Stretch-activated ion channels in smooth muscle: a mechanism for the initiation of stretch-induced contraction

Vascular mechanotransduction

This review aims to survey the current state of mechanotransduction in vascular smooth muscle cells (VSMCs) and endothelial cells (ECs), including their sensing of mechanical stimuli and transduction of mechanical signals that result in the acute functional modulation and longer-term transcriptomic and epigenetic regulation of blood vessels. The mechanosensors discussed include ion channels, plasma membrane-associated structures and receptors, and junction proteins. The mechanosignaling pathways presented include the cytoskeleton, integrins, extracellular matrix, and intracellular signaling molecules. These are followed by discussions on mechanical regulation of transcriptome and epigenetics, relevance of mechanotransduction to health and disease, and interactions between VSMCs and ECs. Throughout this review, we offer suggestions for specific topics that require further understanding. In the closing section on conclusions and perspectives, we summarize what is known and point out the need to treat the vasculature as a system, including not only VSMCs and ECs but also the extracellular matrix and other types of cells such as resident macrophages and pericytes, so that we can fully understand the physiology and pathophysiology of the blood vessel as a whole, thus enhancing the comprehension, diagnosis, treatment, and prevention of vascular diseases.

Smooth muscle contribution to vaginal viscoelastic response

Smooth muscle cells contribute to the mechanical function of various soft tissues, however, their contribution to the viscoelastic response when subjected to multiaxial loading remains unknown. The vagina is a fibromuscular viscoelastic organ that is exposed to prolonged and increased pressures with daily activities and physiologic processes such as vaginal birth. The vagina changes in geometry over time under prolonged pressure, known as creep. Vaginal smooth muscle cells may contribute to creep. This may be critical for the function of vaginal and other soft tissues that experience fluctuations in their biomechanical environment. Therefore, the objective of this study was to develop methods to evaluate the contribution of smooth muscle to vaginal creep under multiaxial loading using extension - inflation tests. The vaginas from wildtype mice (C57BL/6 × 129SvEv; 3-6 months; n = 10) were stimulated with various concentrations of potassium chloride then subjected to the measured in vivo pressure (7 mmHg) for 100 s. In a different cohort of mice (n = 5), the vagina was stimulated with a single concentration of potassium chloride then subjected to 5 and 15 mmHg. A laser micrometer measured vaginal outer diameter in real-time. Immunofluorescence evaluated the expression of alpha-smooth muscle actin and myosin heavy chain in the vaginal muscularis (n = 6). When smooth muscle contraction was activated, vaginal creep behavior increased compared to the relaxed state. However, increased pressure decreased the active creep response. This study demonstrated that extension - inflation protocols can be used to evaluate smooth muscle contribution to the viscoelastic response of tubular soft tissues.

Strains induced in the vagina by smooth muscle contractions

The ability of the vagina to contract gives rise to a set of active mechanical properties that contribute to the complex function of this organ in-vivo. Regional differences in the morphology of the vagina have been long recognized, but the large heterogeneous deformations that the vagina experiences during contractions have never been quantified. Furthermore, there is no consensus regarding differences in contractility along the two primary anatomical directions of the vagina: the longitudinal direction (LD) and the circumferential direction (CD). In this study, square vaginal specimens from healthy virgin rats (n=15) were subjected to isometric planar biaxial tests at four equi-biaxial stretches of 1.0, 1.1, 1.2, and 1.3. Contractions were induced at each stretch by a high concentration potassium solution. The digital image correlation method was used to perform full-field strain measurements during contractions. The vagina was found to undergo significantly higher compressive strains, tensile strains, and contractile forces along the LD than along the CD during contractions. Specifically, when computed over all the applied equi-biaxial stretches, mean (± std. dev.) absolute maximum compressive strains were -(13.43 ± 1.56)% along the LD and -(3.19 ± 0.25)% along the CD, mean absolute maximum tensile strains were (10.92 ± 1.73)% along the LD and (3.62 ± 0.57)% along the CD, and mean maximum contractile forces were 6.24 ± 0.55 mN along the LD and 3.35 ± 0.56 mN along the CD. Moreover, the vaginal tissue appeared to undergo compression in the proximal region and tension in the distal region while kept at constant equi-biaxial stretches. The active mechanical properties of the healthy vagina need to be fully investigated so that detrimental alterations in vaginal contractility, such as those caused by pelvic floor disorders and current treatment strategies, can be prevented. STATEMENT OF SIGNIFICANCE: Contractile forces of the vagina have been measured by several investigators using uniaxial tensile testing methods. Unlike previous studies, in this study planar-biaxial tests of vaginal specimens were performed while the full-field strains of the vagina, as induced by smooth muscle contraction, were measured. The vagina was found to generate significantly larger contractile strains and forces in the longitudinal direction than in the circumferential direction. Knowledge of the contractile mechanics of the healthy vagina is essential to understand the detrimental effects that pelvic organ prolapse and the use of surgical meshes have on the functionality of smooth muscle in the vagina.

Intercellular communication controls agonist-induced calcium oscillations independently of gap junctions in smooth muscle cells

In this study, we report the existence of a communication system among human smooth muscle cells that uses mechanical forces to frequency modulate long-range calcium waves. An important consequence of this mechanical signaling is that changes in stiffness of the underlying extracellular matrix can interfere with the frequency modulation of Ca²⁺ waves, causing smooth muscle cells from healthy human donors to falsely perceive a much higher agonist dose than they actually received. This aberrant sensing of contractile agonist dose on stiffer matrices is completely absent in isolated smooth muscle cells, although the isolated cells can sense matrix rigidity. We show that the intercellular communication that enables this collective Ca²⁺ response in smooth muscle cells does not involve transport across gap junctions or extracellular diffusion of signaling molecules. Instead, our data support a collective model in which mechanical signaling among smooth muscle cells regulates their response to contractile agonists.

Coupling of store-operated calcium entry to vasoconstriction is acid-sensing ion channel 1a dependent in pulmonary but not mesenteric arteries

Although voltage-gated Ca²⁺ channels (VGCC) are a major Ca²⁺ entry pathway in vascular smooth muscle cells (VSMCs), several other Ca²⁺-influx mechanisms exist and play important roles in vasoreactivity. One of these is store-operated Ca²⁺ entry (SOCE), mediated by an interaction between STIM1 and Orai1. Although SOCE is an important mechanism of Ca²⁺ influx in non-excitable cells (cells that lack VGCC); there is debate regarding the contribution of SOCE to regulate VSMC contractility and the molecular components involved. Our previous data suggest acid-sensing ion channel 1a (ASIC1a) is a necessary component of SOCE and vasoconstriction in small pulmonary arteries. However, it is unclear if ASIC1a similarly contributes to SOCE and vascular reactivity in systemic arteries. Considering the established role of Orai1 in mediating SOCE in the systemic circulation, we hypothesize the involvement of ASIC1a in SOCE and resultant vasoconstriction is unique to the pulmonary circulation. To test this hypothesis, we examined the roles of Orai1 and ASIC1a in SOCE- and endothelin-1 (ET-1)-induced vasoconstriction in small pulmonary and mesenteric arteries. We found SOCE is coupled to vasoconstriction in pulmonary arteries but not mesenteric arteries. In pulmonary arteries, inhibition of ASIC1a but not Orai1 attenuated SOCE- and ET-1-induced vasoconstriction. However, neither inhibition of ASIC1a nor Orai1 altered ET-1-induced vasoconstriction in mesenteric arteries. We conclude that SOCE plays an important role in pulmonary, but not mesenteric, vascular reactivity. Furthermore, in contrast to the established role of Orai1 in SOCE in non-excitable cells, the SOCE response in pulmonary VSMCs is largely mediated by ASIC1a.

