Stretch-induced Ca(2+) release via an IP(3)-insensitive Ca(2+) channel

Basal Vascular Smooth Muscle Cell Tone in eNOS Knockout Mice Can Be Reversed by Cyclic Stretch and Is Independent of Age

Introduction and Aims: Endothelial nitric oxide synthase (eNOS) knockout mice develop pronounced cardiovascular disease. In the present study, we describe the alterations in aortic physiology and biomechanics of eNOS knockout and C57Bl/6 control mice at 2–12 months of age, including a thorough physiological investigation of age and cyclic stretch-dependent VSMC contractility and aortic stiffness.

Methods and Results: Peripheral blood pressure and aortic pulse wave velocity were measured in vivo, and aortic biomechanical studies and isometric contractions were investigated ex vivo. Age-dependent progression of aortic stiffness, peripheral hypertension, and aortic contractility in eNOS knockout mice was absent, attenuated, or similar to C57Bl/6 control mice. Voltage-gated calcium channel (VGCC)-dependent calcium influx inversely affected isometric contraction and aortic stiffening by α₁-adrenergic stimulation in eNOS knockout mice. Baseline aortic stiffness was selectively reduced in eNOS knockout mice after ex vivo cyclic stretch exposure in an amplitude-dependent manner, which prompted us to investigate cyclic stretch dependent regulation of aortic contractility and stiffness. Aortic stiffness, both in baseline conditions and after activation of vascular smooth muscle cell (VSMC) contraction, was reduced with increasing cyclic stretch amplitude. This cyclic stretch dependency was attenuated with age, although aged eNOS knockout mice displayed better preservation of cyclic stretch-dependency compared to C57Bl/6 control mice. Store operated calcium entry-medicated aortic stiffening as induced by inhibiting sarcoplasmic reticulum calcium ATPase pumps with 10 µM CPA was most pronounced in the aorta of aged mice and at low cyclic stretch amplitude, but independent of eNOS. Basal aortic tonus and VSMC depolarization were highly dependent on eNOS, and were most pronounced at low cyclic stretch, with attenuation at increasing cyclic stretch amplitude.

Conclusion: eNOS knockout mice display attenuated progression of arterial disease as compared to C57Bl/6 control mice. Basal VSMC tone in eNOS knockout mice could be reduced by ex vivo exposure to cyclic stretch through stretch-dependent regulation of cytosolic calcium. Both baseline and active aortic stiffness were highly dependent on cyclic stretch regulation, which was more pronounced in young versus aged mice. Other mediators of VSMC contraction and calcium handling were dependent on cyclic stretch mechanotransduction, but independent of eNOS.

The Phenotypic Responses of Vascular Smooth Muscle Cells Exposed to Mechanical Cues

During the development of atherosclerosis and other vascular diseases, vascular smooth muscle cells (SMCs) located in the intima and media of blood vessels shift from a contractile state towards other phenotypes that differ substantially from differentiated SMCs. In addition, these cells acquire new functions, such as the production of alternative extracellular matrix (ECM) proteins and signal molecules. A similar shift in cell phenotype is observed when SMCs are removed from their native environment and placed in a culture, presumably due to the absence of the physiological signals that maintain and regulate the SMC phenotype in the vasculature. The far majority of studies describing SMC functions have been performed under standard culture conditions in which cells adhere to a rigid and static plastic plate. While these studies have contributed to discovering key molecular pathways regulating SMCs, they have a significant limitation: the ECM microenvironment and the mechanical forces transmitted through the matrix to SMCs are generally not considered. Here, we review and discuss the recent literature on how the mechanical forces and derived biochemical signals have been shown to modulate the vascular SMC phenotype and provide new perspectives about their importance.

Teucrium polium-induced Vasorelaxation Mediated by Endothelium-dependent and Endothelium-independent Mechanisms in Isolated Rat Thoracic Aorta

Objective:

There are some reports on hypotensive and antispasmodic effects of Teucrium polium L. (Lamiaceae) (TP).

