Evidence for the presence of phospholamban in the endoplasmic reticulum of smooth muscle

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.

Effect of proinflammatory cytokines on regulation of sarcoplasmic reticulum Ca2+ reuptake in human airway smooth muscle

Airway inflammation leads to increased intracellular Ca(2+) ([Ca(2+)](i)) levels in airway smooth muscle (ASM) cells. Sarcoplasmic reticulum Ca(2+) release and reuptake are key components of ASM [Ca(2+)](i) regulation. Ca(2+) reuptake occurs via sarcoendoplasmic reticulum Ca(2+) ATPase (SERCA) and is regulated by the inhibitory protein phospholamban (PLB) in many cell types. In human ASM, we tested the hypothesis that inflammation increases PLB, thus inhibiting SERCA function, and leading to maintained [Ca(2+)](i) levels. Surprisingly, we found that human ASM does not express PLB protein (although mRNA is detectable). Overnight exposure to the proinflammatory cytokines TNFalpha and IL-13 did not induce PLB expression, raising the issue of how SERCA is regulated. We then found that direct SERCA phosphorylation (via CaMKII) occurs in human ASM. In fura-2-loaded human ASM cells, we found that the CaMKII antagonist KN-93 significantly slowed the rate of fall of [Ca(2+)](i) transients induced by ACh or bradykinin (in zero extracellular Ca(2+)), suggesting a role for CaMKII-mediated SERCA regulation. SERCA expression was decreased by cytokine exposure, and the rate of fall of [Ca(2+)](i) transients was slowed in cells exposed to TNFalpha and IL-13. Cytokine effects on Ca(2+) reuptake were unaffected by additional exposure to KN-93. These data indicate that in human ASM, SERCA is regulated by mechanisms such as CaMKII and that airway inflammation maintains [Ca(2+)](i) levels by decreasing SERCA expression and slowing Ca(2+) reuptake.

Regulation of sarcoplasmic reticulum Ca2+ reuptake in porcine airway smooth muscle

Regulation of intracellular Ca(2+) concentration ([Ca(2+)](i)) in airway smooth muscle (ASM) during agonist stimulation involves sarcoplasmic reticulum (SR) Ca(2+) release and reuptake. The sarco(endo)plasmic reticulum Ca(2+)-ATPase (SERCA) is key to replenishment of SR Ca(2+) stores. We examined regulation of SERCA in porcine ASM: our hypothesis was that the regulatory protein phospholamban (PLN) and the calmodulin (CaM)-CaM kinase (CaMKII) pathway (both of which are known to regulate SERCA in cardiac muscle) play a role. In porcine ASM microsomes, we examined the expression and extent of PLN phosphorylation after pharmacological inhibition of CaM (with W-7) vs. CaMKII (with KN-62/KN-93) and found that PLN is phosphorylated by CaMKII. In parallel experiments using enzymatically dissociated single ASM cells loaded with the Ca(2+) indicator fluo 3 and imaged using fluorescence microscopy, we measured the effects of PLN small interfering RNA, W-7, and KN-62 on [Ca(2+)](i) responses to ACh and direct SR stimulation. PLN small interfering RNA slowed the rate of fall of [Ca(2+)](i) transients to 1 microM ACh, as did W-7 and KN-62. The two inhibitors additionally slowed reuptake in the absence of PLN. In other cells, preexposure to W-7 or KN-62 did not prevent initiation of ACh-induced [Ca(2+)](i) oscillations (which were previously shown to result from repetitive SR Ca(2+) release/reuptake). However, when ACh-induced [Ca(2+)](i) oscillations reached steady state, subsequent exposure to W7 or KN-62 decreased oscillation frequency and amplitude and slowed the fall time of [Ca(2+)](i) transients, suggesting SERCA inhibition. Exposure to W-7 completely abolished ongoing ACh-induced [Ca(2+)](i) oscillations in some cells. Preexposure to W-7 or KN-62 did not affect caffeine-induced SR Ca(2+) release, indicating that ryanodine receptor channels were not directly inhibited. These data indicate that, in porcine ASM, the CaM-CaMKII pathway regulates SR Ca(2+) reuptake, potentially through altered PLN phosphorylation.

Reduced expression of Ca2+-regulating proteins in the upper gastrointestinal tract of patients with achalasia

AIM: To compare expression of Ca²⁺-regulating proteins in upper gastrointestinal (GI) tract of achalasia patients and healthy volunteers and to elucidate their role in achalasia.

METHODS: Sarcoplasmic reticulum Ca²⁺ ATPase (SERCA) isoforms 2a and 2b, phospholamban (PLB), calsequestrin (CSQ), and calreticulin (CRT) were assessed by quantitative Western blotting in esophagus and heart of rats, rabbits, and humans. Furthermore, expression profiles of these proteins in biopsies of lower esophageal sphincter and esophagus from patients with achalasia and healthy volunteers were analyzed.

RESULTS: SERCA 2a protein expression was much higher in human heart (cardiac ventricle) compared to esophagus. However, SERCA 2b was expressed predominantly in the esophagus. The highest CRT expression was noted in the human esophagus, while PLB, although highly expressed in the heart, was below our detection limit in upper GI tissue. Compared to healthy controls, CSQ and CRT expression in lower esophageal sphincter and distal esophageal body were significantly reduced in patients with achalasia (P < 0.05).

CONCLUSION: PLB in the human esophagus might be of lesser importance for regulation of SERCA than in heart. Lower expression of Ca²⁺ storage proteins (CSQ and CRT) might contribute to increased lower esophageal sphincter pressure in achalasia, possibly by increasing free intracellular Ca²⁺.

