Desensitization of G-protein-coupled receptors in the cardiovascular system

Randomised clinical trial: A phase 2 double-blind study of namodenoson in non-alcoholic fatty liver disease and steatohepatitis

BACKGROUND

Namodenoson, an A3 adenosine receptor (A3AR) agonist, improved liver function/pathology in non-alcoholic steatohepatitis (NASH) preclinical models.

AIM

To evaluate the efficacy and safety of namodenoson for the treatment of non-alcoholic fatty liver disease (NAFLD) with or without NASH METHODS: This phase 2 study included 60 patients with NAFLD (ALT ≥60 IU/L) who were randomised (1:1:1) to oral namodenoson 12.5 mg b.d. (n = 21), 25 mg b.d. (n = 19), or placebo (n = 20) for 12 weeks (total follow-up: 16 weeks). The main efficacy endpoint involved serum ALT after 12 weeks of treatment.

RESULTS

Serum ALT decreased over time with namodenoson in a dose-dependent manner. The difference between change from baseline (CFB) for ALT in the namodenoson 25 mg b.d. arm vs placebo trended towards significance at 12 weeks (P = 0.066). Serum AST levels also decreased with namodenoson in a dose-dependent manner; at 12 weeks, the CFB for 25 mg b.d. vs placebo was significant (P = 0.03). At Week 12, 31.6% in the namodenoson 25 mg b.d. arm and 20.0% in the placebo arm achieved ALT normalisation (P = 0.405). At week 16, the respective rates were 36.8% and 10.0% (P = 0.038). A3AR expression levels were stable over time across study arms. Both doses of namodenoson were well tolerated with no drug-emergent severe adverse events, drug-drug interactions, hepatotoxicity, or deaths. Three adverse events were considered possibly related to study treatment: myalgia (12.5 mg b.d. arm), muscular weakness (25 mg b.d. arm), and headache (25 mg b.d. arm).

CONCLUSION

A3AR is a valid target; namodenoson 25 mg b.d. was safe and demonstrated efficacy signals (ClinicalTrials.gov #NCT02927314).

Cerebral and systemic hemodynamic effect of recurring seizures

The increase in neuronal activity induced by a single seizure is supported by a rise in the cerebral blood flow and tissue oxygenation, a mechanism called neurovascular coupling (NVC). Whether cerebral and systemic hemodynamics are able to match neuronal activity during recurring seizures is unclear, as data from rodent models are at odds with human studies. In order to clarify this issue, we used an invasive brain and systemic monitoring to study the effects of chemically induced non-convulsive seizures in sheep. Despite an increase in neuronal activity as seizures repeat (Spearman’s ρ coefficient 0.31, P < 0.001), ictal variations of cerebral blood flow remained stable while it progressively increased in the inter-ictal intervals (ρ = 0.06, P = 0.44 and ρ = 0.22; P = 0.008). We also observed a progressive reduction in the inter-ictal brain tissue oxygenation (ρ =  − 0.18; P = 0.04), suggesting that NVC was unable to compensate for the metabolic demand of these closely repeating seizures. At the systemic level, there was a progressive reduction in blood pressure and a progressive rise in cardiac output (ρ =  − 0.22; P = 0.01 and ρ = 0.22; P = 0.01, respectively), suggesting seizure-induced autonomic dysfunction.

Namodenoson in Advanced Hepatocellular Carcinoma and Child-Pugh B Cirrhosis: Randomized Placebo-Controlled Clinical Trial

Namodenoson, an A3 adenosine-receptor agonist, showed promising results in advanced hepatocellular carcinoma (HCC) and moderate hepatic dysfunction (Child-Pugh B; CPB) in a phase I/II clinical study. This phase II study investigated namodenoson as second-line therapy in such patients. Patients were randomized 2:1 to twice a day (BID) namodenoson (25 mg; n = 50) or placebo (n = 28). The primary endpoint (overall survival [OS]) was not met. Median OS was 4.1/4.3 months for namodenoson/placebo (hazard ratio [HR], 0.82; 95% confidence interval [CI] 0.49-1.38; p = 0.46). Pre-planned subgroup analysis of CPB7 patients (34 namodenoson-treated, 22 placebo-treated) showed a nonsignificant improvement in OS/progression-free survival (PFS). OS: 6.9 versus 4.3 months; HR, 0.81; 95% CI: 0.45-1.43, p = 0.46. PFS: 3.5 versus 1.9 months; HR, 0.89; 95% CI: 0.51-1.55, p = 0.67 (log-rank test). The difference in 12-month OS was significant (44% versus 18%, p = 0.028). Response rates were determined in patients for whom ≥ 1 assessment post-baseline was available (34 namodenoson-treated, 21 placebo-treated). Partial response was achieved by 3/34 (8.8%) and 0/21 (0%) patients, respectively. Namodenoson was well-tolerated, with a safety profile comparable to that of the placebo group. No treatment-related deaths were reported; no patients withdrew due to toxicity. In conclusion, namodenoson demonstrated a favorable safety profile and a preliminary efficacy signal in HCC CPB.

The effect of mild agonist stimulation on the platelet reactivity in patients with type 2 diabetes mellitus

BACKGROUND

Patients with type 2 diabetes mellitus (T2DM) have accelerated atherosclerosis as a pro thrombotic state that is associated with the platelet activation priming. Platelets, which undergo the continuous mild stimulation, may lose their sensitivity to react to a strong stimulation. The present study aimed to investigate activation responses of platelets to mild and subsequent strong stimulations in patients with T2DM and healthy individuals.

METHODS

Blood samples, which were taken from 40 patients with T2DM and 35 healthy individuals, were collected into the citrate containing tubes. The samples were subjected to the soft centrifugation to prepare the platelet rich plasma (PRP). Platelets in PRP samples were treated at a low (1 μM) concentration and then at a high (10 μM) concentration of ADP. Before and after stimulation with different doses of ADP, levels of CD62P expression and formation of platelet micro particles (PMPs) were measured using a flow cytometry method.

RESULTS

The platelets from patients with T2DM had higher levels of CD62P expression before any stimulation (P = 0.003) than control samples. Platelets, which underwent the mild stimulation, indicated lower responses to CD62P expression, but higher PMPs formation after stimulation with high dose of ADP. Patients with T2DM had higher platelet micro particles in all states with the ADP stimulation. (P = 0.004, SD: ±74.52).

CONCLUSIONS

The flow cytometry data indicated that platelets were pre-active and associated with metabolic conditions in patients with type 2 diabetes mellitus. The induction of desensitization state helped platelets to reduce the platelet activation and sensitivity to ADP in a diabetic environment. Furthermore, the production of platelets micro-particles was high in the patients; and desensitized platelets were more susceptible to shedding of micro-particles.

Short-term desensitization of muscarinic K+ current in the heart

Acetylcholine (ACh) rapidly increases cardiac K(+) currents (IKACh) by activating muscarinic K(+) (KACh) channels followed by a gradual amplitude decrease within seconds. This phenomenon is called short-term desensitization and its precise mechanism and physiological role are still unclear. We constructed a mathematical model for IKACh to examine the conditions required to reconstitute short-term desensitization. Two conditions were crucial: two distinct muscarinic receptors (m2Rs) with different affinities for ACh, which conferred an IKACh response over a wide range of ACh concentrations, and two distinct KACh channels with different affinities for the G-protein βγ subunits, which contributed to reconstitution of the temporal behavior of IKACh. Under these conditions, the model quantitatively reproduced several unique properties of short-term desensitization observed in myocytes: 1), the peak and quasi-steady states with 0.01-100 μM [ACh]; 2), effects of ACh preperfusion; and 3), recovery from short-term desensitization. In the presence of 10 μM ACh, the IKACh model conferred recurring spontaneous firing after asystole of 8.9 s and 10.7 s for the Demir and Kurata sinoatrial node models, respectively. Therefore, two different populations of KACh channels and m2Rs may participate in short-term desensitization of IKACh in native myocytes, and may be responsible for vagal escape at nodal cells.

