Interactions between inositol 1,4,5-trisphosphate receptors and ryanodine receptors in smooth muscle: one store or two?

PKC Inhibits Sec61 Translocon-Mediated Sarcoplasmic Reticulum Ca2+ Leak in Smooth Muscle Cells

PKC inhibitors stimulate Ca2+ release from internal stores in diverse cell types. Our data indicate that this action cannot be explained by an increased agonist-induced IP3 production or an overloaded SR Ca2+ pool in smooth muscle cells from guinea pig urinary bladder. The incubation of these cells with three different PKC inhibitors, such as Go6976, Go6983, and BIM 1, resulted in a higher SR Ca2+ leak revealed by inhibition of the SERCA pump with thapsigargin. This SR Ca2+ leakage was sensitive to protein translocation inhibitors such as emetine and anisomycin. Since this increased SR Ca2+ leak did not result in a depleted SR Ca2+ store, we have inferred there was a compensatory increase in SERCA pump activity, resulting in a higher steady-state. This new steady-state increased the frequency of Spontaneous Transient Outward Currents (STOCs), which reflect the activation of high conductance, Ca2+-sensitive potassium channels in response to RyR-mediated Ca2+ sparks. This increased STOC frequency triggered by PKC inhibition was restored to normal by inhibiting translocon-mediated Ca2+ leak with emetine. These results suggest a critical role of PKC-mediated translocon phosphorylation in regulating SR Ca2+ steady-state, which, in turn, alters SR Ca2+ releasing activity.

Neuronal microRNAs safeguard ER Ca2+ homeostasis and attenuate the unfolded protein response upon stress

Ca2+ is a critical mediator of neurotransmitter release, synaptic plasticity, and gene expression, but also excitotoxicity. Ca2+ signaling and homeostasis are coordinated by an intricate network of channels, pumps, and calcium-binding proteins, which must be rapidly regulated at all expression levels. Τhe role of neuronal miRNAs in regulating ryanodine receptors (RyRs) and inositol 1,4,5-triphosphate receptors (IP3Rs) was investigated to understand the underlying mechanisms that modulate ER Ca2+ release. RyRs and IP3Rs are critical in mounting and propagating cytosolic Ca2+ signals by functionally linking the ER Ca2+ content, while excessive ER Ca2+ release via these receptors is central to the pathophysiology of a wide range of neurological diseases. Herein, two brain-restricted microRNAs, miR-124-3p and miR-153-3p, were found to bind to RyR1-3 and IP3R3 3'UTRs, and suppress their expression at both the mRNA and protein level. Ca2+ imaging studies revealed that overexpression of these miRNAs reduced ER Ca2+ release upon RyR/IP3R activation, but had no effect on [Ca2+]i under resting conditions. Interestingly, treatments that cause excessive ER Ca2+ release decreased expression of these miRNAs and increased expression of their target ER Ca2+ channels, indicating interdependence of miRNAs, RyRs, and IP3Rs in Ca2+ homeostasis. Furthermore, by maintaining the ER Ca2+ content, miR-124 and miR-153 reduced cytosolic Ca2+ overload and preserved protein-folding capacity by attenuating PERK signaling. Overall, this study shows that miR-124-3p and miR-153-3p fine-tune ER Ca2+ homeostasis and alleviate ER stress responses.

RuBPY decreases intracellular calcium by decreasing influx and increasing storage

RuBPY is a ruthenium complex NO donor that presents a nitrite in its moiety and has been shown to induce vasodilation in various arteries, as well as arterial pressure reduction with no changes in heart rate. Since vascular tone is highly dependent on the cytosolic calcium concentration ([Ca²⁺ ]c), the current study aimed to investigate the effects of RuBPY on the intracellular mobilization of calcium stores of rat aortic vascular smooth muscle cells. Vascular reactivity experiments were performed in isolated aortic rings that were contracted with a high concentration of KCl or phenylephrine (Phe). Moreover, primary cultured vascular smooth muscle cells were used to measure [Ca²⁺ ]c by confocal microscopy. The NO donor RuBPY decreased the [Ca²⁺ ]c and reduced KCl and Phe -induced contractile responses. The selective inhibitor of sarco-endoplasmic Ca-ATPase (SERCA) with thapsigargin impaired the effect of RuBPY on Phe -induced contractile response. RuBPY also reduced caffeine-induced contraction, and the contraction dependent on the capacitive Ca²⁺ influx. Therefore, our results suggest that NO released from RuBPY decreased [Ca²⁺ ]c by calcium influx blockade, and activation of guanylyl-cyclase-cGMP-GK pathway. These results indicate that RuBPY increases Ca²⁺ storage in the sarcoplasmic reticulum by SERCA activation, and also by capacitive Ca²⁺ influx inhibition, which is dependent on the intracellular release of nitric oxide from this compound. This article is protected by copyright. All rights reserved.

The Involvement of Ca(2+) Signal Pathways in Distal Colonic Myocytes in a Rat Model of Dextran Sulfate Sodium-induced Colitis

Background:

Disrupted Ca²⁺ homeostasis contributes to the development of colonic dysmotility in ulcerative colitis (UC), but the underlying mechanisms are unknown. This study aimed to examine the alteration of colonic smooth muscle (SM) Ca²⁺ signaling and Ca²⁺ handling proteins in a rat model of dextran sulfate sodium (DSS)-induced UC.

Methods:

Male Sprague-Dawley rats were randomly divided into control (n = 18) and DSS (n = 17) groups. Acute colitis was induced by 5% DSS in the drinking water for 7 days. Contractility of colonic SM strips (controls, n = 8 and DSS, n = 7) was measured in an organ bath. Cytosolic resting Ca²⁺ levels (n = 3 in each group) and Ca²⁺ transients (n = 3 in each group) were measured in single colonic SM cells. Ca²⁺ handling protein expression was determined by Western blotting (n = 4 in each group). Differences between control and DSS groups were analyzed by a two-sample independent t-test.

