Caveolins and intracellular calcium regulation in human airway smooth muscle

Regulation of intracellular Ca(2+) concentration ([Ca(2+)](i)) is a key factor in airway smooth muscle (ASM) tone. In vascular smooth muscle, specialized membrane microdomains (caveolae) expressing the scaffolding protein caveolin-1 are thought to facilitate cellular signal transduction. In human ASM cells, we tested the hypothesis that caveolae mediate Ca(2+) responses to agonist stimulation. Fluorescence immunocytochemistry with confocal microscopy, as well as Western blot analysis, was used to determine that agonist receptors (M(3) muscarinic, bradykinin, and histamine) and store-operated Ca(2+) entry (SOCE)-regulatory mechanisms colocalize with caveolin-1. Although caveolin-2 coexpressed with caveolin-1, caveolin-3 was absent. In fura 2-loaded ASM cells, [Ca(2+)](i) responses to 1 microM ACh, 10 microM histamine, and 10 nM bradykinin, as well as SOCE, were attenuated (each to a different extent) after disruption of caveolae by the cholesterol-chelating drug methyl-beta-cyclodextrin. Transfection of ASM cells with 50 nM caveolin-1 small interfering RNA significantly weakened caveolin-1 expression and blunted [Ca(2+)](i) responses to bradykinin and histamine, as well as SOCE, but the response to ACh was less intense. These results indicate that caveolae are present in ASM and that caveolin-1 contributes to regulation of [Ca(2+)](i) responses to agonist.     

Sodium-calcium exchange in sarcolemmal vesicles from tracheal smooth muscle

Sarcolemmal vesicles prepared by a new procedure from bovine tracheal smooth muscle were found to have a Na-Ca exchange activity that is significantly higher than that reported for different preparations from other types of smooth muscle. The exchange process system co-purified with 5'-nucleotidase, a plasma membrane marker enzyme, and was significantly enriched (over 100-fold) compared to mitochondria (cytochrome-c oxidase) but only slightly enriched (4-fold) compared to sarcoplasmic reticulum (NADPH-cytochrome-c reductase). The Na+ dependence of Ca2+ transport was demonstrated through both uptake and efflux procedures. The uptake profile with respect to Ca2+ was monotonic with a linear vo VS. vo.S-1 plot. The resultant Km of Ca2+ from the airway sarcolemmal vesicles (20 microM) was similar in magnitude to the Km of cardiac sarcolemmal vesicles (30 microM). Tracheal vesicles demonstrated a Vmax of 0.3-0.5 nmol.mg-1.s-1 which is significantly higher than that reported in preparations from other smooth muscle types. Furthermore, two processes found to stimulate cardiac Na-Ca exchange, pretreatment with either a mixture of dithiothreitol and Fe2+ or with chymotrypsin, were ineffective on the tracheal smooth muscle. Thus, the Na-Ca exchanger identified in tracheal smooth muscle appears to be different from that observed in cardiac muscle, implying that regulation of this activity may also be different.     

Effects of salbutamol on intracellular calcium oscillations in porcine airway smooth muscle

Prakash, Y. S., H. F. M. van der Heijden, M. S. Kannan, andG. C. Sieck. Effects of salbutamol on intracellular calcium oscillations in porcine airway smooth muscle. J. Appl.Physiol. 82(6): 1836-1843, 1997.Relaxation ofairway smooth muscle (ASM) by -adrenoceptor agonists involvesreduction of intracellular Ca²⁺concentration([Ca²⁺]_i).In porcine ASM cells, acetylcholine induces[Ca²⁺]_ioscillations that display frequency modulation by agonist concentration and basal[Ca²⁺]_i.We used real-time confocal microscopy to examine the effect ofsalbutamol (1 nM to 1 µM), a₂-adrenoceptor agonist, on[Ca²⁺]_ioscillations in freshly dissociated porcine ASM cells. Salbutamol decreased the frequency of[Ca²⁺]_ioscillations in a concentration-dependent fashion, completely inhibiting the oscillations at 1 µM. These effects were mimicked by acell-permeant analog of adenosine 3,5-cyclicmonophosphate. The inhibitory effect of salbutamol was partiallyreversed by BAY K 8644. Salbutamol reduced[Ca²⁺]_ieven when sarcoplasmic reticulum (SR)Ca²⁺ reuptake andCa²⁺ influx were blocked.Lanthanum blockade of Ca²⁺ effluxattenuated the inhibitory effect of salbutamol on[Ca²⁺]_i.The[Ca²⁺]_iresponse to caffeine was unaffected by salbutamol. On the basis ofthese results, we conclude that₂-adrenoceptor agonists have little effect on SR Ca²⁺ releasein ASM cells but reduce[Ca²⁺]_iby inhibiting Ca²⁺ influx throughvoltage-gated channels and by enhancingCa²⁺ efflux.

