A functional ryanodine-sensitive intracellular Ca2+ store is present in vascular endothelial cells

Loss of Endothelial Annexin A1 Aggravates Inflammation-Induched Vascular Aging

Chronic inflammation is increasingly considered as the most important component of vascular aging, contributing to the progression of age-related cardiovascular diseases. To delay the process of vascular aging, anti-inflammation may be an effective measure. The anti-inflammatory factor annexin A1 (ANXA1) is shown to participate in several age-related diseases; however, its function during vascular aging remains unclear. Here, an ANXA1 knockout (ANXA1-/-) and an endothelial cell-specific ANXA1 deletion mouse (ANXA1△EC) model are used to investigate the role of ANXA1 in vascular aging. ANXA1 depletion exacerbates vascular remodeling and dysfunction while upregulates age- and inflammation-related protein expression. Conversely, Ac2-26 (a mimetic peptide of ANXA1) supplementation reverses this phenomenon. Furthermore, long-term tumor necrosis factor-alpha (TNF-α) induction of human umbilical vein endothelial cells (HUVECs) increases cell senescence. Finally, the senescence-associated secretory phenotype and senescence-related protein expression, rates of senescence-β-galactosidase positivity, cell cycle arrest, cell migration, and tube formation ability are observed in both ANXA1-knockdown HUVECs and overexpressed ANXA1-TNF-α induced senescent HUVECs. They also explore the impact of formyl peptide receptor 2 (a receptor of ANXA1) in an ANXA1 overexpression inflammatory model. These data provide compelling evidence that age-related inflammation in arteries contributes to senescent endothelial cells that promote vascular aging.

Cell-To-Cell Communication in the Resistance Vasculature

The arterial vasculature can be divided into large conduit arteries, intermediate contractile arteries, resistance arteries, arterioles, and capillaries. Resistance arteries and arterioles primarily function to control systemic blood pressure. The resistance arteries are composed of a layer of endothelial cells oriented parallel to the direction of blood flow, which are separated by a matrix layer termed the internal elastic lamina from several layers of smooth muscle cells oriented perpendicular to the direction of blood flow. Cells within the vessel walls communicate in a homocellular and heterocellular fashion to govern luminal diameter, arterial resistance, and blood pressure. At rest, potassium currents govern the basal state of endothelial and smooth muscle cells. Multiple stimuli can elicit rises in intracellular calcium levels in either endothelial cells or smooth muscle cells, sourced from intracellular stores such as the endoplasmic reticulum or the extracellular space. In general, activation of endothelial cells results in the production of a vasodilatory signal, usually in the form of nitric oxide or endothelial-derived hyperpolarization. Conversely, activation of smooth muscle cells results in a vasoconstriction response through smooth muscle cell contraction. © 2022 American Physiological Society. Compr Physiol 12: 1-35, 2022.

Calcium-Dependent Ion Channels and the Regulation of Arteriolar Myogenic Tone

Arterioles in the peripheral microcirculation regulate blood flow to and within tissues and organs, control capillary blood pressure and microvascular fluid exchange, govern peripheral vascular resistance, and contribute to the regulation of blood pressure. These important microvessels display pressure-dependent myogenic tone, the steady state level of contractile activity of vascular smooth muscle cells (VSMCs) that sets resting arteriolar internal diameter such that arterioles can both dilate and constrict to meet the blood flow and pressure needs of the tissues and organs that they perfuse. This perspective will focus on the Ca²⁺-dependent ion channels in the plasma and endoplasmic reticulum membranes of arteriolar VSMCs and endothelial cells (ECs) that regulate arteriolar tone. In VSMCs, Ca²⁺-dependent negative feedback regulation of myogenic tone is mediated by Ca²⁺-activated K⁺ (BK_Ca) channels and also Ca²⁺-dependent inactivation of voltage-gated Ca²⁺ channels (VGCC). Transient receptor potential subfamily M, member 4 channels (TRPM4); Ca²⁺-activated Cl⁻ channels (CaCCs; TMEM16A/ANO1), Ca²⁺-dependent inhibition of voltage-gated K⁺ (K_V) and ATP-sensitive K⁺ (K_ATP) channels; and Ca²⁺-induced-Ca²⁺ release through inositol 1,4,5-trisphosphate receptors (IP₃Rs) participate in Ca²⁺-dependent positive-feedback regulation of myogenic tone. Calcium release from VSMC ryanodine receptors (RyRs) provide negative-feedback through Ca²⁺-spark-mediated control of BK_Ca channel activity, or positive-feedback regulation in cooperation with IP₃Rs or CaCCs. In some arterioles, VSMC RyRs are silent. In ECs, transient receptor potential vanilloid subfamily, member 4 (TRPV4) channels produce Ca²⁺ sparklets that activate IP₃Rs and intermediate and small conductance Ca²⁺ activated K⁺ (IK_Ca and sK_Ca) channels causing membrane hyperpolarization that is conducted to overlying VSMCs producing endothelium-dependent hyperpolarization and vasodilation. Endothelial IP₃Rs produce Ca²⁺ pulsars, Ca²⁺ wavelets, Ca²⁺ waves and increased global Ca²⁺ levels activating EC sK_Ca and IK_Ca channels and causing Ca²⁺-dependent production of endothelial vasodilator autacoids such as NO, prostaglandin I₂ and epoxides of arachidonic acid that mediate negative-feedback regulation of myogenic tone. Thus, Ca²⁺-dependent ion channels importantly contribute to many aspects of the regulation of myogenic tone in arterioles in the microcirculation.

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

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

IP3 receptor signaling and endothelial barrier function

The endothelium, a monolayer of endothelial cells lining vessel walls, maintains tissue-fluid homeostasis by restricting the passage of the plasma proteins and blood cells into the interstitium. The ion Ca²⁺, a ubiquitous secondary messenger, initiates signal transduction events in endothelial cells that is critical to control of vascular tone and endothelial permeability. The ion Ca²⁺ is stored inside the intracellular organelles and released into the cytosol in response to environmental cues. The inositol 1,4,5-trisphosphate (IP₃) messenger facilitates Ca²⁺ release through IP₃ receptors which are Ca²⁺-selective intracellular channels located within the membrane of the endoplasmic reticulum. Binding of IP₃ to the IP₃Rs initiates assembly of IP₃R clusters, a key event responsible for amplification of Ca²⁺ signals in endothelial cells. This review discusses emerging concepts related to architecture and dynamics of IP₃R clusters, and their specific role in propagation of Ca²⁺ signals in endothelial cells.

Role of inositol 1,4,5-trisphosphate in the regulation of ventricular Ca(2+) signaling in intact mouse heart

Inositol 1,4,5-trisphosphate (InsP(3)R)-mediated Ca(2+) signaling is a major pathway regulating multiple cellular functions in excitable and non-excitable cells. Although InsP(3)-mediated Ca(2+) signaling has been extensively described, its influence on ventricular myocardium activity has not been addressed in contracting hearts at the whole-organ level. In this work, InsP(3)-sensitive intracellular Ca(2+) signals were studied in intact hearts using laser scanning confocal microscopy and pulsed local-field fluorescence microscopy. Intracellular [InsP(3)] was rapidly increased by UV flash photolysis of membrane-permeant caged InsP(3). Our results indicate that the basal [Ca(2+)] increased after the flash photolysis of caged InsP(3) without affecting the action potential (AP)-induced Ca(2+) transients. The amplitude of the basal [Ca(2+)] elevation depended on the intracellular [InsP(3)] reached after the UV flash. Pretreatment with ryanodine failed to abolish the InsP(3)-induced Ca(2+) release (IICR), indicating that this response was not mediated by ryanodine receptors (RyR). Thapsigargin prevented Ca(2+) release from both RyR- and InsP(3)R-containing Ca(2+) stores, suggesting that these pools have similar Ca(2+) reuptake mechanisms. These results were reproduced in acutely isolated cells where photorelease of InsP(3) was able to induce changes in endothelial cells but not in AP-induced transients from cardiomyocytes. Taken together, these results suggest that IICR does not directly regulate cardiac excitation-contraction coupling. To our knowledge, this is the first demonstration of IICR in intact hearts. Consequently, our work provides a reference framework of the spatiotemporal attributes of the IICR under physiological conditions.

