Up-regulation of pressure-activated Ca(2+)-permeable cation channel in intact vascular endothelium of hypertensive rats

Effects of arterial wall stress on vasomotion

Smooth muscle and endothelial cells in the arterial wall are exposed to mechanical stress. Indeed blood flow induces intraluminal pressure variations and shear stress. An increase in pressure may induce a vessel contraction, a phenomenon known as the myogenic response. Many muscular vessels present vasomotion, i.e., rhythmic diameter oscillations caused by synchronous cytosolic calcium oscillations of the smooth muscle cells. Vasomotion has been shown to be modulated by pressure changes. To get a better understanding of the effect of stress and in particular pressure on vasomotion, we propose a model of a blood vessel describing the calcium dynamics in a coupled population of smooth muscle cells and endothelial cells and the consequent vessel diameter variations. We show that a rise in pressure increases the calcium concentration. This may either induce or abolish vasomotion, or increase its frequency depending on the initial conditions. In our model the myogenic response is less pronounced for large arteries than for small arteries and occurs at higher values of pressure if the wall thickness is increased. Our results are in agreement with experimental observations concerning a broad range of vessels.

Regulation of hydraulic conductivity in response to sustained changes in pressure

The present study addresses the effect of a sustained change in pressure on microvascular permeability assessed by hydraulic conductivity ( Lp) measurements from microvessels of the rat mesentery. With a microperfusion technique, transvascular filtration (normalized to surface area; Jv/ S) and Lp were measured in small arterioles (baseline Lp = 0.26 × 10−7 cm·s−1·cmH2O−1) and venules (baseline Lp = 2.88 × 10−7 cm·s−1·cmH2O−1). The main finding of this study is that step increases in microvascular pressure led to time-dependent alterations of Lp. Immediately after a twofold step increase in pressure, Jv/ S increased in proportion to the pressure change. This observation is consistent with Starling's law that predicts filtration proportional to the overall pressure gradient when Lp is constant. However, when Jv/ S measurements continued for 60–90 min past the step in pressure, there was an initial decrease in Jv/ S for 30 min (“sealing effect”) followed by a substantial increase in Jv/ S out to 90 min. The sustained increase in Jv/ S suggests an increase in Lp of 36 ± 7% for small arterioles and 42 ± 5% for small venules ( P < 0.05 for both). In addition, the increase in Lp in response to an increase in pressure was attenuated significantly by nitric oxide synthase inhibition. These results indicate that a pressure-induced mechanical stimulus (possibly Jv) activates a NO-dependent biochemical response that leads to an increase in hydraulic conductivity.

Response of descending vasa recta to luminal pressure

We tested whether luminal perfusion and pressurization induce an endothelial cytoplasmic Ca2+([Ca2+]CYT) response in descending vasa recta (DVR). DVR isolated from the rat outer medulla were cannulated and subjected to free-flow microperfusion (5 nl/min); the onset of which increased [Ca2+]CYTfrom a baseline of 76 ± 13 to 221 ± 65 nM. A graded increase in luminal pressure from 0 to 45 mmHg in stopped-flow experiments induced a parallel increase in [Ca2+]CYTfrom a baseline of 74 ± 24 to 194 ± 33 nM at 45 mmHg, with a tendency for [Ca2+]CYTto plateau at pressures >25 mmHg. The removal of extracellular Ca2+and blockade by either La3+(10 μM) or SKF-96365 (100 μM) eliminated the response. Luminal pressurization to 25 mmHg increased nitric oxide (NO) generation, a response blocked by NO synthase inhibition or removal of extracellular Ca2+. The NO generation was not affected by the superoxide dismutase mimetic tempol. We conclude that DVR endothelia are mechanosensitive and respond to luminal pressure by elevating [Ca2+]CYTand generating NO. That response might augment medullary perfusion and saliuresis.

