Modulation of Ca(2+)-activated Cl- currents in rabbit portal vein smooth muscle by an inhibitor of mitochondrial Ca2+ uptake

Role of ryanodine receptor 2 and FK506-binding protein 12.6 dissociation in pulmonary hypertension

Pulmonary hypertension (PH) is a devastating disease characterized by a progressive increase in pulmonary arterial pressure leading to right ventricular failure and death. A major cellular response in this disease is the contraction of smooth muscle cells (SMCs) of the pulmonary vasculature. Cell contraction is determined by the increase in intracellular Ca2+ concentration ([Ca2+]i), which is generated and regulated by various ion channels. Several studies by us and others have shown that ryanodine receptor 2 (RyR2), a Ca2+-releasing channel in the sarcoplasmic reticulum (SR), is an essential ion channel for the control of [Ca2+]i in pulmonary artery SMCs (PASMCs), thereby mediating the sustained vasoconstriction seen in PH. FK506-binding protein 12.6 (FKBP12.6) strongly associates with RyR2 to stabilize its functional activity. FKBP12.6 can be dissociated from RyR2 by a hypoxic stimulus to increase channel function and Ca2+ release, leading to pulmonary vasoconstriction and PH. More specifically, dissociation of the RyR2-FKBP12.6 complex is a consequence of increased mitochondrial ROS generation mediated by the Rieske iron-sulfur protein (RISP) at the mitochondrial complex III after hypoxia. Overall, RyR2/FKBP12.6 dissociation and the corresponding signaling pathway may be an important factor in the development of PH. Novel drugs and biologics targeting RyR2, FKBP12.6, and related molecules may become unique effective therapeutics for PH.

Cl⁻ channels in smooth muscle cells

In smooth muscle cells (SMCs), the intracellular chloride ion (Cl−) concentration is high due to accumulation by Cl−/HCO3− exchange and Na+–K+–Cl− cotransportation. The equilibrium potential for Cl− (ECl) is more positive than physiological membrane potentials (Em), with Cl− efflux inducing membrane depolarization. Early studies used electrophysiology and nonspecific antagonists to study the physiological relevance of Cl− channels in SMCs. More recent reports have incorporated molecular biological approaches to identify and determine the functional significance of several different Cl− channels. Both "classic" and cGMP-dependent calcium (Ca2+)-activated (ClCa) channels and volume-sensitive Cl− channels are present, with TMEM16A/ANO1, bestrophins, and ClC-3, respectively, proposed as molecular candidates for these channels. The cystic fibrosis transmembrane conductance regulator (CFTR) has also been described in SMCs. This review will focus on discussing recent progress made in identifying each of these Cl− channels in SMCs, their physiological functions, and contribution to diseases that modify contraction, apoptosis, and cell proliferation.

Regulation of permeability across the blood-brain barrier

The blood-brain barrier refers to the very low permeability across microvessels in the Central Nervous System (CNS), created by the interaction between vascular endothelial cells and surrounding cells of the neurovascular unit. Permeability can be modulated (increased and decreased) by a variety of factors including inflammatory mediators, inflammatory cells such as neutrophils and through alterations in the phenotype of blood vessels during angiogenesis and apoptosis. In this chapter, some of these factors are discussed as well as the challenge of treating harmful increases in permeability that result in brain swelling (vasogenic cerebral edema).

Mitochondrial regulation of cytosolic Ca²⁺ signals in smooth muscle

The cytosolic Ca²⁺ concentration ([Ca²⁺]c) controls virtually every activity of smooth muscle, including contraction, migration, transcription, division and apoptosis. These processes may be activated by large (>10 μM) amplitude [Ca²⁺]c increases, which occur in small restricted regions of the cell or by smaller (<1 μM) amplitude changes throughout the bulk cytoplasm. Mitochondria contribute to the regulation of these signals by taking up Ca²⁺. However, mitochondria's reported low affinity for Ca²⁺ is thought to require the organelle to be positioned close to ion channels and within a microdomain of high [Ca²⁺]. In cultured smooth muscle, mitochondria are highly dynamic structures but in native smooth muscle mitochondria are immobile, apparently strategically positioned organelles that regulate the upstroke and amplitude of IP₃-evoked Ca²⁺ signals and IP₃ receptor (IP₃R) cluster activity. These observations suggest mitochondria are positioned within the high [Ca²⁺] microdomain arising from an IP₃R cluster to exert significant local control of channel activity. On the other hand, neither the upstroke nor amplitude of voltage-dependent Ca²⁺ entry is modulated by mitochondria; rather, it is the declining phase of the transient that is regulated by the organelle. Control of the declining phase of the transient requires a high mitochondrial affinity for Ca²⁺ to enable uptake to occur over the normal physiological Ca²⁺ range (<1 μM). Thus, in smooth muscle, mitochondria regulate Ca²⁺ signals exerting effects over a large range of [Ca²⁺] (∼200 nM to at least tens of micromolar) to provide a wide dynamic range in the control of Ca²⁺ signals.

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.

Compartmentalized signalling: Ca2+ compartments, microdomains and the many facets of Ca2+ signalling

Ca(2+) regulates a multitude of cellular processes and does so by partitioning its actions in space and time. In this review, we discuss how Ca(2+) responses are constructed from small quantal (elementary) events that have the potential to propagate to produce large pan-cellular responses. We review how Ca(2+) is compartmentalized both physically and functionally, and describe how each organelle has its own distinct Ca(2+)-handling properties. We explain how coordination of the movement of Ca(2+) between organelles is used to shape and hone Ca(2+) signals. Finally, we provide a number of specific examples of where compartmentation and localization of Ca(2+) are crucial to cell function.

Regulation of plasma membrane calcium fluxes by mitochondria

The role of mitochondria in cell signaling is becoming increasingly apparent, to an extent that the signaling role of mitochondria appears to have stolen the spotlight from their primary function as energy producers. In this chapter, we will review the ionic basis of calcium handling by mitochondria and discuss the mechanisms that these organelles use to regulate the activity of plasma membrane calcium channels and transporters.

Microtubule-associated protein tau in development, degeneration and protection of neurons

As a principal neuronal microtubule-associated protein, tau has been recognized to play major roles in promoting microtubule assembly and stabilizing the microtubules and to maintain the normal morphology of the neurons. Recent studies suggest that tau, upon alternative mRNA splicing and multiple posttranslational modifications, may participate in the regulations of intracellular signal transduction, development and viability of the neurons. Furthermore, tau gene mutations, aberrant mRNA splicing and abnormal posttranslational modifications, such as hyperphosphorylation, have also been found in a number of neurodegenerative disorders, collectively known as tauopathies. Therefore, changes in expression of the tau gene, alternative splicing of its mRNA and its posttranslational modification can modulate the normal architecture and functions of neurons as well as in a situation of tauopathies, such as Alzheimer's disease. The primary aim of this review is to summarize the latest developments and perspectives in our understanding about the roles of tau, especially hyperphosphorylation, in the development, degeneration and protection of neurons.

