Superficial buffer barrier and preferentially directed release of Ca2+ in canine airway smooth muscle

Role of voltage-gated Ca2+ channels and Ano1 Ca2+-activated Cl- channels in M2 muscarinic receptor-dependent contractions of murine airway smooth muscle

Cholinergic tone is elevated in obstructive lung conditions such as chronic obstructive pulmonary disease (COPD) and asthma, but the cellular mechanisms underlying cholinergic contractions of airway smooth muscle (ASM) are still unclear. Some studies report an important role for L-type Ca2+ channels (LTCC) and Ano1 Ca2+-activated Cl- channels (CACC) in these responses, but others dispute their importance. Cholinergic contractions of ASM involve activation of M3Rs, however, stimulation of M2Rs exerts a profound hypersensitization of these responses. Here, we show that M2R-dependent potentiation of cholinergic nerve-evoked contractions of ASM was reversed by the LTCC blocker nifedipine and the Ano1 CACC inhibitors Ani9 and CaCCinh-A01. Carbachol induced sustained contractions of ASM that were converted into oscillatory contractions when M3Rs were blocked with 4-DAMP. The 4-DAMP-resistant contractions were absent in preparations taken from M2R knockout (KO) mice. The remaining M2R-dependent responses, observed in wild-type (WT) mice, were abolished by nifedipine and Ani9. Inhibition of sarcoplasmic endoplasmic reticulum Ca2+ ATPases (SERCA) with thapsigargin increased the amplitude of contractions induced by electrical field stimulation (EFS) and these effects were also reversed by nifedipine and Ani9. Thapsigargin also potentiated contractions of ASM induced by the LTCC activator FPL64176. Therefore, contractions of ASM that involved Ca2+ influx via LTCC were enhanced by inhibition of SERCA. Immunocytochemistry experiments revealed prominent SERCA staining around the periphery of ASM cells. These data indicate that M2R-dependent contractions of ASM involve Ano1 CACC and LTCC by a mechanism involving inhibition of buffering of Ca2+ influx by SERCA.NEW & NOTEWORTHY The role of L-type Ca2+ channels and Ano1 Ca2+-activated Cl- channels in cholinergic contractions of airway smooth muscle is disputed. Here, we show that both channels are involved in M2 muscarinic receptor-dependent contractions of murine airway smooth muscle via inhibition of buffering of Ca2+ influx by sarcoplasmic endoplasmic reticulum Ca2+ ATPases.

ANO1, CaV1.2, and IP3R form a localized unit of EC-coupling in mouse pulmonary arterial smooth muscle

Pulmonary arterial (PA) smooth muscle cells (PASMC) generate vascular tone in response to agonists coupled to Gq-protein receptor signaling. Such agonists stimulate oscillating calcium waves, the frequency of which drives the strength of contraction. These Ca2+ events are modulated by a variety of ion channels including voltage-gated calcium channels (CaV1.2), the Tmem16a or Anoctamin-1 (ANO1)-encoded calcium-activated chloride (CaCC) channel, and Ca2+ release from the sarcoplasmic reticulum through inositol-trisphosphate receptors (IP3R). Although these calcium events have been characterized, it is unclear how these calcium oscillations underly a sustained contraction in these muscle cells. We used smooth muscle-specific ablation of ANO1 and pharmacological tools to establish the role of ANO1, CaV1.2, and IP3R in the contractile and intracellular Ca2+ signaling properties of mouse PA smooth muscle expressing the Ca2+ biosensor GCaMP3 or GCaMP6. Pharmacological block or genetic ablation of ANO1 or inhibition of CaV1.2 or IP3R, or Ca2+ store depletion equally inhibited 5-HT-induced tone and intracellular Ca2+ waves. Coimmunoprecipitation experiments showed that an anti-ANO1 antibody was able to pull down both CaV1.2 and IP3R. Confocal and superresolution nanomicroscopy showed that ANO1 coassembles with both CaV1.2 and IP3R at or near the plasma membrane of PASMC from wild-type mice. We conclude that the stable 5-HT-induced PA contraction results from the integration of stochastic and localized Ca2+ events supported by a microenvironment comprising ANO1, CaV1.2, and IP3R. In this model, ANO1 and CaV1.2 would indirectly support cyclical Ca2+ release events from IP3R and propagation of intracellular Ca2+ waves.

Voltage effects on muscarinic acetylcholine receptor-mediated contractions of airway smooth muscle

Studies have shown that the activity of muscarinic receptors and their affinity to agonists are sensitive to membrane potential. It was reported that in airway smooth muscle (ASM) depolarization evoked by high K+ solution increases contractility through direct effects on M3 muscarinic receptors. In this study, we assessed the physiological relevance of voltage sensitivity of muscarinic receptors on ASM contractility. Our findings reveal that depolarization by high K+ solution induces contraction in intact mouse trachea predominantly through activation of acetylcholine release from embedded nerves, and to a lesser extent by direct effects on M3 receptors. We therefore devised a pharmacological approach to depolarize tissue to various extents in an organ bath preparation, and isolate contraction due exclusively to ASM muscarinic receptors within range of physiological voltages. Our results indicate that unliganded muscarinic receptors do not contribute to contraction regardless of voltage. Utilizing low K+ solution to hyperpolarize membrane potentials during contractions had no effect on liganded muscarinic receptor-evoked contractions, although it eliminated the contribution of voltage-gated calcium channels. However, we found that muscarinic signaling was potentiated by at least 42% at depolarizing voltages (average -12 mV) induced by high K+ solution (20 mmol/L K+ ). In summary, we conclude that contractions evoked by direct activation of muscarinic receptors have negligible sensitivity to physiological voltages. However, contraction activated by cholinergic stimulation can be potentiated by membrane potentials occurring beyond the physiological range of ASM.

