Evidence for myosin light chain kinase mediating noradrenaline-evoked cation current in rabbit portal vein myocytes

High glucose upregulates myosin light chain kinase to induce microfilament cytoskeleton rearrangement in hippocampal neurons

Chronic hyperglycemia leads to myosin light chain kinase (MLCK) upregulation and induces neuronal damage. However, the underlying molecular mechanism of neuronal damage in hyperglycemia has not yet been fully elucidated. In the present study, hippocampal neuronal cells were cultured and treated with a high glucose concentration (45 mmol/l). The results demonstrated that high glucose induced shrinking of the synapses, nuclear shape irregularity and microfilament damage. Filamentous actin (F‑actin) filaments were rearranged, cell apoptosis rate was increased and the protein expression of MLCK and phosphorylated (p)‑MLC was upregulated. The MLCK inhibitor ML‑7 largely reversed the alterations in the microfilament cytoskeleton, inhibited F‑actin depolymerization, reduced apoptosis and downregulated MLCK and p‑MLC protein expression. Overall, these results indicated that high glucose upregulated MLCK to promote F‑actin depolymerization, which induced microfilament cytoskeleton rearrangement in hippocampal neuronal cells.

Voltage Dependence of a Neuromodulator-Activated Ionic Current

The neuromodulatory inward current (IMI) generated by crab Cancer borealis stomatogastric ganglion neurons is an inward current whose voltage dependence has been shown to be crucial in the activation of oscillatory activity of the pyloric network of this system. It has been previously shown that IMI loses its voltage dependence in conditions of low extracellular calcium, but that this effect appears to be regulated by intracellular calmodulin. Voltage dependence is only rarely regulated by intracellular signaling mechanisms. Here we address the hypothesis that the voltage dependence of IMI is mediated by intracellular signaling pathways activated by extracellular calcium. We demonstrate that calmodulin inhibitors and a ryanodine antagonist can reduce IMI voltage dependence in normal Ca(2+), but that, in conditions of low Ca(2+), calmodulin activators do not restore IMI voltage dependence. Further, we show evidence that CaMKII alters IMI voltage dependence. These results suggest that calmodulin is necessary but not sufficient for IMI voltage dependence. We therefore hypothesize that the Ca(2+)/calmodulin requirement for IMI voltage dependence is due to an active sensing of extracellular calcium by a GPCR family calcium-sensing receptor (CaSR) and that the reduction in IMI voltage dependence by a calmodulin inhibitor is due to CaSR endocytosis. Supporting this, preincubation with an endocytosis inhibitor prevented W7 (N-(6-aminohexyl)-5-chloro-1-naphthalenesulfonamide hydrochloride)-induced loss of IMI voltage dependence, and a CaSR antagonist reduced IMI voltage dependence. Additionally, myosin light chain kinase, which is known to act downstream of the CaSR, seems to play a role in regulating IMI voltage dependence. Finally, a Gβγ-subunit inhibitor also affects IMI voltage dependence, in support of the hypothesis that this process is regulated by a G-protein-coupled CaSR.

Regulation of calcium channels in smooth muscle: new insights into the role of myosin light chain kinase

Smooth muscle myosin light chain kinase (MLCK) plays a crucial role in artery contraction, which regulates blood pressure and blood flow distribution. In addition to this role, MLCK contributes to Ca(2+) flux regulation in vascular smooth muscle (VSM) and in non-muscle cells, where cytoskeleton has been suggested to help Ca(2+) channels trafficking. This conclusion is based on the use of pharmacological inhibitors of MLCK and molecular and cellular techniques developed to down-regulate the enzyme. Dissimilarities have been observed between cells and whole tissues, as well as between large conductance and small resistance arteries. A differential expression in MLCK and ion channels (either voltage-dependent Ca(2+) channels or non-selective cationic channels) could account for these observations, and is in line with the functional properties of the arteries. A potential involvement of MLCK in the pathways modulating Ca(2+) entry in VSM is described in the present review.

Myosin light chain kinase regulates hearing in mice by influencing the F-actin cytoskeleton of outer hair cells and cochleae

Myosin light chain kinase (MLCK) phosphorylates myosin regulatory light chains to facilitate its interaction with actin filaments and produce contractile activity. The outer hair cells (OHCs) in the ear contain large amounts of actin and a variety myosins. The stereociliary and somatic motility of OHCs are closely related to hearing. It appears likely that MLCK may play an important role in acoustic trans-duction. In this study, we analyzed, both in vivo and in vitro, the OHCs of mice bearing a specific deletion of the MLCK gene and the OHCs of control mice. The phenotype was assessed by auditory function [acoustic brainstem responses (ABRs) and distortion product otoacoustic emissions (DPOAEs)], inner ear morphology and histology. MLCK-deficient mice aged 6-7 months showed impaired hearing, a 5- to 10-dB sound pressure level (SPL) increase in the ABR thresholds, when responding to clicks and tones of different frequencies (8 and 16 kHz) (P<0.05). The DPOAE amplitudes of 3-month-old MLCK-deficient mice decreased significantly (>10 dB SPL) at low frequencies (4, 5 and 6 kHz). The OHCs in the MLCK-deficient mice increased with abnormal stereocilia. The staining of F-actin and the phosphorylation of the regulatory light chain in MLCK-deficient OHCs was weak. Our results indicate that MLCK may regulate the structure and the motility of stereocilia through F-actin polymerization.

Myosin light chain kinase controls voltage-dependent calcium channels in vascular smooth muscle

The Ca(2+)-dependent kinase myosin light chain kinase (MLCK) is the activator of smooth muscle contraction. In addition, it has been reported to be involved in Ca(2+) channel regulation in cultured cells, and we previously showed that the MLCK inhibitor ML-7 decreases arginine vasopressin (AVP)-induced Ca(2+) influx in rat aorta. This study was designed to investigate whether MLCK is involved in Ca(2+) regulation in resistance artery smooth muscle cell, which plays a major role in the control of blood pressure. As ML compounds were shown to have off-target effects, MLCK was downregulated by transfection with a small interfering RNA targeting MLCK (MLCK-siRNA) in rat small resistance mesenteric artery (RMA) and in the rat embryonic aortic cell line A7r5. Noradrenaline-induced contraction and Ca(2+) signal were significantly depressed in MLCK-siRNA compared to scramble-siRNA-transfected RMA. Contraction and Ca(2+) signal induced by high KCl and voltage-activated Ca(2+) current were also significantly decreased in MLCK-siRNA-transfected RMA, suggesting that MLCK depletion modifies voltage-operated Ca(2+) channels. KCl- and AVP-induced Ca(2+) signals and voltage-activated Ca(2+) current were decreased in MLCK-depleted A7r5 cells. Eventually, real-time quantitative PCR analysis indicated that in A7r5, MLCK controlled mRNA expression of CaV1.2 (L-type) and CaV3.1 (T-type) voltage-dependent Ca(2+) channels. Our results suggest that MLCK controls the transcription of voltage-dependent Ca(2+) channels in vascular smooth muscle cells.

