Pharmacomechanical coupling: the role of calcium, G-proteins, kinases and phosphatases

The role of intracellular calcium and Rho kinase pathways in G protein-coupled receptor-mediated contractions of urinary bladder urothelium and lamina propria

The influence of extracellular and intracellular calcium on smooth muscle contractile activity varies between organs. In response to G protein-coupled receptor (GPCR) stimulation, the urinary bladder detrusor muscle has shown a 70% dependence on extracellular calcium, whereas the urothelium and lamina propria (U&LP) has a 20%-50% dependence. However, as this only accounts for partial contractile activity, the contribution of intracellular calcium and calcium sensitization pathways remains unclear. This study assessed the role of intracellular signaling pathways on GPCR-mediated urinary bladder U&LP contraction. Porcine U&LP responses to activation of the G_q/11-coupled muscarinic, histamine, 5-hydroxytryptamine (serotonin), neurokinin, prostaglandin, and angiotensin II receptors were assessed with three selective inhibitors of store-released intracellular calcium, 2-aminoethyl diphenylborinate (2-APB), cyclopiazonic acid (CPA), and ruthenium red, and three Rho kinase inhibitors, fasudil, Y-27632, and GSK269962. There was no discernible impact on receptor agonist-induced contractions of the U&LP after blocking intracellular calcium pathways, suggesting that this tissue is more sensitive to alterations in the availability of extracellular calcium. However, an alternative mechanism of action for GPCR-mediated contraction was identified to be the activation of Rho kinase, such as when Y-27632 significantly reduced the GPCR-mediated contractile activity of the U&LP by approximately 50% (P < 0.05, n = 8). This suggests that contractile responses of the bladder U&LP do not involve a significant release of calcium from intracellular stores, but that G_q/11-coupled receptor activation causes calcium sensitization via Rho kinase. This study highlights a key role for Rho kinase in the urinary bladder, which may provide a novel target in the future pharmaceutical management of bladder contractile disorders.

Conflicting Nongenomic Effects of Progesterone in the Myometrium of Pregnant Rats

Recently, it has been suggested that progesterone affects the contractile activity of pregnant myometrium via nongenomic pathways; therefore, we aimed to clarify whether progesterone causes and/or inhibits pregnant myometrial contractions via nongenomic pathways. Our in vitro experiments using myometrial strips obtained from rats at 20 days of gestation revealed that progesterone caused myometrial contractions in a concentration- and time-dependent manner at concentrations up to 5 × 10-7 M; however, this effect decreased at concentrations higher than 5 × 10-5 M. Similarly, progesterone enhanced oxytocin-induced contractions up to 5 × 10-7 M and inhibited contractions at concentrations higher than 5 × 10-5 M. Conversely, progesterone did not enhance high-KCl-induced contractions but inhibited contractions in a concentration- and time-dependent manner at concentrations higher than 5 × 10-7 M. We also found that RU486 did not affect progesterone-induced contractions or the progesterone-induced inhibition of high-KCl-induced contractions; however, progesterone-induced contractions were blocked by calcium-free phosphate saline solution, verapamil, and nifedipine. In addition, FPL64176, an activator of L-type voltage-dependent calcium channels, enhanced high-KCl-induced contractions and rescued the decrease in high-KCl-induced contractions caused by progesterone. Together, these results suggest that progesterone exerts conflicting nongenomic effects on the contractions of pregnant myometrium via putative L-type voltage-dependent calcium channels.

Anti-Bronchospasmodic Effect of JME-173, a Novel Mexiletine Analog Endowed With Highly Attenuated Anesthetic Activity

Local anesthetics (LAs), such as lidocaine and mexiletine, inhibit bronchoconstriction in asthmatics, but adverse effects limit their use for this specific clinical application. In this study, we describe the anti-spasmodic properties of the mexiletine analog 2-(2-aminopropoxy)-3,5-dimethyl, 4-Br-benzene (JME-173), which was synthesized and screened for inducing reduced activity on Na⁺ channels. The effectiveness of JME-173 was assessed using rat tracheal rings, a GH3 cell line and mouse cardiomyocytes to access changes in smooth muscle contraction, and Na⁺, and Ca⁺⁺ionic currents, respectively. Bronchospasm and airway hyper-reactivity (AHR) were studied using whole-body barometric plethysmography in A/J mice. We observed that the potency of JME-173 was 653-fold lower than mexiletine in inhibiting Na⁺ currents, but 12-fold higher in inhibiting L-type Ca⁺⁺ currents. JME-173 was also more potent than mexiletine in inhibiting tracheal contraction by carbachol, allergen, extracellular Ca⁺⁺, or sodium orthovanadate provocations. The effect of JME-173 on carbachol-induced tracheal contraction remained unaltered under conditions of de-epithelized rings, β₂-receptor blockade or adenylate cyclase inhibition. When orally administered, JME-173 and theophylline inhibited methacholine-induced bronchospasm at time points of 1 and 3 h post-treatment, while only JME-173 remained active for at least 6 h. In addition, JME-173 also inhibited AHR in a mouse model of lipopolysaccharide (LPS)-induced lung inflammation. Thus, the mexiletine analog JME-173 shows highly attenuated activity on Na⁺ channels and optimized anti-spasmodic properties, in a mechanism that is at least in part mediated by regulation of Ca⁺⁺ inflow toward the cytosol. Thus, JME-173 is a promising alternative for the treatment of clinical conditions marked by life-threatening bronchoconstriction.

The Role of Neurovascular System in Neurodegenerative Diseases

The neurovascular system (NVS), which consisted of neurons, glia, and vascular cells, is a functional and structural unit of the brain. The NVS regulates blood-brain barrier (BBB) permeability and cerebral blood flow (CBF), thereby maintaining the brain's microenvironment for normal functioning, neuronal survival, and information processing. Recent studies have highlighted the role of vascular dysfunction in several neurodegenerative diseases. This is not unexpected since both nervous and vascular systems are functionally interdependent and show close anatomical apposition, as well as similar molecular pathways. However, despite extensive research, the precise mechanism by which neurovascular dysfunction contributes to neurodegeneration remains incomplete. Therefore, understanding the mechanisms of neurovascular dysfunction in disease conditions may allow us to develop potent and effective therapies for prevention and treatment of neurodegenerative diseases. This review article summarizes the current research in the context of neurovascular signaling associated with neurodegenerative diseases, such as Alzheimer's disease (AD), Parkinson's disease (PD), amyotrophic lateral sclerosis (ALS), and Huntington's disease (HD). We also discuss the potential implication of neurovascular factor as a novel therapeutic target and prognostic marker in patients with neurodegenerative conditions. Graphical Abstract.

Stimulation history affects vasomotor responses in rat mesenteric arterioles

Resistance vessels regulate blood flow by continuously adjusting activity of the wall smooth muscle cells. These cells integrate a variety of stimuli from blood, endothelium, autonomic nerves, and surrounding tissues. Each stimulus elicits an intracellular signaling cascade that eventually influences activation of the contractile machinery. The characteristic time scale of each cascade and the sharing of specific reactions between cascades provide for complex behavior when a vessel receives multiple stimuli. Here, we apply sequential stimulation with invariant concentrations of vasoconstrictor (norepinephrine/methoxamine) and vasodilator (SNAP/carbacol) to rat mesenteric vessels in the wire myograph to show that (1) time elapsed between addition of two vasoactive drugs and (2) the sequence of addition may significantly affect final force development. Furthermore, force oscillations (vasomotion) often appear upon norepinephrine administration. Using computational modeling in combination with nitric oxide (NO) inhibition/NO addition experiments, we show that (3) amplitude and number of oscillating vessels increase over time, (4) the ability of NO to induce vasomotion depends on whether it is applied before or after norepinephrine, and (5) emergence of vasomotion depends on the prior dynamical state of the system; in simulations, this phenomenon appears as "hysteresis." These findings underscore the time-dependent nature of vascular tone generation which must be considered when evaluating the vasomotor effects of multiple, simultaneous stimuli in vitro or in vivo.

