Evidence for a significant role of a Gs-triggered mechanism unrelated to the activation of adenylyl cyclase in the cyclic AMP-independent relaxant response of guinea-pig tracheal smooth muscle

Intracellular pathways of calcitonin gene-related peptide-induced relaxation of human coronary arteries: A key role for Gβγ subunit instead of cAMP

BACKGROUND AND PURPOSE

Calcitonin gene-related peptide (CGRP) is a potent vasodilator. While its signalling is assumed to be mediated via increases in cAMP, this study focused on elucidating the actual intracellular signalling pathways involved in CGRP-induced relaxation of human isolated coronary arteries (HCA).

EXPERIMENTAL APPROACH

HCA were obtained from heart valve donors (27 M, 25 F, age 54 ± 2 years). Concentration-response curves to human α-CGRP or forskolin were constructed in HCA segments, incubated with different inhibitors of intracellular signalling pathways, and intracellular cAMP levels were measured with and without stimulation.

RESULTS

Adenylyl cyclase (AC) inhibitors SQ22536 + DDA and MDL-12330A, and PKA inhibitors Rp-8-Br-cAMPs and H89, did not inhibit CGRP-induced relaxation of HCA, nor did the guanylyl cyclase inhibitor ODQ, PKG inhibitor KT5823, EPAC1/2 inhibitor ESI09, potassium channel blockers TRAM-34 + apamin, iberiotoxin or glibenclamide, or the Gαq inhibitor YM-254890. Phosphodiesterase inhibitors induced a concentration-dependent decrease in the response to KCl but did not potentiate relaxation to CGRP. Relaxation to forskolin was not blocked by PKA or AC inhibitors, although AC inhibitors significantly inhibited the increase in cAMP. Inhibition of Gβγ subunits using gallein significantly inhibited the relaxation to CGRP in human coronary arteries.

CONCLUSION

While CGRP signalling is generally assumed to act via cAMP, the CGRP-induced vasodilation in HCA was not inhibited by targeting this intracellular signalling pathway at different levels. Instead, inhibition of Gβγ subunits did inhibit the relaxation to CGRP, suggesting a different mechanism of CGRP-induced relaxation than generally believed.

Selectivity of pharmacological tools: implications for use in cell physiology. A review in the theme: Cell signaling: proteins, pathways and mechanisms

Pharmacological inhibitors are frequently used to identify the receptors, receptor subtypes, and associated signaling pathways involved in physiological cell responses. Based on the effects of such inhibitors conclusions are drawn about the involvement of their assumed target or lack thereof. While such inhibitors can be useful tools for a better physiological understanding, their uncritical use can lead to incorrect conclusions. This article reviews the concept of inhibitor selectivity and its implication for cell physiology. Specifically, we discuss the implications of using inhibitor vs. activator approaches, issues of direct vs. indirect pathway modulation, implications of inverse agonism and biased signaling, and those of orthosteric vs. allosteric, competitive vs. noncompetitive, and reversible vs. irreversible inhibition. Additional problems can result from inconsistent estimates of inhibitor potency and differences in potency between cell-free systems and intact cells. These concepts are illustrated by several examples of inhibitors displaying affinity for related but distinct targets or even unrelated targets. Of note, many of the issues being addressed are also applicable to genetic inhibition strategies. The main practical conclusion following from these concepts is that investigators should be critical in the choice of inhibitor, its concentrations, and its mode of application. When this advice is adhered to, small-molecule pharmacological inhibitors can be important experimental tools in the hand of physiologists.