Evolving mechanisms of vascular smooth muscle contraction highlight key targets in vascular disease

Vascular smooth muscle (VSM) plays an important role in the regulation of vascular function. Identifying the mechanisms of VSM contraction has been a major research goal in order to determine the causes of vascular dysfunction and exaggerated vasoconstriction in vascular disease. Major discoveries over several decades have helped to better understand the mechanisms of VSM contraction. Ca2+ has been established as a major regulator of VSM contraction, and its sources, cytosolic levels, homeostatic mechanisms and subcellular distribution have been defined. Biochemical studies have also suggested that stimulation of Gq protein-coupled membrane receptors activates phospholipase C and promotes the hydrolysis of membrane phospholipids into inositol 1,4,5-trisphosphate (IP3) and diacylglycerol (DAG). IP3 stimulates initial Ca2+ release from the sarcoplasmic reticulum, and is buttressed by Ca2+ influx through voltage-dependent, receptor-operated, transient receptor potential and store-operated channels. In order to prevent large increases in cytosolic Ca2+ concentration ([Ca2+]c), Ca2+ removal mechanisms promote Ca2+ extrusion via the plasmalemmal Ca2+ pump and Na+/Ca2+ exchanger, and Ca2+ uptake by the sarcoplasmic reticulum and mitochondria, and the coordinated activities of these Ca2+ handling mechanisms help to create subplasmalemmal Ca2+ domains. Threshold increases in [Ca2+]c form a Ca2+-calmodulin complex, which activates myosin light chain (MLC) kinase, and causes MLC phosphorylation, actin-myosin interaction, and VSM contraction. Dissociations in the relationships between [Ca2+]c, MLC phosphorylation, and force have suggested additional Ca2+ sensitization mechanisms. DAG activates protein kinase C (PKC) isoforms, which directly or indirectly via mitogen-activated protein kinase phosphorylate the actin-binding proteins calponin and caldesmon and thereby enhance the myofilaments force sensitivity to Ca2+. PKC-mediated phosphorylation of PKC-potentiated phosphatase inhibitor protein-17 (CPI-17), and RhoA-mediated activation of Rho-kinase (ROCK) inhibit MLC phosphatase and in turn increase MLC phosphorylation and VSM contraction. Abnormalities in the Ca2+ handling mechanisms and PKC and ROCK activity have been associated with vascular dysfunction in multiple vascular disorders. Modulators of [Ca2+]c, PKC and ROCK activity could be useful in mitigating the increased vasoconstriction associated with vascular disease.

Multi-scale and multi-physics model of the uterine smooth muscle with mechanotransduction

Preterm labor is an important public health problem. However, the efficiency of the uterine muscle during labor is complex and still poorly understood. This work is a first step towards a model of the uterine muscle, including its electrical and mechanical components, to reach a better understanding of the uterus synchronization. This model is proposed to investigate, by simulation, the possible role of mechanotransduction for the global synchronization of the uterus. The electrical diffusion indeed explains the local propagation of contractile activity, while the tissue stretching may play a role in the synchronization of distant parts of the uterine muscle. This work proposes a multi-physics (electrical, mechanical) and multi-scales (cell, tissue, whole uterus) model, which is applied to a realistic uterus 3D mesh. This model includes electrical components at different scales: generation of action potentials at the cell level, electrical diffusion at the tissue level. It then links these electrical events to the mechanical behavior, at the cellular level (via the intracellular calcium concentration), by simulating the force generated by each active cell. It thus computes an estimation of the intra uterine pressure (IUP) by integrating the forces generated by each active cell at the whole uterine level, as well as the stretching of the tissue (by using a viscoelastic law for the behavior of the tissue). It finally includes at the cellular level stretch activated channels (SACs) that permit to create a loop between the mechanical and the electrical behavior (mechanotransduction). The simulation of different activated regions of the uterus, which in this first "proof of concept" case are electrically isolated, permits the activation of inactive regions through the stretching (induced by the electrically active regions) computed at the whole organ scale. This permits us to evidence the role of the mechanotransduction in the global synchronization of the uterus. The results also permit us to evidence the effect on IUP of this enhanced synchronization induced by the presence of SACs. This proposed simplified model will be further improved in order to permit a better understanding of the global uterine synchronization occurring during efficient labor contractions.

Mechanosensitive Piezo Channels in the Gastrointestinal Tract

Sensation of mechanical forces is critical for normal function of the gastrointestinal (GI) tract and abnormalities in mechanosensation are linked to GI pathologies. In the GI tract there are several mechanosensitive cell types-epithelial enterochromaffin cells, intrinsic and extrinsic enteric neurons, smooth muscle cells and interstitial cells of Cajal. These cells use mechanosensitive ion channels that respond to mechanical forces by altering transmembrane ionic currents in a process called mechanoelectrical coupling. Several mechanosensitive ionic conductances have been identified in the mechanosensory GI cells, ranging from mechanosensitive voltage-gated sodium and calcium channels to the mechanogated ion channels, such as the two-pore domain potassium channels K2P (TREK-1) and nonselective cation channels from the transient receptor potential family. The recently discovered Piezo channels are increasingly recognized as significant contributors to cellular mechanosensitivity. Piezo1 and Piezo2 are nonselective cationic ion channels that are directly activated by mechanical forces and have well-defined biophysical and pharmacologic properties. The role of Piezo channels in the GI epithelium is currently under investigation and their role in the smooth muscle syncytium and enteric neurons is still not known. In this review, we outline the current state of knowledge on mechanosensitive ion channels in the GI tract, with a focus on the known and potential functions of the Piezo channels.

The organelle of differentiation in embryos: the cell state splitter

The cell state splitter is a membraneless organelle at the apical end of each epithelial cell in a developing embryo. It consists of a microfilament ring and an intermediate filament ring subtending a microtubule mat. The microtubules and microfilament ring are in mechanical opposition as in a tensegrity structure. The cell state splitter is bistable, perturbations causing it to contract or expand radially. The intermediate filament ring provides metastability against small perturbations. Once this snap-through organelle is triggered, it initiates signal transduction to the nucleus, which changes gene expression in one of two readied manners, causing its cell to undergo a step of determination and subsequent differentiation. The cell state splitter also triggers the cell state splitters of adjacent cells to respond, resulting in a differentiation wave. Embryogenesis may be represented then as a bifurcating differentiation tree, each edge representing one cell type. In combination with the differentiation waves they propagate, cell state splitters explain the spatiotemporal course of differentiation in the developing embryo. This review is excerpted from and elaborates on "Embryogenesis Explained" (World Scientific Publishing, Singapore, 2016).

Synergistic and antagonistic interplay between myostatin gene expression and physical activity levels on gene expression patterns in triceps Brachii muscles of C57/BL6 mice

Levels of myostatin expression and physical activity have both been associated with transcriptome dysregulation and skeletal muscle hypertrophy. The transcriptome of triceps brachii muscles from male C57/BL6 mice corresponding to two genotypes (wild-type and myostatin-reduced) under two conditions (high and low physical activity) was characterized using RNA-Seq. Synergistic and antagonistic interaction and ortholog modes of action of myostatin genotype and activity level on genes and gene pathways in this skeletal muscle were uncovered; 1,836, 238, and 399 genes exhibited significant (FDR-adjusted P-value < 0.005) activity-by-genotype interaction, genotype and activity effects, respectively. The most common differentially expressed profiles were (i) inactive myostatin-reduced relative to active and inactive wild-type, (ii) inactive myostatin-reduced and active wild-type, and (iii) inactive myostatin-reduced and inactive wild-type. Several remarkable genes and gene pathways were identified. The expression profile of nascent polypeptide-associated complex alpha subunit (Naca) supports a synergistic interaction between activity level and myostatin genotype, while Gremlin 2 (Grem2) displayed an antagonistic interaction. Comparison between activity levels revealed expression changes in genes encoding for structural proteins important for muscle function (including troponin, tropomyosin and myoglobin) and for fatty acid metabolism (some linked to diabetes and obesity, DNA-repair, stem cell renewal, and various forms of cancer). Conversely, comparison between genotype groups revealed changes in genes associated with G1-to-S-phase transition of the cell cycle of myoblasts and the expression of Grem2 proteins that modulate the cleavage of the myostatin propeptide. A number of myostatin-feedback regulated gene products that are primarily regulatory were uncovered, including microRNA impacting central functions and Piezo proteins that make cationic current-controlling mechanosensitive ion channels. These important findings extend hypotheses of myostatin and physical activity master regulation of genes and gene pathways, impacting medical practices and therapies associated with muscle atrophy in humans and companion animal species and genome-enabled selection practices applied to food-production animal species.

