A role for the sarcoplasmic reticulum in Ca2+ extrusion from rabbit inferior vena cava smooth muscle

On a Magical Mystery Tour with 8-Bromo-Cyclic ADP-Ribose: From All-or-None Block to Nanojunctions and the Cell-Wide Web

A plethora of cellular functions are controlled by calcium signals, that are greatly coordinated by calcium release from intracellular stores, the principal component of which is the sarco/endooplasmic reticulum (S/ER). In 1997 it was generally accepted that activation of various G protein-coupled receptors facilitated inositol-1,4,5-trisphosphate (IP3) production, activation of IP3 receptors and thus calcium release from S/ER. Adding to this, it was evident that S/ER resident ryanodine receptors (RyRs) could support two opposing cellular functions by delivering either highly localised calcium signals, such as calcium sparks, or by carrying propagating, global calcium waves. Coincidentally, it was reported that RyRs in mammalian cardiac myocytes might be regulated by a novel calcium mobilising messenger, cyclic adenosine diphosphate-ribose (cADPR), that had recently been discovered by HC Lee in sea urchin eggs. A reputedly selective and competitive cADPR antagonist, 8-bromo-cADPR, had been developed and was made available to us. We used 8-bromo-cADPR to further explore our observation that S/ER calcium release via RyRs could mediate two opposing functions, namely pulmonary artery dilation and constriction, in a manner seemingly independent of IP3Rs or calcium influx pathways. Importantly, the work of others had shown that, unlike skeletal and cardiac muscles, smooth muscles might express all three RyR subtypes. If this were the case in our experimental system and cADPR played a role, then 8-bromo-cADPR would surely block one of the opposing RyR-dependent functions identified, or the other, but certainly not both. The latter seemingly implausible scenario was confirmed. How could this be, do cells hold multiple, segregated SR stores that incorporate different RyR subtypes in receipt of spatially segregated signals carried by cADPR? The pharmacological profile of 8-bromo-cADPR action supported not only this, but also indicated that intracellular calcium signals were delivered across intracellular junctions formed by the S/ER. Not just one, at least two. This article retraces the steps along this journey, from the curious pharmacological profile of 8-bromo-cADPR to the discovery of the cell-wide web, a diverse network of cytoplasmic nanocourses demarcated by S/ER nanojunctions, which direct site-specific calcium flux and may thus coordinate the full panoply of cellular processes.

Na+/Ca2+ Exchanger 1 in Airway Smooth Muscle of Allergic Inflammation Mouse Model

Cytosolic free Ca²⁺ ([Ca²⁺]_cyt) is essential for airway contraction, secretion and remodeling. [Ca²⁺]_cyt homeostasis is controlled by several critical molecules, one of which is the Na⁺/Ca²⁺ exchanger 1 (NCX1) in the plasma membrane. Since little is currently known about NCX1 in the airway smooth muscle and its involvement in airway diseases, the present study was designed to investigate the expression and function of NCX1 in normal airway smooth muscle and its relevance to airway inflammation. Western blot analysis, tracheal smooth muscle contraction, and [Ca²⁺]_cyt measurements were performed in mouse tracheal smooth muscle tissues and primary airway smooth muscle cell cultures. Additional studies were performed in a mouse model of allergic airway inflammation. Our data showed that NCX1 proteins were expressed in the human bronchial smooth muscle cells (HBSMCs), murine airway and whole lung. Carbachol raised [Ca²⁺]_cyt in mouse tracheal smooth muscle cells and induced murine tracheal contraction, all of which were significantly attenuated by KB-R7943, a selective NCX inhibitor. Removal of extracellular Na⁺ increased [Ca²⁺]_cyt in HBSMCs and mouse tracheal SMCs, which was dependent on extracellular Ca²⁺ and sensitive to KB-R7943. TNF-α treatment of HBSMCs significantly upregulated mRNA and protein expression of NCX1 and enhanced NCX activity. Finally, KB-R7943 abolished the airway hyperresponsiveness to methacholine in an ovalbumin-induced mouse model of allergic airway inflammation. Together, these findings indicate that NCX1 in airway smooth muscle may play an important role in the development of airway hyperresponsiveness, and downregulation or inhibition of NCX1 may serve as a potential therapeutic approach for asthma.

