Regulation of sarcoplasmic reticulum gene expression during cardiac and skeletal muscle development

Calcium homeostasis and control of contractility in the developing heart

The Ca2+ concentration within the myocyte is an important determinant of myocardial contractility. Substantial changes in the cellular processes responsible for transport of Ca2+ ions across the sarcolemmal and sarcoplasmic reticulum membranes occur during maturation of the heart. In this article, the mechanisms underlying these changes and their impact on myocardial performance are discussed in detail.

Changes in mRNA levels of the sarcoplasmic/endoplasmic-reticulum Ca(2+)-ATPase isoforms in the rat soleus muscle regenerating from notexin-induced necrosis

The relative mRNA levels corresponding to the different sarcoplasmic/endoplasmic-reticulum Ca(2+)-ATPase isoforms (SERCA1a, SERCA1b, SERCA2a, SERCA2b and SERCA3) were measured by reverse transcriptase-PCR in rat soleus muscles regenerating after notexin-induced necrosis. The succession of appearance of the different types of SERCA mRNA species in regenerating muscle largely recapitulates those observed during normal ontogenesis. The mRNA levels of the muscle-specific isoforms SERCA1a and SERCA2a became very low on the first and third days after injection of the snake venom. It was only on the fifth day of regeneration that the mRNA of the neonatal variant of the fast-twitch skeletal SERCA1b isoform began to rise, well before the other SERCA transcripts. At 7 and 10 days, i.e. at a time when the new myofibres normally become reinnervated, the mRNA level of SERCA1a and SERCA2a increased markedly, but the fast-twitch skeletal SERCA1a isoform was still the most prominent. On day 21, in the advanced stage of regeneration, a switch in the relative expression levels of SERCA1a and SERCA2a mRNA was observed and the ratio of both isoforms became similar to that found in the normal soleus muscles. This was followed by a decline in the level of all SERCA mRNA species, so that on day 28 the levels of the sarcoplasmic/endoplasmatic-reticulum Ca(2+)-pump RNAs was again lower but their ratio remained similar to that of the untreated control soleus.

Salbutamol changes the molecular and mechanical properties of canine skeletal muscle

1. Salbutamol, a beta 2-agonist, increased the weight of the canine latissimus dorsi muscle. It also increased fusion frequency, and decreased time-to-peak tension, half-relaxation time, and total contraction time. These changes in twitch times and fusion frequency were associated with changes in the levels of proteins expressed in slow- and fast-twitch fibres. Salbutamol decreased the levels of the slow-twitch cardiac isoform of sarco-/endoplasmic reticulum Ca(2+)-ATPase (SERCA2a) and phospholamban proteins, and increased the level of the fast-twitch isoform of sarco-/endoplasmic reticulum Ca(2+)-ATPase (SERCA1a). 2. Changes in the levels of SERCA proteins, particularly SERCA1a, could account for most of the increases in calcium uptake rate observed in homogenates of muscles from the salbutamol-treated animals and could partially account for the changes in half-relaxation rates and other twitch times. 3. Changes in the levels of SERCA1a, SERCA2a and phospholamban protein did not always follow changes in the levels of their corresponding mRNAs. Divergence depended upon the SERCA isoform and muscle. The muscles studied were latissimus dorsi and vastus intermedius. 4. Salbutamol did not change the level of myosin heavy chain (HC)-I isoforms in either muscle, suggesting that it did not increase the proportion of slow-twitch fibres in these muscles. It did increase the level of HC-IIx and decrease the level of HC-IIa isoforms in the latissimus dorsi. Salbutamol did not produce these effects in the vastus intermedius. It is of particular interest that salbutamol changed the relative levels of SERCA proteins in the latissimus dorsi muscle without producing significant change in the level of HC-I isoform.

Early postnatal changes in sarcoplasmic reticulum calcium transport function in spontaneously hypertensive rats

This comparative study investigates the relationship between sarcoplasmic reticulum (SR) calcium(Ca2+)-ATPase transport activity and phospholamban (PLB) phosphorylation in whole cardiac homogenates of spontaneously hypertensive rats (SHR) and their parent, normotensive Wistar Kyoto (WKY) strain during early postnatal development at days 1, 3, 6, 12 and at day 40 to ascertain any difference in SR Ca2+ handling before the onset of hypertension. At day 1, the rate of homogenate oxalate-supported Ca2+ uptake was significantly higher in SHR than in WKY (0.25 +/- 0.02 vs 0.12 +/- 0.01 nmoles Ca2+/mg wet ventricular weight/min, respectively; p < 0.001). This interstrain difference disappeared with further developmental increase in SR Ca2+ transport. Western Blot analysis and a semiquantitative ELISA did not reveal any difference in the amount of immunoreactive PLB (per mg of total tissue protein) between strains at any of the ages studied. In addition, levels of phosphorylated PLB formed in vitro in the presence of radiolabelled ATP and catalytic (C) subunit of protein kinase A did not differ between SHR and WKY at days 1, 3, 6 and 12. At day 40, C subunit-catalyzed formation of 32P-PLB was reduced by 66% (p < 0.001) in SHR when compared to age-matched WKY. In the early postnatal period between day 1 and 12 SR Ca(2+)-transport values were linearly related to the respective 32P-PLB levels of both SHR and WKY rats. The results indicate that cardiac SR of SHR can sequester Ca2+ at a much higher rate immediately after birth compared to WKY rats. The disappearance of this interstrain difference with further development suggests that some endogenous neuroendocrine or nutritional factor(s) from the hypertensive mother may exert an influence upon the developing heart in utero resulting in a transiently advanced maturation of the SR Ca2+ transport function in SHR pups at the time of birth.

Cloning and characterization of the 5'-upstream regulatory region of the Ca(2+)-release channel gene of cardiac sarcoplasmic reticulum

To elucidate the transcriptional regulation mechanism for the Ca(2+)-release channel gene of the cardiac sarcoplasmic reticulum (RYR2), we isolated and analyzed the 5'-upstream flanking region of the gene. Sequence analysis indicated that the core promoter region lacks canonical TATA and CAAT boxes, but contains three overlapping GC boxes. A gel shift assay indicated that Sp1 binds to the region containing the GC boxes. Different 5'-deletion constructs in the 5'-flanking region of the RYR2 gene were fused to the luciferase gene, and their promoter activity in rat neonatal cardiac myocytes was subsequently determined. The results revealed the presence of a region containing positive regulatory elements in the 5'-flanking region. Analyses of substitutional mutations introduced into the GC boxes and the regulatory region indicated that in addition to the GC box located at -56 to -51, two regulatory elements (RYR2P1 and RYR2P2) are essential for the promoter activity. These results indicated that Sp1 and transcription factors that bind to RYR2P1 and RYR2P2 cooperatively enhance the expression of the RYR2 gene. In a transient transfection experiment involving promoter-luciferase gene constructs in skeletal muscle cells, we identified a negative regulatory region between positions -209 and -90 that represses the expression of the RYR2 gene in skeletal muscle cells.