     

Sodium-calcium exchange in mammalian heart: current-voltage relation and intracellular calcium concentration

Membrane currents and changes in intracellular calcium ion concentration ([Ca²⁺]_i) have been recorded that can be attributed to the operation of an electrogenic, voltage-dependent sodium-calcium (Na-Ca) exchanger in mammalian heart cells. Single guinea-pig ventricular myocytes under voltage clamp were perfused internally with the fluorescent Ca²⁺-indicator, fura-2, and changes in [Ca²⁺]_i and membrane current that resulted from Na-Ca exchange were isolated through the use of various organic channel blockers (verapamil, TTX), impermeant ions (Cs⁺, Ni²⁺), and inhibitors of sarcoplasmic reticulum (ryanodine). The I-V relation of Na-Ca exchange was obtained from the Ni²⁺-sensitive current elicited by ramp repolarization from +90 mV to –80 mV. Ramps were sufficiently rapid that little change in [Ca²⁺]_i occured during the ramp. The (constant) [Ca²⁺]_i during the ramp was varied over the range 100 nM to 1000 nM by varying the amplitude and duration of a pre-pulse to the ramp. The reversal potential of the Ni²⁺-sensitive ramp current varied linearly with 1n([Ca²⁺])_i. The I-V relations at different [Ca²⁺]_i over the range –60 mV to +140 mV were in reasonable accord with the predictions of a simple, simultaneous scheme of Na-Ca exchange, on the basis that only [Ca²⁺]_i had changed. The relationship between [Ca²⁺]_i and current at a constant membrane voltage was also in accord with this scheme. We suggest that Ca²⁺-fluxes through the exchanger during the cardiac action potential can be understood quantitatively by considering the binding of Ca²⁺ to the exchanger during the [Ca²⁺]_i-transient and the effects of membrane voltage on the exchanger.     

Resting calcium influx in airway smooth muscle

Plasma membrane Ca2+ leak remains the most uncertain of the cellular Ca2+ regulation pathways. During passive Ca2+ influx in non-stimulated smooth muscle cells, basal activity of constitutive Ca2+ channels seems to be involved. In vascular smooth muscle, the 3 following Ca2+ entry pathways contribute to this phenomenon: (i) via voltage-dependent Ca2+ channels, (ii) receptor gated Ca2+ channels, and (iii) store operated Ca2+ channels, although, in airway smooth muscle it seems only 2 passive Ca2+ influx pathways are implicated, one sensitive to SKF 96365 (receptor gated Ca2+ channels) and the other to Ni2+ (store operated Ca2+ channels). Resting Ca2+ entry could provide a sufficient amount of Ca2+ and contribute to resting intracellular Ca2+ concentration ([Ca2+]i), maintenance of the resting membrane potential, myogenic tone, and sarcoplasmic reticulum-Ca2+ refilling. However, further research, especially in airway smooth muscle, is required to better explore the physiological role of this passive Ca2+ influx pathway as it could be involved in airway hyperresponsiveness.     

