Serotonin passes through myoendothelial gap junctions to promote pulmonary arterial smooth muscle cell differentiation

Exploring the Role of Serotonin as an Immune Modulatory Component in Cardiovascular Diseases

Serotonin, also known as 5-hydroxytryptamine (5-HT) is a well-known neurotransmitter in the central nervous system (CNS), but also plays a significant role in peripheral tissues. There is a growing body of evidence suggesting that serotonin influences immune cell responses and contributes to the development of pathological injury in cardiovascular diseases, such as atherosclerosis, as well as other diseases which occur as a result of immune hyperactivity. In particular, high levels of serotonin are able to activate a multitude of 5-HT receptors found on the surface of immune cells, thereby influencing the process of atherosclerotic plaque formation in arteries. In this review, we will discuss the differences between serotonin production in the CNS and the periphery, and will give a brief outline of the function of serotonin in the periphery. In this context, we will particularly focus on the effects of serotonin on immune cells related to atherosclerosis and identify caveats that are important for future research.

PVECs-Derived Exosomal microRNAs Regulate PASMCs via FoxM1 Signaling in IUGR-induced Pulmonary Hypertension

Background Intrauterine growth restriction (IUGR) is closely related to systemic or pulmonary hypertension (PH) in adulthood. Aberrant crosstalk between pulmonary vascular endothelial cells (PVECs) and pulmonary arterial smooth muscle cells (PASMCs) that is mediated by exosomes plays an essential role in the progression of PH. FoxM1 (Forkhead box M1) is a key transcription factor that governs many important biological processes. Methods and Results IUGR-induced PH rat models were established. Transwell plates were used to coculture PVECs and PASMCs. Exosomes were isolated from PVEC-derived medium, and a microRNA (miRNA) screening was proceeded to identify effects of IUGR on small RNAs enclosed within exosomes. Dual-Luciferase assay was performed to validate the predicted binding sites of miRNAs on FoxM1 3' untranslated region. FoxM1 inhibitor thiostrepton was used in IUGR-induced PH rats. In this study, we found that FoxM1 expression was remarkably increased in IUGR-induced PH, and PASMCs were regulated by PVECs through FoxM1 signaling in a non-contact way. An miRNA screening showed that miR-214-3p, miR-326-3p, and miR-125b-2-3p were downregulated in PVEC-derived exosomes of the IUGR group, which were associated with overexpression of FoxM1 and more significant proliferation and migration of PASMCs. Dual-Luciferase assay demonstrated that the 3 miRNAs directly targeted FoxM1 3' untranslated region. FoxM1 inhibition blocked the PVECs-PASMCs crosstalk and reversed the abnormal functions of PASMCs. In vivo, treatment with thiostrepton significantly reduced the severity of PH. Conclusions Transmission of exosomal miRNAs from PVECs regulated the functions of PASMCs via FoxM1 signaling, and FoxM1 may serve as a potential therapeutic target in IUGR-induced PH.

Investigation of Genes and Proteins Expression Associating Serotonin Signaling Pathway in Lung and Pulmonary Artery Tissues of Dogs with Pulmonary Hypertension Secondary to Degenerative Mitral Valve Disease: The Preliminary Study

Simple Summary

Pulmonary hypertension is an unusual elevation of the blood pressure in pulmonary circulation. In dogs, pulmonary hypertension is commonly occurred as a complication of progressive degenerative mitral valve disease especially in senior small-breed dogs. Previous studies revealed that the serotonin signaling pathway is associated with the pathogenesis of pulmonary hypertension. However, research in dogs was scarce. Therefore, the present study aimed to illustrate the understanding of this point by assessing the expression of the targeted genes and proteins related to the serotonin pathway in lung tissues and pulmonary arteries of dogs. Our results showed that the pattern of gene and protein expression was different in canine lung and pulmonary arteries. The targeted proteins in pulmonary artery tissues of the degenerative mitral valve disease dogs with and without pulmonary hypertension tended to be upregulated. In addition, the expression of these protein was increased in the pneumocytes of the alveolar walls, pulmonary alveolar macrophages, and bronchial epithelial cells. A correlation between the targeted gene and protein expression and the echocardiographic data was also founded. Overall results pointed out that the serotonin pathway in lung and pulmonary artery tissues might have different roles in degenerative mitral disease with and without pulmonary hypertension.

Abstract

Pulmonary hypertension (PH) is defined as an increase in pulmonary vascular pressure. It is one of the most common complications that occur as a result of degenerative mitral valve disease (DMVD) in dogs. Serotonin (5-HT) can trigger the development of PH. Accordingly, this study investigated the changes in the expression of genes and proteins associated with local 5-HT signaling in the lungs and pulmonary arteries (PA) of dogs with PH secondary to DMVD. Lung and PA tissue samples were collected from the cadavers of fourteen small-breed dogs and divided into normal (n = 4), DMVD (n = 5) and DMVD with PH (n = 5) groups. Gene expression (tph1, slc6a4 and htr2a) was analyzed by quantitative reverse transcription-polymerase chain reaction (qRT-PCR). The expression of proteins (TPH-1, SERT, 5-HTR2A, ERK1/2 and pERK1/2) was examined by Western blot analysis and immunohistochemical staining. The results showed that the expression of genes and proteins evaluated by qRT-PCR and Western blot analysis in lung and PA tissues did not differ among groups. However, the expression of proteins related to 5-HT signaling tended to be upregulated in PA tissues from DMVD dogs with and without PH. Immunohistochemical examination revealed the overexpression of these proteins in the DMVD and DMVD with PH groups in lung tissue. These findings suggest a local effect of 5-HT signaling in DMVD dogs with and without PH.

Connexins may play a critical role in cigarette smoke-induced pulmonary hypertension

Pulmonary hypertension (PH) is a chronic progressive disease characterized by pulmonary vasoconstriction and remodeling. It causes a gradual increase in pulmonary vascular resistance leading to right-sided heart failure, and may be fatal. Chronic exposure to cigarette smoke (CS) is an essential risk factor for PH group 3; however, smoking continues to be prevalent and smoking cessation is reported to be difficult. A majority of smokers exhibit PH, which leads to a concomitant increase in the risk of mortality. The current treatments for PH group 3 focus on vasodilation and long-term oxygen supplementation, and fail to stop or reverse PH-associated continuous vascular remodeling. Recent studies have suggested that pulmonary vascular endothelial dysfunction induced by CS exposure may be an initial event in the natural history of PH, which in turn may be associated with abnormal alterations in connexin (Cx) expression. The relationship between Cx and CS-induced PH development has not yet been directly investigated. Therefore, this review will describe the roles of CS and Cx in the development of PH and discuss the related downstream pathways. We also discuss the possible role of Cx in CS-induced PH. It is hoped that this review may provide new perspectives for early intervention.

