Capacitative Ca(2+) entry in agonist-induced pulmonary vasoconstriction

Mechanosensitive cation channels in arterial smooth muscle cells are activated by diacylglycerol and inhibited by phospholipase C inhibitor

Mechanosensitive cation channels may be involved in the development of the myogenic tone of arteries. The molecular identity of these channels is not clear, but transient receptor potential channels (TRPCs) are good candidates. In the present study, we searched for mechanosensitive channels at the single-channel level in arterial smooth muscle cells using the patch-clamp technique and investigated the channel properties in the light of properties of TRPCs. With 140 mmol/L CsCl in the pipette solution, application of negative pressures to the back of the pipette induced the activation of channels the open probability of which increased with the amount of negative pressure. The current-voltage relationship was linear in symmetrical ionic conditions, and the single-channel conductances for Cs+, K+, and Na+ were 30, 36, and 27 pS, respectively. When NMDG+ was substituted for Cs+ in the pipette solution, inward currents were abolished, whereas outward currents remained active, indicating that the channels were nonselective to cations. The channel activity was blocked by intracellular Gd3+ and 4,4'-diisothiocyanatostilbene-2,2'-disulfonic acid and increased by diacylglycerol and by cyclopiazonic acid. Phospholipase C inhibitor (U73122) inhibited not only channel activity but also the development of myogenic tone induced by stretching of the basilar arteries. These results suggest that the ion channel responsible for the development of myogenic tone is the 30-pS mechanosensitive cation channel that exhibits properties similar to those of TRPCs.

Role of sarcoplasmic reticulum Ca2+ content in Ca2+ entry of bovine airway smooth muscle cells

Depletion of intracellular Ca(2+) stores induces the opening of an unknown Ca(2+ )entry pathway to the cell. We measured the intracellular free-Ca(2+) concentration ([Ca(2+)]i) at different sarcoplasmic reticulum (SR) Ca(2+) content in fura-2-loaded smooth muscle cells isolated from bovine tracheas. The absence of Ca(2+) in the extracellular medium generated a time-dependent decrement in [Ca(2+)]i which was proportional to the reduction in the SR-Ca(2+) content. This SR-Ca(2+) level was indirectly determined by measuring the amount of Ca(2+) released by caffeine. Ca(2+) restoration at different times after Ca(2+)-free incubation (2, 4, 6 and 10 min) induced an increment of [Ca(2+)]i. This increase in [Ca(2+)]i was considered as Ca(2+) entry to the cell. The rate of this entry was slow (~0.3 nM/s) when SR-Ca(2+) content was higher than 50% (2 and 4 min in Ca(2+)-free medium), and significantly ( p<0.01) accelerated (>1.0 nM/s) when SR-Ca(2+) content was lower than 50% (6 and 10 min in Ca(2+)-free medium). Thapsigargin significantly induced a higher rate of this Ca(2+) entry ( p<0.01). Variations in Ca(2+) influx after SR-Ca(2+) depletion were estimated more directly by a Mn(2+) quench approach. Ca(2+) restoration to the medium 4 min after Ca(2+) removal did not modify the Mn(2+) influx. However, when Ca(2+) was added after 10 min in Ca(2+)-free medium, an increment of Mn(2+) influx was observed, corroborating an increase in Ca(2+) entry. The fast Ca(2+) influx was Ni(2+) sensitive but was not affected by other known capacitative Ca(2+) entry blockers such as La(3+), Mg(2+), SKF 96365 and 2-APB. It was also not affected by the blockage of L-type Ca2(+) channels with methoxyverapamil or by the sustained K(+)-induced depolarisation. The slow Ca(2+) influx was only sensitive to SKF 96365. In conclusion, our results indicate that in bovine airway smooth muscle cells Ca(2+) influx after SR-Ca(2+) depletion has two rates: A) The slow Ca(2+) influx, which occurred in cells with more than 50% of their SR-Ca(2+) content, is sensitive to SKF 96365 and appears to be a non-capacitative Ca(2+) entry; and B) The fast Ca(2+) influx, observed in cells with less than 50% of their SR-Ca(2+) content, is probably a capacitative Ca(2+) entry and was only Ni(2+)-sensitive.