Results:

Average tension and amplitude of spontaneous contractions of colonic muscle strips were significantly enhanced in DSS-treated rats compared with controls (1.25 ± 0.08 g vs. 0.96 ± 0.05 g, P = 0.007; and 2.67 ± 0.62 g vs. 0.52 ± 0.10 g, P = 0.013). Average tensions of carbachol-evoked contractions were much weaker in the DSS group (1.08 ± 0.10 g vs. 1.80 ± 0.19 g, P = 0.006). Spontaneous Ca²⁺ transients were observed in more SM cells from DSS-treated rats (15/30 cells) than from controls (5/36 cells). Peak caffeine-induced intracellular Ca²⁺ release was lower in SM cells of DSS-treated rats than controls (0.413 ± 0.046 vs. 0.548 ± 0.041, P = 0.033). Finally, several Ca²⁺ handling proteins in colonic SM were altered by DSS treatment, including sarcoplasmic reticulum calcium-transporting ATPase 2a downregulation and phospholamban and inositol 1,4,5-trisphosphate receptor 1 upregulation.

Conclusions:

Impaired intracellular Ca²⁺ signaling of colonic SM, caused by alteration of Ca²⁺ handing proteins, contribute to colonic dysmotility in DSS-induced UC.

Ethyl rosmarinate relaxes rat aorta by an endothelium-independent pathway

Ethyl rosmarinate is an ester derivative of rosmarinic acid, a major constituent of Hyptis suaveolens. The present study investigated the vasorelaxant mechanism of ethyl rosmarinate in isolated rat aortic rings using an organ bath system. Ethyl rosmarinate (0.1 µM-3mM) produced concentration-dependent relaxation in aortic rings pre-contracted with phenylephrine (10 µM), exhibiting a pD2 value of 4.56 ± 0.08 and an Emax value of 93.82 ± 5.00% (in endothelium-intact rings), as well as a pD2 value of 4.42 ± 0.05 and an Emax value of 92.10 ± 3.78% (in endothelium-denuded rings). In the endothelium-denuded rings, the vasorelaxant effect of ethyl rosmarinate was reduced by only 4-aminopyridine (1mM); however, this was not the case with tetraethylammonium (5mM), glibenclamide (10 µM), barium chloride (1mM), and 1H-[1,2,4] oxadiazolo [4,3-a]quinoxalin-1-one (ODQ, 1 µM). Ethyl rosmarinate also reduced the contraction induced by phenylephrine (10 µM) and caffeine (20mM) in a Ca(2+)-free solution, and inhibited the contraction induced by increasing extracellular Ca(2+) influx, which was induced by KCl (80 mM). Ethyl rosmarinate (10 µM) inhibits concentration-response curves for phenylephrine, while in the same concentration of ethyl rosmarinate has no effect on contractions induced by increasing concentrations of calcium in the presence of high extracellular potassium. Our results suggests that ethyl rosmarinate induces relaxation in aortic rings via an endothelium-independent pathway, which involves the opening of voltage-gated potassium (Kv) channels and the blockade of both Ca(2+)release from intracellular stores and extracellular Ca(2+) influx. Moreover, ethyl-rosmarinate acts on the extracellular Ca(2+) influx inhibition by interacting with voltage-operated calcium channels (VOCCs) and receptor-operated calcium channels (ROCCs).

Hydrogen sulfide: a novel gaseous signaling molecule and intracellular Ca2+ regulator in rat parotid acinar cells

In addition to nitric oxide (NO), hydrogen sulfide (H2S) is recognized as a crucial gaseous messenger that exerts many biological actions in various tissues. An attempt was made to assess the roles and underlying mechanisms of both gases in isolated rat parotid acinar cells. Ductal cells and some acinar cells were found to express NO and H2S synthases. Cevimeline, a muscarinic receptor agonist upregulated endothelial NO synthase in parotid tissue. NO and H2S donors increased the intracellular Ca2+ concentration ([Ca2+]i). This was not affected by inhibitors of phospholipase C and inositol 1,4,5-trisphosphate receptors, but was decreased by blockers of ryanodine receptors (RyRs), soluble guanylyl cyclase, and protein kinase G. The H2S donor evoked NO production, which was decreased by blockade of NO synthases or phosphoinositide 3-kinase or by hypotaurine, an H2S scavenger. The H2S donor-induced [Ca2+]i increase was diminished by a NO scavenger or the NO synthases blocker. These results suggest that NO and H2S play important roles in regulating [Ca2+]i via soluble guanylyl cyclase-cGMP-protein kinase G-RyRs, but not via inositol 1,4,5-trisphosphate receptors. The effect of H2S may be partially through NO produced via phosphoinositide 3-kinase-Akt-endothelial NO synthase. It was concluded that both gases regulate [Ca2+]i in a synergistic way, mainly via RyRs in rat parotid acinar cells.

Gene expression profiles of patients with cerebral hematoma following spontaneous intracerebral hemorrhage

The present study aimed to investigate the gene functions and expression profiles in perihematomal (PH) brain regions following spontaneous intracerebral hemorrhage. The gene expression profiles were downloaded from the Gene Expression Omnibus database under accession number GSE24265, which includes 11 brain samples from different regions, including four samples from PH areas, four from contralateral grey matter (CG) and three from contralateral white matter (CW). The gene expression profiles were pre-processed and the differentially expressed genes (DEGs) between PH and CG tissue, and PH and CW tissue were identified using R packages. The expression of genes in different tissues was analyzed by hierarchical clustering. Then, the interaction network between the DEGs was constructed using String software. Finally, Gene Ontology was performed and pathway analysis was conducted using FuncAssociate and Expression Analysis Systematic Explorer to identify the gene function. As a result, 399 DEGs were obtained between PH and CG, and 756 DEGs were identified between PH and CW. There were 35 common DEGs between the two groups. These DEGs may be involved in PH edema by regulating the calcium signaling pathway [calcium channel, voltage-dependent, T-type, α1I subunit, Ca²⁺/calmodulin-dependent protein kinase II α (CAMK2A), ryanodine receptor 2 (RYR2) and inositol 1,4,5-trisphosphate receptor, type 1 (ITPR1)], cell proliferation (sphingosine kinase 1), neuron differentiation (Ephrin-A5) or extracellular matrix-receptor interaction [collagen, type I, α 2, laminin B1 (LAMB1), syndecan 2, fibronectin 1 and integrin α5 (ITGA5)]. A number of genes may cooperate to participate in the same pathway, such as ITPR1-RYR2, CAMK2A-RYR2 and ITGA5-LAMB1 interaction pairs. The present study provides several potential targets to decrease hematoma expansion and alleviate neuronal cell death following spontaneous intracerebral hemorrhage.