Pharmacological modulation of sarcoplasmic reticulum function in smooth muscle

The sarco/endoplasmic reticulum (SR/ER) is the primary storage and release site of intracellular calcium (Ca2+) in many excitable cells. The SR is a tubular network, which in smooth muscle (SM) cells distributes close to cellular periphery (superficial SR) and in deeper aspects of the cell (deep SR). Recent attention has focused on the regulation of cell function by the superficial SR, which can act as a buffer and also as a regulator of membrane channels and transporters. Ca2+ is released from the SR via two types of ionic channels [ryanodine- and inositol 1,4,5-trisphosphate-gated], whereas accumulation from thecytoplasm occurs exclusively by an energy-dependent sarco-endoplasmic reticulum Ca2+-ATPase pump (SERCA). Within the SR, Ca2+ is bound to various storage proteins. Emerging evidence also suggests that the perinuclear portion of the SR may play an important role in nuclear transcription. In this review, we detail the pharmacology of agents that alter the functions of Ca2+ release channels and of SERCA. We describe their use and selectivity and indicate the concentrations used in investigating various SM preparations. Important aspects of cell regulation and excitation-contractile activity coupling in SM have been uncovered through the use of such activators and inhibitors of processes that determine SR function. Likewise, they were instrumental in the recent finding of an interaction of the SR with other cellular organelles such as mitochondria. Thus, an appreciation of the pharmacology and selectivity of agents that interfere with SR function in SM has greatly assisted in unveiling the multifaceted nature of the SR.

Mechanisms involved in the cellular calcium homeostasis in vascular smooth muscle: calcium pumps

The regulation of cytosolic Ca2+ homeostasis is essential for cells, and particularly for vascular smooth muscle cells. In this regulation, there is a participation of different factors and mechanisms situated at different levels in the cell, among them Ca2+ pumps play an important role. Thus, Ca2+ pump, to extrude Ca2+; Na+/Ca2+ exchanger; and different Ca2+ channels for Ca2+ entry are placed in the plasma membrane. In addition, the inner and outer surfaces of the plasmalemma possess the ability to bind Ca2+ that can be released by different agonists. The sarcoplasmic reticulum has an active role in this Ca2+ regulation; its membrane has a Ca2+ pump that facilitates luminal Ca2+ accumulation, thus reducing the cytosolic free Ca2+ concentration. This pump can be inhibited by different agents. Physiologically, its activity is regulated by the protein phospholamban; thus, when it is in its unphosphorylated state such a Ca2+ pump is inhibited. The sarcoplasmic reticulum membrane also possesses receptors for 1,4,5-inositol trisphosphate and ryanodine, which upon activation facilitates Ca2+ release from this store. The sarcoplasmic reticulum and the plasmalemma form the superficial buffer barrier that is considered as an effective barrier for Ca2+ influx. The cytosol possesses different proteins and several inorganic compounds with a Ca2+ buffering capacity. The hypothesis of capacitative Ca2+ entry into smooth muscle across the plasma membrane after intracellular store depletion and its mechanisms of inhibition and activation is also commented.

The role of phospholamban and SERCA3 in regulation of smooth muscle-endothelial cell signalling mechanisms: evidence from gene-ablated mice

It is generally agreed that intracellular Ca2+ stores, the sarco(endo)plasmic-reticulum (SER), affect Ca2+ homeostasis and thus contractility of vascular smooth muscle. There is, however, no general consensus as to the magnitude of the SER contribution to Ca2+ handling, the basis for isoforms of the SER Ca(2+)-ATPases (SERCAs) or the role of an SER-associated regulatory protein, phospholamban (PLB). Although the biochemical and cell biological roles of the SER have been intensely studied in vitro, the development of gene-targeted and transgenic mouse models enables one to extend our information to the in vivo levels. A brief review of the role of PLB and SERCA function in vascular and endothelial cell function is presented. Studies on the PLB gene-ablated mouse indicate that vascular contractility is considerably altered. This is mirrored by changes in intracellular Ca2+. Moreover, differences in contractility of the gene-ablated tissues are eliminated by treatment with cyclopiazonic acid, which pharmacologically abolishes SER function by inhibiting the Ca(2+)-ATPase. Thus PLB modulation of sarcoplasmic reticulum (SR) Ca2+ uptake plays a major role in modulating vascular contractility. It is interesting that endothelium-dependent relaxation was decreased in the PLB-deficient aorta. This is surprising in light of the PLB distribution, thought to be limited to cardiac, slow skeletal and smooth muscle. Our data indicate the presence of PLB in endothelial cells and point to an unrecognized pathway for modulation of endothelial cell [Ca2+]i and vascular contractility. Data from smooth muscle tissues of the SERCA3 gene-ablated mouse demonstrate that this isoform affects endothelium-dependent function, but not that of smooth muscle, consistent with its known distribution. This isoform appears to perform a modulatory function, rather than the more essential role of SERCA2. Gene-targeted and transgenic models provide an important avenue for understanding the role of SER in vascular signalling.

Phospholamban: protein structure, mechanism of action, and role in cardiac function

A comprehensive discussion is presented of advances in understanding the structure and function of phospholamban (PLB), the principal regulator of the Ca2+-ATPase of cardiac sarcoplasmic reticulum. Extensive historical studies are reviewed to provide perspective on recent developments. Phospholamban gene structure, expression, and regulation are presented in addition to in vitro and in vivo studies of PLB protein structure and activity. Applications of breakthrough experimental technologies in identifying PLB structure-function relationships and in defining its interaction with the Ca2+-ATPase are also highlighted. The current leading viewpoint of PLB's mechanism of action emerges from a critical examination of alternative hypotheses and the most recent experimental evidence. The potential physiological relevance of PLB function in human heart failure is also covered. The interest in PLB across diverse biochemical disciplines portends its continued intense scrutiny and its potential exploitation as a therapeutic target.

