Epithelial calcium channels: from identification to function and regulation

The epithelial calcium channels TRPV5 and TRPV6 have been studied extensively in the epithelial tissues controlling Ca(2+) homeostasis and exhibit a range of distinctive properties that distinguish them from other transient receptor potential (TRP) channels. These two novel members of the superfamily of TRP channels were cloned from vitamin D-responsive epithelia: kidney, small intestine and placenta, and identified subsequently in tissues like pancreas, bone and prostate. This review addresses the unique properties of these highly Ca(2+)-selective channels and highlights their implications for the process of transepithelial Ca(2+) transport.     

Functional TRPV4 channels are expressed in mouse skeletal muscle and can modulate resting Ca2+ influx and muscle fatigue

Skeletal muscle contraction is basically controlled by Ca(2+) release and its reuptake into the sarcoplasmic reticulum. However, the long-term maintenance of muscle function requires an additional Ca(2+) influx from extracellular. Several mechanisms seem to contribute to the latter process, such as store-operated Ca(2+) entry, stretch-activated Ca(2+) influx and resting Ca(2+) influx. Candidate channels that may control Ca(2+) influx into muscle fibers are the STIM proteins, Orai, and the members of the transient receptor potential (TRP) family of cation channels. Here we show that TRPV4, an osmo-sensitive cation channel of the vanilloid subfamily of TRP channels is functionally expressed in mouse skeletal muscle. Western blot analysis showed the presence of TRPV4-specific bands at about 85 and 100?kDa in all tested muscles. The bands were absent when muscle proteins from TRPV4 deficient mice were analyzed. Using the manganese quench technique, we studied the resting influx of divalent cations into isolated wild-type muscle fibers. The specific TRPV4-channel activator 4α-phorbol-12,13-didecanoate (4α-PDD) stimulated resting influx by about 60% only in wild-type fibers. Electrical stimulation of soleus muscles did not reveal changes in isometric twitch contractions upon application of 4α-PDD, but tetanic contractions (at 120?Hz) were slightly increased by about 15%. When soleus muscles were stimulated with a fatigue protocol, muscle fatigue was significantly attenuated in the presence of 4α-PDD. The latter effect was not observed with muscles from TRPV4(-/-) mice. We conclude that TRPV4 is functionally expressed in mouse skeletal muscle and that TRPV4 activation modulates resting Ca(2+) influx and muscle fatigue.     

The TRPV4 channel: structure-function relationship and promiscuous gating behaviour

Transient receptor potential (TRP) channels provide an enormous variability of Ca(2+) influx mechanisms triggered by a wide range of stimuli. In this review, we discuss the activation properties of the Ca(2+)- and Mg(2+)-permeable TRP channel of the vanilloid subfamily TRPV4. This channel is activated by various physical and chemical stimuli, such as cell swelling, heat, phorbols and, probably, by endogenous ligands, which are able to induce Ca(2+) entry. Not much is known about the regulation of this channel. We will refer only to a mechanism of Ca(2+)-dependent inhibition of TRPV4. Possible functional roles of this channel will be correlated with its observed expression pattern. Finally, we discuss the structural determinants of TRPV4 channel function.     

Expression and functional characterization of transient receptor potential vanilloid-related channel 4 (TRPV4) in rat cortical astrocytes

Cell-cell communication in astroglial syncytia is mediated by intracellular Ca(2+) ([Ca(2+)](i)) responses elicited by extracellular signaling molecules as well as by diverse physical and chemical stimuli. Despite the evidence that astrocytic swelling promotes [Ca(2+)](i) elevation through Ca(2+) influx, the molecular identity of the channel protein underlying this response is still elusive. Here we report that primary cultured cortical astrocytes express the transient receptor potential vanilloid-related channel 4 (TRPV 4), a Ca(2+)-permeable cation channel gated by a variety of stimuli, including cell swelling. Immunoblot and confocal microscopy analyses confirmed the presence of the channel protein and its localization in the plasma membrane. TRPV4 was functional because the selective TRPV4 agonist 4-alpha-phorbol 12,13-didecanoate (4alphaPDD) activated an outwardly rectifying cation current with biophysical and pharmacological properties that overlapped those of recombinant human TRPV4 expressed in COS cells. Moreover, 4alphaPDD and hypotonic challenge promoted [Ca(2+)](i) elevation mediated by influx of extracellular Ca(2+). This effect was abolished by low micromolar concentration of the TRPV4 inhibitor Ruthenium Red. Immunofluorescence and immunogold electron microscopy of rat brain revealed that TRPV4 was enriched in astrocytic processes of the superficial layers of the neocortex and in astrocyte end feet facing pia and blood vessels. Collectively, these data indicate that cultured cortical astroglia express functional TRPV4 channels. They also demonstrate that TRPV4 is particularly abundant in astrocytic membranes at the interface between brain and extracerebral liquid spaces. Consistent with its roles in other tissues, these results support the view that TRPV4 might participate in astroglial osmosensation and thus play a key role in brain volume homeostasis.     