Mechanosensitive channels: multiplicity of families and gating paradigms

Mechanosensitive ion channels are the primary transducers that convert mechanical force into an electrical or chemical signal in hearing, touch, and other mechanical senses. Unlike vision, olfaction, and some types of taste, which all use similar kinds of primary heterotrimeric GTP-binding protein-coupled receptors, mechanosensation relies on diverse types of transducer molecules. Unrelated types of channels can be used for the perception of various mechanical stimuli, not only in distant groups of organisms, but also in separate locations of the same organism. The extreme sensitivity of the transduction mechanism in the auditory system, which relies on an elaborate structure of rigid cilia, filamentous links, and molecular motors to focus force on transduction channels, contrasts with that of the bacterial channel MscL, which is opened by high lateral tension in the membrane and fulfills a safety-valve rather than a sensory function. The spatial scales of conformational movement and force in these two systems are described, and are shown to be consistent with a general physical description of mechanical channel gating. We outline the characteristics of several types of mechanosensitive channels and the functional contexts in which they participate in signaling and cellular regulation in sensory and nonsensory cells.

Effects of static magnetic fields on plasma levels of angiotensin II and aldosterone associated with arterial blood pressure in genetically hypertensive rats

Effects of static magnetic fields (SMFs) on development of hypertension were investigated using young male, stroke resistant, spontaneously hypertensive rats (SHRs) beginning at 7 weeks of age. SHRs were randomly assigned to two different exposure groups or an unexposed group. The SHRs in the exposure groups were constantly exposed to two different types of external SMFs of 3.0-10.0 mT or 8.0-25.0 mT for 12 weeks. The SMFs were generated from permanent magnetic plates attached to the rat cage. The blood pressure (BP) of each rat was determined at weekly intervals using indirect tail-cuff method. The SMFs suppressed and retarded the development of hypertension in both exposed groups to a statistically significant extent for several weeks, as compared with an unexposed group. The antipressor effects were related to the extent of reduction in plasma levels of angiotensin II and aldosterone in the SHRs. These results suggest that the SMFs of mT intensities with spatial gradients could be attributable to suppression of early BP elevation via hormonal regulatory system.

Ionic mechanisms and Ca(2+) regulation in airway smooth muscle contraction: do the data contradict dogma?

In general, excitation-contraction coupling in muscle is dependent on membrane depolarization and hyperpolarization to regulate the opening of voltage-dependent Ca(2+) channels and, thereby, influence intracellular Ca(2+) concentration ([Ca(2+)](i)). Thus Ca(2+) channel blockers and K(+) channel openers are important tools in the arsenals against hypertension, stroke, and myocardial infarction, etc. Airway smooth muscle (ASM) also exhibits robust Ca(2+), K(+), and Cl(-) currents, and there are elaborate signaling pathways that regulate them. It is easy, then, to presume that these also play a central role in contraction/relaxation of ASM. However, several lines of evidence speak to the contrary. Also, too many researchers in the ASM field view the sarcoplasmic reticulum as being centrally located and displacing its contents uniformly throughout the cell, and they have focused almost exclusively on the initial single [Ca(2+)] spike evoked by excitatory agonists. Several recent studies have revealed complex spatial and temporal heterogeneity in [Ca(2+)](i), the significance of which is only just beginning to be appreciated. In this review, we will compare what is known about ion channels in ASM with what is believed to be their roles in ASM physiology. Also, we will examine some novel ionic mechanisms in the context of Ca(2+) handling and excitation-contraction coupling in ASM.

Mechanosensitive cation channels in human leukaemia cells: calcium permeation and blocking effect

Cell-attached and inside-out patch-clamp methods were employed to identify and characterize mechanosensitive (MS) ionic channels in the plasma membrane of human myeloid leukaemia K562 cells. A reversible activation of gadolinium-blockable mechanogated currents in response to negative pressure application was found in 58 % of stable patches (n = 317). I-V relationships measured with a sodium-containing pipette solution showed slight inward rectification. Data analysis revealed the presence of two different populations of channels that were distinguishable by their conductance properties (17.2 +/- 0.3 pS and 24.5 +/- 0.5 pS), but were indistinguishable with regard to their selective and pharmacological properties. Ion-substitution experiments indicated that MS channels in leukaemia cells were permeable to cations but not to anions and do not discriminate between Na(+) and K(+). The channels were fully impermeable to large organic cations such as Tris(+) and N-methyl-D-glucamine ions (NMDG(+)). Ca(2+) permeation and blockade of MS channels were examined using pipettes containing different concentrations of Ca(2+). In the presence of 2 mM CaCl(2), when other cations were impermeant, both outward and inward single-channel currents were observed; the I-V relationship showed a unitary conductance of 7.7 +/- 1.0 pS. The relative permeability value, P(Ca)/P(K), was equal to 0.75, as estimated at physiological Ca(2+) concentrations. Partial or full inhibition of inward Ca(2+) currents through MS channels was observed at higher concentrations of external Ca(2+) (10 or 20 mM). No MS channels were activated when using a pipette containing 90 mM CaCl(2). Monovalent mechanogated currents were not significantly affected by extracellular Ca(2+) at concentrations within the physiological range (0-2 mM), and at some higher Ca(2+) concentrations.