Nogo-A is a negative regulator of CNS angiogenesis

Nogo-A is an important axonal growth inhibitor in the adult and developing CNS. In vitro, Nogo-A has been shown to inhibit migration and cell spreading of neuronal and nonneuronal cell types. Here, we studied in vivo and in vitro effects of Nogo-A on vascular endothelial cells during angiogenesis of the early postnatal brain and retina in which Nogo-A is expressed by many types of neurons. Genetic ablation or virus-mediated knock down of Nogo-A or neutralization of Nogo-A with an antibody caused a marked increase in the blood vessel density in vivo. In culture, Nogo-A inhibited spreading, migration, and sprouting of primary brain microvascular endothelial cells (MVECs) in a dose-dependent manner and induced the retraction of MVEC lamellipodia and filopodia. Mechanistically, we show that only the Nogo-A-specific Delta 20 domain exerts inhibitory effects on MVECs, but the Nogo-66 fragment, an inhibitory domain common to Nogo-A, -B, and -C, does not. Furthermore, the action of Nogo-A Delta 20 on MVECs required the intracellular activation of the Ras homolog gene family, member A (Rho-A)-associated, coiled-coil containing protein kinase (ROCK)-Myosin II pathway. The inhibitory effects of early postnatal brain membranes or cultured neurons on MVECs were relieved significantly by anti-Nogo-A antibodies. These findings identify Nogo-A as an important negative regulator of developmental angiogenesis in the CNS. They may have important implications in CNS pathologies involving angiogenesis such as stroke, brain tumors, and retinopathies.

Rho kinase regulation of vasopressin-induced calcium entry in vascular smooth muscle cell: comparison between rat isolated aorta and cultured aortic cells

In addition to its role in artery contraction, Rho kinase (ROCK) is reported to be involved in the Ca(2+) response to vasoconstrictor agonist in rat aorta. However the signaling pathway mediated by ROCK had not been investigated so far and it was not known whether ROCK also contributed to Ca(2+) signaling in cultured vascular smooth muscle cells (VSMC), which undergo profound phenotypic changes. Our results showed that in VSMC, ROCK inhibition by Y-27632 or H-1152 had no effect on the Ca(2+) response to vasopressin, while in aorta the vasopressin-induced Ca(2+) entry was significantly decreased. The inhibition of myosin light chain kinase (MLCK) by ML-7 depressed the vasopressin-induced Ca(2+) signal in aorta but not in VSMC. The difference in ROCK sensitivity of vasopressin-induced Ca(2+) entry between aorta and VSMC was not related to an alteration of the RhoA/ROCK pathway. However, MLCK expression and activity were depressed in cultured cells compared to aorta. We concluded that the regulation of vasopressin-induced Ca(2+) entry by ROCK in aorta could involve the myosin cytoskeleton and could be prevented by the downregulation of MLCK in VSMC. These results underline the important differences in Ca(2+) regulation between whole tissue and cultured cells.

Effects of commonly used protein kinase inhibitors on vascular contraction and L-type Ca(2+) current

Regulation of smooth muscle contraction is driven by a number of protein kinases: the evidence for this often originates from studies that investigate the effects of extracellularly added specific protein kinase inhibitors. Six compounds, thought to be selective inhibitors of various kinases, were analysed for their effects on vascular L-type Ca(2+) channels because this potential subsidiary activity could strongly influence our understanding of the pathways involved in smooth muscle contraction. Whole-cell L-type Ba(2+) currents [I(Ba(L))] were recorded in single myocytes, and contractile responses were measured from endothelium-denuded rings taken from the rat tail artery. Although ML-7, ML-9, and wortmannin (MLCK inhibitors), HA-1077 and Y-27632 (Rho-associated kinase inhibitors), and GF-109203X (PKC inhibitor) relaxed rings pre-contracted with high KCl in a concentration-dependent manner, their effect on I(Ba(L)) intensity was surprisingly variable. Wortmannin showed negligible effects while HA-1077 and Y-27632 were ineffective. I(Ba(L)) was partly inhibited by GF-109203X and blocked by ML-7 and ML-9 in a concentration-dependent manner, with the blockade by ML-7 being voltage-dependent. Whilst ML-7, ML-9, and GF-109203X sped up the inactivation kinetics of I(Ba(L)), GF-109203X did not modify ML-7- or ML-9-induced effects, with both intensity and kinetics of the current remaining unchanged. In contrast, application of Bay K 8644 on myocytes pre-treated with ML-7 or ML-9 raised I(Ba(L)) beyond control values. In conclusion, ML-7 and ML-9 inhibit L-type Ca(2+) channels via a mechanism independent of MLCK, PKC or Rho kinase activities, and as such caution should be used in employing these agents to elucidate the role of kinases in smooth muscle contraction.

Differential effects of myosin light chain kinase inhibition on contractility, force development and myosin light chain 20 phosphorylation of rat cervical and thoracic duct lymphatics

The intrinsic contractile activity of lymphatics varies in different regions of the body. We have previously shown that cervical lymphatics possess an inherently higher frequency but lower tone at a given pressure when compared to thoracic duct lymphatics. However, the molecular mechanisms modulating the contractile characteristics of these lymphatics are not well understood. Since myosin light chain 20 (MLC(20)) phosphorylation appears to underlie the tonic component of lymphatic contraction, we hypothesized that the thoracic duct would be more sensitive to the modulation of MLC(20) phosphorylation when compared to cervical lymphatics. To test our hypothesis, the contractile activities and MLC(20) phosphorylation of thoracic duct and cervical lymphatics were determined in the absence or presence of the specific myosin light chain kinase (MLCK) inhibitor ML-7 under both isobaric and isometric conditions. Addition of ML-7 at each concentration tested led to a decrease in tone in both vessel types. While ML-7 (10(-6) m) significantly reduced the phasic contraction frequency of cervical lymphatics, it completely stopped phasic contractions of thoracic duct at that concentration. Under isometric conditions the active peak and plateau components of tension were both significantly higher in thoracic duct compared to cervical lymphatics. ML-7 (10(-5) m) significantly decreased both the active peak and plateau tensions of thoracic duct, whereas only the active peak tension of cervical lymphatics was decreased. In thoracic duct MLC(20) di-phosphorylation, but not mono-phosphorylation, was significantly decreased with increasing transmural pressure, whereas in cervical vessels only at the higher pressures tested did MLC(20) di-phosphorylation decrease. ML-7 treatment of the thoracic duct caused a significant decrease in both the mono- and di-phosphorylated forms of MLC(20). However, in cervical vessels ML-7 treatment produced an increase in the mono-phosphorylated MLC(20) form while di-phosphorylated MLC(20) was significantly decreased. These data indicate that thoracic duct has an enhanced sensitivity to MLCK inhibition when compared to cervical lymphatics and while the status of the mono- and di-phosphorylation forms of MLC(20) affects both tonic and phasic components of lymphatic contractions, the pressure-dependent changes in tonic contractions are modulated by the status of the di-phosphorylation of MLC(20) in the lymphatics.