Testosterone-induced relaxation involves L-type and store-operated Ca2+ channels blockade, and PGE 2 in guinea pig airway smooth muscle

In vascular smooth muscle, it has been described that testosterone (TES) produces relaxation by blocking L-type Ca(2+) channels. Recently, we found that L-type Ca(2+) and store-operated Ca(2+) (SOC) channels are the main membranal structures that provide extracellular Ca(2+) for carbachol (CCh)-induced contraction in airway smooth muscle (ASM). We studied the possible interactions between L-type and SOC channels in TES-induced relaxation in guinea pig ASM. TES (10, 32, 100, and 178 μM) induced a complete relaxation of CCh-precontracted tracheal smooth muscle, and indomethacin partially inhibited this response. In single myocytes, the KCl-induced intracellular Ca(2+) increase ([Ca(2+)]i) was decreased by 32 and completely blocked by 100 nM TES. This androgen (32 and 100 μM) significantly diminished (~25 and 49 %, respectively) the capacitative Ca(2+) entry. Myocytes stimulated with CCh produced a transient Ca(2+) peak followed by a sustained plateau. D-600 was added during the plateau phase, and a partial diminution (~35 %) was observed. A greater decrease (~78 %) was seen when 2-aminoethyl diphenylborinate (2-APB, SOC antagonist) was used. The combination of both drugs completely abolished the Ca(2+) plateau induced by CCh. TES (100 μM) also completely abolished the CCh-induced Ca(2+) plateau. Indomethacin significantly diminished this effect of TES. PGE2 and butaprost proportionally decreased the Ca(2+) plateau as indomethacin blocked it. Sarcoplasmic reticulum refilling was partially, dependently, and significantly diminished by TES. We concluded that TES-induced relaxation involves blockade of L-type Ca(2+) channels at nanomolar and SOC channels at micromolar concentration and PGE2 seems to be also involved in this phenomenon.

ROCK1 translocates from non-caveolar to caveolar regions upon KCl stimulation in airway smooth muscle

Airway smooth muscle (ASM) membrane depolarization through KCl opens L-type voltage dependent Ca2+ channels (Ca(v)1.2); its opening was considered the cause of KCl contraction. This substance is used to bypass intracellular second messenger pathways. It is now clear that KCl also activates RhoA/Rho kinase (ROCK) pathway. ROCK isoforms are characterized as ROCK1 and ROCK2. Because ROCK1 seems the most abundant isotype in lung, we studied its participation in KCl stimulated bovine ASM. With methyl-beta-cyclodextrin (MbetaCD) we disrupted caveolae, a membrane compartment considered as the RhoA/ROCK assembly site, and found that KCl contraction was reduced to the same extent (~26%) as Y-27632 (ROCK inhibitor) treated tissues. We confirmed that KCl induces ROCK activation and this effect was annulled by Y-27632 or MbetaCD. In isolated plasmalemma, ROCK1 was localized in non-caveolar membrane fractions in Western blots from control tissues, but it transferred to caveolae in samples from tissues stimulated with KCl. Ca(v)1.2 was found at the non-caveolar membrane fractions in control and MbetaCD treated tissues. In MbetaCD treated tissues stimulated with KCl, contraction was abolished by nifedipine; only the response to Ca(v)1.2 opening remained as the ROCK component disappeared. Our results show that, in ASM, the KCl contraction involves the translocation of ROCK1 from non-caveolar to caveolar regions and that the proper physiological response depends on this translocation.

Caffeine relaxes smooth muscle through actin depolymerization

Caffeine is sometimes used in cell physiological studies to release internally stored Ca(2+). We obtained evidence that caffeine may also act through a different mechanism that has not been previously described and sought to examine this in greater detail. We ruled out a role for phosphodiesterase (PDE) inhibition, since the effect was 1) not reversed by inhibiting PKA or adenylate cyclase; 2) not exacerbated by inhibiting PDE4; and 3) not mimicked by submillimolar caffeine nor theophylline, both of which are sufficient to inhibit PDE. Although caffeine is an agonist of bitter taste receptors, which in turn mediate bronchodilation, its relaxant effect was not mimicked by quinine. After permeabilizing the membrane using β-escin and depleting the internal Ca(2+) store using A23187, we found that 10 mM caffeine reversed tone evoked by direct application of Ca(2+), suggesting it functionally antagonizes the contractile apparatus. Using a variety of molecular techniques, we found that caffeine did not affect phosphorylation of myosin light chain (MLC) by MLC kinase, actin-filament motility catalyzed by MLC kinase, phosphorylation of CPI-17 by either protein kinase C or RhoA kinase, nor the activity of MLC-phosphatase. However, we did obtain evidence that caffeine decreased actin filament binding to phosphorylated myosin heads and increased the ratio of globular to filamentous actin in precontracted tissues. We conclude that, in addition to its other non-RyR targets, caffeine also interferes with actin function (decreased binding by myosin, possibly with depolymerization), an effect that should be borne in mind in studies using caffeine to probe excitation-contraction coupling in smooth muscle.