Ca2+ microdomains in smooth muscle

In smooth muscle, Ca(2+) controls diverse activities including cell division, contraction and cell death. Of particular significance in enabling Ca(2+) to perform these multiple functions is the cell's ability to localize Ca(2+) signals to certain regions by creating high local concentrations of Ca(2+) (microdomains), which differ from the cytoplasmic average. Microdomains arise from Ca(2+) influx across the plasma membrane or release from the sarcoplasmic reticulum (SR) Ca(2+) store. A single Ca(2+) channel can create a microdomain of several micromolar near (approximately 200 nm) the channel. This concentration declines quickly with peak rates of several thousand micromolar per second when influx ends. The high [Ca(2+)] and the rapid rates of decline target Ca(2+) signals to effectors in the microdomain with rapid kinetics and enable the selective activation of cellular processes. Several elements within the cell combine to enable microdomains to develop. These include the brief open time of ion channels, localization of Ca(2+) by buffering, the clustering of ion channels to certain regions of the cell and the presence of membrane barriers, which restrict the free diffusion of Ca(2+). In this review, the generation of microdomains arising from Ca(2+) influx across the plasma membrane and the release of the ion from the SR Ca(2+) store will be discussed and the contribution of mitochondria and the Golgi apparatus as well as endogenous modulators (e.g. cADPR and channel binding proteins) will be considered.

Mitochondria buffer NCX-mediated Ca2+-entry and limit its diffusion into vascular smooth muscle cells

The reverse-mode of the Na(+)/Ca(2+)-exchanger (NCX) mediates Ca(2+)-entry in agonist-stimulated vascular smooth muscle (VSM) and plays a central role in salt-sensitive hypertension. We investigated buffering of Ca(2+)-entry by peripheral mitochondria upon NCX reversal in rat aortic smooth muscle cells (RASMC). [Ca(2+)] was measured in mitochondria ([Ca(2+)](MT)) and the sub-plasmalemmal space ([Ca(2+)](subPM)) with targeted aequorins and in the bulk cytosol ([Ca(2+)](i)) with fura-2. Substitution of extracellular Na(+) by N-methyl-d-glucamine transiently increased [Ca(2+)](MT) ( approximately 2microM) and [Ca(2+)](subPM) ( approximately 1.3microM), which then decreased to sustained plateaus. In contrast, Na(+)-substitution caused a delayed and tonic increase in [Ca(2+)](i) (<100nM). Inhibition of Ca(2+)-uptake by the sarcoplasmic reticulum (SR) (30microM cyclopiazonic acid) or mitochondria (2microM FCCP or 2microM ruthenium red) enhanced the elevation of [Ca(2+)](subPM). These treatments also abolished the delay in the [Ca(2+)](i) response to 0Na(+) and increased its amplitude. Extracellular ATP (1mM) caused a peak and plateau in [Ca(2+)](i), and only the plateau was inhibited by KB-R7943 (10microM), a selective blocker of reverse-mode NCX. Evidence for ATP-mediated NCX-reversal was also found in changes in [Na(+)](i). Mitochondria normally exhibited a transient elevation of [Ca(2+)] in response to ATP, but inhibiting the mitochondrial NCX with CGP-37157 (10microM) unmasked an agonist-induced increase in mitochondrial Ca(2+)-flux. This flux was blocked by KB-R7943. In summary, mitochondria and the sarcoplasmic reticulum co-operate to buffer changes in [Ca(2+)](i) due to agonist-induced NCX reversal.

Regulation of calcium-activated chloride channels in smooth muscle cells: a complex picture is emerging

Calcium-activated chloride channels (ClCa) are ligand-gated anion channels as they have been shown to be activated by a rise in intracellular Ca2+ concentration in various cell types including cardiac, skeletal and vascular smooth muscle cells, endothelial and epithelial cells, as well as neurons. Because ClCa channels are normally closed at resting, free intracellular Ca2+ concentration (approximately 100 nmol/L) in most cell types, they have generally been considered excitatory in nature, providing a triggering mechanism during signal transduction for membrane excitability, osmotic balance, transepithelial chloride movements, or fluid secretion. Unfortunately, the genes responsible for encoding this class of ion channels is still unknown. This review centers primarily on recent findings on the properties of these channels in smooth muscle cells. The first section discusses the functional significance and biophysical and pharmacological properties of ClCa channels in smooth muscle cells, and ends with a description of 2 candidate gene families (i.e., CLCA and Bestrophin) that are postulated to encode for these channels in various cell types. The second section provides a summary of recent findings demonstrating the regulation of native ClCa channels in vascular smooth muscle cells by calmodulin-dependent protein kinase II and calcineurin and how their fine tuning by these enzymes may influence vascular tone.

Depletion of Ca2+ from intracellular stores of the rat portal vein stimulates a tonic contraction

The possibility that Ca2+ store depletion can stimulate contraction of the rat portal vein was investigated in functional experiments. Ca2+ stores were depleted with phenylephrine or cyclopiazonic acid in the absence of extracellular Ca2+ and then washed out for 30 min. Upon re-addition of extracellular Ca2+, a tonic contraction was produced, showing the stimulus for contraction was Ca2+ store depletion. The contractions were abolished by niflumic acid and nifedipine however, indicating they were dependent on depolarization resulting from opening of Ca2+-activated Cl- channels and Ca2+ influx through voltage-gated channels. Cumulative additions of phenylephrine below 3x10(-6) M did not produce tonic contractions but did in high K+ Krebs solution, where levcromakalim had no effect. This showed the tonic contractions were initially prevented by K+ channel opening. Increased Ca2+ entry through voltage-gated channels may therefore stimulate Ca2+-activated Cl- channels. Ca2+ store depletion could stimulate this by opening store-operated non-selective cation channels, resulting in depolarization.