Role of sarcoplasmic reticulum in the myorelaxant activity of nitric oxide donors in guinea pig gastric fundus

The relaxant effect of two nitric oxide (NO) donors: sodium nitroprusside and 3-morpholino-sydnonimine (SIN-1) on circular smooth muscle strips isolated from guinea pig gastric fundus was studied with the view to elucidating the mechanism, which underlies the NO-induced relaxation of this tissue. Both sodium nitroprusside (10(-9)-10(-5) M) and SIN-1 (10(-9)-10(-4) M) suppressed the spontaneous fundus tone and hyperpolarized the muscle cells by about 5 mV. They antagonized the acetylcholine (10(-6) M)-induced tone and exerted their relaxant effects even when Ca2+ influx into the cells was triggered through the Na+/Ca2+ exchanger. Sodium nitroprusside and SIN-1 antagonized the contraction induced by cyclopiazonic acid (10(-5) M), a specific inhibitor of the sarcoplasmic reticulum Ca2+-ATPase. In the presence of high concentrations of sodium nitroprusside or SIN-1, cyclopiazonic acid (10(-5) M) exerted only a slight if any contractile effect. After the complete relaxation induced by sodium nitroprusside or SIN-1, the K+-channel blockers, tetraethylammonium, apamin and charybdotoxin, as well as the Ca2+ ionophore, A 23187, induced high-amplitude contractions, suggesting that the Ca2+ sensitivity of the contractile myofilaments was not affected. The results suggest that NO, released from NO donors increases the sarcoplasmic reticulum Ca2+ uptake thereby enhancing the vectorial sarcoplasmic reticulum Ca2+ release toward the plasmalemma to elicit membrane hyperpolarization and relaxation in guinea pig gastric fundus.

Physiological features of visceral smooth muscle cells, with special reference to receptors and ion channels

Visceral smooth muscle cells (VSMC) play an essential role, through changes in their contraction-relaxation cycle, in the maintenance of homeostasis in biological systems. The features of these cells differ markedly by tissue and by species; moreover, there are often regional differences within a given tissue. The biophysical features used to investigate ion channels in VSMC have progressed from the original extracellular recording methods (large electrode, single or double sucrose gap methods), to the intracellular (microelectrode) recording method, and then to methods for recording from membrane fractions (patch-clamp, including cell-attached patch-clamp, methods). Remarkable advances are now being made thanks to the application of these more modern biophysical procedures and to the development of techniques in molecular biology. Even so, we still have much to learn about the physiological features of these channels and about their contribution to the activity of both cell and tissue. In this review, we take a detailed look at ion channels in VSMC and at receptor-operated ion channels in particular; we look at their interaction with the contraction-relaxation cycle in individual VSMC and especially at the way in which their activity is related to Ca2+ movements and Ca2+ homeostasis in the cell. In sections II and III, we discuss research findings mainly derived from the use of the microelectrode, although we also introduce work done using the patch-clamp procedure. These sections cover work on the electrical activity of VSMC membranes (sect. II) and on neuromuscular transmission (sect. III). In sections IV and V, we discuss work done, using the patch-clamp procedure, on individual ion channels (Na+, Ca2+, K+, and Cl-; sect. IV) and on various types of receptor-operated ion channels (with or without coupled GTP-binding proteins and voltage dependent and independent; sect. V). In sect. VI, we look at work done on the role of Ca2+ in VSMC using the patch-clamp procedure, biochemical procedures, measurements of Ca2+ transients, and Ca2+ sensitivity of contractile proteins of VSMC. We discuss the way in which Ca2+ mobilization occurs after membrane activation (Ca2+ influx and efflux through the surface membrane, Ca2+ release from and uptake into the sarcoplasmic reticulum, and dynamic changes in Ca2+ within the cytosol). In this article, we make only limited reference to vascular smooth muscle research, since we reviewed the features of ion channels in vascular tissues only recently.

Phosphorylation of phospholamban correlates with relaxation of coronary artery induced by nitric oxide, adenosine, and prostacyclin in the pig

The intracellular mechanisms underlying the action of the endogenous vasodilators such as NO/EDRF, adenosine, and prostacyclin acting through cGMP and cAMP, respectively, are not well understood. One important action of cyclic nucleotides in smooth muscle relaxation is to lower the cytosolic Ca2+ concentration by enhanced sequestration into the sarcoplasmic reticulum. The present study was undertaken to elucidate the potential role of phosphorylation of phospholamban, the regulator of sarcoplasmic reticulum Ca2+ pump, for the control of coronary vascular tone by NO/EDRF, adenosine, and prostacyclin. Phospholamban was identified in pig coronary artery preparations by immunofluorescence microscopy, Western blotting and in vitro phosphorylation. Segments of pig coronary artery, with either intact or denuded endothelium, were precontracted with prostaglandin F2alpha (PGF2alpha). In endothelium-denuded preparations 3-morpholinosydnonimine (SIN-1), 5'-N-ethylcarboxiamidoadenosine (NECA), and iloprost (ILO) caused both relaxation and phospholamban phosphorylation with the potency: SIN-1 > NECA > ILO. The regulatory myosin light chain was significantly dephosphorylated only by SIN-1. In endothelium-intact pig coronary artery, L-NAME caused additional vasoconstriction and a decrease in phospholamban phosphorylation, while phosphorylation of myosin light chain remained unchanged. An inverse relationship between phospholamban phosphorylation and vessel tone was obtained. Our findings demonstrate significant phospholamban phosphorylation during coronary artery relaxation evoked by NO, prostacyclin, and adenosine receptor activation. Because of the close correlation between phosphorylation of phospholamban and vessel relaxation, we propose that phospholamban phosphorylation is an important mechanism by which endogenous vasodilators, especially endothelial NO/EDRF, control coronary vascular smooth muscle tone.