Identification of functionally segregated sarcoplasmic reticulum calcium stores in pulmonary arterial smooth muscle

In pulmonary arterial smooth muscle, Ca(2+) release from the sarcoplasmic reticulum (SR) via ryanodine receptors (RyRs) may induce constriction and dilation in a manner that is not mutually exclusive. We show here that the targeting of different sarcoplasmic/endoplasmic reticulum Ca(2+)-ATPases (SERCA) and RyR subtypes to discrete SR regions explains this paradox. Western blots identified protein bands for SERCA2a and SERCA2b, whereas immunofluorescence labeling of isolated pulmonary arterial smooth muscle cells revealed striking differences in the spatial distribution of SERCA2a and SERCA2b and RyR1, RyR2, and RyR3, respectively. Almost all SERCA2a and RyR3 labeling was restricted to a region within 1.5 microm of the nucleus. In marked contrast, SERCA2b labeling was primarily found within 1.5 microm of the plasma membrane, where labeling for RyR1 was maximal. The majority of labeling for RyR2 lay in between these two regions of the cell. Application of the vasoconstrictor endothelin-1 induced global Ca(2+) waves in pulmonary arterial smooth muscle cells, which were markedly attenuated upon depletion of SR Ca(2+) stores by preincubation of cells with the SERCA inhibitor thapsigargin but remained unaffected after preincubation of cells with a second SERCA antagonist, cyclopiazonic acid. We conclude that functionally segregated SR Ca(2+) stores exist within pulmonary arterial smooth muscle cells. One sits proximal to the plasma membrane, receives Ca(2+) via SERCA2b, and likely releases Ca(2+) via RyR1 to mediate vasodilation. The other is located centrally, receives Ca(2+) via SERCA2a, and likely releases Ca(2+) via RyR3 and RyR2 to initiate vasoconstriction.

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.

Ca2+ clearance and contractility in vascular smooth muscle: evidence from gene-altered murine models

The central importance of calcium clearance proteins, and their regulators, in the modulation of myocardial contractility and intracellular Ca(2+) concentration ([Ca(2+)](i)) has long been established. Key players identified include the Na(+)-Ca(2+) exchanger, the Na(+)-K(+) ATPase, the sarco(endo)plasmic reticulum Ca(2+)-ATPase and associated phospholamban. Gene-targeted and transgenic murine models have been critical in the elucidation of their function. The study of these proteins in the regulation of contractile parameters in vascular smooth muscle, on the other hand, is less well studied. More recently, gene-targeted and transgenic models have expanded our knowledge of Ca(2+) clearance proteins and their role in both tonic and phasic smooth muscle contractility. In this review, we will briefly treat the mechanisms which underlie Ca(2+) clearance in smooth muscle. These will be addressed in light of studies using gene-modified mouse models, the results of which will be compared and contrasted with those in the cardiomyocyte. The recently identified human mutations in phospholamban, which lead to dilated cardiomyopathy, are also present in vascular and other smooth muscle. Given the importance of these Ca(2+) clearance systems to modulation of smooth muscle, it is likely that mutations will also lead to smooth muscle pathology.