Airway smooth muscle electrophysiology in a state of flux?

Activation of chloride currents and release of internally sequestered Ca2+ in airway smooth muscle have long been associated with excitation and contraction. Surprisingly, however, two recent publications (Deshpande DA, Wang WC, McIlmoyle EL, Robinett KS, Schillinger RM, An SS, Sham JS, Liggett SB. Nat Med 16: 1299–1304, 2010; Gallos G, Yim P, Chang S, Zhang Y, Xu D, Cook JM, Gerthoffer WT, Emala CW Sr. Am J Physiol Lung Cell Mol Physiol 302: L248–L256, 2012) have linked both events to relaxation. This begs a closer look at our understanding of airway smooth muscle electrophysiology and its contribution to excitation-contraction coupling. This Editorial Focus highlights those two aforementioned studies and several other equally paradoxical findings and proposes some possible reinterpretations of the data and/or new directions of research in which the answers might be found.

cGMP reduces the sarcoplasmic reticulum Ca2+ loading in airway smooth muscle cells: a putative mechanism in the regulation of Ca2+ by cGMP

Ca(2+) and cGMP have opposite roles in many physiological processes likely due to a complex negative feedback regulation between them. Examples of opposite functions induced by Ca(2+) and cGMP are smooth muscle contraction and relaxation, respectively. A main Ca(2+) storage involved in contraction is sarcoplasmic reticulum (SR); nevertheless, the role of cGMP in the regulation of SR-Ca(2+) has not been completely understood. To evaluate this role, intracellular Ca(2+) concentration ([Ca(2+)]i) was determinated by a ratiometric method in isolated myocytes from bovine trachea incubated with Fura-2/AM. The release of Ca(2+) from SR induced by caffeine was transient, whereas caffeine withdrawal was followed by a [Ca(2+)]i undershoot. Caffeine-induced Ca(2+) transient peak and [Ca(2+)]i undershoot after caffeine were reproducible in the same cell. Dibutyryl cGMP (db-cGMP) blocked the [Ca(2+)]i undershoot and reduced the subsequent caffeine peak (SR-Ca(2+) loading). Both, the opening of SR channels with ryanodine (10 μM) and the blockade of SR-Ca(2+) ATPase with cyclopiazonic acid inhibited the [Ca(2+)]i undershoot as well as the SR-Ca(2+) loading. The addition of db-cGMP to ryanodine (10 μM) incubated cells partially restored the SR-Ca(2+) loading. Cyclic GMP enhanced [Ca(2+)]i undershoot induced by the blockade of ryanodine channels with 50 μM ryanodine. In conclusion, the reduction of SR-Ca(2+) content in airway smooth muscle induced by cGMP can be explained by the combination of SR-Ca(2+) loading and the simultaneous release of SR-Ca(2+). The reduction of SR-Ca(2+) content induced by cGMP might be a putative mechanism limiting releasable Ca(2+) in response to a particular stimulus.

Isoprostane-induced airway hyperresponsiveness is dependent on internal Ca2+ handling and Rho/ROCK signaling

We previously reported the ability of isoprostanes to induce airway hyperresponsiveness (AHR). In this study, we examined the signaling mechanisms underlying that phenomenon with the standard muscle bath technique. Responses to a threshold concentration of carbachol (CCh, 3 x 10(-9) M) were significantly augmented by pretreatment for 20 min with 8-isoprostaglandin E(2) (15-E(2t)-IsoP, 10(-6) M): this AHR was obliterated in tissues pretreated with the selective Rho kinase (ROCK) inhibitor Y-27632 added 20 min before isoprostane, but not by cyclopiazonic acid (CPA). Increasing the CCh concentration to 3 x 10(-8) M (still considerably less than the half-maximally effective concentration of CCh) evoked larger contractions that were also augmented significantly by 15-E(2t)-IsoP: this AHR was completely abolished in tissues pretreated with CPA as well as those pretreated with Y-27632. We noted, however, that Y-27632 and CPA profoundly effect baseline tone and the cholinergic response per se, which confounds the interpretation of the data summarized above. We therefore modified the protocol by using combinations of CCh and blocker (CPA, Y-27632, or nifedipine) that were equieffective. In this way, we found that AHR could not be demonstrated under conditions in which Rho/ROCK signaling or Ca(2+) release was abolished (by Y-27632 and CPA, respectively). Likewise, other autacoids that act through G protein-coupled receptors via Rho/ROCK and Ca(2+) release (serotonin, histamine) mimicked this effect of isoprostane, whereas bradykinin did not. We conclude that isoprostane-induced AHR is mediated in part through an action on Rho/ROCK signaling. This novel finding may contribute to a better understanding of the mechanisms underlying AHR and asthma.