Ca2+ transient, cell volume, and microviscosity of the plasma membrane in smooth muscle

Despite pronounced differences by which membrane-depolarizing or phospholipase C-activating stimuli initiate contractile responses, a rise in [Ca2+]i is considered the primary mechanism for induction of smooth muscle contractions. Subsequent to the formation of the well-characterized Ca(2+)4-calmodulin complex, interaction with the catalytic subunit of myosin light chain kinase triggers phosphorylation of 20 kDa myosin light chain and activates actin-dependent Mg2+-ATPase activity, which ultimately leads to the development of tension. The present article reviews the fundamental mechanisms leading to an increase in [Ca2+]i and discusses the biochemical processes involved in the transient and sustained phases of contraction. Moreover, the commentary summarizes current knowledge on the modulatory effect of changes in the microviscosity of the plasma membrane on the Ca2+ transient as well as the contractile response of smooth muscle. Evidence has accumulated that these changes in microviscosity alter the activity of membrane-bound enzymes and affect the generation of endogenous mediators responsible for the regulation of cytosolic Ca2+ concentrations and for the [Ca2+]i-sensitivity of myosin light chain phosphorylation.

Structural perspectives of phospholamban, a helical transmembrane pentamer

Phospholamban is a 52-amino-acid protein that assembles into a pentamer in sarcoplasmic reticulum membranes. The protein has a role in the regulation of the resident calcium ATPase through an inhibitory association that can be reversed by phosphorylation. The phosphorylation of phospholamban is initiated by beta-adrenergic stimulation, identifying phospholamban as an important component in the stimulation of cardiac activity by beta-agonists. In this role of phospholamban that has motivated studies in recent decades. There is evidence that phospholamban may also function as a Ca(2+)-selective ion channel. The structural properties of phospholamban have been studied by mutagenesis, modeling, and spectroscopy, resulting in a new view of the organization of this key molecule in membranes.

The role of sarcoplasmic reticulum and sarcoplasmic reticulum Ca2+-ATPase in the smooth muscle tone of the cat gastric fundus

Circular smooth muscle strips isolated from cat gastric fundus were studied in order to understand whether the sarcoplasmic reticulum (SR) and SR Ca2+-ATPase could play a role in the regulation of the muscle tone. Cyclopiazonic acid (CPA), a specific inhibitor of SR Ca2+-ATPase, caused a significant and sustained increase in muscle tone, depending on the presence of extracellular Ca2+. Nifedipine and cinnarizin only partially suppressed the CPA-induced tonic contraction. Bay K 8644 antagonized the relaxant effect of nifedipine in CPA-contracted fundus. Nitric-oxide-releasing agents sodium nitroprusside and 3-morpholino-sydnonimine completely suppressed the CPA-induced tonic contraction. The blockers of Ca2+-activated K+ channels, tetraethylammonium, charybdotoxin and/or apamin, decreased the contractile effect of CPA. Vanadate increased the tone but did not change significantly the effect of CPA. CPA exerted its contractile effect even when Ca2+ influx was triggered through the Na+/Ca2+ exchanger and the other Ca2+ entry pathways were blocked. Thapsigargin, another specific SR Ca2+-ATPase inhibitor, also increased the muscle tone. The effect of thapsigargin was completely suppressed by sodium nitroprusside and 3-morpholino-sydnonimine and partially by nifedipine. In conclusion, under conditions when the SR Ca2+-ATPase is inhibited, the tissue develops a strong tonic contraction and a large part of this is mediated by Ca2+ influx presumably via nifedipine-sensitive Ca2+ channels. This study suggests the important role of SR Ca2+-ATPase in the modulation of the muscle tone and the function of SR as a "buffer barrier" to Ca2+ entry in the cat gastric fundus smooth muscle.

Signal transduction and regulation in smooth muscle

Smooth muscle cells in the walls of many organs are vital for most bodily functions, and their abnormalities contribute to a range of diseases. Although based on a sliding-filament mechanism similar to that of striated muscles, contraction of smooth muscle is regulated by pharmacomechanical as well as by electromechanical coupling mechanisms. Recent studies have revealed previously unrecognized contractile regulatory processes, such as G-protein-coupled inhibition of myosin light-chain phosphatase, regulation of myosin light-chain kinase by other kinases, and the functional effects of smooth muscle myosin isoforms. Abnormalities of these regulatory mechanisms and isoform variations may contribute to diseases of smooth muscle, and the G-protein-coupled inhibition of protein phosphatase is also likely to be important in regulating non-muscle cell functions mediated by cytoplasmic myosin II.

Calcium and smooth muscle contraction

The fact that smooth muscle exists in almost every hollow organ and is involved in a large number of disease states has led to a vast increase in smooth muscle research, covering areas from testing response to antagonists and agonists to measuring the molecular force generated by a single actin filament. Yet, the exact mechanisms regulating contractile response of smooth muscle remain unsolved. Calcium has been a central player in mediating smooth muscle contraction through binding with calmodulin, although there is evidence showing that under special circumstances smooth muscle can contract without change in intracellular Ca2+. In addition to the major regulatory pathway of Ca(2+)-calmodulin-myosin light chain kinase, there are other thin filament linked regulatory mechanisms in which Ca(2+)-calmodulin dependent phosphorylation of calponin and caldesmon may be involved. Ca2+ sensitivity of smooth muscle contraction may vary under different situations and this has recently been recognized as an important regulatory mechanism. Examples are protein kinase C (PKC) dependent phosphorylation of myosin light chain kinase which results in partial inhibition of contraction, and activation of myosin light chain phosphatase. There is new evidence showing that not only does Ca2+ regulate contraction by regulating the interaction of contractile proteins in smooth muscle, but also that shortening of smooth muscle itself reduces intracellular Ca2+ concentration, via a negative feedback.