New Insights into Pulmonary Hypertension: A Role for Connexin-Mediated Signalling

Pulmonary hypertension is a serious clinical condition characterised by increased pulmonary arterial pressure. This can lead to right ventricular failure which can be fatal. Connexins are gap junction-forming membrane proteins which serve to exchange small molecules of less than 1 kD between cells. Connexins can also form hemi-channels connecting the intracellular and extracellular environments. Hemi-channels can mediate adenosine triphosphate release and are involved in autocrine and paracrine signalling. Recently, our group and others have identified evidence that connexin-mediated signalling may be involved in the pathogenesis of pulmonary hypertension. In this review, we discuss the evidence that dysregulated connexin-mediated signalling is associated with pulmonary hypertension.

Cellular mechanosignaling in pulmonary arterial hypertension

Pulmonary arterial hypertension (PAH) is a vasculopathy characterized by sustained elevated pulmonary arterial pressures in which the pulmonary vasculature undergoes significant structural and functional remodeling. To better understand disease mechanisms, in this review article we highlight recent insights into the regulation of pulmonary arterial cells by mechanical cues associated with PAH. Specifically, the mechanobiology of pulmonary arterial endothelial cells (PAECs), smooth muscle cells (PASMCs) and adventitial fibroblasts (PAAFs) has been investigated in vivo, in vitro, and in silico. Increased pulmonary arterial pressure increases vessel wall stress and strain and endothelial fluid shear stress. These mechanical cues promote vasoconstriction and fibrosis that contribute further to hypertension and alter the mechanical milieu and regulation of pulmonary arterial cells.

Morphology changes induced by intercellular gap junction blocking: A reaction-diffusion mechanism

Complex anatomical form is regulated in part by endogenous physiological communication between cells; however, the dynamics by which gap junctional (GJ) states across tissues regulate morphology are still poorly understood. We employed a biophysical modeling approach combining different signaling molecules (morphogens) to qualitatively describe the anteroposterior and lateral morphology changes in model multicellular systems due to intercellular GJ blockade. The model is based on two assumptions for blocking-induced patterning: (i) the local concentrations of two small antagonistic morphogens diffusing through the GJs along the axial direction, together with that of an independent, uncoupled morphogen concentration along an orthogonal direction, constitute the instructive patterns that modulate the morphological outcomes, and (ii) the addition of an external agent partially blocks the intercellular GJs between neighboring cells and modifies thus the establishment of these patterns. As an illustrative example, we study how the different connectivity and morphogen patterns obtained in presence of a GJ blocker can give rise to novel head morphologies in regenerating planaria. We note that the ability of GJs to regulate the permeability of morphogens post-translationally suggests a mechanism by which different anatomies can be produced from the same genome without the modification of gene-regulatory networks. Conceptually, our model biosystem constitutes a reaction-diffusion information processing mechanism that allows reprogramming of biological morphologies through the external manipulation of the intercellular GJs and the resulting changes in instructive biochemical signals.

Assessment of Platelet and Plasma Serotonin in Canine Pulmonary Hypertension Secondary to Degenerative Mitral Valve Disease

Background: Pulmonary hypertension (PH) is a common complication of degenerative mitral valve disease (DMVD), the most common cardiovascular disease in dogs. Serotonin has been suspected to play a role in the pathogenesis of PH, so this study aimed to investigate the differences in platelet and plasma serotonin between normal, DMVD and DMVD with PH (DMVD+PH) dogs. Materials and Methods: Sixty-two small-breed dogs were enrolled to the study and divided into the normal (n = 22), DMVD (n = 20), and DMVD+PH (n = 20) groups. The platelet and plasma serotonin concentrations were measured by the competitive ELISA. Results: The Kruskal-Wallis revealed the difference among the four groups of normal (179.73 [102.37-352.24] ng/10⁹ platelets), DMVD (325.99 [96.84-407.66] ng/10⁹ platelets), DMVD with intermediate probability of PH (291.11 [106.69-400.84] ng/10⁹ platelets) and DMVD with high probability of PH (35.82 [2.69-126.35] ng/10⁹ platelets) (p = 0.014). The Dunn's post-hoc test showed a decrease in the platelet serotonin concentration of the DMVD dogs with high probability of PH compared to the DMVD group (p = 0.008). The plasma serotonin concentration was not different between normal, DMVD, and DMVD+PH dogs. Conclusion: In conclusion, a decrease in platelet serotonin concentration, which is associated with a degree of PH probability was found in DMVD dogs with PH. Further studies investigating roles of platelet serotonin in PH secondary to DMVD should be performed.

Role of crosstalk between endothelial cells and smooth muscle cells in vascular calcification in chronic kidney disease

Chronic kidney disease (CKD) is a severe health problem worldwide, and vascular calcification (VC) contributes substantially to the cardiovascular morbidity and high mortality of CKD. CKD is often accompanied by a variety of pathophysiological states, such as inflammation, oxidative stress, hyperglycaemia, hyperparathyroidism and haemodynamic derangement, that can cause injuries to smooth muscle cells (SMCs) and endothelial cells (ECs) to promote VC. Similar to SMCs, whose role has been widely explored in VC, ECs may contribute to VC via osteochondral transdifferentiation, apoptosis, etc. In addition, given their location in the innermost layer of the blood vessel lumen and preferential reception of various pro‐calcification stimuli, ECs can pass messages to vascular wall cells and communicate with them. Crosstalk between ECs and SMCs via cytokines through a paracrine mechanism, extracellular vesicles, miRNAs and myoendothelial gap junctions also plays a role in VC. In this review, we emphasize the role of intercellular crosstalk between ECs and SMCs in VC associated with CKD.