Regulatory mechanism of G protein-coupled receptor trafficking to the plasma membrane: a role for mRNA localization

Trafficking and localization of G protein-coupled receptors (GPCRs) to the plasma membrane and its retention in the agonist-naive state are critically important for signaling by these receptors. Agonist-induced desensitization of activated GPCRs and their removal from the cell surface have been studied and reviewed extensively. However, less attention has been given to the regulatory mechanisms and different steps that control the trafficking of newly synthesized receptors to the plasma membrane. It is generally believed that the mRNAs encoding GPCRs are targeted to the endoplasmic reticulum by a cotranslational, signal-sequence recognition particle-dependent pathway that results in protein translation and translocation to the plasma membrane. In this chapter, we discuss the importance of cis-targeting elements and trans-recognition factors in GPCR mRNA translational silencing, trafficking, and localization within the cell and its importance in receptor trafficking to the plasma membrane. Knockdown of the critical trans-recognition factors (RNA-binding proteins) resulted in translation of GPCR mRNAs in the perinuclear region and the receptors failed to traffic to the plasma membrane. Thus, a new paradigm is emerging in GPCR trafficking that suggests a fundamental role for mRNA partitioning to specific cytoplasmic regions for efficient plasma membrane localization of the receptors.

Rediocide A, an Insecticide, induces G-protein-coupled receptor desensitization via activation of conventional protein kinase C

In order to identify small-molecule antagonists of Methuselah (Mth), a Drosophila G-protein-coupled receptor (GPCR) involved in life-span control, a library of natural compounds was screened, and it was found that rediocide A (1), a daphnane ester from the roots of Trigonostemon reidioides and used currently for flea control, potently inhibited calcium mobilization mediated by this receptor. Compound 1 inhibited calcium mobilization in GPCRs other than Mth, indicating that the inhibitory effect was not due to receptor antagonism but rather to a more general mechanism. It was found that 1 can induce GPCR desensitization and internalization, and such effects were mediated by the activation of conventional protein kinase C.

Contribution of the extracellular cAMP-adenosine pathway to dual coupling of β2-adrenoceptors to Gs and Gi proteins in mouse skeletal muscle

β(2)-Adrenoceptor (β(2)-AR) agonists increase skeletal muscle contractile force via activation of G(s) protein/adenylyl cyclases (AC) and increased generation of cAMP. Herein, we evaluated the possible dual coupling of β(2)-AR to G(s) and G(i) proteins and the influence of the β(2)-AR/G(s)-G(i)/cAMP signaling cascade on skeletal muscle contraction. Assuming that the increment of intracellular cAMP is followed by cAMP efflux and extracellular generation of adenosine, the contribution of the extracellular cAMP-adenosine pathway on the β(2)-AR inotropic response was also addressed. The effects of clenbuterol/fenoterol (β(2)-AR agonists), forskolin (AC activator), cAMP/8-bromo-cAMP, and adenosine were evaluated on isometric contractility of mouse diaphragm muscle induced by supramaximal direct electrical stimulation (0.1 Hz, 2 ms duration). Clenbuterol/fenoterol (10-1000 μM), 1 μM forskolin, and 20 μM rolipram induced transient positive inotropic effects that peaked 30 min after stimulation onset, declining to 10 to 20% of peak levels in 30 min. The late descending phase of the β(2)-AR agonist inotropic effect was mimicked by either cAMP or adenosine and abolished by preincubation of diaphragm with pertussis toxin (PTX) (G(i) signaling inhibitor) or the organic anion transporter inhibitor probenecid, indicating a delayed coupling of β(2)-AR to G(i) protein which depends on cAMP efflux. Remarkably, the PTX-sensitive β(2)-AR inotropic effect was inhibited by the A(1) adenosine receptor antagonist 8-cyclopentyl-1,3-dipropylxanthine and ecto-5'-phosphodiesterase inhibitor α,β-methyleneadenosine 5'-diphosphate sodium salt, indicating that β(2)-AR coupling to G(i) is indirect and dependent on A(1) receptor activation. The involvement of the extracellular cAMP-adenosine pathway in β(2)-AR signaling would provide a negative feedback loop that may limit stimulatory G protein-coupled receptor positive inotropism and potential deleterious effects of excessive contractile response.

Extra-long Gαs variant XLαs protein escapes activation-induced subcellular redistribution and is able to provide sustained signaling

Murine models indicate that Gαs and its extra-long variant XLαs, both of which are derived from GNAS, markedly differ regarding their cellular actions, but these differences are unknown. Here we investigated activation-induced trafficking of Gαs and XLαs, using immunofluorescence microscopy, cell fractionation, and total internal reflection fluorescence microscopy. In transfected cells, XLαs remained localized to the plasma membrane, whereas Gαs redistributed to the cytosol after activation by GTPase-inhibiting mutations, cholera toxin treatment, or G protein-coupled receptor agonists (isoproterenol or parathyroid hormone (PTH)(1-34)). Cholera toxin treatment or agonist (isoproterenol or pituitary adenylate cyclase activating peptide-27) stimulation of PC12 cells expressing Gαs and XLαs endogenously led to an increased abundance of Gαs, but not XLαs, in the soluble fraction. Mutational analyses revealed two conserved cysteines and the highly charged domain as being critically involved in the plasma membrane anchoring of XLαs. The cAMP response induced by M-PTH(1-14), a parathyroid hormone analog, terminated quickly in HEK293 cells stably expressing the type 1 PTH/PTH-related peptide receptor, whereas the response remained maximal for at least 6 min in cells that co-expressed the PTH receptor and XLαs. Although isoproterenol-induced cAMP response was not prolonged by XLαs expression, a GTPase-deficient XLαs mutant found in certain tumors and patients with fibrous dysplasia of bone and McCune-Albright syndrome generated more basal cAMP accumulation in HEK293 cells and caused more severe impairment of osteoblastic differentiation of MC3T3-E1 cells than the cognate Gαs mutant (gsp oncogene). Thus, activated XLαs and Gαs traffic differently, and this may form the basis for the differences in their cellular actions.

Cardiac function following prolonged exercise: influence of age

This study sought to determine the influence of age on the left ventricular (LV) response to prolonged exercise (PE; 150 min). LV systolic and diastolic performance was assessed using echocardiography (ECHO) before (pre) and 60 min following (post) exercise performed at 80% maximal aerobic power in young (28 ± 4.5 years; n = 18; mean ± SD) and middle-aged (52 ± 3.9 years; n = 18) participants. LV performance was assessed using two-dimensional ECHO, including speckle-tracking imaging, to determine LV strain (LV S) and LV S rate (LV SR), in addition to Doppler measures of diastolic function. We observed a postexercise elevation in LV S (young: −19.5 ± 2.1% vs. −21.6 ± 2.1%; middle-aged: −19.9 ± 2.3% vs. −20.8 ± 2.1%; P < 0.05) and LV SR (young: −1.19 ± 0.1 vs. −1.37 ± 0.2; middle-aged: −1.20 ± 0.2 vs. −1.38 ± 0.2; P < 0.05) during recovery in both groups. Diastolic function was reduced during recovery, including the LV SR ratio of early-to-late atrial diastolic filling (SRe/a), in young (2.35 ± 0.7 vs. 1.89 ± 0.5; P < 0.01) and middle-aged (1.51 ± 0.5 vs. 1.05 ± 0.2; P < 0.01) participants, as were conventional indices including the E/A ratio. Dobutamine stress ECHO revealed a postexercise depression in LV S in response to increasing dobutamine dose, which was similar in both young (pre-exercise dobutamine 0 vs. 20 μg·kg−1·min−1: −19.5 ± 2.1 vs. −27.2 ± 2.2%; postexercise dobutamine 0 vs. 20 μg·kg−1·min−1: −21.6 ± 2.1 vs. −23.7 ± 2.2%; P < 0.05) and middle-aged participants (pre: −19.9 ± 2.3 vs. −25.3 ± 2.7%; post: −20.8 ± 2.1 vs. −23.5 ± 2.7; P < 0.05). This was despite higher noradrenaline concentrations immediately postexercise in the middle-aged participants compared with young (4.26 ± 2.7 nmol/L vs. 3.00 ± 1.4 nmol/L; P = 0.12). These data indicate that LV dysfunction is observed following PE and that advancing age does not increase the magnitude of this response.