Caveolae and calcium handling, a review and a hypothesis

Caveolae are associated with molecules crucial for calcium handling. This review considers the roles of caveolae in calcium handling for smooth muscle and interstitial cells of Cajal (ICC). Structural studies showed that the plasma membrane calcium pump (PMCA), a sodium-calcium exchanger (NCX1), and a myogenic nNOS appear to be colocalized with caveolin I, the main constituent of these caveolae. Voltage dependent calcium channels (VDCC) are associated but not co-localized with caveolin 1, as are proteins of the peripheral sarcoplasmic reticulum (SR) such as calreticulin. Only the nNOS is absent from caveolin 1 knockout animals. Functional studies in calcium free media sugest that a source of calcium in tonic smooth muscles exists, partly sequestered from extracellular EGTA. This source supported sustained contractions to carbachol using VDCC and dependent on activity of the SERCA pump. This source is postulated to be caveolae, near peripheral SR. New evidence, presented here, suggests that a similar source exists in phasic smooth muscle of the intestine and its ICC. These results suggest that caveolae and peripheral SR are a functional unit recycling calcium through VDCC and controlling its local concentration. Calcium handling molecules associated with caveolae in smooth muscle and ICC were identified and their possible functions also reviewed.

Novel role for K+-dependent Na+/Ca2+ exchangers in regulation of cytoplasmic free Ca2+ and contractility in arterial smooth muscle

Cytoplasmic free Ca2+ ([Ca2+]cyt) is essential for the contraction and relaxation of blood vessels. The role of plasma membrane Na+/Ca2+ exchange (NCX) activity in the regulation of vascular Ca2+ homeostasis was previously ascribed to the NCX1 protein. However, recent studies suggest that a relatively newly discovered K+-dependent Na+/Ca2+ exchanger, NCKX (gene family SLC24), is also present in vascular smooth muscle. The purpose of the present study was to identify the expression and function of NCKX in arteries. mRNA encoding NCKX3 and NCKX4 was demonstrated by RT-PCR and Northern blot in both rat mesenteric and aortic smooth muscle. NCXK3 and NCKX4 proteins were also demonstrated by immunoblot and immunofluorescence. After voltage-gated Ca2+ channels, store-operated Ca2+ channels, and Na+ pump were pharmacologically blocked, when the extracellular Na+ was replaced with Li+ (0 Na+) to induce reverse mode (Ca2+ entry) activity of Na+/Ca2+ exchangers, a large increase in [Ca2+]cyt signal was observed in primary cultured aortic smooth muscle cells. About one-half of this [Ca2+]cyt signal depended on the extracellular K+. In addition, after the activity of NCX was inhibited by KB-R7943, Na+ replacement-induced Ca2+ entry was absolutely dependent on extracellular K+. In arterial rings denuded of endothelium, a significant fraction of the phenylephrine-induced and nifedipine-resistant aortic or mesenteric contraction could be prevented by removal of extracellular K+. Taken together, these data provide strong evidence for the expression of NCKX proteins in the vascular smooth muscle and their novel role in mediating agonist-stimulated [Ca2+]cyt and thereby vascular tone.

Blood pressure fall and increased relaxation of aortic smooth muscle in diabetic rats

This study was designed to identify changes in endothelium-independent relaxation that could contribute to the depressed vascular reactivity and fall in blood pressure (BP) detected in rats after 5 weeks of streptozotocin (STZ)-induced (i.e. type 1) diabetes. Aortic rings were contracted by simultaneous activation of voltage-operated channels (KCl=80 mM) and alpha-adrenergic receptors (phenylephrine 1 microM) and then relaxed by simultaneous exposure to Ca2+-free PSS and 10 microM phentolamine. Additional relaxations were performed under conditions in which the plasma membrane Na-Ca exchanger (PMNaCa) or Ca-pump (PMCA), or the sarcoplasmic reticulum (SR) Ca-pump (SERCA) were blocked, to identify which mechanism(s) could modulate this process. The STZ-diabetic rats had a moderate but significant decrease of BP, and their aortic rings exhibited accelerated relaxation following a biexponential model, with a significantly decreased slow component. In control rats only the inhibition of the PMNaCa could slow down the fast component, while the slow component was insensitive to any blocking maneuver. In contrast, the diabetic animals' slow component was sensitive to the inhibition of both the PMNaCa and the SERCA. The SERCA-sensitive 45Ca2+ uptake by the SR was augmented in the aortas of STZ-treated animals. This hyperactivity of the SERCA, associated with augmented activity of the PMNaCa, at least partly induced by an increase of the plasma membrane Na+/K+-ATPase activity, could explain the decrease in BP and the accelerated aortic relaxation observed in the diabetic rats.