The endoplasmic-sarcoplasmic reticulum of smooth muscle: immunocytochemistry of vas deferens fibers reveals specialized subcompartments differently equipped for the control of Ca2+ homeostasis

Cryosection immunofluorescence and immunogold labeling with antibodies against specific markers were used in rat vas deferens smooth muscle fibers to reveal the molecular arrangement of the endomembrane system (referred to variously in the text as ER or sarcoplasmic reticulum [SR]; S-ER or ER/SR) known to participate in the control of Ca2+ homeostasis. The lumenal ER chaperon, immunoglobulin binding protein (BiP), as well as protein disulfide isomerase, and calreticulin, a Ca2+ binding protein expressed by most eukaryotic cells, appeared to be evenly distributed throughout the entire system (i.e., within [a] the nuclear envelope and the few rough-surfaced cisternae clustered near the nucleus; [b] single elements scattered around in the contractile cytoplasm; and [c] numerous, heterogeneous, mainly smooth-surfaced elements concentrated in the peripheral cytoplasm, part of which is in close apposition to the plasmalemma). All other structures, including nuclei, mitochondria, Golgi complex, and surface caveolae were unlabeled. An even distribution throughout the endomembrane system appeared also for the proteins recognized by anti-ER membrane antibodies. In contrast, calsequestrin (the protein that in striated muscles is believed to be the main actor of the rapidly exchanging Ca2+ storage within the lumen of the sarcoplasmic reticulum) was found preferentially clustered at discrete lumenal sites, most often within peripheral smooth-surfaced elements of moderate electron density. Within these elements dual labeling revealed intermixing of calsequestrin with the other lumenal ER proteins. Moreover, the calsequestrin-rich elements were enriched also in the receptor for inositol 1,4,5-trisphosphate, the second messenger that induces Ca2+ release from intracellular stores. These results document the previously hypothesized molecular heterogeneity of the smooth muscle endomembrane system, particularly in relation to the rapid storage and release of Ca2+.

Expression of phospholamban mRNA during early avian muscle morphogenesis is distinct from that of alpha-actin

We have studied the expression of phospholamban during the early development of chick embryos by in situ hybridization and have compared it to that of alpha-cardiac and alpha-skeletal actin. In adult cross-striated muscles there is only one phospholamban gene and it is expressed exclusively in the heart and slow muscles. In the heart phospholamban transcripts were first detected at stage 14 in the region of presumptive ventricle and at stage 20 in the atrium. In the myotomal portion of the somites phospholamban mRNA was first detected at stage 20, which lagged behind the appearance of the alpha-actins. In the limb rudiments all three mRNAs were barely detectable through stage 24, but increased by stage 28+. However, quantitative analysis of signal intensity at stage 28+ indicated that less phospholamban mRNA is present in the limb bud than in the myotome since for phospholamban the ratio of the signal density in the myotome to that in the limb rudiments was about twice the value of the ratio determined for the alpha-actins. Northern blot analysis of embryonic day 11 chick fast pectoralis muscle showed that phospholamban mRNA was not detected in vivo while alpha-cardiac actin mRNA was. Moreover, no phospholamban mRNA was detected in primary cultures derived from pectoralis muscle of the same age. In concert with previous observations that phospholamban is not detectable at stage 30-32 in wing or thigh muscle, these results suggest that phospholamban mRNA is expressed independently of the alpha-actins in the limb buds during early myogenesis.

Ca(2+)-transport ATPases and their regulation in muscle and brain

Eukaryotic cells express one or more isoforms of a sarco(endo)plasmic reticulum (SERCA) and of a plasma membrane (PMCA) Ca2+ pump. Both the SERCA and PMCA gene transcripts are subject to alternative processing in a differentiation stage-dependent and tissue-dependent manner. The Ca2+ pump isoforms thus generated may present different functional properties. This is exemplified by the SERCA2a and SERCA2b isoforms which differ in their Ca2+ sensitivity. Analysis of the cDNA structures for PMCA1 predicts protein isoforms with variant calmodulin- and phospholipid-binding domains. A comparative study of the tissue-specific mechanisms governing SERCA-PMCA transcript processing and a more detailed study of the functional implication of the PMCA pumps isoform diversity will be challenging subjects for future studies.

Calcium ion homeostasis in smooth muscle

Ca2+ plays an important role in the regulation of smooth-muscle contraction. In this review, we will focus on the various Ca(2+)-transport processes that contribute to the cytosolic Ca2+ concentration. Mainly the functional aspects will be covered. The smooth-muscle inositol 1,4,5-trisphosphate receptor and ryanodine receptor will be extensively discussed. Smooth-muscle contraction also depends on extracellular Ca2+ and both voltage- and Ca(2+)-release-activated plasma-membrane Ca2+ channels will be reviewed. We will finally discuss some functional properties of the Ca2+ pumps that remove Ca2+ from the cytoplasm and of the Ca2+ regulation of the nucleus.

Calcium transport by sarcoplasmic reticulum of vascular smooth muscle: II. Effects of calmodulin and calmodulin inhibitors

The role of calmodulin (CaM) in modulating calcium (Ca) uptake by sarcoplasmic reticulum (SR) of vascular smooth muscle was studied in saponin skinned strips of rat caudal artery. Exogenous CaM concentrations ranging from 0.3-1.8 microM did not statistically change the steady state MgATP-dependent Ca content, the MgATP-independent Ca content, or the oxalate-stimulated Ca influx. Calmidazolium (CDZ), W-7, and trifluoperazine (TFP) were used to examine the potential effect of an endogenous CaM pool on inward Ca transport. The IC50 of these antagonists for inhibition of Ca-CaM-stimulated phosphodiesterase activity and Ca-activated superprecipitation of canine aortic actomyosin was measured and found to be in the low micromolar range with a rank order of potency for inhibition of CDZ greater than TFP greater than W-7. In skinned tissues, micromolar concentrations of antagonists that inhibited CaM-mediated reactions in isolated enzyme systems did not reduce Ca content or oxalate-stimulated Ca influx. At higher concentrations of 100-200 microM, the MgATP-dependent Ca content was significantly reduced by TFP and W-7 but not by CDZ. The order of potency for inhibition of Ca uptake was TFP greater than W-7 greater than CDZ. The MgATP-independent Ca content was significantly decreased only by 200 microM TFP. Although none of these inhibitors significantly altered Ca efflux at concentrations up to 100 microM, Ca release was significantly stimulated by all three at 200 microM. The TFP-stimulated Ca release was partially inhibited by ruthenium red. The results indicate that neither exogenous CaM nor an endogenous CaM pool directly modulates inward Ca transport by the SR of saponin skinned caudal artery. The inhibition of Ca uptake produced by hundred micromolar concentrations of CaM antagonists fails to correlate with the order of and with the potency of inhibition measured in isolated enzyme systems. This suggests that the inhibition of Ca uptake produced by high concentrations of these antagonists may be independent of a specific interaction with CaM. The activation of Ca release by high concentrations of CaM antagonists may involve a nonspecific increase in membrane permeability as well as modulation of a membrane Ca channel.