Calcium absorption across epithelia

Ca(2+) is an essential ion in all organisms, where it plays a crucial role in processes ranging from the formation and maintenance of the skeleton to the temporal and spatial regulation of neuronal function. The Ca(2+) balance is maintained by the concerted action of three organ systems, including the gastrointestinal tract, bone, and kidney. An adult ingests on average 1 g Ca(2+) daily from which 0.35 g is absorbed in the small intestine by a mechanism that is controlled primarily by the calciotropic hormones. To maintain the Ca(2+) balance, the kidney must excrete the same amount of Ca(2+) that the small intestine absorbs. This is accomplished by a combination of filtration of Ca(2+) across the glomeruli and subsequent reabsorption of the filtered Ca(2+) along the renal tubules. Bone turnover is a continuous process involving both resorption of existing bone and deposition of new bone. The above-mentioned Ca(2+) fluxes are stimulated by the synergistic actions of active vitamin D (1,25-dihydroxyvitamin D(3)) and parathyroid hormone. Until recently, the mechanism by which Ca(2+) enter the absorptive epithelia was unknown. A major breakthrough in completing the molecular details of these pathways was the identification of the epithelial Ca(2+) channel family consisting of two members: TRPV5 and TRPV6. Functional analysis indicated that these Ca(2+) channels constitute the rate-limiting step in Ca(2+)-transporting epithelia. They form the prime target for hormonal control of the active Ca(2+) flux from the intestinal lumen or urine space to the blood compartment. This review describes the characteristics of epithelial Ca(2+) transport in general and highlights in particular the distinctive features and the physiological relevance of the new epithelial Ca(2+) channels accumulating in a comprehensive model for epithelial Ca(2+) absorption.     

Histamine-induced Ca(2+) influx via the PLA(2)/lipoxygenase/TRPV1 pathway in rat sensory neurons

Histamine is known to excite a subset of C-fibers and cause itch sensation. Despite its well-defined excitatory action on sensory neurons, intracellular signaling mechanisms are not understood. Previously, we demonstrated that bradykinin excited sensory neurons by activating TRPV1 via the phospholipase A(2) (PLA(2)) and lipoxygenase (LO) pathway. We, thus, hypothesized that histamine excited sensory neurons via the PLA(2)/LO/TRPV1 pathway. Application of histamine elicited a rapid increase in intracellular Ca(2+) ([Ca(2+)](i)) that desensitized slowly in cultured dorsal root ganglion neurons. Histamine-induced [Ca(2+)](i) was dependent on extracellular Ca(2+) and inhibited by capsazepine and by SC0030, competitive antagonists of TRPV1. Quinacrine and nordihydroguaiaretic acid, a PLA(2) and an LO inhibitor, respectively, blocked the histamine-induced Ca(2+) influx in sensory neurons, while indomethacin (a cyclooxygenase inhibitor) did not. We thus conclude that histamine activates TRPV1 after stimulating the PLA(2)/LO pathway, leading to the excitation of sensory neurons. These results further provide an idea for potential use of TRPV1 antagonists as anti-itch drugs.     

