Differential muscarinic receptor mRNA expression by freshly isolated and cultured bovine aortic endothelial cells

Chloroquine differentially modulates coronary vasodilation in control and diabetic mice

BACKGROUND AND PURPOSE

Chloroquine is a traditional medicine to treat malaria. There is increasing evidence that chloroquine not only induces phagocytosis but regulates vascular tone. Few reports investigating the effect of chloroquine on vascular responsiveness of coronary arteries have been made. In this study, we examined how chloroquine affected endothelium-dependent relaxation in coronary arteries under normal and diabetic conditions.

EXPERIMENTAL APPROACH

We isolated coronary arteries from mice and examined endothelium-dependent relaxation (EDR). Human coronary endothelial cells and mouse coronary endothelial cells isolated from control and diabetic mouse (TALLYHO/Jng [TH] mice, a spontaneous type 2 diabetic mouse model) were used for the molecular biological or cytosolic NO and Ca²⁺ measurements.

KEY RESULTS

Chloroquine inhibited endothelium-derived NO-dependent relaxation but had negligible effect on endothelium-derived hyperpolarization (EDH)-dependent relaxation in coronary arteries of control mice. Chloroquine significantly decreased NO production in control human coronary endothelial cells partly by phosphorylating eNOS^Thr495 (an inhibitory phosphorylation site of eNOS) and attenuating the rise of cytosolic Ca²⁺ concentration after stimulation. EDR was significantly inhibited in diabetic mice in comparison to control mice. Interestingly, chloroquine enhanced EDR in diabetic coronary arteries by, specifically, increasing EDH-dependent relaxation due partly to its augmenting effect on gap junction activity in diabetic mouse coronary endothelial cells.

CONCLUSIONS AND IMPLICATIONS

These data indicate that chloroquine affects vascular relaxation differently under normal and diabetic conditions. Therefore, the patients' health condition such as coronary macrovascular or microvascular disease, with or without diabetes, must be taken account into the consideration when selecting chloroquine for the treatment of malaria.

Selective allosteric modulation of muscarinic acetylcholine receptors for the treatment of schizophrenia and substance use disorders

Muscarinic acetylcholine receptor (mAChRs) subtypes represent exciting new targets for the treatment of schizophrenia and substance use disorder (SUD). Recent advances in the development of subtype-selective allosteric modulators have revealed promising effects in preclinical models targeting the different symptoms observed in schizophrenia and SUD. M₁ PAMs display potential for addressing the negative and cognitive symptoms of schizophrenia, while M₄ PAMs exhibit promise in treating preclinical models predictive of antipsychotic-like activity. In SUD, there is increasing support for modulation of mesocorticolimbic dopaminergic circuitry involved in SUD with selective M₄ mAChR PAMs or M₅ mAChR NAMs. Allosteric modulators of these mAChR subtypes have demonstrated efficacy in rodent models of cocaine and ethanol seeking, with indications that these ligand may also be useful for other substances of abuse, as well as in various stages in the cycle of addiction. Importantly, allosteric modulators of the different mAChR subtypes may provide viable treatment options, while conferring greater subtype specificity and corresponding enhanced therapeutic index than orthosteric muscarinic ligands and maintaining endogenous temporo-spatial ACh signaling. Overall, subtype specific mAChR allosteric modulators represent important novel therapeutic mechanisms for schizophrenia and SUD.

Muscarinic M5 receptors trigger acetylcholine-induced Ca2+ signals and nitric oxide release in human brain microvascular endothelial cells

Basal forebrain neurons control cerebral blood flow (CBF) by releasing acetylcholine (Ach), which binds to endothelial muscarinic receptors to induce nitric (NO) release and vasodilation in intraparenchymal arterioles. Nevertheless, the mechanism whereby Ach stimulates human brain microvascular endothelial cells to produce NO is still unknown. Herein, we sought to assess whether Ach stimulates NO production in a Ca²⁺ -dependent manner in hCMEC/D3 cells, a widespread model of human brain microvascular endothelial cells. Ach induced a dose-dependent increase in intracellular Ca²⁺ concentration ([Ca²⁺ ]_i ) that was prevented by the genetic blockade of M5 muscarinic receptors (M5-mAchRs), which was the only mAchR isoform coupled to phospholipase Cβ (PLCβ) present in hCMEC/D3 cells. A comprehensive real-time polymerase chain reaction analysis revealed the expression of the transcripts encoding for type 3 inositol-1,4,5-trisphosphate receptors (InsP₃ R3), two-pore channels 1 and 2 (TPC1-2), Stim2, Orai1-3. Pharmacological manipulation showed that the Ca²⁺ response to Ach was mediated by InsP₃ R3, TPC1-2, and store-operated Ca²⁺ entry (SOCE). Ach-induced NO release, in turn, was inhibited in cells deficient of M5-mAchRs. Likewise, Ach failed to increase NO levels in the presence of l-NAME, a selective NOS inhibitor, or BAPTA, a membrane-permeant intracellular Ca²⁺ buffer. Moreover, the pharmacological blockade of the Ca²⁺ response to Ach also inhibited the accompanying NO production. These data demonstrate for the first time that synaptically released Ach may trigger NO release in human brain microvascular endothelial cells by stimulating a Ca²⁺ signal via M5-mAchRs.

Calcium and electrical signaling in arterial endothelial tubes: New insights into cellular physiology and cardiovascular function

The integral role of the endothelium during the coordination of blood flow throughout vascular resistance networks has been recognized for several decades now. Early examination of the distinct anatomy and physiology of the endothelium as a signaling conduit along the vascular wall has prompted development and application of an intact endothelial "tube" study model isolated from rodent skeletal muscle resistance arteries. Vasodilatory signals such as increased endothelial cell (EC) Ca²⁺ ([Ca²⁺ ]_i ) and hyperpolarization take place in single ECs while shared between electrically coupled ECs through gap junctions up to distances of millimeters (≥2 mm). The small- and intermediate-conductance Ca²⁺ activated K⁺ (SK_Ca /IK_Ca or K_Ca 2.3/K_Ca 3.1) channels function at the interface of Ca²⁺ signaling and hyperpolarization; a bidirectional relationship whereby increases in [Ca²⁺ ]_i activate SK_Ca /IK_Ca channels to produce hyperpolarization and vice versa. Further, the spatial domain of hyperpolarization among electrically coupled ECs can be finely tuned via incremental modulation of SK_Ca /IK_Ca channels to balance the strength of local and conducted electrical signals underlying vasomotor activity. Multifunctional properties of the voltage-insensitive SK_Ca /IK_Ca channels of resistance artery endothelium may be employed for therapy during the aging process and development of vascular disease.

Targeting Pulmonary Endothelial Hemoglobin α Improves Nitric Oxide Signaling and Reverses Pulmonary Artery Endothelial Dysfunction

Pulmonary hypertension is characterized by pulmonary endothelial dysfunction. Previous work showed that systemic artery endothelial cells (ECs) express hemoglobin (Hb) α to control nitric oxide (NO) diffusion, but the role of this system in pulmonary circulation has not been evaluated. We hypothesized that up-regulation of Hb α in pulmonary ECs contributes to NO depletion and pulmonary vascular dysfunction in pulmonary hypertension. Primary distal pulmonary arterial vascular smooth muscle cells, lung tissue sections from unused donor (control) and idiopathic pulmonary artery (PA) hypertension lungs, and rat and mouse models of SU5416/hypoxia-induced pulmonary hypertension (PH) were used. Immunohistochemical, immunocytochemical, and immunoblot analyses and transfection, infection, DNA synthesis, apoptosis, migration, cell count, and protein activity assays were performed in this study. Cocultures of human pulmonary microvascular ECs and distal pulmonary arterial vascular smooth muscle cells, lung tissue from control and pulmonary hypertensive lungs, and a mouse model of chronic hypoxia-induced PH were used. Immunohistochemical, immunoblot analyses, spectrophotometry, and blood vessel myography experiments were performed in this study. We find increased expression of Hb α in pulmonary endothelium from humans and mice with PH compared with controls. In addition, we show up-regulation of Hb α in human pulmonary ECs cocultured with PA smooth muscle cells in hypoxia. We treated pulmonary ECs with a Hb α mimetic peptide that disrupts the association of Hb α with endothelial NO synthase, and found that cells treated with the peptide exhibited increased NO signaling compared with a scrambled peptide. Myography experiments using pulmonary arteries from hypoxic mice show that the Hb α mimetic peptide enhanced vasodilation in response to acetylcholine. Our findings reveal that endothelial Hb α functions as an endogenous scavenger of NO in the pulmonary endothelium. Targeting this pathway may offer a novel therapeutic target to increase endogenous levels of NO in PH.