Regulation of gastrointestinal motility--insights from smooth muscle biology

Gastrointestinal motility results from coordinated contractions of the tunica muscularis, the muscular layers of the alimentary canal. Throughout most of the gastrointestinal tract, smooth muscles are organized into two layers of circularly or longitudinally oriented muscle bundles. Smooth muscle cells form electrical and mechanical junctions between cells that facilitate coordination of contractions. Excitation-contraction coupling occurs by Ca(2+) entry via ion channels in the plasma membrane, leading to a rise in intracellular Ca(2+). Ca(2+) binding to calmodulin activates myosin light chain kinase; subsequent phosphorylation of myosin initiates cross-bridge cycling. Myosin phosphatase dephosphorylates myosin to relax muscles, and a process known as Ca(2+) sensitization regulates the activity of the phosphatase. Gastrointestinal smooth muscles are 'autonomous' and generate spontaneous electrical activity (slow waves) that does not depend upon input from nerves. Intrinsic pacemaker activity comes from interstitial cells of Cajal, which are electrically coupled to smooth muscle cells. Patterns of contractile activity in gastrointestinal muscles are determined by inputs from enteric motor neurons that innervate smooth muscle cells and interstitial cells. Here we provide an overview of the cells and mechanisms that generate smooth muscle contractile behaviour and gastrointestinal motility.

The sigma-1 receptor antagonist PB212 reduces the Ca²⁺-release through the inositol (1, 4, 5)-trisphosphate receptor in SK-N-SH cells

Sigma-1 receptors are specifically located at the endoplasmic reticulum-mitochondrion interface, but upon stimulation by ligands or under prolonged cellular stress, they translocate to other areas of the cell. Sigma-1 receptors are involved in the regulation of intracellular [Ca(2+)] by affecting the Ca(2+)-influx or the release from intracellular stores. In SK-N-SH cells, we measured the affinity of 4-methyl-1-[4-(6-methoxynaphthalen-1-yl)butyl]piperidine (PB212) at sigma-1 receptor by using a competition binding assay with specific radioligand; we obtained a K(i) value=316 ± 19 nM. PB212 also showed an antiproliferative effect in SK-N-SH cells (EC(50)=32 ± 4 μM) but had no effect in MCF7 cells, which only express sigma-2 receptor; these findings suggest that PB212 behaves as a sigma-1 receptor antagonist. We have studied the effect of PB212 on Ca(2+) homeostasis of the SK-N-SH cell line with the fluorescent probe Fura-2. 100 μM PB212 induced a Ca(2+)-efflux from the endoplasmic reticulum through the inositol (1, 4, 5)-trisphosphate (IP(3)) receptor. Moreover, [PB212] ranging from 1 to 100μM reduced the Ca(2+)-response, triggered by carbachol or bradykinin that engage the phospholipase C/IP(3) pathway; such a response is generally increased by sigma-1 receptor agonists. On the other hand, PB212 did not reduce the Ca(2+)-response mediated by IP(3) in LoVo cells, which do not express neither sigma-1 nor sigma-2 receptors, and in MCF7 cells. The fact that the activity of the sigma-1 receptor can be experimentally modulated by agonists and antagonists supports the intriguing hypothesis that some endogenous molecules, unknown at the moment, modulate the sigma-1 receptor and its cellular targets.

Ca2+ responses of pulmonary arterial myocytes to acute hypoxia require release from ryanodine and inositol trisphosphate receptors in sarcoplasmic reticulum

In pulmonary arterial smooth muscle cells (PASMC), acute hypoxia increases intracellular Ca(2+) concentration ([Ca(2+)](i)) by inducing Ca(2+) release from the sarcoplasmic reticulum (SR) and Ca(2+) influx through store- and voltage-operated Ca(2+) channels in sarcolemma. To evaluate the mechanisms of hypoxic Ca(2+) release, we measured [Ca(2+)](i) with fluorescent microscopy in primary cultures of rat distal PASMC. In cells perfused with Ca(2+)-free Krebs Ringer bicarbonate solution (KRBS), brief exposures to caffeine (30 mM) and norepinephrine (300 μM), which activate SR ryanodine and inositol trisphosphate receptors (RyR, IP(3)R), respectively, or 4% O(2) caused rapid transient increases in [Ca(2+)](i), indicating intracellular Ca(2+) release. Preexposure of these cells to caffeine, norepinephrine, or the SR Ca(2+)-ATPase inhibitor cyclopiazonic acid (CPA; 10 μM) blocked subsequent Ca(2+) release to caffeine, norepinephrine, and hypoxia. The RyR antagonist ryanodine (10 μM) blocked Ca(2+) release to caffeine and hypoxia but not norepinephrine. The IP(3)R antagonist xestospongin C (XeC, 0.1 μM) blocked Ca(2+) release to norepinephrine and hypoxia but not caffeine. In PASMC perfused with normal KRBS, acute hypoxia caused a sustained increase in [Ca(2+)](i) that was abolished by ryanodine or XeC. These results suggest that in rat distal PASMC 1) the initial increase in [Ca(2+)](i) induced by hypoxia, as well as the subsequent Ca(2+) influx that sustained this increase, required release of Ca(2+) from both RyR and IP(3)R, and 2) the SR Ca(2+) stores accessed by RyR, IP(3)R, and hypoxia functioned as a common store, which was replenished by a CPA-inhibitable Ca(2+)-ATPase.