Dysfunction of calcium handling by smooth muscle in hypertension

Dysfunction of ion handling, including binding and fluxes (passive and active transport) of physiologically important ions such as potassium, sodium, calcium, and magnesium, by vascular smooth muscle cell membranes has repeatedly been reported to be associated with the pathophysiology of hypertension. The specific purpose of this review is to summarize and evaluate the evidence for alterations of calcium ion (Ca2+) handling by vascular smooth muscle in various forms of hypertension in the animal model on the basis that regulation of cytoplasmic Ca2+ concentration is a complex and yet vitally important process for a normal function of vascular smooth muscle and that derangement of such a regulation may result in excessive retention of cytoplasmic Ca2+, contribute toward increase of total peripheral resistance, and ultimately lead to elevation of blood pressure. Emphasis is placed upon the consideration of the usefulness of the subcellular membrane fractionation technique in studies of binding and transport of Ca2+ by vascular and nonvascular smooth muscle membranes from genetic as well as experimental hypertensive rats. The limitations of the interpretation of data using such an approach are also considered. Decreased active transport of Ca2+ across isolated plasma membrane vesicles from large and small arteries occurs in several but not all forms of hypertension. This membrane abnormality also occurs in nonvascular smooth muscles and other tissues or cells not confined to the cardiovascular system in genetic hypertension, but not in experimental hypertension. A hypothesis of general membrane defects in spontaneous hypertension is proposed.(ABSTRACT TRUNCATED AT 250 WORDS)     

Brain-derived neurotrophic factor enhances calcium regulatory mechanisms in human airway smooth muscle

Neurotrophins (NTs), which play an integral role in neuronal development and function, have been found in non-neuronal tissue (including lung), but their role is still under investigation. Recent reports show that NTs such as brain-derived neurotrophic factor (BDNF) as well as NT receptors are expressed in human airway smooth muscle (ASM). However, their function is still under investigation. We hypothesized that NTs regulate ASM intracellular Ca(2+) ([Ca(2+)](i)) by altered expression of Ca(2+) regulatory proteins. Human ASM cells isolated from lung samples incidental to patient surgery were incubated for 24 h (overnight) in medium (control) or 1 nM BDNF in the presence vs. absence of inhibitors of signaling cascades (MAP kinases; PI3/Akt; NFκB). Measurement of [Ca(2+)](i) responses to acetylcholine (ACh) and histamine using the Ca(2+) indicator fluo-4 showed significantly greater responses following BDNF exposure: effects that were blunted by pathway inhibitors. Western analysis of whole cell lysates showed significantly higher expression of CD38, Orai1, STIM1, IP(3) and RyR receptors, and SERCA following BDNF exposure, effects inhibited by inhibitors of the above cascades. The functional significance of BDNF effects were verified by siRNA or pharmacological inhibition of proteins that were altered by this NT. Overall, these data demonstrate that NTs activate signaling pathways in human ASM that lead to enhanced [Ca(2+)](i) responses via increased regulatory protein expression, thus enhancing airway contractility.     

Sodium-calcium ion exchange in skeletal muscle sarcolemmal vesicles

Summary The Ca²⁺ permeability of rabbit skeletal muscle sarcolemmal vesicles was investigated by means of radioisotope flux measurements. A membrane vesicle fraction highly enriched in sarcolemma, as revealed by enzymatic markers, was obtained from the 22–27% region of sucrose gradients after isopycnic centrifugation. The ability of sarcolemmal vesicles to exchange Na⁺ for Ca²⁺ was investigated by measuring Ca²⁺ influx into and efflux from sarcolemmal vesicles in the presence and absence of a Na⁺ gradient. It was found that Ca²⁺ movements were enhanced in the direction of the higher Na⁺ concentration. When intra- and extravesicular Na⁺ concentrations were high, Na⁺–Na⁺ exchange predominated and Na⁺–Ca²⁺ exchange was low or absent. The presence of the Ca²⁺ ionophore A23187 in the dilution medium resulted in the rapid release of Ca²⁺ and the elimination of the Na⁺-enhanced efflux of Ca²⁺, suggesting that internal rather than bound external Ca²⁺ was exchanged with Na⁺. La³⁺ abolished Na⁺–Ca²⁺ exchange and decreased overall membrane permeability. Na⁺–Ca²⁺ exchange was not due to sarcoplasmic reticulum or mitochondrial contaminants. This investigation suggests that skeletal muscle, like cardiac muscle and neurons, is capable of a transmembranous Na⁺–Ca²⁺ exchange.     