Capacitative calcium entry as a pulmonary specific vasoconstrictor mechanism in small muscular arteries of the rat

(1) The effect of induction of capacitative Ca2+ entry (CCE) upon tone in small (i.d. 200-500 microm) intrapulmonary (IPA), mesenteric (MA), renal (RA), femoral (FA), and coronary arteries (CA) of the rat was examined. (2) Following incubation of IPA with 100 nm thapsigargin (Thg) in Ca2+-free physiological salt solution (PSS), a sustained contraction was observed upon reintroduction of 1.8 mm Ca2+, which was unaffected by either diltiazem (10 microm) or the reverse mode Na+/Ca2+ antiport inhibitor KB-R7943 (10 microm). An identical protocol failed to elicit contraction in MA, RA, or CA, while a small transient contraction was sometimes observed in FA. (3) The effect of this protocol on the intracellular Ca2+ concentration ([Ca2+]i) was assessed using Fura PE3-loaded IPA, MA, and FA. Reintroduction of Ca2+ into the bath solution following Thg treatment in Ca2+-free PSS caused a large, rapid, and sustained increase in [Ca2+]i in all the three types of artery. (4) 100 nm Thg induced a slowly developing noisy inward current in smooth muscle cells (SMC) isolated from IPA, which was due to an increase in the activity of single channels with a conductance of approximately 30 pS. The current had a reversal potential near 0 mV in normal PSS, and persisted when Ca2+-dependent K+ and Cl- currents were blocked; it was greatly inhibited by 1 microm La3+, 1 microm Gd3+, and the IP3 receptor antagonist 2-APB (75 microm), and by replacement of extracellular cations by NMDG+. (5) In conclusion, depletion of intracellular Ca2+ stores with Thg caused capacitative Ca2+ entry in rat small muscular IPA, MA, and FA. However, a corresponding contraction was observed only in IPA. CCE in IPA was associated with the development of a small La3+- and Gd3+-sensitive current, and an increased Mn2+ quench of Fura PE-3 fluorescence. These results suggest that although CCE occurs in a number of types of small arteries, its coupling to contraction appears to be of particular importance in pulmonary arteries.

Pharmacological profile of store-operated channels in cerebral arteriolar smooth muscle cells

1. In this study, we determined a pharmacological profile of store-operated channels (SOCs) in smooth muscle cells of rabbit pial arterioles. Ca(2+)-indicator dyes, fura-PE3 or fluo-4, were used to track [Ca(2+)](i) and 10 micro M methoxyverapamil (D600) was present in all experiments on SOCs to prevent voltage-dependent Ca(2+) entry. Store depletion was induced using thapsigargin or cyclopiazonic acid. 2. SOC-mediated Ca(2+) entry was inhibited concentration dependently by Gd(3+) (IC(50) 101 nM). It was also inhibited by 10 micro M La(3+) (70% inhibition, N=5), 100 micro M Ni(2+) (57% inhibition, N=5), 75 micro M 2-aminoethoxydiphenylborate (66% inhibition, N=4), 100 micro M capsaicin (12% inhibition, N=3) or preincubation with 10 micro M wortmannin (76% inhibition, N=4). It was completely resistant to 1 micro M nifedipine (N=5), 10 micro M SKF96365 (N=6), 10 micro M LOE908 (N=14), 10-100 micro M ruthenium red (N=1+2), 100 micro M sulindac (N=4), 0.5 mM streptomycin (N=3) or 1 : 10,000 dilution Grammostolla spatulata venom (N=4). 3. RT-PCR experiments on isolated arteriolar fragments showed expression of mRNA species for TRPC1, 3, 4, 5 and 6. 4. The pharmacological profile of SOC-mediated Ca(2+) entry in arterioles supports the hypothesis that these SOCs are distinct from tonically active background channels and several store-operated and other nonselective cation channels described in other cells. Similarities with the pharmacology of TRPC1 support the hypothesis that TRPC1 is a subunit of the arteriolar smooth muscle SOC.