All muscarinic acetylcholine receptors (M1-M5) are expressed in murine brain microvascular endothelium

Clinical and experimental studies indicate that muscarinic acetylcholine receptors are potential pharmacological targets for the treatment of neurological diseases. Although these receptors have been described in human, bovine and rat cerebral microvascular tissue, a subtype functional characterization in mouse brain endothelium is lacking. Here, we show that all muscarinic acetylcholine receptors (M₁-M₅) are expressed in mouse brain microvascular endothelial cells. The mRNA expression of M₂, M₃, and M₅ correlates with their respective protein abundance, but a mismatch exists for M₁ and M₄ mRNA versus protein levels. Acetylcholine activates calcium transients in brain endothelium via muscarinic, but not nicotinic, receptors. Moreover, although M₁ and M₃ are the most abundant receptors, only a small fraction of M₁ is present in the plasma membrane and functions in ACh-induced Ca²⁺ signaling. Bioinformatic analyses performed on eukaryotic muscarinic receptors demonstrate a high degree of conservation of the orthosteric binding site and a great variability of the allosteric site. In line with previous studies, this result indicates muscarinic acetylcholine receptors as potential pharmacological targets in future translational studies. We argue that research on drug development should especially focus on the allosteric binding sites of the M₁ and M₃ receptors.

Acetylcholine induces intracellular Ca2+ oscillations and nitric oxide release in mouse brain endothelial cells

Basal forebrain neurons increase cortical blood flow by releasing acetylcholine (Ach), which stimulates endothelial cells (ECs) to produce the vasodilating gasotransmitter, nitric oxide (NO). Surprisingly, the mechanism whereby Ach induces NO synthesis in brain microvascular ECs is unknown. An increase in intracellular Ca²⁺ concentration recruits a multitude of endothelial Ca²⁺-dependent pathways, such as Ca²⁺/calmodulin endothelial NO synthase (eNOS). The present investigation sought to investigate the role of intracellular Ca²⁺ signaling in Ach-induced NO production in bEND5 cells, an established model of mouse brain microvascular ECs, by conventional imaging of cells loaded with the Ca²⁺-sensitive dye, Fura-2/AM, and the NO-sensitive fluorophore, DAF-DM diacetate. Ach induced dose-dependent Ca²⁺ oscillations in bEND5 cells, 300 μM being the most effective dose to generate a prolonged Ca²⁺ burst. Pharmacological manipulation revealed that Ach-evoked Ca²⁺ oscillations required metabotropic muscarinic receptor (mAchR) activation and were patterned by a complex interplay between repetitive ER Ca²⁺ release via inositol-1,4,5-trisphosphate receptors (InsP₃Rs) and store-operated Ca²⁺ entry (SOCE). A comprehensive real time-polymerase chain reaction analysis demonstrated the expression of the transcripts encoding for M3-mAChRs, InsP₃R1 and InsP₃R3, Stim1-2 and Orai2. Next, we found that Ach-induced NO production was hindered by L-NAME, a selective NOS inhibitor, and BAPTA, a membrane permeable intracellular Ca²⁺ buffer. Moreover, Ach-elicited NO synthesis was blocked by the pharmacological abrogation of the accompanying Ca²⁺ spikes. Overall, these data shed novel light on the molecular mechanisms whereby neuronally-released Ach controls neurovascular coupling in blood microvessels.

Acetylcholine released by endothelial cells facilitates flow-mediated dilatation

KEY POINTS

The endothelium plays a pivotal role in the vascular response to chemical and mechanical stimuli. The endothelium is exquisitely sensitive to ACh, although the physiological significance of ACh-induced activation of the endothelium is unknown. In the present study, we investigated the mechanisms of flow-mediated endothelial calcium signalling. Our data establish that flow-mediated endothelial calcium responses arise from the autocrine action of non-neuronal ACh released by the endothelium.

ABSTRACT

Circulating blood generates frictional forces (shear stress) on the walls of blood vessels. These frictional forces critically regulate vascular function. The endothelium senses these frictional forces and, in response, releases various vasodilators that relax smooth muscle cells in a process termed flow-mediated dilatation. Although some elements of the signalling mechanisms have been identified, precisely how flow is sensed and transduced to cause the release of relaxing factors is poorly understood. By imaging signalling in large areas of the endothelium of intact arteries, we show that the endothelium responds to flow by releasing ACh. Once liberated, ACh acts to trigger calcium release from the internal store in endothelial cells, nitric oxide production and artery relaxation. Flow-activated release of ACh from the endothelium is non-vesicular and occurs via organic cation transporters. ACh is generated following mitochondrial production of acetylCoA. Thus, we show ACh is an autocrine signalling molecule released from endothelial cells, and identify a new role for the classical neurotransmitter in endothelial mechanotransduction.

Membrane potential governs calcium influx into microvascular endothelium: integral role for muscarinic receptor activation

In resistance arteries, coupling a rise of intracellular calcium concentration ([Ca(2+)]i) to endothelial cell hyperpolarization underlies smooth muscle cell relaxation and vasodilatation, thereby increasing tissue blood flow and oxygen delivery. A controversy persists as to whether changes in membrane potential (V(m)) alter endothelial cell [Ca(2+)]i. We tested the hypothesis that V(m) governs [Ca(2+)]i in endothelium of resistance arteries by performing Fura-2 photometry while recording and controlling V(m) of intact endothelial tubes freshly isolated from superior epigastric arteries of C57BL/6 mice. Under resting conditions, [Ca(2+)]i did not change when V(m) shifted from baseline (∼-40 mV) via exposure to 10 μM NS309 (hyperpolarization to ∼-80 mV), via equilibration with 145 mm [K(+)]o (depolarization to ∼-5 mV), or during intracellular current injection (±0.5 to 5 nA, 20 s pulses) while V(m) changed linearly between ∼-80 mV and +10 mV. In contrast, during the plateau (i.e. Ca(2+) influx) phase of the [Ca(2+)]i response to approximately half-maximal stimulation with 100 nm ACh (∼EC50), [Ca(2+)]i increased as V(m) hyperpolarized below -40 mV and decreased as V(m) depolarized above -40 mV. The magnitude of [Ca(2+)]i reduction during depolarizing current injections correlated with the amplitude of the plateau [Ca(2+)]i response to ACh. The effect of hyperpolarization on [Ca(2+)]i was abolished following removal of extracellular Ca(2+), was enhanced subtly by raising extracellular [Ca(2+)] from 2 mm to 10 mm and was reduced by half in endothelium of TRPV4(-/-) mice. Thus, during submaximal activation of muscarinic receptors, V(m) can modulate Ca(2+) entry through the plasma membrane in accord with the electrochemical driving force.

Regulation of blood flow in adipose tissue: involvement of the cholinergic system

Acetylcholine (Ach) has vasodilatory actions. However, data are conflicting about the role of Ach in regulating blood flow in subcutaneous adipose tissue (ATBF). This may be related to inaccurate ATBF recording or to the responder/nonresponder (R/NR) phenomenon. We showed previously that healthy individuals are R (ATBF increases postprandially by >50% of baseline BF) or NR (ATBF increases ≤50% postprandially). Our objective was to assess the role of the cholinergic system on ATBF in R and NR subjects. ATBF was manipulated by in situ microinfusion of vasoactive agents (VA) in AT and monitored by the (133)Xenon washout technique (both recognized methods) at the VA site and at the control site. We tested incrementally increasing doses of Ach (10(-5), 10(-3), and 10(-1) mol/l; n = 15) and Ach receptor antagonists (Ra) before and after oral administration of 75-g glucose using atropine (muscarinic Ra; 10(-4) mol/l, n = 13; 10(-5) mol/l, n = 22) and mecamylamine (nicotinic Ra; 10(-3) mol/l, n = 15; 10(-4) mol/l, n = 10). Compared with baseline [2.41 (1.36-2.83) ml·100 g(-1)·min(-1)], Ach increased ATBF dose dependently [3.32 (2.80-5.09), 6.46 (4.36-9.51), and 10.31 (7.98-11.52), P < 0.0001], with no difference between R and NR. Compared with control side, atropine (both concentrations) had no effect on fasting ATBF; only atropine 10(-4) mol/l decreased post-glucose ATBF [iAUC: 1.25 (0.32-2.91) vs. 1.98 (0.64-2.94); P = 0.04]. This effect was further apparent in R. Mecamylamine had no impact on fasting and postglucose ATBF in R and NR. Our results suggest that the cholinergic system is implicated in ATBF regulation, although it has no role in the blunting of ATBF response in NR.