Activation of chloride currents in murine portal vein smooth muscle cells by membrane depolarization involves intracellular calcium release

The present study describes the first characterization of Ca2+-activated Cl− currents ( IClCa) in single smooth muscle cells from a murine vascular preparation (portal veins). IClCa was recorded using the perforated patch version of the whole cell voltage-clamp technique and was evoked using membrane depolarization. Generation of IClCa relied on Ca2+ entry through dihydropyridine-sensitive Ca2+ channels because IClCa was abolished by 1 μM nicardipine and enhanced by raising external Ca2+ concentration or by application of BAY K 8644. IClCa was characterized by the sensitivity to Cl− channel blockers and the effect of altering the external anion on reversal potential. Activation of IClCa after membrane depolarization was dependent on Ca2+ release from intracellular stores. Thus the amplitude of IClCa was diminished by the SR-ATPase inhibitor cyclopiazonic acid, the inositol 1,4,5-trisphosphate receptor antagonist 2-aminoethoxydiphenyl borate (2-APB), and the ryanodine receptor blocker tetracaine. The degree of inhibition produced by the application of 2-APB and tetracaine together was significantly greater than the effect of each agent applied alone. In current-clamp mode, injection of depolarizing current elicited a biphasic action potential, with the later depolarization being sensitive to niflumic acid (NFA; 10 μM). In isometric tension recordings, NFA inhibited spontaneous contractions. These data support a role for this conductance in portal vein excitability.

Function of interstitial cells of Cajal in the rabbit portal vein

Interstitial cells of Cajal (ICCs) were identified in the intact fixed media of the rabbit portal vein (RPV) using c-kit staining. The following experiments were performed using single cell preparations of the enzyme-dispersed vessel. Surviving contacts between the processes of single ICCs and the bodies of smooth muscle cells (SMCs) were observed in electron micrographs and by confocal microscopy. Spontaneous rhythmical [Ca2+]i oscillations were observed in ICCs after loading with the calcium indicator fluo-3 and were associated with depolarizations of the ICCs recorded by tight-seal patch pipette. To investigate signal transmission from ICCs to SMCs in dispersed cell pairs, or within small surviving fragments of the ICC network, an ICC was stimulated under voltage-clamp, while changes in [Ca2+]i in the stimulated cell as well as in a closely adjacent SMC or ICCs were monitored using fast x-y confocal imaging of fluo-3 fluorescence. After stimulation of single voltage-clamped ICC by a depolarizing step similar in duration to depolarizations associated with spontaneous [Ca2+]i oscillations, a depolarization and transient elevation of [Ca2+]i was observed in a closely adjacent SMCs after a delay of up to 4 seconds. In contrast, signal transmission from ICC to ICC was much faster, the delay being less than 200 ms. These results suggest that the an ICC may, in addition to generating an electrical signal (such as a slow wave) and thereby acting as a pacemaker for vascular SMCs of RPV, also release some unknown diffusible substance, which depolarizes the SMCs.

Smooth muscle cells and interstitial cells of blood vessels

A rise in intracellular ionised calcium concentration ([Ca(2+)](i)) at sites adjacent to the contractile proteins is a primary signal for contraction in all types of muscles. Recent progress in the development of imaging techniques with special accent on the fluorescence confocal microscopy and new achievements in the synthesis of organelle- and ion-specific fluorochromes provide an experimental basis for study of the relationship between the structural organisation of the living smooth muscle myocyte and the features of calcium signalling at subcellular level. Applying fluorescent confocal microscopy and tight-seal recording of transmembrane ion currents to freshly isolated vascular myocytes we have demonstrated that: (1) Ca(2+) sparks originate from clustered opening of ryanodine receptors (RyRs) and build up a cell-wide increase in [Ca(2+)](i) upon myocyte excitation; (2) spontaneous Ca(2+) sparks occurred at the highest rate at certain preferred locations, frequent discharge sites (FDS), which are associated with a prominent portion of the sarcoplasmic reticulum (SR) located close to the cell membrane; (3) Ca(2+)-dependent K(+) and Cl(-) channels sense the local changes in [Ca(2+)](i) during a calcium spark and thereby couple changes in [Ca(2+)](i) within a microdomain to changes in the membrane potential, thus affecting excitability of the cell; (4) an intercommunication between RyRs and inositol trisphosphate receptors (IP(3)Rs) is one of the important determinants of intracellular calcium dynamics that, in turn, can modulate the cell membrane potential through differential targeting of calcium dependent membrane ion channels. Furthermore, using immunohystochemical approaches in combination with confocal imaging we identified non-contractile cells closely resembling interstitial cells (ICs) of Cajal (which are considered to be pacemaker cells in the gut) in the wall of portal vein and mesenteric artery. Using electron microscopy, tight-seal recording and fluorescence confocal imaging we obtained information on the morphology of ICs and their possible coupling to smooth muscle cells (SMCs), calcium signalling in ICs and their electrophysiological properties. The functions of these cells are not yet fully understood; in portal vein they may act as pacemakers driving the spontaneous activity of the muscle; in artery they may have other a yet unsuspected functions.

Vascular smooth muscle mitochondria at the cross roads of Ca2+ regulation

Mitochondria play an essential role in the regulation of vascular smooth muscle Ca(2+) signaling being simultaneously integrated in the regulation of ion channels and Ca(2+) transporters, oxygen radical production, metabolite recycling and intracellular redox potential. Mitochondria buffer Ca(2+) from cytoplasmic microdomains to alter the spatio-temporal pattern of Ca(2+) gradients following Ca(2+)-influx and Ca(2+)-release, and thus control site-specific, Ca(2+)-dependent ion channel activation and inactivation. The sub-cellular localization of mitochondria in conjunction with tissue-specific channel expression is fundamental to vascular heterogeneity. The mitochondrial electron transport chain recycles metabolic intermediates that modulate cellular redox potential and produces oxygen radicals in proportion to oxygen tension. Perturbation of specific complexes within the transport chain can affects NADH:NAD and ATP:ADP ratios and radical production, which can in turn influence second messenger metabolism, ion channel gating and Ca(2+)-transporter activity. Mitochondria thus provide the common ground for cross-talk between these regulatory systems that are mutually sensitive to one another. This cross-talk between signaling systems provides a means to render the physiological regulation of vascular tone responsive to complex stimulation by paracrine and endocrine factors, blood pressure and flow, tissue oxygenation and metabolic state.

Mitochondrial modulation of Ca2+ sparks and transient KCa currents in smooth muscle cells of rat cerebral arteries