Hydrogen sulfide increases nitric oxide production with calcium-dependent activation of endothelial nitric oxide synthase in endothelial cells

Hydrogen sulfide (H(2)S) was recently discovered to be synthesized in mammalian tissues by several different enzymes. Numerous studies have shown that H(2)S has vasodilator and antihypertensive effects in the cardiovascular system. However, intracellular mechanisms of the H(2)S-induced vasodilation and its interactions with other endothelium-derived relaxing factors, such as nitric oxide (NO), remain unclear. We investigated whether H(2)S directly regulates endothelial NO synthase (eNOS) activity and NO production in endothelial cells. NaHS, a H(2)S donor, dose-dependently increased NO production in cultured endothelial cells. This effect was abolished by a calcium chelator (BAPTA-AM), but not by the absence of extracellular calcium. The NaHS-induced NO production was partially blocked by inhibitors of ryanodine receptor (dantrolene) or inositol 1,4,5-triphosphate receptor (xestospongin C). NaHS significantly increased intracellular calcium concentrations, and this effect was attenuated by dantrolene or xestospongin C. NaHS induced phosphorylation of eNOS at the activating phosphoserine residue 1179. The NaHS-induced eNOS phosphorylation and NO production were not affected by a PI3K/Akt inhibitor (wortmannin). The data of this study suggest that H(2)S directly acts on endothelial cells to induce eNOS activation and NO production by releasing calcium from the intracellular store in endoplasmic reticulum, which may explain one of mechanisms of its vasodilator function.

Redox regulation of SERCA2 is required for vascular endothelial growth factor-induced signaling and endothelial cell migration

AIMS

Vascular endothelial growth factor (VEGF) increases angiogenesis by stimulating endothelial cell (EC) migration. VEGF-induced nitric oxide ((•)NO) release from (•)NO synthase plays a critical role, but the proteins and signaling pathways that may be redox-regulated are poorly understood. The aim of this work was to define the role of (•)NO-mediated redox regulation of the sarco/endoplasmic reticulum Ca(2+) ATPase (SERCA) in VEGF-induced signaling and EC migration.

RESULTS

VEGF-induced EC migration was prevented by the (•)NO synthase inhibitor, N (G)-nitro-L-arginine methyl ester (LNAME). Either VEGF or (•)NO stimulated endoplasmic reticulum (ER) (45)Ca(2+) uptake, a measure of SERCA activity, and knockdown of SERCA2 prevented VEGF-induced EC migration and (45)Ca(2+) uptake. S-glutathione adducts on SERCA2b, identified immunochemically, were increased by VEGF, and were prevented by LNAME or overexpression of glutaredoxin-1 (Glrx-1). Furthermore, VEGF failed to stimulate migration of ECs overexpressing Glrx-1. VEGF or (•)NO increased SERCA S-glutathiolation and stimulated migration of ECs in which wild-type (WT) SERCA2b was overexpressed with an adenovirus, but did neither in those overexpressing a C674S SERCA2b mutant, in which the reactive cysteine-674 was mutated to a serine. Increased EC Ca(2+) influx caused by VEGF or (•)NO was abrogated by overexpression of Glrx-1 or the C674S SERCA2b mutant. ER store-emptying through the ryanodine receptor (RyR) and Ca(2+) entry through Orai1 were also required for VEGF- and (•)NO-induced EC Ca(2+) influx.

INNOVATION AND CONCLUSIONS

These results demonstrate that (•)NO-mediated activation of SERCA2b via S-glutathiolation of cysteine-674 is required for VEGF-induced EC Ca(2+) influx and migration, and establish redox regulation of SERCA2b as a key component in angiogenic signaling.

Update on vascular endothelial Ca2+ signalling: A tale of ion channels, pumps and transporters

A monolayer of endothelial cells (ECs) lines the lumen of blood vessels and forms a multifunctional transducing organ that mediates a plethora of cardiovascular processes. The activation of ECs from as state of quiescence is, therefore, regarded among the early events leading to the onset and progression of potentially lethal diseases, such as hypertension, myocardial infarction, brain stroke, and tumor. Intracellular Ca²⁺ signals have long been know to play a central role in the complex network of signaling pathways regulating the endothelial functions. Notably, recent work has outlined how any change in the pattern of expression of endothelial channels, transporters and pumps involved in the modulation of intracellular Ca²⁺ levels may dramatically affect whole body homeostasis. Vascular ECs may react to both mechanical and chemical stimuli by generating a variety of intracellular Ca²⁺ signals, ranging from brief, localized Ca²⁺ pulses to prolonged Ca²⁺ oscillations engulfing the whole cytoplasm. The well-defined spatiotemporal profile of the subcellular Ca²⁺ signals elicited in ECs by specific extracellular inputs depends on the interaction between Ca²⁺ releasing channels, which are located both on the plasma membrane and in a number of intracellular organelles, and Ca²⁺ removing systems. The present article aims to summarize both the past and recent literature in the field to provide a clear-cut picture of our current knowledge on the molecular nature and the role played by the components of the Ca²⁺ machinery in vascular ECs under both physiological and pathological conditions.

Calcium signalling in the endothelium

Elevations in cytosolic Ca2+ concentration are the usual initial response of endothelial cells to hormonal and chemical transmitters and to changes in physical parameters, and many endothelial functions are dependent upon changes in Ca2+ signals produced. Endothelial cell Ca2+ signalling shares similar features with other electrically non-excitable cell types, but has features unique to endothelial cells. This chapter discusses the major components of endothelial cell Ca2+ signalling.

Vectorial Ca2+ release via ryanodine receptors contributes to Ca2+ extrusion from freshly isolated rabbit aortic endothelial cells

In this study, we identified ryanodine receptors (RyRs) as a component of a cytosolic Ca(2+) removal pathway in freshly isolated rabbit aortic endothelial cells. In an earlier article, we reported that the sarco(endo)plasmic reticulum Ca(2+)-ATPase (SERCA) and Na(+)/Ca(2+) exchanger (NCX) function in series to extrude cytosolic Ca(2+) to the extracellular space. Here we employed caffeine and ryanodine as modulators of RyR and showed that they act as the linkage between SERCA and NCX in removing Ca(2+) from the cytoplasm. Our data indicate that both 15 mM caffeine and 1 microM ryanodine facilitated Ca(2+) extrusion by activating RyRs while 100 microM ryanodine had the opposite effect by blocking RyRs. A further attempt to investigate RyR pharmacology revealed that in the absence of extracellular Ca(2+), ryanodine at 1 microM, but not 100 microM, stimulated Ca(2+) loss from the endoplasmic reticulum (ER). Blockade of RyR had no effect on the Ca(2+) removal rate when NCX had been previously blocked. In addition, the localization of RyR was determined using confocal microscopy of BODIPY TR-X fluorescent staining. Taken together, our findings suggest that in freshly isolated endothelial cells Ca(2+) is removed in part by transport through SERCA, RyR, and eventually NCX, and that RyR and NCX are in close functional proximity near the plasma membrane. After blockade of this component, Ca(2+) extrusion could be further inhibited by carboxyeosin, indicating a parallel contribution by the plasmalemmal Ca(2+)-ATPase (PMCA).