The Expression of Proteins Related to Serotonin Pathway in Pulmonary Arteries of Dogs Affected With Pulmonary Hypertension Secondary to Degenerative Mitral Valve Disease

Background: Pulmonary hypertension (PH) can cause medial thickening, a hallmark of pulmonary arterial remodeling. The serotonin (5HT) pathway has been suggested as a factor associated with PH by inducing pulmonary arterial smooth muscle cells (SMCs) proliferation, a major cause of medial thickening. This study aims to demonstrate the expression of molecules in the 5HT pathway in the pulmonary artery of dogs affected with PH secondary to degenerative mitral valve disease (DMVD) compared to DMVD and healthy control dogs. Materials and Methods: The study included lung samples from the carcasses of 19 older small-breed dogs (Control n = 5, DMVD n = 7, DMVD+PH n = 7). Lung tissue sections were performed Hematoxylin and Eosin staining for measuring the percentage of medial thickness and immunohistochemistry for evaluating the expression of proteins in the 5HT pathway including serotonin transporter (SERT), serotonin 2A receptor (5HT2A), tryptophan hydroxylase 1 (TPH1), extracellular regulated kinase 1/2 (ERK1/2), and phosphorylated ERK1/2 (pERK1/2). Results: Medial thickening of the pulmonary arteries was found in the DMVD and DMVD+PH groups compared to the control. The medial thickening of the DMVD+PH group was increased significantly compared to that in the DMVD group. Intracytoplasmic expression of proteins related to the 5HT pathway was mainly presented in the medial layer of the pulmonary arteries. The control group showed a low expression of proteins related to the 5HT pathway. An intensive expression of SERT, 5HT2A, TPH1, and ERK1/2 protein was seen in the DMVD and DMVD+PH groups. Interestingly, pERK1/2 was strongly represented only in the DMVD+PH group. Conclusions: Overexpression of proteins related to the 5HT pathway including SERT, 5HT2A, TPH1, ERK1/2, and pERK1/2 was associated with medial remodeling in dogs affected with secondary to DMVD.

Metabolism of vascular smooth muscle cells in vascular diseases

Vascular smooth muscle cells (VSMCs) are the fundamental component of the medial layer of arteries and are essential for arterial physiology and pathology. It is becoming increasingly clear that VSMCs can alter their metabolism to fulfill the bioenergetic and biosynthetic requirements. During vascular injury, VSMCs switch from a quiescent "contractile" phenotype to a highly migratory and proliferative "synthetic" phenotype. Recent studies have found that the phenotype switching of VSMCs is driven by a metabolic switch. Metabolic pathways, including aerobic glycolysis, fatty acid oxidation, and amino acid metabolism, have distinct, indispensable roles in normal and dysfunctional vasculature. VSMCs metabolism is also related to the metabolism of endothelial cells. In the present review, we present a brief overview of VSMCs metabolism and how it regulates the progression of several vascular diseases, including atherosclerosis, systemic hypertension, diabetes, pulmonary hypertension, vascular calcification, and aneurysms, and the effect of the risk factors for vascular disease (aging, cigarette smoking, and excessive alcohol drinking) on VSMC metabolism to clarify the role of VSMCs metabolism in the key pathological process.

Connexin 43 plays a role in proliferation and migration of pulmonary arterial fibroblasts in response to hypoxia

Pulmonary hypertension (PH) is a disease associated with vasoconstriction and remodelling of the pulmonary vasculature. Pulmonary artery fibroblasts (PAFs) play an important role in hypoxic-induced remodelling. Connexin 43 (Cx43) is involved in cellular communication and regulation of the pulmonary vasculature. Using both in vitro and in vivo models of PH, the aims of this study were to (i) investigate the role of Cx43 in hypoxic-induced proliferation and migration of rat PAFs (rPAFs) and rat pulmonary artery smooth muscle cells (rPASMCs) and (ii) determine whether Cx43 expression is dysregulated in the rat sugen5416/hypoxic model of PH. The role of Cx43 in hypoxic-induced proliferation and migration was investigated using Gap27 (a pharmacological inhibitor of Cx43) or genetic knockdown of Cx43 using siRNA. Cx43 protein expression was increased by hypoxia in rPAFs but not rPASMCs. Hypoxic exposure, in the presence of serum, resulted in an increase in proliferation of rPAFs but not rPASMCs. Hypoxic exposure caused migration of rPAFs but not rPASMCs. Phosphorylation of p38 mitogen-activated protein kinase (MAPK) and ERK1/2 were increased by hypoxia in rPAFs. The effects of hypoxia on proliferation, migration and MAPK phosphorylation in rPAFs were attenuated in the presence of Gap27 or Cx43 siRNA. Cx43 protein expression was increased in sugen5416/hypoxic rat lung; this increased expression was not observed in sugen5416/hypoxic rats treated with the MAPK pathway inhibitor GS-444217. In conclusion, Cx43 is involved in the proliferation and migration of rPAFs in response to hypoxia via the MAPK signalling pathway.

Connexins: Key Players in the Control of Vascular Plasticity and Function

Of the 21 members of the connexin family, 4 (Cx37, Cx40, Cx43, and Cx45) are expressed in the endothelium and/or smooth muscle of intact blood vessels to a variable and dynamically regulated degree. Full-length connexins oligomerize and form channel structures connecting the cytosol of adjacent cells (gap junctions) or the cytosol with the extracellular space (hemichannels). The different connexins vary mainly with regard to length and sequence of their cytosolic COOH-terminal tails. These COOH-terminal parts, which in the case of Cx43 are also translated as independent short isoforms, are involved in various cellular signaling cascades and regulate cell functions. This review focuses on channel-dependent and -independent effects of connexins in vascular cells. Channels play an essential role in coordinating and synchronizing endothelial and smooth muscle activity and in their interplay, in the control of vasomotor actions of blood vessels including endothelial cell reactivity to agonist stimulation, nitric oxide-dependent dilation, and endothelial-derived hyperpolarizing factor-type responses. Further channel-dependent and -independent roles of connexins in blood vessel function range from basic processes of vascular remodeling and angiogenesis to vascular permeability and interactions with leukocytes with the vessel wall. Together, these connexin functions constitute an often underestimated basis for the enormous plasticity of vascular morphology and function enabling the required dynamic adaptation of the vascular system to varying tissue demands.

Smooth Muscle Contact Drives Endothelial Regeneration by BMPR2-Notch1-Mediated Metabolic and Epigenetic Changes

RATIONALE

Maintaining endothelial cells (EC) as a monolayer in the vessel wall depends on their metabolic state and gene expression profile, features influenced by contact with neighboring cells such as pericytes and smooth muscle cells (SMC). Failure to regenerate a normal EC monolayer in response to injury can result in occlusive neointima formation in diseases such as atherosclerosis and pulmonary arterial hypertension.

OBJECTIVE

We investigated the nature and functional importance of contact-dependent communication between SMC and EC to maintain EC integrity.

METHODS AND RESULTS

We found that in SMC and EC contact cocultures, BMPR2 (bone morphogenetic protein receptor 2) is required by both cell types to produce collagen IV to activate ILK (integrin-linked kinase). This enzyme directs p-JNK (phospho-c-Jun N-terminal kinase) to the EC membrane, where it stabilizes presenilin1 and releases N1ICD (Notch1 intracellular domain) to promote EC proliferation. This response is necessary for EC regeneration after carotid artery injury. It is deficient in EC-SMC Bmpr2 double heterozygous mice in association with reduced collagen IV production, decreased N1ICD, and attenuated EC proliferation, but can be rescued by targeting N1ICD to EC. Deletion of EC- Notch1 in transgenic mice worsens hypoxia-induced pulmonary hypertension, in association with impaired EC regenerative function associated with loss of precapillary arteries. We further determined that N1ICD maintains EC proliferative capacity by increasing mitochondrial mass and by inducing the phosphofructokinase PFKFB3 (fructose-2,6-bisphosphatase 3). Chromatin immunoprecipitation sequencing analyses showed that PFKFB3 is required for citrate-dependent H3K27 acetylation at enhancer sites of genes regulated by the acetyl transferase p300 and by N1ICD or the N1ICD target MYC and necessary for EC proliferation and homeostasis.