LY294002, but not wortmannin,increases intracellular calcium and inhibits calcium transients in bovine and human airway smooth muscle cells

To characterize the effect that a phosphatidylinositol 3-kinase (PI3-kinase) inhibitor, LY294002, has on cytosolic calcium concentrations ([Ca2+]i), bovine airway smooth muscle cells (BASMC) and cultured human bronchial smooth muscle cells (HBSMC) were loaded with fura 2-AM, imaged as single cells and [Ca2+]i measured ratiometrically. LY294002 (50 microM) increased [Ca2+]i by 294+/-76 nM (P<0.01, n=13) and 230+/-31 nM (P<0.001, n=10) in BASMC and HBSMC, respectively, and increases occurred in the absence of extracellular calcium. In contrast, after pre-treatment with thapsigargin, LY294002 no longer increased [Ca2+]i. This calcium mobilization by LY294002 was associated with a significant functional effect since LY294002 also inhibited calcium transients to carbachol (45+/-23 nM), caffeine (45+/-32 nM), and histamine (20+/-22 nM), with controls of 969+/-190, 946+/-156, and 490+/-28 nM, respectively. Wortmannin, a different PI3-kinase inhibitor, neither increased [Ca2+]i nor inhibited transients. Also, LY294002 increased [Ca2+]i in the presence of wortmannin, U-73122, and xestospongin C. We concluded that LY294002 increased [Ca2+]i, at least in part, by mobilizing intracellular calcium stores and inhibited calcium transients. The effects of LY294002 on [Ca2+]i were not dependent on wortmannin-sensitive PI3-kinases, phospholipase C, or inositol trisphosphate receptors (IP3R). For BASMC and HBSMC, LY294002 has effects on calcium regulation that could be important to recognize when studying PI3-kinases.     

S-nitrosoglutathione-induced decrease in calcium sensitivity of airway smooth muscle

The purpose of this study was to examine whether the nitric oxide donor S-nitrosoglutathione (GSNO) relaxes canine tracheal smooth muscle (CTSM) strips by decreasing Ca(2+) sensitivity [i.e., the amount of force for a given intracellular Ca(2+) concentration ([Ca(2+)](i))]. We further investigated whether GSNO decreases Ca(2+) sensitivity by altering the relationship between regulatory myosin light chain (rMLC) phosphorylation and [Ca(2+)](i) and the relationship between force and rMLC phosphorylation. GSNO (100 microM) relaxed intact CTSM strips contracted with 45 mM KCl by decreasing Ca(2+) sensitivity in comparison to control strips without significantly decreasing [Ca(2+)](i). GSNO reduced the amount of rMLC phosphorylation for a given [Ca(2+)](i) but did not affect the relationship between isometric force and rMLC phosphorylation. These results show that in CTSM strips contracted with KCl, GSNO decreases Ca(2+) sensitivity by affecting the level of rMLC phosphorylation for a given [Ca(2+)](i), suggesting that myosin light chain kinase is inhibited or that smooth muscle protein phosphatases are activated by GSNO.     

Tyrosine kinase-dependent calcium signaling in airway smooth muscle cells

Contractile agonists may stimulate mitogenic responses in airway smooth muscle by mechanisms that involve tyrosine kinases. The role of contractile agonist-evoked activation of tyrosine kinases in contractile signaling is not clear. We addressed this issue using cultured rat airway smooth muscle cells. In these cells, serotonin (5-HT, 1 microM) caused contraction (quantitated by a decrease in cell area), which was blocked by the tyrosine kinase inhibitor genistein (40 microM). Genistein and tyrphostin 23 (40 and 10 microM, respectively) significantly decreased 5-HT-evoked peak Ca(2+) responses, and the effect of genistein could be observed in the absence of extracellular Ca(2+). The specific inhibitor of mitogen-activated protein kinase kinase PD-98059 (30 microM) had no significant effect on peak Ca(2+) levels. Western analysis of cell extracts revealed that 5-HT caused a significant increase in tyrosine phosphorylation of proteins with molecular masses of approximately 70 kDa within 10 s of stimulation but no measurable tyrosine phosphorylation of the gamma isoform of phospholipase C (PLC-gamma). Tyrosine phosphorylation was inhibited by genistein. Furthermore, genistein (40 microM) significantly attenuated 5-HT-induced inositol phosphate production. We conclude that in airway smooth muscle contractile agonists acting on G protein-coupled receptors may activate tyrosine kinase(s), which in turn modulate calcium signaling by affecting, directly or indirectly, PLC-beta activity. It is unlikely that PLC-gamma or the mitogen-activated protein kinase pathway is involved in Ca(2+) signaling to 5-HT.     