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.

Modulation of early [Ca2+]i rise in metabolically inhibited endothelial cells by xestospongin C

When energy metabolism is disrupted, endothelial cells lose Ca(2+) from endoplasmic reticulum (ER) and the cytosolic Ca(2+) concentration ([Ca(2+)](i)) increases. The importance of glycolytic energy production and the mechanism of Ca(2+) loss from the ER were analyzed. Endothelial cells from porcine aorta in culture and in situ were used as models. 2-Deoxy-D-glucose (2-DG, 10 mM), an inhibitor of glycolysis, caused an increase in [Ca(2+)](i) (measured with fura 2) within 1 min when total cellular ATP contents were not yet affected. Stimulation of oxidative energy production with pyruvate (5 mM) did not attenuate this 2-DG-induced rise of [Ca(2+)](i), while this maneuver preserved cellular ATP contents. The inhibitor of ER-Ca(2+)-ATPase, thapsigargin (10 nM), augmented the 2-DG-induced rise of [Ca(2+)](i). Xestospongin C (3 microM), an inhibitor of D-myo-inositol 3-phosphate [Ins(3)P]-sensitive ER-Ca(2+) release, abolished the rise. The results demonstrate that the ER of endothelial cells is very sensitive to glycolytic metabolic inhibition. When this occurs, the ER Ca(2+) store is discharged by opening of the Ins(3)P-sensitive release channel. Xestospongin C can effectively suppress the early [Ca(2+)](i) rise in metabolically inhibited endothelial cells.

Subplasmalemmal ryanodine-sensitive Ca2+ release contributes to Ca2+-dependent K+ channel activation in a human umbilical vein endothelial cell line

The whole-cell configuration of the patch clamp technique was used to assess the involvement of ryanodine-sensitive Ca2+ release (RsCR) in histamine-activated Ca2+-dependent K+ (KCa) channels in the human umbilical vein endothelial cell line EA.hy926. Histamine (10 microM) induced a transient outward current that reached 18.9 +/- 5.5 pA pF-1 at +20 mV. This current was diminished by 1 mM tetraethylammonium or 50 nM iberiotoxin, by 90 % and 80 %, respectively, suggesting that this current results from the stimulation of large-conductance KCa (BKCa) channels. In about 50 % of the cells tested, stimulation of RsCR with 200 nM ryanodine initiated a small outward current that was also sensitive to iberiotoxin. Following the ryanodine-mediated RsCR, the potency of 10 microM histamine to activate KCa channels was reduced by about 60 %. In agreement, an inhibition of RsCR with 25 microM ryanodine diminished KCacurrent in response to histamine by about 70 %. The effect of 100 microM histamine on KCa channel activity was not reduced by previous RsCR with 200 nM ryanodine, or by an inhibition of RsCR by 25 microM ryanodine. Histamine (10 microM)-induced Ca2+ elevation was reduced by 30 % following ryanodine-mediated RsCR, whereas no inhibition occurred in the case of 100 microM histamine stimulation. In cells treated with 10 microM nocodazole for 16 h to collapse the superficial endoplasmic reticulum, 200 nM ryanodine failed to initiate any KCa current. Furthermore, the inhibitory effect of previous RsCR on 10 microM histamine-induced KCa current was not obtained in nocodazole-treated cells. Our data suggest that during moderate cell stimulation (10 microM histamine), subplasmalemmal RsCR greatly contributes to the activation of KCa channels in endothelial cells. Thus, the function of the subplasmalemmal Ca2+ control unit (SCCU) described previously must be extended as a regulator for KCa channels.

Stealth ryanodine-sensitive Ca2+ release contributes to activity of capacitative Ca2+ entry and nitric oxide synthase in bovine endothelial cells

1. The involvement of ryanodine-sensitive Ca2+ release (RsCR) in bradykinin (Bk)-induced Ca2+ release, capacitative Ca2+ entry (CCE) and nitric oxide synthase (NOS) activation was assessed in freshly isolated bovine coronary artery endothelial cells. 2. Using deconvolution microscopy fura-2 was found throughout the whole cytosol, while the cell membrane impermeable dye FFP-18 was exclusively in the cell membrane. Thus, perinuclear ([Ca2+]pn) and subplasmalemmal Ca2+ concentration ([Ca2+]sp) were monitored using fura-2 and FFP-18. 3. Inhibition of Na+-Ca2+ exchange by lowering extracellular Na+ concentration augmented the Bk-induced [Ca2+]pn signal in Ca2+-free solution. This effect was abolished when RsCR was prevented with 25 micromol l-1 ryanodine, while inhibition of RsCR had no effect on Bk-induced increase in [Ca2+]pn without inhibition of Na+-Ca2+ exchange. 4. Initiating RsCR by 200 nmol l-1 ryanodine increased [Ca2+]sp, while [Ca2+]pn remained constant. However, when Na+-Ca2+ exchange was prevented, ryanodine was also able to elevate [Ca2+]pn. 5. Blockage of RsCR diminished Ca2+ extrusion in response to stimulation with Bk in normal Na+-containing solution. 6. Inhibition of RsCR blunted Bk-activated CCE, while inhibition of Na+-Ca2+ exchange during stimulation enhanced CCE. 7. Although direct activation of RsCR failed to activate NOS, inhibition of RsCR diminished the effect of ATP and Bk on NOS, while the effect of thapsigargin remained unchanged. 8. These data suggest that during stimulation subplasmalemmal RsCR occurs, which contributes to the activities of CCE and NOS. Thus, the function of the subplasmalemmal Ca2+ control unit must be extended as a regulator for CCE and NOS.

ACh- and caffeine-induced Ca2+ mobilization and current activation in rabbit arterial endothelial cells

Fura 2 microfluorometry and perforated-patch whole cell recording were carried out simultaneously to investigate the relationship between intracellular free Ca2+ concentration ([Ca2+]i) and membrane current activation in response to ACh and caffeine in freshly dissociated arterial endothelial cells. ACh and caffeine evoked transient increases in [Ca2+]i. The initial increase in [Ca2+]i was accompanied by a transient outward current, which caused membrane hyperpolarization. The amplitudes of the [Ca2+]i transient and outward current were dependent on caffeine concentration (EC50 approximately 1 mM). Cyclopiazonic acid raised resting [Ca2+]i levels by >/=50 nM and failed to completely block caffeine- or ACh-induced [Ca2+]i transients but slowed [Ca2+]i recovery fourfold. The reversal potential of caffeine-induced currents was dependent on external K+ and Cl- concentrations. Caffeine-induced current amplitudes, but not [Ca2+]i responses, were attenuated by external tetraethylammonium, Zn2+, and La3+. A consistent temporal relationship between agonist-activated membrane current and [Ca2+]i increases was not observed, and, in some cases, time differences were greater than expected for simple diffusion of Ca2+ throughout the cell. These results suggest that Ca2+-dependent current activation monitors local [Ca2+]i changes adjacent to the plasmalemma, whereas single-cell photometry provides a measure of global changes in [Ca2+]i.