Subjects and Methods:

The activity of different concentrations of TP extract (1, 2, 4 and 8 mg/ml) was evaluated on contractile responses of isolated aorta to potassium chloride (KCl) and phenylephrine (PE).

Results:

The cumulative concentrations of the extract induced a concentration-dependent relaxation in the aorta precontracted by PE and KCl. Extract-induced vasorelaxations in denuded aortic rings precontracted by PE and KCl at lower concentrations were considerably less than intact aortic rings, but this effect was significantly more at concentrations of 4 mg/ml for PE-, 4 and 8 mg/ml for KCl-induced contractions. All the extract concentrations (except 1 mg/ml) significantly relaxed PE-induced contraction in the presence of N^G-nitro-L-arginine methyl ester. Indomethacin reduced effectively extract-induced vasorelaxation at 1 and 2 mg/ml. The extract reduced PE- and KCl-induced contractions in the presence of cumulative calcium concentrations and after incubation with diltiazem; this vasorelaxant effect of TP was decreased. TP-induced relaxation was inhibited by heparin, ruthenium red, glibenclamide, and tetraethylammonium, but 4-aminopyridine had no effect on TP-induced relaxation.

Conclusion:

TP extract has vasorelaxant effect on isolated rat thoracic aorta which mediated by endothelium-dependent and endothelium-independent mechanisms. The relaxation mainly was mediated by inhibition of calcium influx in vascular smooth muscle cells. It seems that the vasorelaxant effect of extract at lower concentrations was mediated by nitric oxide and prostacyclin.

SUMMARY

The vasodilatory effect of Teucrium polium L. was mediated by several mechanisms. First: Teucrium polium L. inhibited receptor operated ROCC and VDCC. Second: Teucrium polium L. also inhibited KATP and KCa channels. Third: Teucrium polium L. blocked IP3 receptor and reduced the release of calcium from intracellular source. Forth: Teucrium polium L. increased the release on NO and PGI2 from endothelial cells.

Abbreviations Used: ROCC: Receptor operated calcium channels, VDCC: Voltage dependent calcium channels, PLC: Phospholipase C, IP3: 1,4,5 triphosphate inositol, IP3R: IP3 receptors, SR: sarcoplasmic reticulum, RYR: ryanodine receptors, K+ATP: ATP-sensitive potassium channel, K+Ca: Calcium-activated potassium channel, cAMP: Cyclic adenosine monophosphate, cGMP: Cyclic guanosine monophosphate, PGI2: Prostaglandin I2, NO: Nitric oxide

Design of a Novel Equi-Biaxial Stretcher for Live Cellular and Subcellular Imaging

Cells in the body experience various mechanical stimuli that are often essential to proper cell function. In order to study the effects of mechanical stretch on cell function, several devices have been built to deliver cyclic stretch to cells; however, they are generally not practical for live cell imaging. We introduce a novel device that allows for live cell imaging, using either an upright or inverted microscope, during the delivery of cyclic stretch, which can vary in amplitude and frequency. The device delivers equi-biaxial strain to cells seeded on an elastic membrane via indentation of the membrane. Membrane area strain was calibrated to indenter depth and the device showed repeatable and accurate delivery of strain at the scale of individual cells. At the whole cell level, changes in intracellular calcium were measured at different membrane area strains, and showed an amplitude-dependent response. At the subcellular level, the mitochondrial network was imaged at increasing membrane area strains to demonstrate that stretch can lead to mitochondrial fission in lung fibroblasts. The device is a useful tool for studying transient as well as long-term mechanotransduction as it allows for simultaneous stretching and imaging of live cells in the presence of various chemical stimuli.