Endothelial responses to mechanical stress: where is the mechanosensor?

OBJECTIVE

The endothelium is normally subjected to mechanical deformation resulting from shear stress and from strain associated with stretch of the vessel wall. These stimuli are detected by a mechanosensor that initiates a variety of signaling systems responsible for triggering the functional responses. The identity of the mechanosensor has not been established. This article discusses the different mechanisms of mechanosensing that have been proposed and reviews the literature with respect to signaling systems that are activated in response to stress and strain in endothelium.

DATA SOURCES

Published literature related to mechanotransduction, signal transduction pathways initiated by strain in endothelium, and pathophysiologic effects of abnormal shear forces in diseases.

DATA EXTRACTION AND SYNTHESIS

Proposed mechanisms of mechanosensing include stretch-sensitive ion channels, protein kinases associated with the cytoskeleton, integrin-cytoskeletal interactions, cytoskeletal-nuclear interactions, and oxidase systems capable of generating reactive oxygen species. However, the molecular identity of the mechanosensor is not known, nor is it clear whether multiple sensing mechanisms exist.

CONCLUSIONS

Many responses are initiated in cells subjected to mechanical deformation, including alterations in ion channel conductance, activation of signal transduction pathways, and altered expression of specific genes. Future progress in this field will require a critical distinction between cell systems that become activated during mechanical strain and the identity of the cellular mechanosensor that triggers subsequent responses.

Remodeling of resistance arteries in renal failure: effect of endothelin receptor blockade

Remodeling of vessels is a known feature of renal failure, but it is unclear whether this represents an appropriate or inappropriate response to the known changes in blood flow, shear stress, and wall tension. To investigate remodeling in response to variations in blood flow, first-order mesenteric arteries were exposed to high- and low-flow conditions via the ligation of second-order branches, according to the technique described by Pourageaud and De Mey. The resulting changes in vessel geometric features, relative proportions of intima and media, submicroscopic structure, and immunostaining for proliferating cell nuclear antigen (PCNA), endothelin-1 (ET-1), and ET(A) receptors were assessed in first-order mesenteric arteries under low-flow and high-flow conditions. Subtotally nephrectomized (SNX) animals were compared with sham-operated rats. Animals either were left untreated or were treated with the ET(A) receptor antagonist (ET-RA) LU-135252, because of suggestions in the literature that ET is involved in vascular remodeling in uremia. A highly significant increase in intimal thickness was noted in low-flow arteries (4.21 +/- 1.39 microm) of SNX animals, compared with normal-flow arteries (2.06 +/- 0.61 microm), but this increase was not observed in sham-operated rats (1.38 +/- 0.77 in low-flow arteries versus 2.40 +/- 0.35 microm in normal-flow arteries). The increase in intimal thickness in low-flow arteries was abrogated by ET-RA. The medial thickness was increased in untreated SNX animals (19.5 +/- 3.61 microm), compared with sham-operated rats, and this increase was also prevented by ET-RA. The medial thickness was not affected by low flow in either sham-operated or SNX animals. In parallel, the number of PCNA-positive intimal cells was higher in low-flow, but not high-flow, arteries of SNX rats, compared with sham-operated rats. No significant change was observed in sham-operated animals. In the media, the number of PCNA-positive cells was higher in untreated SNX animals than in sham-operated rats. The number was even more markedly increased in high-flow, but not low-flow, vessels. This increase was abrogated by ET-RA. It is concluded that, in uremic animals, the response of the intima to low flow and the response of the media to high flow are exaggerated. Both responses are apparently mediated by ET.