Beta-adrenergic effects on cellular Na, Mg, Ca, K and Cl in vascular smooth muscle: electron probe analysis of rabbit pulmonary artery

The effects of beta-adrenergic stimulation on the cellular content and subcellular distribution of Na, Mg, Ca, K and Cl were determined by electron probe X-ray microanalysis of muscles stimulated with 5-hydroxytryptamine. Isoproterenol caused a significant decrease in cytoplasmic and mitochondrial Na and Cl, and an increase in cytoplasmic Mg. Isoproterenol also significantly decreased total cytoplasmic Ca measured with small diameter probes, without affecting cellular Ca measured with large probes that included the sarcoplasmic reticulum (SR). The decrease in cytoplasmic Na and the effects on cytoplasmic and cellular Ca are consistent with, respectively, beta-adrenergic stimulation of the Na-pump and of Ca-uptake into the SR, but the beta-adrenergic increase in cytoplasmic Mg also raises the possibility of stimulated Na/Mg exchange.

Effects of cyclopiazonic acid, a novel Ca(2+)-ATPase inhibitor, on contractile responses in skinned ileal smooth muscle

1. Effects of cyclopiazonic acid (CPA), a specific inhibitor of the Ca(2+)-ATPase in sarcoplasmic reticulum (SR) of skeletal and cardiac muscles, on contractile responses induced by Ca(2+)-release from intracellular storage sites were examined in the longitudinal smooth muscle strip of the guinea-pig ileum skinned with beta-escin. 2. Ca(2+)-loading of storage sites (Ca(2+)-uptake) was performed in pCa 6.3 solution. The amount of Ca2+ taken up was monitored by use of the amplitude of contraction following application of 25 mM caffeine or 25 microM inositol 1,4,5-trisphosphate (IP3). 3. Contractile responses to caffeine or IP3 were reduced or abolished when the preceding Ca(2+)-uptake was performed in the presence of 0.1-10 microM CPA. The dose of CPA required to inhibit the contraction induced by caffeine or IP3 by 50% was approximately 0.6 microM. The CPA-sensitive Ca(2+)-uptake completely depended upon the presence of ATP in the solution during Ca(2+)-uptake. 4. When 1 microM CPA was added after Ca(2+)-uptake, the subsequent caffeine- or IP3-induced contraction was not significantly affected by the presence of CPA. 5. Acetylcholine-induced contraction was also almost abolished when the preceding Ca(2+)-uptake was performed in the presence of 10 microM CPA. 6. The relationship between pCa and contraction was not affected by the presence of 10 microM CPA in skinned fibres where Ca2+ storage sites had been destroyed by treatment with A23187. The enhancement of contraction in pCa 6.0 solution by calmodulin was not affected by 10 microM CPA.7. These results suggest that CPA selectively inhibits ATP-dependent Ca2"-uptake into intracellular storage sites in skinned ileal smooth muscle strips. CPA appears to be a potent, reversible, and very specific inhibitor of the Ca2+-pump in the storage sites of smooth muscle, and is an extremely valuable pharmacological tool.

Role of phospholamban in NO/EDRF-induced relaxation in rat aorta

The role of endothelium-derived nitric oxide (NO) to cause smooth muscle phospholamban (PLB) phosphorylation was studied in the isolated perfused rat aorta precontracted with norepinephrine using a back-phosphorylation technique. NO-induced relaxation was associated with increased PLB-phosphorylation while norepinephrine as such was ineffective. Removal of endothelium significantly reduced PLB-phosphorylation in indomethacin treated vessels. Stimulation of NO-formation by ATP augmented PLB-phosphorylation in intact vessels but was ineffective in denuded aortas. The results indicate that PLB-phosphorylation of vascular smooth muscle plays an important role in mediating NO-dependent relaxation by enhancing Ca(++)-uptake into sarcoplasmic reticulum.

Annexin VI is associated with calcium-sequestering organelles

Annexin VI is a member of a Ca(2+)-dependent, phospholipid-binding protein family. Although functions for this annexin have been proposed from in vitro studies, most remain controversial. Díaz-Muñoz et al. (J Biol Chem 265:15894, 1990) demonstrated that annexin VI modified, in a Ca(2+)-dependent manner, the gating behavior of the sarcoplasmic reticulum Ca(2+)-release channel, reconstituted into artificial bilayers, by increasing both the open probability and the mean open time. This effect was specific to the trans chamber, which represents the luminal side of the sarcoplasmic reticulum. In agreement with those findings, we show herein that annexin VI produced no effect on Ca(2+)-uptake or -release by intact heavy sarcoplasmic reticulum vesicles (analogous to the cis chamber). We also used monospecific antibodies to evaluate the subcellular localization of annexin VI by immunofluorescent microscopy. Studies in rat skeletal muscle suggest that annexin VI is present surrounding individual myofibrils. Double immunolocalization studies with cultured muscle cells (chick myotubes) using anti-annexin VI and anti-SR Ca(2+)-ATPase antibodies demonstrated superimposable staining patterns. In non-muscle tissue (normal rat kidney (NRK) cells), a punctate, perinuclear anti-annexin VI staining pattern was observed. Collectively, these data suggest that annexin VI may play a regulatory role in the Ca(2+)-release/uptake cycle in the sarcoplasmic reticulum as well as in non-muscle organelles, a key process in stimulus-response systems.