Effect of the M1 Muscarinic Acetylcholine Receptor on Retinal Neuron Number Studied with Gene-Targeted Mice

Pharmacological activation of the M1 muscarinic receptor subtype was suggested to promote the survival of retinal neurons. We examined the hypothesis that the M1 receptor is crucial for retinal neuron survival in vivo by using mice devoid of the M1 receptor gene. Muscarinic receptor gene expression was determined in the retina using real-time PCR. The amount of neurons in the retinal ganglion cell layer and of axons in the optic nerve was determined in retinal wholemounts stained with cresyl blue and in optic nerve cross-sections stained with toluidine blue, respectively. mRNA of all five muscarinic receptor subtypes (M1-M5) was detected in the retina from wild-type mice. Remarkably, M2 and M3 receptor mRNA were most abundant. In retinas from M1 receptor-deficient mice, M4 receptor mRNA expression was increased compared to that of wild-type mice, while no marked changes in the mRNA expression levels of the other muscarinic receptor subtypes were observed. The amount of cells in the retinal ganglion cell layer and the amount of axons in the optic nerve did not differ between M1 receptor-deficient and wild-type mice. The present findings suggest that the M1 receptor is not essential for the survival of retinal neurons in vivo.

Isolation of microvascular endothelial tubes from mouse resistance arteries

The control of blood flow by the resistance vasculature regulates the supply of oxygen and nutrients concomitant with the removal of metabolic by-products, as exemplified by exercising skeletal muscle. Endothelial cells (ECs) line the intima of all resistance vessels and serve a key role in controlling diameter (e.g. endothelium-dependent vasodilation) and, thereby, the magnitude and distribution of tissue blood flow. The regulation of vascular resistance by ECs is effected by intracellular Ca²⁺ signaling, which leads to production of diffusible autacoids (e.g. nitric oxide and arachidonic acid metabolites)^1-3 and hyperpolarization^4,5 that elicit smooth muscle cell relaxation. Thus understanding the dynamics of endothelial Ca²⁺ signaling is a key step towards understanding mechanisms governing blood flow control. Isolating endothelial tubes eliminates confounding variables associated with blood in the vessel lumen and with surrounding smooth muscle cells and perivascular nerves, which otherwise influence EC structure and function. Here we present the isolation of endothelial tubes from the superior epigastric artery (SEA) using a protocol optimized for this vessel.

To isolate endothelial tubes from an anesthetized mouse, the SEA is ligated in situ to maintain blood within the vessel lumen (to facilitate visualizing it during dissection), and the entire sheet of abdominal muscle is excised. The SEA is dissected free from surrounding skeletal muscle fibers and connective tissue, blood is flushed from the lumen, and mild enzymatic digestion is performed to enable removal of adventitia, nerves and smooth muscle cells using gentle trituration. These freshly-isolated preparations of intact endothelium retain their native morphology, with individual ECs remaining functionally coupled to one another, able to transfer chemical and electrical signals intercellularly through gap junctions^6,7. In addition to providing new insight into calcium signaling and membrane biophysics, these preparations enable molecular studies of gene expression and protein localization within native microvascular endothelium.

Calcium and endothelium-mediated vasodilator signaling

Vascular tone refers to the balance between arterial constrictor and dilator activity. The mechanisms that underlie tone are critical for the control of haemodynamics and matching circulatory needs with metabolism, and thus alterations in tone are a primary factor for vascular disease etiology. The dynamic spatiotemporal control of intracellular Ca(2+) levels in arterial endothelial and smooth muscle cells facilitates the modulation of multiple vascular signaling pathways. Thus, control of Ca(2+) levels in these cells is integral for the maintenance of tone and blood flow, and intimately associated with both physiological and pathophysiological states. Hence, understanding the mechanisms that underlie the modulation of vascular Ca(2+) activity is critical for both fundamental knowledge of artery function, and for the development of targeted therapies. This brief review highlights the role of Ca(2+) signaling in vascular endothelial function, with a focus on contact-mediated vasodilator mechanisms associated with endothelium-derived hyperpolarization and the longitudinal conduction of responses over distance.

Activation of muscarinic receptors by non-neuronal acetylcholine

The biological role of acetylcholine and the cholinergic system is revisited based particularly on scientific research early and late in the last century. On the one hand, acetylcholine represents the classical neurotransmitter, whereas on the other hand, acetylcholine and the pivotal components of the cholinergic system (high-affinity choline uptake, choline acetyltransferase and its end product acetylcholine, muscarinic and nicotinic receptors and esterase) are expressed by more or less all mammalian cells, i.e. by the majority of cells not innervated by neurons at all. Moreover, it has been demonstrated that acetylcholine and "cholinergic receptors" are expressed in non-neuronal organisms such as plants and protists. Acetylcholine is even synthesized by bacteria and algae representing an extremely old signalling molecule on the evolutionary timescale. The following article summarizes examples, in which non-neuronal acetylcholine is released from primitive organisms as well as from mammalian non-neuronal cells and binds to muscarinic receptors to modulate/regulate phenotypic cell functions via auto-/paracrine pathways. The examples demonstrate that non-neuronal acetylcholine and the non-neuronal cholinergic system are vital for various types of cells such as epithelial, endothelial and immune cells.

Temperature effects on morphological integrity and Ca²⁺ signaling in freshly isolated murine feed artery endothelial cell tubes

To study Ca(2+) signaling in the endothelium of murine feed arteries, we determined the in vitro stability of endothelial cell (EC) tubes freshly isolated from abdominal muscle feed arteries of male and female C57BL/6 mice (5-9 mo, 25-35 g). We tested the hypothesis that intracellular Ca(2+) concentration ([Ca(2+)](i)) responses to muscarinic receptor activation would increase with temperature. Intact EC tubes (length: 1-2 mm, width: 65-80 μm) were isolated using gentle enzymatic digestion with trituration to remove smooth muscle cells. A freshly isolated EC tube was secured in a chamber and superfused at 24 (room temperature), 32, or 37°C. Using fura-2 dye, [Ca(2+)](i) was monitored (ratio of fluorescence at 340- to 380-nm wavelength) at rest and in response to bolus doses of ACh (20 nmol to 200 μmol). The morphological integrity of EC tubes was preserved at 24 and 32°C. Based on the Ca(2+) K(d) values we determined for fura-2 (174 nM at 24°C and 146 nM at 32°C), resting [Ca(2+)](i) remained stable for 180 min at both 24 and 32°C (27 ± 4 and 34 ± 2 nM, respectively), with peak responses to ACh (20 μmol) increasing from ∼220 nM at 24°C to ∼500 nM at 32°C (P < 0.05). There was no difference in responses to ACh between EC tubes from male versus female mice. When EC tubes were maintained at 37°C (typical in vivo temperature), resting [Ca(2+)](i) increased by ∼30% within 15 min, and gaps formed between individual ECs as they retracted and extruded dye, precluding further study. We conclude that EC tubes enable Ca(2+) signaling to be evaluated in the freshly isolated endothelium of murine feed arteries. While Ca(2+) responses are enhanced by approximately twofold at 32 versus 24°C, the instability of EC tubes at 37°C precludes their study at typical body temperature.

The mechanism of agonist induced Ca2+ signalling in intact endothelial cells studied confocally in in situ arteries

In endothelial cells there remain uncertainties in the details of how Ca²⁺ signals are generated and maintained, especially in intact preparations. In particular the role of the sarco-endoplasmic reticulum Ca²⁺-ATPase (SERCA), in contributing to the components of agonist-induced signals is unclear.

The aim of this work was to increase understanding of the detailed mechanism of Ca²⁺ signalling in endothelial cells using real time confocal imaging of Fluo-4 loaded intact rat tail arteries in response to muscarinic stimulation. In particular we have focused on the role of SERCA, and its interplay with capacitative Ca²⁺ entry (CCE) and ER Ca²⁺ release and uptake. We have determined its contribution to the Ca²⁺ signal and how it varies with different physiological stimuli, including single and repeated carbachol applications and brief and prolonged exposures.

In agreement with previous work, carbachol stimulated a rise in intracellular Ca²⁺ in the endothelial cells, consisting of a rapid initial phase, then a plateau upon which oscillations of Ca²⁺ were superimposed, followed by a decline to basal Ca²⁺ levels upon carbachol removal. Our data support the following conclusions: (i) the size (amplitude and duration) of the Ca²⁺ spike and early oscillations are limited by SERCA activity, thus both are increased if SERCA is inhibited. (ii) SERCA activity is such that brief applications of carbachol do not trigger CCE, presumably because the fall in luminal Ca²⁺ is not sufficient to trigger it. However, longer applications sufficient to deplete the ER or even partial SERCA inhibition stimulate CCE. (iii) Ca²⁺ entry occurs via STIM-mediated CCE and SERCA contributes to the cessation of CCE. In conclusion our data show how SERCA function is crucial to shaping endothelial cell Ca signals and its dynamic interplay with both CCE and ER Ca releases.

