Expression and distribution of the type 1 and type 3 inositol 1,4, 5-trisphosphate receptor in developing vascular smooth muscle

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.

Sarcoplasmic reticulum and calcium signaling in muscle cells: Homeostasis and disease

The sarco/endoplasmic reticulum is an extensive, dynamic and heterogeneous membranous network that fulfills multiple homeostatic functions. Among them, it compartmentalizes, stores and releases calcium within the intracellular space. In the case of muscle cells, calcium released from the sarco/endoplasmic reticulum in the vicinity of the contractile machinery induces cell contraction. Furthermore, sarco/endoplasmic reticulum-derived calcium also regulates gene transcription in the nucleus, energy metabolism in mitochondria and cytosolic signaling pathways. These diverse and overlapping processes require a highly complex fine-tuning that the sarco/endoplasmic reticulum provides by means of its numerous tubules and cisternae, specialized domains and contacts with other organelles. The sarco/endoplasmic reticulum also possesses a rich calcium-handling machinery, functionally coupled to both contraction-inducing stimuli and the contractile apparatus. Such is the importance of the sarco/endoplasmic reticulum for muscle cell physiology, that alterations in its structure, function or its calcium-handling machinery are intimately associated with the development of cardiometabolic diseases. Cardiac hypertrophy, insulin resistance and arterial hypertension are age-related pathologies with a common mechanism at the muscle cell level: the accumulation of damaged proteins at the sarco/endoplasmic reticulum induces a stress response condition termed endoplasmic reticulum stress, which impairs proper organelle function, ultimately leading to pathogenesis.

Calcium signals that determine vascular resistance

Small arteries in the body control vascular resistance, and therefore, blood pressure and blood flow. Endothelial and smooth muscle cells in the arterial walls respond to various stimuli by altering the vascular resistance on a moment to moment basis. Smooth muscle cells can directly influence arterial diameter by contracting or relaxing, whereas endothelial cells that line the inner walls of the arteries modulate the contractile state of surrounding smooth muscle cells. Cytosolic calcium is a key driver of endothelial and smooth muscle cell functions. Cytosolic calcium can be increased either by calcium release from intracellular stores through IP3 or ryanodine receptors, or the influx of extracellular calcium through ion channels at the cell membrane. Depending on the cell type, spatial localization, source of a calcium signal, and the calcium-sensitive target activated, a particular calcium signal can dilate or constrict the arteries. Calcium signals in the vasculature can be classified into several types based on their source, kinetics, and spatial and temporal properties. The calcium signaling mechanisms in smooth muscle and endothelial cells have been extensively studied in the native or freshly isolated cells, therefore, this review is limited to the discussions of studies in native or freshly isolated cells. This article is categorized under: Biological Mechanisms > Cell Signaling Laboratory Methods and Technologies > Imaging Models of Systems Properties and Processes > Mechanistic Models.

PLCγ1-PKCε-IP3R1 signaling plays an important role in hypoxia-induced calcium response in pulmonary artery smooth muscle cells

Hypoxia-induced pulmonary vasoconstriction (HPV) is attributed to an increase in intracellular Ca²⁺ concentration ([Ca²⁺]_i) in pulmonary artery smooth muscle cells (PASMCs). We have reported that phospholipase C-γ1 (PLCγ1) plays a significant role in the hypoxia-induced increase in [Ca²⁺]_i in PASMCs and attendant HPV. In this study, we intended to determine molecular mechanisms for hypoxic Ca²⁺ and contractile responses in PASMCs. Our data reveal that hypoxic vasoconstriction occurs in pulmonary arteries, but not in mesenteric arteries. Hypoxia caused a large increase in [Ca²⁺]_i in PASMCs, which is diminished by the PLC inhibitor U73122 and not by its inactive analog U73433 . Hypoxia augments PLCγ1-dependent inositol 1,4,5-trisphosphate (IP₃) generation. Exogenous ROS, hydrogen peroxide (H₂O₂), increases PLCγ1 phosphorylation at tyrosine-783 and IP₃ production. IP₃ receptor-1 (IP₃R1) knock-down remarkably diminishes hypoxia- or H₂O₂-induced increase in [Ca²⁺]_i. Hypoxia or H₂O₂ increases the activity of IP₃Rs, which is significantly reduced in protein kinase C-ε (PKCε) knockout PASMCs. A higher PLCγ1 expression, activity, and basal [Ca²⁺]_i are found in PASMCs, but not in mesenteric artery smooth muscle cells from mice exposed to chronic hypoxia (CH) for 21 days. CH enhances H₂O₂- and ATP-induced increase in [Ca²⁺]_i in PASMCs and PLC-dependent, norepinephrine-evoked pulmonary vasoconstriction. In conclusion, acute hypoxia uniquely causes ROS-dependent PLCγ1 activation, IP₃ production, PKCε activation, IP₃R1 opening, Ca²⁺ release, and contraction in mouse PASMCs; CH enhances PASM PLCγ1 expression, activity, and function, playing an essential role in pulmonary hypertension in mice.

Smooth Muscle Ion Channels and Regulation of Vascular Tone in Resistance Arteries and Arterioles

Vascular tone of resistance arteries and arterioles determines peripheral vascular resistance, contributing to the regulation of blood pressure and blood flow to, and within the body's tissues and organs. Ion channels in the plasma membrane and endoplasmic reticulum of vascular smooth muscle cells (SMCs) in these blood vessels importantly contribute to the regulation of intracellular Ca2+ concentration, the primary determinant of SMC contractile activity and vascular tone. Ion channels provide the main source of activator Ca2+ that determines vascular tone, and strongly contribute to setting and regulating membrane potential, which, in turn, regulates the open-state-probability of voltage gated Ca2+ channels (VGCCs), the primary source of Ca2+ in resistance artery and arteriolar SMCs. Ion channel function is also modulated by vasoconstrictors and vasodilators, contributing to all aspects of the regulation of vascular tone. This review will focus on the physiology of VGCCs, voltage-gated K+ (KV) channels, large-conductance Ca2+-activated K+ (BKCa) channels, strong-inward-rectifier K+ (KIR) channels, ATP-sensitive K+ (KATP) channels, ryanodine receptors (RyRs), inositol 1,4,5-trisphosphate receptors (IP3Rs), and a variety of transient receptor potential (TRP) channels that contribute to pressure-induced myogenic tone in resistance arteries and arterioles, the modulation of the function of these ion channels by vasoconstrictors and vasodilators, their role in the functional regulation of tissue blood flow and their dysfunction in diseases such as hypertension, obesity, and diabetes. © 2017 American Physiological Society. Compr Physiol 7:485-581, 2017.

Calcium Channels in Vascular Smooth Muscle

Calcium (Ca²⁺) plays a central role in excitation, contraction, transcription, and proliferation of vascular smooth muscle cells (VSMs). Precise regulation of intracellular Ca²⁺ concentration ([Ca²⁺]_i) is crucial for proper physiological VSM function. Studies over the last several decades have revealed that VSMs express a variety of Ca²⁺-permeable channels that orchestrate a dynamic, yet finely tuned regulation of [Ca²⁺]_i. In this review, we discuss the major Ca²⁺-permeable channels expressed in VSM and their contribution to vascular physiology and pathology.