CONCLUSIONS

Thus, SMC-EC contact is required for activation of Notch1 by BMPR2, to coordinate metabolism with chromatin remodeling of genes that enable EC regeneration, and to maintain monolayer integrity and vascular homeostasis in response to injury.

Inhibition of serotonin synthesis: A novel therapeutic paradigm

The rate-limiting enzyme in serotonin synthesis is tryptophan hydroxylase (TPH). There are two independent serotonin systems in the body characterized by two isoforms of TPH, TPH1 and TPH2. While TPH2 synthesizes serotonin in the brain, TPH1 is expressed in the gut and in other peripheral tissues and supplies platelets in the circulation with serotonin. This duality of the serotonin system is enforced by the blood-brain barrier which is impermeable for serotonin. In the brain serotonin acts as neurotransmitter and is a main target for the treatment of psychiatric disorders. In the periphery it is released by platelets at the site of activation and elicits numerous physiological effects. TPH1 deficient mice were shown to be protected from diverse diseases including hemostatic, inflammatory, fibrotic, gastrointestinal, and metabolic disorders and therefore serotonin synthesis inhibition emerged as a reasonable therapeutic paradigm. Recently the first TPH inhibitor, telotristat ethyl, came on the market for the treatment of carcinoid syndrome. This review summarizes the state of development and the therapeutic opportunities of such compounds.

Selective Serotonin Reuptake Inhibitor Use During Pregnancy and Major Malformations: The Importance of Serotonin for Embryonic Development and the Effect of Serotonin Inhibition on the Occurrence of Malformations

Bioelectric signaling is transduced by neurotransmitter pathways in many cell types. One of the key mediators of bioelectric control mechanisms is serotonin, and its transporter SERT, which is targeted by a broad class of blocker drugs (selective serotonin reuptake inhibitors [SSRIs]). Studies showing an increased risk of multiple malformations associated with gestational use of SSRI have been accumulating but debate remains on whether SSRI as a class has the potential to generate these malformations. This review highlights the importance of serotonin for embryonic development; the effect of serotonin inhibition during early pregnancy on the occurrence of multiple diverse malformations that have been shown to occur in human pregnancies; that the risks outweigh the benefits of SSRI use during gestation in populations of mild to moderately depressed pregnant women, which encompass the majority of pregnant depressed women; and that the malformations seen in human pregnancies constitute a pattern of malformations consistent with the known mechanisms of action of SSRIs. We present at least three mechanisms by which SSRI can affect development. These studies highlight the relevance of basic bioelectric and neurotransmitter mechanism for biomedicine.

Connexins in the control of vasomotor function

Vascular endothelial cells, as well as smooth muscle cells, show heterogeneity with regard to their receptor expression and reactivity. For the vascular wall to act as a functional unit, the various cells' responses require integration. Such an integration is not only required for a homogeneous response of the vascular wall, but also for the vasomotor behaviour of consecutive segments of the microvascular arteriolar tree. As flow resistances of individual sections are connected in series, sections require synchronization and coordination to allow effective changes of conductivity and blood flow. A prerequisite for the local coordination of individual vascular cells and different sections of an arteriolar tree is intercellular communication. Connexins are involved in a dual manner in this coordination. (i) By forming gap junctions between cells, they allow an intercellular exchange of signalling molecules and electrical currents. In particular, the spread of electrical currents allows for coordination of cell responses over longer distances. (ii) Connexins are able to interact with other proteins to form signalling complexes. In this way, they can modulate and integrate individual cells' responses also in a channel-independent manner. This review outlines mechanisms allowing the vascular connexins to exert their coordinating function and to regulate the vasomotor reactions of blood vessels both locally, and in vascular networks. Wherever possible, we focus on the vasomotor behaviour of small vessels and arterioles which are the main vessels determining vascular resistance, blood pressure and local blood flow.

Connexin 43 Plays a Role in Pulmonary Vascular Reactivity in Mice

Pulmonary arterial hypertension (PAH) is a chronic condition characterized by vascular remodeling and increased vaso-reactivity. PAH is more common in females than in males (~3:1). Connexin (Cx)43 has been shown to be involved in cellular communication within the pulmonary vasculature. Therefore, we investigated the role of Cx43 in pulmonary vascular reactivity using Cx43 heterozygous (Cx43+/−) mice and 37,43Gap27, which is a pharmacological inhibitor of Cx37 and Cx43. Contraction and relaxation responses were studied in intra-lobar pulmonary arteries (IPAs) derived from normoxic mice and hypoxic mice using wire myography. IPAs from male Cx43+/− mice displayed a small but significant increase in the contractile response to endothelin-1 (but not 5-hydroxytryptamine) under both normoxic and hypoxic conditions. There was no difference in the contractile response to endothelin-1 (ET-1) or 5-hydroxytryptamine (5-HT) in IPAs derived from female Cx43+/−mice compared to wildtype mice. Relaxation responses to methacholine (MCh) were attenuated in IPAs from male and female Cx43+/− mice or by pre-incubation of IPAs with 37,43Gap27. Nω-Nitro-L-arginine methyl ester (l-NAME) fully inhibited MCh-induced relaxation. In conclusion, Cx43 is involved in nitric oxide (NO)-induced pulmonary vascular relaxation and plays a gender-specific and agonist-specific role in pulmonary vascular contractility. Therefore, reduced Cx43 signaling may contribute to pulmonary vascular dysfunction.

A novel mechanism of depression: role for connexins

Major depressive disorder (MDD) is a chronic and debilitating illness that affects over 350 million people worldwide; however, current treatments have failed to cure or prevent the progress of depression. Increasing evidence suggests a crucial role for connexins in MDD. In this review, we have summarised recent accomplishments regarding the role of connexins, gap junctions, and hemichannels in the aetiology of MDD, and discussed the limitations of current research. A blockage of gap junctions or hemichannels induces depressive behaviour. Possible underlying mechanisms include the regulation of neurosecretory functions and synaptic activity by gap junctions and hemichannels. Gap junctions are functionally inhibited under stress conditions. Conversely, hemichannel permeability is increased. Antidepressants inhibit hemichannel permeability; however, they have contrasting effects on the function of gap junctions under normal conditions and can protect them against stress. In conclusion, the blockage of hemichannels concurrent with improvements in gap junction functionality might be potential targets for depression treatment.

