beta-Adrenergic receptor stimulation of L-type Ca2+ channels in rabbit portal vein myocytes involves both alphas and betagamma G protein subunits

Activation of the Thiazide-Sensitive Sodium-Chloride Cotransporter by Beta3-Adrenoreceptor in the Distal Convoluted Tubule

We previously showed that the beta-3 adrenergic receptor (BAR3) is expressed in most segments of the nephron where its agonism promotes a potent antidiuretic effect. We localized BAR3 in distal convoluted tubule (DCT) cells expressing the thiazide-sensitive sodium-chloride cotransporter (NCC). Aim of this study is to investigate the possible functional role of BAR3 on NCC modulation in DCT cells. Here, we found that, in mice, the knockout of BAR3 was paralleled by a significant attenuation of NCC phosphorylation, paralleled by reduced expression and activation of STE-20/SPS1-related proline-alanine-rich kinase (SPAK) and WNKs the main kinases involved in NCC activation. Conversely, in BAR1/2 knockout mice, we found reduced NCC abundance with no changes in the phosphorylation state of NCC. Moreover, selective BAR3 agonism promotes both SPAK and NCC activation in wild-type mouse kidney slices. In conclusion, our findings suggest a novel role for BAR3 in the regulation of NCC in DCT.

Fine tuning by protein kinases of CaV1.2 channel current in rat tail artery myocytes

Seventeen compounds, rather selective, direct or indirect inhibitors and activators of PKA, PKG, and PKC, were analysed for effects on vascular Ca_V1.2 channel current (I_Ca1.2) by using the patch-clamp technique in single rat tail artery myocytes. The aim was to investigate how PKs regulate I_Ca1.2 and disclose any unexpected modulation of Ca_V1.2 channel function by these agents. The cAMP analogues 8-Br-cAMP and 6-Bnz-cAMP partially reduced I_Ca1.2 in dialysed cells, while weakly increasing it under the perforated configuration. The β-adrenoceptor agonist isoproterenol and the adenylate cyclase activator forskolin concentration-dependently increased I_Ca1.2; this effect was reversed by PKA inhibitors H-89 and KT5720, but not by PKI 6-22. The cGMP analogue 8-Br-cGMP, similarly to the NO-donor SNP, moderately reduced I_Ca1.2, this effect being reversed to a slight stimulation under the perforated configuration. Among PKG inhibitors, Rp-8-Br-PET-cGMPS decreased current amplitude in a concentration-dependent manner while Rp-8-Br-cGMPS was ineffective. The non-specific phosphodiesterase inhibitor IBMX increased I_Ca1.2, while H-89, KT5720, and PKI 6-22 antagonized this effect. The PKC activator PMA, but not the diacylglycerol analogue OAG, stimulated I_Ca1.2 in a concentration-dependent manner; conversely, the PKCα inhibitor Gö6976 markedly reduced basal I_Ca1.2 and, similarly to the PKCδ (rottlerin) and PKCε translocation inhibitors antagonised PMA-induced current stimulation. The ensemble of findings indicates that the stimulation of cAMP/PKA, in spite of the paradoxical effect of both 8-Br-cAMP and 6-Bnz-cAMP, or PKC pathways enhanced, while that of cGMP/PKG weakly inhibited I_Ca1.2 in rat tail artery myocytes. Since Rp-8-Br-PET-cGMPS and Gö6976 appeared to block directly Ca_V1.2 channel, their docking to the channel protein was investigated. Both compounds appeared to bind the α_1C subunit in a region involved in Ca_V1.2 channel inactivation, forming an interaction network comparable to that of Ca_V1.2 channel blockers. Therefore, caution should accompany the use of these agents as pharmacological tools to elucidate the mechanism of action of drugs on vascular preparations.