Differential regulation of calcium homeostasis in adenocarcinoma cell line A549 and its Taxol-resistant subclone

Drug resistance is a fundamental problem in cancer chemotherapy. Intracellular calcium concentration ([Ca2+](i)) may play a role in the development of chemoresistance. We investigated the regulatory role of [Ca2+](i) in Taxol resistance in the non-small-cell lung cancer cell line A549 and its chemoresistant subclone A549-T24. Measurement of cytosolic calcium ([Ca2+](c)) in single cells and cell populations revealed similar levels of basal calcium in the two cell lines. However, a reduced response to thapsigargin (a sarcoplasmic/endoplasmic reticulum Ca2+-ATPase (SERCA) inhibitor) in A549-T24 cells compared to the parent cell line suggested a lower ER Ca2+ content in these cells. mRNA expression of SERCA2b and SERCA3, major Ca2+ pumps involved in ER Ca2+ homeostasis, did not significantly differ between the two cell lines, as revealed by RT-PCR. An altered calcium influx pathway in the Taxol-resistant cell line was observed. Modulation of the ER calcium pools using CMC (4-chloro-m-cresol) and ATP revealed lower ryanodine receptor (RyR) and IP(3) receptor (IP(3)R)-sensitive Ca2+ stores in the chemoresistant cell line. Western blot and RT-PCR studies suggested that A549-T24 cells expressed higher levels of the antiapoptotic protein Bcl-2 and the calcium-binding protein sorcin, respectively, in comparison to the parent cell line. Both of these proteins have been previously implicated in chemoresistance, in part, due to their ability to modulate[Ca2+](i). These results suggest that altered intracellular calcium homeostasis may contribute to the Taxol-resistant phenotype.

Ca(2+) oscillations, gradients, and homeostasis in vascular smooth muscle

Vascular smooth muscle shows both plasticity and heterogeneity with respect to Ca(2+) signaling. Physiological perturbations in cytoplasmic Ca(2+) concentration ([Ca(2+)](i)) may take the form of a uniform maintained rise, a transient uniform [Ca(2+)](i) elevation, a transient localized rise in [Ca(2+)](i) (also known as spark and puff), a transient propagated wave of localized [Ca(2+)](i) elevation (Ca(2+) wave), recurring asynchronous Ca(2+) waves, or recurring synchronized Ca(2+) waves dependent on the type of blood vessel and the nature of stimulation. In this overview, evidence is presented which demonstrates that interactions of ion transporters located in the membranes of the cell, sarcoplasmic reticulum, and mitochondria form the basis of this plasticity of Ca(2+) signaling. We focus in particular on how the junctional complexes of plasmalemma and superficial sarcoplasmic reticulum, through the generation of local cytoplasmic Ca(2+) gradients, maintain [Ca(2+)](i) oscillations, couple these to either contraction or relaxation, and promote Ca(2+) cycling during homeostasis.

Ca(2+) regulation in guinea-pig colonic smooth muscle: the role of the Na(+)-Ca(2+) exchanger and the sarcoplasmic reticulum