Exercise training prevents Ca2+ dysregulation in coronary smooth muscle from diabetic dyslipidemic yucatan swine

Aerobic exercise training is known to have profound cardioprotective effects in disease, yet cellular mechanisms remain largely undefined. We tested the hypothesis that increased sarcoplasmic reticulum Ca(2+) buffering and increased voltage-gated Ca(2+) channel density underlie coronary smooth muscle intracellular Ca(2+) (Ca(2+)(i)) dysregulation in diabetic dyslipidemia and that exercise training would prevent these increases. Yucatan swine were maintained in 1) control, 2) alloxan-induced hyperglycemic, 3) high fat/cholesterol fed, 4) hyperglycemic plus high fat/cholesterol fed (diabetic dyslipidemic), and 5) diabetic dyslipidemic plus exercise-trained (treadmill running) conditions. After 20 wk, the heart was removed and smooth muscle cells isolated from the right coronary artery. We utilized fura-2 imaging of Ca(2+)(i) levels to separate the functional role of the sarcoendoplasmic reticulum Ca(2+)-ATPase (SERCA) from the Na(+)-Ca(2+) exchanger and the plasmalemmal Ca(2+)-ATPase, and whole-cell patch clamp to examine voltage-gated Ca(2+) channel current density (i.e., Ca(2+) influx). Results indicated that diabetic dyslipidemia impaired plasmalemmal Ca(2+) efflux, increased basal Ca(2+)(i) levels, increased SERCA protein and sarcoplasmic reticulum Ca(2+)(i) buffering, and elicited an approximately 50% decrease in voltage-gated Ca(2+) channel current density. Exercise training concurrent with the diabetic dyslipidemic state restored plasmalemmal Ca(2+) efflux, SERCA protein, sarcoplasmic reticulum Ca(2+)(i) buffering, and voltage-gated Ca(2+) channel current density to control levels. Interestingly, basal Ca(2+)(i) levels were significantly lower in the exercise-trained group compared with control. Collectively, these results demonstrate a crucial role for exercise in the prevention of diabetic dyslipidemia-induced Ca(2+)(i) dysregulation.

Training-induced sarcoplasmic reticulum Ca2+ unloading occurs without Ca2+ influx

INTRODUCTION

Aerobic exercise training elicits adaptations in coronary smooth muscle that result in a novel intracellular Ca2+ signaling phenomenon termed sarcoplasmic reticulum (SR) Ca2+ unloading. Sarcoplasmic reticulum Ca2+ unloading is defined as a time-dependent depletion and then repletion of the caffeine-sensitive SR Ca2+ store.

PURPOSE

To determine whether Ca2+ influx is necessary to elicit SR Ca2+ unloading.

METHODS

Male, Yucatan swine (8 months old) were maintained: 1) sedentary or 2) exercise trained (treadmill running performed 5 d.wk(-1) for 16 wk). Smooth muscle cells were isolated from the right coronary artery and loaded with the intracellular Ca2+-indicator, fura-2. Sarcoplasmic reticulum Ca2+ content was assessed as the change in the caffeine (5 mM)-induced intracellular Ca2+ peak after a 2-, 5-, 8-, 11- or 13-min recovery from high K+ (depolarization)-induced Ca2+ influx in a physiological (2 mM) Ca2+ solution. The effect of Ca2+ influx on SR Ca2+ unloading was assessed by replacing the 2 mM Ca2+ solution with a virtually Ca2+-free (100 nM) solution during the recovery period.

RESULTS

Consistent with previous studies, SR Ca2+ unloading was not observed in cells from sedentary swine. In cells from exercise-trained swine, SR Ca2+ depletion was observed in both the 2 mM and Ca2+-free solutions, suggesting that Ca2+-induced Ca2+ release was not initiating SR Ca2+ unloading during the recovery period. In addition, the reloading of the SR Ca2+ store occurred even in the Ca2+-free solution, suggesting that exercise training facilitates an internal cycling of Ca2+ between the SR and another intracellular Ca2+ store.

CONCLUSION

In coronary smooth muscle from male swine, Ca2+ influx is not necessary for the exercise training-induced phenomenon, SR Ca2+ unloading.

Regulation of Rho/ROCK signaling in airway smooth muscle by membrane potential and [Ca2+]i

Recently, we have shown that Rho and Rho-activated kinase (ROCK) may become activated by high-millimolar KCl, which had previously been widely assumed to act solely through opening of voltage-dependent Ca(2+) channels. In this study, we explored in more detail the relationship between membrane depolarization, Ca(2+) currents, and activation of Rho/ROCK in bovine tracheal smooth muscle. Ca(2+) currents began to activate at membrane voltages more positive than -40 mV and were maximally activated above 0 mV; at the same time, these underwent time- and voltage-dependent inactivation. Depolarizing intact tissues by KCl challenge evoked contractions that were blocked equally, and in a nonadditive fashion, by nifedipine or by the ROCK inhibitor Y-27632. Other agents that elevate intracellular calcium concentration ([Ca(2+)](i)) by pathways independent of G protein-coupled receptors, namely the SERCA-pump inhibitor cyclopiazonic acid and the Ca(2+) ionophore A-23187, evoked contractions that were also largely reduced by Y-27632. KCl directly increased Rho and ROCK activities in a concentration-dependent fashion that paralleled closely the effect of KCl on tone and [Ca(2+)](i), as well as the voltage-dependent Ca(2+) currents that were measured over the voltage ranges that are evoked by 0-120 mM KCl. Through the use of various pharmacological inhibitors, we ruled out roles for Ca(2+)/calmodulin-dependent CaM kinase II, protein kinase C, and protein kinase A in mediating the KCl-stimulated changes in tone and Rho/ROCK activities. In conclusion, Rho is activated by elevation of [Ca(2+)](i) (although the signal transduction pathway underlying this Ca(2+) dependence is still unclear) and possibly also by membrane depolarization per se.