Inhibition of native TRPC6 channel activity by phosphatidylinositol 4,5-bisphosphate in mesenteric artery myocytes

The present work investigates the effect of phosphatidylinositol-4,5-bisphosphate (PIP(2)) on native TRPC6 channel activity in freshly dispersed rabbit mesenteric artery myocytes using patch clamp recording and co-immunoprecipitation methods. Inclusion of 100 microM diC8-PIP(2) in the patch pipette and bathing solutions, respectively, inhibited angiotensin II (Ang II)-evoked whole-cell cation currents and TRPC6 channel activity by over 90%. In inside-out patches diC8-PIP(2) also inhibited TRPC6 activity induced by the diacylglycerol analogue 1-oleoyl-2-acetyl-sn-glycerol (OAG) with an IC(50) of 7.6 microM. Anti-PIP(2) antibodies potentiated Ang II- and OAG-evoked TRPC6 activity by about 2-fold. Depleters of tissue PIP(2) wortmannin and LY294002 stimulated TRPC6 activity, as did the polycation PIP(2) scavenger poly-L-lysine. Wortmannin reduced Ang II-evoked TRPC6 activity by over 75% but increased OAG-induced TRPC6 activity by over 50-fold. Co-immunoprecipitation studies demonstrated association between PIP(2) and TRPC6 proteins in tissue lysates. Pre-treatment with Ang II, OAG and wortmannin reduced TRPC6 association with PIP(2). These results provide for the first time compelling evidence that constitutively produced PIP(2) exerts a powerful inhibitory action on native TRPC6 channels.

Identification and functional characterization of an aggregation domain in long myosin light chain kinase

The functions of long smooth muscle myosin light chain kinase (L-MLCK), a molecule with multiple domains, are poorly understood. To examine the existence of further potentially functional domains in this molecule, we analyzed its amino acid sequence with a tango program and found a putative aggregation domain located at the 4Ig domain of the N-terminal extension. To verify its aggregation capability in vitro, expressible truncated L-MLCK variants driven by a cytomegalovirus promoter were transfected into cells. As anticipated, only the overexpression of the 4Ig fragment led to particle formation in Colon26 cells. These particles contained 4Ig polymers and actin. Analysis with detergents demonstrated that the particles shared features in common with aggregates. Thus, we conclude that the 4Ig domain has a potent aggregation ability. To further examine this aggregation domain in vivo, eight transgenic mouse lines expressing the 4Ig domain (4Ig lines) were generated. The results showed that the transgenic mice had typical aggregation in the thigh and diaphragm muscles. Histological examination showed that 7.70 +/- 1.86% of extensor digitorum longus myofibrils displayed aggregates with a 36.44% reduction in myofibril diameter, whereas 65.13 +/- 3.42% of diaphragm myofibrils displayed aggregates and the myofibril diameter was reduced by 43.08%. Electron microscopy examination suggested that the aggregates were deposited at the mitochondria, resulting in structural impairment. As a consequence, the oxygen consumption of mitochondria in the affected muscles was also reduced. Macrophenotypic analysis showed the presence of muscular degeneration characterized by a reduction in force development, faster fatigue, decreased myofibril diameters, and structural alterations. In summary, our study revealed the existence of a novel aggregation domain in L-MLCK and provided a direct link between L-MLCK and aggregation. The possible significance and mechanism underlying the aggregation-based pathological processes mediated by L-MLCK are also discussed.

Role of calmodulin and myosin light chain kinase in the activation of carbachol-activated cationic current in murine ileal myocytes

We investigated the effect of calmodulin (CaM) and myosin light chain kinase (MLCK) on murine ileal myocytes using the whole-cell patch-clamp technique. Under the voltage clamp, at the holding potential of -60 mV, 50 micromol/L carbachol (CCh) induced inward currents (I CCh), and spontaneous decay of I CCh occurred. The peak inward currents induced by the repetitive application of CCh (50 micromol/L) tended to decrease in amplitude. Intracellular application of 0.2 mmol/L guanosine 5'-O-(gamma-thio)triphosphate (GTP gammaS) from the patch electrode induced an inward current at a holding potential of -60m V, and the peak inward currents induced by the repetitive application of Cs tended to decrease slightly in amplitude. The amplitude of I CCh was reduced by pretreatment either with W-7, trifluoroperazine, W-5, and melittin (CaM inhibitors) or with ML-7 and ML-9 (selective MLCK inhibitors), and the inhibitory effects were reversible. However, when we pretreated with 50 micromol/L W-7 or 5 micromol/L ML-7 on GTP gammaS-induced inward currents, almost no inhibition was observed in the inward currents. Application of both Rho kinase inhibitor and MLCK inhibitor inhibited GTP gammaS-induced currents. We conclude that CaM and MLCK modulate the activation process of I CCh in murine ileal myocytes and suggest that the classical type transient receptor potential (TRPC) channel 5 might be a candidate for nonselective cationic currents (NSCC) activated by muscarinic stimulation in gastrointestinal smooth muscle cells.

Myosin light chain kinase-independent inhibition by ML-9 of murine TRPC6 channels expressed in HEK293 cells

BACKGROUND AND PURPOSE

Myosin light chain kinase (MLCK) plays a pivotal role in regulation of cellular functions, the evidence often relying on the effects of extracelluarly administered drugs such as ML-9. Here we report that this compound exerts non-specific inhibitory actions on the TRPC6 channel, a transient receptor potential (TRP) protein.

EXPERIMENTAL APPROACH

Macroscopic and single channel currents were recorded from transfected HEK293 cells by patch-clamp techniques.

KEY RESULTS

Cationic currents elicited by carbachol (CCh; 100 microM) in HEK293 cells overexpressing murine TRPC6 (I(TRPC6)) were dose-dependently inhibited by externally applied ML-9 (IC(50)=7.8 microM). This inhibition was voltage-dependent and occurred as fast as external Na(+) removal. Another MLCK inhibitor, wortmannin (3 microM), and MLCK inhibitory peptides MLCK-IP(11-19) (10 microM) and -IP(480-501) (1 microM) showed little effects on I(TRPC6) density and the inhibitory efficacy of ML-9. The extent of the inhibition also unchanged with co-expression of wild-type or a dominant negative mutant of MLCK. Inhibitory effects of ML-9 on I(TRPC6) remained unaffected whether TRPC6 was activated constitutively or by a diacylglycerol analogue OAG (100 microM). Similar rapid inhibition was also observed with a ML-9 relative, ML-7. Intracellular perfusion of ML-9 via patch pipette, dose-dependently suppressed I(TRPC6). In inside-out patch configuration, bath application of ML-9 (and ML-7) rapidly diminished approximately 35pS single TRPC6 channel activities. Contrarily, currents due to TRPC7 expression were rapidly enhanced by externally applied ML-9 and ML-7, which was not prevented by MLCK inhibitory peptides.