Facilitated hyperpolarization signaling in vascular smooth muscle-overexpressing TRIC-A channels

The TRIC channel subtypes, namely TRIC-A and TRIC-B, are intracellular monovalent cation-specific channels and likely mediate counterion movements to support efficient Ca(2+) release from the sarco/endoplasmic reticulum. Vascular smooth muscle cells (VSMCs) contain both TRIC subtypes and two Ca(2+) release mechanisms; incidental opening of ryanodine receptors (RyRs) generates local Ca(2+) sparks to induce hyperpolarization and relaxation, whereas agonist-induced activation of inositol trisphosphate receptors produces global Ca(2+) transients causing contraction. Tric-a knock-out mice develop hypertension due to insufficient RyR-mediated Ca(2+) sparks in VSMCs. Here we describe transgenic mice overexpressing TRIC-A channels under the control of a smooth muscle cell-specific promoter. The transgenic mice developed congenital hypotension. In Tric-a-overexpressing VSMCs from the transgenic mice, the resting membrane potential decreased because RyR-mediated Ca(2+) sparks were facilitated and cell surface Ca(2+)-dependent K(+) channels were hyperactivated. Under such hyperpolarized conditions, L-type Ca(2+) channels were inactivated, and thus, the resting intracellular Ca(2+) levels were reduced in Tric-a-overexpressing VSMCs. Moreover, Tric-a overexpression impaired inositol trisphosphate-sensitive stores to diminish agonist-induced Ca(2+) signaling in VSMCs. These altered features likely reduced vascular tonus leading to the hypotensive phenotype. Our Tric-a-transgenic mice together with Tric-a knock-out mice indicate that TRIC-A channel density in VSMCs is responsible for controlling basal blood pressure at the whole-animal level.

Inositol trisphosphate receptors in smooth muscle cells

Inositol 1,4,5-trisphosphate receptors (IP(3)Rs) are a family of tetrameric intracellular calcium (Ca(2+)) release channels that are located on the sarcoplasmic reticulum (SR) membrane of virtually all mammalian cell types, including smooth muscle cells (SMC). Here, we have reviewed literature investigating IP(3)R expression, cellular localization, tissue distribution, activity regulation, communication with ion channels and organelles, generation of Ca(2+) signals, modulation of physiological functions, and alterations in pathologies in SMCs. Three IP(3)R isoforms have been identified, with relative expression and cellular localization of each contributing to signaling differences in diverse SMC types. Several endogenous ligands, kinases, proteins, and other modulators control SMC IP(3)R channel activity. SMC IP(3)Rs communicate with nearby ryanodine-sensitive Ca(2+) channels and mitochondria to influence SR Ca(2+) release and reactive oxygen species generation. IP(3)R-mediated Ca(2+) release can stimulate plasma membrane-localized channels, including transient receptor potential (TRP) channels and store-operated Ca(2+) channels. SMC IP(3)Rs also signal to other proteins via SR Ca(2+) release-independent mechanisms through physical coupling to TRP channels and local communication with large-conductance Ca(2+)-activated potassium channels. IP(3)R-mediated Ca(2+) release generates a wide variety of intracellular Ca(2+) signals, which vary with respect to frequency, amplitude, spatial, and temporal properties. IP(3)R signaling controls multiple SMC functions, including contraction, gene expression, migration, and proliferation. IP(3)R expression and cellular signaling are altered in several SMC diseases, notably asthma, atherosclerosis, diabetes, and hypertension. In summary, IP(3)R-mediated pathways control diverse SMC physiological functions, with pathological alterations in IP(3)R signaling contributing to disease.

Function and expression of ryanodine receptors and inositol 1,4,5-trisphosphate receptors in smooth muscle cells of murine feed arteries and arterioles

We tested the hypothesis that vasomotor control is differentially regulated between feed arteries and downstream arterioles from the cremaster muscle of C57BL/6 mice. In isolated pressurized arteries, confocal Ca(2+) imaging of smooth muscle cells (SMCs) revealed Ca(2+) sparks and Ca(2+) waves. Ryanodine receptor (RyR) antagonists (ryanodine and tetracaine) inhibited both sparks and waves but increased global Ca(2+) and myogenic tone. In arterioles, SMCs exhibited only Ca(2+) waves that were insensitive to ryanodine or tetracaine. Pharmacological interventions indicated that RyRs are functionally coupled to large-conductance, Ca(2+)-activated K(+) channels (BK(Ca)) in SMCs of arteries, whereas BK(Ca) appear functionally coupled to voltage-gated Ca2+ channels in SMCs of arterioles. Inositol 1,4,5-trisphosphate receptor (IP3R) antagonists (xestospongin D or 2-aminoethoxydiphenyl borate) or a phospholipase C inhibitor (U73122) attenuated Ca(2+) waves, global Ca(2+) and myogenic tone in arteries and arterioles but had no effect on arterial sparks. Real-time PCR of isolated SMCs revealed RyR2 as the most abundant isoform transcript; arteries expressed twice the RyR2 but only 65% the RyR3 of arterioles and neither vessel expressed RyR1. Immunofluorescent localisation of RyR protein indicated bright, clustered staining of arterial SMCs in contrast to diffuse staining in arteriolar SMCs. Expression of IP(3)R transcripts and protein immunofluorescence were similar in SMCs of both vessels with IP(3)R1>>IP(3)R2>IP(3)R3. Despite similar expression of IP(3)Rs and dependence of Ca(2+) waves on IP(3)Rs, these data illustrate pronounced regional heterogeneity in function and expression of RyRs between SMCs of the same vascular resistance network. We conclude that vasomotor control is differentially regulated in feed arteries vs. downstream arterioles.

Calcium signaling in smooth muscle

Changes in intracellular Ca(2+) are central to the function of smooth muscle, which lines the walls of all hollow organs. These changes take a variety of forms, from sustained, cell-wide increases to temporally varying, localized changes. The nature of the Ca(2+) signal is a reflection of the source of Ca(2+) (extracellular or intracellular) and the molecular entity responsible for generating it. Depending on the specific channel involved and the detection technology employed, extracellular Ca(2+) entry may be detected optically as graded elevations in intracellular Ca(2+), junctional Ca(2+) transients, Ca(2+) flashes, or Ca(2+) sparklets, whereas release of Ca(2+) from intracellular stores may manifest as Ca(2+) sparks, Ca(2+) puffs, or Ca(2+) waves. These diverse Ca(2+) signals collectively regulate a variety of functions. Some functions, such as contractility, are unique to smooth muscle; others are common to other excitable cells (e.g., modulation of membrane potential) and nonexcitable cells (e.g., regulation of gene expression).