The serotonin hypothesis in pulmonary hypertension revisited: targets for novel therapies (2017 Grover Conference Series)

Increased synthesis of serotonin and/or activity of serotonin in pulmonary arteries has been implicated in the pathobiology of pulmonary arterial hypertension (PAH). The incidence of PAH associated with diet pills such as aminorex, fenfluramine, and chlorphentermine initially led to the “serotonin hypothesis of pulmonary hypertension.” Over the last couple of decades there has been an accumulation of convincing evidence that targeting serotonin synthesis or signaling is a novel and promising approach to the development of novel therapies for PAH. Pulmonary endothelial serotonin synthesis via tryptophan hydroxlase 1 (TPH1) is increased in patients with PAH and serotonin can act in a paracrine fashion on underlying pulmonary arterial smooth muscle cells (PASMCs), In humans, serotonin can enter PASMCs via the serotonin transporter (SERT) or activate the 5-HT1B receptor; 5-HT1B activation and SERT activity cooperate to induce PASMC contraction and proliferation via activation of downstream proliferative and contractile signaling pathways. Here we will review the current status of the serotonin hypothesis and discuss potential and novel therapeutic targets.

5-HT7 receptor antagonism (SB-269970) attenuates bleomycin-induced pulmonary fibrosis in rats via downregulating oxidative burden and inflammatory cascades and ameliorating collagen deposition: Comparison to terguride

The neurotransmitter 5-hydroxytryptamine (5-HT) is involved in regulation of local tissue inflammation and repair through a set of receptors (5-HT1-7 receptors), which are expressed in the lung. Considering the protective importance of 5-HT receptor antagonists against development of pulmonary fibrosis, we evaluated whether 5-HT7 receptor antagonist (SB-269970) modulates lung inflammatory and fibrogenic processes in comparison with 5-HT2A/B receptor antagonist (terguride), in bleomycin (BLM)-induced idiopathic pulmonary fibrosis (IPF) model. IPF model induced by a single dose of intra-tracheal BLM instillation (5mg/kg), and rats were treated with intraperitoneal injection of SB-269970 (1mg/kg day) or terguride (1.2mg/kg/d). The experiment was carried out on two separate sets of rats that were killed at day 7th and day 21st to evaluate the endpoint of the IPF inflammatory and fibrogenic phases, respectively. During the inflammatory phase 5-HT2A/B and 5-HT7 receptor antagonists attenuated the BLM-induced increase in the lung fluid content, the inflammatory cytokines levels and oxidative stress burden. In the fibrogenic phase, both SB-269970 and terguride reduced the serotonin concentrations in lung homogenates and significantly protected against IPF fibrogenic phase by attenuating collagen deposition and mRNA expression of both transforming growth factor-β1 (TGF- β1), and procollagen type Ӏ (PINP). 5-hydroxytryptamine 5-HT7 receptor antagonist showed more benefits than 5-HT2A/B receptor antagonist on the deleterious effects accompanied BLM instillation. The present study showed involvement of 5-HT7 receptor in the pathophysiology of BLM-induced IPF in rats and identified it as a potential therapeutic target in lung fibrotic disorders.

Identification of a Novel Allosteric Inhibitory Site on Tryptophan Hydroxylase 1 Enabling Unprecedented Selectivity Over all Related Hydroxylases

Pulmonary arterial hypertension (PAH) has demonstrated multi-serotonin receptor dependent pathologies, characterized by increased tone (5-HT_1B receptor) and complex lesions (SERT, 5-HT_1B, 5-HT_2B receptors) of the pulmonary vasculature together with right ventricular hypertrophy, ischemia and fibrosis (5-HT_2B receptor). Selective inhibitors of individual signaling elements – SERT, 5-HT_2A, 5HT_2B, and combined 5-HT2_A/B receptors, have all been tested clinically and failed. Thus, inhibition of tryptophan hydroxylase 1 (TPH1), the rate limiting step in 5-HT synthesis, has been suggested as a more broad, and thereby more effective, mode of 5-HT inhibition. However, selectivity over non-pathogenic enzyme family members, TPH2, phenylalanine hydroxylase, and tyrosine hydroxylase has hampered therapeutic development. Here we describe the site/sequence, biochemical, and biophysical characterization of a novel allosteric site on TPH1 through which selectivity over TPH2 and related aromatic amino acid hydroxylases is achieved. We demonstrate the mechanism of action by which novel compounds selectively inhibit TPH1 using surface plasma resonance and enzyme competition assays with both tryptophan ligand and BH4 co-factor. We demonstrate 15-fold greater potency within a human carcinoid cell line versus the most potent known TPH1/2 non-specific inhibitor. Lastly, we detail a novel canine in vivo system utilized to determine effective biologic inhibition of newly synthesized 5-HT. These findings are the first to demonstrate TPH1-selective inhibition and may pave the way to a truly effective means to reduce pathologic 5-HT and thereby treat complex remodeling diseases such as PAH.

Gap junctional signaling in pattern regulation: Physiological network connectivity instructs growth and form

Gap junctions (GJs) are aqueous channels that allow cells to communicate via physiological signals directly. The role of gap junctional connectivity in determining single-cell functions has long been recognized. However, GJs have another important role: the regulation of large-scale anatomical pattern. GJs are not only versatile computational elements that allow cells to control which small molecule signals they receive and emit, but also establish connectivity patterns within large groups of cells. By dynamically regulating the topology of bioelectric networks in vivo, GJs underlie the ability of many tissues to implement complex morphogenesis. Here, a review of recent data on patterning roles of GJs in growth of the zebrafish fin, the establishment of left-right patterning, the developmental dysregulation known as cancer, and the control of large-scale head-tail polarity, and head shape in planarian regeneration has been reported. A perspective in which GJs are not only molecular features functioning in single cells, but also enable global neural-like dynamics in non-neural somatic tissues has been proposed. This view suggests a rich program of future work which capitalizes on the rapid advances in the biophysics of GJs to exploit GJ-mediated global dynamics for applications in birth defects, regenerative medicine, and morphogenetic bioengineering. © 2016 Wiley Periodicals, Inc. Develop Neurobiol 77: 643-673, 2017.