Impact of heme and heme degradation products on vascular diameter in mouse visual cortex

BACKGROUND

Delayed cerebral vasospasm is the most common cause of mortality and severe neurological impairment in patients who survive subarachnoid hemorrhage. Despite improvements in the field of diagnostic imaging, options for prevention and medical treatment-primarily with the calcium channel antagonist nimodipine or hemodynamic manipulations-are insufficient. Previous studies have suggested that heme and bilirubin oxidation end products, originating from degraded hemoglobin around ruptured blood vessels, are involved in the development of vasospasm by inhibiting large conductance BKC a potassium channels in vascular smooth muscle cells. In this study, we identify individual heme degradation products regulating arteriolar diameter in dependence of BKC a channel activity.

METHODS AND RESULTS

Using differential interference contrast video microscopy in acute brain slices, we determined diameter changes of intracerebral arterioles in mouse visual cortex. In preconstricted vessels, the specific BKC a channel blockers paxilline and iberiotoxin as well as iron-containing hemin caused vasoconstriction. In addition, the bilirubin oxidation end product Z-BOX A showed a stronger vasoconstrictive potency than its regio-isomer Z-BOX B. Importantly, Z-BOX A had the same vasoconstrictive effect, independent of its origin from oxidative degradation or chemical synthesis. Finally, in slices of Slo1-deficient knockout mice, paxilline and Z-BOX A remained ineffective in changing arteriole diameter.

CONCLUSIONS

We identified individual components of the oxidative bilirubin degradation that led to vasoconstriction of cerebral arterioles. The vasoconstrictive effect of Z-BOX A and Z-BOX B was mediated by BKC a channel activity that might represent a signaling pathway in the occurrence of delayed cerebral vasospasm in subarachnoid hemorrhage patients.

A PLCγ1-dependent, force-sensitive signaling network in the myogenic constriction of cerebral arteries

Maintaining constant blood flow in the face of fluctuations in blood pressure is a critical autoregulatory feature of cerebral arteries. An increase in pressure within the artery lumen causes the vessel to constrict through depolarization and contraction of the encircling smooth muscle cells. This pressure-sensing mechanism involves activation of two types of transient receptor potential (TRP) channels: TRPC6 and TRPM4. We provide evidence that the activation of the γ1 isoform of phospholipase C (PLCγ1) is critical for pressure sensing in cerebral arteries. Inositol 1,4,5-trisphosphate (IP3), generated by PLCγ1 in response to pressure, sensitized IP3 receptors (IP3Rs) to Ca(2+) influx mediated by the mechanosensitive TRPC6 channel, synergistically increasing IP3R-mediated Ca(2+) release to activate TRPM4 currents, leading to smooth muscle depolarization and constriction of isolated cerebral arteries. Proximity ligation assays demonstrated colocalization of PLCγ1 and TRPC6 with TRPM4, suggesting the presence of a force-sensitive, local signaling network comprising PLCγ1, TRPC6, TRPM4, and IP3Rs. Src tyrosine kinase activity was necessary for stretch-induced TRPM4 activation and myogenic constriction, consistent with the ability of Src to activate PLCγ isoforms. We conclude that contraction of cerebral artery smooth muscle cells requires the integration of pressure-sensing signaling pathways and their convergence on IP3Rs, which mediate localized Ca(2+)-dependent depolarization through the activation of TRPM4.

Gastrointestinal tone; its genesis and contribution to the physical processes of digestion

BACKGROUND

Myogenic tone has long been recognised as an important component of gastrointestinal motility. Recent work has clarified the cellular mechanisms that engender tone and the neurogenic and mechanical stimuli that modulate it but has also highlighted cellular and regional specialisation in these mechanisms within the GI tract. Smooth muscle in all segments of the gut has the capability of latching, i.e. can generate ongoing specific rather than tetanic tone. This is likely modulated by both direct and indirect input from agonists such as acetylcholine and mechanoreceptors, the latter originating in ICC-IM, smooth muscle cells or elements of the ENS. Tonic contraction can occur in the absence of phasic contractions or concurrent with them, and it can modulate wall compliance and the capacity of particular segments, thereby affecting the level of on-flow and mixing, both luminal and adjacent to the mucosa.

PURPOSE

The review seeks to provide an overview of our understanding of the mechanism by which tone is generated and maintained, highlighting its modulation by neurogenic and mechanical stimuli, its mechanical consequences in the walls of the various segments of the gastrointestinal tract and its contribution to flow and mixing of contained digesta.

Regulation of cerebral blood flow

The control of cerebral blood flow is complex, and only beginning to be elucidated. Studies have identified three key regulatory paradigms. The first is cerebral pressure autoregulation, which maintains a constant flow in the face of changing cerebral perfusion pressure. Flow-metabolism coupling refers to the brains ability to vary blood flow to match metabolic activity. An extensive arborization of perivascular nerves also serves to modulate cerebral blood flow, so-called neurogenic regulation. Central to these three paradigms are two cell types: endothelium and astrocytes. The endothelium produces several vasoactive factors that are germane to the regulation of cerebral blood flow: nitric oxide, endothelium-dependent hyperpolarization factor, the eicosanoids, and the endothelins. Astrocytic foot processes directly abut the blood vessels, and play a key role in regulation of cerebral blood flow. Lastly, new research has been investigating cell-cell communication at the microvascular level. Several lines of evidence point to the ability of the larger proximal vessels to coordinate vasomotor responses downstream.

Expression and physiological roles of TRP channels in smooth muscle cells

Smooth muscles are widely distributed in mammal body through various systems such as circulatory, respiratory, gastro-intestinal and urogenital systems. The smooth muscle cell (SMC) is not only a contractile cell but is able to perform other important functions such as migration, proliferation, production of cytokines, chemokines, extracellular matrix proteins, growth factors and cell surface adhesion molecules. Thus, SMC appears today as a fascinating cell with remarkable plasticity that contributes to its roles in physiology and disease. Most of the SMC functions are dependent on a key event: the increase in intracellular calcium concentration ([Ca(2+)](i)). Calcium entry from the extracellular space is a major step in the elevation of [Ca(2+)](i) in SMC and involves a variety of plasmalemmal calcium channels, among them is the superfamily of transient receptor potential (TRP) proteins. TRPC (canonical), TRPM (melastatin), TRPV (vanilloid) and TRPP (polycystin), are widely expressed in both visceral (airways, gastrointestinal tract, uterus) and vascular (systemic and pulmonary circulation) smooth muscles. Mainly, TRPC, TRPV and TRPM are implicated in a variety of physiological and pathophysiological processes such as: SMC contraction, relaxation, growth, migration and proliferation; control of blood pressure, arterial myogenic tone, pulmonary hypertension, intestinal motility, gastric acidity, uterine activity during parturition and labor. Thus it is becoming evident that TRP are major element of SMC calcium homeostasis and, thus, appear as novel drug targets for a better management of diseases originating from SMC dysfunction.