Arachidonic acid influences intracellular calcium handling in human osteoblasts

The effect of arachidonic acid (AA) on intracellular Ca(2+) concentration ([Ca(2+)]i) in human osteoblasts MG63 was studied. AA caused a concentration-dependent increase in [Ca(2+)]i, mainly due to inward Ca(2+) transport from extracellular environment. Moreover, AA in Ca(2+) -free medium produced a small, transient increase of [Ca(2+)]i, indicating that AA may also trigger Ca(2+) release from intracellular stores. Because the [Ca(2+)]i response to AA was inhibited by the cyclooxygenase (COX) inhibitor indomethacin, we tested the effect of prostaglandins (PGs), products of COX pathway. PGs E1 and E2 caused an increase in [Ca(2+)]i, which, however, was far lower than that obtained with AA. The [Ca(2+)]i response to AA was not inhibited by nifedipine, suggesting that AA did not activate a voltage-dependent Ca(2+) channel. Our results indicate that AA could modulate [Ca(2+)]i in MG63 human osteoblasts, where it may influence Ca(2+) transport across both plasma and endoplasmic membranes. Furthermore, they suggest that osteoblast activity may be modulated by AA.     

Inflammation,caveolae and CD38-mediated calcium regulation in human airway smooth muscle

The pro-inflammatory cytokine tumor necrosis factor-alpha (TNFα) increases expression of CD38 (a membrane-associated bifunctional enzyme regulating cyclic ADP ribose), and enhances agonist-induced intracellular Ca^2 + ([Ca^2 +]_i) responses in human airway smooth muscle (ASM). We previously demonstrated that caveolae and their constituent protein caveolin-1 are important for ASM [Ca^2 +]_i regulation, which is further enhanced by TNFα. Whether caveolae and CD38 are functionally linked in mediating TNFα effects is unknown. In this regard, whether the related cavin proteins (cavin-1 and -3) that maintain structure and function of caveolae play a role is also not known. In the present study, we hypothesized that TNFα effects on CD38 expression and function in human ASM involve caveolae. Caveolar fractions from isolated human ASM cells expressed CD38 and its expression was upregulated by exposure to 20 ng/ml TNFα (48 h). ASM cells expressed cavin-1 and cavin-3, which were also upregulated by TNFα. Knockdown of caveolin-1, cavin-1 or cavin-3 (using siRNA) all significantly reduced CD38 expression and ADP-ribosyl cyclase activity in the presence or absence of TNFα. Furthermore, caveolin-1, cavin-1 and cavin-3 siRNAs reduced [Ca^2 +]_i responses to histamine under control conditions, and blunted the enhanced [Ca^2 +]_i responses in TNFα-exposed cells. These data demonstrate that CD38 is expressed within caveolae and its function is linked to the caveolar regulatory proteins caveolin-1, cavin-1 and -3. The link between caveolae and CD38 is further enhanced during airway inflammation demonstrating the important role of caveolae in regulation of [Ca^2 +]_i and contractility in the airway.     