CONCLUSION AND IMPLICATIONS

These results strongly suggest that ML compounds inhibit TRPC6 channels via a mechanism independent of inhibition of MLCK activity.

Ca2+ signaling in hypoxic pulmonary vasoconstriction: effects of myosin light chain and Rho kinase antagonists

Antagonists of myosin light chain (MLC) kinase (MLCK) and Rho kinase (ROK) are thought to inhibit hypoxic pulmonary vasoconstriction (HPV) by decreasing the concentration of phosphorylated MLC at any intracellular Ca(2+) concentration ([Ca(2+)](i)) in pulmonary arterial smooth muscle cells (PASMC); however, these antagonists can also decrease [Ca(2+)](i). To determine whether MLCK and ROK antagonists alter Ca(2+) signaling in HPV, we measured the effects of ML-9, ML-7, Y-27632, and HA-1077 on [Ca(2+)](i), Ca(2+) entry, and Ca(2+) release in rat distal PASMC exposed to hypoxia or depolarizing concentrations of KCl. We performed parallel experiments in isolated rat lungs to confirm the inhibitory effects of these agents on pulmonary vasoconstriction. Our results demonstrate that MLCK and ROK antagonists caused concentration-dependent inhibition of hypoxia-induced increases in [Ca(2+)](i) in PASMC and HPV in isolated lungs and suggest that this inhibition was due to blockade of Ca(2+) release from the sarcoplasmic reticulum and Ca(2+) entry through store- and voltage-operated Ca(2+) channels in PASMC. Thus MLCK and ROK antagonists might block HPV by inhibiting Ca(2+) signaling, as well as the actin-myosin interaction, in PASMC. If effects on Ca(2+) signaling were due to decreased phosphorylated myosin light chain concentration, their diversity suggests that MLCK and ROK antagonists may have acted by inhibiting myosin motors and/or altering the cytoskeleton in a manner that prevented achievement of required spatial relationships among the cellular components of the response.

Transient receptor potential channels in cardiovascular function and disease

Sustained elevation in the intracellular Ca2+ concentration via Ca2+ influx, which is activated by a variety of mechanisms, plays a central regulatory role for cardiovascular functions. Recent molecular biological research has disclosed an unexpectedly diverse array of Ca(2+-entry channel molecules involved in this Ca2+ influx. These include more than ten transient receptor potential (TRP) superfamily members such as TRPC1, TRPC3-6, TRPV1, TRPV2, TRPV4, TRPM4, TRPM7, and polycystin (TRPP2). Most of them appear to be multimodally activated or modulated and show relevant features to both acute hemodynamic control and long-term remodeling of the cardiovascular system, and many of them have been found to respond not only to receptor stimulation but also to various forms of stimuli. There is good evidence to implicate TRPC1 in neointimal hyperplasia after vascular injury via store-depletion-operated Ca2+ entry. TRPC6 likely contributes to receptor-operated and mechanosensitive Ca2+ mobilizations, being involved in vasoconstrictor and myogenic responses and pulmonary arterial proliferation and its associated disease (idiopathic pulmonary arterial hypertension). Considerable evidence has also been accumulated for unique involvement of TRPV1 in blood flow/pressure regulation via sensory vasoactive neuropeptide release. New lines of evidence suggest that TRPV2 may act as a Ca2+-overloading pathway associated with dystrophic cardiomyopathy, TRPV4 as a mediator of endothelium-dependent hyperpolarization, TRPM7 as a proproliferative vascular Mg2+ entry channel, and TRPP2 as a Ca2+-entry channel requisite for vascular integrity. This review attempts to provide an overview of the current knowledge on TRP proteins and discuss their possible roles in cardiovascular functions and diseases.

Microfilament-binding properties of N-terminal extension of the isoform of smooth muscle long myosin light chain kinase

Myosin light chain kinases (MLCK) phosphorylate the regulatory light chain of myosin II in thick filaments and bind to F-actin-containing thin filaments with high affinity. The ability of short myosin light chain kinase (S-MLCK) to bind F-actin is structurally attributed to the DFRXXL regions in its N-terminus. The long myosin light chain kinase (L-MLCK) has two additional DFRXXL motifs and six Ig-like modules in its N-terminal extension. The six Ig-like modules are capable of binding to stress fibers independently. Our results from the imaging analysis demonstrated that the first two intact Ig-like modules (2Ig) in N-terminal extension of L-MLCK is the minimal binding module required for microfilament binding. Binding assay confirmed that F-actin was able to bind 2Ig. Stoichiometries of 2Ig peptide were similar for myofilament or pure F-actin. The binding affinities were slightly lower than 5DFRXXL peptide as reported previously. Similar to DFRXXL peptides, the 2Ig peptide also caused efficient F-actin bundle formation in vitro. In the living cell, over-expression of 2Ig fragment increased "spike"-like protrusion formation with over-bundled F-actin. Our results suggest that L-MLCK may act as a potent F-actin bundling protein via its DFRXXL region and the 2Ig region, implying that L-MLCK plays a role in cytoskeleton organization.

Muscarinic cationic current in gastrointestinal smooth muscles: signal transduction and role in contraction

1 The muscarinic receptor plays a key role in the parasympathetic nervous control of various peripheral tissues including gastrointestinal tract. The neurotransmitter acetylcholine, via activating muscarinic receptors that exist in smooth muscle, produces its contraction. 2 There is the opening of cationic channels as an underlying mechanism. The opening of cationic channels results in influxes of Ca2+ via the channels into the cell and also via voltage-dependent Ca2+ channels which secondarily opened in response to the depolarization, providing an amount of Ca2+ for activation of the contractile proteins. 3 Electrophysiological and pharmacological studies have shown that the cationic channels as well as muscarinic receptors exist in many visceral smooth muscle cells. However, the activation mechanisms of the cationic channels are still unclear. 4 In this article, we summarize the current knowledge of the muscarinic receptor-operated cationic channels, focusing on the receptor subtype, G protein and other signalling molecules that are involved in activation of these channels and on the molecular characteristics of the channel. This will improve strategies aimed at developing new selective pharmacological agents and understanding the activation mechanism and functions of these channels in physiological systems.