Ion channels as effectors of cyclic nucleotide pathways: Functional relevance for arterial tone regulation

Numerous mediators and drugs regulate blood flow or arterial pressure by acting on vascular tone, involving cyclic nucleotide intracellular pathways. These signals lead to regulation of several cellular effectors, including ion channels that tune cell membrane potential, Ca²⁺ influx and vascular tone. The characterization of these vasocontrictive or vasodilating mechanisms has grown in complexity due to i) the variety of ion channels that are expressed in both vascular endothelial and smooth muscle cells, ii) the heterogeneity of responses among the various vascular beds, and iii) the number of molecular mechanisms involved in cyclic nucleotide signalling in health and disease. This review synthesizes key data from literature that highlight ion channels as physiologically relevant effectors of cyclic nucleotide pathways in the vasculature, including the characterization of the molecular mechanisms involved. In smooth muscle cells, cation influx or chloride efflux through ion channels are associated with vasoconstriction, whereas K⁺ efflux repolarizes the cell membrane potential and mediates vasodilatation. Both categories of ion currents are under the influence of cAMP and cGMP pathways. Evidence that some ion channels are influenced by CN signalling in endothelial cells will also be presented. Emphasis will also be put on recent data touching a variety of determinants such as phosphodiesterases, EPAC and kinase anchoring, that complicate or even challenge former paradigms.

let-7e replacement yields potent anti-arrhythmic efficacy via targeting beta 1-adrenergic receptor in rat heart

Beta-adrenoceptor (β-AR) exerts critical regulation of cardiac function. MicroRNAs (miRNAs) are potentially involved in a variety of biological and pathological processes. This study aimed to investigate the role of miRNA let-7e in the up-regulation of β₁-AR and arrhythmogenesis in acute myocardial infarction (AMI) in rats. β₁-AR expression was significantly up-regulated and let-7a, c, d, e and i were markedly down-regulated in the infarcted heart after 6 and 24 hrs myocardial infarction. Forced expression of let-7e suppressed β₁-AR expression at the protein level, without affecting β₁-AR mRNA level, in neonatal rat ventricular cells (NRVCs). Silencing of let-7e by let-7e antisense inhibitor (AMO-let-7e) enhanced β₁-AR expression at the protein level in NRVCs. Administration of the lentivirus vector containing precursor let-7e (len-pre-let-7e) significantly inhibited β₁-AR expression in rats, whereas len-AMO-let-7e up-regulated β₁-AR relative to the baseline control level, presumably as a result of depression of tonic inhibition of β₁-AR by endogenous let-7e. Len-negative control (len-NC) did not produce significant influence on β₁-AR expression. Len-pre-let-7e also profoundly reduced the up-regulation of β₁-AR induced by AMI and this effect was abolished by len-AMO-let-7e. Importantly, len-pre-let-7e application significantly reduced arrhythmia incidence after AMI in rats and its anti-arrhythmic effect was cancelled by len-AMO-let-7e. Notably, anti-arrhythmic efficacy of len-pre-let-7e was similar to propranolol, a non-selective β-AR blocker and metoprolol, a selective β₁-AR blocker. Down-regulation of let-7e contributes to the adverse increase in β₁-AR expression in AMI and let-7e supplement may be a new therapeutic approach for preventing adverse β₁-AR up-regulation and treating AMI-induced arrhythmia.

The cellular and molecular basis of bitter tastant-induced bronchodilation

Bitter tastants can activate bitter taste receptors on constricted smooth muscle cells to inhibit L-type calcium channels and induce bronchodilation.