Effects of acetylcholine on electrical remodeling of human atrial fibers

Autonomic nervous system activation can result in significant changes of atrial electrophysiology and facilitate induction of atrial fibrillation. By recording influence of different concentrations of acetylcholine (ACh) on atrial fibers (AF), we investigated the role of the increased vagal tone in electrical remodeling in atrial fibrillation. Parameters of action potentials and force contraction (Fc) in atrial fibers were recorded by using standard intracellular microelectrode technique and force transducer. It was found that: (1) ACh at 0.1 μmol/L had no significant influence on spontaneous action potentials (SAPs) and Fc (n=6, P>0.05); ACh at both 1.0 and 10.0 μmol/L shortened action potential duration (APD) and Fc of human AF from right atrium (n=6, P<0.05); there was no significant difference in shortening APD between 10.0 and 1.0 μmol/L of ACh; (2) ACh at 0.1 μmol/L had no significant desensitization (n=6, P>0.05), but ACh at 1.0 and 10.0 μmol/L had desensitization (n=6, P<0.05) to SAPs and Fc. The desensitization of ACh on APD in AF was concentration- and time-dependent. It was shown that APD was longer than the control along with extending time of continuous Tyrode's solution perfusion after desensitization. It is concluded that ACh changes the electrophysiological characteristics of human AF, indicating that increased vagal tone plays a role in the development of a vulnerable substrate for atrial electrical remodeling in atrial fibrillation.

Nonenzymatic rapid control of GIRK channel function by a G protein-coupled receptor kinase

G protein-coupled receptors (GPCRs) respond to agonists to activate downstream enzymatic pathways or to gate ion channel function. Turning off GPCR signaling is known to involve phosphorylation of the GPCR by GPCR kinases (GRKs) to initiate their internalization. The process, however, is relatively slow and cannot account for the faster desensitization responses required to regulate channel gating. Here, we show that GRKs enable rapid desensitization of the G protein-coupled potassium channel (GIRK/Kir3.x) through a mechanism independent of their kinase activity. On GPCR activation, GRKs translocate to the membrane and quench channel activation by competitively binding and titrating G protein βγ subunits away from the channel. Of interest, the ability of GRKs to effect this rapid desensitization depends on the receptor type. The findings thus reveal a stimulus-specific, phosphorylation-independent mechanism for rapidly downregulating GPCR activity at the effector level.

cAMP initiates early phase neuron-like morphology changes and late phase neural differentiation in mesenchymal stem cells

The intracellular second messenger cAMP is frequently used in induction media to induce mesenchymal stem cells (MSCs) into neural lineage cells. To date, an understanding of the role cAMP exerts on MSCs and whether cAMP can induce MSCs into functional neurons is still lacking. We found cAMP initiated neuron-like morphology changes early and neural differentiation much later. The early phase changes in morphology were due to cell shrinkage, which subsequently rendered some cells apoptotic. While the morphology changes occurred prior to the expression of neural markers, it is not required for neural marker expression and the two processes are differentially regulated downstream of cAMP-activated protein kinase A. cAMP enabled MSCs to gain neural marker expressions with neuronal function, such as, calcium rise in response to neuronal activators, dopamine, glutamate, and potassium chloride. However, only some of the cells induced by cAMP responded to the three neuronal activators and further lack the neuronal morphology, suggesting that although cAMP is able to direct MSCs towards neural differentiation, they do not achieve terminal differentiation.

Intermolecular cross-talk between the prostaglandin E2 receptor (EP)3 of subtype and thromboxane A(2) receptor signalling in human erythroleukaemic cells

BACKGROUND AND PURPOSE

In previous studies investigating cross-talk of signalling between prostaglandin (PG)E(2) receptor (EP) and the TPalpha and TPbeta isoforms of the human thromboxane (TX)A(2) receptor (TP), 17-phenyl trinor PGE(2)-induced desensitization of TP receptor signalling through activation of the AH6809 and SC19220-sensitive EP(1) subtype of the EP receptor family, in a cell-specific manner. Here, we sought to further investigate that cross-talk in human erythroleukaemic (HEL) 92.1.7 cells.

EXPERIMENTAL APPROACH

Specificity of 17-phenyl trinor PGE(2) signalling and its possible cross-talk with signalling by TPalpha/TPbeta receptors endogenously expressed in HEL cells was examined through assessment of agonist-induced inositol 1,4,5-trisphosphate (IP)(3) generation and intracellular calcium ([Ca(2+)](i)) mobilization.

KEY RESULTS

While 17-Phenyl trinor PGE(2) led to activation of phospholipase (PL)Cbeta to yield increases in IP(3) generation and [Ca(2+)](i), it did not desensitize but rather augmented that signalling in response to subsequent stimulation with the TXA(2) mimetic U46619. Furthermore, the augmentation was reciprocal. Signalling by 17-phenyl trinor PGE(2) was found to occur through AH6809- and SC19920-insensitive, Pertussis toxin-sensitive, G(i)/G(betagamma)-dependent activation of PLCbeta. Further pharmacological investigation using selective EP receptor subtype agonists and antagonists confirmed that 17-phenyl trinor PGE(2)-mediated signalling and reciprocal cross-talk with the TP receptors occurred through the EP(3), rather than the EP(1), EP(2) or EP(4) receptor subtype in HEL cells.

CONCLUSIONS AND IMPLICATIONS

The EP(1) and EP(3) subtypes of the EP receptor family mediated intermolecular cross-talk to differentially regulate TP receptor-mediated signalling whereby activation of EP(1) receptors impaired or desensitized, while that of EP(3) receptors augmented signalling through TPalpha/TPbeta receptors, in a cell type-specific manner.

Report and recommendations of the workshop of the European Centre for the Validation of Alternative Methods for Drug-Induced Cardiotoxicity

Cardiotoxicity is among the leading reasons for drug attrition and is therefore a core subject in non-clinical and clinical safety testing of new drugs. European Centre for the Validation of Alternative Methods held in March 2008 a workshop on "Alternative Methods for Drug-Induced Cardiotoxicity" in order to promote acceptance of alternative methods reducing, refining or replacing the use of laboratory animals in this field. This review reports the outcome of the workshop. The participants identified the major clinical manifestations, which are sensitive to conventional drugs, to be arrhythmias, contractility toxicity, ischaemia toxicity, secondary cardiotoxicity and valve toxicity. They gave an overview of the current use of alternative tests in cardiac safety assessments. Moreover, they elaborated on new cardiotoxicological endpoints for which alternative tests can have an impact and provided recommendations on how to cover them.

Adenosine receptors and cancer

The A(1), A(2A), A(2B) and A(3) G-protein-coupled cell surface adenosine receptors (ARs) are found to be upregulated in various tumor cells. Activation of the receptors by specific ligands, agonists or antagonists, modulates tumor growth via a range of signaling pathways. The A(1)AR was found to play a role in preventing the development of glioblastomas. This antitumor effect of the A(1)AR is mediated via tumor-associated microglial cells. Activation of the A(2A)AR results in inhibition of the immune response to tumors via suppression of T regulatory cell function and inhibition of natural killer cell cytotoxicity and tumor-specific CD4+/CD8+ activity. Therefore, it is suggested that pharmacological inhibition of A(2A)AR activation by specific antagonists may enhance immunotherapeutics in cancer therapy. Activation of the A(2B)AR plays a role in the development of tumors via upregulation of the expression levels of angiogenic factors in microvascular endothelial cells. In contrast, it was evident that activation of A(2B)AR results in inhibition of ERK1/2 phosphorylation and MAP kinase activity, which are involved in tumor cell growth signals. Finally, A(3)AR was found to be highly expressed in tumor cells and tissues while low expression levels were noted in normal cells or adjacent tissue. Receptor expression in the tumor tissues was directly correlated to disease severity. The high receptor expression in the tumors was attributed to overexpression of NF-kappaB, known to act as an A(3)AR transcription factor. Interestingly, high A(3)AR expression levels were found in peripheral blood mononuclear cells (PBMCs) derived from tumor-bearing animals and cancer patients, reflecting receptor status in the tumors. A(3)AR agonists were found to induce tumor growth inhibition, both in vitro and in vivo, via modulation of the Wnt and the NF-kappaB signaling pathways. Taken together, A(3)ARs that are abundantly expressed in tumor cells may be targeted by specific A(3)AR agonists, leading to tumor growth inhibition. The unique characteristics of these A(3)AR agonists make them attractive as drug candidates.