Involvement of calmodulin and myosin light chain kinase in activation of mTRPC5 expressed in HEK cells

The classic type of transient receptor potential channel (TRPC) is a molecular candidate for Ca(2+)-permeable cation channels in mammalian cells. Because TRPC channels have calmodulin (CaM) binding sites at their COOH termini, we investigated the effect of CaM on mTRPC5. TRPC5 was initially activated by muscarinic stimulation with 50 microM carbachol and then decayed rapidly even in the presence of carbachol. Intracellular CaM (150 microg/ml) increased the amplitude of mTRPC5 current activated by muscarinic stimulation. CaM antagonists (W-7 and calmidazolium) inhibited mTRPC5 currents when they were applied during the activation of mTRPC5. Pretreatment of W-7 and calmidazolium also inhibited the activation of mTRPC5 current. Inhibitors of myosin light chain kinase (MLCK) inhibited the activation of mTRPC5 currents, whereas inhibitors of CaM-dependent protein kinase II did not. Small interfering RNA against cardiac type MLCK also inhibited the activation of mTRPC5 currents. However, inhibitors of CaM or MLCK did not show any effect on GTPgammaS-induced currents. Application of both Rho kinase inhibitor and MLCK inhibitor inhibited GTPgammaS-induced currents. We conclude that CaM and MLCK modulates the activation process of mTRPC5.

Ca2+-calmodulin-dependent myosin light chain kinase is essential for activation of TRPC5 channels expressed in HEK293 cells

Mammalian homologues of Drosophila transient receptor potential (TRP) proteins are responsible for receptor-activated Ca(2+) influx in vertebrate cells. We previously reported the involvement of intracellular Ca(2+) in the receptor-mediated activation of mammalian canonical transient receptor potential 5 (TRPC5) channels. Here we investigated the role of calmodulin, an important sensor of changes in intracellular Ca(2+), and its downstream cascades in the activation of recombinant TRPC5 channels in human embryonic kidney (HEK) 293 cells. Ca(2+) entry through TRPC5 channels, induced upon stimulation of the G-protein-coupled ATP receptor, was abolished by treatment with W-13, an inhibitor of calmodulin. ML-9 and wortmannin, inhibitors of Ca(2+)-calmodulin-dependent myosin light chain kinase (MLCK), and the expression of a dominant-negative mutant of MLCK inhibited the TRPC5 channel activity, revealing an essential role of MLCK in maintaining TRPC5 channel activity. It is important to note that ML-9 impaired the plasma membrane localization of TRPC5 channels. Furthermore, TRPC5 channel activity measured using the whole-cell patch-clamp technique was inhibited by ML-9, whereas TRPC5 channel activity observed in the cell-excised, inside-out patch was unaffected by ML-9. An antibody that recognizes phosphorylated myosin light chain (MLC) revealed that the basal level of phosphorylated MLC under unstimulated conditions was reduced by ML-9 in HEK293 cells. These findings strongly suggest that intracellular Ca(2+)-calmodulin constitutively activates MLCK, thereby maintaining TRPC5 channel activity through the promotion of plasma membrane TRPC5 channel distribution under the control of phosphorylation/dephosphorylation equilibrium of MLC.

Signal transduction pathways and gating mechanisms of native TRP-like cation channels in vascular myocytes

Activation of Ca2+-permeable non-selective cation channels produces an increase in excitability of vascular smooth muscle cells which has an important role in vasoconstriction. These channels are activated by various physiological stimuli including vasoconstrictor agents such as noradrenaline, depletion of internal Ca2+ stores and cell stretching. In addition cation channels have been shown to be constitutively active and these channels are thought to contribute to resting membrane conductance and basal Ca2+ influx in vascular myocytes. Recent evidence has suggested that transient receptor potential (TRP) proteins represent strong candidates for these channels in the vasculature. This review discusses proposed signal transduction pathways and gating mechanisms which link physiological stimuli to opening of cation channels in vascular myocytes. It is apparent that G-protein-coupled pathways linked to stimulation of phospholipase activity have a profound effect on regulating channel activity and that generation of diacylglycerol (DAG) is a central event in these signalling cascades with this triglyceride having a pivotal role in gating cation channels via both PKC-independent and -dependent mechanisms. Moreover phosphorylation processes produced by stimulation of protein kinases have been proposed to have an important role in regulating cation channel activity.

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.

Myosin phosphatase and myosin phosphorylation in differentiating C2C12 cells

C2C12 cells offer a useful model to study the differentiation of non-muscle cells to skeletal muscle cells. Myosin phosphorylation and changes in related enzymes, with an emphasis on myosin phosphatase (MP) were analyzed over the first 6 days of C2C12 differentiation. There was a transition from myosin phosphatase target subunit 1 (MYPT1), predominant in the non-muscle cells to increased expression of MYPT2. Levels of MYPT1/2 were estimated, and both isoforms were higher in non- or partially differentiated cells compared to the concentrations in the differentiated isolated myotubes from day 6. A similar profile of expression was estimated for the type 1 protein phosphatase catalytic subunit, delta isoform (PP1c delta). Phosphatase activities, using phosphorylated smooth and skeletal muscle myosins, were estimated for total cell lysates and isolated myotubes. In general, smooth muscle myosin was the preferred substrate. Although the expression of MYPT1/2 and PP1c delta was considerably reduced in isolated myotubes the phosphatase activities were not reduced to corresponding levels. Most of the MP activity was due to PP1c, as indicated by okadaic acid. In spite of relatively high expression of MYPT1/2 and PP1c delta, marked phosphorylation of non-muscle myosin (over 50% of total myosin) was observed at day 2 (onset of expression of muscle-specific proteins) and both mono- and diphosphorylated light chains were observed. Partial inhibition of MLCK by 1-(5-chloronaphthalene-1-sulphonyl)-1H-hexahydro-1,4-diazepine HCl (ML-9) or by a construct designed from the autoinhibitory domain of MLCK, resulted in an increase in small myotubes (3-5 nuclei) after 3 days of differentiation and a decrease in larger myotubes (compared to control). The effect of ML-9 was not due to a reduction in intracellular Ca2+ levels. These results suggest that phosphorylation of non-muscle myosin is important in growth of myotubes, either in the fusion process to form larger myotubes or indirectly, by its role in sarcomere organization.