The mechanosensitive nature of TRPV channels

Transient receptor potential vanilloid (TRPV) channels are widely expressed in both sensory and nonsensory cells. Whereas the channels display a broad diversity to activation by chemical and physical stimuli, activation by mechanical stimuli is common to many members of this group in both lower and higher organisms. Genetic screening in Caenorhabditis elegans has demonstrated an essential role for two TRPV channels in sensory neurons. OSM-9 and OCR-2, for example, are essential for both osmosensory and mechanosensory (nose-touch) behaviors. Likewise, two Drosophila TRPV channels, NAN and IAV, have been shown to be critical for hearing by the mechanosensitive chordotonal organs located in the flys antennae. The mechanosensitive nature of the channels appears to be conserved in higher organisms for some TRPV channels. Two vertebrate channels, TRPV2 and TRPV4, are sensitive to hypotonic cell swelling, shear stress/fluid flow (TRPV4), and membrane stretch (TRPV2). In the osmosensing neurons of the hypothalamus (circumventricular organs), TRPV4 appears to function as an osmoreceptor, or part of an osmoreceptor complex, in control of vasopressin release, whereas in inner ear hair cells and vascular baroreceptors a mechanosensory role is suggestive, but not demonstrated. Finally, in many nonsensory cells expressing TRPV4, such as vascular endothelial cells and renal tubular epithelial cells, the channel exhibits well-developed local mechanosensory transduction processes where both cell swelling and shear stress/fluid flow lead to channel activation. Hence, many TRPV channels, or combinations of TRPV channels, display a mechanosensitive nature that underlies multiple mechanosensitive processes from worms to mammals.     

Heat sensitization in skin and muscle nociceptors expressing distinct combinations of TRPV1 and TRPV2 protein

Recordings were made from small and medium diameter dorsal root ganglia (DRG) neurons that expressed transient receptor potential (TRP) proteins. Physiologically characterized skin nociceptors expressed either TRPV1 (type 2) or TRPV2 (type 4) in isolation. Other nociceptors co-expressed both TRP proteins and innervated deep tissue sites (gastrocnemius muscle, distal colon; type 5, type 8) and skin (type 8). Subpopulations of myelinated (type 8) and unmyelinated (type 5) nociceptors co-expressed both TRPs. Cells that expressed TRPV1 were excellent transducers of intense heat. Proportional inward currents were obtained from a threshold of approximately 46.5 to approximately 56 degrees C. In contrast, cells expressing TRPV2 alone (52 degrees C threshold) did not reliably transduce the intensity of thermal events. Studies were undertaken to assess the capacity of skin and deep nociceptors to exhibit sensitization to repeated intense thermal stimuli [heat-heat sensitization (HHS)]. Only nociceptors that expressed TRPV2, alone or in combination with TRPV1, exhibited HHS. HHS was shown to be Ca(2+) dependent in either case. Intracellular Ca(2+) dependent pathways to HHS varied with the pattern of TRP protein expression. Cells co-expressing both TRPs modulated heat reactivity through serine/threonine phosphorylation or PLA(2)-dependent pathways. Cells expressing only TRPV2 may have relied on tyrosine kinases for HHS. We conclude that heat sensitization in deep and superficial capsaicin and capsaicin-insensitive C and Adelta nociceptors varies with the distribution of TRPV1 and TRPV2 proteins. The expression pattern of these proteins are specific to subclasses of physiologically identified C and A fiber nociceptors with highly restricted tissue targets.     

TRPV1 shows dynamic ionic selectivity during agonist stimulation

Transient receptor potential vanilloid 1 (TRPV1) is an ion channel that is gated by noxious heat, capsaicin and other diverse stimuli. It is a nonselective cation channel that prefers Ca2+ over Na+. These permeability characteristics, as in most channels, are widely presumed to be static. On the contrary, we found that activation of native or recombinant rat TRPV1 leads to time- and agonist concentration-dependent increases in relative permeability to large cations and changes in Ca2+ permeability. Using the substituted cysteine accessibility method, we saw that these changes were attributable to alterations in the TRPV1 selectivity filter. TRPV1 agonists showed different capabilities for evoking ionic selectivity changes. Furthermore, protein kinase C-dependent phosphorylation of Ser800 in the TRPV1 C terminus potentiated agonist-evoked ionic selectivity changes. Thus, the qualitative signaling properties of TRPV1 are dynamically modulated during channel activation, a process that probably shapes TRPV1 participation in pain, cytotoxicity and neurotransmitter release.     