Relaxation of rat aorta by farrerol correlates with potency to reduce intracellular calcium of VSMCs

Farrerol, isolated from Rhododendron dauricum L., has been proven to be an important multifunctional physiologically active component, but its vasoactive mechanism is not clear. The present study was performed to observe the vasoactive effects of farrerol on rat aorta and to investigate the possible underlying mechanisms. Isolated aortic rings of rat were mounted in an organ bath system and the myogenic effects stimulated by farrerol were studied. Intracellular Ca²⁺ ([Ca²⁺]_in) was measured by molecular probe fluo-4-AM and the activities of L-type voltage-gated Ca²⁺ channels (LVGC) were studied with whole-cell patch clamp in cultured vascular smooth muscle cells (VSMCs). The results showed that farrerol significantly induced dose-dependent relaxation on aortic rings, while this vasorelaxation was not affected by N^G-nitro-l-arginine methylester ester or endothelium denudation. In endothelium-denuded aortas, farrerol also reduced Ca²⁺-induced contraction on the basis of the stable contraction induced by KCl or phenylephrine (PE) in Ca²⁺-free solution. Moreover, after incubation with verapamil, farrerol can induce relaxation in endothelium-denuded aortas precontracted by PE, and this effect can be enhanced by ruthenium red, but not by heparin. With laser scanning confocal microscopy method, the farrerol-induced decline of [Ca²⁺]_in in cultured VSMCs was observed. Furthermore, we found that farrerol could suppress Ca²⁺ influx via LVGC by patch clamp technology. These findings suggested that farrerol can regulate the vascular tension and could be developed as a practicable vasorelaxation drug.

Mechanism of vasorelaxation induced by Achillea wilhelmsii in rat isolated thoracic aorta

BACKGROUND

Achillea wilhelmsii (A. wilhelmsii) is used in Iraninan folk medicine for the treatment of hypertension; also, in previous reports, the hypotensive and antihypertensive effects of this plant have been indicated. The aim of the present study is to investigate the vasorelaxant effect of the hydroalcholic extract of A. wilhelmsii and its underlying mechanisms in isolated rat aorta.

MATERIALS AND METHODS

The effect of the hydroalcholic A. wilhelmsii extract was tested on the contractile response of Wistar rat aorta induced by potassium chloride (KCl) and phenylephrine (PE) using a pressure transducer that is connected to the PowerLab.

RESULTS

The cumulative concentrations of A. wilhelmsii (0.5-8 mg/ml) induced a vasorelaxation both in endothelium-intact and endothelium-denuded aortas precontracted by high K(+) (6 × 10(-2) M) or 10(-6) M PE. A. wilhelmsii, at a concentration of 4 mg/ml, reduced Ca(2+)-induced contraction (P < 0.001 vs. control) after PE or KCl had generated a stable contraction in the Ca(2+)-free solution. Furthermore, after incubation with diltiazem, the vasorelaxant effect of A. wilhelmsii reduced in the endothelium-denuded aortas precontracted by PE or KCl (P < 0.001 vs. control). In contrast, A. wilhelmsii-induced relaxation was not affected by glibenclamide, BaCl2, ruthenium red, methylene blue, or heparin.

CONCLUSIONS

The results showed that A. wilhelmsii had a vasorelaxation effect, which was not endothelium-dependent. The relaxation was mediated by inhibition of extracellular Ca(2+) influx through voltage- and receptor-operated Ca(2+) channels (VDDCs and ROCCs) in vascular smooth muscle cells.

Neuroprotective effect of gadolinium: a stretch-activated calcium channel blocker in mouse model of ischemia-reperfusion injury