Direct measurement of SR release flux by tracking 'Ca2+ spikes' in rat cardiac myocytes

1. Ca2+ release flux across the sarcoplasmic reticulum (SR) during cardiac excitation-contraction coupling was investigated using a novel fluorescence method. Under whole-cell voltage-clamp conditions, rat ventricular myocytes were dialysed with a high concentration of EGTA (4.0 mM, 150 nM free Ca2+), to minimize the residence time of released Ca2+ in the cytoplasm, and a low-affinity, fast Ca2+ indicator, Oregon Green 488 BAPTA-5N (OG-5N; 1.0 mM, Kd approximately 31 microM), to optimize the detection of localized high [Ca2+] in release site microdomains. Confocal microscopy was employed to resolve intracellular [Ca2+] at high spatial and temporal resolution. 2. Analytical and numerical analyses indicated that, under conditions of high EGTA concentration, the free [Ca2+] change is the sum of two terms: one major term proportional to the SR release flux/Ca2+ influx, and the other reflecting the running integral of the released Ca2+. 3. Indeed, the OG-5N transients in EGTA-containing cells consisted of a prominent spike followed by a small pedestal. The OG-5N spike closely resembled the first derivative (d[Ca2+]/dt) of the conventional Ca2+ transient (with no EGTA), and mimicked the model-derived SR Ca2+ release function reported previously. In SR Ca2+-depleted cells, the OG-5N transient also closely followed the waveform of L-type Ca2+ current (ICa). Using ICa as a known source of Ca2+ influx, SR flux can be calibrated in vivo by a linear extrapolation of the ICa-elicited OG-5N signal. 4. The OG-5N image signal was localized to discrete release sites at the Z-line level of sarcomeres, indicating that the local OG-5N spike arises from 'Ca2+ spikes' at transverse (T) tubule-SR junctions (due to the imbalance between calcium ions entering the cytosol and the buffer molecules). 5. Both peak SR release flux and total amount of released Ca2+ exhibited a bell-shaped voltage dependence. The temporal pattern of SR release also varied with membrane voltage: Ca2+ release was most synchronized and produced maximal peak release flux (4.2 mM s-1) at 0 mV; in contrast, maximal total release occurred at -20 mV (71 versus 61 microM at 0 mV), but the localized release signals were partially asynchronous. Since the maximal conventional [Ca2+] transient and contraction were elicited at 0 mV, it appears that not only the amount of Ca2+ released, but also the synchronization among release sites affects the whole-cell Ca2+ transient and the Ca2+-myofilament interaction.

Evidence for mitochondrial Ca(2+)-induced Ca2+ release in permeabilised endothelial cells

Generally most intracellular Ca2+ is stored in the endoplasmic reticulum (ER) and mitochondria. Recently a mitochondrial Ca(2+)-induced Ca2+ release (mCICR) mechanism, unconnected with ryanodine receptors (RyR's), has been shown in tumour cells. The existence of a mitochondrial Ca2+ release mechanism in BAE cells was investigated using saponin-permeabilised BAE cells. When buffered intracellular solution were 'stepped' from 10 nM to 10 microM free Ca2+, the mitochondrial inhibitors CN (2 mM), FCCP (1 microM), and RR (20 microM) significantly reduced total CICR by approximately 25%. The ER Ca(2+)-ATPase inhibitor thapsigargin (100 nM) had no effect. Furthermore, cyclosporin A (200 nM), an inhibitor of the mitochondrial permeability transition pore (PTP), abolished total CICR. Therefore, the novel ryanodine-caffeine insensitive CICR mechanism previously reported in BAE cells involves mitochondrial Ca2 release. It is proposed that in BAE cells, mCICR occurs via the mitochondrial PTP and may be physiologically important in endothelial cell Ca2+ signalling.

Nitric oxide production in human endothelial cells stimulated by histamine requires Ca2+ influx

The causal relationships between cytosolic free-Ca2+ concentration ([Ca2+]i) increases and production of nitric oxide (NO) have been investigated mostly with indirect methods and remain unclear. Here we demonstrate, by direct real-time measurements of [NO] with a porphyrinic microsensor, that Ca2+ entry, but not an increase in [Ca2+]i, is required for triggering of NO production in human endothelial cells. Histamine, ranging from 0.1 to 100 microM, increased both NO production and [Ca2+]i when given in a single dose. However, histamine caused increased NO release but induced progressively smaller [Ca2+]i changes when cumulatively added. In the absence of a transmembrane Ca2+ gradient, no significant NO release was detectable, despite the marked Ca2+ peak induced by histamine. Inhibition of Ca2+ entry by SK&F 96365 abolished histamine-elicited NO production but only reduced the transient [Ca2+]i rise. The suppression of the sustained [Ca2+]i response under these two conditions suggests that NO release was closely associated with Ca2+ entry from the extracellular space. In addition, membrane depolarization, achieved by increasing the extracellular K+ concentration from 5 to 130 mM, reduced both the amplitude of histamine-induced sustained [Ca2+]i elevation and NO production. These results lead us to propose that the availability of numerous Ca2+ ions around the internal side of the plasma membrane would promote the association between nitric oxide synthase and calmodulin, thereby activating the enzyme.

Mobilization of internal Ca2+ by vasoactive intestinal polypeptide in endothelial cells

The aims of the present study were to establish whether vasoactive intestinal polypeptide (VIP) could mobilize internally-stored Ca2+ and whether Ca2+ release could trigger Ca2+ influx from the extracellular space. Bovine pulmonary artery endothelial cells from an established cell line were loaded with fura-2/AM and cells were studied in suspension or were imaged in monolayers at 40-80% confluency. In Ca2+ imaging studies, VIP evoked Ca2+ transients in Ca2+-free medium containing 50 microM EGTA. This was observed in 33 out of 122 cells examined on 29 separate trials. With each cell, the spread of Ca2+ appeared to occur from the periphery of the cell to the central core. Cells which did not respond to VIP responded to other stimulants such as bradykinin, endoplasmic reticulum Ca2+ pump inhibitors, (cyclopiazonic acid and thapsigargin), and endoplasmic reticulum Ca2+ release channel opener, ryanodine. The reintroduction of Ca2+ following VIP-induced Ca2+ release did not evoke a Ca2+ response in 5 cells imaged. Cells in suspension showed typical biphasic responses to bradykinin, thapsigargin or cyclopiazonic acid in the presence of external Ca2+. Stimulation with VIP caused transient Ca2+ responses in Ca2+-free physiological saline containing 50 microM EGTA. However, only 1 out of 4 cells tested showed a response to Ca2+ when it was reintroduced to the bathing medium. This study provided direct evidence for the first time in these bovine endothelial cells for VIP-mediated elevation of cytosolic concentration of Ca2+. The results also suggested that other mechanisms might prevail preventing capacitative Ca2+ entry following the release of internally-stored Ca2+.

Tunicamycin increases intracellular calcium levels in bovine aortic endothelial cells

Tunicamycin is a nucleoside antibiotic that inhibits protein glycosylation and palmitoylation. The therapeutic use of tunicamycin is limited in animals because of its toxic effects, particularly in cerebral vasculature. Tunicamycin decreases palmitoylation of the endothelial isoform of nitric oxide synthase, stimulates nitric oxide synthesis, and increases the concentration of intracellular calcium ([Ca2+]i) in bovine aortic endothelial cells (B. J. Buckley and A. R. Whorton. FASEB J. 11: A110, 1997). In the present study, we investigated the mechanism by which tunicamycin alters [Ca2+]i using the Ca2+-sensitive dye fura 2. We found that tunicamycin increased [Ca2+]i without increasing levels of inositol phosphates. When cells were incubated in the absence of extracellular Ca2+, [Ca2+]i rapidly rose in response to tunicamycin, although a full response was not achieved. The pool of intracellular Ca2+ mobilized by tunicamycin overlapped with that mobilized by thapsigargin. Extracellular nickel blocked a full response to tunicamycin when cells were incubated in the presence of extracellular Ca2+. The effects of tunicamycin on [Ca2+]i were partially reversed by washing out the drug, and the remainder of the response was inhibited by removing extracellular Ca2+. These results indicate that tunicamycin mobilizes Ca2+ from intracellular stores in a manner independent of phospholipase C activation and increases the influx of Ca2+ across the plasma membrane.