Intracellular Ca2+ storage in health and disease: a dynamic equilibrium

Homeostatic control of the endoplasmic reticulum (ER) both as the site for protein handling (synthesis, folding, trafficking, disaggregation and degradation) and as a Ca2+ store is of crucial importance for correct functioning of the cell. Disturbance of the homeostatic control mechanisms leads to a vast array of severe pathologies. The Ca2+ content of the ER is a dynamic equilibrium between active uptake via Ca2+ pumps and Ca2+ release by a number of highly regulated Ca2+-release channels. Regulation of the Ca2+-release channels is very complex and several mechanisms are still poorly understood or controversial. There is increasing evidence that a number of unrelated proteins, either by themselves or in association with other Ca2+ channels, can provide additional Ca2+-leak pathways. The ER is a dynamic organelle and changes in its size and components have been described, either as a result of (de)differentiation processes affecting the secretory capacity of cells, or as a result of adaptation mechanisms to diverse stress conditions such as the unfolded protein response and autophagy. In this review we want to give an overview of the current knowledge of the (short-term) regulatory mechanisms that affect Ca2+-release and Ca2+-leak pathways and of the (long-term) adaptations in ER size and capacity. Understanding of the consequences of these mechanisms for cellular Ca2+ signaling could provide a huge therapeutic potential.

Sarcoplasmic reticulum function in smooth muscle

The sarcoplasmic reticulum (SR) of smooth muscles presents many intriguing facets and questions concerning its roles, especially as these change with development, disease, and modulation of physiological activity. The SR's function was originally perceived to be synthetic and then that of a Ca store for the contractile proteins, acting as a Ca amplification mechanism as it does in striated muscles. Gradually, as investigators have struggled to find a convincing role for Ca-induced Ca release in many smooth muscles, a role in controlling excitability has emerged. This is the Ca spark/spontaneous transient outward current coupling mechanism which reduces excitability and limits contraction. Release of SR Ca occurs in response to inositol 1,4,5-trisphosphate, Ca, and nicotinic acid adenine dinucleotide phosphate, and depletion of SR Ca can initiate Ca entry, the mechanism of which is being investigated but seems to involve Stim and Orai as found in nonexcitable cells. The contribution of the elemental Ca signals from the SR, sparks and puffs, to global Ca signals, i.e., Ca waves and oscillations, is becoming clearer but is far from established. The dynamics of SR Ca release and uptake mechanisms are reviewed along with the control of luminal Ca. We review the growing list of the SR's functions that still includes Ca storage, contraction, and relaxation but has been expanded to encompass Ca homeostasis, generating local and global Ca signals, and contributing to cellular microdomains and signaling in other organelles, including mitochondria, lysosomes, and the nucleus. For an integrated approach, a review of aspects of the SR in health and disease and during development and aging are also included. While the sheer versatility of smooth muscle makes it foolish to have a "one model fits all" approach to this subject, we have tried to synthesize conclusions wherever possible.

Hydrogen peroxide down-regulates inositol 1,4,5-trisphosphate receptor content through proteasome activation

Hydrogen peroxide (H(2)O(2)) is implicated in the regulation of signaling pathways leading to changes in vascular smooth muscle function. Contractile effects produced by H(2)O(2) are due to the phosphorylation of myosin light chain kinase triggered by increases in intracellular calcium (Ca(2+)) from intracellular stores or influx of extracellular Ca(2+). One mechanism for mobilizing such stores involves the phosphoinositide pathway. Inositol 1,4,5-trisphosphate (IP(3)) mobilizes intracellular Ca(2+) by binding to a family of receptors (IP(3)Rs) on the endoplasmic-sarcoplasmic reticulum that act as ligand-gated Ca(2+) channels. IP(3)Rs can be rapidly ubiquitinated and degraded by the proteasome, causing a decrease in cellular IP(3)R content. In this study we show that IP(3)R(1) and IP(3)R(3) are down-regulated when vascular smooth muscle cells (VSMC) are stimulated by H(2)O(2), through an increase in proteasome activity. Moreover, we demonstrate that the decrease in IP(3)R by H(2)O(2) is accompanied by a reduction in calcium efflux induced by IP(3) in VSMC. Also, we observed that angiotensin II (ANGII) induces a decrease in IP(3)R by activation of NADPH oxidase and that preincubation with H(2)O(2) decreases ANGII-mediated calcium efflux and planar cell surface area in VSMC. The decreased IP(3) receptor content observed in cells was also found in aortic rings, which exhibited a decreased ANGII-dependent contraction after treatment with H(2)O(2). Altogether, these results suggest that H(2)O(2) mediates IP(3)R down-regulation via proteasome activity.

Targeting and clustering of IP3 receptors: key determinants of spatially organized Ca2+ signals

Inositol 1,4,5-trisphosphate receptors (IP3R) are intracellular Ca2+ channels that are almost ubiquitously expressed in animal cells. The spatiotemporal complexity of the Ca2+ signals evoked by IP3R underlies their versatility in cellular signaling. Here we review the mechanisms that contribute to the subcellular targeting of IP3R and the dynamic interplay between IP3R that underpin their ability to generate complex intracellular Ca2+ signals.

A computational analysis of localized Ca2+-dynamics generated by heterogeneous release sites

We investigate the role of heterogeneous expression of IP3R and RyR in generating diverse elementary Ca2+ signals. It has been shown empirically (Wojcikiewicz and Luo in Mol. Pharmacol. 53(4):656-662, 1998; Newton et al. in J. Biol. Chem. 269(46):28613-28619, 1994; Smedt et al. in Biochem. J. 322(Pt. 2):575-583, 1997) that tissues express various proportions of IP3 and RyR isoforms and this expression is dynamically regulated (Parrington et al. in Dev. Biol. 203(2):451-461, 1998; Fissore et al. in Biol. Reprod. 60(1):49-57, 1999; Tovey et al. in J. Cell Sci. 114(Pt. 22):3979-3989, 2001). Although many previous theoretical studies have investigated the dynamics of localized calcium release sites (Swillens et al. in Proc. Natl. Acad. Sci. U.S.A. 96(24):13750-13755, 1999; Shuai and Jung in Proc. Natl. Acad. Sci. U.S.A. 100(2):506-510, 2003a; Shuai and Jung in Phys. Rev. E, Stat. Nonlinear Soft Matter Phys. 67(3 Pt. 1):031905, 2003b; Thul and Falcke in Biophys. J. 86(5):2660-2673, 2004; DeRemigio and Smith in Cell Calcium 38(2):73-86, 2005; Nguyen et al. in Bull. Math. Biol. 67(3):393-432, 2005), so far all such studies focused on release sites consisting of identical channel types. We have extended an existing mathematical model (Nguyen et al. in Bull. Math. Biol. 67(3):393-432, 2005) to release sites with two (or more) receptor types, each with its distinct channel kinetics. Mathematically, the release site is represented by a transition probability matrix for a collection of nonidentical stochastically gating channels coupled through a shared Ca2+ domain. We demonstrate that under certain conditions a previously defined mean-field approximation of the coupling strength does not accurately reproduce the release site dynamics. We develop a novel approximation and establish that its performance in these instances is superior. We use this mathematical framework to study the effect of heterogeneity in the Ca2+-regulation of two colocalized channel types on the release site dynamics. We consider release sites consisting of channels with both Ca2+-activation and inactivation ("four-state channels") and channels with Ca2+-activation only ("two-state channels") and show that for the appropriate parameter values, synchronous channel openings within a release site with any proportion of two-state to four-state channels are possible, however, the larger the proportion of two-state channels, the more sensitive the dynamics are to the exact spatial positioning of the channels and the distance between channels. Specifically, the clustering of even a small number of two-state channels interferes with puff/spark termination and increases puff durations or leads to a tonic response.