Calcium homeostasis in single intact smooth muscle cells

We have demonstrated that ISO produces part of its negative inotropic action through activation of the plasmalemmal Na+/K+ pump, and reduction of [Na+]i. This action is mediated by the beta-adrenergic receptor through activation of adenylate cyclase. The reduction of [Na+]i is most probably translated to a change in the contractile state of the cell through activation of the Na+/Ca2+ exchanger. While the exchanger is at equilibrium when the cell is at rest, after ISO it would extrude Ca2+ at the expense of the increased Na+ gradient, resulting in a decrease Ca2+ availability and a reduction in the magnitude of subsequent contractions. We have also seen that the previous calcium history of the myoplasm can influence the time course of future calcium transients. Prolonged large increases in [Ca2+]i can accelerate the rate of its removal and depress basal [Ca2+]i levels. This action is most probably mediated through a Ca2+/calmodulin dependent protein kinase. We have observed that MLCK is both necessary and sufficient to produce contraction of Bufo marinus stomach smooth muscle. There is also evidence that an as yet unidentified Ca(2+)-calmodulin dependent protein kinase is acting to limit the magnitude and the duration of the Ca2+ transient by feeding back on processes involved in Ca2+ signal generation.

Ca2+ extrusion across plasma membrane and Ca2+ uptake by intracellular stores

The aim of this review is to summarize the various systems that remove Ca2+ from the cytoplasm. We will initially focus on the Ca2+ pump and the Na(+)-Ca2+ exchanger of the plasma membrane. We will review the functional regulation of these systems and the recent progress obtained with molecular-biology techniques, which pointed to the existence of different isoforms of the Ca2+ pump. The Ca2+ pumps of the sarco(endo)plasmic reticulum will be discussed next, by summarizing the discoveries obtained with molecular-biology techniques, and by reviewing the physiological regulation of these proteins. We will finally briefly review the mitochondrial Ca(2+)-uptake mechanism.

Expression of endoplasmic-reticulum Ca2(+)-pump isoforms and of phospholamban in pig smooth-muscle tissues

The expression of the gene 2 sarcoplasmic/endoplasmic-reticulum Ca2(+)-pump isoforms (SERCA2a and SERCA2b) and of phospholamban was studied in pig smooth muscle of the stomach, longitudinal ileum, pulmonary artery and aorta. mRNA levels were determined using an RNAase protection assay. The SERCA2 isoforms and phospholamban were tested on Western blots with a panel of antibodies, some of which were isoform-specific. The pig smooth-muscle tissues all contained comparable SERCA2 mRNA levels, but these levels were 10-20-fold lower than SERCA2 mRNA levels in cardiac muscle. Of the SERCA2 mRNAs in smooth muscle, 72-81% encoded the non-muscle isoform (SERCA2b), and Western blot analysis with isoform-specific antibodies confirmed that the SERCA2b isoform is the predominant endoplasmic-reticulum Ca2(+)-pump in smooth muscle. In contrast with SERCA2 mRNA levels, phospholamban mRNA levels varied by 12-fold between the different pig smooth-muscle tissues, with low and very low levels in the pig pulmonary artery and the pig aorta respectively. The differential expression of phospholamban was also confirmed on Western blots. The finding that the phospholamban content varied between the different smooth-muscle tissues whereas the SERCA2 expression remained rather constant indicates that, in pig smooth muscle, the expression of phospholamban is not coupled with that of SERCA2.

Agonist-dependent Ca2+ and Mn2+ entry dependent on state of filling of Ca2+ stores in aortic smooth muscle cells of the rat

1. The properties of intracellular Ca2+ stores of intact- and of saponin-skinned A7r5 (an established cell line from embryonic rat aorta) smooth muscle cells were studied by measuring 45Ca2+ and 54Mn2+ fluxes. 2. Application of 5 microM-vasopressin to intact cells increased the fractional loss of 45Ca2+ in Ca2(+)-free solution by a factor of 5.2. This effect was not influenced by a pre-incubation with 10 microM-ryanodine. Caffeine (25 mM) did not stimulate the fractional loss of 45Ca2+ from intact cells. 3. In skinned cells 10 microM-IP3 (inositol 1,4,5-trisphosphate) and 5 microM-A23187 (a calcium ionophore) released the same amount of 45Ca2+. This release did not require GTP and was not affected by a pre-incubation with 10 microM-ryanodine. Caffeine (25 mM) did not release stored Ca2+. 4. NaF (1 mM) plus 10 microM-AlCl3 inhibited by 72% the 45Ca2+ uptake by the IP3-sensitive store of skinned cells at 0.15 microM-Ca2+. Cyclic AMP-dependent protein kinase did not stimulate this ATP-dependent 45Ca2+ uptake, nor could the presence of phospholamban be demonstrated immunologically. 5. The 45Ca2+ uptake by cells which had been depleted of Ca2+ with 5 microM-vasopressin was 69% higher than the uptake obtained without such proceeding depletion. This enhanced 45Ca2+ uptake did not occur through voltage-operated Ca2+ channels, because blockade of these channels with verapamil, or depolarization of the plasma membrane by increasing [K+] from 5.9 to 59 mM in the presence of verapamil, did not modify this uptake. 6. A similar increase of the 54Mn2+ uptake occurred in intact cells with a depleted Ca2+ store. If, however, the cells were first skinned and subsequently exposed to 54Mn2+, the ATP-dependent 54Mn2+ uptake amounted to less than 6% of the ATP-dependent 45Ca2+ uptake. 7. If intact cells were first exposed to a 45Ca2(+)- or 54Mn2(+)-containing solution, and subsequently skinned in a non-radioactive intracellular solution, the addition of 10 microM-A23187 to these cells released stored Ca2+ or Mn2+. The amount of released Ca2+ was only slightly larger than the amount of released Mn2+. If the intracellular store was depleted before loading, the amount of Ca2+ or Mn2+ released by the ionophore increased by 68 and 28%, respectively. 8. It is concluded that A7r5 smooth muscle cells do not express a Ca2(+)-induced Ca2+ release mechanism, but do contain an IP3-induced Ca2+ release mechanism which can release approximately all intracellularly accumulated 45Ca2+.(ABSTRACT TRUNCATED AT 400 WORDS)