Endothelin and calcium antagonists in the skin microcirculation of patients with coronary artery disease

BACKGROUND

Endothelin, a potent endothelium-derived vasoconstrictor peptide, is elevated in coronary artery disease (CAD); however, its pathophysiological role is uncertain. Calcium antagonists are widely used in patients with CAD. Using laser Doppler flowmetry, we investigated the influence of two endothelin antagonists and the calcium antagonist diltiazem on endogenous and exogenous endothelin in the skin microcirculation of CAD patients and healthy control subjects.

METHODS AND RESULTS

Both endothelin antagonists and diltiazem applied intradermally induced vasodilation in CAD patients, which was more pronounced with the ETA/ETB antagonist than with the ETA antagonist or diltiazem. Exogenous endothelin led to profound vasoconstriction in CAD patients and healthy volunteers. Both endothelin antagonists and diltiazem blunted the vasoconstriction to exogenous endothelin in CAD patients and young healthy volunteers and less so in old healthy volunteers. However, compared with both endothelin antagonists, a 10-times-higher dose of diltiazem was required. Systemic diltiazem (240 mg, slow release) attenuated endothelin-induced vasoconstriction in CAD patients. Neurogenic vasodilation to exogenous endothelin was inhibited by both endothelin antagonists.

CONCLUSIONS

This study demonstrates that endogenous endothelin of CAD patients contributes to the regulation of vascular tone in the skin microcirculation not only through ETA receptors but also possibly through ETB receptors. Diltiazem inhibited endothelin-induced vasoconstriction, but endothelin antagonists were slightly more effective. Thus, endothelin antagonists represent potent new tools to interfere with the vascular effects of endothelin in CAD patients. Future studies must confirm these findings in other areas of the circulation.

Structure-function relationships in the Ca(2+)-ATPase of sarcoplasmic reticulum: facts, speculations and questions for the future

Structural data on the Ca(2+)-ATPase of sarcoplasmic reticulum are integrated with kinetic data on Ca2+ transport. The emphasis is upon ATPase-ATPase interactions, the requirement for phospholipids, and the mechanism of Ca2+ translocation. The possible role of cytoplasmic [Ca2+] in the regulation of the synthesis of Ca(2+)-ATPase is discussed.

Sarco(endo)plasmic reticulum Ca2+ pump and metabolic enzyme expression in rabbit fast-type and slow-type denervated skeletal muscles. A time course study

Recent reports by d'Albis et al. have shown that denervation of 8-day-old rabbit fast-twitch muscle (gastrocnemius) leads to the transformation of the muscle towards a slow phenotype but the changes towards slow-type myosin isoforms and contractile properties of the muscle were temporally uncoordinated. We analyzed the time course of the effects of denervation of the gastrocnemius on the expression of the sarcoplasmic reticulum calcium pump isoforms (SERCA) and on the metabolic state of the muscle. Northern-blot analysis showed a rapid loss of the fast Ca2+ pump isoform (SERCA 1) mRNA from the denervated gastrocnemius which became of the oxidative type. The changes observed were complete as early as 35 days post-natal, i.e at the time when changes in contractile properties were previously observed. Denervation of the slow-twitch soleus led to a 50% decrease in the level of the slow Ca2+ pump isoform (SERCA 2) mRNA and was without effect on the metabolic state of the muscle. These findings extend previous results suggesting that in rabbit, continuous innervation is required for differentiation of fast-twitch muscles but is not an absolute requirement for differentiation of the slow-twitch muscle.

Differential distribution of ryanodine receptor type 3 (RyR3) gene product in mammalian skeletal muscles

Activation of intracellular Ca(2+)-release channels/ryanodine receptors (RyRs) is a fundamental step in the regulation of muscle contraction. In mammalian skeletal muscle, Ca(2+)-release channels containing the type 1 isoform of RyR (RyR1) open to release Ca2+ from the sarcoplasmic reticulum (SR) upon stimulation by the voltage-activated dihydropyridine receptor on the T-tubule/plasma membrane. In addition to RyR1, low levels of the mRNA of the RyR3 isoform have been recently detected in mammalian skeletal muscles. Here we report data on the distribution of the RyR3 gene product in mammalian skeletal muscles. Western-blot analysis of SR of individual muscles indicated that, at variance with the even distribution of the RyR1 isoform, the RyR3 content varies among different muscles, with relatively higher amounts being detected in diaphragm and soleus, and lower levels in abdominal muscles and tibialis anterior. In these muscles RyR3 was localized in the terminal cisternae of the SR. No detectable levels of RyR3 were observed in the extensor digitorum longus. Preferential high content of RyR3 in the diaphragm muscle was observed in several mammalian species. In situ hybridization analysis demonstrated that RyR3 transcripts are not restricted to a specific subset of skeletal-muscle fibres. Differential utilization of the RyR3 isoform in skeletal muscle may be relevant to the modulation of Ca2+ release with respect to specific muscle-contraction properties.

Different mechanisms of Ca2(+)-handling following nicotinic acetylcholine receptor stimulation, P2U-purinoceptor stimulation and K(+)-induced depolarization in C2C12 myotubes

1. The increase in intracellular CA2+ on nicotinic acetylcholine receptor (nAChR) stimulation, P2U-purinoceptor stimulation and K(+)-induced depolarization was investigated in mouse C2C12 myotubes by use of fura-2 fluorescence to characterize the intracellular organisation of Ca2+ releasing stores and Ca(2+)-entry process. 2. Stimulation of nAChRs with carbachol induced a rapid rise in internal Ca2+ (EC50 = 0.85 +/- 0.09 microM), followed by a sustained phase. The Ca2+ response evoked by carbachol (10 microM) was completely blocked by the nAChR antagonist, pancuronium (3 microM), but was not affected by the muscarinic antagonist, atropine (3 microM), or under conditions when Ca2+ entry was blocked by La3+ (50 microM) or diltiazem (10 microM). Addition of pancuronium (3 microM) during the sustained phase of the carbachol-evoked response did not affect this phase. 3. Stimulation of P2U purinoceptors with ATP (1 mM) induced a somewhat higher biphasic Ca2+ response (EC50 of the rapid phase: 8.72 +/- 0.08 microM) than with carbachol. Pretreatment with La3+ abolished the sustained phase of the ATP-induced Ca2+ response, while the response was unaffected by diltiazem or pancuronium. 4. Stimulation of the cells with high K+ (60 mM), producing the same depolarization as with carbachol (10 microM), induced a rapid monophasic Ca2+ response, insensitive to diltiazem, pancuronium or La3+. 5. Under Ca(2+)-free conditions, the sustained phase of the carbachol- and ATP-evoked responses were abolished. Pre-emptying of depolarization-sensitive stores by high K+ under Ca(2+)-free conditions did not affect the carbachol- or ATP-evoked Ca2+ mobilization and vice versa. Preincubation of the cells with ATP in the absence of extracellular Ca2+ decreased the amplitude of the subsequent carbachol-induced Ca2+ response to 11%, while in the reverse procedure the ATP-induced response was decreased to 65%. Ca2+ mobilization evoked by simultaneous addition of optimal concentrations of carbachol and ATP was increased compared to levels obtained with either agonist. 6. Preincubation with high K+ under normal conditions abolished the sustained phase of the ATP-evoked Ca2+ response. The carbachol response consisted only of the sustained phase in the presence of high K+. 7. The carbachol-induced Ca2+ response was completely abolished under low Na+/Ca(2+)-free conditions, while under low Na+ conditions only a sustained Ca2+ response was observed. The ATP- and K(+)-induced responses were changed compared to Ca(2+)-free conditions. 8. ATP (300 microM) induced the formation of Ins(1,4,5)P3 under Ca(2+)-free conditions with a comparable time course to that found for the rise in internal Ca2+. In contrast to ATP, carbachol (10 microM) did not affect Ins(1,4,5)P3 levels under Ca(2+)-free conditions. 9. It is concluded that the Ca2+ release from discrete stores of C2C12 myotubes is induced by stimulation of nAChRs, P2U-purinoceptors and by high K+. Only the P2U-purinoceptor and nAChR activated stores show considerable overlap in releasable Ca2+. Sustained Ca(2+)-entry is activated by stimulation of nAChRs and P2U-purinoceptors via separate ion-channels, which are different from the skeletal muscle nAChR-coupled cation-channel.