Enhanced capacitative calcium entry and TRPC channel gene expression in human LES smooth muscle

Transient receptor potential channel (TRPC) genes encode Ca(2+)-permeable channels mediating capacitative Ca(2+) entry (CCE), which maintains intracellular Ca(2+) stores. We compared TRPC gene expression and CCE in human esophageal body (EB) and lower esophageal sphincter (LES), because these smooth muscles have distinct contractile functions that are likely associated with different Ca(2+) regulatory mechanisms. Circular layer smooth muscle cells were grown in primary culture. Transcriptional expression of TRPC genes was compared by semiquantitative RT-PCR. CCE was measured by fura 2 Ca(2+) fluorescence after blockade of sarcoplasmic reticulum Ca(2+)-ATPase with thapsigargin. mRNA for TRPC1, TRPC3, TRPC4, TRPC5, and TRPC6 was identified in EB and LES. TRPC3 and TRPC4 were more abundant in LES than EB. Basal concentration of free intracellular Ca(2+) ([Ca(2+)](i)) was similar in cells from LES (138 +/- 8 nmol/l) and EB (110 +/- 6 nmol/l) and increased with ACh (10 micromol/l; 650 +/- 28 and 590 +/- 21 nmol/l, respectively). With zero Ca(2+) in bath, thapsigargin (2 micromol/l) increased [Ca(2+)](i) more in LES (550 +/- 22 nmol/l) than EB (250 +/- 15 nmol/l, P < 0.001). Subsequent external application of 1 mmol/l Ca(2+) increased [Ca(2+)](i) more in LES (585 +/- 35 nmol/l) than EB (295 +/- 21 nmol/l, P < 0.001), indicating enhanced CCE in LES. This demonstrates CCE and TRPC transcriptional expression in human esophageal smooth muscle. In LES cells, enhanced CCE and expression of TRPC3 and TRPC4 may contribute to the physiological characteristics that distinguish LES from EB.

TRPC1 store-operated cationic channel subunit

TRPC1 is a membrane protein that is highly conserved in mammals, amphibians and birds. It is widely expressed in cells throughout the body including in the heart and nervous system. Amino acid sequence analysis and over-expression studies indicate it is an ion channel that allows the transmembrane flux of small cations including sodium and calcium. In some cell types it is apparent that at least a fraction of TRPC1 exists in the plasma membrane. Inhibition of TRPC1 expression or block by TRPC1-specific antibody leads to attenuation of the plasma membrane calcium influx that occurs in response to depletion of calcium levels in sarcoplasmic or endoplasmic reticulum. TRPC1 would, therefore, seem to be a key subunit of store-operated channels (SOCs). TRPC1 is, nevertheless, unlikely to act alone. There is good evidence that it can heteromultimerise with the related proteins TRPC4, TRPC5 and polycystin-2; a tetrameric arrangement is envisaged, but not demonstrated. Like its relative in Drosophila, TRPC1 looks likely to function in a signalplex, a protein complex including inositol 1,4,5-triphosphate (IP(3)) receptor, plasma membrane calcium-ATPase, caveolin-1 and calmodulin. Its localisation in membranes is punctate and associated with functionally discrete calcium signals. TRPC1's function may not only be linked to SOCs but also to other cellular events including the nuclear translocation of the NFAT transcription factor. There is still much to be learned about this fundamental protein.