Mitochondria sequester and release calcium (Ca(2+)) and regulate intracellular Ca(2+) concentration ([Ca(2+)](i)) in eukaryotic cells. However, the regulation of different Ca(2+) signalling modalities by mitochondria in smooth muscle cells is poorly understood. Here, we investigated the regulation of Ca(2+) sparks, Ca(2+) waves and global [Ca(2+)](i) by mitochondria in cerebral artery smooth muscle cells. CCCP (a protonophore; 1 microm) and rotenone (an electron transport chain complex I inhibitor; 10 microm) depolarized mitochondria, reduced Ca(2+) spark and wave frequency, and elevated global [Ca(2+)](i) in smooth muscle cells of intact arteries. In voltage-clamped (-40 mV) cells, mitochondrial depolarization elevated global [Ca(2+)](i), reduced Ca(2+) spark amplitude, spatial spread and the effective coupling of sparks to large-conductance Ca(2+)-activated potassium (K(Ca)) channels, and decreased transient K(Ca) current frequency and amplitude. Inhibition of Ca(2+) sparks and transient K(Ca) currents by mitochondrial depolarization could not be explained by a decrease in intracellular ATP or a reduction in sarcoplasmic reticulum Ca(2+) load, and occurred in the presence of diltiazem, a voltage-dependent Ca(2+) channel blocker. Ru360 (10 microm), a mitochondrial Ca(2+) uptake blocker, and lonidamine (100 microm), a permeability transition pore (PTP) opener, inhibited transient K(Ca) currents similarly to mitochondrial depolarization. In contrast, CGP37157 (10 microm), a mitochondrial Na(+)-Ca(2+) exchange blocker, activated these events. The PTP blockers bongkrekic acid and cyclosporin A both reduced inhibition of transient K(Ca) currents by mitochondrial depolarization. These results indicate that mitochondrial depolarization leads to a voltage-independent elevation in global [Ca(2+)](i) and Ca(2+) spark and transient K(Ca) current inhibition. Data also suggest that mitochondrial depolarization inhibits Ca(2+) sparks and transient K(Ca) currents via PTP opening and a decrease in intramitochondrial [Ca(2+)].

Activation of Ca2+-activated Cl- current by depolarizing steps in rabbit urethral interstitial cells

Interstitial cells were isolated from strips of rabbit urethra for study using the amphotericin B perforated-patch technique. Depolarizing steps to -30 mV or greater activated a Ca2+ current (ICa), followed by a Ca2+-activated Cl- current, and, on stepping back to -80 mV, large Cl- tail currents were observed. Both currents were abolished when the cells were superfused with Ca2+-free bath solution, suggesting that Ca2+ influx was necessary for activation of the Cl- current. The Cl- current was also abolished when Ba2+ was substituted for Ca2+ in the bath or the cell was dialyzed with EGTA (2 mM). The Cl- current was also reduced by cyclopiazonic acid, ryanodine, 2-aminoethoxydiphenyl borate (2-APB), and xestospongin C, suggesting that Ca2+-induced Ca2+ release (CICR) involving both ryanodine and inositol 1,4,5-trisphosphate receptors contributes to its activation.

Phasic contractions of the rat portal vein depend on intracellular Ca2+ release stimulated by depolarization

The phasic contraction to phenylephrine of the rat isolated portal vein was investigated using functional studies. Phasic contractions to phenylephrine and caffeine could be produced after several minutes in Ca(2+)-free Krebs solution, which were inhibited by cyclopiazonic acid or ryanodine. The phenylephrine and caffeine contractions were abolished, however, within 10 min in Ca(2+)-free Krebs solution and by nifedipine. This indicated the Ca(2+) stores were depleted in the absence of Ca(2+) influx through voltage-gated channels. The phasic contraction to phenylephrine was also abolished by niflumic acid even in Ca(2+)-free Krebs solution. This showed that the response depended on intracellular Ca(2+) release stimulated directly by depolarization, resulting from opening of Ca(2+)-activated Cl(-) channels, but did not require Ca(2+) influx. In support of this, K(+)-induced phasic contractions were also produced in Ca(2+)-free Krebs solution. The phenylephrine but not K(+)-induced phasic contractions in Ca(2+)-free Krebs solution were inhibited by ryanodine or cyclopiazonic acid. This would be consistent with Ca(2+) release from more superficial intracellular stores (affected most by these agents), probably by inositol 1,4,5-trisphospate, being required to stimulate the phenylephrine depolarization.

Agonist-induced mitochondrial Ca2+ transients in smooth muscle

We investigated the role of mitochondria (MT) in calcium signaling in a culture of rat aortic smooth muscle cells. We used targeted aequorin to selectively measure [Ca2+] in this organelle. Our results reveal that smooth muscle cell stimulation with agonists causes a large, transient increase in mitochondrial [Ca2+] ([Ca2+]m). This large transient can be blocked with inhibitors of the sarco-endoplasmic reticulum Ca2+-ATPase, suggesting a close relationship between the sarcoplasmic reticulum (SR) and the mitochondria. FCCP completely abolished the response to agonists, and targeted mitochondrial GFP revealed a vast tubular network of MT in these cells. When added before stimulation with ATP, IP3 inhibitors partially blocked the ATP-induced rise in mitochondrial Ca2+ release. The role of the Na+/Ca2+ exchanger (NCX) was examined by removing extracellular Na+. This procedure prevented the decrease in the [Ca2+]m transient normally seen on removal of extracellular Ca2+. We propose a functional linkage of MT and SR dependent on a narrow junctional space between the two organelles in which Ca2+ diffusion is restricted. Approximately half of the mitochondria appear to be associated with the superficial SR, which communicates with the extracellular space via NCX.

Mitochondrial Ca2+ uptake is important over low [Ca2+]i range in arterial smooth muscle

Mitochondrial Ca(2+) uptake is usually thought to occur only when intracellular Ca(2+) concentration ([Ca(2+)](i)) is high. We investigated whether mitochondrial Ca(2+) removal participates in shaping [Ca(2+)](i) signals in arterial smooth muscle over a low [Ca(2+)](i) range. [Ca(2+)](i) was measured using fura 2-loaded, voltage-clamped cells from rat femoral arteries. Both diazoxide and carbonyl cyanide m-chlorophenylhydrazone (CCCP) depolarized the mitochondria. Diazoxide application increased resting [Ca(2+)](i), suggesting that Ca(2+) is sequestered in mitochondria. Over a low [Ca(2+)](i) range, diazoxide and CCCP slowed Ca(2+) removal rate, determined after a brief depolarization. When [Ca(2+)](i) was measured during sustained depolarization to -30 mV, CCCP application increased [Ca(2+)](i). When Ca(2+) transients were repeatedly evoked by caffeine applications, CCCP application elevated resting [Ca(2+)](i). Caffeine-induced Ca(2+) transients were compared before and after CCCP application using the half decay time, or time required to reduce increase in [Ca(2+)](i) by 50% (t((1/2))). CCCP treatment significantly increased t((1/2)). These results suggest that Ca(2+) removal to mitochondria in arterial smooth muscle cells may be important at a low [Ca(2+)](i).