Multiple regulation by calcium of murine homologues of transient receptor potential proteins TRPC6 and TRPC7 expressed in HEK293 cells

We investigated, by using the patch clamp technique, Ca2+-mediated regulation of heterologously expressed TRPC6 and TRPC7 proteins in HEK293 cells, two closely related homologues of the transient receptor potential (TRP) family and molecular candidates for native receptor-operated Ca2+ entry channels. With nystatin-perforated recording, the magnitude and time courses of activation and inactivation of carbachol (CCh; 100 microM)-activated TRPC6 currents (I(TRPC6)) were enhanced and accelerated, respectively, by extracellular Ca2+ (Ca2+(o)) whether it was continuously present or applied after receptor stimulation. In contrast, Ca2+(o) solely inhibited TRPC7 currents (I(TRPC7)). Vigorous buffering of intracellular Ca2+ (Ca2+(i)) under conventional whole-cell clamp abolished the slow potentiating (i.e. accelerated activation) and inactivating effects of Ca2+(o), disclosing fast potentiation (EC50: approximately 0.4 mM) and inhibition (IC50: approximately 4 mM) of I(TRPC6) and fast inhibition (IC50: approximately 0.4 mM) of I(TRPC7). This inhibition of I(TRPC6) and I(TRPC7) seems to be associated with voltage-dependent reductions of unitary conductance and open probability at the single channel level, whereas the potentiation of I(TRPC6) showed little voltage dependence and was mimicked by Sr2+ but not Ba2+. The activation process of I(TRPC6) or its acceleration by Ca2+(o) probably involves phosphorylation by calmodulin (CaM)-dependent kinase II (CaMKII), as pretreatment with calmidazolium (3 microM), coexpression of Ca2+-insensitive mutant CaM, and intracellular perfusion of the non-hydrolysable ATP analogue AMP-PNP and a CaMKII-specific inhibitory peptide all effectively prevented channel activation. However, this was not observed for TRPC7. Instead, single CCh-activated TRPC7 channel activity was concentration-dependently suppressed by nanomolar Ca2+(i) via CaM and conversely enhanced by IP3. In addition, the inactivation time course of I(TRPC6) was significantly retarded by pharmacological inhibition of protein kinase C (PKC). These results collectively suggest that TRPC6 and 7 channels are multiply regulated by Ca2+ from both sides of the membrane through differential Ca2+-CaM-dependent and -independent mechanisms.

Alpha1-adrenergic signaling mechanisms in contraction of resistance arteries

Our goal in this review is to provide a comprehensive, integrated view of the numerous signaling pathways that are activated by alpha(1)-adrenoceptors and control actin-myosin interactions (i.e., crossbridge cycling and force generation) in mammalian arterial smooth muscle. These signaling pathways may be categorized broadly as leading either to thick (myosin) filament regulation or to thin (actin) filament regulation. Thick filament regulation encompasses both "Ca(2+) activation" and "Ca(2+)-sensitization" as it involves both activation of myosin light chain kinase (MLCK) by Ca(2+)-calmodulin and regulation of myosin light chain phosphatase (MLCP) activity. With respect to Ca(2+) activation, adrenergically induced Ca(2+) transients in individual smooth muscle cells of intact arteries are now being shown by high resolution imaging to be sarcoplasmic reticulum-dependent asynchronous propagating Ca(2+) waves. These waves differ from the spatially uniform increases in [Ca(2+)] previously assumed. Similarly, imaging during adrenergic activation has revealed the dynamic translocation, to membranes and other subcellular sites, of protein kinases (e.g., Ca(2+)-activated protein kinases, PKCs) that are involved in regulation of MLCP and thus in "Ca(2+) sensitization" of contraction. Thin filament regulation includes the possible disinhibition of actin-myosin interactions by phosphorylation of CaD, possibly by mitogen-activated protein (MAP) kinases that are also translocated during adrenergic activation. An hypothesis for the mechanisms of adrenergic activation of small arteries is advanced. This involves asynchronous Ca(2+) waves in individual SMC, synchronous Ca(2+) oscillations (at high levels of adrenergic activation), Ca(2+) sparks, "Ca(2+)-sensitization" by PKC and Rho-associated kinase (ROK), and thin filament mechanisms.

Distribution and expression of non-muscle myosin light chain kinase in rabbit livers

AIM: To study the distribution and expression of non-muscle myosin light chain kinase (nmMLCK) in rabbit livers.

METHODS: Human nmMLCK N-terminal cDNA was amplified by polymerase chain reaction (PCR) and was inserted into pBKcmv to construct expression vectors. The recombinant plasmid was transformed into XL1-blue. Expression protein was induced by IPTG and then purified by SDS-PAGE and electroelution, which was used to prepare the polycolonal antibody to detect the distribution and expression of nmMLCK in rabbit livers with immunofluorescene techniques.

RESULTS: The polyclonal antibody was prepared, by which nmMLCK expression was detected and distributed mainly in peripheral hepatocytes.

CONCLUSION: nmMLCK can express in hepatocytes peripherally, and may play certain roles in the regulation of hepatic functions.

Synergism between inositol phosphates and diacylglycerol on native TRPC6-like channels in rabbit portal vein myocytes

In rabbit portal vein myocytes noradrenaline activates a non-selective cation current (Icat) which involves a transient receptor potential protein (TRPC6). Previously we have shown that the diaylglycerol (DAG) analogue 1-oleoyl-2-acetyl-sn-glycerol (OAG) stimulates Icat via a protein kinase C (PKC)-independent mechanism, and in the present study we have investigated the interaction between inositol phosphates (InsPs) and OAG on Icat. With whole-cell recording of Icat from freshly isolated rabbit portal vein myocytes the amplitude and rate of activation of noradrenaline-evoked Icat were much greater than those of OAG-induced Icat. Inclusion of inositol 1,4,5-trisphosphate (Ins(1,4,5)P3) in the pipette solution did not evoke Icat but greatly potentiated the amplitude and rate of activation of OAG-induced Icat. With isolated outside-out patches Ins(1,4,5)P3 markedly increased the rate of activation and the open probability of OAG-evoked channel activity, with no change in unitary conductance, channel mean open times or burst durations. The effects of Ins(1,4,5)P3 were mimicked by Ins(2,4,5)P3, 3-F-Ins(1,4,5)P3 and Ins(1,4)P2 but not by Ins(1,3,4,5)P4 and the potentiating effects of InsPs were not inhibited by heparin. Therefore it is concluded that both DAG and InsPs are necessary for full activation of Icat by noradrenaline and the effect of InsPs is via a heparin-insensitive mechanism and represents a novel action of InsPs.