Creatine kinase, energy reserve, and hypertension: from bench to bedside

We hypothesized that human variation in the activity of the ATP regenerating enzyme creatine kinase (CK) activity affects hypertension and cardiovascular disease risk. CK is tightly bound close to ATP-utilizing enzymes including Ca2+-ATPase, myosin ATPase, and Na+/K+-ATPase, where it rapidly regenerates ATP from ADP, H+, and phosphocreatine. Thus, relatively high CK was thought to enhance ATP-demanding processes including resistance artery contractility and sodium retention, and reduce ADP-dependent functions. In a series of studies of our group and others, CK was linked to hypertension and bleeding risk. Plasma CK after rest, used as a surrogate measure for tissue CK, was associated with high blood pressure and failure of antihypertensive therapy in case-control and population studies. Importantly, high tissue CK preceded hypertension in animal models and in humans, and human vascular tissue CK gene expression was strongly associated with clinical blood pressure. In line with this, CK inhibition substantially reduced the contractility of human resistance arteries ex vivo. We also presented evidence that plasma CK reduced ADP-dependent platelet aggregation. In subsequent intervention studies, the oral competitive CK inhibitor beta-guanidinopropionic acid (GPA) reduced blood pressure in spontaneously hypertensive rats (SHRs), and a 1-week trial of sub-therapeutic dose GPA in healthy men was uneventful. Thus, based on theoretical concepts, evidence was gathered in laboratory, case-control, and population studies that high CK is associated with hypertension and with bleeding risk, potentially leading to a new mode of cardiovascular risk reduction with CK inhibition.

Ca2+ signalling in mouse urethral smooth muscle in situ: role of Ca2+ stores and Ca2+ influx mechanisms

KEY POINTS

Contraction of urethral smooth muscle cells (USMCs) contributes to urinary continence. Ca2+ signalling in USMCs was investigated in intact urethral muscles using a genetically encoded Ca2+ sensor, GCaMP3, expressed selectively in USMCs. USMCs were spontaneously active in situ, firing intracellular Ca2+ waves that were asynchronous at different sites within cells and between adjacent cells. Spontaneous Ca2+ waves in USMCs were myogenic but enhanced by adrenergic or purinergic agonists and decreased by nitric oxide. Ca2+ waves arose from inositol trisphosphate type 1 receptors and ryanodine receptors, and Ca2+ influx by store-operated calcium entry was required to maintain Ca2+ release events. Ca2+ release and development of Ca2+ waves appear to be the primary source of Ca2+ for excitation-contraction coupling in the mouse urethra, and no evidence was found that voltage-dependent Ca2+ entry via L-type or T-type channels was required for responses to α adrenergic responses.

ABSTRACT

Urethral smooth muscle cells (USMCs) generate myogenic tone and contribute to urinary continence. Currently, little is known about Ca2+ signalling in USMCs in situ, and therefore little is known about the source(s) of Ca2+ required for excitation-contraction coupling. We characterized Ca2+ signalling in USMCs within intact urethral muscles using a genetically encoded Ca2+ sensor, GCaMP3, expressed selectively in USMCs. USMCs fired spontaneous intracellular Ca2+ waves that did not propagate cell-to-cell across muscle bundles. Ca2+ waves increased dramatically in response to the α1 adrenoceptor agonist phenylephrine (10 μm) and to ATP (10 μm). Ca2+ waves were inhibited by the nitric oxide donor DEA NONOate (10 μm). Ca2+ influx and release from sarcoplasmic reticulum stores contributed to Ca2+ waves, as Ca2+ free bathing solution and blocking the sarcoplasmic Ca2+ -ATPase abolished activity. Intracellular Ca2+ release involved cooperation between ryanadine receptors and inositol trisphosphate receptors, as tetracaine and ryanodine (100 μm) and xestospongin C (1 μm) reduced Ca2+ waves. Ca2+ waves were insensitive to L-type Ca2+ channel modulators nifedipine (1 μm), nicardipine (1 μm), isradipine (1 μm) and FPL 64176 (1 μm), and were unaffected by the T-type Ca2+ channel antagonists NNC-550396 (1 μm) and TTA-A2 (1 μm). Ca2+ waves were reduced by the store operated Ca2+ entry blocker SKF 96365 (10 μm) and by an Orai antagonist, GSK-7975A (1 μm). The latter also reduced urethral contractions induced by phenylephrine, suggesting that Orai can function effectively as a receptor-operated channel. In conclusion, Ca2+ waves in mouse USMCs are a source of Ca2+ for excitation-contraction coupling in urethral muscles.

Regulation of vascular tone homeostasis by NO and H2S: Implications in hypertension

Nitric oxide (NO) and hydrogen sulfide (H2S) are two gasotransmitters that are produced in the vasculature and contribute to the regulation of vascular tone. NO and H2S are synthesized in both vascular smooth muscle and endothelial cells; NO functions primarily through the sGC/cGMP pathway, and H2S mainly through activation of the ATP-dependent potassium channels; both leading to relaxation of vascular smooth muscle cells. A deficit in the NO/H2S homeostasis is involved in the pathogenesis of various cardiovascular diseases, especially hypertension. It is now becoming increasingly clear that there are important interactions between NO and H2S and that have a profound impact on vascular tone and this may provide insights into the new therapeutic interventions. The aim of this review is to provide a better understanding of individual and interactive roles of NO and H2S in vascular biology. Overall, available data indicate that both NO and H2S contribute to vascular (patho)physiology and in regulating blood pressure. In addition, boosting NO and H2S using various dietary sources or donors could be a hopeful therapeutic strategy in the management of hypertension.

The Potential Pathogenic Contributions of Endothelial Barrier and Arterial Contractile Dysfunction to Shock Due to B. anthracis Lethal and Edema Toxins

Shock with B. anthracis infection is particularly resistant to conventional cardiovascular support and its mortality rate appears higher than with more common bacterial pathogens. As opposed to many bacteria that lack exotoxins directly depressing hemodynamic function, lethal and edema toxin (LT and ET respectively) both cause shock and likely contribute to the high lethality rate with B. anthracis. Selective inhibition of the toxins is protective in infection models, and administration of either toxin alone in animals produces hypotension with accompanying organ injury and lethality. Shock during infection is typically due to one of two mechanisms: (i) intravascular volume depletion related to disruption of endothelial barrier function; and (ii) extravasation of fluid and/or maladaptive dilation of peripheral resistance arteries. Although some data suggests that LT can produce myocardial dysfunction, growing evidence demonstrates that it may also interfere with endothelial integrity thereby contributing to the extravasation of fluid that helps characterize severe B. anthracis infection. Edema toxin, on the other hand, while known to produce localized tissue edema when injected subcutaneously, has potent vascular relaxant effects that could lead to pathologic arterial dilation. This review will examine recent data supporting a role for these two pathophysiologic mechanisms underlying the shock LT and ET produce. Further research and a better understanding of these mechanisms may lead to improved management of B. anthracis in patients.

A novel inhibitory effect of oxazol-5-one compounds on ROCKII signaling in human coronary artery vascular smooth muscle cells

The selectivity of (4Z)-2-(4-chloro-3-nitrophenyl)-4-(pyridin-3-ylmethylidene)-1,3-oxazol-5-one (DI) for zipper-interacting protein kinase (ZIPK) was previously described by in silico computational modeling, screening a large panel of kinases, and determining the inhibition efficacy. Our assessment of DI revealed another target, the Rho-associated coiled-coil-containing protein kinase 2 (ROCKII). In vitro studies showed DI to be a competitive inhibitor of ROCKII (Ki, 132 nM with respect to ATP). This finding was supported by in silico molecular surface docking of DI with the ROCKII ATP-binding pocket. Time course analysis of myosin regulatory light chain (LC20) phosphorylation catalyzed by ROCKII in vitro revealed a significant decrease upon treatment with DI. ROCKII signaling was investigated in situ in human coronary artery vascular smooth muscle cells (CASMCs). ROCKII down-regulation using siRNA revealed several potential substrates involved in smooth muscle contraction (e.g., LC20, Par-4, MYPT1) and actin cytoskeletal dynamics (cofilin). The application of DI to CASMCs attenuated LC20, Par-4, LIMK, and cofilin phosphorylations. Notably, cofilin phosphorylation was not significantly decreased with a novel ZIPK selective inhibitor (HS-38). In addition, CASMCs treated with DI underwent cytoskeletal changes that were associated with diminution of cofilin phosphorylation. We conclude that DI is not selective for ZIPK and is a potent inhibitor of ROCKII.

