Differential membrane potential and ion current responses to different types of shear stress in vascular endothelial cells

Abstract

cells (ECs) distinguish among and respond differently to different types of fluid mechanical shear stress. Elucidating the mechanisms governing this differential responsiveness is the key to understanding why early atherosclerotic lesions localize preferentially in arterial regions exposed to low and/or oscillatory flow. An early and very rapid endothelial response to flow is the activation of flow-sensitive K ϩ and Cl Ϫ channels that respectively hyperpolarize and depolarize the cell membrane and regulate several important endothelial responses to flow.We have used whole cell current-and voltage-clamp techniques to demonstrate that flow-sensitive hyperpolarizing and depolarizing currents respond differently to different types of shear stress in cultured bovine aortic ECs. A steady shear stress level of 10 dyn/cm 2 activated both currents leading to rapid membrane hyperpolarization that was subsequently reversed to depolarization. In contrast, a steady shear stress of 1 dyn/cm 2 only activated the hyperpolarizing current. A purely oscillatory shear stress of 0 Ϯ 10 dyn/cm 2 with an oscillation frequency of either 1 or 0.2 Hz activated the hyperpolarizing current but only minimally the depolarizing current, whereas a 5-Hz oscillation activated neither current. These results demonstrate for the first time that flow-activated ion currents exhibit different sensitivities to shear stress magnitude and oscillation frequency. We propose that flow-sensitive ion channels constitute components of an integrated mechanosensing system that, through the aggregate effect of ion channel activation on cell membrane potential, enables ECs to distinguish among different types of flow. ion channels; atherosclerosis; mechanotransduction IN LARGE ARTERIES, the combined effects of arterial geometry, blood flow pulsatility, and vascular wall compliance lead to a complex and highly dynamic flow field that is characterized by a wide range of shear stresses, and, in certain arterial regions, by periodic changes in flow direction (1,4,18,24). This diversity of arterial flow patterns is particularly significant in light of the preferential localization of early atherosclerotic lesions at arterial regions exposed to low and/or oscillatory shear stress (1,18,24,27). Although the biological basis of this correlation remains to be elucidated, shear stress-mediated endothelial dysfunction is likely centrally involved. The local hemodynamic environment in an artery is sensed by the vascular endothelium (34). This is accomplished via an intricate and coordinated sequence of shear stress-induced humoral, metabolic, and structural responses in endothelial cells (ECs) (2,10,12,21,27,39). The mechanisms by which ECs sense fluid mechanical forces and subsequently transduce these forces into biochemical signals remain incompletely understood and are under intense investigation.Of more direct relevance to shear stress involvement in atherogenesis is the fact that ECs, beyond being responsive to shear stress, distinguish among and respond differently to d...

Flow-activated Chloride Channels in Vascular Endothelium

“…Our results also demonstrate that although the Cl Ϫ current initiates immediately upon flow onset, it develops slowly and attains its peak only after ϳ200 s. The slow development of flow-sensitive Cl Ϫ currents is consistent with the observation made in previous studies (11,12) that although both K ϩ and Cl Ϫ channels are activated immediately upon the onset of flow, K ϩ channelmediated membrane hyperpolarization precedes Cl Ϫ channel-mediated depolarization, despite the larger electrochemical gradient for Cl Ϫ than for K ϩ . In response to sustained flow, BAECs exhibit varying degrees of Cl Ϫ current desensitization.…”

“…1, A and B). We had demonstrated in previous studies that the flow-induced outward current is Cl Ϫ -specific (11,12). This was confirmed in this study; in the presence of the Cl Ϫ channel blocker NPPB (100 M), the flow-induced current was completely inhibited (Fig.…”

“…We have recently reported that although flow-sensitive K ϩ channels are equally responsive to steady flow as they are to oscillatory flow with a physiological oscillation frequency of 1 Hz, Cl Ϫ channels respond to steady flow but are largely insensitive to 1-Hz oscillatory flow (12). In this study, we have confirmed the nonresponsiveness of the Cl Ϫ current to 1-Hz oscillatory flow.…”

“…Sudden exposure of ECs to shear stress immediately activates inward-rectifying K ϩ channels, leading to cell membrane hyperpolarization (9,10). Simultaneously, flow stimulates outward-rectifying Cl Ϫ channels whose activation antagonizes the K ϩ channel-mediated hyperpolarization and leads to membrane depolarization (11,12). Although characteristics of flowsensitive K ϩ channels, including their unitary conductance, dependence on the magnitude of applied shear stress, and regulation, have been reported in previous studies (9,13,14), little is known about flow-activated Cl Ϫ channels.…”

Vascular Responses to Shear Stress: The Involvement of Mechanosensors in Endothelial Cells

“…Simultaneously, flow stimulates outward-rectifying Cl -channels whose activation leads to membrane depolarization following the initial K + channel-mediated hyperpolarization [47,48]. Hyperpolarization precedes depolarization despite the fact that the electrochemical driving force for Cl -is larger than that for K + .…”