Bronchodilators are a standard medicine for treating airway obstructive diseases, and β2 adrenergic receptor agonists have been the most commonly used bronchodilators since their discovery. Strikingly, activation of G-protein-coupled bitter taste receptors (TAS2Rs) in airway smooth muscle (ASM) causes a stronger bronchodilation in vitro and in vivo than β2 agonists, implying that new and better bronchodilators could be developed. A critical step towards realizing this potential is to understand the mechanisms underlying this bronchodilation, which remain ill-defined. An influential hypothesis argues that bitter tastants generate localized Ca²⁺ signals, as revealed in cultured ASM cells, to activate large-conductance Ca²⁺-activated K⁺ channels, which in turn hyperpolarize the membrane, leading to relaxation. Here we report that in mouse primary ASM cells bitter tastants neither evoke localized Ca²⁺ events nor alter spontaneous local Ca²⁺ transients. Interestingly, they increase global intracellular [Ca²⁺]_i, although to a much lower level than bronchoconstrictors. We show that these Ca²⁺ changes in cells at rest are mediated via activation of the canonical bitter taste signaling cascade (i.e., TAS2R-gustducin-phospholipase Cβ [PLCβ]- inositol 1,4,5-triphosphate receptor [IP3R]), and are not sufficient to impact airway contractility. But activation of TAS2Rs fully reverses the increase in [Ca²⁺]_i induced by bronchoconstrictors, and this lowering of the [Ca²⁺]_i is necessary for bitter tastant-induced ASM cell relaxation. We further show that bitter tastants inhibit L-type voltage-dependent Ca²⁺ channels (VDCCs), resulting in reversal in [Ca²⁺]_i, and this inhibition can be prevented by pertussis toxin and G-protein βγ subunit inhibitors, but not by the blockers of PLCβ and IP3R. Together, we suggest that TAS2R stimulation activates two opposing Ca²⁺ signaling pathways via Gβγ to increase [Ca²⁺]_i at rest while blocking activated L-type VDCCs to induce bronchodilation of contracted ASM. We propose that the large decrease in [Ca²⁺]_i caused by effective tastant bronchodilators provides an efficient cell-based screening method for identifying potent dilators from among the many thousands of available bitter tastants.

Bitter taste receptors (TAS2Rs), a G-protein-coupled receptor family long thought to be solely expressed in taste buds on the tongue, have recently been detected in airways. Bitter substances can activate TAS2Rs in airway smooth muscle to cause greater bronchodilation than β2 adrenergic receptor agonists, the most commonly used bronchodilators. However, the mechanisms underlying this bronchodilation remain elusive. Here we show that, in resting primary airway smooth muscle cells, bitter tastants activate a TAS2R-dependent signaling pathway that results in an increase in intracellular calcium levels, albeit to a level much lower than that produced by bronchoconstrictors. In bronchoconstricted cells, however, bitter tastants reverse the bronchoconstrictor-induced increase in calcium levels, which leads to the relaxation of smooth muscle cells. We find that this reversal is due to inhibition of L-type calcium channels. Our results suggest that under normal conditions, bitter tastants can activate TAS2Rs to modestly increase calcium levels, but that when smooth muscle cells are constricted, they can block L-type calcium channels to induce bronchodilation. We postulate that this novel mechanism could operate in other extraoral cells expressing TAS2Rs.

β-adrenoceptor regulates miRNA expression in rat heart

Background

MicroRNAs (miRNAs) are noncoding RNAs of 18–25 nucleotides that post-transcriptionally regulate gene expression and are involved in a wide range of physiological and pathological conditions. The β-adrenergic signaling pathway plays a fundamental role in regulation of heart function. The present study was designed to investigate the expression profile of miRNAs and functional implications under conditions of β-adrenoceptor activation or inhibition in rat heart.

Material/Methods

Hemodynamic parameters were measured to assess heart function in Wistar rats treated with isoproterenol (ISO) or propranolol (PRO). miRNA expression was analyzed by miRNA Microarray and confirmed by real-time quantitative reverse transcription PCR (real-time qRT-PCR).

Results

Isoproterenol (ISO, a β-adrenoceptor activator) and propranolol (PRO, a β-adrenoceptor inhibitor) induced differential miRNA expression profiles. Out of 349 miRNAs measured, 43 were upregulated and nine downregulated in the ISO group, while five miRNAs were upregulated and 28 downregulated in PRO group. Among these altered miRNAs in both PRO and ISO groups, 11 were cardiac abundant and 11 showed opposite profiles between the PRO and ISO groups. The recognized anti-hypertrophic miRNAs miR-1, miR-21 and miR-27b, and the pro-hypertrophic miRNAs miR-22, miR-24, miR-199a, miR-212 and miR-214, were upregulated in the ISO group. In the PRO group, pro-hypertrophic miRNA miR-30c was upregulated, whereas miR-212 was downregulated.