Excitation-Contraction Coupling and Excitation-Transcription Coupling in Blood Vessels: Their Possible Interactions in Hypertensive Vascular Remodeling

Vascular smooth muscle cells (VSMC) display considerable phenotype plasticity which can be studied in vivo on vascular remodeling which occurs during acute or chronic vascular injury. In differentiated cells, which represent contractile phenotype, there are characteristic rapid transient changes of intracellular Ca2+ concentration ([Ca2+]i), while the resting cytosolic [Ca2+]i concentration is low. It is mainly caused by two components of the Ca2+ signaling pathways: Ca2+ entry via L-type voltage-dependent Ca2+ channels and dynamic involvement of intracellular stores. Proliferative VSMC phenotype is characterized by long-lasting [Ca2+]i oscillations accompanied by sustained elevation of basal [Ca2+]i. During the switch from contractile to proliferative phenotype there is a general transition from voltage-dependent Ca2+ entry to voltage-independent Ca2+ entry into the cell. These changes are due to the altered gene expression which is dependent on specific transcription factors activated by various stimuli. It is an open question whether abnormal VSMC phenotype reported in rats with genetic hypertension (such as spontaneously hypertensive rats) might be partially caused by a shift from contractile to proliferative VSMC phenotype.

Vascular O-GlcNAcylation augments reactivity to constrictor stimuli by prolonging phosphorylated levels of the myosin light chain

O-GlcNAcylation is a modification that alters the function of numerous proteins. We hypothesized that augmented O-GlcNAcylation levels enhance myosin light chain kinase (MLCK) and reduce myosin light chain phosphatase (MLCP) activity, leading to increased vascular contractile responsiveness. The vascular responses were measured by isometric force displacement. Thoracic aorta and vascular smooth muscle cells (VSMCs) from rats were incubated with vehicle or with PugNAc, which increases O-GlcNAcylation. In addition, we determined whether proteins that play an important role in the regulation of MLCK and MLCP activity are directly affected by O-GlcNAcylation. PugNAc enhanced phenylephrine (PE) responses in rat aortas (maximal effect, 14.2 ± 2 vs 7.9 ± 1 mN for vehicle, n=7). Treatment with an MLCP inhibitor (calyculin A) augmented vascular responses to PE (13.4 ± 2 mN) and abolished the differences in PE-response between the groups. The effect of PugNAc was not observed when vessels were preincubated with ML-9, an MLCK inhibitor (7.3 ± 2 vs 7.5 ± 2 mN for vehicle, n=5). Furthermore, our data showed that differences in the PE-induced contractile response between the groups were abolished by the activator of AMP-activated protein kinase (AICAR; 6.1 ± 2 vs 7.4 ± 2 mN for vehicle, n=5). PugNAc increased phosphorylation of myosin phosphatase target subunit 1 (MYPT-1) and protein kinase C-potentiated inhibitor protein of 17 kDa (CPI-17), which are involved in RhoA/Rho-kinase-mediated inhibition of myosin phosphatase activity. PugNAc incubation produced a time-dependent increase in vascular phosphorylation of myosin light chain and decreased phosphorylation levels of AMP-activated protein kinase, which decreased the affinity of MLCK for Ca(2+)/calmodulin. Our data suggest that proteins that play an important role in the regulation of MLCK and MLCP activity are directly affected by O-GlcNAcylation, favoring vascular contraction.

RhoA-Rho kinase and platelet-activating factor stimulation of ovine foetal pulmonary vascular smooth muscle cell proliferation

OBJECTIVES

Platelet-activating factor (PAF) is produced by pulmonary vascular smooth muscle cells (PVSMC). We studied effects of Rho kinase on PAF stimulation of PVSMC proliferation in an attempt to understand the role of RhoA/Rho kinase on PAF-induced ovine foetal pulmonary vascular remodelling. Our hypothesis is that PAF acts through Rho kinase, as one of its downstream signals, to induce arterial (SMC-PA) and venous (SMC-PV) cell proliferation in the hypoxic lung environment of the foetus, in utero.

MATERIALS AND METHODS

Rho kinase and MAPK effects on PAF receptor (PAFR)-mediated cell population expansion, and PAFR expression, were studied by DNA synthesis, western blot analysis and immunocytochemistry. Effects of constructs T19N and G14V on PAF-induced cell proliferation were also investigated.

RESULTS

Hypoxia increased PVSMC proliferation and Rho kinase inhibitors, Y-27632 and Fasudil (HA-1077) as well as MAPK inhibitors PD 98059 and SB 203580 attenuated PAF stimulation of cell proliferation. RhoA T19N and G14V stimulated cell proliferation, but co-incubation with PAF did not affect proliferative effects of the constructs. PAFR protein expression was significantly downregulated in both cell types by both Y-27632 and HA-1077, with comparable profiles. Also, cells treated with Y-27632 had less PAF receptor fluorescence with significant disruption of cell morphology.

CONCLUSIONS

Our results show that Rho kinase non-specifically modulated PAFR-mediated responses by a translational modification of PAFR protein, and suggest that, in vivo, activation of Rho kinase by PAF may be a further pathway to sustain PAFR-mediated PVSMC proliferation.

Dietary selenium influences calcium release and activation of MLCK in uterine smooth muscle of rats

We sought to elucidate the effects of different concentrations of dietary selenium on calcium ion release, MLCK levels, and muscle contraction in the uterine smooth muscle of rats. The selenium (Se) content of blood and of uterine smooth muscle tissues was detected by fluorescence spectrophotometry. Ca(2+) content was measured by atomic absorption spectroscopy. Calmodulin (CaM) and MLCK RNA and protein levels were analyzed by quantitative real-time polymerase chain reaction and Western blot, respectively. Dietary Se intake increased the Se levels in the blood and in uterine smooth muscle tissues and increased the Ca(2+) concentration in uterine smooth muscle tissues. The addition of Se also promoted CaM expression and enhanced MLCK activation in uterine smooth muscle tissues. In conclusion, Ca(2+), CaM, and MLCK were regulated by Se in uterine smooth muscle; Se plays a major role in regulating smooth muscle contraction in the uterus.

Histamine and H1 -histamine receptors faster venous circulation

The study has analysed the action of histamine in the rabbit venous system and evaluated its potential role in contraction during increased venous pressure. We have found that a great variety exists in histamine sensitivity and H(1) -histamine receptor expression in various types of rabbit veins. Veins of the extremities (saphenous vein, femoral vein, axillary vein) and abdomen (common iliac vein, inferior vena cava) responded to histamine by a prominent, concentration-dependent force generation, whereas great thoracic veins (subclavian vein, superior vena cavas, intrathoracic part of inferior vena cava) and a pelvic vein (external iliac vein) exhibited slight sensitivity to exogenous histamine. The lack of reactivity to histamine was not due to increased activity of nitric oxide synthase (NOS) or heme oxygenase-1. H(1) -histamine receptor expression of veins correlated well with the histamine-induced contractions. Voltage-dependent calcium channels mediated mainly the histamine-induced force generation of saphenous vein, whereas it did not act in the inferior vena cava. In contrast, the receptor-operated channels were not involved in this response in either vein. Tyrosine phosphorylation occurred markedly in response to histamine in the saphenous vein, but not in the inferior vena cava. Histamine induced a prominent ρ kinase activation in both vessels. Protein kinase C and mitogen-activated protein kinase (MAPK) were not implicated in the histamine-induced intracellular calcium sensitization. Importantly, transient clamping of the femoral vein in animals caused a short-term constriction, which was inhibited by H(1) -histamine receptor antagonist in vivo. Furthermore, a significantly greater histamine immunopositivity was detected in veins after stretching compared to the resting state. We conclude that histamine receptor density adapts to the actual requirements of the circulation, and histamine liberated by the venous wall during increased venous pressure contributes to the contraction of vessels, providing a force for the venous return.