Mechanosensitivity of STREX-lacking BKCa channels in the colonic smooth muscle of the mouse

Stretch sensitivity of Ca²(+)-activated large-conductance K(+) channels (BK(Ca)) has been observed in a variety of cell types and considered to be a potential mechanism in mechanoelectric transduction (MET). Mechanical stress is a major stimulator for the smooth muscle in the gastrointestinal (GI) tract. However, much about the role and mechanism of MET in GI smooth muscles remains unknown. The BK(Ca) shows a functional diversity due to intensive Slo I alternative splicing and different α/β-subunit assembly in various cells. The stress-regulated exon (STREX) insert is suggested to be an indispensable domain for the mechanosensitivity of BK(Ca). The purpose of this study was to determine whether the BK(Ca) in colonic myocytes of the adult mouse is sensitive to mechanical stimulation and whether the STREX insert is a crucial segment for the BK(Ca) mechanosensitivity. The α- and β1-subunit mRNAs and the α-subunit protein of the BK(Ca) channels were detected in the colonic muscularis. We found that the BK(Ca) STREX-lacking variant was abundantly expressed in the smooth muscle, whereas the STREX variant was not detectable. We demonstrated that the STREX-lacking BK(Ca) channels were also sensitive to membrane stretch. We suggest that in addition to the STREX domain, there are other additional structures in the channel responsible for mechanically coupling with the cell membrane.

Role of nonreceptor protein tyrosine kinases during phospholipase C-gamma 1-related uterine contractions in the rat

Activated phospholipase C1, produced in response to tyrosine phosphorylation, appears to play an important role during uterine contractions. These studies sought to determine which non-receptor protein tyrosine kinases are involved in the activation of phospholipase C1 in rat uterine tissue. In vitro contraction studies were performed utilizing isoform specific protein tyrosine kinase inhibitors. Western blots were performed utilizing antibodies to phosphotyrosine-phospholipase C1, total phospholipase C1, c-Src kinase and Lck kinase. Spontaneous, stretch-stimulated, and bpV(phen) (tyrosine phosphatase inhibitor) enhanced uterine contractions were significantly suppressed in response to Damnacanthal (Lck kinase inhibitor) and PP1 (c-Src kinase inhibitor). Damnacanthal and PP1 also significantly suppressed bpV(phen)-enhanced tyrosine phosphorylation of phospholipase C1. Western blots confirmed expression of Lck kinase and c-Src kinase in uterine tissue. In conclusion, the Lck and c-Src kinases appear to play an important role in regulating tyrosine phosphorylation of phospholipase C1 and contractile activity in the rat uterus.

Nitric oxide mediates stretch-induced Ca2+ release via activation of phosphatidylinositol 3-kinase-Akt pathway in smooth muscle

BACKGROUND

Hollow smooth muscle organs such as the bladder undergo significant changes in wall tension associated with filling and distension, with attendant changes in muscle tone. Our previous study indicated that stretch induces Ca(2+) release occurs in the form of Ca(2+) sparks and Ca(2+) waves in urinary bladder myocytes. While, the mechanism underlying stretch-induced Ca2+ release in smooth muscle is unknown.

METHODOLOGY/PRINCIPAL FINDINGS

We examined the transduction mechanism linking cell stretch to Ca(2+) release. The probability and frequency of Ca(2+) sparks induced by stretch were closely related to the extent of cell extension and the time that the stretch was maintained. Experiments in tissues and single myocytes indicated that mechanical stretch significantly increases the production of nitric oxide (NO) and the amplitude and duration of muscle contraction. Stretch-induced Ca(2+) sparks and contractility increases were abrogated by the NO inhibitor L-NAME and were also absent in eNOS knockout mice. Furthermore, exposure of eNOS null mice to exogenously generated NO induced Ca(2+) sparks. The soluble guanylyl cyclase inhibitor ODQ did not inhibit SICR, but this process was effectively blocked by the PI3 kinase inhibitors LY494002 and wortmannin; the phosphorylation of Akt and eNOS were up-regulated by 204+/-28.6% and 258+/-36.8% by stretch, respectively. Moreover, stretch significantly increased the eNOS protein expression level.

CONCLUSIONS/SIGNIFICANCE

Taking together, these results suggest that stretch-induced Ca2+ release is NO dependent, resulting from the activation of PI3K/Akt pathway in smooth muscle.

Characteristics of the myogenic behaviour of arteries of the common European frog (Rana temporaria)

Mammalian small arteries exhibit pressure-dependent myogenic behaviour characterised by an active constriction in response to an increased transmural pressure or an active dilatation in response to a decreased transmural pressure. This study aimed to determine whether pressure-dependent myogenic responses are a functional feature of amphibian arteries. Mesenteric and skeletal muscle arteries from the common European frog (Rana temporaria) were cannulated at either end with two fine glass micropipettes in the chamber of an arteriograph. Arterial pressure-diameter relationships (5-40 mmHg) were determined in the presence and absence of Ca2+. All arteries dilated passively with increasing pressure in the absence of Ca2+. In the presence of Ca2+ proximal mesenteric branches and tibial artery branches dilated with increasing transmural pressure but tone (p < 0.05) was evident in both arteries. A clear myogenic response to a step increase or decrease in pressure was observed in small distal arteries (6 of 13 mesenteric and 7 of 10 sciatic branches) resulting in significantly (p < 0.05) narrower diameters in Ca2+ in the range 10-40 mmHg in mesenteric and 20-40 mmHg in sciatic arteries, respectively. The results demonstrate that arteries of an amphibian can generate spontaneous pressure-dependent tone. This is the first study to demonstrate myogenic contractile behaviour in arteries of nonmammalian origin.

Cellular magnesium acquisition: an anomaly in embryonic cation homeostasis

The intracellular dominance of magnesium ion makes clinical assessment difficult despite the critical role of Mg(++) in many key functions of cells and enzymes. There is general consensus that serum Mg(++) levels are not representative of the growing number of conditions for which magnesium is known to be important. There is no consensus method or sample source for testing for clinical purposes. High intracellular Mg(++) in vertebrate embryos results in part from interactions of cations which influence cell membrane transport systems. These are functionally competent from the earliest stages, at least transiently held over from the unfertilized ovum. Kinetic studies with radiotracer cations, osmolar variations, media lacking one or more of the four biological cations, Na(+), Mg(++), K(+), and Ca(++), and metabolic poison 0.05 mEq/L NaF, demonstrated that: (1) all four cations influence the behavior of the others, and (2) energy is required for uptake and efflux on different time scales, some against gradient. Na(+) uptake is energy dependent against an efflux gradient. The rate of K(+) loss is equal with or without fluoride, suggesting a lack of an energy requirement at these stages. Ca(++) efflux took twice as long in the presence of fluoride, likely due in part to intracellular binding. Mg(++) is anomalous in that early teleost vertebrate embryos have an intracellular content exceeding the surrounding sea water, an isolated unaffected yolk compartment, and a clear requirement for energy for both uptake and efflux. The physiological, pathological, and therapeutic roles of magnesium are poorly understood. This will change: (1) when (28)Mg is once again generally available at a reasonable cost for both basic research and clinical assessment, and (2) when serum or plasma levels are determined simultaneously with intracellular values, preferably as part of complete four cation profiles. Atomic absorption spectrophotometry, energy-dispersive x-ray analysis, and inductively coupled plasma emission spectroscopy on sublingual mucosal and peripheral blood samples are potential methods of value for coordinated assessments.