KCl evokes contraction of airway smooth muscle via activation of RhoA and Rho-kinase

Airway smooth muscle (ASM) cells express voltage-dependent Ca2+ channels, primarily of the L-subtype. These may play a role in excitation-contraction coupling of ASM, although other signaling pathways may also contribute: one of these includes Rho and its downstream effector molecule Rho-associated kinase (ROCK). Although voltage-dependent Ca2+ influx and Rho/ROCK signaling have traditionally been viewed as entirely separate pathways, recent evidence in vascular smooth muscle suggest differently. In this study, we monitored contractile activity (muscle baths) in bronchial and/or tracheal preparations from the pig, cow, and human, and further examined Rho and ROCK activities (Western blots and kinase assays) and cytosolic levels of Ca2+ (fluo 4-based fluorimetry) in porcine tracheal myocytes. KCl evoked substantial contractions that were suppressed in tracheal preparations by removal of external Ca2+ or using the selective L-type Ca2+ channel blocker nifedipine; porcine bronchial preparations were much less sensitive, and bovine bronchi were essentially unaffected by 1 microM nifedipine. Surprisingly, KCl-evoked contractions were also highly sensitive to two structurally different ROCK inhibitors: Y-27632 and HA-1077. Furthermore, the inhibitory effects of nifedipine and of the ROCK inhibitors were not additive. KCl also caused marked stimulation of Rho and ROCK activities, and both these changes were suppressed by nifedipine or by removal of external Ca2+. KCl-induced elevation of [Ca2+]i was not affected by Y-27632 but was reversed by NiCl2 or by BAPTA-AM. We conclude that KCl acts in part through stimulation of Rho and ROCK, possibly secondary to voltage-dependent Ca2+ influx.

Enhanced Myosin Phosphatase and Ca2+-Uptake Mediate Adrenergic Relaxation of Airway Smooth Muscle

We examined the mechanisms underlying relaxations evoked by isoproterenol (Iso) in isolated porcine, bovine, or human tracheal and bronchial tissues (TSM and BSM, respectively). Iso had little effect against contractions evoked by high KCl, indicating that it does not directly suppress voltage-dependent Ca(2+)-influx nor directly inhibit myosin light chain kinase. Furthermore, Iso was equally potent against carbachol (CCh) contractions in the presence versus absence of nifedipine (10(-6) M), establishing that the primary action of Iso is not through membrane hyperpolarization. However, Iso relaxations in porcine/bovine BSM were significantly suppressed by inhibitors of the internal Ca(2+) pump (cyclopiazonic acid; 10(-5) M) or of myosin light chain phosphatase (calyculin; 10(-6) M). Myosin light chain phosphatase activity was assayed directly (using (32)P-labeled myosin) and found to be enhanced in a time- and concentration-dependent fashion by Iso. Iso relaxations in human airway tissues, on the other hand, were not significantly affected by either calyculin or cyclopiazonic acid. Thus, we conclude that Iso acts largely in a voltage-independent fashion: in nonhuman airways, this involves enhanced Ca(2+) pump activity (to decrease [Ca(2+)](i)) and myosin light chain phosphatase activation (to decrease Ca(2+)-sensitivity of the contractile apparatus), whereas in human airways the underlying mechanisms are still unclear.

Exercise prevents diabetes-induced impairment in superficial buffer barrier in porcine coronary smooth muscle

In healthy coronary smooth muscle cells, the superficial sarcoplasmic reticulum (SR) buffers rise in intracellular Ca2+ levels. In diabetic dyslipidemia, basal Ca2+ levels are increased, yet Ca2+ influx is decreased and SR Ca2+ uptake is increased. Exercise prevents diabetic dyslipidemia-induced increases in basal Ca2+ levels and decreases in Ca2+ influx. We tested the hypothesis that diabetic dyslipidemia impairs Ca2+ extrusion via a decrease in superficial SR and that exercise will prevent these losses. Male Yucatan swine were maintained in four treatment groups: control, hyperlipidemic, diabetic dyslipidemic, and diabetic dyslipidemic plus aerobically exercise trained. Intracellular Ca2+ levels were measured during depolarization-induced Ca2+ influx and caffeine-induced SR Ca2+ release. Na+/Ca2+ exchanger and plasmalemmal Ca2+-ATPase activity were assessed by inhibition with low extracellular Na+ and 5,6-carboxyeosin, respectively. Superficial SR was quantified using the internal membrane dye 3,3′-dihexyloxacarbocyanine iodide (DiOC6) and novel analysis techniques. We found that, in diabetic dyslipidemia, Ca2+ extrusion was impaired and superficial SR was decreased. Exercise prevented the diabetic dyslipidemia-induced decrease in superficial SR and restored plasmalemmal Ca2+ extrusion. On the basis of these results, we conclude exercise attenuates the diabetic dyslipidemia-induced impairment in intracellular Ca2+ regulation.