Ca2+ handling is altered when arterial myocytes progress from a contractile to a proliferative phenotype in culture

Phenotypic modulation of vascular myocytes is important for vascular development and adaptation. A characteristic feature of this process is alteration in intracellular Ca(2+) handling, which is not completely understood. We studied mechanisms involved in functional changes of inositol 1,4,5-trisphosphate (IP(3))- and ryanodine (Ry)-sensitive Ca(2+) stores, store-operated Ca(2+) entry (SOCE), and receptor-operated Ca(2+) entry (ROCE) associated with arterial myocyte modulation from a contractile to a proliferative phenotype in culture. Proliferating, cultured myocytes from rat mesenteric artery have elevated resting cytosolic Ca(2+) levels and increased IP(3)-sensitive Ca(2+) store content. ATP- and cyclopiazonic acid [CPA; a sarco(endo)plasmic reticulum Ca(2+)-ATPase (SERCA) inhibitor]-induced Ca(2+) transients in Ca(2+)-free medium are significantly larger in proliferating arterial smooth muscle cells (ASMCs) than in freshly dissociated myocytes, whereas caffeine (Caf)-induced Ca(2+) release is much smaller. Moreover, the Caf/Ry-sensitive store gradually loses sensitivity to Caf activation during cell culture. These changes can be explained by increased expression of all three IP(3) receptors and a switch from Ry receptor type II to type III expression during proliferation. SOCE, activated by depletion of the IP(3)/CPA-sensitive store, is greatly increased in proliferating ASMCs. Augmented SOCE and ROCE (activated by the diacylglycerol analog 1-oleoyl-2-acetyl-sn-glycerol) in proliferating myocytes can be attributed to upregulated expression of, respectively, transient receptor potential proteins TRPC1/4/5 and TRPC3/6. Moreover, stromal interacting molecule 1 (STIM1) and Orai proteins are upregulated in proliferating cells. Increased expression of IP(3) receptors, SERCA2b, TRPCs, Orai(s), and STIM1 in proliferating ASMCs suggests that these proteins play a critical role in an altered Ca(2+) handling that occurs during vascular growth and remodeling.

Jak2 tyrosine kinase prevents angiotensin II-mediated inositol 1,4,5 trisphosphate receptor degradation

In addition to its role as a vasoconstrictor, angiotensin II also acts as a potent growth factor by activating several tyrosine kinases, including Jak2. Interestingly, Jak2 has been linked to similar cardiovascular pathologies as have been previously linked to the renin-angiotensin system. Identifying the downstream targets of Jak2 via the AT(1) receptor may therefore elucidate its role in the progression of various pathologies. Previously, microarray analysis from our laboratory identified the Type 1 inositol 1,4,5 trisphosphate (IP(3)) receptor as a potential target of Jak2 following chronic stimulation by angiotensin II. Therefore, we hypothesized that Jak2 regulates IP(3) receptor expression in response to angiotensin II. To test this hypothesis, rat aortic smooth muscle (RASM) cells over-expressing a dominant negative (DN) Jak2 protein were used. The Jak2-dependent signaling in these cells is reduced approximately 90% when compared to RASM control cells. Analysis of protein expression showed that the IP(3) receptor was degraded approximately 2-fold (P<0.05) in cells lacking functional Jak2 within 1 h of treatment by angiotensin II. Notably, degradation of the IP(3) receptor was reversible since protein levels were restored to normal following 2 h of recovery from angiotensin II. To eliminate the possibility of clonal artifact in the DN cells, wild type RASM cells were treated with the Jak2 pharmacological inhibitor, AG490. We found that angiotensin II treatment degraded IP(3) receptor in AG490-treated cells, but not in the vehicle controls. Treatment with lactacystin, a proteasome inhibitor, completely blocked angiotensin II-mediated degradation of IP(3) receptor, thereby suggesting that the degradation occurs through a proteasome-dependent mechanism. Moreover, the degradation of IP(3) receptor in DN cells correlated with a significant loss of intracellular calcium mobilization when treated with angiotensin II (DN 27.4+/-1.1% vs. WT 42.2+/-4.7%; n=5, P=0.002). We next examined through what mechanism Jak2 regulates the IP(3) receptor. When wild type RASM cells were treated with PP2, an Src-family inhibitor, IP(3) receptor expression was markedly reduced. Since previous data show that Fyn, a downstream target of Jak2, is able to phosphorylate the IP(3) receptor at Tyr 353, we believe our data suggest that Jak2 prevents the angiotensin II-mediated IP(3) receptor degradation through the activation of Fyn. In conclusion, these data suggest that Jak2 has a protective role in maintaining IP(3) receptor expression, potentially through activation of Fyn and subsequent phosphorylation of the IP(3) receptor.

Calcium events in smooth muscles and their interstitial cells; physiological roles of sparks

The observation of spontaneous sporadic releases of packets of stored calcium made 20 years ago has opened up a number of new concepts in smooth muscle physiology: (1) the calcium release sites are ryanodine and inositol 1,4,5-trisphosphate (IP3) receptor channels which contribute to cell-wide increases in [Ca2+]i in response to cell depolarization, activation of IP3-generating receptors, or other stimuli; (2) changes in [Ca2+]i act back on the cell membrane to activate or modulate K+, Cl- and cation channel activity so affecting contraction, in arterial smooth muscle for example affecting blood pressure; (3) IP3 production is voltage dependent and is believed to contribute to pacemaker potentials and to refractory periods which control the rhythmical motility of many hollow organs. Most smooth muscle tissues contain interstitial cells (ICs) in addition to contractile smooth muscle cells (SMCs). The interactions of these internal mechanisms, and in turn the interactions of SMCs and ICs in various smooth muscle tissues, are major factors in determining the unique physiological profiles of individual smooth muscles.

Regulation of muscarinic cationic current in myocytes from guinea-pig ileum by intracellular Ca2+ release: a central role of inositol 1,4,5-trisphosphate receptors