The present study was designed to investigate the potential of gadolinium, a stretch-activated calcium channel blocker in ischemic reperfusion (I/R)-induced brain injury in mice. Bilateral carotid artery occlusion of 12 min followed by reperfusion for 24 h was given to induce cerebral injury in male Swiss mice. Cerebral infarct size was measured using triphenyltetrazolium chloride staining. Memory was assessed using Morris water maze test and motor incoordination was evaluated using rota-rod, lateral push, and inclined beam walking tests. In addition, total calcium, thiobarbituric acid reactive substance (TBARS), reduced glutathione (GSH), and acetylcholinesterase (AChE) activity were also estimated in brain tissue. I/R injury produced a significant increase in cerebral infarct size. A significant loss of memory along with impairment of motor performance was also noted. Furthermore, I/R injury also produced a significant increase in levels of TBARS, total calcium, AChE activity, and a decrease in GSH levels. Pretreatment of gadolinium significantly attenuated I/R-induced infarct size, behavioral and biochemical changes. On the basis of the present findings, we can suggest that opening of stretch-activated calcium channel may play a critical role in ischemic reperfusion-induced brain injury and that gadolinium has neuroprotective potential in I/R-induced injury.

Endothelium-independent relaxation and contraction of rat aorta induced by ethyl acetate extract from leaves of Morus alba (L.)

AIM OF THE STUDY

Based on screening for vasoactive traditional Chinese medicinal herbs, the present study was performed to investigate the vasoactive effects of an ethyl acetate extract from leaves of Morus alba (L.) (ELM) on rat thoracic aorta and the mechanisms underlying these effects.

MATERIALS AND METHODS

Isolated rat thoracic rings were mounted in an organ bath system and the effects of ELM on their responses were evaluated.

RESULTS

ELM (0.125-32.000g/l) induced a concentration-dependent relaxation (P<0.01 vs. control) both in endothelium-intact and -denuded aortas precontracted by high K(+) (6 x 10(-2)M) or 10(-6)M phenylephrine (PE). In endothelium-denuded aortas, ELM at the EC(50) concentration reduced Ca(2+)-induced contraction (P<0.01 vs. control) after PE or KCl had generated a stable contraction in Ca(2+)-free solution. And after incubation with verapamil, ELM induced contraction in endothelium-denuded aortas precontracted by PE (P<0.01 vs. control); this was abolished by ruthenium red (P<0.01 vs. ELM-treated endothelium-denuded group; P>0.05 vs. control), but not by heparin (P>0.01 vs. ELM-treated endothelium-denuded group; P<0.01 vs. control).

CONCLUSIONS

The results showed that ELM had dual vasoactive effects, and the relaxation was greater than the contraction. The relaxation was mediated by inhibition of voltage- and receptor-dependent Ca(2+) channels in vascular smooth muscle cells, while the contraction occurred via activation of ryanodine receptors in the sarcoplasmic reticulum.

Distinct apical and basolateral membrane requirements for stretch-induced membrane traffic at the apical surface of bladder umbrella cells

Epithelial cells respond to mechanical stimuli by increasing exocytosis, endocytosis, and ion transport, but how these processes are initiated and coordinated and the mechanotransduction pathways involved are not well understood. We observed that in response to a dynamic mechanical environment, increased apical membrane tension, but not pressure, stimulated apical membrane exocytosis and ion transport in bladder umbrella cells. The exocytic response was independent of temperature but required the cytoskeleton and the activity of a nonselective cation channel and the epithelial sodium channel. The subsequent increase in basolateral membrane tension had the opposite effect and triggered the compensatory endocytosis of added apical membrane, which was modulated by opening of basolateral K(+) channels. Our results indicate that during the dynamic processes of bladder filling and voiding apical membrane dynamics depend on sequential and coordinated mechanotransduction events at both membrane domains of the umbrella cell.

Role of AQP2 in activation of calcium entry by hypotonicity: implications in cell volume regulation