Exercise training modulates the effects of lipoproteins on acetylcholine-induced endothelial calcium signaling in rat aortas

Exercise training improves vascular endothelial functions, while oxidized low-density lipoproteins (oxLDLs) impede them. We proposed that exercise training might influence the endothelial sensitivity to lipoprotein-induced vascular changes. Male Wistar rats either exercised on a leveled treadmill for 8 weeks or remained sedentary as the control. The endothelial intracellular calcium level (EC [Ca(2+)](i)) in vitro was examined using dissected aortic segments treated with different lipoproteins, including native low-density lipoprotein (nLDL), various oxLDLs, and high-density lipoprotein (HDL). Our results indicated that i) none of the various lipoproteins directly evoked EC [Ca(2+)](i) elevation; ii) the acetylcholine-evoked EC [Ca(2+)](i) elevation in the control group was increased by nLDL and progressively suppressed by oxLDLs with increasing degrees of oxidation; iii) exercise training ameliorated the oxLDL-induced suppressive effects on acetylcholine-evoked EC [Ca(2+)](i) elevation; iv) HDL potentiated the acetylcholine-evoked EC [Ca(2+)](i) elevation in vessel segments from exercised rats but not those from control rats; and v) when HDL was present, the suppressive effects of extensively modified oxLDLs were reduced. Furthermore, comparing with the effects of various lipoproteins on EC calcium signaling, the lipoprotein effects on endothelium-dependent vasorelaxing response appeared to be similar but less pronounced. Taken together, one of the beneficial effects of exercise training on vascular functions might be to make blood vessels more resistant to oxLDLs and more sensitive to HDL.

Muscarinic receptor knockout mice confirm involvement of M3 receptor in endothelium-dependent vasodilatation in mouse arteries

The aim of the study was to determine which cholinergic muscarinic receptor subtype is responsible for the endothelium-dependent vasodilatation evoked by acetylcholine (ACh) in mouse arteries. Endothelium-dependent relaxations were evaluated using isometric tension measurement of ring from femoral and aortic artery of M1, M2, and M3 knockout (KO) mice. Rings of femoral and aortic artery from M3 KO mice did not exhibit relaxation at the opposite of rings from M1+M2 KO and wild-type (WT) mice, which were relaxed by ACh. The proportion of endothelial cells responsive to ACh, as manifested by an increase in cytosolic free calcium ([Ca]i), was also observed on the intima of aorta wall in vitro by using laser line confocal microscopy. Of the cells from M3 KO mice and M1+M2 KO mice, 4% and 23%, respectively, responded to ACh in comparison with 20 % in WT mice. These results show that in the endothelium from femoral and aortic artery, the larger proportion of cells that express M3 receptor is responsible for the specificity of the M3 receptor subtype for endothelium-dependent relaxation caused by ACh.

Acetylcholine beyond neurons: the non-neuronal cholinergic system in humans

Animal life is controlled by neurons and in this setting cholinergic neurons play an important role. Cholinergic neurons release ACh, which via nicotinic and muscarinic receptors (n- and mAChRs) mediate chemical neurotransmission, a highly integrative process. Thus, the organism responds to external and internal stimuli to maintain and optimize survival and mood. Blockade of cholinergic neurotransmission is followed by immediate death. However, cholinergic communication has been established from the beginning of life in primitive organisms such as bacteria, algae, protozoa, sponge and primitive plants and fungi, irrespective of neurons. Tubocurarine- and atropine-sensitive effects are observed in plants indicating functional significance. All components of the cholinergic system (ChAT, ACh, n- and mAChRs, high-affinity choline uptake, esterase) have been demonstrated in mammalian non-neuronal cells, including those of humans. Embryonic stem cells (mice), epithelial, endothelial and immune cells synthesize ACh, which via differently expressed patterns of n- and mAChRs modulates cell activities to respond to internal or external stimuli. This helps to maintain and optimize cell function, such as proliferation, differentiation, formation of a physical barrier, migration, and ion and water movements. Blockade of n- and mACHRs on non-innervated cells causes cellular dysfunction and/or cell death. Thus, cholinergic signalling in non-neuronal cells is comparable to cholinergic neurotransmission. Dysfunction of the non-neuronal cholinergic system is involved in the pathogenesis of diseases. Alterations have been detected in inflammatory processes and a pathobiologic role of non-neuronal ACh in different diseases is discussed. The present article reviews recent findings about the non-neuronal cholinergic system in humans.

Basic and clinical aspects of non-neuronal acetylcholine: overview of non-neuronal cholinergic systems and their biological significance

Acetylcholine (ACh) is a phylogenetically ancient molecule involved in cell-to-cell signaling in almost all life-forms on earth. Cholinergic components, including ACh, choline acetyltransferase, acetylcholinesterase, and muscarinic and nicotinic ACh receptors (mAChRs and nAChRs, respectively) have been identified in numerous non-neuronal cells and tissues, including keratinocytes, cancer cells, immune cells, urinary bladder, airway epithelial cells, vascular endothelial cells, and reproductive organs, among many others. Stimulation of the mAChRs and nAChRs elicits cell-specific functional and biochemical effects. These findings support the notion that non-neuronal cholinergic systems are expressed in certain cells and tissues and are involved in the regulation of their function and that cholinergic dysfunction is related to the pathophysiology of certain diseases. They also provide clues for development of drugs with novel mechanisms of action.

Gene expression changes of prostanoid synthases in endothelial cells and prostanoid receptors in vascular smooth muscle cells caused by aging and hypertension

The present study was designed to assess whether or not changes in genomic expression of cyclooxygenases (COX-1, COX-2), endothelial nitric oxide synthase (eNOS), and prostanoid synthases in the endothelium and of prostanoid receptors in vascular smooth muscle contribute to the occurrence of endothelium-dependent contractions during aging and hypertension. Gene expression was quantified by real-time PCR using isolated endothelial cells and smooth muscle cells (SMC) from the aorta of Wistar-Kyoto and spontaneously hypertensive rats. Genes for all known prostanoid synthases and receptors were present in endothelial cells and SMC, respectively. Aging caused overexpression of eNOS, COX-1, COX-2, thromboxane synthase, hematopoietic-type prostaglandin D synthase, membrane prostaglandin E synthase-2, and prostaglandin F synthase in endothelial cells and COX-1 and prostaglandin E2 (EP)4 receptors in SMC. Hypertension augmented the expression of COX-1, prostacyclin synthase, thromboxane synthase, and hematopoietic-type prostaglandin D synthase in endothelial cells and prostaglandin D2 (DP), EP3, and EP4 receptors in SMC. The increase in genomic expression of endothelial COX-1 explains why in aging and hypertension the endothelium has greater propensity to release cyclooxygenase-derived vasoconstrictive prostanoids. The expression of prostacyclin synthase was by far the most abundant, explaining why the majority of the COX-1-derived endoperoxides are transformed into prostacyclin, substantiating the role of prostacyclin as an endothelium-derived contracting factor. The expression of thromboxane synthase was increased in the cells of aging or hypertensive rats, explaining why the prostanoid can contribute to endothelium-dependent contractions. It is uncertain whether the gene modifications caused by aging and hypertension directly contribute to endothelium-dependent contractions or rather to vascular aging and the vascular complications of the hypertensive process.

Effects of increased intracellular Cl- concentration on membrane responses to acetylcholine in the isolated endothelium of guinea pig mesenteric arteries

ACh-induced membrane responses in vascular endothelial cells that have been reported vary between preparations from a sustained hyperpolarization to a transient hyperpolarization followed by a depolarization; the reason for this variation is unknown. Using the perforated whole-cell clamp technique, we investigated ACh-induced membrane currents in freshly isolated endothelial layers having a resting membrane potential of less negative than -10 mV. A group of cells was electrically isolated using a wide-bore micropipette, and their membrane potential was well controlled. ACh activated K(+) and Cl(-) currents simultaneously. The K(+) current was blocked by a combination of charybdotoxin and apamin and appears to result from the opening of IK(Ca) and SK(Ca) channels. The Cl(-) current was partially blocked by tamoxifen, niflumic acid, or DIDS and appears to be produced by Ca(2+)-activated Cl(-) channels. When the pipettes contained 20 mM Cl(-), the ACh-induced K(+) conductance started decreasing during a 1-min application of ACh while the Cl(-) conductance continued, making the ACh-induced hyperpolarization sustained. When the pipettes contained 150 mM Cl(-), both conductances started decreasing during a 1-min application of ACh, making the ACh-induced hyperpolarization small and transient. [Cl(-)](i) is very likely modified by experimental procedures such as the cell isolation and the intracellular dialysis with the pipette solution. Such a variability in [Cl(-)](i) may be one of the reasons for the variations in the ACh-induced membrane response.