Mitochondrial Ca2+ uptake regulates the excitability of myenteric neurons

We investigated the role of mitochondria in the regulation of intracellular Ca2+ ([Ca2+]i) and excitability of myenteric neurons in guinea pig ileum, using microelectrodes and fura-2 [Ca2+]i measurements. In AH/Type-II neurons, action potentials evoke ryanodine-sensitive increases in [Ca2+]i that activate Ca2+-dependent K+ channels and slow afterhyperpolarizations (AH) lasting approximately 15 sec. Exposure to the protonophore carbonyl cyanide p-(trifluoromethoxy)phenylhydrazone (FCCP; 1 microm) had no significant effect on the membrane potential or resting [Ca2+]i. However, action potentials elicited in the presence of FCCP triggered a sustained (>5 min) increase in [Ca2+]i and a compound hyperpolarization (13.4 +/- 1.5 mV). The respiratory chain blockers antimycin A and rotenone (10 microm) had similar effects that developed more slowly. Depletion of the intracellular Ca2+ stores with thapsigargin (2 microm) or ryanodine (10 microm) greatly attenuated the hyperpolarization caused by FCCP. S/Type-I neurons that do not have AH were hyperpolarized by mitochondrial inhibition independently of action potentials. Blockade of the F0F1 ATPase by oligomycin (10 microm) had variable effects on myenteric neurons. The majority of AH/Type-II neurons were hyperpolarized by oligomycin, most likely by activating ATP-dependent K+ channels. This hyperpolarization was not triggered by action potential firing and not accompanied by an increase in [Ca2+]i. MitoTracker staining revealed a dense mitochondrial network particularly in myenteric AH/Type-II neurons, supporting the importance of mitochondrial Ca2+ buffering in this subset of neurons. The data indicate that mitochondrial uptake of Ca2+ released from the endoplasmic reticulum sets [Ca2+]i and the activity of Ca2+-dependent conductances, thus regulating the excitability of myenteric neurons.

A Ca2+-permeable non-selective cation channel activated by depletion of internal Ca2+ stores in single rabbit portal vein myocytes

In vascular smooth muscle cells many agonists cause the release of Ca2+ ions from internal stores. An important problem concerns the mechanism by which the intracellular stores are refilled subsequent to depletion. In the present study, we describe the properties of a Ca2+-permeable non-selective cation channel current that is activated in rabbit portal vein myocytes by depletion of internal Ca2+ stores. Application of cyclopiazonic acid (CPA), which depletes internal Ca2+ stores, activated whole-cell currents that had a reversal potential (E(r)) of about +50 mV in 1.5 mM external Ca2+ (Ca2+o). In 0 mM Ca2+o, the currents were larger and E(r) was approximately 0 mV. Application of CPA and caffeine during cell-attached recording activated single inward channel currents at negative potentials, which had a slope conductance of 2-3 pS and an E(r) of +20 mV. The slope conductance in 0 and 110 mM Ca2+o was 7 and 1.5 pS, respectively, and E(r) values indicated that these non-selective cation channels are highly permeable to Ca2+ ions. Bath application of the cell-permeant Ca2+ chelator, BAPTA-AM, also activated similar currents, indicating that these channels are not activated by Ca2+. Spontaneous channel currents with similar properties to store-operated channels were observed in some patches. Application of W-7, an inhibitor of the Ca2+-binding protein calmodulin, also activated similar Ca2+-permeable channel currents. In conclusion, it is demonstrated that agents that deplete Ca2+ stores and inhibit calmodulin binding activate Ca2+-permeable non-selective cation channel currents in rabbit portal vein myocytes. These channels may have an important role in vascular smooth muscle in providing an influx of Ca2+ to refill depleted internal Ca2+ stores and appear to possess different characteristics to store-operated channels described in other vascular smooth muscle preparations.

Role of mitochondria in Ca(2+) oscillations and shape of Ca(2+) signals in pancreatic acinar cells

We studied the role of mitochondria in Ca(2+) signals in fura-2 loaded exocrine pancreatic acinar cells. Mitochondrial depolarization in response to carbonylcyanide-p-tryfluoromethoxyphenyl hydrazone or rotenone (assessed by confocal microscopy using rhodamine-123) induced a partial but statistically significant reduction in the decay of Ca(2+) signals under different experimental conditions. Spreading of Ca(2+) waves evoked by the pancreatic secretagogue cholecystokinin cholecystokinin octapeptide was accelerated by mitochondrial inhibitors, whereas the cytosolic Ca(2+) concentration ([Ca(2+)](i)) oscillations in response to physiological levels of this hormone were suppressed by rotenone and carbonylcyanide-p-tryfluoromethoxyphenyl hydrazone. Oligomycin, an inhibitor of mitochondrial ATP synthase, did no affect either propagation of calcium waves nor [Ca(2+)](i) oscillations. Individual mitochondria of rhod-2 loaded acinar cells showed heterogeneous matrix Ca(2+) concentration increases in response to oscillatory and maximal levels of cholecystokinin octapeptide. On the other hand, using Ba(2+) for unequivocal study of capacitative calcium entry we found that mitochondrial inhibitors did not affect this process. Our results show that although the role of mitochondria as a Ca(2+) clearing system in exocrine cells is quantitatively secondary, they play an essential role in the spatial propagation of Ca(2+) waves and in the development of [Ca(2+)](i) oscillations.

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.

Invited review: arteriolar smooth muscle mechanotransduction: Ca(2+) signaling pathways underlying myogenic reactivity

The smooth muscle of arterioles responds to an increase in intraluminal pressure with vasoconstriction and with vasodilation when pressure is decreased. Such myogenic vasoconstriction provides a level of basal tone that enables arterioles to appropriately adjust diameter in response to neurohumoral stimuli. Key in this process of mechanotransduction is the role of changes in intracellular Ca(2+). However, it is becoming clear that considerable complexity exists in the spatiotemporal characteristics of the Ca(2+) signal and that changes in intracellular Ca(2+) may play roles other than direct effects on the contractile process via activation of myosin light-chain phosphorylation. The involvement of Ca(2+) may extend to modulation of ion channels and release of Ca(2+) from the sarcoplasmic reticulum, alterations in Ca(2+) sensitivity, and coupling between cells within the vessel wall. The purpose of this brief review is to summarize the current literature relating to Ca(2+) and the arteriolar myogenic response. Consideration is given to coupling of Ca(2+) changes to the mechanical stimuli, sources of Ca(2+), involvement of ion channels, and spatiotemporal aspects of intracellular Ca(2+) signaling.