Biochemical and molecular aspects of vascular adrenergic regulation of blood pressure in the elderly

Hypertension, orthostatic hypotension, arterial insufficiency, and atherosclerosis are common disorders in the elderly that lead to significant morbidity and mortality. One common factor to these conditions is an age-related decline in vascular beta-adrenergic receptor-mediated function and subsequent cAMP generation. Presently, there is no single cellular factor that can explain this age-related decline, and thus, the primary cause of this homeostatic imbalance is yet to be identified. However, the etiology is clearly associated with an age-related change in the ability of beta-adrenergic receptor to respond to agonist at the cellular level in the vasculature. This paper will review what is presently understood regarding the molecular and biochemical basis of age-impaired beta-adrenergic receptor-mediated signaling. A fundamental understanding of why β-AR-mediated vasorelaxation is impaired with age will provide new insights and innovative strategies for the management of multiple clinical disorders.

Chemical genetics of zipper-interacting protein kinase reveal myosin light chain as a bona fide substrate in permeabilized arterial smooth muscle

Zipper-interacting protein kinase (ZIPK) has been implicated in Ca(2+)-independent smooth muscle contraction, although its specific role is unknown. The addition of ZIPK to demembranated rat caudal arterial strips induced an increase in force, which correlated with increases in LC(20) and MYPT1 phosphorylation. However, because of the number of kinases capable of phosphorylating LC(20) and MYPT1, it has proven difficult to identify the mechanism underlying ZIPK action. Therefore, we set out to identify bona fide ZIPK substrates using a chemical genetics method that takes advantage of ATP analogs with bulky substituents at the N(6) position and an engineered ZIPK capable of utilizing such substrates. (32)P-Labeled 6-phenyl-ATP and ZIPK-L93G mutant protein were added to permeabilized rat caudal arterial strips, and substrate proteins were detected by autoradiography following SDS-PAGE. Mass spectrometry identified LC(20) as a direct target of ZIPK in situ for the first time. Tissues were also exposed to 6-phenyl-ATP and ZIPK-L93G in the absence of endogenous ATP, and putative ZIPK substrates were identified by Western blotting. LC(20) was thereby confirmed as a direct target of ZIPK; however, no phosphorylation of MYPT1 was detected. We conclude that ZIPK is involved in the regulation of smooth muscle contraction through direct phosphorylation of LC(20).

Phosphorylated HSP20 modulates the association of thin-filament binding proteins: caldesmon with tropomyosin in colonic smooth muscle

Small heat shock proteins HSP27 and HSP20 have been implicated in regulation of contraction and relaxation in smooth muscle. Activation of PKC-α promotes contraction by phosphorylation of HSP27 whereas activation of PKA promotes relaxation by phosphorylation of HSP20 in colonic smooth muscle cells (CSMC). We propose that the balance between the phosphorylation states of HSP27 and HSP20 represents a molecular signaling switch for contraction and relaxation. This molecular signaling switch acts downstream on a molecular mechanical switch [tropomyosin (TM)] regulating thin-filament dynamics. We have examined the role of phosphorylation state(s) of HSP20 on HSP27-mediated thin-filament regulation in CSMC. CSMC were transfected with different HSP20 phosphomutants. These transfections had no effect on the integrity of actin cytoskeleton. Cells transfected with 16D-HSP20 (phosphomimic) exhibited inhibition of acetylcholine (ACh)-induced contraction whereas cells transfected with 16A-HSP20 (nonphosphorylatable) had no effect on ACh-induced contraction. CSMC transfected with 16D-HSP20 cDNA showed significant decreases in 1) phosphorylation of HSP27 (ser78); 2) phosphorylation of PKC-α (ser657); 3) phosphorylation of TM and CaD (ser789); 4) ACh-induced phosphorylation of myosin light chain; 5) ACh-induced association of TM with HSP27; and 6) ACh-induced dissociation of TM from caldesmon (CaD). We thus propose the crucial physiological relevance of molecular signaling switch (phosphorylation state of HSP27 and HSP20), which dictates 1) the phosphorylation states of TM and CaD and 2) their dissociations from each other.

Physiopathology of splanchnic vasodilation in portal hypertension

In liver cirrhosis, the circulatory hemodynamic alterations of portal hypertension significantly contribute to many of the clinical manifestations of the disease. In the physiopathology of this vascular alteration, mesenteric splanchnic vasodilation plays an essential role by initiating the hemodynamic process. Numerous studies performed in cirrhotic patients and animal models have shown that this splanchnic vasodilation is the result of an important increase in local and systemic vasodilators and the presence of a splanchnic vascular hyporesponsiveness to vasoconstrictors. Among the molecules and factors known to be potentially involved in this arterial vasodilation, nitric oxide seems to have a crucial role in the physiopathology of this vascular alteration. However, none of the wide variety of mediators can be described as solely responsible, since this phenomenon is multifactorial in origin. Moreover, angiogenesis and vascular remodeling processes also seem to play a role. Finally, the sympathetic nervous system is thought to be involved in the pathogenesis of the hyperdynamic circulation associated with portal hypertension, although the nature and extent of its role is not completely understood. In this review, we discuss the different mechanisms known to contribute to this complex phenomenon.

Role of thin-filament regulatory proteins in relaxation of colonic smooth muscle contraction

Coordinated regulation of smooth muscle contraction and relaxation is required for colonic motility. Contraction is associated with phosphorylation of myosin light chain (MLC(20)) and interaction of actin with myosin. Thin-filament regulation of actomyosin interaction is modulated by two actin-binding regulatory proteins: tropomyosin (TM) and caldesmon (CaD). TM and CaD are known to play crucial role in actomyosin interaction promoting contraction. Contraction is associated with phosphorylation of the small heat shock protein HSP27, concomitant with the phosphorylation of TM and CaD. Phosphorylation of HSP27 is attributed as being the prime modulator of thin-filament regulation of contraction. Preincubation of colonic smooth muscle cells (CSMC) with the relaxant neurotransmitter vasoactive intestinal peptide (VIP) showed inhibition in phosphorylation of HSP27 (ser78). Attenuation of HSP27 phosphorylation can result in modulation of thin-filament-mediated regulation of contraction leading to relaxation; thus the role of thin-filament regulatory proteins in a relaxation milieu was investigated. Preincubation of CSMC with VIP exhibited a decrease in phosphorylation of TM and CaD. Furthermore, CSMC preincubated with VIP showed a reduced association of TM with HSP27 and with phospho-HSP27 (ser78) whereas there was reduced dissociation of TM from CaD and from phospho-CaD. We thus propose that, in addition to alteration in phosphorylation of MLC(20), relaxation is associated with alterations in thin-filament-mediated regulation that results in termination of contraction.

Calcium-independent phospholipase A2 participates in KCl-induced calcium sensitization of vascular smooth muscle

In vascular smooth muscle, KCl not only elevates intracellular free Ca(2+) ([Ca(2+)](i)), myosin light chain kinase activity and tension (T), but also can inhibit myosin light chain phosphatase activity by activation of rhoA kinase (ROCK), resulting in Ca(2+) sensitization (increased T/[Ca(2+)](i) ratio). Precisely how KCl causes ROCK-dependent Ca(2+) sensitization remains to be determined. Using Fura-2-loaded isometric rings of rabbit artery, we found that the Ca(2+)-independent phospholipase A(2) (iPLA(2)) inhibitor, bromoenol lactone (BEL), reduced the KCl-induced tonic but not early phasic phase of T and potentiated [Ca(2+)](i), reducing Ca(2+) sensitization. The PKC inhibitor, GF-109203X (> or =3 microM) and the pseudo-substrate inhibitor of PKCzeta produced a response similar to BEL. BEL reduced basal and KCl-stimulated myosin phosphatase phosphorylation. Whereas BEL and H-1152 produced strong inhibition of KCl-induced tonic T (approximately 50%), H-1152 did not induce additional inhibition of tissues already inhibited by BEL, suggesting that iPLA(2) links KCl stimulation with ROCK activation. The cPLA(2) inhibitor, pyrrolidine-1, inhibited KCl-induced tonic increases in [Ca(2+)](i) but not T, whereas the inhibitor of 20-HETE production, HET0016, acted like the ROCK inhibitor H-1152 by causing Ca(2+) desensitization. These data support a model in which iPLA(2) activity regulates Ca(2+) sensitivity.