To study the contribution of the Na(+)-Ca(2+) exchanger to Ca(2+) regulation and its interaction with the sarcoplasmic reticulum (SR), changes in cytoplasmic Ca(2+) concentration ([Ca(2+)](c)) were measured in single, voltage clamped, smooth muscle cells. Increases in [Ca(2+)](c) were evoked by either depolarisation (-70 mV to 0 mV) or by release from the SR by caffeine (10 mM) or flash photolysis of caged InsP(3) (InsP(3)). Depletion of the SR of Ca(2+) (verified by the absence of a response to caffeine and InsP(3)) by either ryanodine (50 microM), to open the ryanodine receptors (RyRs), or thapsigargin (500 nM) or cyclopiazonic acid (CPA, 10 microM), to inhibit the SR Ca(2+) pumps, reduced neither the magnitude of the Ca(2+) transient nor the relationship between the influx of and the rise in [Ca(2+)](c) evoked by depolarisation. This suggested that Ca(2+)-induced Ca(2+) release (CICR) from the SR did not contribute significantly to the depolarisation-evoked rise in [Ca(2+)](c). However, although Ca(2+) was not released from it, the SR accumulated the ion following depolarisation since ryanodine and thapsigargin each slowed the rate of decline of the depolarisation-evoked Ca(2+) transient. Indeed, the SR Ca(2+) content increased following depolarisation as assessed by the increased magnitude of the [Ca(2+)](c) levels evoked each by InsP(3) and caffeine, relative to controls. The increased SR Ca(2+) content following depolarisation returned to control values in approximately 12 min via Na(+)-Ca(2+) exchanger activity. Thus inhibition of the Na(+)-Ca(2+) exchanger by removal of external Na(+) (by either lithium or choline substitution) prevented the increased SR Ca(2+) content from returning to control levels. On the other hand, the Na(+)-Ca(2+) exchanger did not appear to regulate bulk average Ca(2+) directly since the rates of decline in [Ca(2+)](c), following either depolarisation or the release of Ca(2+) from the SR (by either InsP(3) or caffeine), were neither voltage nor Na(+) dependent. Thus, no evidence for short term (seconds) control of [Ca(2+)](c) by the Na(+)-Ca(2+) exchanger was found. Together, the results suggest that despite the lack of CICR, the SR removes Ca(2+) from the cytosol after its elevation by depolarisation. This Ca(2+) is then removed from the SR to outside the cell by the Na(+)-Ca(2+) exchanger. However, the exchanger does not contribute significantly to the decline in bulk average [Ca(2+)](c) following transient elevations in the ion produced either by depolarisation or by release from the store.

Effect of atorvastatin on intracellular calcium uptake in coronary smooth muscle cells from diabetic pigs fed an atherogenic diet

Intracellular Ca(2+) store loading has been shown to alter proliferation and apoptosis of several cell types. In addition, HMG-CoA reductase inhibitors (i.e. atorvastatin) are effective in treating diabetic dyslipidemic patients. Thus, we hypothesized that chronic atorvastatin treatment would prevent increased Ca(2+) uptake into intracellular Ca(2+) stores in vascular smooth muscle cells from diabetic dyslipidemic pigs. Male Yucatan pigs were divided into four groups for 20 weeks-- (1) low fat fed (control); (2) hyperlipidemic (F); (3) alloxan-induced diabetic dyslipidemic (DF); and (4) diabetic dyslipidemic pigs treated with atorvastatin (DFA). The F, DF, and DFA groups were fed a high fat/cholesterol diet. Cells were isolated from the coronary artery and the myoplasmic Ca(2+) (Ca(m)) response measured using single cell fura-2 imaging. The Ca(m) response to caffeine (5 mM to release Ca(2+) from the sarcoplasmic reticulum, SR) and ionomycin (10 microM; to release the total Ca(2+) store) was determined in either the presence of low Na (19Na; inhibits Na(+)-Ca(2+) exchange), thapsigargin (TSG; inhibits the SR Ca(2+) pump), and a 19Na+TSG solution. Low Na induced the uptake of Ca(2+) into both SR and non-SR Ca(2+) stores in the DF group, but not the DFA group. Furthermore, after depletion of the SR Ca(2+) store with TSG, 19Na evoked Ca(2+) uptake into non-SR Ca(2+) stores in all three groups except in the DFA group. In summary, this study demonstrates that atorvastatin prevents the enhanced uptake of Ca(2+) by SR and non-SR Ca(2+) stores in diabetic dyslipidemic pigs.