Interleukin-1beta induces calcium transients and enhances basal and store operated calcium entry in human myometrial smooth muscle

We have previously reported increased protein expression of sarcoplasmic reticulum calcium ATPase (SERCA) 2b in myometrium from women in labor at term, but the stimulus for this change is unknown. Proinflammatory cytokines have been implicated in the cascade of events leading to preterm and term labor, and we hypothesize that interleukin (IL)-1beta may induce changes in key calcium homeostatic mechanisms and, in turn, augment myometrial contractility before labor. The aim of the present study was to investigate the long-term effects of IL-1beta on SERCA 2b protein expression, calcium mobilization from intracellular stores, and store-operated calcium entry. Myometrial biopsies were obtained, with consent, from women undergoing elective cesarean section at term. Primary-cultured human myometrial smooth muscle (HMSM) cells were exposed to IL-1beta (10 ng/ml) for 24 h or to culture medium alone (control). Cells were subsequently used in Western blot studies or loaded with fura-2 to assess calcium dynamics using fluorescent digital imaging. The present study clearly demonstrated that IL-1beta significantly increased SERCA 2b protein expression in HMSM cells. Cyclopiazonic acid-induced calcium transients were also augmented, predominantly by activation of lanthanum-sensitive, store-operated calcium entry. HMSM cell excitability was enhanced, as evidenced by increased basal calcium entry and the initiation of spontaneous calcium transients in 37% of IL-1beta-treated cells. IL-1beta modulation of calcium mobilization may be an important mechanism in the cascade of events preparing the pregnant uterus for labor.

Evidence for the expression of transient receptor potential proteins in guinea pig airway smooth muscle cells

OBJECTIVE

The present study investigates the expression of transient receptor potential (TRPC) proteins in airway smooth muscle (ASM) cells in order to determine whether these proteins may be candidate molecular counterparts of plasma membrane Ca2+-permeable channels involved in the contraction of ASM.

METHODS

Expression of TRPC mRNA was detected using specific primers and RT-PCR. Expression of the TRPC1, TRPC3 and TRPC6 proteins was detected using antibodies in immunoprecipitation and Western blot.

RESULTS

Guinea pig ASM cells exhibited thapsigargin- and acetylcholine-initiated Ca2+ inflow but none by 1-oleoyl-2-acetyl-sn-glycerol. mRNA encoding each of the TRPC1 to TRPC6 proteins was detected in ASM cells. mRNA encoding TRPC1, TRPC3, TRPC4 and TRPC6 was detected in ASM cells at a concentration approximately equivalent to that in guinea pig brain. mRNA encoding TRPC2 and TRPC5 was more abundant in ASM cells than in brain. The TRPC1 protein, but not the TRPC3 or TRPC6 proteins, was detected in extracts of ASM cells, while all three proteins were detected in brain.

CONCLUSION

The results provide evidence for a low level of expression of the TRPC1 to TRPC6 proteins in ASM cells. These proteins may function as store-operated Ca2+ and/or second messenger-activated non-selective cation channels in ASM cells.

PDGF stimulates pulmonary vascular smooth muscle cell proliferation by upregulating TRPC6 expression

Capacitative Ca(2+) entry (CCE) through store-operated Ca(2+) (SOC) channels plays an important role in returning Ca(2+) to the sarcoplasmic reticulum (SR) and regulating cytosolic free Ca(2+) concentration ([Ca(2+)](cyt)). A rise in [Ca(2+)](cyt) and sufficient Ca(2+) in the SR are required for pulmonary artery smooth muscle cell (PASMC) proliferation. We tested the hypothesis that platelet-derived growth factor (PDGF)-mediated PASMC growth involves upregulation of c-Jun and TRPC6, a transient receptor potential cation channel. In rat PASMC, PDGF (10 ng/ml for 0.5-48 h) phosphorylated signal transducer and activator of transcription (STAT3), increased mRNA and protein levels of c-Jun, and stimulated cell proliferation. PDGF treatment also upregulated TRPC6 expression and augmented CCE, elicited by passive depletion of Ca(2+) from the SR using cyclopiazonic acid. Furthermore, overexpression of c-Jun stimulated TRPC6 expression and CCE amplitude in PASMC. Downregulation of TRPC6 using an antisense oligonucleotide specifically for human TRPC6 decreased CCE and inhibited PDGF-mediated PASMC proliferation. These results suggest that PDGF-mediated PASMC proliferation is associated with c-Jun/STAT3-induced upregulation of TRPC6 expression. The resultant increase in CCE raises [Ca(2+)](cyt), facilitates return of Ca(2+) to the SR, and enhances PASMC growth.