Conclusions

β-adrenoceptor intervention alters miRNA expression profile, and miRNAs may be involved in the β-adrenoceptor signaling pathway. Cardiomyocyte hypertrophy is a balanced process between pro-hypertrophic and anti-hypertrophic regulation and involves, at the very least, miRNA participation.

Modulation of distinct isoforms of L-type calcium channels by G(q)-coupled receptors in Xenopus oocytes: antagonistic effects of Gβγ and protein kinase C

L-type voltage dependent Ca(2+) channels (L-VDCCs; Ca(v)1.2) are crucial in cardiovascular physiology. In heart and smooth muscle, hormones and transmitters operating via G(q) enhance L-VDCC currents via essential protein kinase C (PKC) involvement. Heterologous reconstitution studies in Xenopus oocytes suggested that PKC and G(q)-coupled receptors increased L-VDCC currents only in cardiac long N-terminus (NT) isoforms of α(1C), whereas known smooth muscle short-NT isoforms were inhibited by PKC and G(q) activators. We report a novel regulation of the long-NT α(1C) isoform by Gβγ. Gβγ inhibited whereas a Gβγ scavenger protein augmented the G(q)--but not phorbol ester-mediated enhancement of channel activity, suggesting that Gβγ acts upstream from PKC. In vitro binding experiments reveal binding of both Gβγ and PKC to α(1C)-NT. However, PKC modulation was not altered by mutations of multiple potential phosphorylation sites in the NT, and was attenuated by a mutation of C-terminally located serine S1928. The insertion of exon 9a in intracellular loop 1 rendered the short-NT α(1C) sensitive to PKC stimulation and to Gβγ scavenging. Our results suggest a complex antagonistic interplay between G(q)-activated PKC and Gβγ in regulation of L-VDCC, in which multiple cytosolic segments of α(1C) are involved.

The promise of inhibition of smooth muscle tone as a treatment for erectile dysfunction: where are we now?

Ten years ago, the inhibition of Rho kinase by intracavernosal injection of Y-27632 was found to induce an erectile response. This effect did not require activation of nitric oxide-mediated signaling, introducing a novel target pathway for the treatment of erectile dysfunction (ED), with potential added benefit in cases where nitric oxide bioavailability is attenuated (and thus phosphodiesterase type 5 (PDE5) inhibitors are less efficacious). Rho-kinase antagonists are currently being developed and tested for a wide range of potential uses. The inhibition of this calcium-sensitizing pathway results in blood vessel relaxation. It is also possible that blockade of additional smooth muscle contractile signaling mechanisms may have the same effect. In this review, we conducted an extensive search of pertinent literature using PUBMED. We have outlined the various pathways involved in the maintenance of penile smooth muscle tone and discussed the current potential benefit for the pharmacological inhibition of these targets for the treatment of ED.

Role of oxidative stress and nitric oxide in atherothrombosis

During the last decade basic and clinical research has highlighted the central role of reactive oxygen species (ROS) in cardiovascular disease. Enhanced production or attenuated degradation of ROS leads to oxidative stress, a process that affects endothelial and vascular function, and contributes to vascular disease. Nitric oxide (NO), a product of the normal endothelium, is a principal determinant of normal endothelial and vascular function. In states of inflammation, NO production by the vasculature increases considerably and, in conjunction with other ROS, contributes to oxidative stress. This review examines the role of oxidative stress and NO in mechanisms of endothelial and vascular dysfunction with an emphasis on atherothrombosis.