Actions of 4-chloro-3-ethyl phenol on internal Ca2+ stores in vascular smooth muscle and endothelial cells

1. Recently, 4-chloro-3-ethyl phenol (CEP) has been shown to cause the release of internally stored Ca2+ apparently through ryanodine-sensitive Ca2+ channels, in fractionated skeletal muscle terminal cisternae and in a variety of non-excitable cell types. Its action on smooth muscle is unknown. In this study, we characterized the actions of CEP on vascular contraction in endothelium-denuded dog mesenteric artery. We also determined its ability to release Ca2+, by use of Ca2+ imaging techniques, on dog isolated mesenteric artery smooth muscle cells and on bovine cultured pulmonary artery endothelial cells. 2. After phenylephrine-(PE, 10 microM) sensitive Ca2+ stores were depleted by maximal PE stimulation in Ca2+-free medium, the action of CEP on refilling of the emptied PE stores was tested, by first pre-incubating the endothelium-denuded artery in CEP for 15 min before Ca2+ was restored for a 30 min refilling period. At the end of this period, Ca2+ and CEP were removed, and the arterial ring was tested again with PE to assess the degree of refilling of the internal Ca2+ store. 3. In a concentration-dependent manner (30, 100 and 300 microM), CEP significantly reduced the size of the post-refilling PE contraction (49.4, 28.9 and 5.7% of control, respectively) in Ca2+-free media. This suggests that Ca2+ levels are reduced in the internal stores by CEP treatment. CEP alone did not cause any contraction either in Ca2+-containing or Ca2+-free Krebs solution. 4. Restoring Ca2+ in the presence of PE caused a large contraction, which reflects PE-induced influx of extracellular Ca2+. The contraction of tissues pretreated with 300 microM CEP was significantly less compared with controls. However, tissues pretreated with 30 and 100 microM CEP were unaffected. Washout of CEP over 30 min produced complete recovery of responses to PE in Ca2+-free and Ca2+-containing medium suggesting a rapid reversal of CEP effects. 5. Concentration-response curves were constructed for PE, 5-hydroxytryptamine (5-HT) and K+ in the absence of and after 30 min pre-incubation with 30, 100 and 300 microM CEP. In all cases, CEP caused a concentration-dependent depression of the maximum response to PE (84.8, 43.4 and 11.6% of control), 5-HT (65.4, 25.7 and 6.9% of control) and K+ (77.6, 41.1 and 10.8% of control). 6. Some arterial rings were pre-incubated with ryanodine (30 microM) for 30 min before the construction of PE concentration-response curves. In Ca2+-free Krebs solution, ryanodine alone did not cause any contraction. However, 58% (11 out of 19) of the tissues tested with ryanodine developed contraction (6.9+/-1.2% of 100 mM K+ contraction, n=11) in the presence of external Ca2+. EC50 values for PE in ryanodine-treated tissues (1.7+/-0.25 microM, n=16) were not significantly different from controls (2.5+/-0.41 microM, n=22). Maximum contractions to PE (118.5+/-4.4% of 100 mM K+ contraction, n=16) were also unaffected by ryanodine when compared to controls (129+/-4.2%, n=23). 7. When fura-2 loaded smooth muscle cells (n=13) and endothelial cells (n=27) were imaged for Ca2+ distribution, it was observed that 100 and 300 microM CEP in Ca2+-free medium caused Ca2+ release in both cell types. Smooth muscle cells showed a small decrease in cell length. Addition of EGTA (5 mM) reversed the effect of CEP on intracellular Ca2+ to control values. 8. These data show, for the first time in vascular smooth muscle and endothelial cells, that CEP releases Ca2+ more rapidly than ryanodine. Unlike ryanodine, CEP caused no basal contraction but depressed contractions to PE, 5-HT and K+. The lack of basal contraction may result from altered responsiveness of the contractile system to intracellular Ca2+ elevation.

Central role of intracellular calcium stores in acute flow- and agonist-evoked endothelial nitric oxide release

1. We have used a cascade bioassay system and isolated arterial ring preparations to investigate the contribution of Ca2+ release from endothelial intracellular stores to nitric oxide (NO) production evoked by increases in shear stress and by acetylcholine in rabbit aorta. 2. Experiments were performed before and following incubation with either the endoplasmic reticulum Ca(2+)-ATPase inhibitors cyclopiazonic acid (CPA, 10 microM) and thapsigargin (TSG, 1 microM) or ryanodine (30, 100 microM) which binds to a specific endoplasmic reticulum Ca(2+)-release channel. 3. In cascade bioassay all three agents induced relaxations of the recipient ring (CPA, 24.4 +/- 3.8%; TSG, 51.5 +/- 10.6%; ryanodine, 17.4 +/- 1.6%) which were significantly attenuated by preincubation of the donor with 100 microM NG-nitro-L-arginine methyl ester (L-NAME). However, in isolated rings, only CPA and TSG induced L-NAME-sensitive relaxations (CPA 52.7 +/- 6.5%; TSG 61.3 +/- 7%). 4. Addition of superoxide dismutase (SOD) to the donor perfusate evoked relaxations of the recipient ring in cascade bioassay (13.3 +/- 1.4%, n = 22). Prior administration of SOD attenuated relaxations to TSG (23.2 +/- 3.8% n = 4) and ryanodine (1.7 +/- 0.8%, n = 4), and pre-incubation with TSG and ryanodine blunted SOD-induced responses (4 +/- 1.5%, n = 4 and 8.9 +/- 1.1%, n = 4, respectively). By contrast, no interaction was observed between the relaxations evoked by SOD and CPA. In isolated rings, SOD exerted no direct relaxant and did not modulate relaxations to CPA, TSG or ryanodine. 5. In cascade bioassay studies time-averaged shear stress was manipulated with dextran (1-4% w/v, 800000 MW) to increase perfusate viscosity. NO-dependent relaxation of the recipient ring induced by increased perfusate viscosity was significantly attenuated by CPA (P < 0.01; n = 6) and TSG (P < 0.05; n = 7), but not by ryanodine (n = 6). 6. Endothelium-dependent relaxations to acetylcholine (0.1-30 microM) in cascade bioassay and in isolated aortic ring preparations were markedly attenuated by pretreatment with CPA and TSG, but were unaffected by ryanodine. Ryanodine and CPA caused only a small attenuation of endothelium-independent relaxations to sodium nitroprusside (0.001-10 microM), whereas TSG had no effect. 7. We conclude that release of Ca2+ from CPA- and TSG-sensitive endothelial stores is necessary for NO release evoked by acute flow changes and agonists in rabbit abdominal aorta. Ca(2+)-induced Ca2+ release via the ryanodine-sensitive release channel plays no direct role in these responses. Free radical interactions may complicate the interpretation of findings in cascade bioassay compared with isolated ring preparations.