Ionic mechanisms and Ca(2+) regulation in airway smooth muscle contraction: do the data contradict dogma?

In general, excitation-contraction coupling in muscle is dependent on membrane depolarization and hyperpolarization to regulate the opening of voltage-dependent Ca(2+) channels and, thereby, influence intracellular Ca(2+) concentration ([Ca(2+)](i)). Thus Ca(2+) channel blockers and K(+) channel openers are important tools in the arsenals against hypertension, stroke, and myocardial infarction, etc. Airway smooth muscle (ASM) also exhibits robust Ca(2+), K(+), and Cl(-) currents, and there are elaborate signaling pathways that regulate them. It is easy, then, to presume that these also play a central role in contraction/relaxation of ASM. However, several lines of evidence speak to the contrary. Also, too many researchers in the ASM field view the sarcoplasmic reticulum as being centrally located and displacing its contents uniformly throughout the cell, and they have focused almost exclusively on the initial single [Ca(2+)] spike evoked by excitatory agonists. Several recent studies have revealed complex spatial and temporal heterogeneity in [Ca(2+)](i), the significance of which is only just beginning to be appreciated. In this review, we will compare what is known about ion channels in ASM with what is believed to be their roles in ASM physiology. Also, we will examine some novel ionic mechanisms in the context of Ca(2+) handling and excitation-contraction coupling in ASM.

Intracellular stores maintain stable cytosolic Ca(2+) gradients in epithelial cells by active Ca(2+) redistribution

A stable localized region of high calcium concentration near the plasma membrane has been postulated to exist as an outcome of prolonged calcium influx and to play a crucial role in regulation of cellular life. However, the mechanism supporting this phenomenon is a perplexing problem. We show here that a sustained localized region of high cytosolic Ca(2+) concentration is formed near the plasma membrane. Calcium influx, calcium uptake by intracellular stores and calcium release from the stores are essential for this phenomenon. Our results strongly suggest that the mechanism of formation of stable calcium gradient near the plasma membrane involves a process of active redistribution-uptake of entering calcium into intracellular stores and its release from the stores toward the plasma membrane.

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

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

Muscarinic excitation-contraction coupling mechanisms in tracheal and bronchial smooth muscles

We investigated the mechanisms underlying muscarinic excitation-contraction coupling in canine airway smooth muscle using organ bath, fura 2 fluorimetric, and patch-clamp techniques. Cyclopiazonic acid (CPA) augmented the responses to submaximal muscarinic stimulation in both tracheal (TSM) and bronchial smooth muscles (BSM), consistent with disruption of the barrier function of the sarcoplasmic reticulum. During maximal stimulation, however, CPA evoked substantial relaxation in TSM but not BSM. CPA reversal of carbachol tone persisted in the presence of tetraethylammoium or high KCl, suggesting that hyperpolarization is not involved; CPA relaxations were absent in tissues preconstricted with KCl alone or by permeabilization with beta-escin, ruling out a nonspecific effect on the contractile apparatus. Peak contractions were sensitive to inhibitors of tyrosine kinase (genistein) or Rho kinase (Y-27632). Sustained responses were dependent on Ca(2+) influx in TSM but not BSM; this influx was sensitive to Ni(2+) but not La(3+). In conclusion, there are several mechanisms underlying excitation-contraction coupling in airway smooth muscle, the relative importance of which varies depending on tissue and degree of stimulation.

Invited review: mechanisms of calcium handling in smooth muscles

The concentration of cytoplasmic Ca(2+) regulates the contractile state of smooth muscle cells and tissues. Elevations in global cytoplasmic Ca(2+) resulting in contraction are accomplished by Ca(2+) entry and release from intracellular stores. Pathways for Ca(2+) entry include dihydropyridine-sensitive and -insensitive Ca(2+) channels and receptor and store-operated nonselective channels permeable to Ca(2+). Intracellular release from the sarcoplasmic reticulum (SR) is accomplished by ryanodine and inositol trisphosphate receptors. The impact of Ca(2+) entry and release on cytoplasmic concentration is modulated by Ca(2+) reuptake into the SR, uptake into mitochondria, and extrusion into the extracellular solution. Highly localized Ca(2+) transients (i.e., sparks and puffs) regulate ionic conductances in the plasma membrane, which can provide feedback to cell excitability and affect Ca(2+) entry. This short review describes the major transport mechanisms and compartments that are utilized for Ca(2+) handling in smooth muscles.