Investigation of mechanisms involved in (-)-borneol-induced vasorelaxant response on rat thoracic aorta

The monoterpene (-)-borneol is present in essential oils of several medicinal plants. The aim of this study was to evaluate (-)-borneol effects on rat thoracic aorta artery rings. The cumulative addition of (-)-borneol (10(-9) -3 × 10(-4)  M) on a phenylephrine-induced pre-contraction (10(-6)  M) promoted a vasorelaxant effect in a concentration-dependent manner and independent of vascular endothelium. A similar effect was obtained on KCl-induced pre-contractions (80 mM). (-)-Borneol (10(-5) -3 × 10(-4 ) M) inhibited contractions induced by cumulative addition of CaCl2 (10(-6) -3 × 10(-2)  M) in depolarizing medium without Ca(2+) in a concentration-dependent manner. On S-(-) Bay K 8644-induced pre-contractions (10(-7)  M), (-)-borneol did not induce significant changes compared with KCl-induced pre-contractions. In a Ca(2+) -free medium, (-)-borneol (10(-5) , 10(-4) or 10(-3)  M) interfered in calcium mobilization from phenylephrine (10(-6)  M)- or caffeine (20 mM)-sensitive intracellular stores. The involvement of K(+) channels was evaluated by tetraethylammonium (3 mM), 4-aminopyridine (1 mM) and glibenclamide (10(-5)  M) pre-treatment, and (-)-borneol-induced vasorelaxation was markedly attenuated. Thus, this vasorelaxant effect can probably be attributed to calcium influx blockade through voltage-operated calcium channels (CaV L), calcium mobilization from intracellular stores and potassium channels activation.

Endothelin 1 stimulates Ca2+-sparks and oscillations in retinal arteriolar myocytes via IP3R and RyR-dependent Ca2+ release

PURPOSE

To investigate endothelin 1 (Et1)-dependent Ca(2+)-signaling at the cellular and subcellular levels in retinal arteriolar myocytes.

METHODS

Et1 responses were imaged from Fluo-4-loaded smooth muscle in isolated segments of rat retinal arteriole using confocal laser microscopy.

RESULTS

Basal [Ca(2+)](i), subcellular Ca(2+)-sparks, and cellular Ca(2+)-oscillations were all increased during exposure to Et1 (10 nM). Ca(2+)-spark frequency was also increased by 90% by 10 nM Et1. The increase in oscillation frequency was concentration dependent and was inhibited by the EtA receptor (Et(A)R) blocker BQ123 but not by the EtB receptor antagonist BQ788. Stimulation of Ca(2+)-oscillations by Et1 was inhibited by a phospholipase C blocker (U73122; 10 μM), two inhibitors of inositol 1,4,5-trisphosphate receptors (IP(3)Rs), xestospongin C (10 μM), 2-aminoethoxydiphenyl borate (100 μM), and tetracaine (100 μM), a blocker of ryanodine receptors (RyRs).

CONCLUSIONS

Et1 stimulates Ca(2+)-sparks and oscillations through Et(A)Rs. The underlying mechanism involves the activation of phospholipase C and both IP(3)Rs and RyRs, suggesting crosstalk between these Ca(2+)-release channels. These findings suggest that phasic Ca(2+)-oscillations play an important role in the smooth muscle response to Et1 within the retinal microvasculature and support an excitatory, proconstrictor role for Ca(2+)-sparks in these vessels.

Ryanodine receptors selectively contribute to the formation of taste-evoked calcium signals in mouse taste cells

The peripheral taste system uses multiple signaling pathways to transduce a stimulus into an output signal that activates afferent neurons. All of these signaling pathways depend on transient increases in intracellular calcium, but current understanding of these calcium signals is not well developed. Using molecular and physiological techniques, this study establishes that ryanodine receptors (RyRs), specifically isoform 1, are expressed in taste cells and that their physiological function differs among cell types employing different signaling pathways. RyR1 contributes to some taste-evoked signals that rely on calcium release from internal stores but can also supplement the calcium signal that is initiated by opening voltage-gated calcium channels. In taste cells expressing both signaling pathways, RyR1 contributes to the depolarization-induced calcium signal but not to the calcium signal that depends on calcium release from stores. These data suggest that RyR1 is an important regulator of calcium signaling and that its physiological role in taste cells is dictated by the nature of the calcium signaling mechanisms expressed.

Minding the calcium store: Ryanodine receptor activation as a convergent mechanism of PCB toxicity

Chronic low-level polychlorinated biphenyl (PCB) exposures remain a significant public health concern since results from epidemiological studies indicate that PCB burden is associated with immune system dysfunction, cardiovascular disease, and impairment of the developing nervous system. Of these various adverse health effects, developmental neurotoxicity has emerged as a particularly vulnerable endpoint in PCB toxicity. Arguably the most pervasive biological effects of PCBs could be mediated by their ability to alter the spatial and temporal fidelity of Ca2+ signals through one or more receptor-mediated processes. This review will focus on our current knowledge of the structure and function of ryanodine receptors (RyRs) in muscle and nerve cells and how PCBs and related non-coplanar structures alter these functions. The molecular and cellular mechanisms by which non-coplanar PCBs and related structures alter local and global Ca2+ signaling properties and the possible short and long-term consequences of these perturbations on neurodevelopment and neurodegeneration are reviewed.

cAMP/PKA-dependent increases in Ca Sparks, oscillations and SR Ca stores in retinal arteriolar myocytes after exposure to vasopressin

PURPOSE

To investigate the effects of arginine vasopressin (AVP) on Ca(2+) sparks and oscillations and on sarcoplasmic reticulum (SR) Ca(2+) content in retinal arteriolar myocytes.