Comparative study of calcium ion dynamics and contractile response in rat middle cerebral and basilar arteries

The objective of this study was to compare intracellular calcium concentration ([Ca(2+)](i)) and contractile responses in isolated rat middle cerebral artery (MCA) with those in basilar artery (BA) employing real-time confocal laser microscopy. KCl elicited transient [Ca(2+)](i) elevation and sustained contraction in both arteries; moreover, nearly equal responses were evident in both arteries. Application of 5-hydroxytryptamine (5-HT), vasopressin (VP), and alpha,beta-methylene adenosine 5'-triphosphate (alpha,beta-me ATP) also induced elevation of [Ca(2+)](i) and contraction in both arteries. The maximum response of 5-HT and VP necessary to increase [Ca(2+)](i) and to constrict the MCA was less in comparison to the BA; however, a linear relationship emerged between the maximum response of [Ca(2+)](i) and that of contraction. Additionally, the slope of the correlation regression line of MCA was nearly identical to that of BA. On the other hand, cyclopiazonic acid (CPA)-induced Ca(2+) release from store sites following contraction of MCA was distinct from that of BA. In MCA, velocity of [Ca(2+)](i) elevation in smooth muscle cells and Ca(2+)-wave propagation along smooth muscle cells induced by 5-HT were slower than those in BA. These observations revealed that different regions of arteries along the same cerebral tissue may display distinct [Ca(2+)](i) response; moreover, this difference may be one reason for the distinct contractile response.

Deletion of SM-B, the high ATPase isoform of myosin, upregulates the PKC-mediated signal transduction pathway in murine urinary bladder smooth muscle

Detrusor smooth muscle (DSM) hypertrophy induced by partial bladder outlet obstruction (PBOO) is associated with changes in the NH2-terminal myosin heavy chain isoform from predominantly SM-B to SM-A, alteration in the Ca2+ sensitization pathway, and the contractile characteristics from phasic to tonic in rabbits. We utilized the SM-B knockout (KO) mouse to determine whether a shift from SM-B to SM-A without PBOO is associated with changes in the signal transduction pathway mediated via PKC and CPI-17, which keeps the myosin phosphorylation (MLC20) level high by inhibiting the myosin phosphatase. DSM strips from SM-B KO mice generated more force in response to electrical field stimulation, KCl, carbachol, and phorbol 12,13-dibutyrate than that of age-matched wild-type mice. There was no difference in the ED50 for carbachol but the maximum response was greater for the SM-B KO mice. DSM from SM-B KO mice revealed increased mass and hypertrophy. The KO mice also showed an overexpression of PKC-alpha, increased levels of phospho-CPI-17, and an elevated level of IP3 and DAG upon stimulation with carbachol. Two-dimensional gel electrophoresis revealed an increased level of MLC20 phosphorylation in response to carbachol. Together, these changes may be responsible for the higher level of force generation and maintenance by the DSM from the SM-B KO bladders. In conclusion, our data show that ablation of SM-B is associated with alteration of PKC-mediated signal transduction and CPI-17-mediated Ca2+ sensitization pathway that regulate smooth muscle contraction. Interestingly, similar changes are also present in PBOO-induced DSM compensatory response in the rabbit model in which SM-B is downregulated.

Activation of adenosine 2A receptors preserves structure and function of podocytes

Adenosine 2A receptor (A(2A)R) activation was recently shown to be renoprotective in diabetic nephropathy. A(2A)R are found in glomeruli and have been shown to associate with the podocyte cytoskeletal protein alpha-actinin-4, but the effect of their activation on podocyte structure and function is unknown. Podocyte injury was induced in C57BL/6 mice with puromycin aminonucleoside, and the selective A(2A)R agonist ATL313 was found to attenuate the resulting albuminuria and foot process fusion. The selective A(2A)R antagonist ZM241385 reversed the effects of ATL313. In vitro, A(2A)R mRNA and protein were expressed in a conditionally immortalized podocyte cell line, and A(2A)R-like immunoreactivity co-localized with the actin cytoskeleton. Treatment with ATL313 also blocked the increased podocyte permeability to albumin and disruption of the actin cytoskeleton that accompanied puromycin aminonucleoside-induced injury in vitro. ATL313 was ineffective, however, in the presence of the A(2A)R antagonist and in A(2A)R-deficient podocytes. It was concluded that A(2A)R activation reduces glomerular proteinuria, at least in part, by preserving the normal structure of podocyte foot processes, slit diaphragms, and actin cytoskeleton.

The mechanism underlying the contractile effect of a chemotactic peptide, formyl-Met-Leu-Phe on the guinea-pig Taenia coli

1 The contractile mechanism of N-formyl-L-methionyl-L-leucyl-L-phenylalanine (fMLP) was investigated in the guinea-pig Taenia coli, by simultaneously monitoring the changes in the cytosolic Ca(2+) concentration ([Ca(2+)](i)) and force. 2 fMLP induced a significant elevation of [Ca(2+)](i) and force at concentrations higher than 10 nM. The maximal response was obtained at a concentration of higher than 1 microM. 3 fMLP (10 microM) augmented the force development induced by a stepwise increment of the extracellular Ca(2+) concentration during 60 mM K(+) depolarization, while it had no effect on the [Ca(2+)](i) elevation, and thus produced a greater force for a given elevation of [Ca(2+)](i) than 60 mM K(+) depolarization. 4 The removal of extracellular Ca(2+) completely abolished the fMLP-induced contraction. The fMLP-induced [Ca(2+)](i) elevation was inhibited substantially but not completely by 10 microM diltiazem, partly by 10 microM SK&F 96365, and completely by their combination. 5 Y27632, a specific inhibitor of rho-kinase, had no significant effect on the fMLP-induced [Ca(2+)](i) elevation and force development. 6 Chenodeoxycholic acid, a formyl peptide receptor antagonist, specifically abolished the fMLP-induced contraction but not high K(+)- or carbachol-induced contractions. 7 A dual lipoxygenase/cyclooxygenase inhibitor, a 5-lipoxygenase inhibitor, a nonselective leukotriene receptor antagonist, and a selective type 1 cysteinyl-containing leukotriene receptor antagonist specifically reduced the fMLP-induced contraction. 8 We suggest that the low-affinity-type fMLP receptor and lipoxygenase metabolites of arachidonic acid are involved in the fMLP-induced contraction in the guinea-pig T. coli. This contraction mainly depends on the [Ca(2+)](i) elevation due to Ca(2+) influx and the enhancement of Ca(2+) sensitivity in the contractile apparatus.

Heme oxygenase-2 products activate IKCa: role of CO and iron in guinea pig portal vein smooth muscle cells

Hemin (10 microM) and carbon monoxide (CO) increased iberiotoxin-blockable IKCa in portal vein smooth muscle cells. CO-induced IKCa activation was abolished by 10 microM ODQ, 10 microM cyclopiazonic acid and 1 microM KT5823. The hemin-induced effect on IKCa was abolished by pretreatment with Sn-protoporphyrin IX, a heme oxygenase inhibitor and Fe2+ chelator but was insensitive to inhibitors of soluble guanylate cyclase (GC) and cGMP-dependent protein kinase (PKG). There was no effect of hemin on IKCa in the presence of 3 microM dithiotreitol into the bath or 3 mM glutathione into the pipette solution. Superoxide dismutase (1000 U/ml) or catalase (3000 U/ml) added into the pipette solution also abolished the effect of hemin on IKCa in this tissue. Additionally, 10 microM hemin could not influence IKCa in Ca2+-free external solution or in the presence of 30 microM SKF 95356. It was concluded that CO increases IKCa via its "conventional" signaling pathway, which involves soluble GC and PKG activation and subsequent stimulation of sarcoplasmic reticulum Ca2+ pump activity resulting in Ca2+-dependent activation of IKCa due to the accumulation of Ca2+ into the space near the plasma membrane. On the other hand, internally produced CO could not yield the same IKCa increase, while Fe2+ derived from heme oxygenase 2-dependent degradation of hemin in portal vein smooth muscle cells gives rise to reactive oxygen species namely hydroxyl and superoxide radicals. Both radicals are responsible for the SKF 95356-sensitive non-selective cation channel activation, the Ca2+ influx and the subsequent increase of Ca2+ concentration near the plasma membrane that augments the KCa channel activity.