Interaction of the epithelial Ca2+ channels TRPV5 and TRPV6 with the intestine- and kidney-enriched PDZ protein NHERF4

The epithelial Ca²⁺ channels TRPV5 and TRPV6 constitute the apical Ca²⁺ influx pathway in epithelial Ca²⁺ transport. PDZ proteins have been demonstrated to play a crucial role in the targeting or anchoring of ion channels and transporters in the apical domain of the cell. In this study, we describe the identification of NHERF4 (Na-P_i Cap2/IKEPP/PDZK2) as a novel TRPV5- and TRPV6-associated PDZ protein. NHERF4 was identified using two separate yeast two-hybrid screens with the carboxyl termini of TRPV5 and TRPV6 as bait. Binding of the carboxyl termini of TRPV5 and TRPV6 with NHERF4 was confirmed by GST pull-down assays using in-vitro-translated NHERF4 or lysates of Xenopus laevis oocytes expressing NHERF4. Furthermore, the interaction was confirmed by GST pull-down and co-immunoprecipitation assays using in-vitro-translated full-length TRPV5 and Xenopus oocytes or HEK293 cells co-expressing NHERF4 and TRPV5/TRPV6, respectively. The fourth PDZ domain of NHERF4 was sufficient for the interaction, although PDZ domain 1 also contributed to the binding. The binding site for NHERF4 localized in a conserved region in the carboxyl terminus of TRPV5 and was distinct from the binding site of the PDZ protein NHERF2. NHERF4 predominantly localized at the plasma membrane of X. laevis oocytes and HeLa cells. This localization was independent of the presence of TRPV5. Therefore, we hypothesize a role for this novel PDZ protein as a putative plasma membrane scaffold for the epithelial Ca²⁺ channels.     

Transient receptor potential V1 regulates activation and modulation of transient receptor potential A1 by Ca2+

The transient receptor potential A1 (TRPA1) channel contributes to nociceptive signaling in certain pain models. It has been suggested that Ca(2+), which activates and modulates TRPA1, could play a critical regulatory role in this process. Since TRPA1 and transient receptor potential V1 (TRPV1) channels are co-expressed and interact in neurons, we investigated whether activation and modulation of TRPA1 by Ca(2+) is regulated by TRPV1. Cell-attached recordings showed that TRPA1 is activated by extracellular Ca(2+) ([Ca(2+)](e)) in concentration-response fashion. This activation, especially by 2 mM [Ca(2+)](e) was substantially suppressed by co-expression with TRPV1. Inside-out recordings demonstrated that intracellular Ca(2+) ([Ca(2+)](i))-triggered activation of TRPA1 was attenuated by the presence of TRPV1 only at 2 mM [Ca(2+)](e), but not in Ca(2+)-free conditions. Further, depletion of internal Ca(2+) stores by thapsigargin generated TRPA1-mediated currents, which is affected by TRPV1 in both Chinese hamster ovary cells and sensory neurons. Since mustard oil current (I(MO)) is modulated by [Ca(2+)](e), we next examined whether alterations in the Ca(2+)-permeability of TRPV1 by mutating Y671 effect I(MO) properties. First it was demonstrated that the mutations in TRPV1 did not affect association of the TRPA1 and TRPV1 channels. However, these TRPV1 mutations, particularly Y671K, altered the following characteristics of TRPA1: magnitude of I(MO) in presence and absence of [Ca(2+)](e); the influence of [Ca(2+)](e) on the voltage-dependency of I(MO), and open probability of single-channel I(MO). In summary, activation of TRPA1 by [Ca(2+)](e) and [Ca(2+)](i) is controlled by the TRPV1 channel, and characteristics of I(MO) depend on Ca(2+) permeability of the TRPV1 channel.     

TRPV channels as thermosensory receptors in epithelial cells

Temperature-sensitive transient receptor potential vanilloid (TRPV) ion channels are critical contributors to normal pain and temperature sensation and therefore represent attractive targets for pain therapy. When these channels were first discovered, most attention was focused on their potential contributions to direct thermal activation of peripheral sensory neurons. However, recent anatomical, physiological, and behavioral studies have provided evidence that TRPV channels expressed in skin epithelial cells may also contribute to thermosensation in vitro and in vivo. Here, we review these studies and speculate on possible communication mechanisms from cutaneous epithelial cells to sensory neurons.     