The mechanism of phenylephrine-mediated [Ca(2+)](i) oscillations underlying tonic contraction in the rabbit inferior vena cava

1. We characterized the mechanisms in vascular smooth muscle cells (VSMCs) that produce asynchronous, wave-like Ca(2+) oscillations in response to phenylephrine (PE). Confocal imaging was used to observe [Ca(2+)](i) in individual VSMCs of intact inferior vena cava (IVC) from rabbits. 2. It was found that the Ca(2+) waves were initiated by Ca(2+) release from the sarcoplasmic reticulum (SR) via inositol 1,4,5-trisphosphate-sensitive SR Ca(2+) release channels (IP(3)R channels) and that refilling of the SR Ca(2+) store through the sarcoplasmic-endoplasmic reticulum Ca(2+)-ATPase (SERCA) was required for maintained generation of the repetitive Ca(2+) waves. 3. Blockade of L-type voltage-gated Ca(2+) channels (L-type VGCCs) with nifedipine reduced the frequency of PE-stimulated [Ca(2+)](i) oscillations, while additional blockade of receptor-operated channels/store-operated channels (ROCs/SOCs) with SKF96365 abolished the remaining oscillations. Parallel force measurements showed that nifedipine inhibited PE-induced tonic contraction by 27 % while SKF96365 abolished it. This indicates that stimulated Ca(2+) entry refills the SR to support the recurrent waves of SR Ca(2+) release and that both L-type VGCCs and ROCs/SOCs contribute to this process. 4. Application of the Na(+)-Ca(2+) exchanger (NCX) inhibitors 2',4'-dichlorobenzamil (forward- and reverse-mode inhibitor) and KB-R7943 (reverse-mode inhibitor) completely abolished the nifedipine-resistant component of [Ca(2+)](i) oscillations and markedly reduced PE-induced tone. 5. Thus, we conclude that each Ca(2+) wave depends on initial SR Ca(2+) release via IP(3)R channels followed by SR Ca(2+) refilling through SERCA. Na(+) entry through ROCs/SOCs facilitates Ca(2+) entry through the NCX operating in the reverse mode, which refills the SR and maintains PE-induced [Ca(2+)](i) oscillations. In addition some Ca(2+) entry through L-type VGCCs and ROCs/SOCs serves to modulate the frequency of the oscillations and the magnitude of force development.

Rat arterial smooth muscle devoid of ryanodine receptor function: effects on cellular Ca(2+) handling

The roles of intracellular Ca(2+) stores and ryanodine (Ry) receptors for vascular Ca(2+) homeostasis and viability were investigated in rat tail arterial segments kept in organ culture with Ry (10 - 100 microM) for up to 4 days. Acute exposure to Ry or the non-deactivating ryanodine analogue C(10)-O(eq) glycyl ryanodine (10 microM) eliminated Ca(2+) release responses to caffeine (20 mM) and noradrenaline (NA, 10 microM), whereas responses to NA, but not caffeine, gradually returned to normal within 4 days of exposure to RY: Ry receptor protein was detected on Western blots in arteries cultured either with or without RY: Brief Ca(2+) release events (sparks) were absent after culture with Ry, whereas Ca(2+) waves still occurred. The propagation velocity of waves was equal ( approximately 19 microm s(-1)) in tissue cultured either with or without RY: Inhibition of Ca(2+) accumulation into the sarcoplasmic reticulum (SR) by culture with caffeine (5 mM), cyclopiazonic acid or thapsigargin (both 10 microM) decreased contractility due to Ca(2+)-induced cell damage. In contrast, culture with Ry did not affect contractility. Removal of Ca(2+) from the cytosol following a Ca(2+) load was retarded after Ry culture. Thapsigargin reduced the rate of Ca(2+) removal in control cultured rings, but had no effect after Ry culture. It is concluded that intracellular Ca(2+) stores recover during chronic Ry treatment, while Ry receptors remain non-functional. Ry receptor activity is required for Ca(2+) sparks and for SR-dependent recovery from a Ca(2+) load, but not for Ca(2+) waves or basal Ca(2+) homeostasis.