Nandrolone-pretreatment enhances cardiac beta(2)-adrenoceptor expression and reverses heart contractile down-regulation in the post-stress period of acute-stressed rats

To investigate whether nandrolone decanoate (ND)-pretreatment can modulate (1) beta-adrenoceptor expression and (2) myocardial contractility in response to beta-adrenoceptors stimulation with isoproterenol (ISO), in hearts of both normal and stressed rats. Rats were treated with 15 mg/(kgday) of Deca-Durabolin (ND, 1 ml i.m.) or with vehicle (oil) for 14 days. The day after the last injection, the dose-response to ISO (1 x 10(-8), 5 x 10(-8) and 10(-7)M), was studied in isolated rat hearts harvested from unstressed animals (unstressed+vehicle (control) or unstressed+ND) or from stressed animals (stressed+vehicle or stressed+ND): acute stress protocol consisted in restrain for 1h immediately before sacrifice. ND-pretreatment increased beta(2)-adrenoceptor expression. In baseline conditions all hearts had a similar left ventricular developed pressure (LVDP) and maximum rate of increase of LVDP (dP/dt(max)). In hearts of unstressed+vehicle or unstressed+ND, ISO caused a similar increase in LVDP (+90-100%) and dP/dt(max) (+120-150%). However, hearts of stressed+vehicle animals showed a marked depression of inotropic response to ISO (i.e. for ISO 1 x 10(-8),-55% in LVDP response versus unstressed). Yet, in hearts of stressed+ND-animals the effect of stress was reversed, showing the highest response to ISO (i.e. for ISO 1 x 10(-7), +30% LVDP response versus unstressed). The ND-induced beta(2)-adrenoceptor overexpression does not affect ISO-response in unstressed animals. However, acute stress induces a down-regulation of ISO-response, which is reversed by ND-pretreatment. Since the physiological post-stress down-regulation of adrenergic-response is absent after nandrolone treatment, the heart may be exposed to a sympathetic over-stimulation. This might represent a risk for cardiovascular incidents in anabolic steroid addicts under stressing conditions.

Ca2+mobilization through dorsal root ganglion Ca2+-sensing receptor stably expressed in HEK293 cells

The rat dorsal root ganglion (DRG) Ca2+-sensing receptor (CaR) was stably expressed in-frame as an enhanced green fluorescent protein (EGFP) fusion protein in human embryonic kidney (HEK)293 cells, and is functionally linked to changes in intracellular Ca2+concentration ([Ca2+]i). RT-PCR analysis indicated the presence of the message for the DRG CaR cDNA. Western blot analysis of membrane proteins showed a doublet of 168–175 and 185 kDa, consistent with immature and mature forms of the CaR.EGFP fusion protein, respectively. Increasing extracellular [Ca2+] ([Ca2+]e) from 0.5 to 1 mM resulted in increases in [Ca2+]ilevels, which were blocked by 30 μM 2-aminoethyldiphenyl borate. [Ca2+]e-response studies indicate a Ca2+sensitivity with an EC50of 1.75 ± 0.10 mM. NPS R-467 and Gd3+activated the CaR. When [Ca2+]ewas successively raised from 0.25 to 4 mM, peak [Ca2+]i, attained with 0.5 mM, was reduced by ∼50%. Similar reductions were observed with repeated applications of 10 mM Ca2+, 1 and 10 μM NPS R-467, or 50 and 100 μM Gd3+, indicating desensitization of the response. Furthermore, Ca2+mobilization increased phosphorylated protein kinase C (PKC)α levels in the cells. However, the PKC activator, phorbol myristate acetate did not inhibit CaR-mediated Ca2+signaling. Rather, a spectrum of PKC inhibitors partially reduced peak responses to Cae2+. Treatment of cells with 100 nM PMA for 24 h, to downregulate PKC, reduced [Ca2+]itransients by 49.9 ± 5.2% (at 1 mM Ca2+) and 40.5 ± 6.5% (at 2 mM Ca2+), compared with controls. The findings suggest involvement of PKC in the pathway for Ca2+mobilization following CaR activation.

Internalization and distribution of three alpha1-adrenoceptor subtypes in HEK293A cells before and after agonist stimulation

AIM

To examine the subcellular distribution of the 3 alpha1-adrenoceptor (alpha1-AR) subtypes and their internalization and trafficking upon agonist stimulation in human embryonic kidney 293A cells.

METHODS

Confocal real-time imaging, enzyme linked immunosorbent assay (ELISA) and whole cell [3H]-prazosin binding assay were applied to detect the distribution and localization of the 3 alpha1-AR subtypes.

RESULTS

alpha1A-AR was found both on the cell surface and in the cytoplasm; alpha1BAR, however, was predominantly detected on the cell surface, while alpha1D-AR was detected mainly in the intracellular compartments. After stimulation with phenylephrine, localization changes were detected by confocal microscopy for alpha1A- and alpha1B-AR,but the localization of alpha1D-AR were unaffected. Phenylephrine stimulation promoted a more rapid internalization of alpha1B-AR than alpha1A-AR. alpha1D-AR internalization was detected only by ELISA. Whole cell [3H]-prazosin binding assay showed that alpha1A-AR functional receptors were detected both on the cell surface and in the cytoplasm; alpha1B-AR, however, were detected predominantly on the cell surface, while alpha1D-AR were detected mainly in intracellular compartments. Phenylephrine stimulation promoted internalization of alpha1A- and alpha1B-AR.

CONCLUSION

Phenylephrine stimulation induced changes in the localization of the 3 alpha1-AR.

Serotonergic pre-treatments block in vitro serotonergic phase shifts of the mouse suprachiasmatic nucleus circadian clock

The suprachiasmatic nucleus (SCN) contains a circadian clock that maintains its time-generating and phase-modulating capacities in vitro. Previous studies report clear differences in the ability of serotonergic stimuli to phase-shift the SCN clock when applied directly to the SCN either in vivo or in vitro: while mice and rat circadian clocks are readily phase-shifted by serotonin (5-HT) or 5-HT agonists applied in vitro, hamster and mice circadian clocks respond inconsistently to 5-HT agonists injected directly into the SCN in vivo. Here we have investigated one possible explanation for these differences: that the SCN isolated in vitro experiences reduced endogenous 5-HT signaling, which increases clock sensitivity to subsequent 5-HT stimulation. For these experiments we treated mouse SCN brain slices with low concentrations of compounds that increase serotonin signaling: 5-HT, a 5-HT agonist (8-OH-DPAT), the 5-HT precursor, l-tryptophan, or the 5-HT re-uptake inhibitor, fluoxetine. Pretreatment with each of these substances completely blocked subsequent phase-shifts induced by mid-subjective day treatment with either 5-HT or 8-OH-DPAT, while they did not block phase-shifts induced by the adenylate cyclase activator, forskolin. Time-course data on l-tryptophan-induced inhibition are consistent with this treatment inducing receptor internalization, while timing of the recovery from inhibition is consistent with receptor reinsertion. Together these data support the hypothesis that SCN clock sensitivity to serotonergic phase modulation is affected by the amount of prior serotonin signaling present in the SCN, and that this signaling alters the density of surface 5-HT receptors on SCN clock neurons.

Cysteinyl-leukotrienes in the regulation of beta2-adrenoceptor function: an in vitro model of asthma

Background

The response to β₂-adrenoceptor agonists is reduced in asthmatic airways. This desensitization may be in part due to inflammatory mediators and may involve cysteinyl-leukotrienes (cysteinyl-LTs). Cysteinyl-LTs are pivotal inflammatory mediators that play important roles in the pathophysiology of asthma, allergic rhinitis, and other inflammatory conditions. We tested the hypothesis that leukotriene D₄ (LTD₄) and allergen challenge cause β₂-adrenoceptor desensitization through the activation of protein kinase C (PKC).

Methods

The isoproterenol-induced cAMP accumulation was evaluated in human airway smooth muscle cell cultures challenged with exogenous LTD₄ or the PKC activator phorbol-12-myristate-13-acetate with or without pretreatments with the PKC inhibitor GF109203X or the CysLT₁R antagonist montelukast. The relaxant response to salbutamol was studied in passively sensitized human bronchial rings challenged with allergen in physiological salt solution (PSS) alone, or in the presence of either montelukast or GF109203X.

Results

In cell cultures, both LTD₄ and phorbol-12-myristate-13-acetate caused significant reductions of maximal isoproterenol-induced cAMP accumulation, which were fully prevented by montelukast and GF109203X, respectively. More importantly, GF109203X also prevented the attenuating effect of LTD₄ on isoproterenol-induced cAMP accumulation. In bronchial rings, both montelukast and GF109203X prevented the rightward displacement of the concentration-response curves to salbutamol induced by allergen challenge.

Conclusion

LTD₄ induces β₂-adrenoceptor desensitization in human airway smooth muscle cells, which is mediated through the activation of PKC. Allergen exposure of sensitized human bronchi may also cause a β₂-adrenoceptor desensitization through the involvement of the CysLT₁R-PKC pathway.