TRPV5 and TRPV6 in Ca2+ (re)absorption: regulating Ca2+ entry at the gate

Many physiological functions rely on the exact maintenance of body Ca²⁺ balance. Therefore, the extracellular Ca²⁺ concentration is tightly regulated by the concerted actions of intestinal Ca²⁺ absorption, exchange of Ca²⁺ to and from bone, and renal Ca²⁺ reabsorption. Renal distal convoluted and connecting tubular cells as well as duodenal epithelial cells are unique in their ability to mediate transcellular (re)absorption of Ca²⁺ at large and highly variable rates. Two members of the transient receptor potential (TRP) superfamily, TRP vanilloid (TRPV)5 and TRPV6, are specialized epithelial Ca²⁺ channels responsible for the critical Ca²⁺ entry step in transcellular Ca²⁺ (re)absorption in intestine and kidney, respectively. Because transcellular Ca²⁺ transport is fine-tuned to the bodys specific requirements, regulation of the transmembrane Ca²⁺ flux through TRPV5/6 is of particular importance and has, therefore, to be conspicuously controlled. We present an overview of the current knowledge and recent advances concerning the coordinated regulation of Ca²⁺ influx through the epithelial Ca²⁺ channels TRPV5 and TRPV6 in transcellular Ca²⁺ (re)absorption.     

Emerging roles of calcium‐activated K channels and TRPV4 channels in lung oedema and pulmonary circulatory collapse

It has been suggested that the transient receptor potential cation (TRP) channel subfamily V (vanilloid) type 4 (TRPV4) and intermediate conductance calcium‐activated potassium (KCa3.1) channels contribute to endothelium‐dependent vasodilation. Here, we summarize very recent evidence for a synergistic interplay of TRPV4 and KCa3.1 channels in lung disease. Among the endothelial Ca²⁺‐permeable TRPs, TRPV4 is best characterized and produces arterial dilation by stimulating Ca²⁺‐dependent nitric oxide synthesis and endothelium‐dependent hyperpolarization. Besides these roles, some TRP channels control endothelial/epithelial barrier functions and vascular integrity, while KCa3.1 channels provide the driving force required for Cl^? and water transport in some cells and most secretory epithelia. The three conditions, increased pulmonary venous pressure caused by left heart disease, high inflation pressure and chemically induced lung injury, may lead to activation of TRPV4 channels followed by Ca²⁺ influx leading to activation of KCa3.1 channels in endothelial cells ultimately leading to acute lung injury. We find that a deficiency in KCa3.1 channels protects against TRPV4‐induced pulmonary arterial relaxation, fluid extravasation, haemorrhage, pulmonary circulatory collapse and cardiac arrest in vivo. These data identify KCa3.1 channels as crucial molecular components in downstream TRPV4 signal transduction and as a potential target for the prevention of undesired fluid extravasation, vasodilatation and pulmonary circulatory collapse.     

TRPV channels and vascular function

Transient receptor potential channels, of the vanilloid subtype (TRPV), act as sensory mediators, being activated by endogenous ligands, heat, mechanical and osmotic stress. Within the vasculature, TRPV channels are expressed in smooth muscle cells, endothelial cells, as well as in peri-vascular nerves. Their varied distribution and polymodal activation properties make them ideally suited to a role in modulating vascular function, perceiving and responding to local environmental changes. In endothelial cells, TRPV1 is activated by endocannabinoids, TRPV3 by dietary agonists and TRPV4 by shear stress, epoxyeicosatrienoic acids (EETs) and downstream of Gq-coupled receptor activation. Upon activation, these channels contribute to vasodilation via nitric oxide, prostacyclin and intermediate/small conductance potassium channel-dependent pathways. In smooth muscle, TRPV4 is activated by endothelial-derived EETs, leading to large conductance potassium channel activation and smooth muscle hyperpolarization. Conversely, smooth muscle TRPV2 channels contribute to global calcium entry and may aid constriction. TRPV1 and TRPV4 are expressed in sensory nerves and can cause vasodilation through calcitonin gene-related peptide and substance P release as well as mediating vascular function via the baroreceptor reflex (TRPV1) or via increasing sympathetic outflow during osmotic stress (TRPV4). Thus, TRPV channels play important roles in the regulation of normal and pathological cellular function in the vasculature.     