Multiple Sp1 binding sites in the cardiac/slow twitch muscle sarcoplasmic reticulum Ca2+-ATPase gene promoter are required for expression in Sol8 muscle cells

The rabbit cardiac/slow twitch muscle sarcoplasmic reticulum Ca2+-ATPase (SERCA2) gene encodes a Ca2+ transport pump whose expression is regulated during skeletal and cardiac muscle development and in response to various pathophysiological and hormonal states. Employing transient transfection analyses in Sol8 muscle cells, we have identified two positive regulatory regions, one distal (-1810 base pair (bp) to -1110 bp) and one proximal (-284 bp to -72 bp), within the SERCA2 promoter. The proximal promoter region from -284 bp to -80 bp was shown to confer muscle-specific expression to a heterologous promoter in Sol8 cells. This region is highly GC-rich containing the consensus sequence for four Sp1 elements (GGGCGG) and three Sp1-like elements (GGGAGG). DNase I footprint analysis with Sol8 nuclear extracts and purified Sp1 protein showed the protection of the seven Sp1 binding sites. In addition, site-directed mutagenesis of the Sp1 consensus sites demonstrated that Sp1 sites are essential for the muscle-specific expression of the SERCA2 promoter. Furthermore, we demonstrate that cotransfection of an Sp1 expression vector together with SERCA2-CAT constructs can up-regulate SERCA2 promoter activity. These results imply that the Sp1 transcription factor plays an important role in the transcriptional regulation of SERCA2 within muscle cells.

Effects of thapsigargin and cyclopiazonic acid on intracellular calcium activity in newborn rat cardiomyocytes during their development in primary culture

The effects of specific inhibitors of sarcoplasmic reticulum (SR) calcium ATPase, thapsigargin (TG), and cyclopiazonic acid (CPA) were investigated on the resting and transient levels of intracellular free calcium concentrations recorded in Indo-1-loaded ventricular myocytes of newborn rat heart in primary culture. The calcium transients were induced by caffeine (10 mM) or high potassium (100 mM) solutions. In 2 day- as in 7-day-old cultured cells, the calcium transients induced by 10 mM caffeine were blocked dose dependently by TG and CPA. The dose-response curves suggest that TG was more efficient than CPA and that both drugs were more efficient in 7-day- than in 2-day-old cells. The calcium transients induced by 100 mM K+ were also strongly inhibited by these agents. The lack of effect on sarcolemmal calcium currents, as shown by whole-cell patch-clamp experiments, suggests that these drugs affect only SR function. In cells exhibiting spontaneous activity, the associated calcium transients were not affected by TG or CPA at the beginning of the culture (2-day-old cells) but were fully blocked at the end (7-day-old cells). These results confirm that TG and CPA specifically inhibit the cardiac SR Ca2+ pump without affecting the sarcolemmal calcium current. Their blocking effect of the calcium transients as a function of the developmental stage of neonatal cardiac cells in culture suggests an increasing role of the SR in the regulation of intracellular calcium. This argues for developmental changes of the SR through the differentiation and maturation of newborn cardiomyocytes at the early stage of the postnatal life, leading to a predominant role of the SR in excitation-contraction coupling mechanisms in adult cells.

Chronic intrauterine pulmonary hypertension alters endothelin receptor activity in the ovine fetal lung

Although endothelin (ET) contributes to the regulation of pulmonary vascular tone in the normal fetus, little is known about its role in pulmonary hypertension in the perinatal period. To examine the role of the ETB receptor in the normal ovine fetal lung, we studied the hemodynamic effects of ET-3 (a selective ETB receptor agonist) before and after RES-701 (a selective ETB receptor antagonist). RES-701 (10 mu g/min for 10 min) did not change basal pulmonary tone and blocked pulmonary vasodilation to ET-3 (500 ng/min for 10 min). To examine the effects of experimental perinatal pulmonary hypertension on activity of the ETA and ETB receptors, we studied the hemodynamic effects of ET-3, ET-1 (a nonselective ETA and ETB receptor agonist), and BQ 123 (a selective ETA receptor antagonist) in 12 chronically prepared late gestation fetal lambs after partial ligation of the ductus arteriosus. Serial changes in the pulmonary vascular effects of these agents were measured early (1-3 d) and late (7-10 d) after partial ductus arteriosus ligation. Left lung total pulmonary resistance in the normal late-gestation fetus was 0.62 +/- 0.01 mm Hg/ml/min (n = 4). After partial ductus arteriosus ligation, total pulmonary resistance increased to 1.2 +/- 0.3 (early; p < 0.05 versus normal), and progressively rose to 1.9 +/- 0.2 mm Hg/ml/min (late; p < 0.05 versus early). Intrapulmonary infusion of ET-3 (500 ng/min for 10 min) increased pulmonary blood flow from 94 +/- 11 to 183 +/- 17 mL/min in the normal fetus, but had no effect during late pulmonary hypertension. Infusions of ET-1 (50 ng/min for 30 min) caused transient pulmonary vasodilation followed by vasoconstriction during early pulmonary hypertension. During late pulmonary hypertension, however, infusion of ET-1 caused predominantly vasoconstriction. Pulmonary vasodilation to BQ 123 (100 mu g/min for 10 min) was greater during late than early pulmonary hypertension (43 versus 21%; p < 0.05). After 10 d of ductus arteriosus ligation, immunoreactive ET-1 content in whole lung tissue was 3-fold higher in hypertensive (n = 7) than control (n = 10) lungs (p < 0.05). We conclude that the ETB receptor contributes little to regulation of basal vascular tone in the normal ovine fetal lung and that chronic intrauterine pulmonary hypertension causes the loss of ETB-mediated vasodilation, progressive ETA-mediated vasoconstriction, and increased lung ET-1 content. We speculate that diminished ETB receptor-mediated vasodilation in combination with enhanced ETA receptor-mediated vasoconstriction and increased ET-1 production contributes to high pulmonary vascular resistance in perinatal pulmonary hypertension.