Ion channels and their functional role in vascular endothelium

Endothelial cells (EC) form a unique signal-transducing surface in the vascular system. The abundance of ion channels in the plasma membrane of these nonexcitable cells has raised questions about their functional role. This review presents evidence for the involvement of ion channels in endothelial cell functions controlled by intracellular Ca(2+) signals, such as the production and release of many vasoactive factors, e.g., nitric oxide and PGI(2). In addition, ion channels may be involved in the regulation of the traffic of macromolecules by endocytosis, transcytosis, the biosynthetic-secretory pathway, and exocytosis, e.g., tissue factor pathway inhibitor, von Willebrand factor, and tissue plasminogen activator. Ion channels are also involved in controlling intercellular permeability, EC proliferation, and angiogenesis. These functions are supported or triggered via ion channels, which either provide Ca(2+)-entry pathways or stabilize the driving force for Ca(2+) influx through these pathways. These Ca(2+)-entry pathways comprise agonist-activated nonselective Ca(2+)-permeable cation channels, cyclic nucleotide-activated nonselective cation channels, and store-operated Ca(2+) channels or capacitative Ca(2+) entry. At least some of these channels appear to be expressed by genes of the trp family. The driving force for Ca(2+) entry is mainly controlled by large-conductance Ca(2+)-dependent BK(Ca) channels (slo), inwardly rectifying K(+) channels (Kir2.1), and at least two types of Cl( -) channels, i.e., the Ca(2+)-activated Cl(-) channel and the housekeeping, volume-regulated anion channel (VRAC). In addition to their essential function in Ca(2+) signaling, VRAC channels are multifunctional, operate as a transport pathway for amino acids and organic osmolytes, and are possibly involved in endothelial cell proliferation and angiogenesis. Finally, we have also highlighted the role of ion channels as mechanosensors in EC. Plasmalemmal ion channels may signal rapid changes in hemodynamic forces, such as shear stress and biaxial tensile stress, but also changes in cell shape and cell volume to the cytoskeleton and the intracellular machinery for metabolite traffic and gene expression.

Impaired function of endothelial pressure-activated cation channel in salt-sensitive genetic hypertension

Mechanosensitive ion channels have been suggested to act as endothelial mechanosensors for hemodynamic forces. The present study tested the hypothesis that the pressure-activated cation channel (PAC), a novel type of endothelial mechanosensitive ion channel, is involved in salt sensitivity in the Sabra rat model of hypertension. Groups of Sabra salt-sensitive (SBH/y) and salt-resistant (SBN/y) rats were loaded with deoxycorticosterone-acetate (DOCA)-salt for 8 wk or were fed a regular diet. Single channel function of PAC in SBH/y and SBN/y rats was investigated in intact endothelium of mesenteric artery using the patch-clamp technique. After DOCA-salt treatment, the SBH/y rats showed a full hypertensive response, whereas SBN/y rats were normotensive. Rats of both strains that received a regular diet were normotensive. In endothelium of both Sabra rats, Ca(2+) permeable PAC that was activated by positive pipette pressures was identified. Apparent PAC density (percentage of patches with PAC activity) was reduced in hypertensive SBH/y rats that were loaded with DOCA-salt compared with salt-loaded normotensive SBN/y rats (6 +/- 2% versus 24 +/- 8%, respectively; P < 0.05). In normotensive SBH/y and SBN/y rats that received a regular diet, PAC density was not altered. Mechanosensitivity and unitary conductance of endothelial PAC were similar in both strains under a regular diet as well as salt loading with DOCA-salt. In conclusion, the decreased density of PAC in mesenteric endothelium from hypertensive SBH/y rats indicates an impaired ion channel regulation. The defective PAC function presumably leads to an impaired mechanosensitive Ca(2+) entry and might contribute to endothelial dysfunction and high BP in this type of salt-sensitive genetic hypertension.