Signaling through NO and cGMP-dependent protein kinases

The gaseous molecule nitric oxide (NO) modulates a large variety of physiological functions including vascular tone, intestinal motility, platelet aggregation, proliferation, apoptosis, and neurotransmission. NO initiates diverse cellular signaling cascades which comprise nitrosylation of proteins, adenosine 5'-diphosphate (ADP)-ribosylation, or stimulation of soluble guanylyl cyclases which catalyze intracellular guanosine 3',5'-cyclic monophosphate (cGMP) synthesis. cGMP activates cGMP-dependent protein kinases (cGK) which mediate localized and global signaling. Furthermore, cGMP regulates the activity of phosphodiesterases (PDE) which modulate the duration and amplitude of cyclic nucleotide signaling. Two different types of cGK are expressed in mammals, cGKI and cGKII. Activation of the NO/cGMP/cGKI pathway induces relaxation of smooth muscle by lowering the cytosolic calcium level and/or by calcium desensitization of the contractile elements.

Mg2+ blocks myogenic tone but not K+-induced constriction: role for SOCs in small arteries

The effects of Mg(2+) and nifedipine (Nif) on vasoconstriction and Ca(2+) transients were studied in intact, pressurized rat mesenteric arteries with myogenic tone. Changes in cytosolic Ca(2+) concentration ([Ca(2+)](cyt)) were measured with confocal microscopy in fluo 4-AM loaded, individual myocytes. Myogenic tone was abolished by 10 mM Mg(2+) or 0.3 microM Nif. Contractions induced by 75 mM K(+) depolarization were blocked by 0.3 microM Nif, but not by 10 mM Mg(2+). Phenylephrine (PE; 5 microM) evoked sustained [Ca(2+)](cyt) elevation and vasoconstriction with superimposed Ca(2+) oscillations and vasomotion. The subsequent addition of 10 mM Mg(2+) or 0.3 microM Nif reduced [Ca(2+)](cyt) and abolished plateau vasoconstriction. When added before PE, both Mg(2+) and Nif abolished the PE-evoked Ca(2+) oscillations and vasomotion. Mg(2+) dilated the PE-constricted arteries after a brief (< or =180-240 s) vasoconstriction, but Nif did not. Both agents also abolished the vasoconstriction attributed to Ca(2+) entry through store-operated channels (SOCs) during internal Ca(2+) store refilling that followed store depletion. The data suggest that Ca(2+) entry through SOCs helps maintain both myogenic tone and alpha(1)-adrenoceptor-induced tonic vasoconstriction.

Protein kinase Calpha participates in activation of store-operated Ca2+ channels in human glomerular mesangial cells

Protein kinase C (PKC) plays an important role in activating store-operated Ca2+ channels (SOC) in human mesangial cells (MC). The present study was performed to determine the specific isoform(s) of conventional PKC involved in activating SOC in MC. Fura 2 fluorescence ratiometry showed that the thapsigargin-induced Ca2+ entry (equivalent to SOC) was significantly inhibited by 1 microM Gö-6976 (a specific PKCalpha and betaI inhibitor) and PKCalpha antisense treatment (2.5 nM for 24-48 h). However, LY-379196 (PKCbeta inhibitor) and 2,2',3,3',4,4'-hexahydroxy-1,1'-biphenyl-6,6'-dimethanoldimethyl ether (HBDDE; PKCalpha and gamma inhibitor) failed to affect thapsigargin-evoked activation of SOC. Single-channel analysis in the cell-attached configuration revealed that Gö-6976 and PKCalpha antisense significantly depressed thapsigargin-induced activation of SOC. However, LY-379196 and HBDDE did not affect the SOC responses. In inside-out patches, application of purified PKCalpha or betaI, but not betaII or gamma, significantly rescued SOC from postexcision rundown. Western blot analysis revealed that thapsigargin evoked a decrease in cytosolic expression with a corresponding increase in membrane expression of PKCalpha and gamma. However, the translocation from cytosol to membranes was not detected for PKCbetaI or betaII. These results suggest that PKCalpha participates in the intracellular signaling pathway for activating SOC upon release of intracellular stores of Ca2+.