5-HT2B receptor antagonists attenuate myofibroblast differentiation and subsequent fibrotic responses in vitro and in vivo

Pulmonary fibrosis is characterized by excessive accumulation of connective tissue, along with activated extracellular matrix (ECM)‐producing cells, myofibroblasts. The pathological mechanisms are not well known, however serotonin (5‐HT) and 5‐HT class 2 (5‐HT₂) receptors have been associated with fibrosis. The aim of the present study was to investigate the role of 5‐HT_2B receptors in fibrosis, using small molecular 5‐HT_2B receptor antagonists EXT5 and EXT9, with slightly different receptor affinity. Myofibroblast differentiation [production of alpha‐smooth muscle actin (α‐SMA)] and ECM synthesis were quantified in vitro, and the effects of the receptor antagonists were evaluated. Pulmonary fibrosis was also modeled in mice by subcutaneous bleomycin administrations (under light isoflurane anesthesia), and the effects of receptor antagonists on tissue density, collagen‐producing cells, myofibroblasts and decorin expression were investigated. In addition, cytokine expression was analyzed in serum. Lung fibroblasts displayed an increased α‐SMA (P < 0.05) and total proteoglycan production (P < 0.01) when cultured with TGF‐β1 together with 5‐HT, which were significantly reduced with both receptor antagonists. Following treatment with EXT5 or EXT9, tissue density, expression of decorin, number of collagen‐producing cells, and myofibroblasts were significantly decreased in vivo compared to bleomycin‐treated mice. Receptor antagonization also significantly reduced systemic levels of TNF‐α and IL‐1β, indicating a role in systemic inflammation. In conclusion, 5‐HT_2B receptor antagonists have potential to prevent myofibroblast differentiation, in vitro and in vivo, with subsequent effect on matrix deposition. The attenuating effects of 5‐HT_2B receptor antagonists on fibrotic tissue remodeling suggest these receptors as novel targets for the treatment of pulmonary fibrosis.

Vascular structural and functional changes: their association with causality in hypertension: models, remodeling and relevance

Essential hypertension is a complex multifactorial disease process that involves the interaction of multiple genes at various loci throughout the genome, and the influence of environmental factors such as diet and lifestyle, to ultimately determine long-term arterial pressure. These factors converge with physiological signaling pathways to regulate the set-point of long-term blood pressure. In hypertension, structural changes in arteries occur and show differences within and between vascular beds, between species, models and sexes. Such changes can also reflect the development of hypertension, and the levels of circulating humoral and vasoactive compounds. The role of perivascular adipose tissue in the modulation of vascular structure under various disease states such as hypertension, obesity and metabolic syndrome is an emerging area of research, and is likely to contribute to the heterogeneity described in this review. Diversity in structure and related function is the norm, with morphological changes being causative in some beds and states, and in others, a consequence of hypertension. Specific animal models of hypertension have advantages and limitations, each with factors influencing the relevance of the model to the human hypertensive state/s. However, understanding the fundamental properties of artery function and how these relate to signalling mechanisms in real (intact) tissues is key for translating isolated cell and model data to have an impact and relevance in human disease etiology. Indeed, the ultimate aim of developing new treatments to correct vascular dysfunction requires understanding and recognition of the limitations of the methodologies used.

Sphingosine-1-phosphate is involved in the occlusive arteriopathy of pulmonary arterial hypertension

Despite several advances in the pathobiology of pulmonary arterial hypertension (PAH), its pathogenesis is not completely understood. Current therapy improves symptoms but has disappointing effects on survival. Sphingosine-1-phosphate (S1P) is a lysophospholipid synthesized by sphingosine kinase 1 (SphK1) and SphK2. Considering the regulatory roles of S1P in several tissues leading to vasoconstriction, inflammation, proliferation, and fibrosis, we investigated whether S1P plays a role in the pathogenesis of PAH. To test this hypothesis, we used plasma samples and lung tissue from patients with idiopathic PAH (IPAH) and the Sugen5416/hypoxia/normoxia rat model of occlusive PAH. Our study revealed an increase in the plasma concentration of S1P in patients with IPAH and in early and late stages of PAH in rats. We observed increased expression of both SphK1 and SphK2 in the remodeled pulmonary arteries of patients with IPAH and PAH rats. Exogenous S1P stimulated the proliferation of cultured rat pulmonary arterial endothelial and smooth-muscle cells. We also found that 3 weeks of treatment of late-stage PAH rats with an SphK1 inhibitor reduced the increased plasma levels of S1P and the occlusive pulmonary arteriopathy. Although inhibition of SphK1 improved cardiac index and the total pulmonary artery resistance index, it did not reduce right ventricular systolic pressure or right ventricular hypertrophy. Our study supports that S1P is involved in the pathogenesis of occlusive arteriopathy in PAH and provides further evidence that S1P signaling may be a novel therapeutic target.

Endothelial and Smooth Muscle Cell Interactions in the Pathobiology of Pulmonary Hypertension

In the pulmonary vasculature, the endothelial and smooth muscle cells are two key cell types that play a major role in the pathobiology of pulmonary vascular disease and pulmonary hypertension. The normal interactions between these two cell types are important for the homeostasis of the pulmonary circulation, and any aberrant interaction between them may lead to various disease states including pulmonary vascular remodeling and pulmonary hypertension. It is well recognized that the endothelial cell can regulate the function of the underlying smooth muscle cell by releasing various bioactive agents such as nitric oxide and endothelin-1. In addition to such paracrine regulation, other mechanisms exist by which there is cross-talk between these two cell types, including communication via the myoendothelial injunctions and information transfer via extracellular vesicles. Emerging evidence suggests that these nonparacrine mechanisms play an important role in the regulation of pulmonary vascular tone and the determination of cell phenotype and that they are critically involved in the pathobiology of pulmonary hypertension.

Expression and role of connexin-based gap junctions in pulmonary inflammatory diseases

Connexins are transmembrane proteins that can generate intercellular communication channels known as gap junctions. They contribute to the direct movement of ions and larger cytoplasmic solutes between various cell types. In the lung, connexins participate in a variety of physiological functions, such as tissue homeostasis and host defence. In addition, emerging evidence supports a role for connexins in various pulmonary inflammatory diseases, such as asthma, pulmonary hypertension, acute lung injury, lung fibrosis or cystic fibrosis. In these diseases, the altered expression of connexins leads to disruption of normal intercellular communication pathways, thus contributing to various pathophysiological aspects, such as inflammation or tissue altered reactivity and remodeling. The present review describes connexin structure and organization in gap junctions. It focuses on connexins in the lung, including pulmonary bronchial and arterial beds, by looking at their expression, regulation and physiological functions. This work also addresses the issue of connexin expression alteration in various pulmonary inflammatory diseases and describes how targeting connexin-based gap junctions with pharmacological tools, synthetic blocking peptides or genetic approaches, may open new therapeutic perspectives in the treatment of these diseases.