Regulation of calcium binding proteins calreticulin and calsequestrin during differentiation in the myogenic cell line L6

In this report we defined the structural and temporal limits within which calreticulin and calsequestrin participate in the muscle cell phenotype, in the L6 model myogenic system. Calreticulin and calsequestrin are two Ca2+ binding proteins thought to participate in intracellular Ca2+ homeostasis. We show that calsequestrin protein and mRNA were expressed when L6 cells were induced to differentiate, during which time the level of expression of calreticulin protein did not change appreciably. Calreticulin mRNA levels, however, were constant throughout L6 cell differentiation except for slight decline in the mRNA levels at the very late stages of L6 differentiation (day 11-12). We also show that the two Ca2+ binding proteins are coexpressed in differentiated L6 cells. Based on its mobility in SDS-PAGE, L6 rat skeletal muscle cells in culture expressed cardiac isoform of calsequestrin. In the mature rat skeletal muscle, calreticulin and calsequestrin were localized to sarcoplasmic reticulum (SR). Calreticulun, but not calsequestrin, staining was also observed in the perinuclear region. These data suggest that expression of calreticulin and calsequestrin may be under different control during myogenesis in rat L6 cells in culture.

Skeletal muscle sarcoplasmic reticulum phenotype in myotonic dystrophy

In this study we investigated the sarcoplasmic reticulum (SR), alongside myofibrillar phenotype, in muscle samples from five Myotonic Dystrophy (DM) patients and five control individuals. DM muscles exhibited as a common feature, a decrease in the slow isoform of myosin heavy chain (MHC) and of troponin C in myofibrils. We observed a match between myofibrillar changes and changes in SR membrane markers specific to fiber type, i.e. the fast (SERCA1) Ca(2+)-ATPase isoform increased concomitantly with a decrease of protein phospholamban (PLB), which in native SR membranes colocalizes with the slow (SERCA2a) SR Ca(2+)-ATPase, and regulates its activity depending on phosphorylation by protein kinases. Our results outline a cellular process selectively affecting slow-twitch fibers, and non-degenerative in nature, since neither the total number of Ca(2+)-pumps or of ryanodine receptor/Ca(2+)-release channels, or their ratio to the dihydropyridine receptor/voltage sensor in junctional transverse tubules, were found to be significantly changed in DM muscle. The only documented, apparently specific molecular changes associated with this process in the SR of DM muscle, are the defective expression of the slow/cardiac isoform of Ca(2+)-binding protein calsequestrin, together with an increased phosphorylation activity of membrane-bound 60 kDa Ca(2+)-calmodulin (CaM) dependent protein kinase. Enhanced phosphorylation of PLB by membrane-bound Ca(2+)-CaM protein kinase also appeared to be most pronounced in biopsy from a patient with a very high CTG expansion, as was the overall 'slow-to-fast' transformation of the same muscle biopsy. Animal studies showed that endogenous Ca(2+)-CaM protein kinase exerts a dual activatory role on SERCA2a SR Ca(2+)-ATPase, i.e. either by direct phosphorylation of the Ca(2+)-ATPase protein, or mediated by phosphorylation of PLB. Our results seem to be consistent with a maturational-related abnormality and/or with altered modulatory mechanisms of SR Ca(2+)-transport in DM slow-twitch muscle fibers.

Cardiac troponin I phosphorylation increases the rate of cardiac muscle relaxation

Cardiac troponin (Tn) I (CTnI), compared with skeletal TnI, contains extra amino acids (32 to 33) at its amino terminus, including two adjacent serine residues. These two serine residues are believed to be phosphorylated by protein kinase A (PKA) upon stimulation of the heart by beta-agonists. In this study, we found that phosphorylation of a cardiac skinned muscle preparation by PKA, mainly at CTnI, results in a decrease in the Ca2+ sensitivity of muscle contraction. The pCa50 decreased by approximately 0.27 +/- 0.06 pCa units upon phosphorylation. To study cardiac muscle relaxation, we used diazo-2, a photolabile Ca2+ chelator with a low Ca2+ affinity in its intact form that is converted to a high-affinity form after photolysis. We found that the rate of cardiac muscle relaxation increased from a time of half-relaxation (t1/2) = 110 +/- 10 milliseconds to t1/2 = 70 +/- 8 milliseconds after CTnI phosphorylation. This result demonstrates that CTnI phosphorylation can be linked with the increased rate of muscle relaxation in a relatively intact muscle preparation. Since CTnI phosphorylation has been shown previously to affect the Ca2+ affinity and Ca2+ off-rate of CTnC in vitro, it is likely that the faster relaxation seen here reflects faster dissociation of Ca2+ from cardiac TnC (CTnC). Model calculations show that increased dissociation of Ca2+ from CTnC, coupled with the faster uptake of Ca2+ by the sarcoplasmic reticulum stimulated by PKA phosphorylation of phospholamban, can account for the faster relaxation seen in the inotropic response of the heart to catecholamines.

Transcriptional regulation of phospholamban gene and translational regulation of SERCA2 gene produces coordinate expression of these two sarcoplasmic reticulum proteins during skeletal muscle phenotype switching

Chronic 1 Hz stimulation of the canine latissimus dorsi muscle produced a time-dependent switch from the fast-twitch to the slow-twitch phenotype. This included changes in the proteins of the sarcoplasmic reticulum. After 3 days of muscle stimulation, there was down-regulation of fast-twitch Ca-ATPase (SERCA1a) mRNA and induction of slow-twitch Ca-ATPase (SERCA2a) mRNA; most changes in both mRNAs were nearly complete after 14 days of stimulation. Although the induction of phospholamban mRNA began after 3 days of muscle stimulation, its up-regulation was not completed until the muscle had been stimulated for 42 days. The time course of expression of SERCA2a protein was very different from that of SERCA2a mRNA, suggesting that SERCA2 gene expression is regulated at the translational as well as the transcriptional level. The time course of expression of phospholamban protein closely followed that of phospholamban mRNA, suggesting that this gene is under transcriptional control. Thus coordinated expression of SERCA2a and phospholamban proteins is achieved via translational control of the SERCA2 gene and transcriptional control of the phospholamban gene.