A simple method for rapid separation of endothelial and smooth muscle mRNA reveals Na/K+ -ATPase alpha-subunit distribution in rat arteries

BACKGROUND/AIMS

The endothelium has been recognized as a key component in the regulation of blood vessels. We designed a simple procedure to separate endothelial and smooth muscle RNA from rat aorta and mesenteric artery and used this method to establish the distribution of Na(+)/K(+)-ATPase alpha-subunit isoforms (NaKalpha1, NaKalpha2 and NaKalpha3) within the arterial wall.

METHODS

Rat aorta was perfused with Tripure, a reagent for RNA isolation, yielding 3 successive RNA fractions (E1-E3) and the remaining tissular RNA (Ao[E-]). A similar procedure was applied to the mesenteric artery. Gene expression was studied by semiquantitative reverse-transcription polymerase chain reaction.

RESULTS

Compared to unperfused aorta (Ao[E+]), typical endothelial mRNAs were enriched 3- to 5-fold in E1-E3 but almost absent in Ao[E-], whereas smooth muscle mRNAs were low in E1-E3 but similarly expressed in Ao[E-] and Ao[E+]. NaKalpha1 was uniformly expressed in all fractions, NaKalpha2 closely followed the expression pattern of smooth muscle markers and NaKalpha3 expression was weak and attributable to blood contamination. Comparable results were obtained with the mesenteric artery.

CONCLUSION

We conclude that, in aorta and mesenteric artery, Tripure perfusion allows for a rapid and reliable separation of endothelial mRNA from smooth muscle mRNA, and that endothelium only expresses NaKalpha1, whereas smooth muscle expresses NaKalpha1 and NaKalpha2, but not NaKalpha3.

Effect of short-term organoid culture on the pharmaco-mechanical properties of rat extra- and intrapulmonary arteries

1 Organoid cultured explants from differentiated tissues have gained renewed interest in the undertaking of physiological and pharmacological studies. In the work herein, we examined the pharmaco-mechanical properties of an in vitro model consisting of organoid cultured rings derived from rat extra- and intrapulmonary arteries, over a period of 4 days in culture. 2 Mechanical changes were quantified using isometric tension measurements on both fresh and cultured pulmonary arterial tissues, with experiments performed in the presence or absence of 10% foetal calf serum. Conventional histochemical and immunofluorescent stainings were also performed to assess tissue structure integrity and apoptosis. 3 The explants developed spontaneous rhythmic contractions (SRC) in approximately half of the vessels. SRC amplitude and time course were modified by conditions and agents acting on membrane potential (high-potassium solutions--levcromakalim, a potassium channel opener), while nitrendipine, an L-type calcium channel blocker, suppressed SRC. 4 Cultured explants also developed a hyper-reactivity to high potassium challenges (10-40 mM). Whereas contraction to serotonin (5-HT) was enhanced in intrapulmonary arteries, contraction to endothelin-1 remained unchanged after 4 days of culture. Serum did not alter contractile properties during the culture period. 5 Endothelial-dependent relaxation was maintained in response to A23187 500 microM, but was abolished in response to 10 microM carbamylcholine. 6 Histological and immuno-histological analyses revealed the absence of hypertrophied vascular wall or apoptosis. 7 In conclusion, the contractile phenotype as well as tissue structure integrity of organoid explants remain essentially intact during short-term culture, making this model suitable for pharmaco-genomic studies.

Overexpression ofTRPC1enhances pulmonary vasoconstriction induced by capacitative Ca2+entry

Transient receptor potential (TRP) cation channels are a critical pathway for Ca2+entry during pulmonary artery (PA) smooth muscle contraction. However, whether canonical TRP (TRPC) subunits and which TRP channel isoforms are involved in store depletion-induced pulmonary vasoconstriction in vivo remain unclear. This study was designed to test whether overexpression of the human TRPC1 gene ( hTRPC1) in rat PA enhances pulmonary vasoconstriction due to store depletion-mediated Ca2+influx. The hTRPC1 was infected into rat PA rings with an adenoviral vector. RT-PCR and Western blot analyses confirmed the mRNA and protein expression of hTRPC1 in the arterial rings. The amplitude of active tension induced by 40 mM K+(40K) in PA rings infected with an empty adenoviral vector (647 ± 88 mg/mg) was similar to that in PA rings infected with hTRPC1 (703 ± 123 mg/mg, P = 0.3). However, the active tension due to capacitative Ca2+entry (CCE) induced by cyclopiazonic acid was significantly enhanced in PA rings overexpressing hTRPC1 (91 ± 13% of 40K-induced contraction) compared with rings infected with an empty adenoviral vector (61 ± 14%, P < 0.001). Endothelial expression of hTRPC1 was not involved since the CCE-induced vasoconstriction was also enhanced in endothelium-denuded PA rings infected with the adenoviral vector carrying hTRPC1. These observations demonstrate that hTRPC1 is an important Ca2+-permeable channel that mediates pulmonary vasoconstriction when PA smooth muscle cell intracellular Ca2+stores are depleted.

Modulation of Ca2+-activated K+ channel in renal artery endothelium in situ by nitric oxide and reactive oxygen species

BACKGROUND

Endothelium-derived nitric oxide and reactive oxygen species (ROS) have been proposed to regulate vascular tone by complex mechanisms, including the modulation of ion channel function. In endothelial function itself, activation of Ca2+-activated K+ channels (KCa) plays a crucial role by inducing hyperpolarization, which promotes membrane potential-driven Ca2+ influx and Ca2+-dependent synthesis of vasodilatory factors. In the present study, we tested whether nitric oxide and ROS modulate endothelial KCa function.

METHODS

By employing the patch-clamp technique in endothelium of porcine renal arteries in situ, we identified a large-conductance Ca2+-activated K+ channel (big K+ channel, BKCa) with a conductance of 297 +/- 6 pS.

RESULTS

Channel activity was strongly controlled by the membrane potential and the cytosolic Ca2+ concentration (EC50 3.1 +/- 0.5 micromol/L Ca2+ at 0 mV). Channel activity was inhibited by Ba2+ and iberiotoxin. At submicromolar [Ca2+]i, nitric oxide induced a dose-dependent stimulation of BKCa activity with a 10-fold increase at the highest dose tested (1 micromol/L). A similar stimulation was achieved by the nitric oxide donors, sodium nitroprusside (SNP), and diethylamine nitric oxide complex (DEA-NO). In contrast, ROS and, in particular, hydrogen peroxide (H2O2) led to dose-dependent inactivation of BKCa with an IC50 of 80 +/- 6 nmol/L and 1.1 +/- 0.4 micromol/L, respectively. In isolated porcine renal arteries, bradykinin-induced vasodilation was significantly reduced by either iberiotoxin or H2O2.

CONCLUSION

Direct stimulation of endothelial BKCa by nitric oxide might represent a novel mechanism of autocrine regulation of endothelial function and points to a positive feedback mechanism by promoting hyperpolarization and nitric oxide production itself. The ROS-induced inhibition of BKCa could be part of the cellular mechanisms by which ROS impairs endothelium-dependent vasodilation.

Hypoxia activates a background conductance in freshly isolated pulmonary arterial endothelial cells of the rat

Utilising the patch-clamp recording technique we have demonstrated for the first time the effects of hypoxia on the background current in pulmonary arterial endothelial cells. Electrophysiological studies revealed the presence of a novel oxygen-sensitive, non-selective cation conductance (I(NSC)) in these cells. The inward component of I(NSC) was significantly potentiated by hypoxia. Both the inward and outward components of I(NSC) were inhibited by both La(3+) and Gd(3+). Hypoxic activation of I(NSC) may provide an important Ca(2+) influx pathway essential for the release of a pulmonary-selective vasoconstrictor pivotal to the sustained phase of hypoxic pulmonary vasoconstriction.