Effects of cyclic nucleotide dependent protein kinases on the endoplasmic reticulum Ca2+ pump of bovine pulmonary artery

This paper describes the stimulation by cyclic nucleotide dependent protein kinases on the Ca2+ uptake by isolated endoplasmic reticulum (ER) vesicles from the bovine main pulmonary artery. This ER fraction has previously been shown to be highly enriched in phospholamban, a protein kinase substrate that has been well characterized in cardiac sarcoplasmic reticulum (SR), where its phosphorylation is accompanied by an increased rate of Ca2+ uptake. As previously observed for the phosphorylation of phospholamban, the stimulation of the rate of Ca uptake was as high with cGMP dependent protein kinase as with cAMP dependent protein kinase. The effect of phosphorylation of the ER membranes from smooth muscle on the Ca2+ uptake was smaller than that seen in cardiac SR, and it was only observed if albumin was included during the isolation of the membranes. This relatively small effect is probably not due to a lower ratio of phospholamban to Ca2(+)-transport enzyme in the ER membranes as compared to cardiac SR. Several alternative explanations are discussed.

Cyclic AMP and mechanisms of vasodilation

Cyclic AMP and the mechanism of vasodilation have been reviewed by first discussing the enzymes involved (adenyl cyclase, cyclic nucleotide phosphodiesterases, cyclic AMP-dependent protein kinase) and then agents that increase cAMP in smooth muscle. Two mechanisms of vasodilation are described: (i) effects on contractile proteins; (ii) effects on Ca2+ levels. Evidence for compartments of cAMP is also presented.

The expression of canine cardiac phospholamban in heterologous systems

A synthetic phospholamban gene has been cloned and expressed in Escherichia coli, producing both native phospholamban and a fusion protein with 81 amino acids of the influenza virus NS1 protein. Both the native phospholamban and fusion proteins produced extensive cell lysis upon induction of gene expression, but only the native protein underwent spontaneous pentamer formation in E. coli. Translation in vitro of mRNA produced by transcription in vitro of phospholamban cDNA was used to demonstrate the spontaneous aggregation of phospholamban to form pentamers in this system also, both in the presence and absence of exogenous microsomes from canine pancreas or heart. Phospholamban produced by translation in vitro was apparently susceptible to proteolysis by enzymes present in the particulate fractions in these experiments.

cDNA cloning and sequencing of phospholamban from pig stomach smooth muscle

Phospholamban cDNA from pig stomach smooth muscle was cloned and sequenced. The 737-nucleotide-residue cDNA contained an open reading frame of 156 nucleotide residues encoding a peptide of 52 amino acid residues (Mr 6080). This peptide shares 100% sequence identity with dog cardiac-muscle phospholamban. It differs from rabbit cardiac-muscle and slow-twitch skeletal-muscle phospholamban only at position 2, which is a glutamic acid residue in rabbit phospholamban, but an aspartic acid residue in the pig smooth-muscle protein. Northern-blot analysis reveals the presence of several phospholamban mRNAs in smooth muscle, but a 900-nucleotide-residue and a 2800-nucleotide-residue transcript predominate.

Evidence for two isoforms of the endoplasmic-reticulum Ca2+ pump in pig smooth muscle

cDNA clones coding for the endoplasmic reticulum Ca2+-transport ATPase have been cloned from a pig smooth-muscle cDNA library. The transcripts can be divided into two classes which differ in their 3' ends due to alternative splicing of the primary gene transcript. The class 1 cDNA encodes a protein of 997 amino acids (Mr 110,000). The class 2 protein (1042 amino acids; Mr 115,000) is completely identical to the class 1 protein except that the four C-terminal amino acids of the class 1 protein are replaced in the class 2 protein with a tail of 49 amino acids. Comparison of these sequences with other Ca2+ pump sequences reveals that the class 1 isoform corresponds to the sarcoplasmic reticulum Ca2+ pump of slow-twitch skeletal/cardiac muscle, whereas the class 2 protein corresponds to a Ca2+ pump recently detected in non-muscle tissues.

Phospholamban is a good substrate for cyclic GMP-dependent protein kinase in vitro, but not in intact cardiac or smooth muscle

1. Cyclic GMP-dependent protein kinase phosphorylates purified phospholamban. It also phosphorylates phospholamban present in vesicles of cardiac sarcoplasmic reticulum and smooth muscle microsomal fractions, and in transformants of Escherichia coli which contain a plasmid into which a gene encoding phospholamban has been inserted. 2. In vitro the phospholamban present in cardiac sarcoplasmic reticulum membranes is a better substrate for cyclic GMP-dependent protein kinase than for cyclic AMP-dependent protein kinase. 3. Studies using [32P]Pi to label the cellular ATP in intact cardiac or smooth muscle failed to demonstrate that phosphorylation of phospholamban occurs in response to stimuli which increase intracellular cyclic GMP. Possible reasons for this functional separation between increased cyclic GMP and phosphorylation of phospholamban are discussed.

Regulation of calcium concentration in voltage-clamped smooth muscle cells

The regulation of intracellular calcium concentration in single smooth muscle cells was investigated by simultaneously monitoring electrical events at the surface membrane and calcium concentration in the cytosol. Cytosolic calcium concentration rose rapidly during an action potential or during a voltage-clamp pulse that elicited calcium current; a train of voltage-clamp pulses caused further increases in the calcium concentration up to a limit of approximately 1 microM. The decline of the calcium concentration back to resting levels occurred at rates that varied with the calcium concentration in an apparently saturable manner. Moreover, the rate of decline at any given calcium concentration was enhanced after a higher, more prolonged increase of calcium. The process responsible for this enhancement persisted for many seconds after the calcium concentration returned to resting levels. Thus, the magnitude and duration of a calcium transient appear to regulate the subsequent calcium removal.