Telokin mediates Ca2+-desensitization through activation of myosin phosphatase in phasic and tonic smooth muscle

Telokin, a 17 kDa smooth muscle specific protein, consists of the C-terminal domain of MLCK, is phosphorylated by PKA and PKG at Ser13 in vivo (Wu et al. (1998) J Biol Chem 273: 11362-11369; Walker et al. (2001) J. Biol Chem 276: 24519-24524) and is proposed to induce Ca2+-desensitization through activation of myosin phosphatase (Wu et al. (1998) J. Biol Chem 273: 11362-11369). Telokin is reported to be highly expressed in phasic with only trace amounts in tonic smooth muscle. In alpha-toxin permeabilized femoral artery, 5 microM 8-Br-cGMP induced a two-fold increase in telokin phosphorylation and a maximal 30% relaxation of Ca2+-activated force compared to a 90% relaxation in phasic ileum muscle consistent with the relative amounts of telokin expressed in ileum, 27+/-4.6 microM SEM compared to 6+/-1.7 microM SEM, in femoral artery. Recombinant Wt telokin and the phospho-telokin mutant, S13D relaxed telokin-depleted femoral artery, by 38+/-8% SEM and 60+/-20% SEM, respectively. 8-Br-cGMP increased the rate and decreased the amplitude of force development initiated by photolysis of caged ATP in alpha-toxin permeabilized ileum and femoral artery smooth muscle, consistent with a cGMP-induced increase in phosphatase activity. Similarly, in telokin depleted ileum, recombinant S13D mutant telokin significantly increased the rate (0.08+/-0.01 s-1 vs. 014+/-0.02 s-1) and decreased force amplitude. In conclusion, our data support a role for telokin in cyclic nucleotide-induced relaxation of not only phasic, but also tonic smooth muscle and that this relaxation is mediated by activation of myosin phosphatase activity leading to a decrease in myosin light chain phosphorylation.

Cyclic AMP-mediated inhibition of gallbladder contractility: role of K+ channel activation and Ca2+ signaling

We have examined the mechanisms of cAMP-induced gallbladder relaxation by recording isometric tension and membrane potential in the intact tissue, and global intracellular calcium concentrations ([Ca(2+)](i)) and F-actin content in isolated myocytes. Both the phosphodiesterase (PDE) inhibitor, IBMX (100 microM) and the adenylate cyclase activator, forskolin (2 microM) caused decreases in basal tone that exhibited similar kinetics. IBMX and forskolin both caused concentration dependent, right-downward shifts in the concentration-response curves of KCl and cholecystokinin (CCK). IBMX and forskolin elicited a membrane hyperpolarization that was almost completely inhibited by the ATP-sensitive K(+) channel (K(ATP)) channel blocker, glibenclamide (10 microM). IBMX also induced an increase in large-conductance Ca(2+)-dependent K(+) (BK) channel currents, although the simultaneous blockade of BK and K(ATP) channels did not block IBMX- and forskolin-induced relaxations. Ca(2+) influx activated by L-type Ca(2+) channel activation or store depletion was also impaired by IBMX and forskolin, indicating a general impairment in Ca(2+) entry mechanisms. IBMX also decreases [Ca(2+)](i) transients activated by CCK and 3,6-Di-O-Bt-IP(4)-PM, a membrane permeable analog of inositol triphosphate, indicating an impairment in Ca(2+) release through IP(3) receptors. Ionomycin-induced [Ca(2+)](i) transients were not altered by IBMX, but the contractile effects of the Ca(2+) ionophore were reduced in the presence of IBMX, suggesting that cAMP can decrease Ca(2+) sensitivity of the contractile apparatus. A depolymerization of the thin filament could be reason for this change, as forskolin induced a decrease in F-actin content. In conclusion, these findings suggest that multiple, redundant intracellular processes are affected by cAMP to induce gallbladder relaxation.

Dominant role of smooth muscle L-type calcium channel Cav1.2 for blood pressure regulation

Blood pressure is regulated by a number of key molecules involving G-protein-coupled receptors, ion channels and monomeric small G-proteins. The relative contribution of these different signaling pathways to blood pressure regulation remains to be determined. Tamoxifen-induced, smooth muscle-specific inactivation of the L-type Cav1.2 Ca2+ channel gene in mice (SMAKO) reduced mean arterial blood pressure (MAP) in awake, freely moving animals from 120 +/- 4.5 to 87 +/- 8 mmHg. Phenylephrine (PE)- and angiotensin 2 (AT2)-induced MAP increases were blunted in SMAKO mice, whereas the Rho-kinase inhibitor Y-27632 reduced MAP to the same extent in control and SMAKO mice. Depolarization-induced contraction was abolished in tibialis arteries of SMAKO mice, and development of myogenic tone in response to intravascular pressure (Bayliss effect) was absent. Hind limb perfusion experiments suggested that 50% of the PE-induced resistance is due to calcium influx through the Cav1.2 channel. These results show that Cav1.2 calcium channels are key players in the hormonal regulation of blood pressure and development of myogenic tone.

Theophylline attenuates Ca2+ sensitivity and modulates BK channels in porcine tracheal smooth muscle

Theophylline, a nonselective phosphodiesterase inhibitor, has long been regarded as a major bronchodilator in the treatment of human asthma. Using front-surface fluorometry with fura-2 and alpha-toxin permeabilization, the effects of theophylline on intracellular Ca2+ concentration ([Ca2+]i), tension development and Ca2+ sensitivity of the contractile apparatus were investigated in porcine tracheal smooth muscle strips. Application of theophylline induced a relaxation without a significant decrease in [Ca2+]i when strips were precontracted by 40 mm K+ depolarization, while theophylline significantly decreased both [Ca2+]i and tension induced by carbachol. The effects of theophylline on the increases in [Ca2+]i and tension induced by carbachol were significantly inhibited by iberiotoxin, an inhibitor of large-conductance Ca2+-activated K+ channels. In the absence of extracellular Ca2+, theophylline significantly attenuated carbachol-induced transient increases in tension development, while it did not affect carbachol-induced transient increase in [Ca2+]i. The [Ca2+]i-force relationship, which was determined by cumulative applications of extracellular Ca2+ (0-5 mm) during 40 mm K+ depolarization, was significantly shifted to the right by theophylline. In alpha-toxin permeabilized strips, theophylline significantly increased the EC50 value of [Ca2+]i for contraction and enhanced the effect of cAMP, but not of cGMP. These results indicate that theophylline induces relaxation of the porcine tracheal smooth muscle through an activation of BK channels, and a resultant decrease in [Ca2+]i and an attenuation of Ca2+ sensitivity, presumably through the action of cAMP.

Protein kinase network in the regulation of phosphorylation and dephosphorylation of smooth muscle myosin light chain

The contraction of smooth muscle is regulated primarily by intracellular Ca2+ signal. It is well established that the elevation of the cytosolic Ca2+ level activates myosin light chain kinase, which phosphorylates 20 kDa regulatory myosin light chain and activates myosin ATPase. The simultaneous measurement of cytosolic Ca2+ concentration and force development revealed that the alteration of the Ca2+-sensitivity of the contractile apparatus as well as the Ca2+ signal plays a critical role in the regulation of smooth muscle contraction. The fluctuation of an extent of myosin phosphorylation for a given change in Ca2+ concentration is considered to contribute to the major mechanisms regulating the Ca2+-sensitivity. The level of myosin phosphorylation is determined by the balance between phosphorylation and dephosphorylation. The phosphorylation level for a given Ca2+ elevation is increased either by Ca2+-independent activation of phosphorylation process or inhibition of dephosphorylation. In the last decade, the isolation and cloning of myosin phosphatase facilitated the understanding of regulatory mechanism of dephosphorylation process at the molecular level. The inhibition of myosin phosphatase can be achieved by (1) alteration of hetrotrimeric structure, (2) phosphorylation of 110 kDa regulatory subunit MYPT1 at the specific site and (3) inhibitory protein CPI-17 upon its phosphorylation. Rho-kinase was first identified to phosphorylate MYPT1, and later many kinases were found to phosphorylate MYPT1 and inhibit dephosphorylation of myosin. Similarly, the phosphorylation of CPI-17 can be catalysed by multiple kinases. Moreover, the myosin light chain can be phosphorylated by not only authentic myosin light chain kinase in a Ca2+-dependent manner but also by multiple kinases in a Ca2+-independent manner, thus adding a novel mechanism to the regulation of the Ca2+-sensitivity by regulating the phosphorylation process. It is now clarified that the protein kinase network is involved in the regulation of myosin phosphorylation and dephosphorylation. However, the physiological role of each component remains to be determined. One approach to accomplish this purpose is to investigate the effects of the dominant negative mutants of the signalling molecule on the smooth muscle contraction. In this regards, a protein transduction technique utilizing the cell-penetrating peptides would provide a useful tool. In the preliminary study, we succeeded in introducing a fragment of MYPT1 into the arterial strips, and found enhancement of contraction.