Expression of phospholamban in C2C12 cells and regulation of endogenous SERCA1 activity

Phospholamban (PLB) is a regulator of the sarcoplasmic reticulum Ca(2+)-ATPase (SERCA2) expressed in cardiac, slow-twitch skeletal, and smooth muscles. Phospholamban is not expressed in the sarcoplasmic reticulum of fast-twitch skeletal muscle, but it can regulate the sarcoplasmic reticulum Ca(2+)-ATPase activity (SERCA1) expressed in this muscle, in vitro. To determine whether phospholamban can regulate SERCA1 activity in its native membrane environment, phospholamban was stably transfected into a cell line (C2C12) derived from murine fast-twitch skeletal muscle. Differentiation of C2C12 myoblasts to myotubes was associated with induction of SERCA1 expression, assessed by Western blotting analysis using Ca(2+)-ATPase isoform specific antibodies. The expressed phospholamban protein was localized in the microsomal fraction isolated from C2C12 myotubes. To determine the effect of phospholamban expression on SERCA1 activity, microsomes were isolated from transfected and nontransfected C2C12 cell myotubes, and the initial rates of 45Ca(2+)-uptake were determined over a wide range of Ca2+ concentrations (0.1-10 microM). Expression of phospholamban was associated with inhibition of the initial rates of Ca(2+)-uptake at low [Ca2+] and this resulted in a decrease in the affinity of SERCA1 for Ca2+ (0.27 +/- 0.02 microM in nontransfected vs. 0.41 +/- 0.03 microM in PLB transfected C2C12 cells). These findings indicate that phospholamban expression in C2C12 cells is associated with inhibition of the endogenous SERCA1 activity and provide evidence that phospholamban is capable of regulating this Ca(2+)-ATPase isoform in its native membrane environment.

In vivo adenovirus-mediated gene transfer via the pulmonary artery of rats

Gene transfer into the pulmonary vasculature has the potential to be a powerful technique for both investigation of pulmonary pathophysiology and development of genetic therapies for pulmonary vascular disease. To evaluate the potential for in vivo pulmonary arterial gene transfer, we infused adenoviral vectors into the left pulmonary artery of Sprague-Dawley and cotton rats. Access to the left pulmonary artery was obtained by a percutaneous transcatheter approach or through thoracotomy and pulmonary arteriotomy. With the thoracotomy approach, both pulmonary arterial inflow and pulmonary venous outflow were occluded during vector influsion and throughout a subsequent 20-minute dwell period. The success of gene transfer was assessed by staining for evidence of recombinant gene expression in lungs excised at time points ranging from 48 to 72 hours after virus infusion. With the thoracotomy technique, pulmonary gene transfer was successful in 15% of surviving Sprague-Dawley rats and 30% of surviving cotton rats. Percutaneous catheter-based pulmonary gene transfer was not successful. In rats with pulmonary gene transfer, 1% to 8% of total left lung cells expressed the recombinant gene. Recombinant gene expression was found in endothelial cells (0.2% to 18% of total transduced cells), smooth muscle cells (0% to 3%), macrophages (1% to 7%), airway epithelial cells (2% to 50%), and alveolar epithelial cells (38% to 94%). Investigation of the low rate of successful gene transfer in individual animals suggested that insufficient physical contact between the virions and pulmonary cells was the most likely cause. In vivo gene transfer into the rat pulmonary vasculature can be accomplished with adenovirus vectors.(ABSTRACT TRUNCATED AT 250 WORDS)

Patterns of expression of sarcoplasmic reticulum Ca(2+)-ATPase and phospholamban mRNAs during rat heart development

This study reports the clonal analysis and sequence of rat phospholamban (PLB) cDNA clones and the temporal appearance and patterns of distribution of the mRNAs encoding sarcoplasmic/endoplasmic reticulum Ca(2+)-ATPase (SERCA2) and PLB in the developing rat heart determined by in situ hybridization. Both proteins play a critical role in the contraction-relaxation cycle of the heart. SERCA2 mRNA is already abundantly present in the first stage studied, in the cardiogenic plate of the 9-day-old presomite embryo, before the occurrence of the first contractions. This very early expression makes it an excellent marker for the study of early heart development. Subsequently, SERCA2 mRNA becomes expressed in a craniocaudal gradient, being highest at the venous pole and decreasing in concentration toward the arterial pole of the heart. PLB mRNA can be detected in hearts from 12 days of development onward in a virtually opposite gradient. In essence, these patterns do not change during further development. PLB mRNA levels remain highest in the ventricle and outflow tract, whereas SERCA2 mRNA prevails in the inflow tract and atrium, although the difference between atrium and ventricle becomes less pronounced. These observations are compatible with a model in which the upstream part of the heart (inflow tract and atrium) would have a greater capacity to clear calcium and hence would have a longer duration of the diastole than the downstream compartments (atrioventricular canal, ventricle, and outflow tract), similar to the observed pattern of contraction of the embryonic heart. The sinoatrial and atrioventricular nodes do not reveal an expression pattern of SERCA2 and PLB mRNA that allows one to distinguish them from the surrounding atrial working myocardium. However, the ventricular part of the conduction system, comprising atrioventricular bundle and bundle branches, are almost devoid of SERCA2 mRNA.

Developmental and tissue-specific regulation of rabbit skeletal and cardiac muscle calcium channels involved in excitation-contraction coupling

Two types of calcium channels signal excitation-contraction (E-C) coupling in striated muscle: dihydropyridine receptors (DHPRs, voltage-gated L-type calcium channels on the transverse tubule) and ryanodine receptors (RyRs, calcium release channels on the sarcoplasmic reticulum). Sarcolemmal depolarization activates the DHPR; subsequently, the RyR is activated and releases calcium that activates muscle contraction. We show in the present study that expression of the E-C coupling calcium channels is upregulated during myogenic development in the rabbit. Skeletal and cardiac muscle isoforms of the following genes were examined: the DHPR alpha 1, alpha 2, beta, and gamma subunits and the RyR. Distinct cardiac and skeletal muscle-specific cDNAs were isolated, encoding each of the DHPR subunits and the RyR. The skeletal muscle DHPR alpha 1, alpha 2, beta, and gamma subunits and the cardiac DHPR alpha 1 subunit mRNA levels increased on the day of birth and at the adult stage compared with fetal levels. The skeletal and cardiac RyR mRNA levels increased on the day of birth and at adult stages compared with fetal levels. Ryanodine binding sites increased in both skeletal and cardiac muscle. We now provide a molecular explanation for the physiological "maturation" of the E-C coupling apparatus observed at the day of birth and during early postnatal development in both skeletal and cardiac muscles. Low levels of calcium channel expression in fetal cardiac and skeletal muscle make these tissues more sensitive to pharmacological therapy with calcium channel blockers, a phenomenon that has been reported in human neonates.