Calcium sparklets regulate local and global calcium in murine arterial smooth muscle

In arterial smooth muscle, protein kinase Calpha (PKCalpha) coerces discrete clusters of L-type Ca2+ channels to operate in a high open probability mode, resulting in subcellular domains of nearly continual Ca2+ influx called 'persistent Ca2+ sparklets'. Our previous work suggested that steady-state Ca2+ entry into arterial myocytes, and thus global [Ca2+]i, is regulated by Ca2+ influx through clusters of L-type Ca2+ channels operating in this persistently active mode in addition to openings of solitary channels functioning in a low-activity mode. Here, we provide the first direct evidence supporting this 'Ca2+ sparklet' model of Ca2+ influx at a physiological membrane potential and external Ca2+ concentration. In support of this model, we found that persistent Ca2+ sparklets produced local and global elevations in [Ca2+]i. Membrane depolarization increased Ca2+ influx via low-activity and high-activity persistent Ca2+ sparklets. Our data indicate that Ca2+ entering arterial smooth muscle through persistent Ca2+ sparklets accounts for approximately 50% of the total dihydropyridine-sensitive (i.e. L-type Ca2+ channel) Ca2+ influx at a physiologically relevant membrane potential (-40 mV) and external Ca2+ concentration (2 mm). Consistent with this, inhibition of basal PKCalpha-dependent persistent Ca2+ sparklets decreased [Ca2+]i by about 50% in isolated arterial myocytes and intact pressurized arteries. Taken together, these data support the conclusion that in arterial smooth muscle steady-state Ca2+ entry and global [Ca2+]i are regulated by low-activity and PKCalpha-dependent high-activity persistent Ca(2+) sparklets.

Conditional stimulation of type V and VI adenylyl cyclases by G protein betagamma subunits

In a yeast two-hybrid screen of mouse brain cDNA library, using the N-terminal region of human type V adenylyl cyclase (hACV) as bait, we identified G protein beta2 subunit as an interacting partner. Additional yeast two-hybrid assays showed that the Gbeta(1) subunit also interacts with the N-terminal segments of hACV and human type VI adenylyl cyclase (hACVI). In vitro adenylyl cyclase (AC) activity assays using membranes of Sf9 cells expressing hACV or hACVI showed that Gbetagamma subunits enhance the activity of these enzymes provided either Galpha(s) or forskolin is present. Deletion of residues 77-151, but not 1-76, in the N-terminal region of hACVI obliterated the ability of Gbetagamma subunits to conditionally stimulate the enzyme. Likewise, activities of the recombinant, engineered, soluble forms of ACV and ACVI, which lack the N termini, were not enhanced by Gbetagamma subunits. Transfection of the C terminus of G protein receptor kinase 2 to sequester endogenous Gbetagamma subunits attenuated the ability of isoproterenol to increase cAMP accumulation in COS-7 cells overexpressing hACVI even when G(i) was inactivated by pertussis toxin. Therefore, we conclude that the N termini of human hACV and hACVI are necessary for interactions with, and regulation by, Gbetagamma subunits both in vitro and in intact cells. Moreover, Gbetagamma subunits derived from a source(s) other than G(i) are necessary for the full activation of hACVI by isoproterenol in intact cells.

Signaling pathway underlying stimulation of L-type Ca2+ channels in rabbit portal vein myocytes by recombinant Gbetagamma subunits