High-salt diet depresses acetylcholine reactivity proximal to NOS activation in cerebral arteries

Rats were fed a low-salt (LS; 0.4% NaCl) or high-salt (HS; 4.0% NaCl) diet for 3 days, and the responses of isolated cerebral arteries to acetylcholine (ACh), the nitric oxide (NO)-dependent dilator bradykinin, and the NO donor 6-(2-hydroxy-1-methyl-2-nitrosohydrazino)-N-methyl-1-hex-anamine (NOC-9) were determined. ACh-induced vasodilation and NO release, assessed with the fluorescent NO indicator 4,5-diaminofluorescein (DAF-2) diacetate, were eliminated with the HS diet. Inhibition of cyclooxygenase, cytochrome P-450 epoxygenase, and acetylcholinesterase did not alter ACh responses. Bradykinin and NOC-9 caused a similar dilation in cerebral arteries of all groups. Arteries from animals on LS or HS diets exhibited similar levels of basal superoxide (O(2)(-)) production, assessed by dihydroethidine fluorescence, and ACh responses were unaffected by O(2)(-) scavengers. Muscarinic type 3 receptor expression was unaffected by dietary salt intake. These results indicate that 1) a HS diet attenuates ACh reactivity in cerebral arteries by inhibiting NO release, 2) this attenuation is not due to production of a cyclooxygenase-derived vasoconstrictor or elevated O(2)(-) levels, and 3) alteration(s) in ACh signaling are located upstream from NO synthase.

Ophidian envenomation strategies and the role of purines

Snake envenomation employs three well integrated strategies: prey immobilization via hypotension, prey immobilization via paralysis, and prey digestion. Purines (adenosine, guanosine and inosine) evidently play a central role in the envenomation strategies of most advanced snakes. Purines constitute the perfect multifunctional toxins, participating simultaneously in all three envenomation strategies. Because they are endogenous regulatory compounds in all vertebrates, it is impossible for any prey organism to develop resistance to them. Purine generation from endogenous precursors in the prey explains the presence of many hitherto unexplained enzyme activities in snake venoms: 5'-nucleotidase, endonucleases (including ribonuclease), phosphodiesterase, ATPase, ADPase, phosphomonoesterase, and NADase. Phospholipases A(2), cytotoxins, myotoxins, and heparinase also participate in purine liberation, in addition to their better known functions. Adenosine contributes to prey immobilization by activation of neuronal adenosine A(1) receptors, suppressing acetylcholine release from motor neurons and excitatory neurotransmitters from central sites. It also exacerbates venom-induced hypotension by activating A(2) receptors in the vasculature. Adenosine and inosine both activate mast cell A(3) receptors, liberating vasoactive substances and increasing vascular permeability. Guanosine probably contributes to hypotension, by augmenting vascular endothelial cGMP levels via an unknown mechanism. Novel functions are suggested for toxins that act upon blood coagulation factors, including nitric oxide production, using the prey's carboxypeptidases. Leucine aminopeptidase may link venom hemorrhagic metalloproteases and endogenous chymotrypsin-like proteases with venom L-amino acid oxidase (LAO), accelerating the latter. The primary function of LAO is probably to promote prey hypotension by activating soluble guanylate cyclase in the presence of superoxide dismutase. LAO's apoptotic activity, too slow to be relevant to prey capture, is undoubtedly secondary and probably serves principally a digestive function. It is concluded that the principal function of L-type Ca(2+) channel antagonists and muscarinic toxins, in Dendroaspis venoms, and acetylcholinesterase in other elapid venoms, is to promote hypotension. Venom dipeptidyl peptidase IV-like enzymes probably also contribute to hypotension by destroying vasoconstrictive peptides such as Peptide YY, neuropeptide Y and substance P. Purines apparently bind to other toxins which then serve as molecular chaperones to deposit the bound purines at specific subsets of purine receptors. The assignment of pharmacological activities such as transient neurotransmitter suppression, histamine release and antinociception, to a variety of proteinaceous toxins, is probably erroneous. Such effects are probably due instead to purines bound to these toxins, and/or to free venom purines.

Acute exercise enhances vasorelaxation by modulating endothelial calcium signaling in rat aortas

The role of endothelial calcium signaling in exercise-enhanced ACh-induced vasorelaxation was examined using male Wistar rats (8~10 wk old) that were divided into control and exercise groups. The exercised animals ran on a treadmill with progressive increments of speed until exhaustion. After decapitation, aortic rings were dissected and loaded with fura 2-AM. After being mounted on a tissue flow chamber, vessels were precontracted with phenylephrine, and ACh-induced endothelial calcium elevation and vasorelaxation were determined simultaneously under an epifluorescence microscope equipped with ratio imaging capability. Our results showed that 1) there was logarithmic correlation between endothelial calcium elevation and vasorelaxation; 2) acute exercise enhanced ACh-induced endothelial calcium elevation and vasorelaxation without altering their relationship; 3) pretreatment with N(omega)-nitro-L-arginine markedly reduced ACh-induced vasorelaxation in both groups but suppressed the calcium response only in the exercise group; and 4) the exercise effect on endothelial calcium elevation was abolished by Ca2+-free buffer or gadolinium. In conclusion, acute exercise increases ACh-induced vasorelaxation by increasing the endothelial calcium influx and the calcium-dependent nitric oxide release.

Ion channels and their functional role in vascular endothelium

Endothelial cells (EC) form a unique signal-transducing surface in the vascular system. The abundance of ion channels in the plasma membrane of these nonexcitable cells has raised questions about their functional role. This review presents evidence for the involvement of ion channels in endothelial cell functions controlled by intracellular Ca(2+) signals, such as the production and release of many vasoactive factors, e.g., nitric oxide and PGI(2). In addition, ion channels may be involved in the regulation of the traffic of macromolecules by endocytosis, transcytosis, the biosynthetic-secretory pathway, and exocytosis, e.g., tissue factor pathway inhibitor, von Willebrand factor, and tissue plasminogen activator. Ion channels are also involved in controlling intercellular permeability, EC proliferation, and angiogenesis. These functions are supported or triggered via ion channels, which either provide Ca(2+)-entry pathways or stabilize the driving force for Ca(2+) influx through these pathways. These Ca(2+)-entry pathways comprise agonist-activated nonselective Ca(2+)-permeable cation channels, cyclic nucleotide-activated nonselective cation channels, and store-operated Ca(2+) channels or capacitative Ca(2+) entry. At least some of these channels appear to be expressed by genes of the trp family. The driving force for Ca(2+) entry is mainly controlled by large-conductance Ca(2+)-dependent BK(Ca) channels (slo), inwardly rectifying K(+) channels (Kir2.1), and at least two types of Cl( -) channels, i.e., the Ca(2+)-activated Cl(-) channel and the housekeeping, volume-regulated anion channel (VRAC). In addition to their essential function in Ca(2+) signaling, VRAC channels are multifunctional, operate as a transport pathway for amino acids and organic osmolytes, and are possibly involved in endothelial cell proliferation and angiogenesis. Finally, we have also highlighted the role of ion channels as mechanosensors in EC. Plasmalemmal ion channels may signal rapid changes in hemodynamic forces, such as shear stress and biaxial tensile stress, but also changes in cell shape and cell volume to the cytoskeleton and the intracellular machinery for metabolite traffic and gene expression.

Heterogeneity in hand veins responses to acetylcholine is not associated with polymorphisms in the G-protein beta3-subunit (C825T) and endothelial nitric oxide synthase (G894T) genes but with serum low density lipoprotein cholesterol

Vascular responses to acetylcholine (ACh) are notoriously variable, the reason for this phenomenon is unknown. We tested the hypothesis that the variability in venous response to acetylcholine may be associated with two recently identified genetic polymorphisms for proteins involved in the signal transduction pathway, i.e. the G-protein beta3-subunit (GNB3) and endothelial nitric oxide synthase (eNOS). The dorsal hand vein technique was used in 37 healthy subjects. Hand veins were preconstricted with the alpha1-adrenoceptor agonist phenylephrine and the venodilator response to local ACh infusion was measured with and without comedication of acetylsalicylic acid or co-infusion of N(G)-monomethyl-L-arginine (L-NMMA). In addition, all subjects received routine laboratory tests and 26 of them were genotyped for the C825T polymorphism of the GNB3 gene and for the G894T polymorphism of the eNOS gene. A striking variability in venous response to ACh was found with dilation observed in the low ACh concentration range and reduced dilation or even constriction at high concentrations. ACh-induced venodilation was mediated by muscarinic receptors and abolished in the presence of both acetylsalicylic acid and L-NMMA suggesting dependence on endothelium. We did not find any association of the variability in ACh response with GNB3 or eNOS allele status. On the other hand, a significant positive correlation between ACh responsiveness and low density lipoprotein-cholesterol status was detected. Two recently discovered gene polymorphisms are not responsible for the profound heterogeneity in venodilator response to ACh. Surprisingly, this variability appears to relate to the lipid status of the subjects. The exact nature of this new finding requires further study.