Regulation of splicing is responsible for the expression of the muscle-specific 2a isoform of the sarco/endoplasmic-reticulum Ca(2+)-ATPase

Tissue-specific alternative processing of sarco/endoplasmic reticulum Ca(2+)-ATPase 2 (SERCA2) transcripts generates functionally different Ca2+ pump isoforms in muscle compared with non-muscle tissues. In non-muscle cells, the SERCA2 pre-mRNA can be polyadenylated at a site located between the donor and acceptor splice site of an intron which is only removed in muscle tissues. To define the cis-active elements involved in differential processing, we constructed a minigene (pCM beta SERCA2) containing the 3' end of the SERCA2 gene. When stably transfected into a myogenic cell line, minigene transcripts were differentially processed depending on the differentiation state of the cells. This proves that the essential elements required for regulated processing are present in the construct. Furthermore, co-transfection of the pCM beta SERCA2 minigene and a myogenin expression vector in a fibroblast cell line induced muscle-specific splicing of transcripts from pCM beta SERCA2. This shows that trans-acting factor(s) responsible for muscle-specific processing can be induced by one of the important regulatory genes of muscle differentiation. Inactivation of the non-muscle poly(A) site did not induce splicing in non-muscle cells. This excludes a simple competition model between splicing and polyadenylation, but it is consistent with splicing being very inefficient in non-muscle cells. Moreover, splicing could be induced in non-muscle cells by optimizing the muscle-specific donor splice site and/or by shortening the intron length. We therefore propose that expression of the muscle-specific SERCA2a isoform is the result of activation of an otherwise inefficient splicing process.

Characterization study of the ryanodine receptor and of calsequestrin isoforms of mammalian skeletal muscles in relation to fibre types

We have investigated high-affinity ryanodine-binding sites in membrane preparations from representative fast-twitch and slow-twitch muscles of the rabbit and rat, as well as from human mixed muscle. Our results, obtained in high-ionic strength binding buffer, demonstrate extensive similarities in binding affinity for [3H]ryanodine (Kd: about 10 nM) and a two-fold to four-fold difference in membrane density of the ryanodine receptor between fast-twitch and slow-twitch muscle of the rat and rabbit, respectively. The [3H]ryanodine-pCa relationship for the Ca(2+)-activation curve of ryanodine binding was found to be similar for all mammalian muscles, as tested at 20 nM ryanodine. With 10 mM caffeine or 50 microM doxorubicin the pCa for half-maximal activation of [3H]ryanodine binding invariably shifted from an average pCa value of 6.5 to pCa 7.1-7.3. IC50 values for the inhibition of [3H]ryanodine binding by Ruthenium Red, a Ca(2+)-release channel blocker, did not differ significantly (range 0.3-1.0 microM). The Ca(2+)-dependence curve (range 1 nM-10 mM free Ca2+) that we have observed at 5 nM ryanodine, for [3H]ryanodine binding to terminal cisternae from rabbit fast-twitch, as well as slow-twitch muscle, is bell-shaped and differs from that obtained with cardiac terminal cisternae from the same species. Cardiac ryanodine receptor is also clearly distinguishable for electrophoretic mobility, Cleveland's peptide maps, and, most strikingly, for total lack of cross-reactivity with polyclonal antibody to fast skeletal RyR. By the same properties, the ryanodine receptor of fast- and slow-twitch muscle appear to be the same or a similar protein. On investigating the composition of calsequestrin in rat and human skeletal muscles, both in membrane-bound form and after purification by phenyl-Sepharose chromatography, we have been able to show that, independent of the animal species, the cardiac isoform, as characterized by the identical amino-terminal amino-acid sequence, pattern of immunoreactivity, and lack of Ca(2+)-dependent shift in mobility on SDS-PAGE, is exclusively expressed in slow-twitch fibres, together with the main fast-skeletal calsequestrin isoform. While our experimental findings strongly argue for the presence of only one population of skeletal-specific Ca(2+)-release channels in junctional terminal cisternae of mammalian fast-twitch and slow-twitch muscle, they at the same time suggest the existence of differences in calsequestrin modulation of Ca(2+)-release, depending on its isoform composition.

Steady-state mRNA levels of the sarcolemmal Na(+)-Ca2+ exchanger peak near birth in developing rabbit and rat hearts

To functionally compensate for an underdeveloped sarcoplasmic reticulum in immature cardiomyocytes, it has been proposed that the sarcolemmal Na(+)-Ca2+ exchanger may assume a more predominant role for regulating cytosolic Ca2+. Previous studies using sarcolemma prepared from developing rabbit hearts demonstrated that Na(+)-dependent Ca2+ uptake and exchanger protein content were highest at birth and declined postnatally. To further investigate the significance of the Na(+)-Ca2+ exchanger during normal myocardial development, steady-state mRNA levels of the cardiac Na(+)-Ca2+ exchanger were quantitated by Northern blot and slot-blot analyses using poly(A+) RNA isolated from rabbit and rat ventricles at various fetal and postnatal ages. Northern analyses were performed with a 1.35-kb guinea pig cardiac Na(+)-Ca2+ exchanger cDNA probe. Exchanger mRNA levels were quantitated by densitometric scans of the slot blots, and results were normalized by reprobing the same blots with 32P 5'-end-labeled oligo(dT). In both species, exchanger mRNA levels peaked near birth and declined postnatally. Maximal levels were approximately sixfold greater in the late fetal rabbit (gestational day 29) and eightfold greater in the early newborn rat (postnatal day 1) compared with adults of the respective species. The parallel changes in exchanger mRNA and protein levels suggest that developmental regulation of cardiac Na(+)-Ca2+ exchanger expression involves pretranslational control mechanisms. These results support the concept that during normal cardiac development, Na(+)-Ca2+ exchanger expression is maximal near the time of birth and then declines postnatally as Ca2+ regulation by the sarcoplasmic reticulum reaches functional maturity.

Switching of the dominant calcium sequestering protein during skeletal muscle differentiation

A major Ca(2+)-storing protein in endoplasmic reticulum (ER) of non-muscle cells is calreticulin (CR), which is considered to be functionally homologous to calsequestrin. Calsequestrin is a Ca(2+)-binding protein in sarcoplasmic reticulum (SR) of striated muscle, which stores Ca2+ during muscle relaxation. In order to investigate the expression and distribution of calsequestrin and calreticulin during skeletal muscle differentiation, cultured chick embryonic skeletal muscles were observed by immunofluorescence using anti-calsequestrin, anti-calreticulin, anti-desmin, and anti-sarcomeric myosin antibodies and rhodamine-phalloidin. Within 6 hours in culture, myoblasts started to express desmin. Desmin-positive cells demonstrated the reticular staining of calreticulin, as did desmin-negative cells. Around fusion, calsequestrin and sarcomeric myosin started to appear in desmin-positive cells. The expression of calsequestrin slightly preceded that of sarcomeric myosin. As the myotubes matured, the fluorescent dots of calsequestrin increased and spread to the cell periphery along the myofibrils, while the reticular pattern of calreticulin gradually disappeared. Double labeling showed that calsequestrin colocalized with calreticulin. In mature myotubes, anti-calsequestrin staining demonstrated many dots along myofibrils, whereas calreticulin was barely seen except at the perinuclear region. These results suggest that the expression of calsequestrin and calreticulin are switched during skeletal muscle differentiation.