The dynamics of carbachol (CCh)-induced [Ca(2+)](i) changes was related to the kinetics of muscarinic cationic current (mI(cat)) and the effect of Ca(2+) release through ryanodine receptors (RyRs) and inositol 1,4,5-trisphosphate receptors (IP(3)Rs) on mI(cat) was evaluated by fast x-y or line-scan confocal imaging of [Ca(2+)](i) combined with simultaneous recording of mI(cat) under whole-cell voltage clamp. When myocytes freshly isolated from the longitudinal layer of the guinea-pig ileum were loaded with the Ca(2+)-sensitive indicator fluo-3, x-y confocal imaging revealed CCh (10 microM)-induced Ca(2+) waves, which propagated from the cell ends towards the myocyte centre at 45.9 +/- 8.8 microms(-1) (n = 13). Initiation of the Ca(2+) wave preceded the appearance of any measurable mI(cat) by 229 +/- 55 ms (n = 7). Furthermore, CCh-induced [Ca(2+)](i) transients peaked 1.22 +/- 0.11s (n = 17) before mI(cat) reached peak amplitude. At -50 mV, spontaneous release of Ca(2+) through RyRs, resulting in Ca(2+) sparks, had no effect on CCh-induced mI(cat) but activated BK channels leading to spontaneous transient outward currents (STOCs). In addition, Ca(2+) release through RyRs induced by brief application of 5 mM caffeine was initiated at the cell centre but did not augment mI(cat) (n = 14). This was not due to an inhibitory effect of caffeine on muscarinic cationic channels (since application of 5 mM caffeine did not inhibit mI(cat) when [Ca(2+)](i) was strongly buffered with Ca(2+)/BAPTA buffer) nor was it due to an effect of caffeine on other mechanisms possibly involved in the regulation of Ca(2+) sensitivity of muscarinic cationic channels (since in the presence of 5 mM caffeine, photorelease of Ca(2+) upon cell dialysis with 5 mM NP-EGTA/3.8 mM Ca(2+) potentiated mI(cat) in the same way as in control). In contrast, IP(3)R-mediated Ca(2+) release upon flash photolysis of "caged" IP(3) (30 microM in the pipette solution) augmented mI(cat) (n = 15), even though [Ca(2+)](i) did not reach the level required for potentiation of mI(cat) during photorelease of Ca(2+) (n = 10). Intracellular calcium stores were visualised by loading of the myocytes with the low-affinity Ca(2+) indicator fluo-3FF AM and consisted of a superficial sarcoplasmic reticulum (SR) network and some perinuclear formation, which appeared to be continuous with the superficial SR. Immunostaining of the myocytes with antibodies to IP(3)R type 1 and to RyRs revealed that IP(3)Rs are predominant in the superficial SR while RyRs are confined to the central region of the cell. These results suggest that IP(3)R-mediated Ca(2+) release plays a central role in the modulation of mI(cat) in the guinea-pig ileum and that IP(3) may sensitise the regulatory mechanisms of the muscarinic cationic channels gating to Ca(2+).

Endothelin-1- and depolarization-induced differential regulation of cAMP response element-binding protein in proliferating and developed vascular smooth muscle

Regulation of the transcription factor cAMP response element-binding protein (CREB) is important for gene expression in vascular smooth muscle (VSM). This study has examined the intracellular regulation of CREB by endothelin-1 (ET-1) and depolarization in native VSM, comparing proliferative and fully differentiated phenotypes. Portal veins from neonatal (proliferative) and 6-week-old (differentiated) rats were stimulated with ET-1 or K(+). In both phenotypes, CREB activation was increased by ET-1 although the time course was prolonged in neonatal VSM. This paralleled a prolonged ET-1-induced [Ca(2+)](i) increase. ET-1-induced CREB activation was dependent on extracellular signal-regulated kinase 1/2 (ERK1/2) activation and inhibited by BAPTA but not by a calmodulin-dependent protein kinase (CamK) inhibitor. In contrast, CREB activation induced by depolarization in both neonatal and developed VSM was significantly reduced by CaMK inhibition and by ERK1/2 inhibition. Therefore, CREB activation is regulated differentially in VSM depending upon stimulus; however, this is not altered in different growth states.

Smooth muscle cells and interstitial cells of blood vessels

A rise in intracellular ionised calcium concentration ([Ca(2+)](i)) at sites adjacent to the contractile proteins is a primary signal for contraction in all types of muscles. Recent progress in the development of imaging techniques with special accent on the fluorescence confocal microscopy and new achievements in the synthesis of organelle- and ion-specific fluorochromes provide an experimental basis for study of the relationship between the structural organisation of the living smooth muscle myocyte and the features of calcium signalling at subcellular level. Applying fluorescent confocal microscopy and tight-seal recording of transmembrane ion currents to freshly isolated vascular myocytes we have demonstrated that: (1) Ca(2+) sparks originate from clustered opening of ryanodine receptors (RyRs) and build up a cell-wide increase in [Ca(2+)](i) upon myocyte excitation; (2) spontaneous Ca(2+) sparks occurred at the highest rate at certain preferred locations, frequent discharge sites (FDS), which are associated with a prominent portion of the sarcoplasmic reticulum (SR) located close to the cell membrane; (3) Ca(2+)-dependent K(+) and Cl(-) channels sense the local changes in [Ca(2+)](i) during a calcium spark and thereby couple changes in [Ca(2+)](i) within a microdomain to changes in the membrane potential, thus affecting excitability of the cell; (4) an intercommunication between RyRs and inositol trisphosphate receptors (IP(3)Rs) is one of the important determinants of intracellular calcium dynamics that, in turn, can modulate the cell membrane potential through differential targeting of calcium dependent membrane ion channels. Furthermore, using immunohystochemical approaches in combination with confocal imaging we identified non-contractile cells closely resembling interstitial cells (ICs) of Cajal (which are considered to be pacemaker cells in the gut) in the wall of portal vein and mesenteric artery. Using electron microscopy, tight-seal recording and fluorescence confocal imaging we obtained information on the morphology of ICs and their possible coupling to smooth muscle cells (SMCs), calcium signalling in ICs and their electrophysiological properties. The functions of these cells are not yet fully understood; in portal vein they may act as pacemakers driving the spontaneous activity of the muscle; in artery they may have other a yet unsuspected functions.

Sphingosine 1-phosphate induces CREB activation in rat cerebral artery via a protein kinase C-mediated inhibition of voltage-gated K+ channels

Sphingosine 1-phosphate (S1P) is a potential mitogenic stimulus for vascular smooth muscle. S1P promotes an increase in the intracellular calcium concentration ([Ca(2+)](i)) in cerebral arteries, however S1P effects on regulation of gene expression are not known. Activation of the Ca(2+)-dependent transcription factor, cAMP response element-binding protein (CREB), is associated with smooth muscle proliferation. The aim of this study was to examine the Ca(2+)-dependent mechanisms involved in S1P-induced CREB activation in cerebral artery. Western blotting and immunofluorescence with a phospho-CREB antibody were used to detect CREB activation in Sprague-Dawley rat cerebral arteries. Whole-cell patch clamp recording and single cell imaging of [Ca(2+)](i) were performed on freshly isolated cerebral artery myocytes. S1P increased activation of CREB in the nucleus of cerebral arteries. This activation was mediated by Ca(2+)/calmodulin-dependent protein kinase and was dependent on an increase in [Ca(2+)](i) via two mechanisms: (i) intracellular Ca(2+) release via an inositol 1,4,5-trisphosphate (InsP(3))-dependent pathway and (ii) Ca(2+) entry through voltage-dependent Ca(2+) channels (VDCC). Activation of the VDCC occurred through S1P-induced inhibition (approximately 50%) of the voltage-gated potassium (K(+)) current. This inhibition was via a protein kinase C-mediated pathway resulting in tyrosine phosphorylation of at least one isoform of the Kv channel (Kv 1.2). These results demonstrate that S1P can activate the transcription factor CREB through different Ca(2+)-dependent pathways including intracellular Ca(2+) release and inhibition of voltage-gated K(+) channels leading to Ca(2+) influx. Our findings suggest a potential role for S1P in regulation of gene expression in vascular smooth muscle.