Distribution of TRPV1- and TRPV2-immunoreactive afferent nerve endings in rat trachea

Nociception in the trachea is important for respiratory modulation. We investigated the distribution, neurochemical characteristics, and origin of nerve endings with immunoreactivity for candidate sensor channels, TRPV1 and TRPV2, in rat trachea. In the epithelial layer, the intraepithelial nerve endings and dense subepithelial network of nerve fibers were immunoreactive for TRPV1. In contrast, TRPV2 immunoreactivity was observed mainly in nerve fibers of the tracheal submucosal layer and in several intrinsic ganglion cells in the peritracheal plexus. Double immunostaining revealed that some TRPV1-immunoreactive nerve fibers were also immunoreactive for substance P or calcitonin gene-related peptide, but neither neuropeptide colocalized with TRPV2. Injection of the retrograde tracer, fast blue, into the tracheal wall near the thoracic inlet demonstrated labeled neurons in the jugular, nodose, and dorsal root ganglia at segmental levels of C2-C8. In the jugular and nodose ganglia, 59.3% (70/118) and 10.7% (17/159), respectively, of fast blue-labeled neurons were immunoreactive for TRPV1, compared to 8.8% (8/91) and 2.6% (5/191) for TRPV2-immunoreactive. Our results indicate that TRPV1-immunoreactive nerve endings are important for tracheal nociception, and the different expression patterns of TRPV1 and TRPV2 with neuropeptides may reflect different subpopulations of sensory neurons.     

Time- and concentration-dependent activation of TRPA1 by hydrogen sulfide in rat DRG neurons

Hydrogen sulfide (H(2)S) is considered as a gasotransmitter. Although several reports have shown that H(2)S stimulates sensory neurons, the primary targets of H(2)S remain controversial. We investigated the effects of H(2)S on cultured sensory neurons isolated from rat dorsal root ganglion (DRG) using Ca(2+) imaging and whole-cell voltage-clamp techniques. Brief (2 min) application of NaHS (1mM), a donor of H(2)S, evoked marked increases in intracellular Ca(2+) concentration ([Ca(2+)](i)) in a subset of DRG neurons. These neurons also responded to both capsaicin and mustard oil (MO), transient receptor potential vanilloid 1 (TRPV1) and ankyrin 1 (TRPA1) agonists, respectively. The NaHS-evoked [Ca(2+)](i) increases were inhibited by a removal of external Ca(2+) and antagonists for TRPA1, but not for TRPV1 or voltage-dependent Ca(2+) channels. At -80 mV, NaHS evoked inward currents in MO-sensitive neurons, which were also inhibited by a TRPA1 antagonist. Even at lower concentration (≤1 μM), the 10-min application of NaHS increased [Ca(2+)](i) in a time- and concentration-dependent manner. These results suggest that H(2)S stimulates sensory neurons via activation of TRPA1. Endogenous H(2)S may be involved in physiological processes through TRPA1.     

A novel Ca2+ influx pathway activated by mechanical stretch in human airway smooth muscle cells

In response to mechanical stretch, airway smooth muscle exhibits various cellular functions such as contraction, proliferation, and cytoskeletal remodeling, all of which are implicated in the pathophysiology of asthma. We tested the hypothesis that mechanical stretch of airway smooth muscle cells increases intracellular Ca(2+) concentration ([Ca(2+)](i)) by activating stretch-activated (SA) nonselective cation channels. A single uniaxial stretch (3 s) was given to human bronchial smooth muscle cells cultured on an elastic silicone membrane. After the mechanical stretch, a transient increase in [Ca(2+)](i) was observed. The [Ca(2+)](i) increase was significantly dependent on stretch amplitude. The augmented [Ca(2+)](i) due to stretch was completely abolished by removal of extracellular Ca(2+) and was markedly attenuated by an application of Gd(3+), an inhibitor of SA channels, or ruthenium red, a transient receptor potential vanilloid (TRPV) inhibitor. In contrast, the stretch-induced rises of [Ca(2+)](i) were not altered by other Ca(2+) channel inhibitors such as nifedipine, BTP-2, and SKF-96365. Moreover, the [Ca(2+)](i) increases were not affected by indomethacin, a cyclooxygenase inhibitor, U-73122, a phospholipase C inhibitor, or xestospongin C, an inhibitor of the inositol-trisphosphate receptor. These findings demonstrate that a novel Ca(2+) influx pathway activated by mechanical stretch, possibly through the Ca(2+)-permeable SA channel activated directly by stretch rather than by indirect mechanisms via intracellular messenger production, is involved in human airway smooth muscle cells. A molecular candidate for the putative SA channel may be one of the members of the TRPV channel family. Thus, abnormal Ca(2+) homeostasis in response to excessive mechanical strain would contribute to the pathogenesis of asthma.