We previously reported in a rat cortical collecting duct cell line (RCCD1) that the presence of aquaporin 2 (AQP2) in the cell membrane is critical for the rapid activation of regulatory volume decrease mechanisms (RVD) (Ford et al. Biol Cell 97: 687–697, 2005). The aim of our present work was to investigate the signaling pathway that links AQP2 to this rapid RVD activation. Since it has been previously described that hypotonic conditions induce intracellular calcium ([Ca2+]i) increases in different cell types, we tested the hypothesis that AQP2 could have a role in activation of calcium entry by hypotonicity and its implication in cell volume regulation. Using a fluorescent probe technique, we studied [Ca2+]iand cell volume changes in response to a hypotonic shock in WT-RCCD1(not expressing aquaporins) and in AQP2-RCCD1(transfected with AQP2) cells. We found that after a hypotonic shock only AQP2-RCCD1cells exhibit a substantial increase in [Ca2+]i. This [Ca2+]iincrease is strongly dependent on extracellular Ca2+and is partially inhibited by thapsigargin (1 μM) indicating that the rise in [Ca2+]ireflects both influx from the extracellular medium and release from intracellular stores. Exposure of AQP2-RCCD1cells to 100 μM gadolinium reduced the increase in [Ca2+]isuggesting the involvement of a mechanosensitive calcium channel. Furthermore, exposure of cells to all of the above described conditions impaired rapid RVD. We conclude that the expression of AQP2 in the cell membrane is critical to produce the increase in [Ca2+]iwhich is necessary to activate RVD in RCCD1cells.

Stretch-elicited calcium responses in the intact mouse thoracic aorta

Stretch-elicited intracellular calcium ([Ca(2+)](i)) changes in individual smooth muscle cells in a ring of aorta were measured simultaneously with the force developed by the ring. A phasic increase in [Ca(2+)](i) was observed in 30% of the cells and a sustained one in 10%. Depletion of intracellular calcium store by thapsigargin and caffeine decreased phasic and increased sustained calcium responses. The inhibition of calcium entry either by stretching the aorta in a calcium-free medium or by the inhibition of stretch-activated, non-selective cationic channels by 5 microM GsMtx-4 toxin, decreased the proportion of sustained [Ca(2+)](i) responses but increased transient responses. In this condition, a third of the cells responded to stretch by a bursts of [Ca(2+)](i) spikes. The decrease of calcium influx triggered the generation of burst of calcium spikes after the application of stretch steps to the vascular wall. We conclude that progressive recruitment of smooth muscle cells is the mechanism underlying the force-generating part of the myogenic response. Two types of stretch-elicited calcium responses were observed during the recruitment of the smooth muscle cells. One was a phasic calcium discharge generated by the sarcoplasmic reticulum. The second was a tonic response produced by the activation of the stretch-sensitive cationic channels allowing extracellular Ca(2+) entry.

Molecular basis of the effects of mechanical stretch on vascular smooth muscle cells

The pulsatile nature of blood pressure and flow creates hemodynamic stimuli in the forms of cyclic stretch and shear stress, which exert continuous influences on the constituents of the blood vessel wall. Vascular smooth muscle cells (VSMCs) use multiple sensing mechanisms to detect the mechanical stimulus resulting from pulsatile stretch and transduce it into intracellular signals that lead to modulations of gene expression and cellular functions, e.g., proliferation, apoptosis, migration, and remodeling. The cytoskeleton provides a structural framework for the VSMC to transmit mechanical forces between its luminal, abluminal, and junctional surfaces, as well as its interior, including the focal adhesion sites, the cytoplasm, and the nucleus. VSMCs also respond differently to the surrounding structural environment, e.g., two-dimensional versus three-dimensional matrix. In vitro studies have been conducted on cultured VSMCs on deformable substrates to elucidate the molecular mechanisms by which the cells convert mechanical inputs into biochemical events, eventually leading to functional responses. The knowledge gained from research on mechanotransduction in vitro, in conjunction with verifications under in vivo conditions, will advance our understanding of the physiological and pathological processes involved in vascular remodeling and adaptation in health and disease.