Distinct PKA and Epac compartmentalization in airway function and plasticity

Asthma and chronic obstructive pulmonary disease (COPD) are obstructive lung diseases characterized by airway obstruction, airway inflammation and airway remodelling. Next to inflammatory cells and airway epithelial cells, airway mesenchymal cells, including airway smooth muscle cells and (myo)fibroblasts, substantially contribute to disease features by the release of inflammatory mediators, smooth muscle contraction, extracellular matrix deposition and structural changes in the airways. Current pharmacological treatment of both diseases intends to target the dynamic features of the endogenous intracellular suppressor cyclic AMP (cAMP). This review will summarize our current knowledge on cAMP and will emphasize on key discoveries and paradigm shifts reflecting the complex spatio-temporal nature of compartmentalized cAMP signalling networks in health and disease. As airway fibroblasts and airway smooth muscle cells are recognized as central players in the development and progression of asthma and COPD, we will focus on the role of cAMP signalling in their function in relation to airway function and plasticity. We will recapture on the recent identification of cAMP-sensing multi-protein complexes maintained by cAMP effectors, including A-kinase anchoring proteins (AKAPs), proteins kinase A (PKA), exchange protein directly activated by cAMP (Epac), cAMP-elevating seven-transmembrane (7TM) receptors and phosphodiesterases (PDEs) and we will report on findings indicating that the pertubation of compartmentalized cAMP signalling correlates with the pathopysiology of obstructive lung diseases. Future challenges include studies on cAMP dynamics and compartmentalization in the lung and the development of novel drugs targeting these systems for therapeutic interventions in chronic obstructive inflammatory diseases.

Activation of endogenous GABAA channels on airway smooth muscle potentiates isoproterenol-mediated relaxation

Reactive airway disease predisposes patients to episodes of acute smooth muscle mediated bronchoconstriction. We have for the first time recently demonstrated the expression and function of endogenous ionotropic GABA(A) channels on airway smooth muscle cells. We questioned whether endogenous GABA(A) channels on airway smooth muscle could augment beta-agonist-mediated relaxation. Guinea pig tracheal rings or human bronchial airway smooth muscles were equilibrated in organ baths with continuous digital tension recordings. After pretreatment with or without the selective GABA(A) antagonist gabazine (100 muM), airway muscle was contracted with acetylcholine or beta-ala neurokinin A, followed by relaxation induced by cumulatively increasing concentrations of isoproterenol (1 nM to 1 muM) in the absence or presence of the selective GABA(A) agonist muscimol (10-100 muM). In separate experiments, guinea pig tracheal rings were pretreated with the large conductance K(Ca) channel blocker iberiotoxin (100 nM) after an EC(50) contraction with acetylcholine but before cumulatively increasing concentrations of isoproterenol (1 nM to 1 uM) in the absence or presence of muscimol (100 uM). GABA(A) activation potentiated the relaxant effects of isoproterenol after an acetylcholine or tachykinin-induced contraction in guinea pig tracheal rings or an acetylcholine-induced contraction in human endobronchial smooth muscle. This muscimol-induced potentiation of relaxation was abolished by gabazine pretreatment but persisted after blockade of the maxi K(Ca) channel. Selective activation of endogenous GABA(A) receptors significantly augments beta-agonist-mediated relaxation of guinea pig and human airway smooth muscle, which may have important therapeutic implications for patients in severe bronchospasm.

Pharmacological characterization of the β-adrenoceptor that mediates the relaxant response to noradrenaline in guinea-pig tracheal smooth muscle

Pharmacological characteristics of beta-adrenoceptors (beta-ARs) mediating noradrenaline-induced relaxation were investigated in guinea-pig tracheal smooth muscle. The inhibitory effects of several types of beta-AR antagonists on noradrenaline-induced relaxation against histamine contraction were scrutinized with Schild plot analysis. The concentration-response curve for noradrenaline obtained in the absence of phentolamine and uptake inhibitors was competitively antagonized by all of the beta-AR antagonists used in this study (propranolol, bupranolol, atenolol, butoxamine and ICI-118,551). However, their pA2 values were markedly less than the expected values for beta1-AR and beta2-AR. On the other hand, pA2 values of ICI-118,551 (6.85) determined in the presence of phentolamine suggested a contribution of a beta1 -AR rather than beta2 -AR. In the presence of phentolamine and uptake inhibitors (desipramine and deoxycorticosterone), the Schild plot for atenolol was a better fit, with two distinct straight lines. The pA2 values of atenolol provided by the regression were: approximately 7.0, which corresponds to the expected beta1-AR value, and approximately 6.5, which was 3 times less than the expected value for beta1 -AR, and thus the possible presence of two classes of beta1 -AR (beta1(Low) and beta1(High)) was suggested. This view was also supported by Schild plot analysis for propranolol, which fit two straight lines each with a slope of 1.0. The present findings indicate that beta1 -ARs contributing to noradrenaline-elicited relaxation in guinea-pig tracheal smooth muscle exhibit diverse pharmacological characteristics and may be subdivided into at least two classes with distinct affinities for atenolol. Under physiological conditions, beta1(Low) rather than beta1(High) seems to play a more significant role in noradrenaline-regulated airway smooth muscle tone.