METHODS

Fluo-4-loaded smooth muscle in intact segments of freshly isolated porcine retinal arteriole was imaged by confocal laser microscopy. SR Ca(2+) store content was assessed by recording caffeine-induced Ca(2+) transients with microfluorimetry and fura-2.

RESULTS

The frequencies of Ca(2+) sparks and oscillations were increased both during exposure to, and 10 minutes after washout of AVP (10 nM). Caffeine transients were increased in amplitude 10 and 90 minutes after a 3-minute application of AVP. Both AVP-induced Ca(2+) transients and the enhancement of caffeine responses after AVP washout were inhibited by SR 49059, a V(1a) receptor blocker. Forskolin, an activator of adenylyl cyclase, also persistently enhanced caffeine transients. Rp-8-HA-cAMPS, a membrane-permeant PKA inhibitor, prevented enhancement of caffeine transients by both AVP and forskolin. Forskolin, but not AVP, produced a reversible, Rp-8-HA-cAMPS insensitive reduction in basal [Ca(2+)](i).

CONCLUSIONS

AVP activates a cAMP/PKA-dependent pathway via V(1a) receptors in retinal arteriolar smooth muscle. This effect persistently increases SR Ca(2+) loading, upregulating Ca(2+) sparks and oscillations, and may favor prolonged agonist activity despite receptor desensitization.

The contribution of inositol 1,4,5-trisphosphate and ryanodine receptors to agonist-induced Ca(2+) signaling of airway smooth muscle cells

The relative contribution of inositol 1,4,5-trisphosphate (IP(3)) receptors (IP(3)Rs) and ryanodine receptors (RyRs) to agonist-induced Ca(2+) signaling in mouse airway smooth muscle cells (SMCs) was investigated in lung slices with phase-contrast or laser scanning microscopy. At room temperature (RT), methacholine (MCh) or 5-hydroxytryptamine (5-HT) induced Ca(2+) oscillations and an associated contraction in small airway SMCs. The subsequent exposure to an IP(3)R antagonist, 2-aminoethoxydiphenyl borate (2-APB), inhibited the Ca(2+) oscillations and induced airway relaxation in a concentration-dependent manner. 2-APB also inhibited Ca(2+) waves generated by the photolytic release of IP(3). However, the RyR antagonist ryanodine had no significant effect, at any concentration, on airway contraction or agonist- or IP(3)-induced Ca(2+) oscillations or Ca(2+) wave propagation. By contrast, a second RyR antagonist, tetracaine, relaxed agonist-contracted airways and inhibited agonist-induced Ca(2+) oscillations in a concentration-dependent manner. However, tetracaine did not affect IP(3)-induced Ca(2+) release or wave propagation nor the Ca(2+) content of SMC Ca(2+) stores as evaluated by Ca(2+)-release induced by caffeine. Conversely, both ryanodine and tetracaine completely blocked agonist-independent slow Ca(2+) oscillations induced by KCl. The inhibitory effects of 2-APB and absence of an effect of ryanodine on MCh-induced airway contraction or Ca(2+) oscillations of SMCs were also observed at 37 degrees C. In Ca(2+)-permeable SMCs, tetracaine inhibited agonist-induced contraction without affecting intracellular Ca(2+) levels indicating that relaxation also resulted from a reduction in Ca(2+) sensitivity. These results indicate that agonist-induced Ca(2+) oscillations in mouse small airway SMCs are primary mediated via IP(3)Rs and that tetracaine induces relaxation by both decreasing Ca(2+) sensitivity and inhibiting agonist-induced Ca(2+) oscillations via an IP(3)-dependent mechanism.

Mechanisms of agonist-induced constriction in isolated human pulmonary arteries

We determined the calcium signalling pathways involved in the mechanisms of contraction of the vasoconstrictive agonists KCl, U46619 and PDBu in isolated human pulmonary arteries. The influence of gender, vessel diameter and age of the patients was also investigated. Human pulmonary arteries (n = 86) were loaded in a wire myograph and maintained at a tension equivalent to the in vivo pressure of 17.5 mm Hg, bubbled with 95%O2/5%CO2 to maintain pH 7.4 in physiological saline solution (PSS). Cumulative concentration-response curves were obtained to KCl (100 microM-100 mM), U46619 (1 nM-1 microM) or PDBu (1 nM-1 microM), before or after a 30 min incubation with either the voltage-gated calcium channel (VGCC) blocker nifedipine (10 microM), the store-operated calcium channel (SOCC) blocker SK&F96365 (50 microM) or in calcium-free PSS (-Ca2+PSS). The KCl response was partially blocked in -Ca2+PSS and with nifedipine. The U46619 response was partially blocked in -Ca2+PSS and with nifedipine and SK&F96365. Incubation in -Ca2+PSS had no effect on the response to PDBu. Endothelial intact arteries responded significantly higher to U46619 than endothelial denuded arteries. This study demonstrates that KCl induces pulmonary vasoconstriction via activation of extracellular calcium entry through VGCCs, U46619 induces pulmonary vasoconstriction predominantly via activation of VGCCs and PDBu induces pulmonary vasoconstriction via a calcium-independent pathway.