Negative regulation of rho signaling by insulin and its impact on actin cytoskeleton organization in vascular smooth muscle cells: role of nitric oxide and cyclic guanosine monophosphate signaling pathways

Recent studies from our laboratory have shown that insulin induces relaxation of vascular smooth muscle cells (VSMCs) via stimulation of myosin phosphatase and inhibition of Rho kinase activity. In this study, we examined the mechanism whereby insulin inhibits Rho signaling and its impact on actin cytoskeleton organization. Incubation of confluent serum-starved VSMCs with thrombin or phenylephrine (PE) caused a rapid increase in glutathione S-transferase-Rhotekin-Rho binding domain-associated RhoA, Rho kinase activation, and actin cytoskeleton organization, which was blocked by preincubation with insulin. Preexposure to N(G)-monomethyl L-arginine acetate (L-NMMA), a nitric oxide synthase inhibitor, and Rp-8 CPT-cyclic guanosine monophosphate (RpcGMP), a cyclic guanosine monophosphate (cGMP) antagonist, attenuated the inhibitory effect of insulin on RhoA activation and restored thrombin-induced Rho kinase activation, and site-specific phosphorylation of the myosin-bound regulatory subunit (MBS(Thr695)) of myosin-bound phosphatase (MBP), and caused actin fiber reorganization. In contrast, 8-bromo-cGMP, a cGMP agonist, mimicked the inhibitory effects of insulin and abolished thrombin-mediated Rho activation. Insulin inactivation of RhoA was accompanied by inhibition of isoprenylation via reductions in geranylgeranyl transferase-1 activity as well as increased RhoA phosphorylation, which was reversed by pretreatment with RpcGMP and L-NMMA. We conclude that insulin may inhibit Rho signaling by affecting posttranslational modification of RhoA via nitric oxide/cGMP signaling pathway to cause MBP activation, actin cytoskeletal disorganization, and vasodilation.

Urocortin relaxes rat tail arteries by a PKA-mediated reduction of the sensitivity of the contractile apparatus for calcium

1. Urocortin is an endogenous vasodilator although the mechanism of vasorelaxation is not completely understood. The hypothesis that an alteration of smooth muscle calcium concentration is involved was tested using isometric tension recording and calcium fluorimetry. The relationship between contraction and intracellular calcium was also estimated. 2. Urocortin produced a concentration dependent relaxation (pD(2) 8.59+/-0.06, n=6) of vessels pre-contracted with a physiological salt solution containing 42 mM KCl (42 mM K-PSS). 3. Removal of the endothelium did not alter the effect of urocortin, pD(2) was 8.49+/-0.11, n=5. 4. Corticotropin-releasing factor relaxed 42 mM K-PSS pre-contracted vessels with less potency compared to urocortin (pD(2) 6.99+/-0.28, n=5). 5. Urocortin at 100 nM relaxed vessels pre-contracted with 42 mM K-PSS by 59.6+/-4.6% (n=8) and vessels pre-contracted with 500 nM noradrenaline by 25.2+/-6.8% (n=6). Both effects were not accompanied by a change in the intracellular calcium concentration. 6. Urocortin at 100 nM produced a significant rightward shift of 0.33+/-0.07 units of normalized intracellular calcium (n=5) of the relationship between tension and intracellular calcium. 7. The urocortin-induced relaxation was considerably reduced in the presence of 0.3 mM Rp-8-CPT-cAMPS, a cyclic AMP-dependent protein kinase (PKA) inhibitor. 8. The PKA-activator Sp-5,6-DCl-cBIMPS relaxed 42 mM K-PSS pre-contracted vessels (pD(2) 4.98+/-0.07, n=6). Sp-5,6-DCl-cBIMPS at 0.1 mM relaxed vessels by 85.3+/-2.5% (n=5), but did not change the intracellular calcium concentration. 9. In conclusion, the data show that urocortin is a potent, endothelium-independent dilator of rat tail arteries and suggest that this effect is mediated by PKA causing a reduction of the sensitivity of the contractile apparatus for calcium.

Noradrenaline-induced changes in intracellular Ca(2+) and tension in mesenteric arteries from diabetic rats

1. The purpose of this investigation was to determine whether enhanced contractile responses to noradrenaline (NA) of mesenteric arteries from rats with chronic streptozotocin-induced diabetes are associated with increases in mean cytosolic [Ca(2+)]i. 2. [Ca(2+)]i was measured with fura 2-AM, and was monitored simultaneously with tension in perfused endothelium-denuded mesenteric arterial rings from 12 - 14 week diabetic rats and age- and gender-matched control rats. 3. Basal [Ca(2+)]i (expressed as R(n), the normalized fura 2 ratio) was not significantly different in arteries from control and diabetic rats. Similarly, no differences between control and diabetic arteries in the tension or [Ca(2+)]i responses to 80 mM KCl in the presence of phentolamine were detected. 4. The rate of tension development, peak tension and integrated tension in response to 30 microM NA were all significantly greater in diabetic than control arteries. However, this was not associated with enhancement of the corresponding [Ca(2+)]i responses in the diabetic arteries. 5. Peak contractile responses to perfusion with both 0.3 and 3 microM NA, but peak [Ca(2+)]i only in response to 0.3 microM NA, were significantly greater in diabetic than control arteries. 6. NA (30 microM) produced a greater increase in both peak tension and [Ca(2+)]i in diabetic than control arteries perfused with Ca(2+)-free solution containing 1 mM EGTA. Neither the rate nor the magnitude of NA-induced Ca(2+) influx appeared to be altered in the diabetic arteries. 7. The enhanced sustained contractile response of diabetic arteries to NA appears to be dissociated from increases in [Ca(2+)]i, and may be due to other factors, such as an increase in the Ca(2+) sensitivity of the contractile proteins.

The mechanism for the contraction induced by leukotriene C4 in guinea-pig taenia coli

The mechanism underlying the LTC(4)-induced contraction of guinea-pig taenia coli was determined using the simultaneous measurements of [Ca(2+)](i) and force in whole muscle preparations. Additional experiments were performed in receptor coupled permeabilized preparation. For comparison purposes, the contraction which was induced by a typical G-protein mediated agonist, carbachol was also characterized. LTC(4) induced a contraction in the guinea-pig taenia coli in a concentration-dependent manner. The maximal response was obtained at 100 nM and the EC(50) value was 5.4+/-1.9 nM. Both LTC(4) and carbachol induced increases in [Ca(2+)](i) and force. The maximum force induced by 100 nM LTC(4) was significantly smaller than that induced by 10 microM carbachol, although an increase in [Ca(2+)](i) produced by both agonists was similar. In the permeabilized preparations, carbachol, but not LTC(4), induced an additional force development at a fixed Ca(2+) concentration. LTC(4) induced no increase in [Ca(2+)](i) and force in the Ca(2+)-free solution, while carbachol induced transient increases in both [Ca(2+)](i) and force in a Ca(2+)-free solution. Both diltiazem and SK&F 96365 significantly inhibited the LTC(4)- and carbachol-induced increases in [Ca(2+)](i) and force in normal PSS. The inhibitory pattern of [Ca(2+)](i) by these drugs was also similar. We thus conclude that LTC(4) induces the contraction of the guinea-pig taenia coli mainly through Ca(2+) influx via both the diltiazem-sensitive and SK&F 96365-sensitive Ca(2+) channels, without affecting either the Ca(2+)-sensitivity or the intracellular Ca(2+) release. These results indicated that the mechanism underlying the LTC(4)-induced contraction differs greatly from that for conventional G-protein mediated agonists, such as carbachol.