In previous studies, we (Callaghan B, Koh SD, and Keef KD, Circ Res 94: 626-633, 2004) have shown that voltage-dependent L-type Ca(2+) channels (Cav) in portal vein myocytes are enhanced when muscarinic M2 receptors are activated with ACh. Current stimulation was coupled to the G protein subunit Gbetagamma along with the downstream mediators phosphatidylinositol-3-kinase (PI3K), protein kinase C (PKC), and c-Src. The present study was designed to determine whether the same second messenger pathway could be identified when exogenous recombinant Gbetagamma subunits are introduced into cells. Smooth muscle myocytes were freshly isolated from rabbit portal vein, and Cav currents were recorded by using the patch-clamp technique. Dialysis of cells with recombinant Gbetagamma (50 nM) significantly increased Cav currents (141%). Nifedipine (1 microM) reduced both control and stimulated currents by approximately 90%. The enhancement of current by Gbetagamma was equivalent to that produced by ACh (142%), whereas the PKC activator phorbol 12,13-dibutyrate (PdBu) gave rise to greater current stimulation (192%). Current stimulation with Gbetagamma, ACh, and PdBu were not associated with changes in the voltage dependence of activation or inactivation. The PI3K inhibitor LY-294002 (20 microM) reduced peak currents by 32% in cells dialyzed with Gbetagamma, whereas the inactive analog LY-303511 resulted in a small but significant reduction in current (12%). The c-Src inhibitor PP2 (1 microM) also significantly reduced currents (34%), whereas the inactive analog PP3 was without effect. These data provide further evidence for the hypothesis that Gbetagamma leads to stimulation of Cav currents in rabbit portal vein myocytes via a signaling pathway that includes PI3K, PKC, and c-Src.

Role of GPR40 in fatty acid action on the beta cell line INS-1E

GPR40 is a G protein-coupled receptor expressed preferentially in beta cells, that has been implicated in mediating free fatty acid-stimulated insulin release. GPR40 RNAi impaired the ability of palmitic acid (PA) to increase both insulin secretion and intracellular calcium ([Ca2+]i). The PA-dependent [Ca2+]i increase was attenuated by inhibitors of Galphaq, PLC, and SERCA. Thus GPR40 activates the Galphaq pathway, leading to release of Ca2+ from the ER. Yet the GPR40-dependent [Ca2+]i rise was dependent on extracellular Ca2+ and elevated glucose, and was blocked by inhibition of L-type calcium channels (LTCC) or opening of the K(ATP) channel; this suggests that GPR40 promotes Ca2+ influx through up-regulation of LTCC pre-activated by glucose and membrane depolarization. Taken together, the data indicate that GPR40 mediates the increase in [Ca2+]i and insulin secretion through the Galphaq-PLC pathway, resulting in release of Ca2+ from the ER and leading to up-regulation of Ca2+ influx via LTCC.

Hypotonic swelling stimulates L-type Ca2+ channel activity in vascular smooth muscle cells through PKC

It has been suggested that L-type Ca(2+) channels play an important role in cell swelling-induced vasoconstriction. However, there is no direct evidence that Ca(2+) channels in vascular smooth muscle are modulated by cell swelling. We tested the hypothesis that L-type Ca(2+) channels in rabbit portal vein myocytes are modulated by hypotonic cell swelling via protein kinase activation. Ba(2+) currents (I(Ba)) through L-type Ca(2+) channels were recorded in smooth muscle cells freshly isolated from rabbit portal vein with the conventional whole cell patch-clamp technique. Superfusion of cells with hypotonic solution reversibly enhanced Ca(2+) channel activity but did not alter the voltage-dependent characteristics of Ca(2+) channels. Bath application of selective inhibitors of protein kinase C (PKC), Ro-31-8425 or Go-6983, prevented I(Ba) enhancement by hypotonic swelling, whereas the specific protein kinase A (PKA) inhibitor KT-5720 had no effect. Bath application of phorbol 12,13-dibutyrate (PDBu) significantly increased I(Ba) under isotonic conditions and prevented current stimulation by hypotonic swelling. However, PDBu did not have any effect on I(Ba) when cells were first exposed to hypotonic solution. Furthermore, downregulation of endogenous PKC by overnight treatment of cells with PDBu prevented current enhancement by hypotonic swelling. These data suggest that hypotonic cell swelling can enhance Ca(2+) channel activity in rabbit portal vein smooth muscle cells through activation of PKC.