Physiological controls of large-scale patterning in planarian regeneration: a molecular and computational perspective on growth and form

Planaria are complex metazoans that repair damage to their bodies and cease remodeling when a correct anatomy has been achieved. This model system offers a unique opportunity to understand how large-scale anatomical homeostasis emerges from the activities of individual cells. Much progress has been made on the molecular genetics of stem cell activity in planaria. However, recent data also indicate that the global pattern is regulated by physiological circuits composed of ionic and neurotransmitter signaling. Here, we overview the multi-scale problem of understanding pattern regulation in planaria, with specific focus on bioelectric signaling via ion channels and gap junctions (electrical synapses), and computational efforts to extract explanatory models from functional and molecular data on regeneration. We present a perspective that interprets results in this fascinating field using concepts from dynamical systems theory and computational neuroscience. Serving as a tractable nexus between genetic, physiological, and computational approaches to pattern regulation, planarian pattern homeostasis harbors many deep insights for regenerative medicine, evolutionary biology, and engineering.

Connexin-mediated regulation of the pulmonary vasculature

Pulmonary arterial hypertension (PAH) is a complex, multi-factorial disorder characterized by both constriction and remodelling of the distal pulmonary vasculature. This leads to increased pulmonary pressures and eventually right heart failure. Current drugs, which primarily target the vasoconstriction, serve only to prolong life and novel therapies targeting both the vasoconstriction and the remodelling are required. Aberrant signalling between cells of the pulmonary vasculature has been associated with the development of PAH. In particular, endothelial dysfunction can lead to hyperplasia of the underlying medial layer. Connexins are a family of transmembrane proteins which can form intercellular communication channels known as gap junctions. This review will discuss recent evidence which shows that connexins play a role in regulation of the pulmonary vasculature and that dysregulation of connexins may contribute to PAH pathogenesis. Interaction of connexins with signalling pathways relevant to the pathogenesis of PAH, such as bone morphogenetic protein (BMP), serotonin and oestrogen are discussed.

Involvement of gap junctions between smooth muscle cells in sustained hypoxic pulmonary vasoconstriction development: a potential role for 15-HETE and 20-HETE

In response to hypoxia, the pulmonary artery normally constricts to maintain optimal ventilation-perfusion matching in the lung, but chronic hypoxia leads to the development of pulmonary hypertension. The mechanisms of sustained hypoxic pulmonary vasoconstriction (HPV) remain unclear. The aim of this study was to determine the role of gap junctions (GJs) between smooth muscle cells (SMCs) in the sustained HPV development and involvement of arachidonic acid (AA) metabolites in GJ-mediated signaling. Vascular tone was measured in bovine intrapulmonary arteries (BIPAs) using isometric force measurement technique. Expression of contractile proteins was determined by Western blot. AA metabolites in the bath fluid were analyzed by mass spectrometry. Prolonged hypoxia elicited endothelium-independent sustained HPV in BIPAs. Inhibition of GJs by 18β-glycyrrhetinic acid (18β-GA) and heptanol, nonspecific blockers, and Gap-27, a specific blocker, decreased HPV in deendothelized BIPAs. The sustained HPV was not dependent on Ca(2+) entry but decreased by removal of Ca(2+) and by Rho-kinase inhibition with Y-27632. Furthermore, inhibition of GJs decreased smooth muscle myosin heavy chain (SM-MHC) expression and myosin light chain phosphorylation in BIPAs. Interestingly, inhibition of 15- and 20-hydroxyeicosatetraenoic acid (HETE) synthesis decreased HPV in deendothelized BIPAs. 15-HETE- and 20-HETE-stimulated constriction of BIPAs was inhibited by 18β-GA and Gap-27. Application of 15-HETE and 20-HETE to BIPAs increased SM-MHC expression, which was also suppressed by 18β-GA and by inhibitors of lipoxygenase and cytochrome P450 monooxygenases. More interestingly, 15,20-dihydroxyeicosatetraenoic acid and 20-OH-prostaglandin E2, novel derivatives of 20-HETE, were detected in tissue bath fluid and synthesis of these derivatives was almost completely abolished by 18β-GA. Taken together, our novel findings show that GJs between SMCs are involved in the sustained HPV in BIPAs, and 15-HETE and 20-HETE, through GJs, appear to mediate SM-MHC expression and contribute to the sustained HPV development.

A novel method for inducing nerve growth via modulation of host resting potential: gap junction-mediated and serotonergic signaling mechanisms

A major goal of regenerative medicine is to restore the function of damaged or missing organs through the implantation of bioengineered or donor-derived components. It is necessary to understand the signals and cues necessary for implanted structures to innervate the host, as organs devoid of neural connections provide little benefit to the patient. While developmental studies have identified neuronal pathfinding molecules required for proper patterning during embryogenesis, strategies to initiate innervation in structures transplanted at later times or alternate locations remain limited. Recent work has identified membrane resting potential of nerves as a key regulator of growth cone extension or arrest. Here, we identify a novel role of bioelectricity in the generation of axon guidance cues, showing that neurons read the electric topography of surrounding cells, and demonstrate these cues can be leveraged to initiate sensory organ transplant innervation. Grafts of fluorescently labeled embryological eye primordia were used to produce ectopic eyes in Xenopus laevis tadpoles. Depolarization of host tissues through anion channel activation or other means led to a striking hyperinnervation of the body by these ectopic eyes. A screen of possible transduction mechanisms identified serotonergic signaling to be essential for hyperinnervation to occur, and our molecular data suggest a possible model of bioelectrical control of the distribution of neurotransmitters that guides nerve growth. Together, these results identify the molecular components of bioelectrical signaling among cells that regulates axon guidance, and suggest novel biomedical and bioengineering strategies for triggering neuronal outgrowth using ion channel drugs already approved for human use.

Prenatal programming of pulmonary hypertension induced by chronic hypoxia or ductal ligation in sheep

Pulmonary hypertension of the newborn is caused by a spectrum of functional and structural abnormalities of the cardiopulmonary circuit. The existence of multiple etiologies and an incomplete understanding of the mechanisms of disease progression have hindered the development of effective therapies. Animal models offer a means of gaining a better understanding of the fundamental basis of the disease. To that effect, a number of experimental animal models are being used to generate pulmonary hypertension in the fetus and newborn. In this review, we compare the mechanisms associated with pulmonary hypertension caused by two such models: in utero ligation of the ductus arteriosus and chronic perinatal hypoxia in sheep fetuses and newborns. In this manner, we make direct comparisons between ductal ligation and chronic hypoxia with respect to the associated mechanisms of disease, since multiple studies have been performed with both models in a single species. We present evidence that the mechanisms associated with pulmonary hypertension are dependent on the type of stress to which the fetus is subjected. Such an analysis allows for a more thorough evaluation of the disease etiology, which can help focus clinical treatments. The final part of the review provides a clinical appraisal of current treatment strategies and lays the foundation for developing individualized therapies that depend on the causative factors.