FKBP12.6 and cADPR regulation of Ca2+ release in smooth muscle cells

Intracellular Ca2+ release through ryanodine receptors (RyRs) plays important roles in smooth muscle excitation-contraction coupling, but the underlying regulatory mechanisms are poorly understood. Here we show that FK506 binding protein of 12.6 kDa (FKBP12.6) associates with and regulates type 2 RyRs (RyR2) in tracheal smooth muscle. FKBP12.6 binds to RyR2 but not other RyR or inositol 1,4,5-trisphosphate receptors, and FKBP12, known to bind to and modulate skeletal RyRs, does not associate with RyR2. When dialyzed into tracheal myocytes, cyclic ADP-ribose (cADPR) alters spontaneous Ca2+ release at lower concentrations and produces macroscopic Ca2+ release at higher concentrations; neurotransmitter-evoked Ca2+ release is also augmented by cADPR. These actions are mediated through FKBP12.6 because they are inhibited by molar excess of recombinant FKBP12.6 and are not observed in myocytes from FKBP12.6-knockout mice. We also report that force development in FKBP12.6-null mice, observed as a decrease in the concentration/tension relationship of isolated trachealis segments, is impaired. Taken together, these findings point to an important role of the FKBP12.6/RyR2 complex in stochastic (spontaneous) and receptor-mediated Ca2+ release in smooth muscle.

The influence of different InsP(3) receptor isoforms on Ca(2+) signaling in tracheal smooth muscle cells

In airway myocytes, like in many cells, Ca(2+) signaling is controlled by inositol 1,4,5-trisphosphate (InsP(3)) via InsP(3) receptors (InsP(3)R) located in the sarco-endoplasmic reticulum. Three types of InsP(3)R exist, labeled Types 1, 2, and 3, which differ in their gating kinetics. We analyze a possible impact of the different gating kinetics of Type 1 and Type 3 InsP(3)R on the time course of cytosolic Ca(2+) concentration in tracheal smooth muscle cells upon agonist stimulation. Previous experimental data in rat tracheal myocytes showed that upon gradually increased stimulation with acetylcholine (ACh), a contractile agonist that acts via InsP(3) production, signal spikes, several spikes with declining maxima, and sustained oscillations appear. Our model reproduces the time courses of cytosolic Ca(2+) measured in tracheal myocytes. Moreover, by postulating slight variations in the model parameters which determine the total number of receptors expressed and the ratio between Type 1 and Type 3 InsP(3)R, it offers an explanation to the experimental observation of qualitatively different responses of cells within a presumably homogeneous tissue.

Crosstalk between ryanodine receptors and IP(3) receptors as a factor shaping spontaneous Ca(2+)-release events in rabbit portal vein myocytes

In smooth muscle cells freshly isolated from rabbit portal vein, there was only one site discharging the majority of spontaneous Ca(2+)-release events; the activity of this single site was studied using laser scanning confocal imaging after loading the cells with the fluorescent Ca(2+) indicator fluo-4 acetoxymethyl ester. Localised spontaneous Ca(2+)-release events visualised by line-scan imaging revealed two predominant spatiotemporal patterns: (i) small-amplitude, fast events similar to Ca(2+) sparks in cardiomyocytes and (ii) larger and slower events. The sum of two Gaussian profiles was well fitted to the amplitude histogram (peak frequencies at 1.8 and 3.2 F/F(0)) and spatial spread (full width at half-maximal amplitude) histogram (peak frequencies at 2 and 3.8 microm) for the 230 localised Ca(2+)-release events analysed. The existence of two populations of Ca(2+)-release events was also supported by the histograms of the rise times and half-decay times, which revealed modes at 38 and 65 ms, respectively. Shifting the scan line along the z-axis during imaging from a single discharge site suggested that the appearance of two populations of Ca(2+)-release events is not due to out-of-focus imaging. Both small and large events persisted upon 3-5 min exposure to 1-5 microM nicardipine, but were abolished after 10-15 min exposure to 50-100 microM ryanodine, 0.1 microM thapsigargin or 10 microM cyclopiazonic acid. Only small-amplitude, fast events persisted in the presence of inhibitors of inositol 1,4,5-trisphosphate (IP(3))-induced Ca(2+) release, 10 microM xestospongin C or 30 microM 2-aminoethoxy-diphenylborate (2-APB), or in the presence of 2.5 microM U-73122 (a phospholipase C (PLC) inhibitor). Coupling between neighbouring Ca(2+)-release domains giving rise to spontaneous [Ca(2+)](i) waves was abolished in the presence of 2-APB. Examination of the saltatory propagation of the waves suggested that the critical factor that determines propagation between domains is a time-dependent change in the sensitivity of ryanodine receptors and/or IP(3) receptors to Ca(2+), which can give rise to 'loose coupling' between release sites. These results suggest that activation of IP(3) receptors (due to the tonic activity of PLC and ongoing production of IP(3)) recruits neighbouring domains of ryanodine receptors, leading to larger Ca(2+) releases and saltatory propagation of [Ca(2+)](i) waves in portal vein myocytes.

Developmental changes in ryanodine- and IP(3)-sensitive Ca(2+) pools in ovine basilar artery

To explore the hypothesis that cerebrovascular maturation alters ryanodine- and inositol 1,4,5-trisphosphate (IP(3))-sensitive Ca(2+) pool sizes, we measured total intracellular Ca(2+) with (45)Ca and the fractions of intracellular Ca(2+) released by IP(3) and/or caffeine in furaptra-loaded permeabilized basilar arteries from nonpregnant adult and term fetal (139-141 days) sheep. Ca(2+) mass (nmol/mg dry weight) was similar in adult (1.60 +/- 0.18) and fetal (1.71 +/- 0.16) arteries in the pool sensitive to IP(3) alone but was significantly lower for adult (0.11 +/- 0.01) than for fetal (1.22 +/- 0.11) arteries in the pool sensitive to ryanodine alone. The pool sensitive to both ryanodine and IP(3) was also smaller in adult (0.14 +/- 0.01) than in fetal (0.85 +/- 0.08) arteries. Because the Ca(2+) fraction in the ryanodine-IP(3) pool was small in both adult (5 +/- 1%) and fetal (7 +/- 4%) arteries, the IP(3) and ryanodine pools appear to be separate in these arteries. However, the pool sensitive to neither IP(3) nor ryanodine was 10-fold smaller in adult (0.87 +/- 0.10) than in fetal (8.78 +/- 0.81) arteries, where it accounted for 72% of total intracellular membrane-bound Ca(2+). Thus, during basilar artery maturation, intracellular Ca(2+) mass plummets in noncontractile pools, decreases modestly in ryanodine-sensitive pools, and remains constant in IP(3)-sensitive pools. In addition, age-related increases in IP(3) efficacy must involve factors other than IP(3) pool size alone.

Smooth muscle cell response to mechanical injury involves intracellular calcium release and ERK1/ERK2 phosphorylation

We have investigated possible signaling pathways coupled to injury-induced ERK1/2 activation and the subsequent initiation of vascular rat smooth muscle cell migration and proliferation. Aortic smooth muscle cells were cultured to confluency and subjected to in vitro injury under serum-free conditions. In fluo-4-loaded cells, injury induced a rapid wave of intracellular Ca(2+) release that propagated about 200 microm in radius from the injured zone, reached a peak in about 20 s, and subsided to the baseline within 2 min. The wave was abolished by prior treatment with the sarcoplasmic reticulum ATPase inhibitor thapsigargin, but not by omission of extracellular Ca(2+). ERK1/2 activation reached a peak at 10 min after injury and was inhibited by the MEK1 inhibitor PD98059, as well as by thapsigargin, fluphenazine, genistein, and the Src inhibitor PP2. These inhibitors also reduced [(3)H]thymidine incorporation and migration of cells into the injured area determined at 48 h after injury. These results show that mechanical injury to vascular smooth muscle cells induces a Ca(2+) wave which is dependent on intracellular Ca(2+) release. Furthermore, the injury activates ERK1/2 phosphorylation as well as cell migration and replication.