Postnatal development of rabbit fast-twitch skeletal muscle: accumulation, isoform transition and fibre distribution of calsequestrin

The time-course of disappearance of slow-cardiac calsequestrin (CS) and that of appearance of the skeletal CS isoform were investigated in developing fast-twitch skeletal muscle of the rabbit between postnatal days 1 and 60, along with changes in density of the ryanodine receptor (RyR)/Ca2+ release channel. Western blot data on skeletal muscle membranes, purification of two CS isoforms by phenyl-Sepharose chromatography, and their immunolocalization in muscle fibres, all show that both CS isoforms are coexpressed in neonatal muscles. Our results, at the protein level, indicate that the turning off of synthesis of cardiac CS and its total disappearance from fast-twitch fibres take place at critical periods between two and four weeks postnatally, i.e. past changes in the respective mRNA. In contrast, the accumulation in muscle membranes of both the RyR and the skeletal CS isoform proceeds steadily up to one month, to reach adult values at about two months of age. These findings seem to argue that myogenic factors, in addition to the morphogenetic influence on the sarcoplasmic reticulum from the neural input to the muscle, may be involved in the developmental transition of CS isoforms in mammalian fast-twitch muscle fibres.

Endothelin-1 in adult respiratory distress syndrome

Endothelin-1 (ET-1), a potent vasoconstrictor peptide produced by endothelial cells and degraded predominantly in the pulmonary vasculature, has been implicated in the development of various organ dysfunctions. To determine the pathophysiologic role of ET-1 in adult respiratory distress syndrome (ARDS) and the impact of impaired lung function on transpulmonary peptide handling, we compared plasma levels and pulmonary ET-1 balance in 14 patients with ARDS and in seven healthy control subjects. To obtain comparable conditions in both groups, the ET-1 level was raised in the control group by exogenous infusion (0.4 pmol/kg/min) to 9.4 +/- 0.8 pmol/L. ARDS was accompanied by a hyperdynamic circulatory pattern with increased cardiac output and depressed total vascular resistance but, simultaneously, pulmonary hypertension. Venous ET-1 concentration was massively increased in ARDS (9.8 +/- 1.2 versus 2.1 +/- 0.2 pmol/L, p < 0.001). In control subjects, the lung cleared the major fraction of ET-1 (fractional extraction 43 +/- 8.8%, uptake 12.5 +/- 2.5 pmol/min). In contrast, in ARDS there was a pronounced pulmonary releases into the circulation (32.8 +/- 10.3 pmol/min). We conclude that ET-1 concentrations are elevated in ARDS as the result of both increased formation and decreased disposal. Lung failure affects not only gas exchange but also nonrespiratory, metabolic pulmonary functions.

Targeting gene expression to specific cardiovascular cell types in transgenic mice

Transgenic techniques, which allow the introduction of exogenous genes into the genome of experimental animals, promise to bridge the gap between the in vitro observations made by molecular and cellular biologists on cardiac and vascular cells in tissue culture and the physiology and pathology of the whole organ system. One such application of these techniques is tissue targeting: by genetic manipulation to direct expression of a protein--such as a signaling peptide, a growth factor receptor, or an oncogene involved in cell growth--to a tissue where it normally would not be expressed (or where expression is tightly controlled) by fusing it to the transcriptional control sequences of another gene normally expressed in that tissue. In the cardiovascular system, regulatory sequences for cardiomyocyte-specific proteins, vascular endothelium-specific proteins, and smooth muscle-specific proteins can be used to target heterologous genes to their respective tissues in transgenic animals. The effects that such perturbations have on organ physiology and intracellular and intercellular communication can be observed by applying established physiological and molecular approaches. In this review, we highlight some tissue-specific genes from cardiac and vascular cell types whose regulatory sequences may be used to target heterologous proteins; we discuss neutral "reporter" proteins and signal transduction components as paradigms for the application of this technique; and we briefly touch on the potentials and pitfalls of transgenic approaches to molecular physiology.

Characterization of promoter elements of the rabbit cardiac sarcoplasmic reticulum Ca(2+)-ATPase gene required for expression in cardiac muscle cells

The sarcoplasmic reticulum Ca(2+)-ATPase (SERCA2) plays a critical role in the contractile performance of cardiac and slow-twitch skeletal muscle by restoring cytosolic calcium to low resting levels during the contractile cycle. We have previously shown that SERCA2 expression in the heart is altered by a number of pathophysiological stimuli. In an effort to define molecular mechanisms regulating expression of the SERCA2 gene in cardiac muscle cells, deletions of a 1460-bp promoter fragment were generated and inserted into a luciferase reporter plasmid. Promoter constructs were transiently transfected into embryonic cardiocytes and skeletal muscle cell lines Sol 8 and C2C12 in vitro and injected into adult myocardium in vivo. Results demonstrate that sequences from the transcription start site to -284 are both necessary and sufficient for high-level transcription of the reporter gene in differentiating muscle cells and in fetal cardiocytes in culture. We further demonstrate that this promoter fragment is highly active in vivo when injected into rat hearts, suggesting that the same regulatory elements are functional in vivo as well as in vitro. The region of the gene from -284 to -658 exerts a modest positive effect in cardiocytes and Sol 8 myotubes but exerts a negative effect in C2C12 fast skeletal muscle cells. This initial analysis of transcriptional regulation of the SERCA2 gene will serve as a foundation for the study of alterations of expression of the gene in pathological conditions.

Expression of endothelin-1 in the lungs of patients with pulmonary hypertension

BACKGROUND

Pulmonary hypertension is characterized by an increase in vascular tone or an abnormal proliferation of muscle cells in the walls of small pulmonary arteries. Endothelin-1 is a potent endothelium-derived vasoconstrictor peptide with important mitogenic properties. It has therefore been suggested that endothelin-1 may contribute to increases in pulmonary arterial tone or smooth-muscle proliferation in patients with pulmonary hypertension. We studied the sites and magnitude of endothelin-1 production in the lungs of patients with various causes of pulmonary hypertension.

METHODS

We studied the distribution of endothelin-1-like immunoreactivity (by immunocytochemical analysis) and endothelin-1 messenger RNA (by in situ hybridization) in lung specimens from 15 control subjects, 11 patients with plexogenic pulmonary arteriopathy (grades 4 through 6), and 17 patients with secondary pulmonary hypertension and pulmonary arteriopathy of grades 1 through 3.

RESULTS

In the controls, endothelin-1-like immunoreactivity was rarely seen in vascular endothelial cells. In the patients with pulmonary hypertension, endothelin-1-like immunoreactivity was abundant, predominantly in endothelial cells of pulmonary arteries with medial thickening and intimal fibrosis. Likewise, endothelin-1 messenger RNA was increased in the patients with pulmonary hypertension and was expressed primarily at sites of endothelin-1-like immunoreactivity. There was a strong correlation between the intensity of endothelin-1-like immunoreactivity and pulmonary vascular resistance in the patients with plexogenic pulmonary arteriopathy, but not in those with secondary pulmonary hypertension.

CONCLUSIONS

Pulmonary hypertension is associated with the increased expression of endothelin-1 in vascular endothelial cells, suggesting that the local production of endothelin-1 may contribute to the vascular abnormalities associated with this disorder.