Alpha1-, alpha2- and beta-adrenoceptors in the urinary bladder, urethra and prostate

1 We have systematically reviewed the presence, functional responses and regulation of alpha(1)-, alpha(2)- and beta-adrenoceptors in the bladder, urethra and prostate, with special emphasis on human tissues and receptor subtypes. 2 Alpha(1)-adrenoceptors are only poorly expressed and play a limited functional role in the detrusor. Alpha(1)-adrenoceptors, particularly their alpha(1A)-subtype, show a more pronounced expression and promote contraction of the bladder neck, urethra and prostate to enhance bladder outlet resistance, particularly in elderly men with enlarged prostates. Alpha(1)-adrenoceptor agonists are important in the treatment of symptoms of benign prostatic hyperplasia, but their beneficial effects may involve receptors within and outside the prostate. 3 Alpha(2)-adrenoceptors, mainly their alpha(2A)-subtype, are expressed in bladder, urethra and prostate. They mediate pre-junctional inhibition of neurotransmitter release and also a weak contractile effect in the urethra of some species, but not humans. Their overall post-junctional function in the lower urinary tract remains largely unclear. 4 Beta-adrenoceptors mediate relaxation of smooth muscle in the bladder, urethra and prostate. The available tools have limited the unequivocal identification of receptor subtypes at the protein and functional levels, but it appears that the beta(3)- and beta(2)-subtypes are important in the human bladder and urethra, respectively. Beta(3)-adrenoceptor agonists are promising drug candidates for the treatment of the overactive bladder. 5 We propose that the overall function of adrenoceptors in the lower urinary tract is to promote urinary continence. Further elucidation of the functional roles of their subtypes will help a better understanding of voiding dysfunction and its treatment.

Involvement of reduced sensitivity to Ca in beta-adrenergic action on airway smooth muscle

BACKGROUND

It is well known that beta-adrenoceptor agonists (beta-agonists) cause relaxation in airway smooth muscle mediated by a reduction in the concentration of intracellular Ca2+ ([Ca2+](i)). However, little is currently known regarding whether reduced sensitization to Ca2+ is involved in the beta-adrenergic relaxation.

OBJECTIVE

This study was designed to determine the intracellular mechanisms underlying suppression of Ca2+ sensitization in beta-adrenergic relaxation (Ca(2+)-independent relaxation by beta-agonists). Methods Isometric tension and [Ca2+](i) were simultaneously measured in fura-2-loaded strips isolated from guinea-pig tracheal smooth muscles. The relationships between tension and [Ca2+](i) were examined in the inhibitory action of isoprenaline (ISO) and other cAMP-related agents against methacholine-induced contraction.