Enhanced L-type Ca2+ channel current density in coronary smooth muscle of exercise-trained pigs is compensated to limit myoplasmic free Ca2+ accumulation

We hypothesized that enhanced voltage-gated Ca2+ channel current (VGCC) density in coronary smooth muscle cells of exercise-trained miniature Yucatan pigs is compensated by other cellular Ca2+ regulatory mechanisms to limit net myoplasmic free Ca2+ accumulation. Whole-cell voltage clamp experiments demonstrated enhanced VGCC density in smooth muscle cells freshly dispersed from coronary arteries of exercise-trained vs. sedentary animals. In separate experiments using fura-2 microfluorometry, we measured depolarization-induced (80 mM KCl) accumulation of myoplasmic free Ba2+ and free Ca2+. Both maximal rate and net accumulation of free Ba2+ in response to membrane depolarization were increased in smooth muscle cells isolated from exercise-trained pigs, consistent with an increased VGCC density. Depolarization also produced an enhanced maximal rate of free Ca2+ accumulation in cells of exercise-trained pigs; however, net accumulation of free Ca2+ was not significantly increased suggesting enhanced Ca2+ influx was compensated to limit net free Ca2+ accumulation. Inhibition of sarco-endoplasmic reticulum Ca2+-transporting ATPase (SERCA; 10 microM cyclopiazonic acid) and/or sarcolemmal Na+-Ca2+ exchange (low extracellular Na+) suggested neither mechanism compensated the enhanced VGCC in cells of exercise-trained animals. Local Ca2+-dependent inactivation of VGCC, assessed by buffering myoplasmic Ca2+ with EGTA in the pipette and using Ca2+ and Ba2+ as charge carriers, was not different between cells of sedentary and exercise-trained animals. Our findings indicate that increased VGCC density is compensated by other cellular Ca2+ regulatory mechanisms to limit net myoplasmic free Ca2+ accumulation in smooth muscle cells of exercise-trained animals. Further, SERCA, Na+-Ca2+ exchange and local Ca2+-dependent inactivation of VGCC do not appear to function as compensatory mechanisms. Additional potential compensatory mechanisms include Ca2+ extrusion via plasma membrane Ca2+-ATPase, mitochondrial uptake, myoplasmic Ca2+-binding proteins and other sources of VGCC inactivation.

Hypoxia inhibits the Na(+)/Ca(2+) exchanger in pulmonary artery smooth muscle cells

The cellular mechanisms underlying hypoxic pulmonary vasoconstriction are not fully understood. We examined the effect of hypoxia on Ca(2+) efflux from the cytosol in single Fura-2-loaded pulmonary artery myocytes. During mild hypoxia (pO(2)=50-60 Torr), peak [Ca(2+)](i) was increased and the rate of Ca(2+) removal from the cytosol was markedly slowed after stimuli that elevated [Ca(2+)](i). Removal of extracellular Na(+) potentiated the peak [Ca(2+)](i) rise and slowed the Ca(2+) decay rate in cells recorded under normoxic conditions; it did not further slow the Ca(2+) decay rate or potentiate the [Ca(2+)](i) increase in hypoxic cells. An Na(+)/Ca(2+) exchange current was recorded in isolated pulmonary artery myocytes. Switching from Li(+) to Na(+) (130 mM) revealed an inward current with reversal potential consistent with the Na(+)/Ca(2+) exchange current in cells in which [Ca(2+)](i) was clamped at 1 microM similar currents, although smaller, were observed with normal resting [Ca(2+)](i) using the perforated patch clamp technique. The Na(+)/Ca(2+) exchange current was markedly inhibited in myocytes exposed to mild hypoxia. RT-PCR revealed the expression of specific alternatively spliced RNAs of NCX1 in rat pulmonary arteries. These findings provide an enhanced understanding of the molecular mechanisms underlying hypoxic sensing in pulmonary arteries.