Mechanisms of the lysophosphatidic acid-induced increase in [Ca(2+)](i) in skeletal muscle cells

Although lysophosphatidic acid (LPA) is known to increase intracellularfree calcium concentration ([Ca²⁺]_i) in different cell types, the effect of LPA on the skeletal muscle cells is not known. The present study was therefore undertaken to examine the effect of LPA on the [Ca²⁺]_i in C2C12 cells. LPA induced a concentration and time dependent increase in [Ca²⁺]_i, which was inhibited by VPC12249, VPC 32183 and dioctanoyl glycerol pyrophosphate, LPA1/3 receptor antagonists. Pertussis toxin, a Gⁱ protein inhibitor, also inhibited the LPA-induced increase in [Ca²⁺]_i. Inhibition of tyrosine kinase activities with tyrphostin A9 and genistein also prevented the increase in [Ca²⁺]_i due to LPA. Likewise, wortmannin and LY 294002, phosphatidylinositol 3-kinase (PI3-K) inhibitors, inhibited [Ca²⁺]_i response to LPA. The LPA effect was also attenuated by ethylene glycolbis(β-aminoethylether)-N,N,N′,N′-tetraacetic acid (EGTA), an extracellular Ca²⁺ chelator, Ni²⁺ and KB-R7943, inhibitors of the Na⁺-Ca²⁺ exchanger; the receptor operated Ca²⁺ channel (ROC) blockers, 2-aminoethoxydiphenyl borate and SK&F 96365. However, the L-type Ca²⁺ channel blockers, verapamil and diltiazem; the store operated Ca²⁺ channel blockers, La³⁺ and Gd³⁺; a sarcoplasmic reticulum calcium pump inhibitor, thapsigargin; an inositol trisphosphate receptor antagonist, xestospongin and a phospholipase C inhibitor, U73122, did not prevent the increase [Ca²⁺]_i due to LPA. Our data suggest that the LPA-induced increase in [Ca²⁺]_i might occur through Gⁱ-protein coupled LPA_1/3 receptors that may be linked to tyrosine kinase and PI3-K, and may also involve the Na⁺-Ca²⁺ exchanger as well as the ROC. In addition, LPA stimulated C2C12 cell proliferation via PI3-K. Thus, LPA may be an important phospholipid in the regulation of [Ca²⁺]_i and growth of skeletal muscle cells.

Ca2+ signalling in cardiogenesis

A variety of InsP3-dependent Ca2+ signals coded in time, amplitude and space occur during the process of cardiac cell differentiation and heart development. Studies performed in both embryos and embryonic stem cell differentiating in cardiomyocytes have uncovered that Ca2+ regulates multiple steps of these biological processes in vertebrates. These include secretion of cardiogenic factors, cardiac transcriptional cascades and in turn gene expression, myofibrillogenesis and initiation of embryonic pacemaker activity. Evidence is accumulating to foresee Ca2+ as a major second messenger in directing the fate of stem cells and patterning the heart in the embryo.

Redox control of calcium channels: from mechanisms to therapeutic opportunities

Calcium plays an integral role in cellular function. It is a well-recognized second messenger necessary for signaling cellular responses, but in excessive amounts can be deleterious to function, causing cell death. The main route by which calcium enters the cytoplasm is either from the extracellular compartment or internal addistores via calcium channels. There is good evidence that calcium channels can respond to pharmacological compounds that reduce or oxidize thiol groups on the channel protein. In addition, reactive oxygen species such as hydrogen peroxide and superoxide that can mediate oxidative pathology also mediate changes in channel function via alterations of thiol groups. This review looks at the structure and function of calcium channels, the evidence that changes in cellular redox state mediate changes in channel function, and the role of redox modification of channels in disease processes. Understanding how redox modification of the channel protein alters channel structure and function is providing leads for the design of therapeutic interventions that target oxidative stress responses.

Caveolae in smooth muscles: nanocontacts

Smooth muscle cell (SMC) caveolae have been investigated by quantitative and qualitative analysis of transmission electron microscopy (TEM) images of rat stomach, bladder and myometrium, guinea pig taenia coli, human ileum, and rat aortic SMCs. Ultrathin (below 30 nm) serial sections were used for examination of caveolar morphology and their connections with SMC organelles. Average caveolar diameter was smaller in vascular SMCs (70 nm, n=50) than in visceral SMCs (77 nm, n=100), but with the same morphology. Most of the caveolae, featured as flask-shaped plasma membrane (PM) invaginations, opened to the extracellular space through a 20 nm stoma (21, 3nm) having a 7 nm thick diaphragm. A small percentage of caveolae (3%), gathered as grape-like clusters, did not open directly to the extracellular space, but to irregular PM pockets having a 20-30 nm opening to the extracellular space. In visceral SMCs, caveolae were disposed in 4 - 6 rows, parallel to myofilaments, whilst aortic SMCs caveolae were arranged as clusters. This caveolar organization in rows or clusters minimizes the occupied volume, providing more space for the contractile machinery. The morphometric analysis of relative volumes (% of cell volume) showed that caveolae were more conspicuous in visceral than in vascular SMCs (myometrium - 2.40%; bladder - 3.66%, stomach - 2.61%, aorta - 1.43%). We also observed a higher number of caveolae per length unit of cell membrane in most visceral SMCs compared to vascular SMCs (myometrium - 1.06/μm, bladder - 0.74/μm, aorta - 0.57/μm, stomach - 0.48/μm). Caveolae increase the cellular perimeter up to 15% and enlarge the surface area of the plasma membrane about 80% in SMCs. Three-dimensional reconstructions (15μ³) showed that most caveolae, in both visceral and vascular SMCs, have nanocontacts with SR (87%), or with mitochondria (10%), and only 3%, apparently, have no contact with these organelles. Usually, 15 nm wide junctional spaces exist between caveolae and SR, some of them with nanostructural links between each other or with mitochondria: direct contacts (space < 2 nm or none) and molecular links, so called ‘feet’ (about 12 nm electron dense structures between organellar membranes). Direct contacts possibly allow molecular translocation between the two membranes. Electron-dense ‘feet’-like structures suggest a molecular link between these organelles responsible for intracellular Ca²⁺ homeostasis (excitation-contraction coupling or pharmaco-mechan-ical coupling). Close appositions (∼15 nm) have also been observed between caveolae and perinuclear SR cisterna, suggesting that caveolae might be directly implicated in excitation-transcription coupling.