Comparative capacitative calcium entry mechanisms in canine pulmonary and renal arterial smooth muscle cells

Experiments were performed to determine whether capacitative Ca(2+) entry (CCE) can be activated in canine pulmonary and renal arterial smooth muscle cells (ASMCs) and whether activation of CCE parallels the different functional structure of the sarcoplasmic reticulum (SR) in these two cell types. The cytosolic [Ca(2+)] was measured by imaging fura-2-loaded individual cells. Increases in the cytosolic [Ca(2+)] due to store depletion in pulmonary ASMCs required simultaneous depletion of both the inositol 1,4,5-trisphosphate (InsP(3))- and ryanodine (RY)-sensitive SR Ca(2+) stores. In contrast, the cytosolic [Ca(2+)] rises in renal ASMCs occurred when the SR stores were depleted through either the InsP(3) or RY pathways. The increase in the cytosolic [Ca(2+)] due to store depletion in both pulmonary and renal ASMCs was present in cells that were voltage clamped and was abolished when cells were perfused with a Ca(2+)-free bathing solution. Rapid quenching of the fura-2 signal by 100 microM Mn(2+) following SR store depletion indicated that extracellular Ca(2+) entry increased in both cell types and also verified that activation of CCE in pulmonary ASMCs required the simultaneous depletion of the InsP(3)- and RY-sensitive SR Ca(2+) stores, while CCE could be activated in renal ASMCs by the depletion of either of the InsP(3)- or RY-sensitive SR stores. Store depletion Ca(2+) entry in both pulmonary and renal ASMCs was strongly inhibited by Ni(2+) (0.1-10 mM), slightly inhibited by Cd(2+) (200-500 microM), but was not significantly affected by the voltage-gated Ca(2+) channel (VGCC) blocker nisoldipine (10 microM). The non-selective cation channel blocker Gd(3+) (100 microM) inhibited a portion of the Ca(2+) entry in 6 of 18 renal but not pulmonary ASMCs. These results provide evidence that SR Ca(2+) store depletion activates CCE in parallel with the organization of intracellular Ca(2+) stores in canine pulmonary and renal ASMCs.

Discrete store-operated calcium influx into an intracellular compartment in rabbit arteriolar smooth muscle

This study tested the hypothesis that store-operated channels (SOCs) exist as a discrete population of Ca2+ channels activated by depletion of intracellular Ca(2+) stores in cerebral arteriolar smooth muscle cells and explored their direct contractile function. Using the Ca2+ indicator fura-PE3 it was observed that depletion of sarcoplasmic reticulum (SR) Ca2+ by inhibition of SR Ca2+-ATPase (SERCA) led to sustained elevation of [Ca2+]i that depended on extracellular Ca2+ and slightly enhanced Mn2+ entry. Enhanced background Ca2+ influx did not explain the raised [Ca2+]i in response to SERCA inhibitors because it had marked gadolinium (Gd3+) sensitivity, which background pathways did not. Effects were not secondary to changes in membrane potential. Thus SR Ca2+ depletion activated SOCs. Strikingly, SOC-mediated Ca2+ influx did not evoke constriction of the arterioles, which were in a resting state. This was despite the fura-PE3-indicated [Ca2+]i rise being greater than that evoked by 20 mM [K+]o (which did cause constriction). Release of endothelial vasodilators did not explain the absence of SOC-mediated constriction, nor did a change in Ca2+ sensitivity of the contractile proteins. We suggest SOCs are a discrete subset of Ca2+ channels allowing Ca2+ influx into a 'non-contractile' compartment in cerebral arteriolar smooth muscle cells.