Reduced store-operated Ca(2+) currents in rat basophilic leukaemia cells cultured under serum-free conditions

Influx of Ca(2+) represents an important regulatory signal in the process of cell proliferation. However, little is known about how Ca(2+) entry changes during the cell-cycle. Patch-clamp experiments and microfluorimetry show that store-operated Ca(2+) entry was substantially reduced in rat basophilic leukaemia cells cultured for 24h under serum-free conditions. Likewise, retinoic acid treatment blocked Ca(2+) influx activated by store depletion via inositol 1,4,5-trisphosphate. Both procedures are known to arrest cells at the G0/G1 boundary of the cell-cycle and induced a reduction in 5-bromo 2'-deoxyuridine incorporation into DNA. Ca(2+) release from the stores remained unaltered and two types of K(+) currents were not affected in cells after serum starvation. The specific reduction in Ca(2+) entry was not detected when using aphidicolin, 5-fluorouracil or thymidine to synchronise the cell-cycle. These data suggest that store-operated Ca(2+) influx changed during cell-cycle progression which might have important implications for cell growth.

Effects of maturation and acute hypoxia on receptor-IP(3) coupling in ovine common carotid arteries

Whereas previous studies have established that many mechanisms mediating pharmacomechanical coupling are subject to regulation, evidence of physiological regulation of the coupling efficiency between receptor activation and second-messenger production is scarce. The present studies address the hypothesis that acute hypoxia and maturation can influence the mass of second-messenger production for each activated agonist-bound receptor ("receptor gain"). For this assessment, receptor density and agonist affinity values were used to calculate 5-hydroxytryptamine (5-HT) concentrations that would produce standardized numbers of bound receptors (8.5 fmol/mg protein) in each experimental group and thus minimize effects of age or hypoxia on receptor density or agonist affinity. After 3 min of exposure to these 5-HT concentrations, normoxic magnitudes of contraction were similar (as %potassium maxima) in fetal (50 +/- 14%) and adult (40 +/- 9%) arteries, but hypoxia (PO(2) approximately 9--12 Torr for 30 min) depressed contractile tensions with a significantly different time course and magnitude in fetal (30 +/- 10%) and adult (17 +/- 11%) arteries (P < 0.05). Basal inositol 1,4,5-trisphosphate (IP(3)) values (in pmol/mg protein) were significantly greater in fetal (94 +/- 16) than in adult (44 +/- 6) arteries, and integrated areas above baseline for the IP(3) time courses (in nmol-s/mg protein) were significantly greater in fetal than in adult arteries both in normoxic (14.3 +/- 1.8 vs. 9.1 +/- 1.6) and hypoxic (15.0 +/- 2.1 vs. 8.6 +/- 1.2) conditions (P < 0.05). Hypoxia altered the IP(3) time courses both in the fetus and the adult but had no significant effect on IP(3 )mobilization or receptor gain. These data demonstrate that for the 5-HT(2a) receptor predominant in this preparation, receptor gain can be experimentally determined, is not influenced by acute hypoxia, but is greater in fetal than in adult ovine carotid arteries.

Direct visualization of sarcoplasmic reticulum regions discharging Ca(2+)sparks in vascular myocytes

Localized Ca(2+)-release events, Ca(2+)sparks, have been suggested to be the 'elementary building blocks' of the calcium signalling system in all types of muscles. In striated muscles these occur at regular intervals along the fibre corresponding to the sarcomeric structures which do not exist in smooth muscle. We showed previously that in visceral and vascular myocytes Ca(2+)sparks occurred much more frequently at certain sites (frequent discharge sites [FDSs]). In this paper, we have related the position of FDSs to the distribution of the sarcoplasmic reticulum in the same living myocyte. The three-dimensional distribution of the SR in freshly isolated rabbit portal vein myocytes was visualized by means of high-resolution confocal imaging after staining with DiOC(6)and/or BODIPY TR-X ryanodine. Both fluorochromes revealed a similar staining pattern indicating a helical arrangement of well-developed superficial SR which occupied about 6% of the cell volume. Computing the frequency of spontaneous Ca(2+)sparks detected by means of fluo-4 fluorescence revealed that in about 70% of myocytes there was only one major FDS located on a prominent portion of superficial SR network usually within 1-2 microm of the nuclear envelope, although a few sparks occurred at other sites scattered generally in superficial locations throughout the cell. Polarized mitochondria were readily identified by accumulation of tetramethylrhodamine ethyl ester (TMRE). These were closely associated with the SR network in extra-nuclear regions. TMRE staining, however, failed to reveal any mitochondria near the FDS-related SR element. When observed, propagating [Ca(2+)](i)waves and associated myocyte contractions were initiated at FDSs. This study provide first insight into the three-dimensional arrangement of the SR in living smooth muscle cells and relates the peculiarity of the structural organization of the myocyte to the features of Ca(2+)signalling at subcellular level.

Subtype identification and functional properties of inositol 1,4, 5-trisphosphate receptors in heart and aorta

One of the major mechanisms by which hormones elevate intracellular Ca(2+)levels is by generating the second messenger inositol 1,4, 5-trisphosphate (InsP(3)), which activates a Ca(2+)channel (InsP(3)receptor) located in the endoplasmic reticulum (ER). This study undertakes to identify the InsP(3)receptor subtypes (isoforms) in heart and aorta and to characterize their functional properties. The InsP(3)receptor isoforms were identified from rat heart and aorta tissues using both reverse-transcriptase polymerase chain reaction (RT-PCR) to assess the presence of mRNA for the different isoforms and immunochemistry using InsP(3)receptor isoform-specific antibodies. Functional studies included ligand binding experiments using [(3)H]InsP(3)and InsP(3)-induced Ca(2+)release studies using Fluo-3 as the Ca(2+)sensing dye. All three isoforms of the InsP(3)receptor were identified using RT-PCR and immunochemical analyses. [(3)H]InsP(3)binding studies using microsomes derived from these tissues showed that heart had a 3-fold lower abundance of InsP(3)receptors than aorta, while both have considerably lower abundance than the well characterized cerebellar microsomes. The affinity of the InsP(3)binding to the receptor was also different in the three tissues. In cerebellum the K(d)was 60 nM, while aorta had a much higher K(d)of 220 nM. Heart microsomes, appeared to show two classes of binding affinity with K(d)s of 150 nM and 60 nM. Furthermore, the effects of free [Ca(2+)] on [(3)H]InsP(3)binding levels were also different for the three tissues. InsP(3)binding to both cerebellar and aorta microsomes decreased by 90% and 60%, respectively, above 30 nM free [Ca(2+)], while InsP(3)binding to heart was relatively insensitive to changes in [Ca(2+)]. At maximal InsP(3)concentrations, aorta microsomes were able to release about 5% of the accumulated Ca(2+), compared to 25% by cerebellar microsomes. Heart microsomes, however, showed only very little InsP(3)-induced Ca(2+)release ( <0.5%). The EC(50)concentration for InsP(3)-induced Ca(2+)release was 1.2 micro M for aorta while that for cerebellum was 0.3 micro M. Known agonists of the cerebellar InsP(3)receptor such as 3-deoxy InsP(3)and adenophostin A were also able to mobilize Ca(2+)from aorta microsomes. In addition, the competitive antagonist heparin and the non-competitive antagonists of the cerebellar InsP(3)receptor, tetracaine and tetrahexylammonium chloride, were also able to block InsP(3)-induced Ca(2+)release from aorta microsomes.