Nitric oxide induces transient Ca2+ changes in endothelial cells independent of cGMP

Ca2+ changes induced by nitric oxide (NO.) were investigated in cultured human endothelial cells. Sodium nitroprusside (SNP) (1-100 mumol/L) and S-Nitroso-N-acetylpenicillamine (SNAP) (100 mumol/L) were used as NO. donors. The cytoplasmatic Ca2+ concentration was calculated using ratiometric FURA2 fluorescence measurements. Both NO. donors caused transient oscillatory Ca2+ changes, which were not detectable in the presence of oxyhemoglobin (50 mumol/L). Digital ratio imaging revealed initiation sites within cells where Ca2+ increases started spreading, which indicates that nonuniformly distributed targets might be involved in these reactions. Calcium was released from intracellular stores as indicated by experiments performed in Ca(2+)-free buffer. L-type Ca(2+)-channel blocker diltiazem (100 mumol/L) was not able to block these responses. NO.-induced Ca2+ release from intracellular stores caused capacitative Ca2+ entry. Both thapsigargin (1 mumol/L) and cyclopiazonic acid (10 mumol/L) inhibited the SNP response completely, whereas neither ryanodine (up to 100 mumol/L) nor dantrolene (100 mumol/L) was able to inhibit Ca2+ changes induced by SNP, indicating that primarily inositol 1,4,5-triphosphate (IP3)-dependent stores are released upon stimulation with NO.. A small inhibitory effect of ATP- and SNP-induced peak [Ca2+]i increase was measured in the presence of both caffeine (20 mmol/L) and procaine (1 mmol/L). Evidence is presented that cGMP is not involved in NO.-induced Ca2+ signals, as neither inhibitors of guanylate cyclase (methylene blue and LY 83583) nor cell permeant analogues of cGMP altered or simulated [Ca2+] changes. An inhibitor of cGMP-dependent protein kinase was also ineffective. We therefore propose that endothelial cells have specific targets proximal or at IP3 receptors to induce Ca2+ changes in endothelial cells stimulated with NO..

Sources of Ca2+ in relation to generation of acetylcholine-induced endothelium-dependent hyperpolarization in rat mesenteric artery

1. The aim of the present study was to identify the sources of Ca2+ contributing to acetylcholine (ACh)-induced release of endothelium-derived hyperpolarizing factor (EDHF) from endothelial cells of rat mesenteric artery and to assess the pathway involved. The changes in membrane potentials of smooth muscles by ACh measured with the microelectrode technique were evaluated as a marker for EDHF release. 2. ACh elicited membrane hyperpolarization of smooth muscle cells in an endothelium-dependent manner. The hyperpolarizing response was not affected by treatment with 10 microM indomethacin, 300 microM NG-nitro-L-arginine or 10 microM oxyhaemoglobin, thereby indicating that the hyperpolarization is not mediated by prostanoids or nitric oxide but is presumably by EDHF. 3. In the presence of extracellular Ca2+, 1 microM ACh generated a hyperpolarization composed of the transient and sustained components. By contrast, in Ca(2+)-free medium, ACh produced only transient hyperpolarization. 4. Pretreatment with 100 nM thapsigargin and 3 microM cyclopiazonic acid, endoplasmic reticulum Ca(2+)-ATPase inhibitors, completely abolished ACh-induced hyperpolarization. Pretreatment with 20 mM caffeine also markedly attenuated ACh-induced hyperpolarization. However, the overall pattern and peak amplitude of hyperpolarization were unaffected by pretreatment with 1 microM ryanodine. 5. In the presence of 5 mM Ni2+ or 3 mM Mn2+, the hyperpolarizing response to ACh was transient, and the sustained component of hyperpolarization was not observed. On the other hand, 1 microM nifedipine had no effect on ACh-induced hyperpolarization. 6. ACh-induced hyperpolarization was nearly completely eliminated by 500 nM U-73122 or 200 microM 2-nitro-4-carboxyphenyl-N, N-diphenylcarbamate, inhibitors of phospholipase C, but was unchanged by 500 nM U-73343, an inactive form of U-73122. Pretreatment with 20 nM staurosporine, an inhibitor of protein kinase C, did not modify ACh-induced hyperpolarization. 7. These results indicate that the ACh-induced release of EDHF from endothelial cells of rat mesenteric artery is possibly initiated by Ca2+ release from inositol 1,4,5-trisphosphate (IP3)-sensitive Ca2+ pool as a consequence of stimulation of phospholipid hydrolysis due to phospholipase C activation, and maintained by Ca2+ influx via a Ni(2+)- and Mn(2+)-sensitive pathway distinct from L-type Ca2+ channels. The Ca(2+)-influx mechanism seems to be activated following IP3-induced depletion of the pool.

Ca2+ influx does more than provide releasable Ca2+ to maintain repetitive spiking in human umbilical vein endothelial cells

We investigated why oscillations of intracellular Ca2+ concentrations ([Ca2+]i) in endothelial cells challenged by sub-maximal histamine run down in Ca(2+)-free medium despite stores retaining most of their Ca2+. One explantation is that only a small subpopulation of the Ca2+ stores oscillate and are completely emptied of Ca2+. To investigate if influx refills an empty store subpopulation, we differentiated between cations entering the cell and those released from internal stores by using extracellular Sr2+ as a Ca2+ surrogate; we distinguished between [Sr2+]i and [Ca2+]i by using the larger effect of Sr2+ on fura 2 fluorescence at 360 nm (F360). Ca2+ was still available for release when oscillations had run down since oscillations promptly reappeared on addition of Sr2+o and these were predominantly of Ca2+ (indicated by F360 changes). Also, totally depleting Ca2+ stores inhibited Sr(2+)-induced oscillations, suggesting that Sr2+ entry leads to Ca2+ release. In contrast, Ba2+o was unable to stimulate oscillations. Finally, oscillations generated by photolytic release of inositol trisphosphate (IP3) analogues were similarly sensitive to extracellular Ca2+ and Sr2+. We conclude that stores (or a sub-population) are not completely depleted of Ca2+ when oscillations run down in Ca(2+)-free medium. Bivalent cation entry therefore maintains sensitivity to IP3, possibly by maintaining luminal bivalent cation levels.

Extrusion mechanisms of intracellular Ca2+ in human aortic endothelial cells

Although participation of the plasma membrane Ca2+ pump in Ca2+ efflux has been generally accepted, the existence of Na(2+)-Ca2+ exchange in endothelial cells is still controversial. In the present experiments, the role of Na(+)-Ca2+ exchange and Ca(2+)-ATPase were examined in cultured human aortic endothelial cells loaded with fura. In Ca(2+)-free solution, the declining phase of [Ca2+]i in response to histamine was clearly slowed with low Na+ solution or Ni2+. Vanadate also slightly slowed the declining phase. The declining phase was much more clearly slowed with La3+. The response to thapsigargin, a specific endoplasmic reticulum Ca2+ ATPase inhibitor, was much more clearly prolonged by those interventions. These results strongly imply the presence of Na(2+)-Ca2+ exchange and Ca2+ ATPase in the plasma membrane, and suggest that not only Ca2+ uptake into the internal stores but also Na(+)-Ca2+ exchange and plasma membrane Ca2+ ATPase have a physiological role in reducing [Ca2+]i elevated by receptor agonists and endoplasmic reticulum Ca(2+)-ATPase inhibitors in cultured human aortic endothelial cells.

Simultaneous multicompartment intracellular Ca2+ measurements in the perfused heart using 19F NMR spectroscopy

Although Ca2+ transport regulation at subcellular organelles is of great interest, only limited methodology has been available for measuring organellar [Ca2+] levels. The present study employs the 19F NMR resonance frequency of 4F-BAPTA to measure free [Ca2+]. In 4F-BAPTA loaded perfused rabbit hearts, two 19F NMR resonances were clearly observed. The frequency of one was consistent with cytosolic [Ca2+] levels. Responses to agents that after sarcoplasmic reticulum function identified the other resonance as originating from that organelle. The experiments demonstrate the utility of NMR shift indicator methodology in obtaining simultaneous multi-compartment intracellular [Ca2+] measurements and in enabling organellar [Ca2+] measurements to be made from within intact living tissue.