Regulation of cellular communication by signaling microdomains in the blood vessel wall

It has become increasingly clear that the accumulation of proteins in specific regions of the plasma membrane can facilitate cellular communication. These regions, termed signaling microdomains, are found throughout the blood vessel wall where cellular communication, both within and between cell types, must be tightly regulated to maintain proper vascular function. We will define a cellular signaling microdomain and apply this definition to the plethora of means by which cellular communication has been hypothesized to occur in the blood vessel wall. To that end, we make a case for three broad areas of cellular communication where signaling microdomains could play an important role: 1) paracrine release of free radicals and gaseous molecules such as nitric oxide and reactive oxygen species; 2) role of ion channels including gap junctions and potassium channels, especially those associated with the endothelium-derived hyperpolarization mediated signaling, and lastly, 3) mechanism of exocytosis that has considerable oversight by signaling microdomains, especially those associated with the release of von Willebrand factor. When summed, we believe that it is clear that the organization and regulation of signaling microdomains is an essential component to vessel wall function.

Role of asymmetric methylarginine and connexin 43 in the regulation of pulmonary endothelial function

Abstract Circulating levels of asymmetric dimethylarginine (ADMA), a nitric oxide synthase inhibitor, are increased in patients with idiopathic pulmonary hypertension (IPAH). We hypothesized that ADMA abrogates gap junctional communication, required for the coordinated regulation of endothelial barrier function and angiogenesis, and so contributes to pulmonary endothelial dysfunction. The effects of ADMA on expression and function of gap junctional proteins were studied in human pulmonary artery endothelial cells; pulmonary endothelial microvascular cells from mice deficient in an enzyme metabolizing ADMA, dimethylarginine dimethylaminohydrolase I (DDAHI); and blood-derived endothelial-like cells from patients with IPAH. Exogenous and endogenous ADMA inhibited protein expression and membrane localization of connexin 43 (Cx43) in a nitric oxide/soluble guanosine monophosphate/c-jun-dependent manner in pulmonary endothelial cells, resulting in the inhibition of gap junctional communication, increased permeability, and decreased angiogenesis. The effects of ADMA were prevented by overexpression of DDAHI or Cx43 and by treatment with rotigaptide. Blood-derived endothelial-like cells from IPAH patients displayed a distinct disease-related phenotype compared to cells from healthy controls, characterized by reduced DDAHI expression, increased ADMA production, and abnormal angiogenesis. In summary, we show that ADMA induces pulmonary endothelial dysfunction via changes in expression and activity of Cx43. Cells from IPAH patients exhibit abnormal DDAHI/Cx43 signaling as well as differences in gap junctional communication, barrier function, and angiogenesis. Strategies that promote DDAHI/Cx43 signaling may have an endothelium-protective effect and be beneficial in pulmonary vascular disease.

Gap junctions support the sustained phase of hypoxic pulmonary vasoconstriction by facilitating calcium sensitization

AIMS

To determine the role of gap junctions (GJs) in hypoxic pulmonary vasoconstriction (HPV).

METHODS AND RESULTS

Studies were performed in rat isolated intrapulmonary arteries (IPAs) mounted on a myograph and in anaesthetized rats. Hypoxia induced a biphasic HPV response in IPAs preconstricted with prostaglandin F2α (PGF2α, 3 µM) or 20 mM K⁺. The GJ inhibitors 18β-glycyrrhetinic acid (18β-GA, 30 µM), heptanol (3.5 mM), or 2-aminoethoxydiphenyl borate (2-APB) (75 µM) had little effect on the transient Phase 1 of HPV, but abolished the sustained Phase 2 which is associated with Ca²⁺ sensitization. The voltage-dependent Ca²⁺ channel blocker diltiazem (10 µM) had no effect on HPV, and did not alter the inhibitory action of 18β-GA. Sustained HPV is enhanced by high glucose (15 mM) via potentiation of Ca²⁺ sensitization, in the presence of high glucose 18β-GA still abolished sustained HPV. Simultaneous measurement of tension and intracellular Ca²⁺ using Fura PE-3 demonstrated that whilst 18β-GA abolished tension development during sustained HPV, it did not affect the elevation of intracellular Ca²⁺. Consistent with this, 18β-GA abolished hypoxia-induced phosphorylation of the Rho kinase target MYPT-1. In anaesthetized rats hypoxia caused a biphasic increase in systolic right ventricular pressure. Treatment with oral 18β-GA (25 mg/kg) abolished the sustained component of the hypoxic pressor response.

CONCLUSION

These results imply that GJs are critically involved in the signalling pathways leading to Rho kinase-dependent Ca²⁺ sensitization during sustained HPV, but not elevation of intracellular Ca²⁺, and may explain the dependence of the former on an intact endothelium.

Endothelial control of vasodilation: integration of myoendothelial microdomain signalling and modulation by epoxyeicosatrienoic acids

Endothelium-derived epoxyeicosatrienoic acids (EETs) are fatty acid epoxides that play an important role in the control of vascular tone in selected coronary, renal, carotid, cerebral and skeletal muscle arteries. Vasodilation due to endothelium-dependent smooth muscle hyperpolarization (EDH) has been suggested to involve EETs as a transferable endothelium-derived hyperpolarizing factor. However, this activity may also be due to EETs interacting with the components of other primary EDH-mediated vasodilator mechanisms. Indeed, the transfer of hyperpolarization initiated in the endothelium to the adjacent smooth muscle via gap junction connexins occurs separately or synergistically with the release of K(+) ions at discrete myoendothelial microdomain signalling sites. The net effects of such activity are smooth muscle hyperpolarization, closure of voltage-dependent Ca(2+) channels, phospholipase C deactivation and vasodilation. The spatially localized and key components of the microdomain signalling complex are the inositol 1,4,5-trisphosphate receptor-mediated endoplasmic reticulum Ca(2+) store, Ca(2+)-activated K(+) (KCa), transient receptor potential (TRP) and inward-rectifying K(+) channels, gap junctions and the smooth muscle Na(+)/K(+)-ATPase. Of these, TRP channels and connexins are key endothelial effector targets modulated by EETs. In an integrated manner, endogenous EETs enhance extracellular Ca(2+) influx (thereby amplifying and prolonging KCa-mediated endothelial hyperpolarization) and also facilitate the conduction of this hyperpolarization to spatially remote vessel regions. The contribution of EETs and the receptor and channel subtypes involved in EDH-related microdomain signalling, as a candidate for a universal EDH-mediated vasodilator mechanism, vary with vascular bed, species, development and disease and thus represent potentially selective targets for modulating specific artery function.