Calcium-dependent inhibition of L, N, and P/Q Ca2+ channels in chromaffin cells: role of mitochondria

The hypothesis that the buffering of Ca(2+) by mitochondria could affect the Ca(2+)-dependent inhibition of voltage-activated Ca(2+) channels, (I(Ca)), was tested in voltage-clamped bovine adrenal chromaffin cells. The protonophore carbonyl cyanide m-chlorophenyl-hydrazone (CCCP), the blocker of the Ca(2+) uniporter ruthenium red (RR), and a combination of oligomycin plus rotenone were used to interfere with mitochondrial Ca(2+) buffering. In cells dialyzed with an EGTA-free solution, peak I(Ca) generated by 20 msec pulses to 0 or +10 mV, applied at 15 sec intervals, from a holding potential of -80 mV, decayed rapidly after superfusion of cells with 2 microm CCCP (tau = 16.7 +/- 3 sec; n = 8). In cells dialyzed with 14 mm EGTA, CCCP did not provoke I(Ca) loss. Cell dialysis with 4 microm ruthenium red or cell superfusion with oligomycin (3 microm) plus rotenone (4 microm) also accelerated the decay of I(Ca). After treatment with CCCP, decay of N- and P/Q-type Ca(2+) channel currents occurred faster than that of L-type Ca(2+) channel currents. These data are compatible with the idea that the elevation of the bulk cytosolic Ca(2+) concentration, [Ca(2+)](c), causes the inhibition of L- and N- as well as P/Q-type Ca(2+) channels expressed by bovine chromaffin cells. This [Ca(2+)](c) signal appears to be tightly regulated by rapid Ca(2+) uptake into mitochondria. Thus, it is plausible that mitochondria might efficiently regulate the activity of L, N, and P/Q Ca(2+) channels under physiological stimulation conditions of the cell.

Intracellular calcium dynamics--sparks of insight

Calcium ions are of key importance in a large number of cellular functions. In the past decade a large variety of cells have been found to show localized increases in the intracellular calcium concentration named calcium sparks. In this brief review, the methodology of detecting calcium sparks by confocal microscopy is summarized. Some of the properties of calcium sparks in muscle (cardiac, skeletal and smooth muscles), neurons, nerve terminals and oocytes aredescribed. Speculations are put forward regarding their possible role in microcontrol of cell function.

Calcium-dependent inhibition of L, N, and P/Q Ca2+ channels in chromaffin cells: role of mitochondria

The hypothesis that the buffering of Ca(2+) by mitochondria could affect the Ca(2+)-dependent inhibition of voltage-activated Ca(2+) channels, (I(Ca)), was tested in voltage-clamped bovine adrenal chromaffin cells. The protonophore carbonyl cyanide m-chlorophenyl-hydrazone (CCCP), the blocker of the Ca(2+) uniporter ruthenium red (RR), and a combination of oligomycin plus rotenone were used to interfere with mitochondrial Ca(2+) buffering. In cells dialyzed with an EGTA-free solution, peak I(Ca) generated by 20 msec pulses to 0 or +10 mV, applied at 15 sec intervals, from a holding potential of -80 mV, decayed rapidly after superfusion of cells with 2 microm CCCP (tau = 16.7 +/- 3 sec; n = 8). In cells dialyzed with 14 mm EGTA, CCCP did not provoke I(Ca) loss. Cell dialysis with 4 microm ruthenium red or cell superfusion with oligomycin (3 microm) plus rotenone (4 microm) also accelerated the decay of I(Ca). After treatment with CCCP, decay of N- and P/Q-type Ca(2+) channel currents occurred faster than that of L-type Ca(2+) channel currents. These data are compatible with the idea that the elevation of the bulk cytosolic Ca(2+) concentration, [Ca(2+)](c), causes the inhibition of L- and N- as well as P/Q-type Ca(2+) channels expressed by bovine chromaffin cells. This [Ca(2+)](c) signal appears to be tightly regulated by rapid Ca(2+) uptake into mitochondria. Thus, it is plausible that mitochondria might efficiently regulate the activity of L, N, and P/Q Ca(2+) channels under physiological stimulation conditions of the cell.

Chronic reduction in complex I function alters calcium signaling in SH-SY5Y neuroblastoma cells

Sporadic, non-familial Parkinson's disease is characterized by a 15-30% reduction in complex I activity of the electron transport chain. A pharmacological model of reduced complex I activity was created by prolonged treatment of SH-SY5Y cells with low doses (5-20 nM) of rotenone, a selective inhibitor of complex I. Short-term (less than 2 week) exposure to rotenone did not influence calcium signaling, production of reactive oxygen species, or mitochondrial morphology. However, following 2 weeks of rotenone exposure, SH-SY5Y cells showed unusual calcium dynamics, specifically multiple calcium responses to carbachol, a muscarinic agonist. These secondary calcium responses were not seen in control SH-SY5Y cells and were dependent upon calcium influx. Mitochondrial membrane potential was also reduced in low dose rotenone-treated cells. These results demonstrate that a chronic, partial reduction in complex I activity, such as that seen in Parkinson's disease, can alter cell signaling events and perhaps increase the susceptibility of cells to calcium overload and subsequent cell death.

Direct visualization of sarcoplasmic reticulum regions discharging Ca(2+)sparks in vascular myocytes

Localized Ca(2+)-release events, Ca(2+)sparks, have been suggested to be the 'elementary building blocks' of the calcium signalling system in all types of muscles. In striated muscles these occur at regular intervals along the fibre corresponding to the sarcomeric structures which do not exist in smooth muscle. We showed previously that in visceral and vascular myocytes Ca(2+)sparks occurred much more frequently at certain sites (frequent discharge sites [FDSs]). In this paper, we have related the position of FDSs to the distribution of the sarcoplasmic reticulum in the same living myocyte. The three-dimensional distribution of the SR in freshly isolated rabbit portal vein myocytes was visualized by means of high-resolution confocal imaging after staining with DiOC(6)and/or BODIPY TR-X ryanodine. Both fluorochromes revealed a similar staining pattern indicating a helical arrangement of well-developed superficial SR which occupied about 6% of the cell volume. Computing the frequency of spontaneous Ca(2+)sparks detected by means of fluo-4 fluorescence revealed that in about 70% of myocytes there was only one major FDS located on a prominent portion of superficial SR network usually within 1-2 microm of the nuclear envelope, although a few sparks occurred at other sites scattered generally in superficial locations throughout the cell. Polarized mitochondria were readily identified by accumulation of tetramethylrhodamine ethyl ester (TMRE). These were closely associated with the SR network in extra-nuclear regions. TMRE staining, however, failed to reveal any mitochondria near the FDS-related SR element. When observed, propagating [Ca(2+)](i)waves and associated myocyte contractions were initiated at FDSs. This study provide first insight into the three-dimensional arrangement of the SR in living smooth muscle cells and relates the peculiarity of the structural organization of the myocyte to the features of Ca(2+)signalling at subcellular level.