Rho-kinase--mediated contraction of isolated stress fibers

It is widely accepted that actin filaments and the conventional double-headed myosin interact to generate force for many types of nonmuscle cell motility, and that this interaction occurs when the myosin regulatory light chain (MLC) is phosphorylated by MLC kinase (MLCK) together with calmodulin and Ca(2+). However, recent studies indicate that Rho-kinase is also involved in regulating the smooth muscle and nonmuscle cell contractility. We have recently isolated reactivatable stress fibers from cultured cells and established them as a model system for actomyosin-based contraction in nonmuscle cells. Here, using isolated stress fibers, we show that Rho-kinase mediates MLC phosphorylation and their contraction in the absence of Ca(2+). More rapid and extensive stress fiber contraction was induced by MLCK than was by Rho-kinase. When the activity of Rho-kinase but not MLCK was inhibited, cells not only lost their stress fibers and focal adhesions but also appeared to lose cytoplasmic tension. Our study suggests that actomyosin-based nonmuscle contractility is regulated by two kinase systems: the Ca(2+)-dependent MLCK and the Rho-kinase systems. We propose that Ca(2+) is used to generate rapid contraction, whereas Rho-kinase plays a major role in maintaining sustained contraction in cells.

Mechanism of CGRP-induced relaxation in rat intramural coronary arteries

1. This study investigates the mechanism of CGRP-induced relaxation in intramural coronary arteries by determining the effect of CGRP on cytosolic Ca(2+) concentration ([Ca(2+)](i)) using FURA-2 technique. 2. CGRP concentration-dependently (10 pM - 100 nM) decreased the [Ca(2+)](i) and tension of coronary arteries precontracted with either U46619 or BAY K 8644, and also of resting coronary arteries in PSS. In 36 mM K(+)-depolarized arteries, CGRP reduced only the tension without affecting the [Ca(2+)](i). 3. In 300 nM U46619- precontracted arteries, pretreatment with 10 microM thapsigargin significantly (P<0.05) attenuated the CGRP-induced reduction in the tension (but not [Ca(2+)](i)). 4. In 300 nM U46619-precontracted arteries, pretreatment with either 100 nM charybdotoxin or 100 nM iberiotoxin or 10 nM felodipine significantly (P<0.05) attenuated the CGRP-induced reduction in both [Ca(2+)](i) and tension. In contrast, 1 microM glibenclamide did not affect the CGRP-induced responses in these coronary arteries. 5. In resting coronary arteries, only pretreatment with the combination of 1 microM glibenclamide and 100 nM charybdotoxin attenuated the CGRP-induced decrease in the [Ca(2+)](i) and tension, suggesting a different mechanism of action for CGRP in resting coronary arteries. 6. We conclude that CGRP relaxes precontracted rat coronary arteries via three mechanisms: (1) a decrease in [Ca(2+)](i) by inhibiting the Ca(2+) influx through membrane hyperpolarization mediated partly by activation of the large conductance Ca(2+)-activated potassium channels, (2) a decrease in [Ca(2+)](i) presumably by sequestrating cytosolic Ca(2+) into thapsigargin-sensitive Ca(2+) storage sites and (3) a decrease in the Ca(2+)-sensitivity of the contractile apparatus. In resting coronary arteries, however, there seems to be an interplay between different types of K(+) channels.

Early radiation effects on muscarinic receptor-induced secretory responsiveness of the parotid gland in the freely moving rat

Although the salivary glands have a low rate of cell turnover, they are relatively radiosensitive. To study the possible mechanism behind this inherent radiosensitivity, a rat model was developed in which saliva can be collected after local irradiation of the parotid gland without the use of anesthetics or stressful handling. Saliva secretion was induced by the partial muscarinic receptor agonist pilocarpine (0.03-3 mg/kg) with or without pretreatment with the beta-adrenoceptor antagonist propranolol (2.5 mg/kg), or the full muscarinic receptor agonist methacholine (0.16-16 mg/min), and measured during 5 min per drug dose before and 1, 3, 6 and 10 days after irradiation. The maximal secretory response induced by pilocarpine plus propranolol was increased compared to that with pilocarpine alone but did not reach the level of methacholine-induced secretion, which was about five times higher. One day after irradiation a decrease in maximal pilocarpine-induced secretion was observed (-22%) using the same dose of pilocarpine that induces 50% of the maximal response (ED(50)), in both the absence and presence of propranolol, indicating that the receptor-drug interaction was not affected by the radiation at this time. The secretory response to methacholine 1 day after irradiation, however, was normal. At day 3 after irradiation, the maximal methacholine-induced secretion was also affected, whereas pilocarpine (+/-propranolol)-induced maximal secretion decreased further. At day 6 after irradiation, maximal secretory responses had declined to approximately 50% regardless of the agonist used, whereas ED(50) values were still unaffected. No net acinar cell loss was observed within the first 10 days after irradiation, and this therefore could not account for the loss in function. The results indicate that radiation does not affect cell number or receptor-drug interaction, but rather signal transduction, which eventually leads to the impaired response. We hypothesize that the early radiation effect, within 3 days, may be membrane damage affecting the receptor-G-protein signaltransfer. Later critical damage, however, is probably of a different nature and may be located in the second-messenger signal transduction pathway downstream from the G protein, not necessarily involving cellular membranes.

Signal transduction by G-proteins, rho-kinase and protein phosphatase to smooth muscle and non-muscle myosin II

We here review mechanisms that can regulate the activity of myosin II, in smooth muscle and non-muscle cells, by modulating the Ca2+ sensitivity of myosin regulatory light chain (RLC) phosphorylation. The major mechanism of Ca2+ sensitization of smooth muscle contraction and non-muscle cell motility is through inhibition of the smooth muscle myosin phosphatase (MLCP) that dephosphorylates the RLC in smooth muscle and non-muscle. The active, GTP-bound form of the small GTPase RhoA activates a serine/threonine kinase, Rho-kinase, that phosphorylates the regulatory subunit of MLCP and inhibits phosphatase activity. G-protein-coupled release of arachidonic acid may also contribute to inhibition of MLCP acting, at least in part, through the Rho/Rho-kinase pathway. Protein kinase C(s) activated by phorbol esters and diacylglycerol can also inhibit MLCP by phosphorylating and thereby activating CPI-17, an inhibitor of its catalytic subunit; this mechanism is independent of the Rho/Rho-kinase pathway and plays only a minor, transient role in the G-protein-coupled mechanism of Ca2+ sensitization. Ca2+ sensitization by the Rho/Rho-kinase pathway contributes to the tonic phase of agonist-induced contraction in smooth muscle, and abnormally increased activation of myosin II by this mechanism is thought to play a role in diseases such as high blood pressure and cancer cell metastasis.

From pharmacomechanical coupling to G-proteins and myosin phosphatase

A brief summary of recent studies of pharmacomechanical coupling is presented, with emphasis on the role of GTP-binding proteins and Ca(2+)-independent regulation of contraction (Ca(2+)-sensitization/desensitization) through regulatory myosin light chain (MLC20) phosphorylation and dephosphorylation. Pharmacomechanical regulation of cytosolic [Ca2+] is largely, though not solely, controlled by the phosphatidylinositol cascade and Ca(2+)-pumps of the plasma membrane and the sarcoplasmic reticulum. The monomeric GTPase, RhoA, is a major upstream component of Ca(2+)-sensitization. Its crystal structure and apparently obligatory translocation to the plasma membrane for activation of its downstream effectors are described. Inhibition of RhoA activity by a membrane-permeant ADP-ribosylating bacterial exoenzyme, DC3B, causes severe depression of the tonic component of agonist-induced contraction, suggesting that this component is largely due to Ca(2+)-sensitization. A relatively specific inhibitor (Y27632) of Rho-kinase, a downstream effector of Ca(2+)-sensitization (Uehata et al 1997), also inhibits oxytoxin-induced Ca(2+)-sensitization of myometrium. The major mechanism of physiological, G-protein-coupled Ca(2+)-sensitization is through inhibition of smooth muscle myosin phosphatase (SMPP-1M), whereas conventional or novel protein kinase Cs play very little or no role in this process. Mechanisms of Ca(2+)-desensitization include inhibition of myosin light chain kinase and activation of SMPP-1M. Activation of SMPP-1M in phasic smooth muscle can be attributed, at least in part, to the synergistic phosphatase activating activities of a cyclic nucleotide-dependent kinase and its major substrate, telokin.