Interactions between inositol trisphosphate and Ca2+ dependent Ca2+ release mechanisms on the endoplasmic reticulum of permeabilised bovine aortic endothelial cells

In this paper we describe data from cultured bovine aortic endothelial (BAE) cells demonstrating a Ca2+ induced Ca2+ release (CICR) process which appears to have pharmacological properties different from CICR mechanisms in other cell types. CICR was measured in saponin permeabilised cells in which the internal stores had been preloaded with 45Ca2+. Step increases in the free Ca2+ concentration of the bathing solution, from 10 nM up to 10 microM were found to increase 45Ca2+ loss. This process was completely inhibited by ruthenium red. Caffeine induced a small release of 45Ca2+ and the response to a subsequent stimulation with a Ca2+ step was reduced. In intact cells, ryanodine activated small oscillations in intracellular Ca2+ in the presence, but not the absence, of external Ca2+. However, in permeabilised cells, ryanodine had no effect on either basal efflux or the increased efflux of 45Ca2+ seen following a step increase in free Ca2+. These data suggest the operation of a ruthenium red sensitive but ryanodine insensitive CICR mechanism on the endoplasmic reticulum (ER) which may also be modulated by caffeine. An IP3 dependent 45Ca2+ release was also observed. In the presence of ruthenium red, the IP3 induced 45Ca2+ release was reduced suggesting that CICR may operate to amplify the magnitude of the IP3 response. The Ca2+ dependence of the IP3 induced release was also measured. Co-operativity between IP3 and Ca2+ could not be detected between 100-300 nM Ca2+. The results suggest that the regulation of IP3 induced Ca2+ release may be different in BAE cells, and point to the operation of a 'novel' CICR process and to complex interactions between Ca2+ release systems in BAE cells.

Effects of phenol on vascular smooth muscle in rabbit mesenteric resistance arteries

Although phenol has long been used clinically as a neurolytic agent or as a preservative for injections, little information is available regarding its direct vascular action. We therefore studied the effects of phenol (0.1 μM-2mM) on isolated rabbit small mesenteric arteries, using isometric tension recording methods. All experiments were performed on endothelium-denuded strips. Phenol (≥10 μM) generated transsient contractions in a concentration-dependent manner in both normal Krebs and Ca(2+)-free solutions with EC50 values (concentrations that produced 50% of the maximal response) of 39.8 μM and 99.7 μM, respectively. Depletion of intracellular Ca(2+) stores by A23187 or ryanodine completely elimited the phenol-induced contractions. When caffeine (10 mM) and noradrenaline (NA, 10μM) were consecutively applied in Ca(2+)-free solution with an interval of 7 min (sufficient to prevent caffeine-induced inhibition of Ca(2+) sensitivity), caffeine eliminated the contractions induced by subsequent application of NA. In similar experiments where phenol (1 mM) and NA (10 μM) were consecutively applied in Ca(2+)-free solution, phenol significantly inhibited contractions induced by subsequent application of NA. Phenol (0.1 mM, ∼EC65), applied in the presence of either 128 mM K(+) or NA (10 μM), produced transient vasoconstrictions superimposed on both high K(+)-and NA-induced contractions, but had a lesser effect on maintenance of these contractions. The vascular responses to high K(+), NA, and caffeine after washout of phenol were not significantly different from those before application of phenol (up to 2 mM). The results suggest that phenol stimulates Ca(2+) release from intracellular Ca(2+) stores, which are sensitive to both caffine and NA in this resistance artery. The effect does not appear to reflect a toxic effect on vascular smooth muscle. It seems unlikely that phenol causes adverse hemodynamic changes because of the observed direct vascular action.

5-HT2B receptor-mediated calcium release from ryanodine-sensitive intracellular stores in human pulmonary artery endothelial cells

1. We have characterized the 5-hydroxytryptamine (5-HT)-induced calcium signalling in endothelial cells from the human pulmonary artery. Using RT-PCR we show, that of all cloned G-protein coupled 5-HT receptors, these cells express only 5-HT1D beta, 5-HT2B and little 5-HT4 receptor mRNA. 2. In endothelial cells 5-HT inhibits the formation of adenosine 3':5'-cyclic monophosphate (cyclic AMP) via 5-HT1D beta receptors but fails to activate phosphoinositide (PI) turnover. However, the latter pathway is strongly activated by histamine. 3. Despite the lack of detectable inositol phosphate (IP) formation in human pulmonary artery endothelial cells, 5-HT (pD2 = 5.82 +/- 0.06, n = 6) or the selective 5-HT2 agonist, 1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane (DOI) (pD2 = 5.66 +/- 0.03, n = 7) elicited transient calcium signals comparable to those evoked by histamine (pD2 = 6.44 +/- 0.01, n = 7). Since 5-HT2A and 5-HT2C receptor mRNAs are not detectable in pulmonary artery endothelial cells, activation of 5-HT2B receptors is responsible for the transient calcium release. The calcium transients are independent of the inhibition of adenylate cyclase, since DOI does not stimulate 5-HT1D beta receptors. 4. Both, the 5-HT- and histamine-stimulated calcium signals were also observed when the cells were placed in calcium-free medium. This indicates that 5-HT triggers calcium release from intracellular stores. 5. Heparin is an inhibitor of the IP3-activated calcium release channels on the endoplasmic reticulum. Intracellular infusion of heparin through patch pipettes in voltage clamp experiments failed to block 5-HT-induced calcium signals, whereas it abolished the histamine response. This supports the conclusion that the 5-HT-induced calcium release is independent of IP3 formation. 6. Unlike the histamine response, the 5-HT response was sensitive to micromolar concentrations of ryanodine and, to a lesser extent, ruthenium red. This implies that 5-HT2B receptors trigger calcium release from a ryanodine-sensitive calcium pool. 7. It has been postulated that cyclic ADP-ribose (cADPR) is a soluble second messenger which activates ryanodine receptors. However, calcium signals similar to the 5-HT response could not be elicited by intracellular infusion with cADPR. Furthermore, the subsequent application of 5-HT or DOI elicited a calcium signal that was not affected by the above pretreatment. 8. We conclude that human 5-HT2B receptors stimulate calcium release from intracellular stores through a novel pathway, which involves activation of ryanodine receptors, and is independent of PI-hydrolysis and cADPR.

Volatile anaesthetic actions on norepinephrine-induced contraction of small splanchnic resistance arteries

The aim of this study was to investigate volatile anaesthetic action on small splanchnic resistance arteries. Employing isometric tension recording, we studied the effects of clinically relevant concentrations (0.25-1.25 minimum alveolar concentration (MAC)) of isoflurane, sevoflurane and enflurane on contractions induced by norepinephrine (NE), a sympathetic neurotransmitter, in the rabbit small mesenteric artery. Rhythmic oscillations were observed in contractile responses to NE. Both isoflurane (> or = 0.25 MAC, 0.5% (approximately 0.11 mM)) and sevoflurane (> or = 0.75 MAC, 2.8% (approximately 0.38 mM)) inhibited the NE (10 microM)-induced contraction with concomitant inhibition of average amplitude of the oscillations. Only enflurane (> or = 0.25 MAC, 0.7% (approximately 0.20 mM)) generated vasoconstriction superimposed on the NE-induced contraction; however, the vasoconstriction was transient and was followed by vasorelaxation. Concurrently, enflurane (> or = 0.25 MAC) strongly inhibited the average amplitude of the oscillations; higher concentrations (> or = 1.0 MAC) of enflurane completely eliminated the oscillations. The frequency of the NE-induced oscillations was less affected by the anaesthetics. The observed vasodilator action of these anaesthetics in small resistance arteries may contribute to their hypotensive effects in vivo. The potent inhibition of the rhythmic oscillations also may play a role in volatile anaesthetic-induced alterations in cardiovascular homeostasis.