A mathematical model for simultaneous spatio-temporal dynamics of calcium and inositol 1,4,5-trisphosphate in Madin-Darby canine kidney epithelial cells

The landmark paper by Hirose et al. (Hirose, K., Kadowaki, S., Tanabe, M., Takeshima, H., Iino, M., Science 284:1527-1530, 1999) presented experimental investigations to show that not only can calcium upregulate IP(3), but that it can also have an inhibitory effect on IP(3). In this paper, we present a preliminary model, which is consistent with these experiments. This model includes positive and negative feedback between calcium and IP(3) and is able to reproduce more precisely the data presented in Hirose et al. (Hirose, K., Kadowaki, S., Tanabe, M., Takeshima, H., Iino, M., Science 284:1527-1530, 1999). In the second part of the paper, the intracellular and intercellular calcium movement in Madin-Darby canine kidney epithelial cells is investigated. With the aid of the model we are able to identify the aspects of IP(3) and calcium signalling, which should be studied further experimentally before refining the model.

The influence of hypotonicity on large-conductance calcium-activated potassium channels in human retinal pigment epithelial cells

The aim of this study was to characterize the effects of hypotonicity on the activity of large-conductance Ca(2+)-activated K+ (BK(Ca)) channels in human retinal pigment epithelial (RPE R-50) cells. Effects of hypotonicity on ion currents were investigated with the aid of the patch-clamp technique. A regulatory volume decrease in response to a hypotonic solution (200 mOsm/L) was observed that could be blunted by paxilline. In whole-cell current recordings, a hypotonic solution (200 mOsm/L) reversibly increased the amplitude of K+ outward currents (I(K)). The increase of I(K) could be reversed by iberiotoxin (200 nM), paxilline (1 microM), or tetrandrine (5 microM), but not by glibenclamide (10 microM), disulphonic acid (DIDS) (100 microM), or dequalinium dichloride (10 microM). In RPE R-50 cells pretreated with thapsigargin, aristolochic acid, or pertussis toxin, the increased amplitude of I(K) in response to hypotonicity was unaltered. In cell-attached patches, an increase in BK(Ca)-channel activity was observed during hypotonicity-induced cell swelling. The enhanced channel activity elicited under this condition was mainly mediated by an increase in the number of long-lived openings. These findings support the evidence for the coupling of volume swelling to the functional activity of BK(Ca) channels.

Hyperosmotic stress stimulates inositol 1,4,5-trisphosphate and inositol 1,3,4,5-tetrakisphosphate formation independently of bis-diphosphoinositol tetrakisphosphate modulation

Hyperosmotic stress induces water diffusion out of the cell, resulting in cell shrinkage, and leading to DNA damage, cell cycle arrest, and cytoskeletal reorganization. A previous report showed that low concentrations of sorbitol (200mM) could increase up to 25-fold the concentration of InsP(8) in animal cells. Here, we investigate the effect of sorbitol (200mM) on the inositol 1,4,5-trisphosphate (InsP(3)) and inositol 1,3,4,5-tetrakisphosphate (InsP(4)) pathway. A 3- to 4-fold increase in InsP(3) and InsP(4) levels after sorbitol challenge was observed. It was prevented by the phospholipase C inhibitor U-73122 but was insensitive to the MAP kinase inhibitor U0126. We also observed an increase in the free intracellular [Ca(2+)] and the occurrence of Ca(2+) oscillations in response to sorbitol. A hyperosmotic stress could therefore affect the levels of both hyperphosphorylated inositol phosphates and InsP(3)/InsP(4)-signalling molecules.