Lipid stress at play: mechanosensitivity of voltage-gated channels

Membrane stretch modulates the activity of voltage-gated channels (VGCs). These channels are nearly ubiquitous among eukaryotes and they are present, too, in prokaryotes, so the potential ramifications of VGC mechanosensitivity are diverse. In situ traumatic stretch can irreversibly alter VGC activity with lethal results but that is pathology. This chapter discusses the reversible responses of VGCs to stretch, with the general relation of stretch stimuli to other forms of lipid stress, and briefly, with some irreversible stretch effects (=stretch trauma). A working assumption throughout is that mechanosensitive (MS) VGC motions-that is, motions that respond reversibly to bilayer stretch-are susceptible to other forms of lipid stress, such as the stresses produced when amphiphilic molecules (anesthetics, lipids, alcohols, and lipophilic drugs) are inserted into the bilayer. Insofar as these molecules change the bilayer's lateral pressure profile, they can be termed bilayer mechanical reagents (BMRs). The chapter also discusses the MS VGC behavior against the backdrop of eukaryotic channels more widely accepted as "MS channels"--namely, the transient receptor potential (TRP)-based MS cation channels.

Hypotonic stress influence the membrane potential and alter the proliferation of keratinocytes in vitro

Keratinocyte proliferation and differentiation is strongly influenced by mechanical forces. We investigated the effect of osmotic changes in the development of HaCaT cells in culture using intracellular calcium measurements, electrophysiological recordings and molecular biology techniques. The application of hypotonic stress (174 mOsmol/l) caused a sustained hyperpolarization of HaCaT cells from a resting potential of -27 +/- 4 to -51 +/- 9 mV. This change was partially reversible. The surface membrane channels involved in the hyperpolarization were identified as chloride channels due to the lack of response in the absence of the anion. Cells responded with an elevation of intracellular calcium concentration to hypotonic stress, which critically depended on external calcium. The presence of phorbol-12-myristate-13-acetate in the culture medium for 12 h augmented the subsequent response to hypotonic stress. A sudden switch from iso- to hypotonic solution increased cell proliferation and suppressed the production of involucrin, filaggrin and transglutaminase, markers of keratinocyte differentiation. It is concluded that sudden mechanical forces increase the proliferation of keratinocytes through alterations in their membrane potential and intracellular calcium concentration. These changes together with additional modifications in channel expression and intracellular signalling mechanisms could underlie the increased proliferation of keratinocytes in hyperproliferative skin diseases.

Mechanosensitive ion channels in interstitial cells of Cajal and smooth muscle of the gastrointestinal tract

Normal gastrointestinal (GI) motility is required to mix digestive enzymes and food and to move content along the GI tract. Underlying the complex motor patterns of the gut are electrical events that reflect ion flux across cell membranes. Smooth muscle electrical activity is directly influenced by GI interstitial cells of Cajal, whose rhythmic oscillations in membrane potential in part determine the excitability of GI smooth muscle and its response to neuronal input. Coordinated activity of the ion channels responsible for the conductances that underlie ion flux in both smooth muscle and interstitial cells is a requisite for normal motility. These conductances are regulated by many factors, including mechanical stress. Recent studies have revealed mechanosensitivity at the level of the ion channels, and the mechanosensor within the channel has been identified in many cases. This has led to better comprehension of the role of mechanosensitive conductances in normal physiology and will undoubtedly lead to understanding of the consequences of disturbances in these conductances.

Actin cytoskeletons regulate the stretch-induced increase of Ca2+ current in human gastric myocytes

Using the whole-cell and single channel recording techniques, the influence of actin cytoskeletons on L-type Ca2+ current was investigated in human gastric smooth muscle cells. In isotonic condition, an actin depolymerizer cytochalasin D (Cyt-D) markedly decreased the whole-cell current (I(Ba)) without changing steady-state voltage dependency and single channel conductance. Intracellular dialysis of phalloidin, an actin polymerizer, significantly increased the I(Ba). Hypotonic stretch (222 mOsm/L) of the myocytes increased the I(Ba), and Cyt-D significantly inhibited the I(Ba) increase by the stretch. Phalloidin was without effect on the I(Ba) increase by the stretch. Phalloidin antagonized the Cyt-D inhibition of the stretch-induced I(Ba) increase. Neither heterotrimeric G protein modifiers (GTPgammaS and GDPbetaS) nor rho GTPase inhibitor (C3 exoenzyme) influenced the stretch-induced responses. These results reveal that the integrity of the actin cytoskeleton is an important factor which determines the activity of L-type Ca2+ channels and a response to stretch.

A steady-state electrochemical model of vascular smooth muscle cells

A model of the steady-state electrochemical response of vascular smooth muscle cells to external stimuli is presented, which accounts for K, Na, and Ca fluxes. The results of the model are broadly in accordance with experimental data 1), at various transmural pressures; 2), with channel and pump blockade; and 3), under manipulation of external ionic concentrations. The model exhibits dual stable states which sometimes coexist, and abrupt transitions between these states may account for nongraded responses in arteries as external potassium or pressure is varied. The simulations suggest that changes in the intracellular sodium concentration ([Na]i) often accompany smooth muscle responses. For example, [Na]i values vary threefold over the range of pressures from 10 to 100 mmHg.

Two-pore-domain potassium channels in smooth muscles: new components of myogenic regulation

Gastrointestinal (GI) smooth muscles are influenced by many levels of regulation, including those provided by enteric motor neurones, hormones and paracrine substances. The integrated contractile responses to these regulatory mechanisms depend heavily on the state of excitability of smooth muscle cells. Resting ionic conductances and myogenic responses to agonists and physical parameters, such as stretch, are important in establishing basal excitability. This review discusses the role of 2-pore-domain K+ channels in contributing to background conductances and in mediating responses of GI muscles to enteric inhibitory nerve stimulation and stretch. Murine GI muscles express TREK-1 channels and display a stretch-dependent K+ (SDK) conductance that is also activated by nitric oxide via a cGMP-dependent mechanism. Cloning and expression of mTREK-1 produced an SDK conductance that was activated by cGMP-dependent phosphorylation at serine-351. GI muscle cells also express TASK-1 and TASK-2 channels that are inhibited by lidocaine and external acidification. These conductances appear to provide significant background K+ permeability that contributes to the negative resting potentials of GI muscles.

Ion channels in smooth muscle: regulators of intracellular calcium and contractility

Smooth muscle (SM) is essential to all aspects of human physiology and, therefore, key to the maintenance of life. Ion channels expressed within SM cells regulate the membrane potential, intracellular Ca2+ concentration, and contractility of SM. Excitatory ion channels function to depolarize the membrane potential. These include nonselective cation channels that allow Na+ and Ca2+ to permeate into SM cells. The nonselective cation channel family includes tonically active channels (Icat), as well as channels activated by agonists, pressure-stretch, and intracellular Ca2+ store depletion. Cl--selective channels, activated by intracellular Ca2+ or stretch, also mediate SM depolarization. Plasma membrane depolarization in SM activates voltage-dependent Ca2+ channels that demonstrate a high Ca2+ selectivity and provide influx of contractile Ca2+. Ca2+ is also released from SM intracellular Ca2+ stores of the sarcoplasmic reticulum (SR) through ryanodine and inositol trisphosphate receptor Ca2+ channels. This is part of a negative feedback mechanism limiting contraction that occurs by the Ca2+-dependent activation of large-conductance K+ channels, which hyper polarize the plasma membrane. Unlike the well-defined contractile role of SR-released Ca2+ in skeletal and cardiac muscle, the literature suggests that in SM Ca2+ released from the SR functions to limit contractility. Depolarization-activated K+ chan nels, ATP-sensitive K+ channels, and inward rectifier K+ channels also hyperpolarize SM, favouring relaxation. The expression pattern, density, and biophysical properties of ion channels vary among SM types and are key determinants of electrical activity, contractility, and SM function.