Nitric oxide: its identity and role in blood pressure control

In response to biochemical factors like catecholamines, bradykinins, histamine and physical factors like shear stress, endothelial cells release a non prostanoid factor, called endothelium derived relaxing factor (EDRF), which relaxes vascular smooth muscle. Since this discovery in 1980, many additional agents have been shown to stimulate release of EDRF from endothelium. Biological and chemical evidence has supported the proposal that EDRF is actually nitric oxide (NO). Research on the synthesis, inhibition and physiological roles of nitric oxide (NO) has led to studies of its involvement in blood pressure homeostasis and immune functions.

Caffeine-evoked, calcium-sensitive membrane currents in rabbit aortic endothelial cells

1. Single cell photometry and whole-cell patch clamp recording were used to study caffeine-induced intracellular Ca2+ signals and membrane currents, respectively, in endothelial cells freshly dissociated from rabbit aorta. 2. Caffeine (5 mM) evoked a transient increase in [Ca2+]i in fura-2-loaded endothelial cells. Pretreatment of cells with 10 microM ryanodine did not alter resting [Ca2+]i but irreversibly inhibited the caffeine-induced rise in [Ca2+]i. The caffeine-induced increase in [Ca2+]i was not attenuated by the removal of extracellular Ca2+ and did not stimulate the rate of Mn2+ quench of fura-2 fluorescence. 3. Bath application of caffeine evoked a dose- and voltage-dependent outward current. The rate of onset and amplitude of the caffeine-evoked outward current increased with higher caffeine concentrations and membrane depolarization. The relationship between caffeine-evoked current amplitude and membrane potential was non linear, suggesting that the channels underlying the current are voltage-sensitive. 4. In the absence of extracellular Ca2+, the amplitude of the caffeine-evoked outward current was reduced by approximately 50% but the duration of the current was prolonged compared to that observed in the presence of external Ca2+. Ca(2+)-free external solutions produced an unexpected increase in both the frequency and amplitude of spontaneous transient outward currents (STOCs). 5. Inclusion of heparin (10 micrograms ml-1) in the patch pipette abolished the acetylcholine (ACh)-induced outward current but failed to inhibit either STOCs or the caffeine-evoked outward current in native endothelial cells. In the absence of extracellular Ca2+, heparin did not affect either STOCs or the caffeine-induced outward current. 6. Externally applied tetraethylammonium ions (TEA, 3-10mM) reversibly inhibited unitary Ca2+-activated K+ currents and STOCs in endothelial cells but failed to inhibit completely the outward current evoked by 20 mM caffeine.7. Bath application of 0.1 mM zinc ion (Zn2+), a chloride channel blocker, did not affect unitary currents or STOCs but reduced the amplitude of the caffeine-evoked current by >75% compared to control. Replacement of extracellular NaCl with Na gluconate also reduced the amplitude of the caffeine-induced outward current. Bath application of 0.1 mM Zn2+ and 10 mM TEA completely blocked the caffeine-evoked outward current in endothelial cells.8. Caffeine-induced Ca2+ release from intracellular stores evokes a transient rise in [Ca2+1, which is correlated with a large, transient outward current. The ionic dependence and inhibition of the caffeine sensitive current by TEA and Zn2+ suggests that Ca2+-activated K+ and Cl- conductances contribute to the caffeine response in rabbit aortic endothelial cells.

Regulation of the adenylyl cyclase signaling system in various types of cultured endothelial cells

We studied the effects of modulators of the adenylyl cyclase pathway on the accumulation of cAMP in endothelial cells isolated from bovine aortas, pig pulmonary arteries, human umbilical veins, and human subcutaneous adipose microvessels. In addition to quantitative differences in the basal levels, cAMP stimulation in different endothelial cell types varied in sensitivity and magnitude in response to both the direct adenylyl cyclase activator forskolin and the beta-adrenergic receptor agonist isoproterenol. Furthermore, the ubiquitous phosphodiesterase inhibitor IBMX differentially enhanced both the basal and the stimulated cAMP levels in the various cell types. Histamine caused an elevation of cAMP only in bovine aortic endothelial cells and in human umbilical vein endothelial cells. Treatment of the cells with cholera and pertussis toxins, which uniquely affect G-protein subunits, resulted in divergent elevation of cAMP in the various cells. Thus, in each cell type, a distinct profile of regulation of the cAMP levels was found. Our results suggest that the adenylyl cyclase signaling system in various types of endothelial cells can be differentially regulated at the levels of receptors, G-proteins, adenylyl cyclase, and phosphodiesterase.

Adrenomedullin stimulates two signal transduction pathways, cAMP accumulation and Ca2+ mobilization, in bovine aortic endothelial cells

The biological action of adrenomedullin, a novel hypotensive peptide, on bovine aortic endothelial cells, was examined. The specific binding of adrenomedullin to these cells was observed, and adrenomedullin was found to induce intracellular cAMP accumulation in a dose-dependent manner. EC50 for the cAMP accumulation was about 100 times lower than the apparent IC50 for the binding assay. Adrenomedullin also induced increase of intracellular free Ca2+ in endothelial cells in a dose-dependent manner. The Ca2+ response to adrenomedullin was biphasic with an initial transient increase due to the release from thapsigargin-sensitive intracellular Ca2+ storage and a prolonged increase by influx through the ion channel on the plasma membrane. This intracellular free Ca2+ increase resulted from phospholipase C activation and inositol 1,4,5-trisphosphate formation, and seemed to cause nitric oxide synthase activation by monitoring intracellular cGMP accumulation. Both cAMP accumulation and Ca2+ increased responses to adrenomedullin were mediated by cholera toxin-sensitive G protein, but the two signal transduction pathways were independent. Thus, the results suggest that adrenomedullin elicits the hypotensive effect through at least two mechanisms, a direct action on vascular smooth muscle cells to increase intracellular cAMP and an action on endothelial cells to stimulate nitric oxide release, with both leading to vascular relaxation.

Heterogeneity of caffeine- and bradykinin-sensitive Ca2+ stores in vascular endothelial cells

The filling state of Ca2+ stores in endothelial cells regulates Ca2+ entry. The functional relationship between the major Ca2+ stores [i.e. Ins(1,4,5)P3-sensitive (= bradykinin-sensitive stores, 'BsS') and caffeine-sensitive stores] is unknown. In pig right-coronary-artery endothelial cells, caffeine failed to release Ca2+ in 68% of the cells (quiet-responders), but increased bradykinin (Bk)-induced Ca2+ release 2.5-fold. In Bk-pre-stimulated cells, caffeine increased Ca2+ release upon a second stimulation with Bk 3.2-fold. In quiet-responders caffeine alone did not affect net Ca2+ storage, whereas Bk or caffeine followed by Bk decreased the intracellular Ca2+ pool to 45% and 15%, respectively. Blockade of the endoplasmic-reticulum Ca2+ pump by thapsigargin unmasked the effect of caffeine in quiet-responders, resulting in a transient increase in intracellular free Ca2+ concentration ([Ca2+]i). In 37% of the cells caffeine alone transiently increased [Ca2+]i and depleted BsS. This study suggests a heterogeneity in functional organization of endothelial Ca2+ stores. In quiet-responders, caffeine translocates Ca2+ towards the BsS, whereas in overt-responders caffeine empties the BsS.