Role of sarcoplasmic reticulum in control of membrane potential and nitrergic response in opossum lower esophageal sphincter

1. We previously demonstrated that a balance of Ca2+-activated Cl- current (ICl(Ca)) and K+ current activity sets the resting membrane potential of opossum lower esophageal sphincter (LES) circular smooth muscle at approximately -41 mV, which leads to continuous spike-like action potentials and the generation of basal tone. Ionic mechanisms underlying this basal ICl(Ca) activity and its nitrergic regulation remain unclear. Recent studies suggest that spontaneous Ca2+ release from sarcoplasmic reticulum (SR) and myosin light chain kinase (MLCK) play important roles. The current study investigated this possibility. Conventional intracellular recordings were performed on circular smooth muscle of opossum LES. Nerve responses were evoked by electrical square wave pulses of 0.5 ms duration at 20 Hz. 2. In the presence of nifedipine (1 microm), substance P (1 microm), atropine (3 microm) and guanethidine (3 microm), intracellular recordings demonstrated a resting membrane potential (MP) of -38.1+/-0.7 mV (n=25) with spontaneous membrane potential fluctuations (MPfs) of 1-3 mV. Four pulses of nerve stimulation induced slow inhibitory junction potentials (sIJPs) with an amplitude of 6.1+/-0.3 mV and a half-amplitude duration of 1926+/-147 ms (n=25). 3. 1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one (ODQ), a specific guanylyl cyclase inhibitor, abolished sIJPs, but had no effects on MPfs. Caffeine, a ryanodine receptor agonist, hyperpolarized MP and abolished sIJPs and MPfs. Ryanodine (20 microm) inhibited the sIJP and induced biphasic effects on MP, an initial small hyperpolarization followed by a large depolarization. sIJPs and MPfs were also inhibited by cyclopiazonic acid, an SR Ca2+ ATPase inhibitor. Specific ICl(Ca) and MLCK inhibitors hyperpolarized the MP and inhibited MPfs and sIJPs. 4. These data suggest that (1). spontaneous release of Ca2+ from the SR activates ICl(Ca), which in turn contributes to resting membrane potential; (2). MLCK is involved in activation of ICl(Ca); (3). inhibition of ICl(Ca) is likely to underlie sIJPs induced by nitrergic innervation.

Store-operated Ca2+-permeable non-selective cation channels in smooth muscle cells

Over twenty years ago it was shown that depletion of the intracellular Ca2+ store in smooth muscle triggered a Ca2+ influx mechanism. The purpose of this review it to describe recent electrophysiological data which indicate that Ca2+ influx occurs through discrete ion channels in the plasmalemma of smooth muscle cells. The effect of external Ca2+ on the amplitude and reversal potential of whole-cell and single channel currents suggests that there are at least two, and probably more, distinct store-operated channels (SOCs) which have markedly different permeabilities to Ca2+ ions. Two activation mechanisms have been identified which involve Ca2+ influx factor and protein kinase C (PKC) activation via diacylglycerol. In addition, in rabbit portal vein cells there is evidence that stimulation of alpha-adrenoceptors can stimulate SOC opening via PKC in a store-independent manner. There is at present little knowledge on the molecular identity of SOCs but it has been proposed that TRPC1 may be a component of the functional channel. We also summarise the data showing that SOCs may be involved in contraction and cell proliferation of smooth muscle. Finally, we highlight the similarities and differences of SOCs and receptor-operated cation channels that are present in native rabbit portal vein myocytes.

Activation of store-operated channels by noradrenaline via protein kinase C in rabbit portal vein myocytes

In the present study we have investigated the role of diacylglycerol (DAG) and protein kinase C (PKC) in mediating activation of Ca(2+)-permeable store-operated channels (SOCs) by noradrenaline in rabbit portal vein smooth muscle cells. With cell-attached recording, bath application of noradrenaline, 1-oleoyl-acetyl-sn-glycerol (OAG) and phorbol 12,13-dibutyrate (PDBu) evoked single channel currents. The biophysical properties of these channel currents were similar to those of the channel currents activated by depletion of internal Ca(2+) stores with cyclopiazonic acid (CPA). The activation of SOCs in cell-attached recording by noradrenaline, OAG, PDBu, CPA and the acetoxymethyl ester form of BAPTA (BAPTA-AM) was markedly inhibited by the PKC inhibitors chelerythrine and RO-31-8220. In isolated outside-out patches CPA did not evoke SOCs but noradrenaline stimulated SOC activity, which was reduced by about 90 % by PKC inhibitors. The addition of the serine/threonine phosphatase inhibitors calyculin A and microcystin also stimulated SOCs in isolated outside-out patches. It is concluded that in rabbit portal vein myocytes, noradrenaline activates SOCs via DAG and PKC, possibly by a store-independent mechanism. In addition in this cell type it appears that PKC and phosphorylation may play an important role in stimulating SOC activity in response to depletion of internal Ca(2+) stores by CPA and BAPTA-AM.

Recovery from muscarinic modulation of M current channels requires phosphatidylinositol 4,5-bisphosphate synthesis

Suppression of M current channels by muscarinic receptors enhances neuronal excitability. Little is known about the molecular mechanism of this inhibition except the requirement for a specific G protein and the involvement of an unidentified diffusible second messenger. We demonstrate here that intracellular ATP is required for recovery of KCNQ2/KCNQ3 current from muscarinic suppression, with an EC(50) of approximately 0.5 mM. Substitution of nonhydrolyzable ATP analogs for ATP slowed or prevented recovery. ADPbetaS but not ADP also prevented the recovery. Receptor-mediated inhibition was irreversible when recycling of agonist-sensitive pools of phosphatidylinositol-4,5-bisphosphate (PIP(2)) was blocked by lipid kinase inhibitors. Lipid phosphorylation by PI 4-kinase is required for recovery from muscarinic modulation of M current.

Role of Ca2+-activated Cl- channels and MLCK in slow IJP in opossum esophageal smooth muscle

The possible contribution of Ca2+-activated Cl- channel [I(Cl(Ca))] and myosin light-chain kinase (MLCK) to nonadrenergic, noncholinergic slow inhibitory junction potentials (sIJP) was studied using conventional intracellular microelectrode recordings in circular smooth muscle of opossum esophageal body and guinea pig ileum perfused with Krebs solution containing atropine (3 microM), guanethidine (3 microM), and substance P (1 microM). In opossum esophageal circular smooth muscle, resting membrane potential (MP) was -51.9 +/- 0.7 mV (n = 89) with MP fluctuations of 1-3 mV. A single square-wave nerve stimulation of 0.5 ms duration and 80 V induced a sIJP with amplitude of 6.3 +/- 0.2 mV, half-amplitude duration of 635 +/- 19 ms, and rebound depolarization amplitude of 2.4 +/- 0.1 mV (n = 89). 9-Anthroic acid (A-9-C), niflumic acid (NFA), wortmannin, and 1-(5-chloronaphthalene-1-sulfonyl)-1H-hexahydro-1,4-diazepine (ML-9) abolished MP fluctuations, sIJP, and rebound depolarization in a concentration-dependent manner. A-9-C and NFA but not wortmannin and ML-9 hyperpolarized MP. In guinea pig ileal circular smooth muscle, nerve stimulation elicited an IJP composed of both fast (fIJP) and slow (sIJP) components, followed by rebound depolarization. NFA (200 microM) abolished sIJP and rebound depolarization but left the fIJP intact. These data suggest that in the tissues studied, activation of I(Cl(Ca)), which requires MLCK, contributes to resting MP, and that closing of I(Cl(Ca)) is responsible for sIJP.