Platelet-activating factor mobilizes intracellular calcium in vascular smooth muscle cells

The effect of platelet-activating factor (PAF) on polyphosphoinositide metabolism and 45Ca2+ efflux was examined in a vascular smooth muscle cell line (A7r5). PAF stimulated a rapid but transient production of inositol trisphosphate and inositol bisphosphate which, in the presence of lithium, resulted in an accumulation of inositol monophosphate. In addition, PAF induced a rapid efflux of 45Ca2+ from preloaded cells, an effect which was concentration-dependent. These data suggest that PAF mobilizes intracellular Ca2+ via the production of inositol trisphosphate.     

Caveolin-1 in cytokine-induced enhancement of intracellular Ca(2+) in human airway smooth muscle

Diseases such as asthma are characterized by airway hyperresponsiveness. Enhanced airway smooth muscle (ASM) intracellular Ca(2+) ([Ca(2+)](i)) response to agonist stimulation leading to increased airway constriction has been suggested to contribute to airway hyperresponsiveness. Caveolae are flask-shaped plasma membrane invaginations that express the scaffolding protein caveolin and contain multiple proteins important in [Ca(2+)](i) signaling (e.g., agonist receptors, ion channels). We recently demonstrated that caveolae and caveolin-1 are important in [Ca(2+)](i) regulation in human ASM. Proinflammatory cytokines such as tumor necrosis factor (TNF)-α and interleukin (IL)-13 modulate [Ca(2+)](i) in ASM. We hypothesized that cytokine upregulation of caveolar signaling in ASM contributes to enhanced agonist-induced [Ca(2+)](i) in inflammation. Enzymatically dissociated human ASM cells were exposed to medium (control), 20 ng/ml TNF-α, or 50 ng/ml IL-13 for 24 h. Caveolae-enriched membrane fractions displayed substantial increase in caveolin-1 and -2 expressions by TNF-α and IL-13. Transfection with caveolin-1-mRed DNA substantially accelerated and increased plasma membrane caveolin-1 expression by TNF-α and to a lesser extent by IL-13. Caveolin-1 enhancement was inhibited by nuclear factor-κB and mitogen-activated protein kinase inhibitors. In fura 2-loaded ASM cells, [Ca(2+)](i) responses to 1 μM ACh, 10 μM histamine, or 10 nM bradykinin were all exaggerated by TNF-α as well as IL-13 exposure. However, disruption of caveolae using caveolin-1 suppression via small-interfering RNA resulted in significant blunting of agonist-induced [Ca(2+)](i) responses of vehicle and TNF-α-exposed cells. These functional data were correlated to the presence of TNFR(1) receptor (but not the IL-4/IL-13 receptor) within caveolae. Overall, these results indicate that caveolin-1 plays an important role in airway inflammation by modulating the effect of specific cytokines on [Ca(2+)](i).     

Regulation of intracellular calcium in human esophageal smooth muscles

We have investigated sources ofCa²⁺ contributing to excitation ofhuman esophageal smooth muscle, using fura 2 to study cytosolic freeCa²⁺ concentration([Ca²⁺]_i)in dispersed cells and contraction of intact muscles. Acetylcholine (ACh) caused an initial peak rise of[Ca²⁺]_ifollowed by a plateau accompanied by reversible contraction. Removal ofextracellular Ca²⁺ or addition ofdihydropyridine Ca²⁺ channelblockers reduced the plateau phase but did not prevent contraction.Caffeine also caused elevation of[Ca²⁺]_iand blocked responses to ACh. Undershoots of[Ca²⁺]_iwere apparent after ACh or caffeine. Blockade of the sarcoplasmic reticular Ca²⁺-ATPase bycyclopiazonic acid (CPA) reduced the ACh-evoked increase of[Ca²⁺]_iand abolished the undershoot, indicating involvement ofCa²⁺ stores. When contraction wasstudied in intact muscles, removal ofCa²⁺ or addition of nifedipinereduced, but did not abolish, carbachol (CCh)-induced contraction.Elevation of extracellular K⁺caused contraction that was inhibited by nifedipine, although CCh stillelicited contraction. CPA caused contraction and suppressed theCCh-induced contraction, whereas ryanodine reduced CCh-induced contraction. Our studies provide evidence that muscarinic excitation ofhuman esophagus involves both release ofCa²⁺ from intracellular stores andinflux of Ca²⁺.