Acetylcholine increases the free intracellular calcium concentration in podocytes in intact rat glomeruli via muscarinic M(5) receptors

The effects of acetylcholine (ACh) on the free intracellular calcium concentration ([Ca2+](i)) of microdissected glomeruli were investigated using fura-2 fluorescence digital imaging and two-photon confocal microscopy. ACh caused a concentration-dependent [Ca2+](i) increases with an initial peak followed by a sustained plateau, which was suppressed by reduced extracellular Ca2+ concentrations. The [Ca2+](i) plateau was not affected by the L-type Ca2+ channel blocker nicardipine, whereas gadolinium and lanthanum (both at 1 microM) blocked the plateau. Diphenylacetoxy-N:-methylpiperidine methiodide (100 nM), an M(3)/M(5) receptor antagonist, and pirenzepine (1 microM), an M(1) receptor antagonist, completely inhibited the effect of ACh. [Ca2+](i) measurements using two-photon excitation of fluo-3 and staining of the cells with calcein/acetoxymethyl ester, for observation of the capillary network together with the glomerular cells, showed that [Ca2+](i) was increased in single podocytes. Immunohistochemical studies did not demonstrate M(3) receptor expression in glomerular cells. M(1) receptors could be detected only in the parietal sheet of Bowman's capsule, whereas M(5) receptors were found only in podocytes. The data show that ACh increases [Ca2+](i) in podocytes of intact glomeruli, most likely via muscarinic M(5) receptors.

Invited review: effects of flow on vascular endothelial intracellular calcium signaling of rat aortas ex vivo

To study the effects of flow on in situ endothelial intracellular calcium concentration ([Ca(2+)](i)) signaling, rat aortic rings were loaded with fura 2, mounted on a tissue flow chamber, and divided into control and flow-pretreated groups. The latter was perfused with buffer at a shear stress of 50 dyns/cm(2) for 1 h. Endothelial [Ca(2+)](i) responses to ACh or shear stresses were determined by ratio image analysis. Moreover, ACh-induced [Ca(2+)](i) elevation responses were measured in a calcium-free buffer, or in the presence of SKF-96365, to elucidate the role of calcium influx in the flow effects. Our results showed that 1) ACh increased endothelial [Ca(2+)](i) in a dose-dependent manner, and these responses were incremented by flow-pretreatment; 2) the differences in ACh-induced [Ca(2+)](i) elevation between control and flow-pretreated groups were abolished by SKF-96365 or by Ca(2+)-free buffer; and 3) in the presence of 10(-5) M ATP, shear stress induced dose-dependent [Ca(2+)](i) elevation responses that were not altered by flow-pretreatment. In conclusion, flow-pretreatment augments the ACh-induced endothelial calcium influx in rat aortas ex vivo.

Effects of chronic exercise on calcium signaling in rat vascular endothelium

Chronic exercise enhances endothelium-dependent vasodilating responses. To investigate whether this is due to a change in endothelial Ca(2+) signaling, we examined intracellular Ca(2+) concentration ([Ca(2+)](i)) level in rat aortic endothelium in response to acetylcholine (ACh) or ATP. Four-week-old male Wistar rats were divided into control and exercise groups. The exercised animals ran on a treadmill at a moderate intensity for 60 min/day, 5 day/wk, for 10 wk. Rat aortas were then excised and loaded with fura 2. After the aortas were mounted on a flow chamber, these specimens were observed under an epifluorescence microscope equipped with ratio-imaging capability. Our results showed that 1) chronic exercise increased both ACh- and ATP-induced [Ca(2+)](i) responses; 2) ACh induced heterogeneous [Ca(2+)](i) elevation in individual endothelial cells; and 3) the exercise effect on ACh-evoked endothelial [Ca(2+)](i) elevation was inhibited by the Ca(2+) influx blocker SKF-96365, by a Ca(2+)-free buffer, or by high concentrations of extracellular K(+). We conclude that chronic exercise increases ACh-induced [Ca(2+)](i) elevation in rat aortic endothelium in situ, possibly by facilitating Ca(2+) influx.

Heterogeneity of [Ca(2+)](i) signaling in intact rat aortic endothelium

Most existing knowledge about [Ca(2+)](i) signaling in vascular endothelium has been based on studies using endothelial cells cultured in vitro. To examine how endothelial cells behave in situ, we have developed a method to monitor single-cell [Ca(2+)](i) from Fura-2-loaded rat aortic segments. Fluorescence ratio images from large numbers of endothelial cells were acquired by using a flow chamber mounted on a dual-wavelength fluorescence microscope. Our results showed that either acetylcholine or histamine reversibly activated the vascular endothelium by eliciting M(3) or H(1) receptor-mediated [Ca(2+)](i) increases, respectively. The acetylcholine-evoked endothelial [Ca(2+)](i) elevation at the branch site (intercostal orifice) was much more pronounced than that at the non-branch area. However, endothelium at the branch site was relatively insensitive to histamine. Both acetylcholine-sensitive and histamine-sensitive endothelial cells were arranged in belts aligned along flow lines and were intercalated with each other. Data analyzed from 400 endothelial cells located at the non-branch site showed drastically heterogeneous [Ca(2+)](i) responses to a fixed concentration of either acetylcholine or histamine, differing by two orders of magnitude in individual cells. As a conclusion, vascular endothelial cells appear to have their own characteristic [Ca(2+)](i) 'fingerprint' to various agonists and they may function coordinately in situ.

Expression of muscarinic receptor genes in the human coronary artery

The authors investigated the role of muscarinic receptors in functional control of coronary arteries affected by intimal thickening due to arteriosclerosis. They first examined the genetic subtypes of muscarinic receptors expressed in human coronary arteries. Twelve samples of human coronary artery, obtained by autopsy from eight subjects, were examined for the expression of four genetic subtypes of muscarinic receptor, m1 to m4, by reverse transcription and polymerase chain reaction (RT-PCR). Two subtypes, m2 and m3, were found to be expressed in the coronary artery. The m2 gene was expressed in seven of the 12 vessels, and m3 in eight of the 12. Expression of both m2 and m3 genes was observed in five of the 12 vessels. Neither the m1 nor m4 was expressed in these samples. These results indicate that the m2 and m3 genes are mainly expressed in the coronary arteries and suggest that these patterns of expression are differentially controlled to induce the diversity of contraction/relaxation reactions induced in the coronary arteries by acetylcholine.

Organoid culture of cannulated rat resistance arteries: effect of serum factors on vasoactivity and remodeling

We developed an organoid culture technique to study the mechanisms involved in arterial remodeling. Resistance arteries were isolated from rat cremaster muscle and mounted in a pressure myograph at 75 mmHg. Vessels were studied during a 4-day culture period in DMEM with either 2% albumin, 10% heat-inactivated FCS (HI-FCS) or 10% dialyzed HI-FCS (12 kDa cut off) added to the perfusate. The albumin group showed a gradual loss of endothelial function and integrity, whereas smooth muscle agonist and myogenic responses were retained. No remodeling was observed. Vessels cultured in the presence of serum showed a progressive constriction. Smooth muscle responses and substance P-induced endothelium-dependent dilation were maintained. An inward remodeling of 17 +/- 4% in the HI-FCS group and 26 +/- 3% in the dialyzed HI-FCS group was found, while media cross-sectional areas were unchanged. These data show that pressurized resistance arteries can be maintained in culture for several days and undergo eutrophic remodeling in vitro in the presence of high molecular weight serum factors.

Functional acetylcholine muscarinic receptor subtypes in human brain microcirculation: identification and cellular localization

Acetylcholine is an important regulator of local cerebral blood flow. There is, however, limited information available on the possible sites of action of this neurotransmitter on brain intraparenchymal microvessels. In this study, a combination of molecular and functional approaches was used to identify which of the five muscarinic acetylcholine receptors (mAChR) are present in human brain microvessels and their intimately associated astroglial cells. Microvessel and capillary fractions isolated from human cerebral cortex were found by reverse transcriptase-polymerase chain reaction to express m2, m3, and, occasionally, m1 and m5 receptor subtypes. To localize these receptors to a specific cellular compartment of the vessel wall, cultures of human brain microvascular endothelial and smooth muscle cells were used, together with cultured human brain astrocytes. Endothelial cells invariably expressed m2 and m5 receptors, and occasionally the m1 receptor; smooth muscle cells exhibited messages for all except the m4 mAChR subtypes, whereas messages for all five muscarinic receptors were identified in astrocytes. In all three cell types studied, acetylcholine induced a pirenzepine-sensitive increase (62% to 176%, P<0.05 to 0.01) in inositol trisphosphate, suggesting functional coupling of m1, m3, or m5 mAChR to a phospholipase C signaling cascade. Similarly, coupling of m2 or m4 mAChR to adenylate cyclase inhibition in endothelial cells and astrocytes, but not in smooth muscle cells, was demonstrated by the ability of carbachol to significantly reduce (44% to 50%, P<0.05 to 0.01) the forskolin-stimulated increase in cAMP levels. This effect was reversed by the mAChR antagonist AFDX 384. The results indicate that microvessels are able to respond to neurally released acetylcholine and that mAChR, distributed in different vascular and astroglial compartments, could regulate cortical perfusion and, possibly, blood-brain barrier permeability, functions that could become jeopardized in neurodegenerative disorders such as Alzheimer's disease.