Muscarinic M2 Receptor Stimulation of Cav1.2b Requires Phosphatidylinositol 3-Kinase, Protein Kinase C, and c-Src

This study investigated regulation of L-type calcium channels (Cav1.2b) by acetylcholine (ACh) in rabbit portal vein myocytes. Whole-cell currents were recorded using 5 mmol/L barium as charge carrier. ACh (10 μmol/L) increased peak currents by 40%. This effect was not reversed by the selective muscarinic M3 receptor antagonist 4-DAMP (100 nmol/L) but was blocked by the M2 receptor antagonist methoctramine (5 μmol/L). The classical and novel protein kinase C (PKC) antagonist calphostin C (50 nmol/L) abolished ACh responses, whereas the classical PKC antagonist Gö6976 (200 nmol/L) had no effect. ACh responses were also abolished by the phosphatidylinositol 3-kinase (PI3K) inhibitor LY294002 (20 μmol/L), by the c-Src inhibitor PP2 (10 μmol/L) (but not the inactive analogue PP3), and by dialyzing cells with an antibody to the G-protein subunit Gβγ. Cells dialyzed with c-Src had significantly greater currents than control cells. Current enhancement persisted in the presence of LY294002, suggesting that c-Src is downstream of PI3K. Phorbol 12,13-dibutyrate (PDBu, 0.1 μmol/L) increased currents by 74%. This effect was abolished by calphostin C and reduced by Gö6976. The PDBu response was also reduced by PP2, and the PP2-insensitive component was blocked by Gö6976. In summary, these data suggest that ACh enhances Cav1.2b currents via M2 receptors that couple sequentially to Gβγ, PI3K, a novel PKC, and c-Src. PDBu stimulates the novel PKC/c-Src pathway along with a second pathway that is independent of c-Src and involves a classical PKC.

Sphingosine-1-phosphate stimulates contraction of human airway smooth muscle cells

The bioactive sphingolipid sphingosine-1-phosphate (S1P) that is increased in airways of asthmatic subjects markedly induced contraction of human airway smooth muscle (HASM) cells embedded in collagen matrices in a Gi-independent manner. Dihydro-S1P, which binds to S1P receptors, also stimulated contractility. S1P induced formation of stress fibers, contraction of individual HASM cells, and stimulated myosin light chain phosphorylation, which was inhibited by the Rho-associated kinase inhibitor Y-27632. S1P-stimulated HASM cell contractility was independent of the ERK1/2 and PKC signaling pathways, important regulators of airway smooth muscle contraction. However, removal of extracellular calcium completely blocked S1P-mediated contraction and Y-27632 reduced it. S1P also induced calcium mobilization that was not desensitized by repeated additions. Pretreatment with thapsigargin to deplete InsP3-sensitive calcium stores partially blocked increases in [Ca2+]i induced by S1P, yet did not inhibit S1P-stimulated contraction. In sharp contrast, the L-type calcium channel blocker verapamil markedly decreased S1P-induced HASM cell contraction, supporting a role for calcium influx from extracellular sources. Collectively, our results suggest that S1P may regulate HASM contractility, important in the pathobiology of asthma.

Different effects of mibefradil and amlodipine on coronary vessels and during beta-adrenergic stimulation in conscious dogs

Coronary effects of Ca -channel blockers mibefradil and amlodipine were compared in conscious dogs. Ten dogs were instrumented for measurement of aortic and left ventricular pressures, circumflex coronary blood flow velocity (CBFv), and coronary diameter (CD). A permanent catheter was implanted in the circumflex coronary artery. At doses having no systemic effects (7.5-150 micro g/kg), mibefradil and amlodipine increased CBFv and CD dose dependently. At the same dose, mibefradil increased less CBFv than amlodipine. However, for a similar increase in CBFv induced by amlodipine, mibefradil increased CD more. BAY K8644, an L-type Ca -channel agonist, prevented the CBFv and CD responses to amlodipine, but minimally affected the coronary responses to mibefradil. Intracoronary isoproterenol (6 ng/kg) increased LV dP/dt max, CBFv, and CD. Amlodipine markedly altered these responses, while mibefradil did not affect LV inotropic response and slightly altered CBFv response to isoproterenol. Thus, in conscious dogs, both mibefradil and amlodipine exert coronary vasodilation, with different patterns on coronary conductance and resistance vessels and during beta-adrenergic stimulation. These differences could be related to their actions on different Ca channels.