Diurnal rhythm in the levels of the serotonin 5-HT1A receptors in the crayfish eyestalk

The crayfish eyestalk (ES) has been postulated as a possible circadian clock. 5-Hydroxytryptamine (5-HT) has been shown to play the role of a neurotransmitter or a modulator in the ES. However, little is known about the 5-HT receptors in the ES. The purpose of this work is to determine the specific binding sites using [(3)H]8-hydroxy-2(di-n-propylamino)tetralin ([(3)H]8-OH-DPAT), a specific agonist of the 5-HT(1A) receptor, and to characterize the diurnal rhythm in the binding by an autoradiography procedure in the crayfish ES. Data show the presence of a circadian rhythmicity in the level of the 5-HT(1A) receptors, principally in two regions: (a) the complex retina (R)-lamina ganglionaris (LG), with the acrophase at dusk and (b) the medulla terminalis (MT), where it was in antiphase. It is suggested that (1) the expression of levels of 5-HT(1A) receptors is modulated by light-dark (LD) cycles, (2) the level of 5-HT(1A) receptors in the R-LG and MT are in antiphase during the 24-h cycle, and (3) there is a different mechanism of action of LD cycles in each of these two anatomical regions of the crayfish ES.

Agonist-induced Internalization of the Caenorhabditis elegans Muscarinic Acetylcholine Receptor GAR-3 in Chinese Hamster Ovary Cells

Many membrane-bound neurotransmitter receptors are known to be internalized by exposure to agonist. This agonist-induced receptor internalization is considered to play important roles in receptor-mediated signaling. Here we investigated the internalization of GAR-3, a Caenorhabditis elegans muscarinic acetylcholine receptor, using cultured mammalian cells. When Chinese hamster ovary cells stably expressing GAR-3 were treated with carbachol, GAR-3 was internalized in a dose- and time-dependent manner. Approximately 60% of the cell surface receptor was internalized by exposure to 1 mM carbachol for 1 h. Carbachol-induced GAR-3 internalization was suppressed by treatment with hypertonic sucrose, which blocks the formation of clathrin-coated pits. Overexpression of a dominant-negative dynamin mutant (DynK44A), but not of a dominant-negative beta-arrestin mutant (Arr319-418), substantially inhibited carbachol-induced internalization of GAR-3. Thus, these data suggest that GAR-3 undergoes agonist-induced internalization via a clathrin- and dynamin-dependent but beta-arrestin-independent pathway. Depletion of Ca2+ by simultaneous treatment of the cells with BAPTA/AM (Ca2+ mobilization blocker) and EGTA (Ca2+ influx blocker) almost completely blocked agonist-induced GAR-3 internalization. Moreover, treatment of the cells with the Ca2+ ionophore A23187 led to GAR-3 internalization in the absence of agonist. These results indicate that Ca2+ plays a critical role in GAR-3 internalization. We tested whether the third intracellular (i3) loop of GAR-3 is involved in agonist-stimulated receptor internalization. A GAR-3 deletion mutant lacking a large central portion of the i3 loop exhibited an internalization pattern comparable to that of the wild type, suggesting that the central i3 loop is not required for the internalization of GAR-3.

Integration of calcium with the signaling network in cardiac myocytes

Calcium has evolved as global intracellular messenger for signal transduction in the millisecond time range by reversibly binding to calcium-sensing proteins. In the cardiomyocyte, ion pumps, ion exchangers and channels keep the cytoplasmic calcium level at rest around approximately 100 nM which is more than 10,000-fold lower than outside the cell. Intracellularly, calcium is mainly stored in the sarcoplasmic reticulum, which comprises the bulk of calcium available for the heartbeat. Regulation of cardiac function including contractility and energy production relies on a three-tiered control system, (i) immediate and fast feedback in response to mechanical load on a beat-to-beat basis (Frank-Starling relation), (ii) more sustained regulation involving transmitters and hormones as primary messengers, and (iii) long-term adaptation by changes in the gene expression profile. Calcium signaling over largely different time scales requires its integration with the protein kinase signaling network which is governed by G-protein-coupled receptors, growth factor and cytokine receptors at the surface membrane. Short-term regulation is dominated by the beta-adrenergic system, while long-term regulation with phenotypic remodeling depends on sustained signaling by growth factors, cytokines and calcium. Mechanisms and new developments in intracellular calcium handling and its interrelation with the MAPK signaling pathways are discussed in detail.

Enhanced isoproterenol-induced cardiac hypertrophy in transgenic rats with low brain angiotensinogen

We have previously shown that a permanent deficiency in the brain renin-angiotensin system (RAS) may increase the sensitivity of the baroreflex control of heart rate. In this study we aimed at studying the involvement of the brain RAS in the cardiac reactivity to the beta-adrenoceptor (beta-AR) agonist isoproterenol (Iso). Transgenic rats with low brain angiotensinogen (TGR) were used. In isolated hearts, Iso induced a significantly greater increase in left ventricular (LV) pressure and maximal contraction (+dP/dt(max)) in the TGR than in the Sprague-Dawley (SD) rats. LV hypertrophy induced by Iso treatment was significantly higher in TGR than in SD rats (in g LV wt/100 g body wt, 0.28 +/- 0.004 vs. 0.24 +/- 0.004, respectively). The greater LV hypertrophy in TGR rats was associated with more pronounced downregulation of beta-AR and upregulation of LV beta-AR kinase-1 mRNA levels compared with those in SD rats. The decrease in the heart rate (HR) induced by the beta-AR antagonist metoprolol in conscious rats was significantly attenuated in TGR compared with SD rats (-9.9 +/- 1.7% vs. -18.1 +/- 1.5%), whereas the effect of parasympathetic blockade by atropine on HR was similar in both strains. These results indicate that TGR are more sensitive to beta-AR agonist-induced cardiac inotropic response and hypertrophy, possibly due to chronically low sympathetic outflow directed to the heart.

CysLT1 receptor is a target for extracellular nucleotide-induced heterologous desensitization: a possible feedback mechanism in inflammation

Both cysteinyl-leukotrienes and extracellular nucleotides mediate inflammatory responses via specific G-protein-coupled receptors, the CysLT and the P2Y receptors, respectively. Since these mediators accumulate at sites of inflammation, and inflammatory cells express both classes of receptors, their responses are likely to be crossregulated. We investigated the molecular basis of desensitization and trafficking of the CysLT1 receptor constitutively and transiently expressed in the human monocyte/macrophage-like U937 or COS-7 cells in response to LTD4 or nucleotides. Exposure to agonist induced a rapid homologous desensitization of the CysLT1 receptor [as measured by the reduction in the maximal agonist-induced intracellular cytosolic Ca2+ ([Ca2+]i) transient], followed by receptor internalization (as assessed by equilibrium binding and confocal microscopy). Activation of P2Y receptors with ATP or UDP induced heterologous desensitization of the CysLT1 receptor. Conversely, LTD4-induced CysLT1 receptor activation had no effect on P2Y receptor responses, which suggests that the latter have a hierarchy in producing desensitizing signals. Furthermore, ATP/UDP-induced CysLT1 receptor desensitization was unable to cause receptor internalization, induced a faster recovery of CysLT1 functionality and was dependent upon protein kinase C. By contrast, homologous desensitization, which is probably dependent upon G-protein-receptor kinase 2 activation, induced a fast receptor downregulation and, accordingly, a slower recovery of CysLT1 functionality. Hence, CysLT1 receptor desensitization and trafficking are differentially regulated by the CysLT1 cognate ligand or by extracellular nucleotides. This crosstalk may have a profound physiological implication in the regulation of responses at sites of inflammation, and may represent just an example of a feedback mechanism used by cells to fine-tune their responses.

Once and future signaling: G protein-coupled receptor kinase control of neuronal sensitivity

G protein-coupled receptors (GPCRs) are the most numerous class of cell surface receptor, and substances acting through GPCRs mediate many critical signaling events and physiological processes. GPCR sensitivity and signaling is dynamic, responding rapidly to adjust to changes in the ambient level of stimulation of target cells. One important mediator of such receptor sensitivity is the family of GPCR kinases (GRKs). Like heterotrimeric G proteins, GRKs recognize agonist-bound, activated receptors, and this recognition promotes catalytic activation of GRKs, resulting in the preferential phosphorylation of activated receptors. GRK-phosphorylated receptors are then targeted by arrestin proteins, which bind to phosphorylated receptors. Arrestin-bound receptors are uncoupled from heterotrimeric G proteins, resulting in decreased sensitivity to further receptor stimulation (desensitization). Arrestin-bound receptors are also accelerated into internalization pathways and linked to distinct arrestin-mediated signaling pathways. GRKs thus serve as gatekeepers for receptors, terminating some signaling pathways and initiating others. One major outstanding question concerning GRKs understanding the mechanisms by which any particular receptor subtype (of the 800 or so in the body) is regulated by a specific GRK(s), and the consequences of this specificity. An understanding of this regulatory specificity could allow targeting of GRK function to ameliorate diseases involving GPCR dysregulation.