Endoplasmic reticulum contribution to the relaxant effect of cGMP- and cAMP-elevating agents in feline aorta

The contribution of endoplasmic reticulum (ER) and phosphorylation of phospholamban (PLB) to the relaxant effect of cGMP- and cAMP-elevating agents was studied in feline aorta. Sodium nitroprusside (NP, 100 microM) completely relaxed contracture induced by 10 microM norepinephrine. This NP-induced relaxation was partially prevented by tetraethylammonium, suggesting that a fraction of NP-induced relaxation was mediated by activation of K(+) channels. In the absence and presence of tetraethylammonium, the relaxant effect of NP was associated with a significant increase in Ser(16) phosphorylation of PLB immunodetected by phosphorylation site-specific antibodies. The relaxant effect of NP on aortic strips precontracted with 80 mM KCl was significantly reduced by 1 microM thapsigargin. This decrease, which represents the ER contribution to the relaxant effect of NP, reached 23 +/- 9% at 100 microM NP and was closely associated with a dose-dependent increase in Ser(16) phosphorylation (128 +/- 49% over control at 100 microM NP). Effects of NP were associated with a significant increase in activity of protein kinase G and were mimicked by 8-bromo-cGMP. Forskolin produced a dose-dependent relaxant effect on KCl-induced contracture, which reached 64 +/- 8% at 50 microM and was associated with an increase in phosphorylation of Ser(16) residue of PLB (88 +/- 18% over control). Thapsigargin reduced this relaxant effect by 38 +/- 9%. 8-Bromo-cAMP mimicked effects of forskolin. The ER-mediated relaxant effect and the increase in Ser(16) phosphorylation produced by forskolin were partially blocked by the protein kinase A inhibitor H-89 (5 microM). The results indicate that ER partially contributes to the relaxant effect of NP and forskolin in feline aorta. This effect may be mediated by the associated increase in Ser(16) phosphorylation of PLB.

The role of the sarcoplasmic reticulum as a Ca2+ sink in rat uterine smooth muscle cells

1. The mechanisms responsible for removing calcium ions from the cytoplasm were investigated in single rat uterine myocytes using indo-1. 2. Trains of depolarizing voltage-clamp pulses increased [Ca2+]i. The rate of decay of [Ca2+]i was slowed by inhibition of the sarcoplasmic reticulum (SR) Ca2+-ATPase with cyclopiazonic acid (CPA). However, if the sarcolemmal Na+-Ca2+ exchanger and Ca2+-ATPase were inhibited then recovery of [Ca2+]i was abolished showing that the SR Ca2+-ATPase alone cannot produce decay of [Ca2+]i. 3. In another series of experiments, Ca2+ release from the SR was induced with carbachol in a Ca2+-free solution. Under these conditions responses to repeated applications of carbachol could be obtained. In the presence of CPA, however, only the first application was effective. This suggests that the SR Ca2+-ATPase sequesters a significant amount of Ca2+ into the SR. 4. CPA slowed the rate of decay of [Ca2+]i following carbachol addition by > 50 %. Again, however, after a brief transient fall, decay was abolished when the Na+-Ca2+ exchanger and sarcolemmal Ca2+-ATPase were inhibited. 5. These data show that, although the SR Ca2+-ATPase contributes to the decay of [Ca2+]i, it cannot function effectively in the absence of Ca2+ removal from the cell. These data are discussed in the context of the superficial buffer barrier model in which Ca2+ is taken up into the SR and then released very close to sarcolemmal Ca2+ extrusion sites, i.e. the SR acting in series with the surface membrane extrusion mechanisms. We also suggest that the amount of filling of the SR influences the rate of Ca2+ removal.