Effects of cyclopiazonic acid on cytosolic calcium in bovine airway smooth muscle cells

In many cells, inhibition of sarcoplasmic reticulum (SR) Ca2+-ATPase activity induces a steady-state increase in cytosolic calcium concentration ([Ca2+]i) that is sustained by calcium influx. The goal was to characterize the response to inhibition of SR Ca2+-ATPase activity in bovine airway smooth muscle cells. Cells were dispersed from bovine trachealis and loaded with fura 2-AM (0.5 microM) for imaging of single cells. Cyclopiazonic acid (CPA; 5 microM) inhibited refilling of both caffeine- and carbachol-sensitive calcium stores. In the presence of extracellular calcium, CPA caused a transient increase in [Ca2+]i from 166 +/- 11 to 671 +/- 100 nM, and then [Ca2+]i decreased to a sustained level (CPA plateau; 236 +/- 19 nM) significantly above basal. The CPA plateau spontaneously declined toward basal levels after 10 min and was attenuated by discharging intracellular calcium stores. When CPA was applied during sustained stimulation with caffeine or carbachol, decreases in [Ca2+]i were observed. We concluded that the CPA plateau depended on the presence of SR calcium and that SR Ca2+-ATPase activity contributed to sustained increases in [Ca2+]i during stimulation with caffeine and, to a lesser extent, carbachol.

Extracellular cyclic ADP-ribose potentiates ACh-induced contraction in bovine tracheal smooth muscle

Cyclic ADP-ribose (cADPR), a universal calcium releaser, is generated from NAD(+) by an ADP-ribosyl cyclase and is degraded to ADP-ribose by a cADPR hydrolase. In mammals, both activities are expressed as ectoenzymes by the transmembrane glycoprotein CD38. CD38 was identified in both epithelial cells and smooth myocytes isolated from bovine trachea. Intact tracheal smooth myocytes (TSMs) responded to extracellular cADPR (100 microM) with an increase in intracellular calcium concentration ([Ca(2+)](i)) both at baseline and after acetylcholine (ACh) stimulation. The nonhydrolyzable analog 3-deaza-cADPR (10 nM) elicited the same effects as cADPR, whereas the cADPR antagonist 8-NH(2)-cADPR (10 microM) inhibited both basal and ACh-stimulated [Ca(2+)](i) levels. Extracellular cADPR or 3-deaza-cADPR caused a significant increase of ACh-induced contraction in tracheal smooth muscle strips, whereas 8-NH(2)-cADPR decreased it. Tracheal mucosa strips, by releasing NAD(+), enhanced [Ca(2+)](i) in isolated TSMs, and this increase was abrogated by either NAD(+)-ase or 8-NH(2)-cADPR. These data suggest the existence of a paracrine mechanism whereby mucosa-released extracellular NAD(+) plays a hormonelike function and cADPR behaves as second messenger regulating calcium-related contractility in TSMs.

Mechanical signals and mechanosensitive modulation of intracellular [Ca(2+)] in smooth muscle

We tested the hypothesis that strain is the primary mechanical signal in the mechanosensitive modulation of intracellular Ca(2+) concentration ([Ca(2+)](i)) in airway smooth muscle. We found that [Ca(2+)](i) was significantly correlated with muscle length during isotonic shortening against 20% isometric force (F(iso)). When the isotonic load was changed to 50% F(iso), data points from the 20 and 50% F(iso) experiments overlapped in the length-[Ca(2+)](i) relationship. Similarly, data points from the 80% F(iso) experiments clustered near those from the 50% F(iso) experiments. Therefore, despite 2.5- and 4-fold differences in external load, [Ca(2+)](i) did not deviate much from the length-[Ca(2+)](i) relation that fitted the 20% F(iso) data. Maximal inhibition of sarcoplasmic reticular (SR) Ca(2+) uptake by 10 microM cyclopiazonic acid (CPA) did not significantly change [Ca(2+)](i) in carbachol-induced isometric contractions and isotonic shortening. CPA also did not significantly change myosin light-chain phosphorylation or force redevelopment when carbachol-activated muscle strips were quickly released from optimal length (L(o)) to 0.5 L(o). These results are consistent with the hypothesis and suggest that SR Ca(2+) uptake is not the underlying mechanism.

Control of the mode of excitation-contraction coupling by Ca(2+) stores in bovine trachealis muscle

Full muscarinic stimulation in bovine tracheal smooth muscle caused a sustained contraction and increase in intracellular Ca(2+) concentration ([Ca(2+)](i)) that was largely resistant to inhibition by nifedipine. Depletion of internal Ca(2+) stores with cyclopiazonic acid resulted in an increased efficacy of nifedipine to inhibit this contraction and the associated increase in [Ca(2+)](i). Thus internal Ca(2+) store depletion promoted electromechanical coupling between full muscarinic stimulation and muscle contraction to the detriment of pharmacomechanical coupling. A similar change in coupling mode was induced by ryanodine even when it did not significantly modify the initial transient increase in [Ca(2+)](i) induced by this stimulation, indicating that depletion of internal stores was not necessary to induce the change in excitation-contraction coupling mode. Blockade of the Ca(2+)-activated K(+) channel by tetraethylammonium, charybdotoxin, and iberiotoxin all induced the change in excitation-contraction coupling mode. These results suggest that in this preparation, Ca(2+) released from the ryanodine-sensitive Ca(2+) store, by activating Ca(2+)-activated K(+) channels, plays a central role in determining the expression of the pharmacomechanical coupling mode between muscarinic excitation and the Ca(2+) influx necessary for the maintenance of tone.