Calcium waves in intact guinea pig gallbladder smooth muscle cells

Intracellular Ca(2+) waves and spontaneous transient depolarizations were investigated in gallbladder smooth muscle (GBSM) whole mount preparations with intact mucosal layer [full thickness (FT)] by laser confocal imaging of intracellular Ca(2+) and voltage recordings with microelectrodes, respectively. Spontaneous Ca(2+) waves arose most often near the center, but sometimes from the extremities, of GBSM cells. They propagated regeneratively by Ca(2+)-induced Ca(2+) release involving inositol 1,4,5-trisphosphate [Ins(1,4,5)P(3)] receptors and were not affected by TTX and atropine (ATS). Spontaneous Ca(2+) waves and spontaneous transient depolarizations were more prevalent in FT than in isolated muscularis layer preparations and occurred with similar pattern in GBSM bundles. Ca(2+) waves were abolished by the Ins(1,4,5)P(3) receptor inhibitors 2-aminoethoxydiphenyl borate and xestospongin C and by caffeine and cyclopiazonic acid. These events were reduced by voltage-dependent calcium channels (VDCCs) inhibitors diltiazem and nifedipine, by PLC inhibitor U-73122, and by thapsigargin and ryanodine. ACh, CCK, and carbachol augmented Ca(2+) waves and induced Ca(2+) flashes. The actions of these agonists were inhibited by U-73122. These results indicate that in GBSM, discharge and propagation of Ca(2+) waves depend on sarco(endo)plasmic reticulum (SR) Ca(2+) release via Ins(1,4,5)P(3) receptors, PLC activity, Ca(2+) influx via VDCCs, and SR Ca(2+) concentration. Neurohormonal enhancement of GBSM excitability involves PLC-dependent augmentation and synchronization of SR Ca(2+) release via Ins(1,4,5)P(3) receptors. Ca(2+) waves likely reflect the activity of a fundamental unit of spontaneous activity and play an important role in the excitability of GBSM.

Mechanisms of agonist-induced constriction in isolated human mesenteric arteries

We determined the calcium signalling pathways involved in the mechanisms of contraction of the vasoconstrictive agonists KCl, U46619 and PDBu in isolated human mesenteric arteries. The influence of gender, vessel diameter and age of the patients was also investigated. Human mesenteric arteries (n = 76) were loaded in a wire myograph and maintained at a tension equivalent to the in vivo pressure of 100 mm Hg, bubbled with 95%O2/5%CO2 to maintain pH 7.4 in physiological saline solution (PSS). Cumulative concentration-response curves were obtained to KCl (100 microM-100 mM), U46619 (1 nM-1 microM) or PDBu (1 nM-1 microM), before or after a 30 min incubation with either the voltage-gated calcium channel (VGCC) blocker nifedipine (10 microM), the store-operated calcium channel (SOCC) blocker SK&F96365 (50 microM) or in calcium-free PSS (-Ca2+ PSS). The KCl response was abolished in -Ca2+ PSS and with nifedipine. The U46619 response was partially blocked in -Ca2+ PSS and with nifedipine and predominantly blocked by SK&F96365. Incubation in -Ca2+ PSS had no effect on the response to PDBu. Arteries from male patients responded significantly higher to KCl than arteries from female patients. This study demonstrates that KCl induces mesenteric vasoconstriction via activation of VGCCs, U46619 induces mesenteric vasoconstriction via activation of SOCCs, but also VGCCs and PDBu induce mesenteric vasoconstriction via a calcium-independent pathway.

Spontaneous electrical rhythmicity and the role of the sarcoplasmic reticulum in the excitability of guinea pig gallbladder smooth muscle cells

Spontaneous action potentials and Ca(2+) transients were investigated in intact gallbladder preparations to determine how electrical events propagate and the cellular mechanisms that modulate these events. Rhythmic phasic contractions were preceded by Ca(2+) flashes that were either focal (limited to one or a few bundles), multifocal (occurring asynchronously in several bundles), or global (simultaneous flashes throughout the field). Ca(2+) flashes and action potentials were abolished by inhibiting sarcoplasmic reticulum (SR) Ca(2+) release via inositol (1,4,5)-trisphosphate [Ins(1,4,5)P(3)] channels with 2-aminoethoxydiphenyl borate and xestospongin C or by inhibiting voltage-dependent Ca(2+) channels (VDCCs) with nifedipine or diltiazem or nisoldipine. Inhibiting ryanodine channels with ryanodine caused multiple spikes superimposed upon plateaus of action potentials and extended quiescent periods. Depletion of SR Ca(2+) stores with thapsigargin or cyclopiazonic acid increased the frequency and duration of Ca(2+) flashes and action potentials. Acetylcholine, carbachol, or cholecystokinin increased synchronized and increased the frequency of Ca(2+) flashes and action potentials. The phospholipase C (PLC) inhibitor U-73122 did not affect Ca(2+) flash or action potential activity but inhibited the excitatory effects of acetylcholine on these events. These results indicate that Ca(2+) flashes correspond to action potentials and that rhythmic excitation in the gallbladder is multifocal among gallbladder smooth muscle bundles and can be synchronized by excitatory agonists. These events do not depend on PLC activation, but agonist stimulation involves activation of PLC. Generation of these events depends on Ca(2+) entry via VDCCs and on Ca(2+) mobilization from the SR via Ins(1,4,5)P(3) channels.