Calcium signalling in sarcoplasmic reticulum, cytoplasm and mitochondria during activation of rabbit aorta myocytes

This study investigated the relationship between cytoplasmic, mitochondrial, and sarcoplasmic reticulum (SR) [Ca(2+)] in rabbit aorta smooth muscle cells, following cell activation. Smooth muscle cells were loaded with the Ca(2+)-sensitive fluorescent indicator Mag-Fura-2-AM, and then either permeabilized by exposure to saponin, or dialyzed with a patch pipette in the whole-cell configuration to remove cytoplasmic indicator. When the intracellular solution contained millimolar EGTA or BAPTA, activation of SR Ca(2+)release through IP(3)or ryanodine receptors induced a decrease in the [Ca(2+)] reported by Mag-Fura-2. However, when EGTA was present at < or =100 microM, the same stimuli caused an increase in the [Ca(2+)] reported by Mag-Fura-2. The increase in [Ca(2+)] caused by phenylephrine or caffeine was delayed, and prolonged, with respect to the cytoplasmic Ca(2+)transient. Evidence is presented that this Mag-Fura-2 signal reflected a rise in mitochondrial [Ca(2+)]. Agents that inhibit mitochondrial function, such as FCCP or cyanide in combination with oligomycin B, converted the increase in organelle Mag-Fura-2 fluorescence to a decrease, while also prolonging the cytoplasmic Ca(2+)transient. There was considerable similarity between the localization of Mag-Fura-2 fluorescence and the mitochondria-selective indicator tetramethylrhodamine ethyl ester. Thus, we propose that there is close functional integration between the SR and mitochondria in aorta smooth muscle cells, with mitochondria taking up Ca(2+)from the cytoplasm following cell activation.

Mitochondrial Ca2+ homeostasis during Ca2+ influx and Ca2+ release in gastric myocytes from Bufo marinus

1. The Ca(2+)-sensitive fluorescent indicator rhod-2 was used to monitor mitochondrial Ca2+ concentration ([Ca2+]m) in gastric smooth muscle cells from Bufo marinus. In some studies, fura-2 was used in combination with rhod-2, allowing simultaneous measurement of cytoplasmic Ca2+ concentration ([Ca2+]i) and [Ca2+]m, respectively. 2. During a short train of depolarizations, which causes Ca2+ influx from the extracellular medium, there was an increase in both [Ca2+]i and [Ca2+]m. The half-time (t1/2) to peak for the increase in [Ca2+]m was considerably longer than the t1/2 to peak for the increase in [Ca2+]i. [Ca2+]m remained elevated for tens of seconds after [Ca2+]i had returned to its resting value. 3. Stimulation with caffeine, which causes release of Ca2+ from the sarcoplasmic reticulum (SR), also produced increases in both [Ca2+]i and [Ca2+]m. The values of t1/2 to peak for the increase in [Ca2+] in both cytoplasm and mitochondria were similar; however, [Ca2+]i returned to baseline values much faster than [Ca2+]m. 4. Using a wide-field digital imaging microscope, changes in [Ca2+]m were monitored within individual mitochondria in situ, during stimulation of Ca2+ influx or Ca2+ release from the SR. 5. Mitochondrial Ca2+ uptake during depolarizing stimulation caused depolarization of the mitochondrial membrane potential. The mitochondrial membrane potential recovered considerably faster than the recovery of [Ca2+]m. 6. This study shows that Ca2+ influx from the extracellular medium and Ca2+ release from the SR are capable of increasing [Ca2+]m in smooth muscle cells. The efflux of Ca2+ from the mitochondria is a slow process and appears to be dependent upon the amount of Ca2+ in the SR.

Neuronal Ca2+ -activated Cl- channels--homing in on an elusive channel species

Ca2+ -activated Cl- channels control electrical excitability in various peripheral and central populations of neurons. Ca2+ influx through voltage-gated or ligand-operated channels, as well as Ca2+ release from intracellular stores, have been shown to induce substantial Cl- conductances that determine the response to synaptic input, spike rate, and the receptor current of various kinds of neurons. In some neurons, Ca2+ -activated Cl- channels are localized in the dendritic membrane, and their contribution to signal processing depends on the local Cl- equilibrium potential which may differ considerably from those at the membranes of somata and axons. In olfactory sensory neurons, the channels are expressed in ciliary processes of dendritic endings where they serve to amplify the odor-induced receptor current. Recent biophysical studies of signal transduction in olfactory sensory neurons have yielded some insight into the functional properties of Ca2+ -activated Cl- channels expressed in the chemosensory membrane of these cells. Ion selectivity, channel conductance, and Ca2+ sensitivity have been investigated, and the role of the channels in the generation of receptor currents is well understood. However, further investigation of neuronal Ca2+ -activated Cl- channels will require information about the molecular structure of the channel protein, the regulation of channel activity by cellular signaling pathways, as well as the distribution of channels in different compartments of the neuron. To understand the physiological role of these channels it is also important to know the Cl- equilibrium potential in cells or in distinct cell compartments that express Ca2+ -activated Cl- channels. The state of knowledge about most of these aspects is considerably more advanced in non-neuronal cells, in particular in epithelia and smooth muscle. This review, therefore, collects results both from neuronal and from non-neuronal cells with the intent of facilitating research into Ca2+ -activated Cl- channels and their physiological functions in neurons.

Physiology of rat retinal pericytes: modulation of ion channel activity by serum-derived molecules

1. Pericytes, which are contractile cells located on the outer wall of microvessels, are thought to be particularly important in the retina where the ratio of these cells to vascular endothelial cells is the highest of any tissue. Retinal pericytes are of interest since they may regulate capillary blood flow and because their selective loss is an early event in diabetic retinopathy, which is a common sight-threatening disorder associated with dysfunction of the blood-retinal barrier. 2. Although a breakdown in the vascular endothelial barrier is a frequent pathophysiological event, knowledge of the effects of blood-derived molecules on pericyte function is limited. Based on the premise that ion channels play a vital role in cellular function, we examined the effect of serum on the ionic currents of retinal pericytes. To do this, we used the perforated-patch configuration of the patch-clamp technique to monitor the whole-cell currents of pericytes located on freshly isolated rat retinal microvessels. 3. Exposure to serum reversibly activated inward and outward currents in virtually all of the sampled retinal pericytes. Two types of sustained conductances were induced by serum. These were a calcium-permeable non-specific cation (NSC) current and a voltage-dependent potassium current. In addition, exposure to serum increased the activity of chloride channels which caused transient depolarizing currents. 4. Associated with the activation of these conductances, the membrane potential showed a sustained decrease of 10 +/- 2 mV from -56 mV to -46 mV and, also, transient depolarizations to near -30 mV. The serum-induced depolarizations can activate the voltage-gated calcium channels expressed by the retinal pericytes. 5. Calcium-permeable NSC channels appear to play a critical role in the response of pericytes to serum-derived molecules. Consistent with this, activation of the chloride and potassium channels was sensitive to SK&F 96365, which is a blocker of NSC channels. In addition, chloride and potassium channel activation was dependent on extracellular calcium. 6. The effects of serum on the activity of channels in retinal pericytes were qualitatively mimicked by insulin-like growth factor-1 (IGF-1), which is a normal constituent of the blood. 7. There are significant differences in the effects of serum on retinal pericytes compared with vascular smooth muscle cells. Serum activated sustained conductances in retinal pericytes but not in the vascular smooth muscle cells. This suggests a fundamental difference in the mechanisms by which serum-derived molecules affect these two types of cells. 8. We conclude that serum-derived molecules, such as IGF-1, can activate several types of ion channels in retinal pericytes. These changes in channel activity are likely to influence pericyte function at sites of a breakdown in the blood-retinal barrier.