RESULTS

The concentration-inhibition curve for ISO against methacholine in tension was significantly dissociated from the curve for ISO in [Ca2+](i). In ISO-induced relaxation, a reduction in tension was significantly greater than that in [Ca2+](i.) This phenomenon was mimicked by other cAMP-related agents: forskolin and dibutyryl-cAMP. In contrast, the inhibitory action of SKF-96365, a non-selective inhibitor of Ca(2+) channels, was associated with that in [Ca2+](i). In the presence of Rp-cAMPS, an inhibitor of protein kinase A (PKA), ISO caused an equivalent relaxation with less reduction in [Ca2+](i). The effects of ISO were not affected by Y-27632, an inhibitor of Rho-kinase, or by bisindolylmaleimide, an inhibitor of protein kinase C. ISO failed to inhibit contraction elicited by calyculin A, an inhibitor of myosin phosphatase. Conclusion beta-Adrenergic action antagonizes not only Ca2+ mobilization but also Ca2+ sensitization in methacholine-induced contraction. The cAMP/PKA-independent, G(s)-direct action is more potent in Ca(2+)-independent relaxation by beta-agonists than the cAMP/PKA-dependent pathway. Moreover, myosin phosphatase is a fundamentally affected protein in the reduced response to Ca2+ mediated by beta-agonist. Our results may provide evidence that this Ca2+ desensitization is a novel target for a reliever medication using rapid-acting beta-agonists in acute asthma management.

New insights into beta-adrenoceptors in smooth muscle: distribution of receptor subtypes and molecular mechanisms triggering muscle relaxation

1. The beta-adrenoceptor is currently classified into beta(1), beta(2) and beta(3) subtypes and all three subtypes are expressed in smooth muscle. Each beta-adrenoceptor subtype exhibits tissue-specific distribution patterns, which may be a determinant controlling the mechanical functions of corresponding smooth muscle. Airway and uterine smooth muscles abundantly express the beta(2)-adrenoceptor, the physiological significance of which is established as a fundamental regulator of the mechanical activities of these muscles. Recent pharmacomechanical and molecular approaches have revealed roles for the beta(3)-adrenoceptor in the gastrointestinal tract and urinary bladder smooth muscle. 2. The beta-adrenoceptor is a G(s)-protein-coupled receptor and its activation elevates smooth muscle cAMP. A substantial role for a cAMP-dependent mechanism(s) is generally believed to be the key trigger for eliciting beta-adrenoceptor-mediated relaxation of smooth muscle. Downstream effectors activated via a cAMP-dependent mechanism(s) include plasma membrane K(+) channels, such as the large-conductance, Ca(2+)-activated K(+) (MaxiK) channel. 3. Beta-Adrenoceptor-mediated relaxant mechanisms also include cAMP-independent signalling pathways. This view is supported by numerous pharmacological and electrophysiological lines of evidence. In airway smooth muscle, direct activation of the MaxiK channel by G(s)alpha is a mechanism by which stimulation of beta(2)-adrenoceptors elicits muscle relaxation independently of the elevation of cAMP. 4. The cAMP-independent mechanism(s) is also substantial in beta(3)-adrenoceptor-mediated relaxation of gastrointestinal tract smooth muscle. However, in the case of the beta(3)-adrenoceptor, a delayed rectified K(+) channel rather than the MaxiK channel seems to mediate, in part, cAMP-independent relaxant mechanisms. 5. In the present article, we review the distribution of beta-adrenoceptor subtypes in smooth muscle tissues and discuss the molecular mechanisms by which each subtype elicits muscle relaxation, focusing on the roles of cAMP and plasma membrane K(+) channels.

Involvement of cyclic AMP-dependent and -independent mechanisms in the relaxation of rat detrusor muscle via beta-adrenoceptors