Analysis of the rabbit cardiac/slow twitch muscle sarcoplasmic reticulum calcium ATPase (SERCA2) gene promoter

The rabbit cardiac/slow twitch muscle sarcoplasmic reticulum (SR) Ca2+ ATPase (SERCA2) gene encodes a Ca2+ transport pump whose expression is regulated during skeletal/cardiac muscle development and by different pathophysiological states of the heart. This study was designed to delineate cis-acting regulatory elements involved in SERCA2 gene expression. A series of unidirectionally deleted fragments of the upstream 1,460 bp SERCA2 promoter were linked to the chloramphenicol acetyltransferase (CAT) reporter gene. Transient DNA transfection experiments performed with these constructs in C2C12 muscle cells and NIH3T3 fibroblasts revealed a 17 bp upstream promoter element (UPE) important for transcription of the SERCA2 gene in skeletal muscle cells. These studies have also identified a strong (muscle specific) negative regulatory region located upstream of nucleotide -658. Gel mobility shift and southwestern analyses using the 17 bp UPE have revealed a specific DNA binding complex referred to as Ca2+ ATPase promoter factor -1 (CaPF1). The binding factor has an approximate M(r) of 43 kDa. Comparison of CaPF1 with known transcription factors suggests that the CaPF1 complex may be a novel DNA-binding transcription factor which plays a role in SERCA2 gene regulation in vivo.

Alternative processing of the gene transcripts encoding a plasma-membrane and a sarco/endoplasmic reticulum Ca2+ pump during differentiation of BC3H1 muscle cells

The effect of differentiation on the RNA processing of the PMCA1 gene encoding a plasma-membrane Ca2+ pump and of the SERCA2 gene encoding a sarco(endo)plasmic reticulum Ca2+ pump was studied in the myogenic BC3H1 cell line. A differentiation stage-dependent change in the RNA processing was observed for both genes. Proliferating myoblasts only expressed the non-muscle mRNA isoform whereas in differentiated cells muscle-specific processing became activated. The switch to muscle-specific RNA processing for both the PMCA1 and SERCA2 genes was found to be linked to the myogenic conversion of the BC3H1 cells. Our results furthermore indicated that the myogenic RNA processing could be reversed for both types of Ca2+ pumps since the expression of the PMCA1 and SERCA2 muscle-specific messengers was rapidly down-regulated by cycloheximide treatment.

Ryanodine receptors: how many, where and why?

Ryanodine receptors are intracellular Ca2+ channels that have been known for more than a decade to have a role in releasing Ca2+ from the sarcoplasmic reticulum to regulate contraction in skeletal and cardiac muscle fibres. Vincenzo Sorrentino and Pompeo Volpe review some recent developments: the ryanodine receptor channels have now been found to be expressed in the central nervous system, and the cloning of a third ryanodine receptor gene (RYR3) has revealed that this new isoform is widely expressed in several tissues and cells. In consequence, the view of ryanodine receptors as Ca2+ channels of muscle cells is rapidly changing, and these channels seem set to take a more central position on the stage of intracellular Ca2+ signalling.

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.

Alterations in sarcoplasmic reticulum gene expression in human heart failure. A possible mechanism for alterations in systolic and diastolic properties of the failing myocardium

Recent studies have shown that intracellular Ca2+ handling is abnormal in the myocardium of patients with end-stage heart failure. Muscles from the failing hearts showed a prolonged Ca2+ transient and a diminished capacity to restore a low resting Ca2+ level during diastole. Accordingly, we examined whether this defect in Ca2+ transport function is due to alterations in sarcoplasmic reticulum gene expression. We determined the messenger RNA (mRNA) levels of sarcoplasmic reticulum Ca2+ transport proteins in failing human hearts from 17 cardiac transplant recipients with a diagnosis of dilated cardiomyopathy, primary pulmonary hypertension, or ischemic heart disease. The expression levels of each mRNA were compared with each other and then correlated with that of atrial natriuretic factor (ANF) mRNA in the failing ventricle. The mRNA levels for the calcium release channel (ryanodine receptor, RYR2), Ca2+ uptake pump (Ca(2+)-ATPase, SERCA2 isoform), and phospholamban differed significantly between heart samples but showed an inverse relation with that of ventricular ANF mRNA. In contrast, calsequestrin mRNA levels remained unchanged in these failing hearts. In addition, beta-myosin and alpha-cardiac actin mRNA levels also showed an inverse relation with ANF mRNA levels. These changes were observed in both right and left ventricles of hearts with congestive heart failure due to dilated cardiomyopathy, primary pulmonary hypertension, or ischemic heart disease. The results are consistent with the hypothesis that abnormal calcium handling in the sarcoplasmic reticulum of failing hearts is due to the altered expression of the genes encoding sarcoplasmic reticulum proteins.

Endothelin peptides: biological actions and pathophysiological significance in the lung

Endothelins (ETs) are a family of novel regulatory peptides. Besides their effects on the cardiovascular system, which have been extensively described, several lines of evidence suggest an important role for ETs in regulating pulmonary functions. ETs are present in the pulmonary tissues, bronchoalveolar space and pulmonary circulation. Release of ETs from macrophages, endothelial and epithelial cells is modulated by a variety of chemical and physical stimuli and is regulated at the level of transcription or translation. Specific endothelin receptors have been identified in the airways as well as in the pulmonary vasculature. ETs are among the most potent bronchoconstrictors yet described. In the pulmonary circulation, ETs can elicit both vasodilation and vasoconstriction and can enhance vascular permeability. ETs could also modulate activation of inflammatory cells. Enhanced ET expression and/or production have been detected in asthma, certain pulmonary tumors, shock states associated with adult respiratory distress syndrome and pulmonary hypertension. These findings suggest that by regulating pulmonary vascular and airway tone, activation of inflammatory cells and cellular growth and/or differentiation, ETs may play an important role in pulmonary pathophysiology.

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.

Application of high resolution two-dimensional polyacrylamide gel electrophoresis of polypeptides from cultured neonatal rat cardiomyocytes: regulation of protein synthesis by catecholamines

High resolution two-dimensional polyacrylamide gel electrophoresis (2-D PAGE) was applied to cultured neonatal rat heart muscle cells, incubated for 72 h at 37 degrees C in serum-free medium, either in the absence or in the presence of 0.1 microM norepinephrine. After silver staining, about 340 and 550 protein spots could be seen in cardiomyocytes, cultured either in the absence or presence of norepinephrine. Of these spots, 141 could be further characterized according to isoelectric point and molecular weight, with 71 protein spots being present under both conditions. In cells cultivated in presence of norepinephrine, 58 new protein spots appeared, whereas 12 spots disappeared, and 22 spots increased (whereas 3 spots decreased) in intensity. In comparison with 2-D PAGE of rat cardiomyocytes, the protein pattern of the intact heart of neonatal rats is incongruent. 2-D PAGE of polypeptides of cultured neonatal rat cardiomyocytes may be a suitable tool to study the regulation of protein synthesis by various stimuli with relevance to cardiac growth adaptation, inotropy and heart failure.