Cardiac transplantation and resistance artery myogenic tone

Transplantation is an effective treatment for end-stage heart disease; however, most grafts eventually fail by progressive cardiac failure. Primarily, failure is ischemic due to the occlusive nature of transplant vascular disease (TVD). Early after transplantation and preceding TVD, alterations in coronary physiology such as reduced vascular myogenic tone occur. Resistance arteries possess an inherent ability to constrict in response to transmural pressure; this constrictive response (myogenic tone) is important in fluid homeostasis. Recent evidence suggests that a decline in myogenic tone leads to deficits in cardiac contractility. Factors that reduce myogenic tone in transplantation include constitutive nitric oxide synthase and inducible nitric oxide synthase catalyzed, NO-mediated vasodilation as well as deficits in arterial contractile function. Reduced myogenic tone in allograft resistance arteries increases coronary blood flow such that hydrostatic pressure surpasses oncotic pressure, causing cardiac interstitial edema. This generalized edema decreases ventricular compliance leading to heart failure during the course of acute immune rejection of the graft. Cyclosporine A treatment reduces immune mediated dysregulation of myogenic tone, resulting in reduced interstitial edema and improved cardiac function. In this review, we discuss aspects of TVD and myogenic tone signaling mechanisms and how aberrations in myogenic regulation of arterial tone contribute to functional changes observed in cardiac transplant.

Vascular ENaC proteins are required for renal myogenic constriction

The myogenic response is an essential component of renal blood flow autoregulation and is the inherent ability of vascular smooth muscle cells (VSMCs) to contract in response to increases in intraluminal pressure. Although mechanosensitive ion channels are thought to initiate VSMC stretch-induced contraction, their molecular identity is unknown. Recent reports suggest degenerin/epithelial Na(+) channels (DEG/ENaC) may form mechanotransducers in sensory neurons and VSMCs; however, the role of DEG/ENaC proteins in myogenic constriction of mouse renal arteries has not been established. To test the hypothesis that DEG/ENaC proteins are required for myogenic constriction in renal vessels, we first determined expression of ENaC transcripts and proteins in mouse renal VSMCs. Then, we determined pressure- and agonist-induced constriction and changes in vascular smooth muscle cytosolic Ca(2+) and Na(+) in isolated mouse renal interlobar arteries following DEG/ENaC inhibition with amiloride and benzamil. We detect alpha-, beta-, and gammaENaC transcript and protein expression in cultured mouse renal VSMC. In contrast, we detect only beta- and gamma- but not alphaENaC protein in freshly dispersed mrVMSC. Selective DEG/ENaC inhibition, with low doses of amiloride and benzamil, abolishes pressure-induced constriction and increases in cytosolic Ca(2+) and Na(+) without diminishing agonist-induced responses in isolated mouse interlobar arteries. Our findings indicate that DEG/ENaC proteins are required for myogenic constriction in mouse interlobar arteries and are consistent with our hypothesis that DEG/ENaC proteins may be components of mechanosensitive ion channel complexes required for myogenic vasoconstriction.

Characterization of stretch-activated calcium permeable cation channels in freshly isolated myocytes of the cricket (Gryllus bimaculatus) lateral oviduct

Stretch-activated channels (SACs) were investigated in myocytes isolated from the lateral oviduct in cricket Gryllus bimaculatus using the cell-attached or excised inside-out patch clamp technique. Application of both negative and positive pressure (10-100 cm H(2)O) into the patch pipettes induced the unitary channel current openings. The open probability (NPo) of the channel increased when negative pressure applied into the patch pipettes increased. The single channel conductance for this channel was approximately 20 pS with 140 mM Na(+), K(+), or Cs(+) in the patch pipettes and was approximately 13 pS with 100mM Ca(2+) or Ba(2+) in the patch pipettes. External application of Gd(3+), La(3+), Cd(2+) and Zn(2+)inhibited the channel with the IC(50) values of 14, 15, 28, and 18 microM respectively. Interestingly external application of TEA, a specific blocker of K(+) channel, also inhibited this channel with IC(50) value of 8.8mM. These results show for the first time the presence of stretch activated Ca(2+)-permeable nonselective cation channel in myocytes isolated from the cricket lateral oviduct. The physiological significance of this channel in oviposition behavior is discussed.

Stretch-activated channels: a mini-review. Are stretch-activated channels an ocular barometer?

All cells are subject to physical forces by virtue of their position in a dynamically changing environment. This review outlines the various putative 'mechanosensors', or sensors of pressure cells possess, and discusses in particular the role stretch-activated membrane channels play in pressure recognition and transduction. The widespread occurrence of these channels is discussed and these 'mechanosensors' are related to pressure-related diseases, in particular, glaucoma.

Identification and spatiotemporal characterization of spontaneous Ca2+ sparks and global Ca2+ oscillations in retinal arteriolar smooth muscle cells

PURPOSE

To identify spontaneous Ca(2+) sparks and global Ca(2+) oscillations in microvascular smooth muscle (MVSM) cells within intact retinal arterioles and to characterize their spatiotemporal properties and physiological functions.

METHODS

Retinal arterioles were mechanically dispersed from freshly isolated rat retinas and loaded with Fluo-4, a Ca(2+)-sensitive dye. Changes in [Ca(2+)](i) were imaged in MVSM cells in situ by confocal scanning laser microscopy in x-y mode or line-scan mode.

RESULTS

The x-y scans revealed discretely localized, spontaneous Ca(2+) events resembling Ca(2+) sparks and more global and prolonged Ca(2+) transients, which sometimes led to cell contraction. In line scans, Ca(2+) sparks were similar to those previously described in other types of smooth muscle, with an amplitude (DeltaF/F(0)) of 0.81 +/- 0.04 (mean +/- SE), full duration at half maximum (FDHM) of 23.62 +/- 1.15 ms, full width at half maximum (FWHM) of 1.25 +/- 0.05 mum, and frequency of 0.56 +/- 0.06 seconds(-1). Approximately 35% of sparks had a prolonged tail (>80 ms), similar to the Ca(2+)"embers" described in skeletal muscle. Sparks often summated to generate global and prolonged Ca(2+) elevations on which Ca(2+) sparks were superimposed. These sparks occurred more frequently (2.86 +/- 025 seconds(-1)) and spread farther across the cell (FWHM = 1.67 +/- 0.08 microm), but were smaller (DeltaF/F(0) = 0.69 +/- 0.04).

CONCLUSIONS

Retinal arterioles generate Ca(2+) sparks with characteristics that vary during different phases of the spontaneous Ca(2+)-signaling cycle. Sparks summate to produce sustained Ca(2+) transients associated with contraction and thus may play an important excitatory role in initiating vessel constriction. This deserves further study, not least because Ca(2+) sparks appear to inhibit contraction in many other smooth muscle cells.

Ionic mechanism for contractile response to hyposmotic challenge in canine basilar arteries

A hyposmotic challenge elicited contraction of isolated canine basilar arteries. The contractile response was nearly abolished by the removal of extracellular Ca(2+) and by the voltage-dependent Ca(2+) channel (VDCC) blocker nicardipine, but it was unaffected by thapsigargin, which depletes intracellular Ca(2+) stores. The contraction was also inhibited by Gd(3+) and ruthenium red, cation channel blockers, and Cl(-) channel blockers DIDS and niflumic acid. The reduction of extracellular Cl(-) concentrations enhanced the hypotonically induced contraction. Patch-clamp analysis showed that a hyposmotic challenge activated outwardly rectifying whole cell currents in isolated canine basilar artery myocytes. The reversal potential of the current was shifted toward negative potentials by reductions in intracellular Cl(-) concentration, indicating that the currents were carried by Cl(-). Moreover, the currents were abolished by 10 mM BAPTA in the pipette solution and by the removal of extracellular Ca(2+). Taken together, these results suggest that a hyposmotic challenge activates cation channels, which presumably cause Ca(2+) influx, thereby activating Ca(2+)-activated Cl(-) channels. The subsequent membrane depolarization is likely to increase Ca(2+) influx through VDCC and elicit contraction.