We investigated the cAMP-dependent and -independent mechanisms of relaxation via beta-adrenoceptor in rat detrusor muscle with and without pre-contraction. A microdialysis technique was used to measure detrusor tension and cAMP level on the same detrusor tissue. In non-contracted tissue, isoproterenol, clenbuterol (beta2-adrenoceptor agonist) and FR165101, ((8S)-8-{[(2R)-2-(3-chlorophenyl)-2-hydroxyethyl]amino}-6,7,8,9-tetrahydro-5H-benzocyclohepten-2-yl)oxy]acetic acid hydrochloride (beta3-adrenoceptor agonist) relaxed detrusor muscle and cAMP levels also increased in a concentration dependent manner. SQ22536 (adenylyl cyclase inhibitor) markedly suppressed relaxation, suggesting that beta-adrenoceptor-mediated relaxation may be attributed mainly to cAMP-dependent mechanism. In high K+ pre-contracted tissue, although relaxation advanced in a concentration dependent manner, cAMP production reached a plateau at concentrations of more than 10(-7) M. SQ22536 had only a small inhibitory effect. However, large-conductance, Ca2+-activated K+ (BK(Ca)) channel inhibitors, charybdotoxin and iberiotoxin markedly suppressed relaxation. These results suggest that in addition to cAMP-dependent pathway, BK(Ca) channels are involved in the beta-adrenoceptor agonists-induced relaxation in pre-contracted detrusor muscle.

Role of β2-adrenoceptors (ß-AR), but not ß1-, β3-AR and endothelial nitric oxide, in β-AR-mediated relaxation of rat intrapulmonary artery

The aim of this study was to analyze beta-adrenoceptor (beta-AR)-mediated relaxation in rat intralobar pulmonary artery. The relaxant responses of beta-AR agonists were characterized using beta-AR antagonists in prostaglandin F2alpha (PGF2alpha)-precontracted arteries. The role of nitric oxide (NO) and endothelium in beta-AR-mediated relaxation was also investigated. Isoprenaline (a non-selective beta-AR agonist) and salbutamol (a selective beta2-AR agonist) induced vasorelaxation. ICI 118551 (a selective beta2-AR antagonist) antagonized the effect of both isoprenaline and salbutamol (pA2 values of 9.57 and 9.51 respectively). In contrast, atenolol (1 microM) and CGP 20712A (0.1 microM), two beta1-AR antagonists, did not modify the relaxing effect of isoprenaline. The response to isoprenaline obtained in the presence of nadolol (10 microM, a beta1/beta2-AR antagonist) was not further inhibited by SR 59230A (1 microM, a selective beta3-AR antagonist). The non-beta1/beta2-AR agonists studied (BRL 37344, SR 58611A, and CGP 12177A) did not elicit vasorelaxation. Relaxation to isoprenaline and salbutamol was unaffected by L-N(G)-nitro-arginine methyl ester (100 microM, an inhibitor of NO synthase) or after endothelium removal. These results demonstrate the role of beta2-AR in mediating relaxation in rat intralobar pulmonary artery precontracted with PGF2alpha. They indicate that beta2-AR-mediated relaxation in this artery is NO- and endothelium-independent. Furthermore, they do not provide evidence of a relaxant role of either beta1- or beta3-AR in PGF2alpha-precontracted rat intrapulmonary artery.

MaxiK channel roles in blood vessel relaxations induced by endothelium-derived relaxing factors and their molecular mechanisms

The endothelium of blood vessels plays a crucial role in the regulation of blood flow by controlling mechanical functions of underlying vascular smooth muscle. The regulation by the endothelium of vascular smooth muscle relaxation and contraction is mainly achieved via the release of vasoactive substances upon stimulation with neurohumoural substances and physical stimuli. Nitric oxide (NO) and prostaglandin I2 (prostacyclin, PGI2) are representative endothelium-derived chemicals that exhibit powerful blood vessel relaxation. NO action involves activation of soluble guanylyl cyclase and PGI2 action is initiated by the stimulation of a cell-surface receptor (IP receptor, IPR) that is coupled with Gs-protein-adenylyl cyclase cascade. Many studies on the mechanisms by which NO and PGI2 elicit blood vessel relaxation have highlighted a role of the large conductance, Ca2+-activated K+ (MaxiK, BKCa) channel in smooth muscle as their common downstream effector. Furthermore, their molecular mechanisms have been unravelled to include new routes different from the conventionally approved intracellular pathways. MaxiK channel might also serve as a target for endothelium-derived hyperpolarizing factor (EDHF), the non-NO, non-PGI2 endothelium-derived relaxing factor in some blood vessels. In this brief article, we review how MaxiK channel serves as an endothelium-vascular smooth muscle transducer to communicate the chemical signals generated in the endothelium to control blood vessel mechanical functions and discuss their molecular mechanisms.