Roles of CaM kinase II and phospholamban in SNP-induced relaxation of murine gastric fundus smooth muscles

The mechanisms by which nitric oxide (NO) relaxes smooth muscles are unclear. The NO donor sodium nitroprusside (SNP) has been reported to increase the Ca2+ release frequency (Ca2+ sparks) through ryanodine receptors (RyRs) and activate spontaneous transient outward currents (STOCs), resulting in smooth muscle relaxation. Our findings that caffeine relaxes and hyperpolarizes murine gastric fundus smooth muscles and increases phospholamban (PLB) phosphorylation by Ca2+/calmodulin (CaM)-dependent protein kinase II (CaM kinase II) suggest that PLB phosphorylation by CaM kinase II participates in smooth muscle relaxation by increasing sarcoplasmic reticulum (SR) Ca2+ uptake and the frequencies of SR Ca2+ release events and STOCs. Thus, in the present study, we investigated the roles of CaM kinase II and PLB in SNP-induced relaxation of murine gastric fundus smooth muscles. SNP hyperpolarized and relaxed gastric fundus circular smooth muscles and activated CaM kinase II. SNP-induced CaM kinase II activation was prevented by KN-93. Ryanodine, tetracaine, 2-aminoethoxydiphenylborate, and cyclopiazonic acid inhibited SNP-induced fundus smooth muscle relaxation and CaM kinase II activation. The Ca2+-activated K+ channel blockers iberiotoxin and apamin inhibited SNP-induced hyperpolarization and relaxation. The soluble guanylate cyclase inhibitor 1H-[1,2,4]oxadiazolo-[4,3-alpha]quinoxalin-1-one inhibited SNP-induced relaxation and CaM kinase II activation. The membrane-permeable cGMP analog 8-bromo-cGMP relaxed gastric fundus smooth muscles and activated CaM kinase II. SNP increased phosphorylation of PLB at Ser16 and Thr17. Thr17 phosphorylation of PLB was inhibited by cyclopiazonic acid and KN-93. Ser16 and Thr17 phosphorylation of PLB was sensitive to 1H-[1,2,4]oxadiazolo-[4,3-alpha]quinoxalin-1-one. These results demonstrate a novel pathway linking the NO-soluble guanylyl cyclase-cGMP pathway, SR Ca2+ release, PLB, and CaM kinase II to relaxation in gastric fundus smooth muscles.

IP3-mediated Ca2+ increases do not involve the ryanodine receptor, but ryanodine receptor antagonists reduce IP3-mediated Ca2+ increases in guinea-pig colonic smooth muscle cells

Smooth muscle responds to IP3-generating (sarcolemma acting) neurotransmitters and hormones by releasing Ca2+ from the sarcoplasmic reticulum (SR) via IP3 receptors (IP3Rs). This release may propagate as Ca2+ waves. The Ca2+ signal emanating from IP3 generation may be amplified by its activating further Ca2+ release from ryanodine receptors (RyRs) in the process of Ca2+-induced Ca2+ release (CICR). Evidence for this proposal has relied largely on the use of blocking drugs such as ryanodine, tetracaine and dantrolene, reportedly specific inhibitors of RyRs. Here we have examined whether or not Ca2+ released via IP3Rs subsequently activates RyRs. In addition, the specificity of the blocking agents has been assessed by determining the extent of their ability to block IP3-mediated Ca2+ release under conditions in which RyRs were not activated. IP3-evoked Ca2+ release and Ca2+ waves did not require or activate RyRs. However, the RyR blocking drugs inhibited IP3-mediated Ca2+ signals at concentrations thought to be selective for RyRs. In single colonic smooth muscle cells, voltage clamped in the whole cell configuration, carbachol (CCh) evoked propagating Ca2+ waves which were not inhibited by ryanodine when the sarcolemma potential was -70 mV. At -20 mV, at which potential the SR Ca2+ content was increased and RyRs activated, ryanodine inhibited the Ca2+ waves. Photolysed caged IP3 increased [Ca2+]c; ryanodine, by itself, did not reduce the IP3-evoked [Ca2+]c increase when the sarcolemma potential was maintained at -70 mV. However, after activation of RyRs by caffeine, in the continued presence of ryanodine, the IP3-evoked [Ca2+]c increase was inhibited. In other experiments, RyRs were activated (as evidenced by the occurrence of spontaneous transient outward currents) by depolarizing the sarcolemma to -20 mV and again ryanodine was effective in inhibiting IP3-evoked Ca2+ increase. Thus while ineffective by itself, ryanodine inhibited IP3-evoked Ca2+ increases, presumably by causing persistent opening of the channel and depleting the SR of Ca2+, after RyRs were activated. These experiments establish that IP3-evoked Ca2+ release and Ca2+ waves do not activate RyRs; had they done so ryanodine would have inhibited the Ca2+ increase. However, under conditions where ryanodine was ineffective against the IP3-evoked Ca2+ transient (i.e. when RyRs were not activated, e.g. at a membrane potential of -70 mV) tetracaine and dantrolene each blocked IP3-evoked Ca2+ increases. The results show that although IP3-mediated Ca2+ release does not activate RyRs, RyR blockers can inhibit IP3-mediated Ca2+ signals.

Interplay between ryanodine and IP3 receptors in ATP-stimulated mouse luteinized-granulosa cells

In mouse luteinized-granulosa cells (MGLC), ATP induces an increase in intracellular Ca2+ concentration by stimulating phospholipase C (PLC) associated with purinergic receptors, leading to production of inositol 1,4,5-trisphosphate (IP3) and subsequent release of Ca2+ from intracellular stores. In this study, we examined the cross-talk between the ryanodine receptors (RyR) and IP3 receptors (IP3R) in response to ATP in MGLC. Specifically, the effect of RyR modulators on ATP response was examined. The results showed that ATP-induced intracellular calcium elevation was abolished by inhibitors of the RyR, such as dantrolene (25 microM) and ryanodine (80 microM). When the MGLC were stimulated with activators of RyR, 2 microM ryanodine and 10 mM caffeine, the ATP-elicited response was decreased. These actions were independent of IP3 production stimulated by ATP. Hence, ATP-induced intracellular Ca2+ mobilization involves the coordinated action of both types of calcium release channels (CRCs). Using fluorescent probes, it was shown that IP3R is uniformly distributed throughout the cell; in contrast, RyR is mainly found around the nuclei. It is concluded that the IP3R and the RyR are functionally associated, and both play a role in the pattern of Ca2+ increase observed during purinergic stimulation of MGLC. This coupling may provide a highly efficient amplification mechanism for ATP stimulation of Ca2+ mobilization.