Selective restoration of calcium coupling to muscarinic M(3) receptors in contractile cultured airway myocytes

We previously demonstrated that after several days of serum deprivation about one-sixth of confluent cultured canine tracheal myocytes acquire an elongated, structurally and functionally contractile phenotype. These myocytes demonstrated significant shortening on ACh exposure. To evaluate the mechanism by which these myocytes acquire responsiveness to ACh, we assessed receptor-Ca(2+) coupling using fura 2-AM fluorescence imaging and muscarinic receptor expression using Western analysis. Cells were grown to confluence in 10% fetal bovine serum and then maintained for 7-13 days in serum-free medium. A fraction of serum-deprived cells exhibited reproducible intracellular Ca(2+) mobilization in response to ACh that was uniformly absent from airway myocytes before serum deprivation. The Ca(2+) response to 10(-4) M ACh was ablated by inositol 1,4,5-trisphosphate (IP(3)) receptor blockade using 10(-6) M xestospongin C but not by removal of extracellular Ca(2+). Also, 10(-7) M atropine or 10(-7) M 4-diphenylacetoxy-N-methylpiperidine completely blocked the response to ACh, but intracellular Ca(2+) mobilization was not ablated by 10(-6) M pirenzepine or 10(-6) M methoctramine. In contrast, 10(-5) M bradykinin (BK) was without effect in these ACh-responsive myocytes. Interestingly, myocytes that did not respond to ACh demonstrated robust increases in intracellular Ca(2+) on exposure to 10(-5) M BK that were blocked by removal of extracellular Ca(2+) and were only modestly affected by IP(3) receptor blockade. Serum deprivation increased the abundance of M(3) receptor protein and of BK(2) receptor protein by two- to threefold in whole cell lysates within 2 days of serum deprivation, whereas M(2) receptor protein fell by >75%. An increase in M(3) receptor abundance and restoration of M(3) receptor-mediated Ca(2+) mobilization occur concomitant with reacquisition of a contractile phenotype during prolonged serum deprivation. These data demonstrate plasticity in muscarinic surface receptor expression and function in a subpopulation of airway myocytes that show mutually exclusive physiological and pharmacological diversity with other cells in the same culture.

NO(+) but not NO radical relaxes airway smooth muscle via cGMP-independent release of internal Ca(2+)

We compared the effects of two redox forms of nitric oxide, NO(+) [liberated by S-nitroso-N-acetyl-penicillamine (SNAP)] and NO. [liberated by 3-morpholinosydnonimine (SIN-1) in the presence of superoxide dismutase], on cytosolic concentration of Ca(2+) ([Ca(2+)](i); single cells) and tone (intact strips) obtained from human main stem bronchi and canine trachealis. SNAP evoked a rise in [Ca(2+)](i) that was unaffected by removing external Ca(2+) but was markedly reduced by depleting the internal Ca(2+) pool using cyclopiazonic acid (10(-5) M). Dithiothreitol (1 mM) also antagonized the Ca(2+) transient as well as the accompanying relaxation. SNAP attenuated responses to 15 and 30 mM KCl but not those to 60 mM KCl, suggesting the involvement of an electromechanical coupling mechanism rather than a direct effect on the contractile apparatus or on Ca(2+) channels. SNAP relaxations were sensitive to charybdotoxin (10(-7) M) or tetraethylammonium (30 mM) but not to 4-aminopyridine (1 mM). Neither SIN-1 nor 8-bromoguanosine 3',5'-cyclic monophosphate had any significant effect on resting [Ca(2+)](i), although both of these agents were able to completely reverse tone evoked by carbachol (10(-7) M). We conclude that NO(+) causes release of internal Ca(2+) in a cGMP-independent fashion, leading to activation of Ca(2+)-dependent K(+) channels and relaxation, whereas NO. relaxes the airways through a cGMP-dependent, Ca(2+)-independent pathway.

Ca(2+)-dependent K(+) channels and Na(+)-K(+)-ATPase mediate H(2)O(2)- and superoxide-induced relaxations in canine trachealis

We examined the ionic mechanisms underlying the responses of canine trachealis to superoxide (generated in vitro by using xanthine oxidase or added exogenously) and peroxide (generated spontaneously in vitro by the dismutation of superoxide or added exogenously). Although neither had any effect on resting tone, both triggered relaxations in carbachol-precontracted tissues. These relaxations were eliminated by catalase but were much less sensitive to the hydroxyl radical scavenger dimethylthiourea, indicating they were mediated primarily by peroxide. These relaxations were decreased in magnitude and/or slowed by nifedipine (10(-6) M), ouabain (10(-6) M), or tetraethylammonium (25 mM), but not by 4-aminopyridine (5 mM), and were small or absent in tissues precontracted with 30 mM KCl. Finally, peroxide triggered membrane hyperpolarization and elevated cytosolic concentration of Ca(2+) (primarily via release from the internal store). Thus peroxide-mediated relaxations seem to involve Ca(2+) release, opening of Ca(2+)-dependent K(+) channels, hyperpolarization, closure of Ca(2+) channels, and relaxation. In addition, some other free radical (hydroxyl radical?) may activate the Na(+)-K(+) pump, also hyperpolarizing the membrane and causing relaxation.