Role of nitric oxide and protein kinase C in the tachyphylaxis to vasopressin in rat aortic rings

The contribution of endothelium-derived mediators and protein kinase C in the tachyphylaxis to arginine vasopressin (AVP) was assessed in the rat aorta. Endothelium-intact (E+) and denuded rings (E-) obtained from the rat thoracic aorta were exposed to three administrations of a supramaximal concentration of AVP (100 nM), lasting 20 min and 45 min apart. N-Omega-nitro-L-arginine (NNLA), a non-selective inhibitor of all isoforms of NO synthase, and AMT, a selective inhibitor for the inducible (iNOS) and neuronal (nNOS) isoforms, diminished the tachyphylaxis to AVP significantly in both E+ and in E- rings. No iNOS could be detected by Western blots in freshly isolated rings or in rings exposed to AVP, despite a strong signal in rings isolated from LPS-treated rats, while nNOS could be constitutively detected. Inhibition of prostaglandins or epoxyeicosatrienoic acids (EETs) synthesis by diclofenac or clotrimazole, respectively, had no effect on tachyphylaxis while combination of these agents diminished tachyphylaxis in E+ only. Combination of NNLA, diclofenac and clotrimazole blocked completely the tachyphylaxis. Inhibition of PKC by either chelerythrine or bisindolylmaleimide I-HCl (BisI) led to a significant diminution of AVP tachyphylaxis only in E-. Activation of PKC with phorbol-12-myristate-13-acetate (PMA) simulated tachyphylaxis to AVP in E- only, effect blocked by the NO donor, SNP. In conclusion, NO produced from constitutive nNOS present in vascular smooth muscle cells participates in tachyphylaxis to AVP. PKC is involved in this tachyphylaxis only in E- rings, the presence of NO probably diminishing the effects of this kinase.

Use of vasopressors in the treatment of cardiac arrest

Intervention for cardiac arrest may require intervention for electrical abnormalities or hemodynamic instability. These actions can result in ineffective cardiac functioning and systemic hypotension. Vasopressors are capable of improving severe hypotension that can result from reduced cardiovascular contractility or heart rate. These vasopressor actions are critical to successful resuscitation efforts for patients.

Agonist-specific regulation of the delta-opioid receptor

Delta opioid receptor (DOR) agonists are attractive potential analgesics, since these compounds exhibit strong antinociceptive activity with relatively few side effects. In the past decade, several novel classes of delta-opioid agonists have been synthesized. Recent experimental data indicate that structurally distinct opioid agonists interact differently with the delta-opioid receptor. Consequently, individual agonist-bound DOR conformations may interact differently with intracellular proteins. In the present paper, after a brief review of the cellular processes that contribute to homologous desensitization of the DOR signaling, we shall focus on experimental data demonstrating that chemically different agonists differ in their ability to phosphorylate, internalize, and/or down-regulate the DOR. Homologous regulation of the opioid receptor signaling is thought to play an important role in the development of opioid tolerance. Therefore, agonist-specific differences in DOR regulation suggest that by further chemical modification, delta-selective opioid analgesics can be designed that exhibit a reduced propensity for analgesic tolerance.

Acute desensitization of GIRK current in rat atrial myocytes is related to K+ current flow

We have investigated the acute desensitization of acetylcholine-activated GIRK current (I(K(ACh))) in cultured adult rat atrial myocytes. Acute desensitization of I(K(ACh)) is observed as a partial relaxation of current with a half-time of < 5 s when muscarinic M2 receptors are stimulated by a high concentration (> 2 micromol l(-1)) of ACh. Under this condition experimental manoeuvres that cause a decrease in the amplitude of I(K(ACh)), such as partial block of M2 receptors by atropine, intracellular loading with GDP-beta-S, or exposure to Ba2+, caused a reduction in desensitization. Acute desensitization was also identified as a decrease in current amplitude and a blunting of the response to saturating [ACh] (20 micromol l(-1)) when the current had been partially activated by a low concentration of ACh or by stimulation of adenosine A1 receptors. A reduction in current analogous to acute desensitization was observed when ATP-dependent K+ current (I(K(ATP))) was activated either by mitochondrial uncoupling using 2,4-dinitrophenole (DNP) or by the channel opener rilmakalim. Adenovirus-driven overexpression of Kir2.1, a subunit of constitutively active inwardly rectifying K+ channels, resulted in a large Ba2+-sensitive background K+ current and a dramatic reduction of ACh-activated current. Adenovirus-driven overexpression of GIRK4 (Kir3.4) subunits resulted in an increased agonist-independent GIRK current paralleled by a reduction in I(K(ACh)) and removal of the desensitizing component. These data indicate that acute desensitization depends on K+ current flow, independent of the K+ channel species, suggesting that it reflects a reduction in electrochemical driving force rather than a bona fide signalling mechanism. This is supported by the observation that desensitization is paralleled by a significant negative shift in reversal potential of I(K(ACh)). Since the ACh-induced hyperpolarization shows comparable desensitization properties as I(K(ACh)), this novel current-dependent desensitization is a physiologically relevant process, shaping the time course of parasympathetic bradycardia.

Sustained beta1-adrenergic stimulation modulates cardiac contractility by Ca2+/calmodulin kinase signaling pathway

A tenet of beta1-adrenergic receptor (beta1AR) signaling is that stimulation of the receptor activates the adenylate cyclase-cAMP-protein kinase A (PKA) pathway, resulting in positive inotropic and relaxant effects in the heart. However, recent studies have suggested the involvement of Ca2+/calmodulin-dependent protein kinase II (CaMKII) in beta1AR-stimulated cardiac apoptosis. In this study, we determined roles of CaMKII and PKA in sustained versus short-term beta1AR modulation of excitation-contraction (E-C) coupling in cardiac myocytes. Short-term (10-minute) and sustained (24-hour) beta1AR stimulation with norepinephrine similarly enhanced cell contraction and Ca2+ transients, in contrast to anticipated receptor desensitization. More importantly, the sustained responses were largely PKA-independent, and were sensitive to specific CaMKII inhibitors or adenoviral expression of a dominant-negative CaMKII mutant. Biochemical assays revealed that a progressive and persistent CaMKII activation was associated with a rapid desensitization of the cAMP/PKA signaling. Concomitantly, phosphorylation of phospholamban, an SR Ca2+ cycling regulatory protein, was shifted from its PKA site (16Ser) to CaMKII site (17Thr). Thus, beta1AR stimulation activates dual signaling pathways mediated by cAMP/PKA and CaMKII, the former undergoing desensitization and the latter exhibiting sensitization. This finding may bear important etiological and therapeutical ramifications in understanding beta1AR signaling in chronic heart failure.

Monte Carlo simulations of receptor dynamics: insights into cell signaling

Many receptor-level processes involve the diffusion and reaction of receptors with other membrane-localized molecules. Monte Carlo simulation is a powerful technique that allows us to track the motions and discrete reactions of individual receptors, thus simulating receptor dynamics and the early events of signal transduction. In this paper, we discuss simulations of two receptor processes, receptor dimerization and G-protein activation. Our first set of simulations demonstrates how receptor dimerization can create clusters of receptors via partner switching and the relevance of this clustering for receptor cross-talk and integrin signaling. Our second set of simulations investigates the activation and desensitization of G-protein coupled receptors when either a single agonist or both an agonist and an antagonist are present. For G-protein coupled receptor systems in the presence of an agonist alone, the dissociation rate constant of agonist is predicted to affect the ratio of G-protein activation to receptor phosphorylation. Similarly, this ratio is affected by the antagonist dissociation rate constant when both agonist and antagonist are present. The relationship of simulation predictions to experimental findings and potential applications of our findings are also discussed.

