonlinear Transduction in a Spider Mechanoreceptor

The nonlinear dynamics of the mechanoelectrical transduction in an arthropod mechanoreceptor (cuticular slit sense organ of the spider Cupiennius salei) were studied using Volterra kernel measurements and the recently proposed method of principal dynamic modes. Since mechanoreceptors must operate with sufficient gain sensitivity to rapidly varying displacement stimuli over a broad bandwidth and for a wide range of amplitudes, the experimental data were generated by applying pseudorandom broadband mechanical displacements of various mean levels to the cuticular slits. The recorded response data were intracellular current and potential. The purpose of this paper is to demonstrate the use of the principal dynamic mode (PDM) methodology in elucidating the nonlinear dynamics of a spider mechanoreceptor. The results obtained demonstrate that two PDMs suffice to provide a complete nonlinear dynamic model of this insect mechanoreceptor. The first PDM resembles the first-order kernel and has a low pass characteristic (position dependent), while the second PDM has a high-pass characteristic (velocity-dependent) and resides entirely in the second-order kernel (nonlinear adaptation). This study may serve as an example of the proper use of this new methodology for the analysis of nonlinear physiological systems. © 1999 Biomedical Engineering Society. PAC99: 8719Bb, 8719Nn, 8716Uv     

A Novel System to Study the Impact of Epithelial Barriers on Cellular Metabolism

Medications introduced into the systematic circulation must be transported across biological barriers such as skin, gastrointestinal, or bronchial epithelia, which can alter their kinetic and metabolic profiles. It is, therefore, important to understand diffusion kinetics across barrier membranes when choosing a dosing regime that will elicit the greatest cellular response. An in vitro system that combines membrane transport studies with a downstream cell culture chamber has been developed. The system has been tested with skin and a small intestine model (Caco-2 cell monolayers) as barriers, the peroxovanadium compound [VO(O₂)₂ 1, 10 phenanthroline] bpV(phen), as the test chemical, Hep-G2 (liver) as the test cells, and glucose consumption as the test assay. Peroxovanadium has insulin mimetic properties and has been previously demonstrated to effectively lower blood glucose levels in diabetic rats when administered transdermally. A dose of 10 mM bpV(phen) placed on the skin epidermis with a continuous iontophoretic current of 0.5 mA/cm² for 4.5 h led to a net 22% increase in glucose consumption by Hep-G2 cells. The same dose of bpV(phen) passively diffusing across a Caco-2 cell monolayer led to an increase in glucose consumption by Hep-G2 cells of 23%. This system is highly versatile and can be used to study many other processes, involving a variety of biological membranes, cell types, chemicals and assays, making it a valuable research tool. © 2000 Biomedical Engineering Society. PAC00: 8716Uv, 8715Vv