Non-selective cationic channels of smooth muscle and the mammalian homologues of Drosophila TRP

Throughout the body there are smooth muscle cells controlling a myriad of tubes and reservoirs. The cells show enormous diversity and complexity compounded by a plasticity that is critical in physiology and disease. Over the past quarter of a century we have seen that smooth muscle cells contain--as part of a gamut of ion-handling mechanisms--a family of cationic channels with significant permeability to calcium, potassium and sodium. Several of these channels are sensors of calcium store depletion, G-protein-coupled receptor activation, membrane stretch, intracellular Ca2+, pH, phospholipid signals and other factors. Progress in understanding the channels has, however, been hampered by a paucity of specific pharmacological agents and difficulty in identifying the underlying genes. In this review we summarize current knowledge of these smooth muscle cationic channels and evaluate the hypothesis that the underlying genes are homologues of Drosophila TRP (transient receptor potential). Direct evidence exists for roles of TRPC1, TRPC4/5, TRPC6, TRPV2, TRPP1 and TRPP2, and more are likely to be added soon. Some of these TRP proteins respond to a multiplicity of activation signals--promiscuity of gating that could enable a variety of context-dependent functions. We would seem to be witnessing the first phase of the molecular delineation of these cationic channels, something that should prove a leap forward for strategies aimed at developing new selective pharmacological agents and understanding the activation mechanisms and functions of these channels in physiological systems.

Imaging calcium entering the cytosol through a single opening of plasma membrane ion channels: SCCaFTs—fundamental calcium events

Recently, it has become possible to record the localized fluorescence transient associated with the opening of a single plasma membrane Ca(2+) permeable ion channel using Ca(2+) indicators like fluo-3. These Single Channel Ca(2+) Fluorescence Transients (SCCaFTs) share some of the characteristics of such elementary events as Ca(2+) sparks and Ca(2+) puffs caused by Ca(2+) release from intracellular stores (due to the opening of ryanodine receptors and IP(3) receptors, respectively). In contrast to intracellular Ca(2+) release events, SCCaFTs can be observed while simultaneously recording the unitary channel currents using patch-clamp techniques to verify the channel openings. Imaging SCCaFTs provides a way to examine localized Ca(2+) handling in the vicinity of a channel with a known Ca(2+) influx, to obtain the Ca(2+) current passing through plasma membrane cation channels in near physiological solutions, to localize Ca(2+) permeable ion channels on the plasma membrane, and to estimate the Ca(2+) currents underlying those elementary events where the Ca(2+) currents cannot be recorded. Here we review studies of these fluorescence transients associated with caffeine-activated channels, L-type Ca(2+) channels, and stretch-activated channels. For the L-type Ca(2+) channel, SCCaFTs have been termed sparklets. In addition, we discuss how SCCaFTs have been used to estimate Ca(2+) currents using the rate of rise of the fluorescence transient as well as the signal mass associated with the total fluorescence increase.

Analysis of factors that determine the compliance of rat jejunum to distension in vivo

Distension of the intestine is commonly used to elicit reflex responses at other sites in the gastrointestinal tract, and also to evaluate pain of intestinal origin. The sensory neurones, that initiate the reflexes or pain responses, react to the forces generated in the wall of the intestine. Thus, the responses of the intestine at the site of distension, particularly changes in contractile activity, influence the signals from the gut. In the present work we have analysed the relationship between distension and pressure changes in the jejunum of the rat, in vivo. Isovolumic distension for 5 min caused an initial pressure increase which declined quickly in the first 30 s, and then declined more slowly. Phasic pressure increases were superimposed on the baseline pressure change. Hexamethonium blocked the phasic pressure increases, whereas the initial rapid and subsequent slower pressure decline during distension persisted. Inhibition of nitric oxide synthase (NOS) increased intraluminal pressure and caused increased frequency and irregularity of phasic pressure increases. However, the decline in jejunal pressure during distension was not changed by inhibition of NOS. The pressure decline during isovolumic distension was similar whether saline or paraffin oil were used to distend the intestine, indicating that the decline was not due to increased hydrostatic pressure causing water and electrolyte to cross the mucosal epithelium from the lumen to the intestinal interstitium. Hyoscine had no significant effect on the pressure profile when the intestine was distended. However, when the systemic or the local circulation of the jejunum was infused with nicardipine, the pressure that was achieved during isovolumic distension was less, although the rate of change in pressure during the slow decline was similar. It is concluded that distension evokes phasic pressure increases in the jejunum, that are nerve-mediated, and increases the tension in the wall through a stretch-activated increase in contractile force generated by the circular muscle. The decline in pressure during maintained distension is primarily a consequence of visco-elastic properties of the wall of the intestine.

Mechanobiology of force transduction in dermal tissue

BACKGROUND/AIMS

The influence of mechanical forces on skin has been examined since 1861 when Langer first reported the existence of lines of tension in cadaver skin. Internal tension in the dermis is not only passively transferred to the epidermis but also gives rise to active cell-extracellular matrix and cell-cell mechanical interactions that may be an important part of the homeostatic processes that are involved in normal skin metabolism. The purpose of this review is to analyse how internal and external mechanical loads are applied at the macromolecular and cellular levels in the epidermis and dermis.

METHODS

A review of the literature suggests that internal and external forces applied to dermal cells appear to be involved in mechanochemical transduction processes involving both cell-cell and cell-extra-cellular matrix (ECM) interactions. Internal forces present in dermis are the result of passive tension that is incorporated into the collagen fiber network during development. Active tension generated by fibroblasts involves specific interactions between cell membrane integrins and macromolecules found in the ECM, especially collagen fibrils. Forces appear to be transduced at the cell-ECM interface via re-arrangement of cytoskeletal elements, activation of stretch-induced changes in ion channels, cell contraction at adherens junctions, activation of cell membrane-associated secondary messenger pathways and through growth factor-like activities that influence cellular proliferation and protein synthesis.

CONCLUSIONS

Internal and external mechanical loading appears to affect skin biology through mechanochemical transduction processes. Further studies are needed to understand how mechanical forces, energy storage and conversion of mechanical energy into changes in chemical potential of small and large macromolecules may occur and influence the metabolism of dermal cells.

20-Hydroxyeicosatetraenoic acid potentiates stretch-induced contraction of canine basilar artery via PKC alpha-mediated inhibition of KCa channel

1. The present study was undertaken to elucidate whether PKCalpha plays a role in the mechanism of the stretch-induced contraction potentiated by 20-hydroxyeicosatetraenoic acid (20-HETE). The effects of 20-HETE on the canine basilar artery were compared with those of iberiotoxin, a blocker of large conductance Ca(2+)-activated K(+) channels (K(Ca) channels), as this blocker was shown earlier to sensitize these arteries to mechanical stretch. 2. Slow stretch at rates of 0.1 to 3 mm s(-1) did not produce any contraction in normal physiological solution. 3. In the presence of 20-HETE, the slow stretch could produce contraction, which was inhibited by nicardipine, a 1,4-dihydropyridine Ca(2+) channel blocker, and gadolinium, a blocker of stretch-activated cation channels. 4. 20-HETE inhibited whole-cell K(+) current and depolarized the membrane by approximately 10 mV. These effects of 20-HETE were similar to those of iberiotoxin. 5. Calphostin C, an inhibitor of protein kinase C (PKC), inhibited the action of 20-HETE, but not that of iberiotoxin. 6. In response to 20-HETE PKCalpha isoform was translocated from the cytosol to the membrane fraction, which translocation was inhibited by calphostin C. 7. These results suggest that 20-HETE induced sensitization of the canine basilar artery to stretch was caused by PKCalpha-mediated inhibition of K(Ca) channel activity.