Regulation of M2 Muscarinic Acetylcholine Receptor Expression and Signaling by Prolonged Exposure to Allosteric Modulators

The effects of prolonged exposure of M(2) muscarinic acetylcholine receptors (mAChRs), stably expressed in Chinese hamster ovary cells, to the allosteric modulators gallamine, alcuronium, and heptane-1,7-bis (dimethyl-3'-phthalimidopropyl)-ammonium bromide (C(7)/3'-phth) were compared with the effects of the agonist carbachol (CCh) and antagonists atropine and N-methylscopolamine (NMS). Intact cell saturation binding assays using [(3)H]NMS found that pretreatment of the cells for 24 h with CCh caused a significant down-regulation of receptor number, whereas atropine, NMS, and all three allosteric modulators caused receptor up-regulation. Functional assays using a cytosensor microphysiometer to measure whole-cell metabolic rate found no acute effects of gallamine on receptor signaling, whereas atropine seemed to behave as an inverse agonist. Pretreatment of the cells with gallamine (20 microM) or atropine (20 nM) resulted in a significant enhancement of the maximal effect evoked by CCh. In contrast, CCh (100 microM) pretreatment resulted in a significant reduction in maximal receptor signaling capacity. Time-course experiments revealed that the effects of atropine and gallamine on receptor up-regulation are only visualized after at least 12-h ligand exposure, compared with the more rapid effects of CCh, which achieve steady-state down-regulation within 90 min. Additional experiments monitoring CCh-mediated M(2) mAChR internalization in the presence of gallamine revealed that part of the mechanism underlying the effects of the modulator on receptor expression may involve a change in receptor internalization properties. These findings suggest that, like orthosteric ligands, G protein-coupled receptor allosteric modulators also are able to mediate long-term effects on receptor regulation.

Cellular mechanisms in sympatho-modulation of the heart

Cardiovascular function relies on complex servo-controlled regulation mechanisms that involve both fast-acting feedback responses and long-lasting adaptations affecting the gene expression. The adrenergic system, with its specific receptor subtypes and intracellular signalling cascades provides the major regulatory system, while the parasympathetic system plays a minor role. At the molecular level, Ca(2+) acts as the general signal trigger for the majority of cell activities including contraction, metabolism and growth. During recent years, important new results have emerged allowing an integrated view of how the multifarious Ca(2+)-signalling mechanisms transmit adrenergic impulses to intracellular target sites. These insights into cellular and molecular mechanisms are pivotal in improving pharmacological control of the sympathetic responses to surgical trauma and perioperative stress. They are examined in detail in this review, with particular emphasis being given to the differences in intracellular signalling between cardiomyocytes and vascular smooth muscle cells.

The Effects of Short-Term Immobilization Stress on Muscarinic Receptors, β-Adrenoceptors, and Adenylyl Cyclase in Different Heart Regions

Heart muscarinic receptors (MR) and beta-adrenoceptors (BAR) belong to a large family of G-protein-coupled receptors. Although the role of catecholamines in the stress has been under keen investigation for many years, the effects of immobilization on this pair of receptors, considering their almost completely opposite actions in the heart, are not yet known. We have studied the effects of short-term immobilization (for 120 min) with different times of decapitation after the end of the immobilization period (0, 3, and 24 h) on MR, BAR (beta(1)-AR and beta(2)-AR using radioligand binding studies), and adenylyl cyclase (AC; using high-pressure liquid chromatography detection of cAMP) in different heart regions (left and right atria with or without cardiac ganglion cells [auriculae], septum, and left and right ventricles). The effects of one immobilization period were first apparent after 24 h. Stress brought about a downregulation of MR and BAR with decrease in AC activity. These effects were regionally specific and were predominantly expressed in the right atria, which is rich in ganglia cells, and in the right ventricles. Our results indicate that stressful stimuli can influence not only BAR, but MR, and that AC activity can also be affected. This finding is in good agreement with our previous hypothesis that parallel changes are possible in the number of this pair of receptors on cell membranes.

Rest insertion combined with high-frequency loading enhances osteogenesis

Mechanical loading can significantly affect skeletal adaptation. High-frequency loading can be a potent osteogenic stimulus. Additionally, insertion of rest periods between consecutive loading bouts can be a potent osteogenic stimulus. Thus we investigated whether the insertion of rest-periods between short-term high-frequency loading bouts would augment adaptation in the mature murine skeleton. Right tibiae of skeletally mature (16 wk) female C57BL/6 mice were loaded in cantilever bending at peak of 800 μϵ, 30 Hz, 5 days/wk for 3 wk. Left tibiae were the contralateral control condition. Mice were randomly assigned into one of two groups: continuous high-frequency (CT) stimulation for 100 s ( n = 9), or 1-s pulses of high-frequency stimuli followed by 10 s of rest (RI) for 100 s ( n = 9). Calcein labels were administered on days 1 and 21; label incorporation was used to histomorphometrically assess periosteal and endosteal indexes of adaptation. Periosteal surface referent bone formation rate (pBFR/BS) was significantly enhanced in CT (>88%) and RI (>126%) loaded tibiae, relative to control tibiae. Furthermore, RI tibiae had significantly greater pBFR/BS, relative to CT tibiae (>72%). The endosteal surface was not as sensitive to mechanical loading as the periosteal surface. Thus short-term high-frequency loading significantly elevated pBFR/BS, relative to control tibiae. Furthermore, despite the 10-fold reduction in cycle number, the insertion of rest periods between bouts of high-frequency stimuli significantly augmented pBFR/BS, relative to tibiae loaded continually. Optimization of osteogenesis in response to mechanical loading may underpin the development of nonpharmacological regiments designed to increase bone strength in individuals with compromised bone structures.

Protein kinase C regulates functional coupling of beta1-adrenergic receptors to Gi/o-mediated responses in cardiac myocytes

The effect of protein kinase C (PKC) activation on beta1-adrenergic receptor (beta1-AR) regulation of the cardiac L-type Ca2+ current (ICa,L) was studied using the whole-cell patch clamp technique. Treatment of guinea pig ventricular myocytes with phorbol-12,13-dibutyrate (PDBu) caused a significant decrease in ICa,L sensitivity to stimulation by submaximal beta1-AR activation using isoproterenol (Iso). This decrease in sensitivity was also associated with the ability of higher concentrations of Iso to directly inhibit the stimulatory response. PDBu treatment produced similar effects on H2 histamine receptor-mediated ICa,L responses. In the presence of PDBu, higher concentrations of Iso inhibited the histamine stimulated ICa,L, and this effect was blocked by a selective beta1-AR antagonist. Higher concentrations of histamine also inhibited the Iso stimulated ICa,L, and this effect was blocked by a selective H2 receptor antagonist. The effects of PDBu were blocked by the PKC inhibitor bisindolylmaleimide I, and they were not mimicked by the inactive phorbol ester 4alpha-phorbol-12,13-didecanoate. The inhibitory effects of Iso and histamine were significantly reduced when Gi/o mediated responses were blocked with pertussis toxin. These results suggest that PKC promotes coupling of cardiac beta1-adrenergic and H2 histamine receptors to Gi/o mediated inhibitory responses.

The α, but Not the β, Isoform of the Human Thromboxane A2 Receptor Is a Target for Nitric Oxide-mediated Desensitization

In humans, thromboxane A2 signals through two thromboxane A2 receptor (TP) isoforms termed TP alpha and TP beta. Signaling by TP alpha, but not TP beta, is subject to prostacyclin-induced desensitization mediated by direct protein kinase (PK) A phosphorylation where Ser329 represents the phosphotarget (Walsh, M. T., Foley, J. F., and Kinsella, B. T. (2000) J. Biol. Chem. 275, 20412-20423). In the current study, the effect of the vasodilator nitric oxide (NO) on intracellular signaling by the TP isoforms was investigated. The NO donor 3-morpholinosydnonimine, HCl (SIN-1) and 8-bromo-guanosine 3',5'-cyclic monophosphate (8-Br-cGMP) functionally desensitized U46619-mediated calcium mobilization and inositol 1,4,5-trisphosphate generation by TP alpha whereas signaling by TP beta was unaffected by either agent. NO-mediated desensitization of TP alpha signaling occurred through a PKG-dependent, PKA- and PKC-independent mechanism. TP alpha, but not TP beta, was efficiently phosphorylated by PKG in vitro and underwent NO/PKG-mediated phosphorylation in whole cells. Deletion/site-directed mutagenesis and metabolic labeling studies identified Ser331 as the target residue of NO-induced PKG phosphorylation of TP alpha. Although TP alpha S331A was insensitive to NO/PKG-desensitization, similar to wild type TP alpha its signaling was fully desensitized by the prostacyclin receptor agonist cicaprost occurring through a PKA-dependent mechanism. Conversely, signaling by TP alpha S329A was insensitive to cicaprost stimulation whereas it was fully desensitized by NO/PKG signaling. In conclusion, TP alpha undergoes both NO- and prostacyclin-mediated desensitization that occur through entirely independent mechanisms involving direct PKG phosphorylation of Ser331, in response to NO, and PKA phosphorylation of Ser329, in response to prostacyclin, within the unique carboxyl-terminal tail domain of TP alpha. On the other hand, signaling by TP beta is unaffected by either NO or prostacyclin.