Novel regulation of the A-type K+ current in murine proximal colon by calcium-calmodulin-dependent protein kinase II

1. The kinetics of inactivation of delayed rectifier K+ current in murine colonic myocytes differed in amphotericin-permeabilized patch and conventional patch clamp. The difference was accounted for by Ca2+ buffering. 2. Calcium-calmodulin-dependent protein kinase II (CaMKII) inhibitors increased the rate of inactivation and slowed recovery from inactivation of the outward current. This was seen in single steps and in the envelope of the current tails. The effect was largely on the TEA-insensitive component of current. 3. Dialysis of myocytes with autothiophosphorylated CaMKII slowed inactivation. This effect was reversed by addition of CaMKII inhibitor. 4. Antibodies revealed CaMKII-like immunoreactivity in murine colonic myocytes and other cells. Immunoblots identified a small protein with CaMKII-like immunoreactivity in homogenates of colonic muscle. 5. We conclude that CaMKII regulates delayed rectifier K+ currents in murine colonic myocytes. The changes in the delayed rectifier current may participate in the Ca2+-dependent regulation of gastrointestinal motility.

Excitation-contraction coupling in gastrointestinal and other smooth muscles

The main contributors to increases in [Ca2+]i and tension are the entry of Ca2+ through voltage-dependent channels opened by depolarization or during action potential (AP) or slow-wave discharge, and Ca2+ release from store sites in the cell by the action of IP3 or by Ca(2+)-induced Ca(2+)-release (CICR). The entry of Ca2+ during an AP triggers CICR from up to 20 or more subplasmalemmal store sites (seen as hot spots, using fluorescent indicators); Ca2+ waves then spread from these hot spots, which results in a rise in [Ca2+]i throughout the cell. Spontaneous transient releases of store Ca2+, previously detected as spontaneous transient outward currents (STOCs), are seen as sparks when fluorescent indicators are used. Sparks occur at certain preferred locations--frequent discharge sites (FDSs)--and these and hot spots may represent aggregations of sarcoplasmic reticulum scattered throughout the cytoplasm. Activation of receptors for excitatory signal molecules generally depolarizes the cell while it increases the production of IP3 (causing calcium store release) and diacylglycerols (which activate protein kinases). Activation of receptors for inhibitory signal molecules increases the activity of protein kinases through increases in cAMP or cGMP and often hyperpolarizes the cell. Other receptors link to tyrosine kinases, which trigger signal cascades interacting with trimeric G-protein systems.

Release of Ca2+ from the sarcoplasmic reticulum increases mitochondrial [Ca2+] in rat pulmonary artery smooth muscle cells

1. The Ca2+-sensitive fluorescent indicator rhod-2 was used to measure mitochondrial [Ca2+] ([Ca2+]m) in single smooth muscle cells from the rat pulmonary artery, while simultaneously monitoring cytosolic [Ca2+] ([Ca2+]i) with fura-2. 2. Application of caffeine produced an increase in [Ca2+]i and also increased [Ca2+]m. The increase in [Ca2+]m occurred after the increase in [Ca2+]i, and remained elevated for a considerable time after [Ca2+]i had returned to resting values. 3. The protonophore carbonyl cyanide p-(trifluoromethoxy)phenylhydrazone (FCCP), which causes the mitochondrial membrane potential to collapse, markedly attenuated the increase in [Ca2+]m following caffeine application and also increased the half-time for recovery of [Ca2+]i to resting values. 4. Activation of purinoceptors with ATP also produced increases in both [Ca2+]i and [Ca2+]m in these smooth muscle cells. In some cells, oscillations in [Ca2+]i were observed during ATP application, which produced corresponding oscillations in [Ca2+]m and membrane currents. 5. This study provides direct evidence that Ca2+ release from the sarcoplasmic reticulum, either through ryanodine or inositol 1,4, 5-trisphosphate (InsP3) receptors, increases both cytosolic and mitochondrial [Ca2+] in smooth muscle cells. These results have potential implications both for the role of mitochondria in Ca2+ regulation in smooth muscle, and for understanding how cellular metabolism is regulated.

Cellular and molecular basis for electrical rhythmicity in gastrointestinal muscles

Regulation of gastrointestinal (GI) motility is intimately coordinated with the modulation of ionic conductance expressed in GI smooth muscle and nonmuscle cells. Interstitial cells of Cajal (ICC) act as pacemaker cells and possess unique ionic conductances that trigger slow wave activity in these cells. The slow wave mechanism is an exclusive feature of ICC: Smooth muscle cells may lack the basic ionic mechanisms necessary to generate or regenerate slow waves. The molecular identification of the components for these conductances provides the foundation for a complete understanding of the ionic basis for GI motility. In addition, this information will provide a basis for the identification or development of therapeutics that might act on these channels. It is much easier to study these conductances and develop blocking drugs in expression systems than in native GI muscle cells. This review focuses on the relationship between ionic currents in native GI smooth muscle cells and ICC and their molecular counterparts.

Hypoxia and smooth muscle function: key regulatory events during metabolic stress

Hypoxia rapidly reduces force in many smooth muscles and we review recent data that shed light on the mechanisms involved. As many regulated cellular processes are integrated to co-ordinate smooth muscle contractility, the processes responsible for decreased force output with altered metabolism are also likely to be many, acting in concert, rather than the actions of one altered parameter. Nevertheless the aim of this study is to elucidate the hierarchical series of events that contribute to reduced smooth muscle force production during altered metabolism. We conclude that in many phasic smooth muscles the decrease in force can be attributed to impaired electro-mechanical coupling whereby the Ca2+ transient is reduced. A direct effect of hypoxia on the Ca2+ channel may be of key importance. In tonic vascular smooth muscles KATP channels may also play a role in the integrated functional responses to hypoxia. There are also many examples of force being reduced, in tonically activated preparations, without a fall in steady-state Ca2+; indeed it usually increases. We examine the roles of altered [ATP], pH, myosin phosphorylation, inorganic phosphate and proteolytic activity on the [Ca2+]-force relationship during hypoxia. We find no defining force-inhibitory role for any one factor acting alone, and suggest that force most probably falls as a result of the combination of myriad factors.