The developing relationship between receptor-operated and store-operated calcium channels in smooth muscle

Contraction of smooth muscle is initiated, and to a lesser extent maintained, by a rise in the concentration of free calcium in the cell cytoplasm ([Ca(2+)](i)). This activator calcium can originate from two intimately linked sources--the extracellular space and intracellular stores, most notably the sarcoplasmic reticulum. Smooth muscle contraction activated by excitatory neurotransmitters or hormones usually involves a combination of calcium release and calcium entry. The latter occurs through a variety of calcium permeable ion channels in the sarcolemma membrane. The best-characterized calcium entry pathway utilizes voltage-operated calcium channels (VOCCs). However, also present are several types of calcium-permeable channels which are non-voltage-gated, including the so-called receptor-operated calcium channels (ROCCs), activated by agonists acting on a range of G-protein-coupled receptors, and store-operated calcium channels (SOCCs), activated by depletion of the calcium stores within the sarcoplasmic reticulum. In this article we will review the electrophysiological, functional and pharmacological properties of ROCCs and SOCCs in smooth muscle and highlight emerging evidence that suggests that the two channel types may be closely related, being formed from proteins of the Transient Receptor Potential Channel (TRPC) family.

Preliminary research on myosin light chain kinase in rabbit liver

AIM: To study preliminarily the properties of myosin light chain kinase (MLCK) in rabbit liver.

METHODS: The expression of MLCK was detected by reverse transcription-polymerase chain reaction (RT-PCR); the MLCK was obtained from rabbit liver, and its activity was analyzed by γ-³² P incorporation technique to detect the phosphorylation of myosin light chain.

RESULTS: MLCK was expressed in rabbit liver, and the activity of the enzyme was similar to rabbit smooth muscle MLCK, and calmodulin- dependent. When the concentration was 0.65 mg •L¯¹, the activity was at the highest level.

CONCLUSION: MLCK expressed in rabbit liver may catalyze the phosphorylation of myosin light chain, which may play important roles in the regulation of hepatic cell functions.

Reconstitution in lipid bilayer of smooth muscle cation channels activated through a GTP-binding protein

Reconstitution of G-protein-coupled receptor activated cation channels into the lipid bilayer was attempted with plasma membrane vesicles prepared from guinea-pig ileal smooth muscle using the purification technique previously applied to the large conductance Ca2+-dependent and ATP-sensitive K+ channels (Toro et al., 1990). Under Na+-rich conditions, incorporation of plasma membrane vesicles into the bilayer produced GTPgammaS (100 microM)-activatable channel activities that are inhibited by GDPbetaS (1 mM), sensitive to Ca2+ and enhanced by depolarization. The reversal potential and unitary conductance (tens of picosiemens) of these channels varied in a manner dependent on Na+ concentration, but not affected by Cl-. These results strongly indicate that the reconstituted channels activated by GTPgammaS belong to a class of voltage-dependent, Ca2+-sensitive cation-selective channels that are activated through a G-protein, and correspond most likely to the muscarinic receptor-activated cation channels previously identified in the same preparation. These results also suggest potential usefulness of bilayer incorporation technique to investigate the receptor-operated cation channels in smooth muscle.

Comparison of spontaneous and noradrenaline-evoked non-selective cation channels in rabbit portal vein myocytes

The properties of non-selective cation channels were studied in rabbit portal vein smooth muscle cells in K+-free conditions with patch pipette techniques. In about 45 % of isolated outside-out patches spontaneous channel currents with a unitary conductance of 23 pS were observed. The reversal potential was +11 mV which was shifted to more negative values and the unitary conductance was reduced when part of the external Na+ was replaced by Tris. At negative potentials the probability of opening (Po) was low and the open time distributions were described by two exponentials with time constants of about 1 and 7 ms. At positive potentials Po and the longer mean open time were greatly increased. The channel exhibited bursting behaviour and the burst duration distributions were described by two exponentials with time constants of about 3 and 15 ms. At positive potentials the longer burst duration was increased substantially. Application of noradrenaline and the diacylglycerol analogue 1-oleoyl-2-acetyl-sn-glycerol (OAG) to the external membrane of quiescent patches evoked single-channel activity with a unitary conductance of about 23 pS and with similar kinetic behaviour to that of the spontaneous channel currents. In conclusion, noradrenaline and OAG activate non-selective cation channel currents in excised patches of rabbit portal vein myocytes with a unitary conductance and kinetic properties that are similar to those of spontaneous channel currents. These channels have two open states and exhibit bursting behaviour. It is suggested that these channels underlie the whole-cell currents evoked by noradrenaline and OAG.

Agents that increase tyrosine phosphorylation activate a non-selective cation current in single rabbit portal vein smooth muscle cells

The effects of agents that increase tyrosine phosphorylation were studied with whole-cell recording of membrane currents in single smooth muscle cells from the rabbit portal vein. In K+-free conditions with the chloride equilibrium potential at about -50 mV, intracellular application via the patch pipette of 200 microM sodium orthovanadate (Na3VO4), which inhibits tyrosine phosphatases, activated a "noisy" inward current at a holding potential of -50 mV. Intracellular dialysis with 100 microM (pY)EEI, a peptide activator of the cytosolic tyrosine kinase pp60c-src, and bath application of 5 microM insulin, which activates receptor-coupled tyrosine kinases, also evoked a "noisy" inward current. The current-voltage relationships and the reversal potential (about +10 mV) of the Na3VO4-, pp60c-src- and insulin-induced currents were similar to those of the noradrenaline-evoked non-selective cation current (Icat). The inward currents evoked by noradrenaline, Na3VO4, (pY)EEI and insulin were all greatly potentiated when the bathing calcium concentration was reduced from 1.5 mM to 50 microM. The single channel conductance estimated from spectral density analysis of the whole-cell current was about 20 pS for noradrenaline, Na3VO4, (pY)EEI and insulin. Moreover for all agents the spectra were described by the sum of two Lorentzians with similar corner frequencies. Noradrenaline-evoked Icat was inhibited to a similar degree by the tyrosine kinase inhibitors genistein and tyrphostin 23 and their inactive analogues daidzein and tyrphostin A1, respectively. In the presence of Na3VO4, application of noradrenaline evoked a cation current of similar peak amplitude to control Icat although the rate of decay of Icat was enhanced in the presence of Na3VO4. This study shows that stimulation of both cytosolic and receptor-coupled tyrosine kinases evokes a non-selective cation current and the conductance is similar to that activated by noradrenaline.

Electrophysiological recording methods used in vascular biology

Vascular tone can be regulated by drugs that alter the activities of membrane ionic channels located in endothelial or smooth muscle cells in the vascular wall. This review examines the methods that are available to investigate the activities and pharmacological modulation of ion channels in vascular cells. They range from classical sucrose-gap and sharp-microelectrode techniques for studies of intact vessels, to the now widely used patch-clamp techniques for voltage-clamp recording of single-channel and macroscopic currents in isolated cells. Each method is described, along with examples of applications and discussion of potential problems and limitations.