Can a GDP-liganded G-protein be active?

The replacement of GDP bound to the alpha-subunit of a G-protein by GTP is generally considered a crucial step in the activation of effectors by a G-protein. New data by Uğur et al. (2005) (p. 720) raise the possibility that for the heterotrimeric G-protein Gs, GDP-liganded Gs is able to activate the effector adenylyl cyclase as potently and effectively as when Gs is in its GTP bound form. We summarize here the evidence that GTP is necessary for effector activation by G-proteins and discuss potential implications and limitations of data to the contrary.

cAMP-independent mechanism is significantly involved in beta2-adrenoceptor-mediated tracheal relaxation

The role of cAMP in the beta2-adrenoceptor-mediated relaxation in response to salbutamol was examined in guinea pig tracheal smooth muscle. The concentration-dependent salbutamol-induced relaxation was antagonized in a competitive fashion by a beta2-selective adrenoceptor antagonist, butoxamine, with a pA2 value of 6.90. Salbutamol (10 microM) elevated the tracheal smooth muscle cAMP content by about fivefold, a response which was significantly inhibited by an adenylyl cyclase inhibitor, 9-(tetrahydro-2-furanyl)-9H-purin-6-amine (SQ 22,536, 100 microM). However, the salbutamol-elicited relaxation was not diminished by SQ 22,536 (100 microM). These results provide evidence for the first time that a cAMP-independent mechanism(s) is involved in beta2-adrenoceptor-mediated tracheal smooth muscle relaxation in the guinea pig.

MaxiK channel mediates beta2-adrenoceptor-activated relaxation to isoprenaline through cAMP-dependent and -independent mechanisms in guinea-pig tracheal smooth muscle

We examined the contribution of large-conductance, Ca(2+)-sensitive K+ (MaxiK) channel to beta2-adrenoceptor-activated relaxation to isoprenaline in guinea-pig tracheal smooth muscle focusing on the role for cAMP in the coupling between beta2-adrenoceptor and MaxiK channel. Isoprenaline-elicited relaxation was confirmed to be mediated through beta2-type of adrenoceptor since the response was antagonized in a competitive fashion by a beta2-selective adrenoceptor antagonist butoxamine with a pA2 value of 6.56. Isoprenaline-induced relaxation was significantly potentiated by a selective inhibitor of cyclic AMP-specific phosphodiesterase, Ro-20-1724 (0.1-1 microM). cAMP-dependent mediation of MaxiK channel in the relaxant response to isoprenaline was evidenced since the potentiated response to isoprenaline by the presence of Ro-20-1724 (1 microM) was inhibited by the channel selective blocker, iberiotoxin (IbTx, 100 nM). This concept was supported by the finding that the relaxation to a membrane permeable cAMP analogue, 8-bromo-cAMP (1 mM), was susceptible to the inhibition by IbTx. On the other hand, isoprenaline-induced relaxation was not practically diminished by an adenylyl cyclase inhibitor SQ 22,536 (100 microM). However, isoprenaline-induced relaxation in the presence of SQ 22,536 was suppressed by IbTx. Characteristics of isoprenaline-induced relaxant response, i.e., impervious to SQ 22,536 but susceptible to IbTx, were practically mimicked by cholera toxin (CTX, 5 microg/ml), an activator of adenylyl cyclase coupled-heterotrimeric guanine nucleotide-binding regulatory protein Gs. These findings indicate that in guinea-pig tracheal smooth muscle: 1) MaxiK channel substantially mediates beta2-adrenoceptor-activated relaxation; 2) both cAMP-dependent and -independent mechanisms underlie the functional coupling between beta2-adrenoceptor and MaxiK channel to induce muscle relaxation; and 3) direct regulation of MaxiK channel by Gs operates in cAMP-independent coupling between beta2-adrenoceptor and this ion channel.