Coupling of M2 muscarinic receptor to L-type Ca channel via c-src kinase in rabbit colonic circular smooth muscle

BACKGROUND & AIMS

The L-type Ca(2+) channel is a major pathway for Ca(2+) influx in colonic smooth muscle and is modulated by endogenous levels of nonreceptor tyrosine kinase, c-src. Tyrosine kinases are also activated by G-protein-coupled receptors (GPCR). This study determined whether muscarinic receptor couples to Ca(2+) channels via c-src kinase.

METHODS

Currents were measured in rabbit colonic smooth muscle cells and in transfected HEK293 cells by patch-clamp technique. Tyrosyl phosphorylated proteins were detected by Western blots and the interaction of c-src with the c-terminus of alpha subunit of Ca(2+) channel was determined by a GST pull-down assay.

RESULTS

Methacholine (10 micromol/L) enhanced Ca(2+) channel currents by 30% under conditions whereby the M(3) receptor pathway was blocked by either 4-DAMP or by intracellular dialysis with anti-Galphaq antibody. Similar effects were observed by blocking intracellular Ca(2+) release with heparin. Enhancement was abolished by intracellular anti-Galphai antibody and by the c-src inhibitor, PP2 but unaffected by the inactive analog PP3. Immunoblot with anti-src antibody revealed increased src phosphorylation by muscarinic receptor stimulation. Purified c-src directly associated with the c-terminus of alpha1c subunit of the Ca(2+) channel. In M(2) receptor transfected HEK293 cells, currents were enhanced 2-fold by carbachol.

CONCLUSIONS

These studies demonstrate stimulation of Ca(2+) current in colonic smooth muscle cells by M2 receptor coupled to Galphai-G protein and c-src activation. They also suggest a central role of c-src kinase in the cross-talk between tyrosine kinase receptor and GPCR.

Sphingosine 1-phosphate evokes calcium signals in C2C12 myoblasts via Edg3 and Edg5 receptors

Sphingosine 1-phosphate (SPP) is a bioactive lipid that exerts multiple biological effects in a large variety of cell types, acting as either an intracellular messenger or an extracellular ligand coupled to Edg-family receptors (where Edg stands for endothelial differentiation gene). Here we report that in C(2)C(12) myoblasts SPP elicited significant Ca(2+) mobilization. Analysis of the process using a confocal laser-scanning microscope showed that the Ca(2+) response occurred in a high percentage of cells, despite variations in amplitude and kinetics. Quantitative analysis of SPP-induced Ca(2+) transients performed with a spectrophotofluorimeter showed that the rise in Ca(2+) was strictly dependent on availability of extracellular Ca(2+). Cell treatment with pertussis toxin partially prevented the Ca(2+) response induced by SPP, indicating that G(i)-coupled-receptors were involved. Indeed, SPP action was shown to be mediated by agonist-specific Edg receptors. In particular, suramin, an antagonist of the SPP-specific receptor Edg3, as well as down-regulation of Edg3 by cell transfection with antisense oligodeoxyribonucleotides (ODN), significantly reduced agonist-mediated Ca(2+) mobilization. Moreover, an antisense ODN designed to inhibit Edg5 expression also decreased the SPP-induced rise in Ca(2+), although to a lesser extent than that observed by inhibiting Edg3. On the contrary, the SPP response was unaffected in myoblasts loaded with antisense ODN specific for Edg1. Remarkably, the concomitant inhibition of Edg3 and Edg5 receptors abolished the SPP-induced Ca(2+) increase, supporting the notion that Ca(2+) mobilization in C(2)C(12) cells induced by SPP is a receptor-mediated process that involves Edg3 and Edg5, but not Edg1.