Mechanism of Contraction of Rat Isolated Tail Arteries by Hyposmotic Solutions

Contraction induced by hyposmotic swelling was examined in rat tail arteries mounted on a myograph containing a modified Krebs physiological saline solution (PSS) containing 50 mM mannitol (300 mosm/l). Hyposmotic swelling was induced by removing mannitol. In arteries having basal tone or arteries precontracted with K(+) or the thromboxane mimetic U-46619, removal of mannitol caused a concentration dependent contraction of rat tail arteries. Concurrent measurement of tension and intracellular calcium [Ca(2+)](i )in arteries loaded with fura-2 showed that both tension and [Ca(2+)](i) increased on exposure to a hyposmotic solution. Removal of endothelium or inhibition of nitric oxide and cyclooxygenase together did not affect contractile responses. Removal of extracellular Ca(2+) abolished the contractile response to hyposmotic solution and NiCl(2), a nonspecific inhibitor of Ca(2+) influx pathways, blocked the rise in [Ca(2+)](i) and tension in response to a hyposmotic solution. Verapamil and nisoldipine, inhibitors of Ca(v)1.2 (L-type) calcium channels significantly reduced the contractile response to a hyposmotic solution. Addition of NiCl(2) to nisoldipine caused an additional inhibition of the response to a hyposmotic solution. Inhibition of calcium release from the sarcoplasmic reticulum by ryanodine or cyclopiazonic acid (CPA) did not cause any change in the tension response to a hyposmotic solution. CPA did not significantly inhibit the response to a hyposmotic solution in the presence of N(G)-methyl-L-arginine, oxyhaemoglobin and indomethacin. We conclude that contraction induced by a hyposmotic solution is largely due to Ca(v)1.2 calcium channels although other Ca(2+) influx pathways also contribute.

Colloidal effect of albumin on the placental lactogen and chorionic gonadotrophin releases from human term placental explants

Albumin has been reported to stimulate the release of placental lactogen and chorionic gonadotrophin from human term placental explants within physiological concentrations. This study aimed at characterizing further its effect on the placental hormonal secretion. The placental lactogen and chorionic gonadotrophin secretory response of incubated explants to 5% albumin was reproduced by colloidal agents, i.e., dextran (4.5%) and polygelin (4%), indicating that a rise in colloidal osmotic pressure can elicit hormonal release from the syncytiotrophoblast. Their secretory effects were not modified by the absence of extracellular calcium or the presence of verapamil in the medium. The three agents also provoked a marked increase in (45)calcium outflow from preloaded and perifused explants that persisted in absence of extracellular calcium. These data indicate that the triggering effect of albumin on placental lactogen and chorionic gonadotrophin release can be partly reproduced by colloidal agents and is independent of extracellular calcium.

Paracellular pathway in the shell epithelium ofAnodonta cygnea

Ultrastructural study of cell-cell connections in the outer mantle epithelium (OME) on high-pressure-frozen specimens revealed zonula adherens, septate junctions and gap junctions in Anodonta cygnea. In order to evaluate the permeability of the paracellular pathway, the OME was incubated under gradients of lanthanum and calcium. After lanthanum incubation (4 mM) from the basal side, the septate junctions were penetrated completely by this tracer. When applied from the apical side, lanthanum deposits were located similarly over the entire length of the septate junctions up to the first dilatations of the intercellular space. Calcium deposits were also present in paracellular areas only when OME had been incubated simultaneously with calcium (6 mM) and lanthanum (4 mM) gradients. Lanthanum and calcium deposits were detected with ESI (Electron Spectroscopic Imaging) and identified with EELS (Electron Energy Loss Spectroscopy). On the other hand, electrophysiological observations showed a 48% reduction of conductance when the OME was bathed on both sides with solutions containing lanthanum (4 mM) and calcium (6 mM), compared to bathing with lanthanum-free solution (control). The conductance reduction was 52% when calcium was removed from the control solution. Supported by morphological and physiological evidence, it appears that, under in vivo conditions, calcium ions may diffuse paracellularly from the haemolymph towards the extrapallial fluid and vice-versa across the septate junctions in the OME of A. cygnea. Permeability of the septate junctions depended proportionally on the calcium concentration in fluids.