Calcium signaling mediated by P2Y receptors in mouse taste cells

Evidence implicates a number of neuroactive substances and their receptors in mediating complex cell-to-cell communications in the taste bud. Recently, we found that ATP, a ubiquitous neurotransmitter/neuromodulator, mobilizes intracellular Ca2+ in taste cells by activating P2Y receptors. Here, P2Y receptor-cellular response coupling was characterized in detail using single cell ratio photometry and the inhibitory analysis. The sequence of underlying events was shown to include ATP-dependent activation of PLC, IP3 production, and IP3 receptor-mediated Ca2+ release followed by Ca2+ influx. Data obtained favor SOC channels rather than receptor-operated channels as a pathway for Ca2+ influx that accompanies Ca2+ release. Intracellular Ca2+ mobilized by ATP is apparently extruded by the plasma membrane Ca2+-ATPase, while a contribution of the Na+/Ca2+ exchange and other mechanisms of Ca2+ clearance is negligible. Cyclic AMP-dependent phosphorylation is likely to control a gain of the phosphoinositide cascade involved in ATP transduction. ATP-responsive taste cells are abundant in circumvallate, foliate, and fungiform papillae. Taken together, our observations point to a putative role for ATP as a neurotransmitter operative in the taste bud.

A3 adenosine receptor activation in melanoma cells: association between receptor fate and tumor growth inhibition

Activation of the Gi protein-coupled A3 adenosine receptor (A3AR) has been implicated in the inhibition of melanoma cell growth by deregulating protein kinase A and key components of the Wnt signaling pathway. Receptor activation results in internalization/recycling events that play an important role in turning on/off receptor-mediated signal transduction pathways. Thus, we hereby examined the association between receptor fate, receptor functionality, and tumor growth inhibition upon activation with the agonist 1-deoxy-1-[6-[[(3-iodophenyl)-methyl]amino]-9H-purine-9-yl]-N-methyl-beta-D-ribofuranuronamide (IB-MECA). Results showed that melanoma cells highly expressed A3AR on the cell surface, which was rapidly internalized to the cytosol and "sorted" to the endosomes for recycling and to the lysosomes for degradation. Receptor distribution in the lysosomes was consistent with the down-regulation of receptor protein expression and was followed by mRNA and protein resynthesis. At each stage, receptor functionality was evidenced by the modulation in cAMP level and the downstream effectors protein kinase A, glycogen synthase kinase-3beta, c-Myc, and cyclin D1. The A3AR antagonist MRS 1523 counteracted the internalization process as well as the modulation in the expression of the signaling proteins, demonstrating that the responses are A3AR-mediated. Supporting this notion are the in vivo studies showing tumor growth inhibition upon IB-MECA treatment and reverse of this response when IB-MECA was given in combination with MRS 1523. In addition, in melanoma tumor lesions derived from IB-MECA-treated mice, the expression level A3AR and the downstream key signaling proteins were modulated in the same pattern as was seen in vitro. Altogether, our observations tie the fate of A3AR to modulation of downstream molecular mechanisms leading to tumor growth inhibition both in vitro and in vivo.

Short-Term Desensitization of G-Protein-Activated, Inwardly Rectifying K+ (GIRK) Currents in Pyramidal Neurons of Rat Neocortex

Whole cell recordings from acutely isolated rat neocortical pyramidal cells were performed to study the kinetics and the mechanisms of short-term desensitization of G-protein-activated, inwardly rectifying K+ (GIRK) currents during prolonged application (5 min) of baclofen, adenosine, or serotonin. Most commonly, desensitization of GIRK currents was characterized by a biphasic time course with average time constants for fast and slow desensitization in the range of 8 and 120 s, respectively. The time constants were independent of the agonist used to evoke the current. The biphasic time course was preserved in perforated-patch recordings, indicating that neither component of desensitization is attributable to cell dialysis. Desensitization of GIRK currents displayed a strong heterologous component in that application of a second agonist substantially reduced the responsiveness to a test agonist. Fast desensitization, but not slow desensitization, was lost in cells loaded with GDP, suggesting that the hydrolysis cycle of G proteins might underlie the initial, rapid current decline. Hydrolysis of phosphatidylinositol biphosphate is an unlikely candidate underlying short-term desensitization, because both components of desensitization were preserved in the presence of the phospholipase C inhibitor U73122. We conclude that short-term desensitization does neither result from receptor downregulation nor from altered channel gating but might involve modifications of the G-protein-dependent pathway that serves to translate receptor activation into channel opening.

Endothelial and Microvascular Function

The endothelium is more than just a passive vessel lining. New advances have revealed and expanded the multifactorial role of the endothelium in the homeostatic regulation of the microvasculature, including control of primary hemostasis, blood coagulation and fibrinolysis, platelet and leukocyte interactions with the vessel wall, lipoprotein metabolism, presentation of histocompatibility antigens, regulation of vascular tone and growth, and regulation of blood pressure. It possesses numerous receptors and releases compounds that affect the regulation of vascular tone and contribute to vascular permeability. Many crucial vasoactive endogenous compounds are formed in the endothelial cells to control the functions of vascular smooth muscle cells and circulating blood cells. Gap junctions facilitate the exchange of metabolites, ions, and other messenger molecules among endothelial cells and smooth muscle cells, and regulate cell growth. Among the numerous regulatory systems affecting microvascular function are the cholinergic and adrenergic (α1, α2, and β) systems. Flow-metabolism coupling is affected by a variety of signaling systems, including adenosine, oxygen, carbon dioxide, lactate, nitric oxide, and others. Agents such as the angiotensin system and endothelin, as well as others, play a role in autoregulation (maintenance of constant flow in the face of changing pressure). All of these are discussed in detail.

A quantitative analysis of mRNA expression of alpha 1 and beta-adrenoceptor subtypes and their functional roles in human normal and obstructed bladders

PURPOSE

We compared the expression level of alpha 1-adrenoceptor (AR) subtype mRNA to that of beta-AR subtype mRNA in control and obstructed human bladders, and examined whether alpha 1-AR mediated contraction and beta-AR mediated relaxation of human detrusor muscle are altered by bladder outlet obstruction.

MATERIALS AND METHODS

A real-time quantitative reverse transcriptase-polymerase chain reaction based method was used to quantify alpha1 and beta-AR subtype mRNA expression. The contractile response to alpha-AR agonists and the relaxant effect of beta-AR agonists were examined using an isometric contraction technique.

RESULTS

The mRNA of alpha 1a, alpha 1b, alpha 1d, beta 1 and beta 2-AR mRNAs was expressed at low levels, while beta 3-AR was the most highly expressed subtype at the mRNA level in control and obstructed bladders. The expression level of alpha 1 and beta-AR subtype mRNA was not significantly different between the 2 groups. The mean contractile response to 10(-4) M phenylephrine +/- SEM was only 4.4% +/- 1.4% and 5.2% +/- 1.4% of the 10(-7) M carbachol induced contraction in the control and obstructed groups, respectively. Contractile responses to phenylephrine were not significantly increased in obstructed bladder. Isoproterenol and the beta 3-AR selective agonist L755,507 relaxed human detrusor muscle in a concentration dependent manner (10(-9) to 10(-4) M). However, the beta 1/beta 2-AR agonist dobutamine and the beta 2-AR selective agonist clenbuterol did not produce relaxation at a concentration of 10(-9) to 10(-5) M in the 2 groups.

CONCLUSIONS

These findings indicate that neither up-regulation of alpha1-ARs nor down-regulation of beta-ARs occurs and relaxation mediating beta 3-ARs are by far predominant in the human obstructed bladder. Therefore, it is not likely that bladder alpha1-ARs are responsible for detrusor overactivity and storage symptoms in patients with benign prostatic obstruction.