Stretch-sensitive switching among different channel sublevels of an endothelial cation channel

A mechanosensitive Ca(2+)-permeable cation channel was recorded by patch clamp in isolated rat aortic endothelial cells. A low level of channel activity could be observed after seal formation. The channel displayed some inward rectification and had a conductance for inward current of approx. 32 pS in Ca(2+)-free pipette and bath solutions. Negative suction of -10 to -20 mmHg increased the probability of the channel being open. When the negative pressure in the pipette was raised to -35 to -45 mmHg, the channel underwent an abrupt transition to a large conductance substate that was interrupted occasionally by two other low conductance levels. Under this condition, the overwhelming majority of openings and closings were between a main level of 83 pS and the closed level. Compared to the 32 pS substate, the 83 pS large conductance substate had shorter mean open and closed times. The two channel substates had similar ionic selectivity and both were sensitive to the inhibition of cGMP and protein kinase G. This is the first demonstration showing that mechanostress can change the single channel conductance level of an ion channel in eukaryotic cells.

Molecular basis of mechanotransduction in living cells

The simplest cell-like structure, the lipid bilayer vesicle, can respond to mechanical deformation by elastic membrane dilation/thinning and curvature changes. When a protein is inserted in the lipid bilayer, an energetic cost may arise because of hydrophobic mismatch between the protein and bilayer. Localized changes in bilayer thickness and curvature may compensate for this mismatch. The peptides alamethicin and gramicidin and the bacterial membrane protein MscL form mechanically gated (MG) channels when inserted in lipid bilayers. Their mechanosensitivity may arise because channel opening is associated with a change in the protein's membrane-occupied area, its hydrophobic mismatch with the bilayer, excluded water volume, or a combination of these effects. As a consequence, bilayer dilation/thinning or changes in local membrane curvature may shift the equilibrium between channel conformations. Recent evidence indicates that MG channels in specific animal cell types (e.g., Xenopus oocytes) are also gated directly by bilayer tension. However, animal cells lack the rigid cell wall that protects bacteria and plants cells from excessive expansion of their bilayer. Instead, a cortical cytoskeleton (CSK) provides a structural framework that allows the animal cell to maintain a stable excess membrane area (i.e., for its volume occupied by a sphere) in the form of membrane folds, ruffles, and microvilli. This excess membrane provides an immediate membrane reserve that may protect the bilayer from sudden changes in bilayer tension. Contractile elements within the CSK may locally slacken or tighten bilayer tension to regulate mechanosensitivity, whereas membrane blebbing and tight seal patch formation, by using up membrane reserves, may increase membrane mechanosensitivity. In specific cases, extracellular and/or CSK proteins (i.e., tethers) may transmit mechanical forces to the process (e.g., hair cell MG channels, MS intracellular Ca(2+) release, and transmitter release) without increasing tension in the lipid bilayer.

Ion channels and the control of blood pressure

Ion channels exist in all cells and are enormously varied in structure, function and regulation. Some progress has been made in understanding the role that ion channels play in the control of blood pressure, but the discipline is still in its infancy. Ion channels provide many different targets for intervention in disorders of blood pressure and exciting advances have been made in this field. It is possible that new drugs, as well as antisense nucleotide technology or gene therapy directed towards ion channels, may form a new class of treatments for high and low blood pressure in the future.

Mechanosensitive Ca2+ oscillations and STOC activation in endothelial cells

Activation of ion channels and the increase in intracellular Ca2+ concentration [Ca2+]i play a key role in endothelial responses to hemodynamic forces and subsequent vasoregulation. In bovine aortic endothelial cells subjected to shear stress in a parallel flow chamber, we demonstrate shear stress activation of hyperpolarizing K+ currents that occur simultaneously with oscillating increases of [Ca2+]i. Oscillating K+ currents, also known as spontaneous transient outward currents (STOC), were regulated in frequency and amplitude by the rate of shear stress in a range from 5 to 18 dyn/cm2. Activation of STOC depended on Ca2+ influx; current depended on the extracellular Ca2+ concentration and was blocked by 50 microM Gd3+. Emptying of Ca2+ stores by BHQ abolished current responses to shear stress. STOC activation was significantly reduced by cell dialysis with ryanodine (20 microM), but not heparin (200 microg/ml). Shear stress-induced STOC activation was also observed in the intact endothelium. The endothelial response to shear stress involves oscillating [Ca2+]i increase and STOC activation, which depend on Ca2+ influx-induced Ca2+ release from ryanodine-sensitive stores, demonstrating a new signaling pathway in endothelial mechanotransduction.