Stretch-induced calcium sensitization of rat lymphatic smooth muscle

The relationships between smooth muscle calcium and isometric tension generation to spontaneous lymphatic pump activity and its modulation by stretch equivalent from 0 to approximately 6 cmH2O were investigated. Excised preparations of the rat thoracic duct were mounted on a wire myograph and loaded with the calcium-sensitive fluorochrome indo-1. Calcium-dependent fluorescence and isometric force were simultaneously recorded. The thoracic duct segments developed spontaneous rhythmic contractile activity. Each contraction was preceded by an increase in intracellular calcium. When the vessels were normalized and stabilized at a preload equal to 3 cmH2O, the peak generation in tension occurred 0.70 +/- 0.11 s after that of calcium. Incremental stretch enhanced the frequency of the phasic activity and amplitude of isometric force generation but not the basal calcium level or the amplitude of the calcium transient. These findings suggest that stretch enhances lymphatic pump activity by increasing the pacemaker activity and the calcium sensitivity of the contractile apparatus.

Effects of emodin on intracellular Ca2+ signaling in the circular smooth muscle cells of rat colon

AIM

To investigate whether emodin has any effects on circular smooth muscle cells of rat colon and to examine the underlying mechanisms.

METHODS

Smooth muscle cells were isolated from the circular muscle layers of Wistar rat colon and cell length was measured by computerized image micrometry. Intracellular Ca²⁺ ([Ca²⁺]i) signaling was studied in smooth muscle cells using Ca²⁺ indicator Fluo-3 AM by laser-scanning confocal microscopy.

RESULTS

Emodin dose-dependently induced smooth muscle cells contraction, caused a large, transient increase in [Ca²⁺]i followed by a _Sustained elevation in [Ca²⁺]i. Emodin-induced increase in [Ca²⁺]i was unaffected by nifedipine, a votage-gated Ca²⁺-channel antagonist, and the _Sustained phase of rising of [Ca²⁺]i was attenuated by extracellular Ca²⁺ removal with EGTA solution. Inhibiting Ca²⁺ release from ryanodine-sensitive intracellular stores by ryanodine reduced the _Peak increase in [Ca²⁺]i. However, the application of heparine, an antagonist of IP3R, nearly abolished the _Peak increase in [Ca²⁺]i induced by emodin.

CONCLUSION

Emodin has direct excitatory effect on circular smooth muscle cells from rat colon and its effect is mediated through Ca²⁺-dependent pathways. Furthermore, emodin-induced Peak [Ca²⁺]i increase may be attributable to the Ca²⁺ release from IP₃ sensitive stores, which promotes Ca²⁺ release from ryanodine-sensitive stores through CICR mechanism. Additionally, Ca²⁺ influx from extracellular medium contributes to the _Sustained increase in [Ca²⁺]i.

The role of the cytoskeleton in capacitative calcium entry in myenteric glia

Capacitative calcium entry (CCE) is the process by which intracellular calcium is replenished from the external milieu upon depletion of intracellular stores. CCE is thought to participate in chemotaxis, proliferation and cell signalling. A physical interaction between intracellular stores and the plasma membrane is postulated to regulate CCE. We hypothesized that cytoskeletal disruption alters this interaction, inhibiting CCE in enteric glia. Cultured myenteric glia from neonatal guinea-pigs were treated with cytochalasin D (10 micro mol L-1), a microfilament disrupting agent, nocodazole (20 micro mol L-1), a microtubule disrupting agent, or vehicle (dimethyl sulphoxide). Intracellular calcium changes were measured using fura-2 microfluorimetry. To evaluate the rate of cation re-entry, barium was substituted for calcium because barium is not sequestered internally. Cytochalasin D-treated glia had diminished CCE responses (57 +/- 3 nmol L-1) compared with controls (97 +/- 7 nmol L-1) as did nocodazole-treated glia (30 +/- 2 nmol L-1) vs controls (77 +/- 6 nmol L-1). The proportion of cells demonstrating CCE abolition was greater in the cytochalasin (50 +/- 8%) and nocodazole-treated (89 +/- 2%) groups compared with controls (21 +/- 2%, 40 +/- 9%, respectively). Cytochalasin D and nocodazole treatment diminished the rate of cation re-entry based on diminished barium entry in treated vs control cells. From this study, we conclude that disruption of cytoskeletal elements diminishes calcium influx essential to calcium store repletion in myenteric glia.