Voltage-dependent K(+) current in capillary endothelial cells isolated from guinea pig heart

Capillary fragments were isolated from guinea pig hearts, and their electrical properties were studied using the perforated-patch and cell-attached mode of the patch-clamp technique. A voltage-dependent K(+) current was discovered that was activated at potentials positive to -20 mV and showed a sigmoid rising phase. For depolarizing voltage steps from -128 to +52 mV, the time to peak was 71 +/- 5 ms (mean +/- SE) and the amplitude of the current was 3.7 +/- 0.5 pA/pF in the presence of 5 mM external K(+). The time course of inactivation was exponential with a time constant of 7.2 +/- 0.5 s at +52 mV. The current was blocked by tetraethylammonium (inhibitory constant approximately 3 mM) but was not affected by charybdotoxin (1 microM) or apamin (1 microM). In the cell-attached mode, depolarization-activated single-channel currents were found that inactivated completely within 30 s; the single-channel conductance was 12.3 +/- 2.4 pS. The depolarization-activated K(+) current described here may play a role in membrane potential oscillations of the endothelium.

Expression of P2Y receptors in cell lines derived from the human lung

1. Northern blotting experiments have been performed with RNA extracted from several cell lines derived from the human lung in order to detect P2Y1, P2Y2, P2Y4 and P2Y6 mRNA. We have investigated the 1HAEo- and 16HBE14o- epithelial cell lines derived from the airway epithelium, the A549 cell line displaying properties of type II alveolar epithelial cells, the CALU-3 serous cells, the 6CFSMEo- submucosal cells and the HASMSC1 airway smooth muscle cells. We have also evaluated one pancreatic epithelial cell line called CFPAC-1. These experiments revealed that P2Y2 and P2Y6 mRNA are co-expressed in the IHAEo-, 16HBE14o- and A549 epithelial cell lines. The CFPAC-1 pancreatic cell line was strongly positive for the P2Y2 receptor. No signal was obtained for the P2Y1 and P2Y4 receptors. 2. We have then performed RT-PCR experiments with specific oligonucleotides of these last two P2Y receptors with the RNA used for the Northern blotting experiments. P2Y4 mRNA was detected in five cell lines: 1HAEo-, 16HBE14o-, 6CFSMEo-, HASMSC1 and CFPAC-1. P2Y1 mRNA was only detected in the CALU-3 cell line. 3. Inositol trisphosphates assays have identified a response typical of the P2Y2 receptor in the 1HAEo- and the 16HBE14o- airway epithelial cell lines which co-express P2Y2 and P2Y6 mRNA. By contrast, the 6CFSMEo- submucosal cells expressed a UTP-specific response which displayed pharmacological characteristics compatible with the human P2Y4 receptor: in particular, there was no response to UDP or ATP and the UTP effect was totally inhibited by pertussis toxin.

Endocardial endothelium mediates luminal ACh-NO signaling in isolated frog heart

ACh exerted a biphasic effect in the in vitro working heart of Rana esculenta. High concentrations (10(-7) M) of ACh depressed stroke volume (SV) and stroke work (SW) by approximately 30% with a shorter systolic phase and reduced peak pressure. Doses from 10(-10) M induced a positive response peaking at 10(-8) M (SV: +8.6%; SW: +6. 5%) and a prolonged systolic phase without affecting peak pressure. Atropine and pirenzepine blocked both the positive and the negative effects of ACh. Pretreatment with Triton X-100 (0.1 ml, 0.05%) or with nitric oxide (NO)-cGMP pathway antagonists (NG-nitro-L-arginine, NG-nitro-L-arginine methyl ester, NG-monomethyl-L-arginine, and 1H-[1,2,4]oxadiazolo-[4,3-a]quinoxalin-1-one) abolished the positive and negative cholinergic effects. Infusion of 8-bromoguanosine 3', 5'-cyclic monophosphate reverted the positive effect of ACh to a negative effect. Milrinone blocked the positive inotropism but did not change the negative cholinergic response. The NO donor 3-morpholinosydnonimine generated a biphasic dose-response curve with a maximum positive effect at 10(-8) M (SV: +8%; SW: +5.6%; systolic phase: +28 ms) and a negative effect at 5 x 10(-8) M (SV and SW: about -12%; systolic phase: -70 ms; peak pressure: -1.50 mm). We conclude that in the avascular frog heart the endocardial endothelium mediates the inotropic effect of luminal cholinergic stimuli via a NO-cGMP pathway.

Muscarinic receptor subtypes in the porcine lung during postnatal development

The responsiveness of the pulmonary circulation to acetylcholine changes in the newborn piglet. Therefore muscarinic receptors have been studied in the developing porcine lung from birth to adulthood using ligand binding, Northern blotting and in situ hybridisation. Maximal binding capacity of [N-methyl-3H] scopolamine and the affinity of the receptor in lung membranes increased between birth and 16 days (p < 0.05). Subtype affinity changed with age, but always M3, > M1 > M2. Northern blots of porcine muscarinic receptor subtypes showed m1, m2 and m3 mRNA present in lung membranes. m2 mRNA was present at all ages and decreased with age. m1 mRNA was absent at birth and m3 mRNA was absent at 3 days. Autoradiographic localisation showed ligand binding to the parenchyma and airway smooth muscle and nerves, there was no binding to intrapulmonary vessels. In situ hybridisation localised mRNA of all three subtypes to the smooth muscle cells of both vessels and airways. Changes in the receptor subtypes may explain the pharmacological changes during postnatal adaptation.

P2Y- and P2U-mediated increases in internal calcium in single bovine aortic endothelial cells in primary culture

Increases in intracellular calcium ([Ca2+]i) to ATP, ADP, AMP, adenosine, UTP, 2-methylthio ATP (2-MeSATP), 2-methylthio ADP (2-MeSADP) and alpha,beta-methylene ATP (alpha,beta-meATP) were investigated in single bovine aortic endothelial cells (BAEC) in primary culture using Indo-1. Evidence was obtained for the presence of P2Y and P2U, but not P2X receptors. Normalized concentration-effect curves for ATP, UTP and 2-MeSATP were biphasic in shape. At 10 nM, the agonist rank order was UTP > ATP approximately 2-MeSATP, while above 1 microM, it was ATP > or = UTP > or = 2-MeSATP. No cross-desensitization between responses to P2U and P2Y receptors was observed in normal external solution. However, when internal Ca2+ stores were depleted by exposure to 2-MeSATP or UTP in Ca2+-free solution and agonists then re-applied in presence of external Ca2+, homologous but not heterologous desensitization was seen. In the same conditions, heterologous desensitization was observed for UTP after ATP but not for ATP after UTP. Taken together, the results are consistent with the coexistence of P2Y and P2U receptors in primary-cultured BAEC and suggest that upon activation, different intracellular signaling pathways could be involved in increasing [Ca2+]i.

Mechanisms of neointima formation--lessons from experimental models

The lamina intima of an artery is the region between the endothelial cell surface and the internal elastic lamina, which forms the luminal margin of the media. In humans the intima of atherosclerosis-prone arteries becomes thicker due to accumulation of smooth muscle cells, which originate from the media. The introduction of percutaneous transluminal coronary angioplasty (PTCA) boosted scientific interest in intimal thickening, because restenosis remains an unresolved problem of this intervention. In order to unravel the mechanisms of intimal thickening there is a need for appropriate animal models. A brief overview of these models is given together with factors that control proliferation and/or migration. Despite intensive research on neointima formation, an effective therapy for restenosis has not emerged to date. This may be due to the fact that other processes, such as acute elastic recoil and chronic constrictive remodeling may contribute to lumen narrowing as well. Other limitations of neointima models are related to species and anatomical differences. Most studies are performed in arteries that are either lesion-free, or contain relatively mild plaques, in contrast to the complicated, stenotic lesions that are the substrate for human PTCA. Other differences are the severity of the injury and incorporation of a mural fibrin-rich thrombus. Nevertheless, studies based on superficial injury, like the frequently used balloon denudation model, are useful. There are similarities with angioplasty, such as endothelial cell damage and proliferation of medial and intimal smooth muscle cells. The use of techniques such as differential display, gene transfer and application of antisense oligonucleotides may provide new therapeutic approaches to reduce neointima formation.