The structure and function of normally mineralizing avian tendons

The leg tendons of certain avian species normally calcify. The gastrocnemius, or Achilles, tendon of the domestic turkey, Meleagris gallopavo, is one such example. Its structure and biomechanical properties have been studied to model the adaptive nature of this tendon to external forces, including the means by which mineral deposition occurs and the functional role mineralization may play in this tissue. Structurally, the distal rounded, thick gastrocnemius bifurcates into two smaller proximal segments that mineralize with time. Mineral deposition occurs at or near the bifurcation, proceeding in a distal-to-proximal direction along the segments toward caudal and medial muscle insertions of the bird hip. Mineral formation appears mediated first by extracellular matrix vesicles and later by type I collagen fibrils. Biomechanical analyses indicate lower tensile strength and moduli for the thick distal gastrocnemius compared to narrow, fan-shaped proximal segments. Tendon mineralization here appears to be strain-induced, the muscle forces causing matrix deformation leading conceptually to calcium binding through the exposure of charged groups on collagen, release of sequestered calcium by proteoglycans, and increased diffusion. Functionally, the mineralized tendons limit further tendon deformation, reduce tendon strain at a given stress, and provide greater load-bearing capacity to the tissue. They also serve as important and efficient elastic energy storage reservoirs, increasing the amount of stored elastic energy by preventing flexible type I collagen regions from stretching and preserving muscle energy during locomotion of the animals.

alpha(1C) (Ca(V)1.2) L-type calcium channel mediates mechanosensitive calcium regulation

Smooth muscle exhibits mechanosensitivity independent of neural input, suggesting that mechanosensitive pathways reside within smooth muscle cells. The native L-type calcium current recorded from human intestinal smooth muscle is modulated by stretch. To define mechanosensitive mechanisms involved in the regulation of smooth muscle calcium entry, we cloned the alpha(1C) L-type calcium channel subunit (Ca(V)1.2) from human intestinal smooth muscle and expressed the channel in a heterologous system. This channel subunit retained mechanosensitivity when expressed alone or coexpressed with a beta(2) calcium channel subunit in HEK-293 or Chinese hamster ovary cells. The heterologously expressed human cardiac alpha(1C) splice form also demonstrated mechanosensitivity. Inhibition of kinase signaling did not affect mechanosensitivity of the native channel. Truncation of the alpha(1C) COOH terminus, which contains an inhibitory domain and a proline-rich domain thought to mediate mechanosensitive signaling from integrins, did not disrupt mechanosensitivity of the expressed channel. These data demonstrate mechanical regulation of calcium entry through molecularly identified L-type calcium channels in mammalian cells and suggest that the mechanosensitivity resides within the pore forming alpha(1C)-subunit.

Hyposmotic membrane stretch potentiated muscarinic receptor agonist-induced depolarization of membrane potential in guinea-pig gastric myocytes

AIM: To investigate the relationship between hyposmotic membrane stretch and muscarinic receptor agonist-induced depolarization of membrane potential in antral gastric circular myocytes of guinea-pig.

METHODS: Using whole cell patch-clamp technique recorded membrane potential and current in single gastric myocytes isolated by collagenase.

RESULTS: Hyposmotic membrane stretch hyperpolarized membrane potential from -60.0 mV ± 1.0 mV to -67.9 mV ± 1.0 mV. TEA (10 mmol/L), a nonselective potassium channel blocker significantly inhibited hyposmotic membrane stretch-induced hyperpolarization. After KCl in the pipette and NaCl in the external solution were replaced by CsCl to block the potassium current, hyposmotic membrane stretch depolarized the membrane potential from -60.0 mV ± 1.0 mV to -44.8 mV ± 2.3 mV (P < 0.05), and atropine (1 μmol/L) inhibited the depolarization of the membrane potential. Muscarinic receptor agonist Carbachol depolarized membrane potential from -60.0 mV ± 1.0 mV to -50.3 mV ± 0.3 mV (P < 0.05) and hyposmotic membrane stretch potentiated the depolarization. Carbachol induced muscarinic current (I_cch) was greatly increased by hyposmotic membrane stretch.

CONCLUSION: Hyposmotic membrane stretch potentiated muscarinic receptor agonist-induced depolarization of membrane potential, which is related to hyposmotic membrane stretch-induced increase of muscarinic current.

Cellular signalling in arteriolar myogenic constriction: involvement of tyrosine phosphorylation pathways

1. An increase in transmural pressure in arterioles results in a shortening of vascular smooth muscle cells, with subsequent constriction of the vessel. The mechanisms underlying this myogenic contraction are not fully understood; however, the obligatory role of increases in intracellular [Ca(2+)] and myosin light chain phosphorylation have been demonstrated. 2. The myogenic response shows a relationship with smooth muscle cell membrane potential and influx of extracellular Ca(2+) through voltage-operated Ca(2+) channels (VOCC). Mechanically sensitive channels and possibly release of Ca(2+) from intracellular stores may play a role. However, there are other components of myogenic contraction that cannot be explained by a Ca(2+)-MLCK mechanism, for example the initial sensing of alterations in transmural pressure, whether sustained myogenic constriction involves myofilament Ca(2+) sensitization or remodelling of the vessel wall in response to a maintained increase in transmural pressure. 3. In an attempt to investigate these areas, recent studies have examined a role for tyrosine phosphorylation pathways in pressure-induced contraction of arterioles. In rat pressurized cremaster arterioles, tyrosine kinase inhibitors dilated vessels showing spontaneous myogenic tone and tyrosine phosphatase inhibitors caused vasoconstriction. However, pressure-induced myogenic constriction of vessels persisted in the presence of these agents. Biochemical studies revealed that phosphotyrosine formed at a relatively slow rate (significant after 5 min, with maximal increase after approximately 15 min) in response to increased vessel transmural pressure, in contrast with myosin light chain phosphorylation or the time-course of myogenic constriction itself (maximum within 1 min). 4. Taken together, these observations support the idea of a role for tyrosine phosphorylation pathways in longer-term responses to increased transmural pressure rather than acute myogenic constriction. Phosphotyrosine formation was also more closely correlated to vessel wall tension (pressure x diameter) than the diameter of the arterioles alone. The identity of the tyrosine-phosphorylated proteins requires further investigation; however, there is some evidence supporting roles for cSrc-type tyrosine kinases and p44 mitogen-activated protein kinase. The longer-term responses of blood vessels to increased transmural pressure that may involve tyrosine phosphorylation pathways include maintenance of myogenic constriction and vessel wall remodelling.

Stretch-induced calcium release in smooth muscle

Smooth muscle cells undergo substantial increases in length, passively stretching during increases in intraluminal pressure in vessels and hollow organs. Active contractile responses to counteract increased transmural pressure were first described almost a century ago (Bayliss, 1902) and several mechanisms have been advanced to explain this phenomenon. We report here that elongation of smooth muscle cells results in ryanodine receptor-mediated Ca(2+) release in individual myocytes. Mechanical elongation of isolated, single urinary bladder myocytes to approximately 120% of slack length (DeltaL = 20) evoked Ca(2+) release from intracellular stores in the form of single Ca(2+) sparks and propagated Ca(2+) waves. Ca(2+) release was not due to calcium-induced calcium release, as release was observed in Ca(2+)-free extracellular solution and when free Ca(2+) ions in the cytosol were strongly buffered to prevent increases in [Ca(2+)](i). Stretch-induced calcium release (SICR) was not affected by inhibition of InsP(3)R-mediated Ca(2+) release, but was completely blocked by ryanodine. Release occurred in the absence of previously reported stretch-activated currents; however, SICR evoked calcium-activated chloride currents in the form of transient inward currents, suggesting a regulatory mechanism for the generation of spontaneous currents in smooth muscle. SICR was also observed in individual myocytes during stretch of intact urinary bladder smooth muscle segments. Thus, longitudinal stretch of smooth muscle cells induces Ca(2+) release through gating of RYR. SICR may be an important component of the physiological response to increases in luminal pressure in smooth muscle tissues.