Cardioprotection induced by AT1R blockade after reperfused myocardial infarction: association with regional increase in AT2R, IP3R and PKCepsilon proteins and cGMP

BACKGROUND

We hypothesized that the cardioprotective effect of angiotensin II (AngII) type 1 receptor (AT(1)R) blockade during in vivo ischemia-reperfusion (IR) might be associated with an increase in AngII type 2 receptor (AT(2)R) protein, as well as 1,4,5-inositol trisphosphate type 2 receptor (IP(3)R) and protein kinase C(epsilon) (PKC(epsilon)) proteins and cyclic guanosine 3',5' monophosphate (cGMP).

METHODS AND RESULTS

We studied the effects of the AT(1)R blocker, candesartan, on in vivo left ventricular (LV) systolic and diastolic function and remodeling (echocardiogram/Doppler) and hemodynamics during canine reperfused anterior infarction (90-minute ischemia, 120-minute reperfusion), and ex vivo infarct size and AT(1)R/AT(2)R, IP(3)R, and PKC(epsilon) proteins (immunoblots), and cGMP (enzyme immunoassay). Compared with controls, candesartan (1 mg/kg intravenously over 30-minute preischemia) inhibited the AngII pressor response, decreased preload and afterload, improved LV systolic and diastolic function, limited LV remodeling, decreased infarct size (55% vs 27% risk; P <.000003), markedly increased AT(2)R, IP(3)R, and PKC(epsilon) proteins in the infarct zone, but not the AT(1)R protein, and increased infarct more than noninfarct cGMP.

CONCLUSIONS

The overall results suggest that cardioprotective effects of AT(1)R blockade on acute IR injury might involve AT(2)R activation and downstream signaling via IP(3)R, PKC(epsilon), and cGMP.

Cerebral artery sarcoplasmic reticulum Ca(2+) stores and contractility: changes with development

To test the hypothesis that sarcoplasmic reticulum (SR) Ca(2+) stores play a key role in norepinephrine (NE)-induced contraction of fetal and adult cerebral arteries and that Ca(2+) stores change with development, we performed the following study. In main branch middle cerebral arteries (MCA) from near-term fetal ( approximately 140 days) and nonpregnant adult sheep, we measured NE-induced contraction and intracellular Ca(2+) concentration ([Ca(2+)](i)) in the absence and presence of different blockers. In adult MCA, after thapsigargin (10(-6) M), the NE-induced responses of tension and [Ca(2+)](i) were 37 +/- 5 and 47 +/- 7%, respectively, of control values (P < 0.01 for each). In the fetal artery, in contrast, this treatment resulted in no significant changes from control. When this was repeated in the absence of extracellular Ca(2+), adult MCA increases in tension and [Ca(2+)](i) were 32 +/- 5 and 13 +/- 3%, respectively, of control. Fetal cerebral arteries, however, showed essentially no response. Ryanodine (RYN, 3 x 10(-6) to 10(-5) M) resulted in increases in tension and [Ca(2+)](i) in both fetal and adult MCA similar to that seen with NE. For both adult and fetal MCA, the increased tension and [Ca(2+)](i) responses to RYN were essentially eliminated in the presence of zero extracellular Ca(2+). These findings provide evidence that in fetal MCA, in contrast to those in the adult, SR Ca(2+) stores are of less importance in NE-induced contraction, with such contraction being almost wholly dependent on Ca(2+) flux via plasma membrane L-type Ca(2+) channels. In addition, they suggest that in both adult and fetal MCA, the RYN receptor is coupled to the plasma membrane Ca(2+)-activated K(+) channel and/or L-type Ca(2+) channel.

Expression and distribution of InsP(3) receptor subtypes in proliferating vascular smooth muscle cells

The expression and distribution of types 1, 2, and 3 inositol 1,4, 5-trisphosphate receptor (InsP(3)R) in proliferating, primary cultures of rat aortic smooth muscle were compared to fully developed and differentiated rat aortic smooth muscle. Subtype-specific InsP(3)R antibodies revealed that the expression of type 1 InsP(3)R was similar in cultured aortic cells and aorta homogenate but expression of type 2 and 3 InsP(3)R subtypes was increased 3-fold in cultured aortic cells. The distribution of the type 1 InsP(3)R was located throughout the cytoplasm; type 2 InsP(3)R was found closely associated with the nucleus and at the plasma membrane; type 3 InsP(3)R was distributed predominantly around the nucleus. Alterations in InsP(3)R subtype expression and localization may have important functions in regulating intracellular calcium release around the nucleus when vascular smooth muscle cells switch to a more proliferating phenotype.

Cardioprotection after angiotensin II type 1 blockade involves angiotensin II type 2 receptor expression and activation of protein kinase C-epsilon in acutely reperfused myocardial infarction in the dog. Effect of UP269-6 and losartan on AT1 and AT2-receptor expression and IP3 receptor and PKCepsilon proteins

To determine whether cardioprotection after chronic angiotensin II (Ang II) type 1 (AT(1)) receptor blockade involves Ang II type 2 (AT(2)) receptor expression and protein kinase C-epsilon (PKC(epsilon)) activation, we measured in vivo haemodynamics and left ventricular (LV) remodelling and dysfunction (echocardiogram/ Doppler) and ex vivo AT(1)/AT(2)-receptor expression, IP(3)R (1, 4, 5-inositol trisphosphate type 2 receptor) and PKC(epsilon) proteins in dogs with acutely reperfused (90 minutes ischaemia, 90 minutes reperfusion) myocardial infarction (MI) following seven days of AT(1)-receptor blockade with oral losartan or UP269-6. The animals were randomised to sham; sham + losartan or UP269-6; MI alone; MI + losartan; MI + UP269-6. More marked AT(1)-receptor blockade with UP269-6 (greater inhibition of Ang II pressor responses) was associated with a smaller increase in preload, less systolic and diastolic dysfunction, less infarct expansion, and smaller LV diastolic and systolic volumes. However, both AT(1)-receptor antagonists decreased infarct size. Importantly, MI decreased AT(1)-receptor and AT(2)-receptor expression while MI after AT(1)-receptor antagonism increased AT(1)-receptor (mRNA, not protein) and AT(2)-receptor (mRNA and protein) expression as well as IP(3)R and PKC(epsilon) proteins and cyclic guanosine 3', 5' monophosphate (cGMP). These results suggest that cardioprotection induced by chronic AT(1)-receptor antagonism involves enhanced AT(2)-receptor expression and possibly downstream signalling through IP(3)R, PKC(epsilon) and cGMP.