Ion channels in pulmonary arterial hypertension

Pulmonary arterial hypertension (PAH) is a hemodynamic abnormality that ultimately results in mortality due to right heart failure. Although the clinical manifestations of primary and secondary PAH are diverse, medial hypertrophy and arterial vasoconstriction are key components in the vascular remodeling leading to PAH. Abnormalities in the homeostasis of intracellular Ca(2+), transmembrane flux of ions, and membrane potential may play significant roles in the processes leading to pulmonary vascular remodeling. Decreased activity of K(+) channels causes membrane depolarization, leading to Ca(2+) influx. The elevated cytoplasmic Ca(2+) is a major trigger for pulmonary vasoconstriction and an important stimulus for vascular smooth muscle proliferation. Dysfunctional K(+) channels have also been linked to inhibition of apoptosis and contribute further to the medial hypertrophy. This review focuses on the relative role of K(+) and Ca(2+) ions and channels in human pulmonary artery smooth muscle cells in the development of PAH.

Inhibition of endogenous TRP1 decreases capacitative Ca2+ entry and attenuates pulmonary artery smooth muscle cell proliferation

Pulmonary vascular medial hypertrophy due to proliferation of pulmonary artery smooth muscle cells (PASMC) greatly contributes to the increased pulmonary vascular resistance in pulmonary hypertension patients. A rise in cytosolic free Ca2+ concentration ([Ca2+]cyt) is an important stimulus for cell growth in PASMC. Resting [Ca2+]cyt, intracellularly stored [Ca2+], capacitative Ca2+ entry (CCE), and store-operated Ca2+ currents (I(SOC)) are greater in proliferating human PASMC than in growth-arrested cells. Expression of TRP1, a transient receptor potential gene proposed to encode the channels responsible for CCE and I(SOC), was also upregulated in proliferating PASMC. Our aim was to determine if inhibition of endogenous TRP1 gene expression affects I(SOC) and CCE and regulates cell proliferation in human PASMC. Cells were treated with an antisense oligonucleotide (AS, for 24 h) specifically designed to cleave TRP1 mRNA and then returned to normal growth medium for 40 h before the experiments. Then, mRNA and protein expression of TRP1 was downregulated, and amplitudes of I(SOC) and CCE elicited by passive depletion of Ca2+ from the sarcoplasmic reticulum using cyclopiazonic acid were significantly reduced in the AS-treated PASMC compared with control. Furthermore, the rate of cell growth was decreased by 50% in AS-treated PASMC. These results indicate that TRP1 may encode a store-operated Ca2+ channel that plays a critical role in PASMC proliferation by regulating CCE and intracellular [Ca2+](cyt).

Sequential opening of IP(3)-sensitive Ca(2+) channels and SOC during alpha-adrenergic activation of rabbit vena cava

Alpha(1)-aderenoceptor-mediated constriction of rabbit inferior vena cava (IVC) is signaled by asynchronous wavelike Ca(2+) oscillations in the in situ smooth muscle. We have shown previously that a putative nonselective cationic channel (NSCC) is required for these oscillations. In this report, we show that the application of 2-aminoethoxyphenyl borate (2-APB) to antagonize inositol 1,4,5-trisphosphate (InsP(3))-sensitive Ca(2+) release channels (IP(3)R channels) can prevent the initiation and abolish ongoing alpha(1)-aderenoceptor-mediated tonic constriction of the venous smooth muscle by inhibiting the generation of these intracellular Ca(2+) concentration ([Ca(2+)](i)) oscillations. The observed effects of 2-APB can only be attributed to its selective inhibition on the IP(3)R channels, not to its slight inhibition of the L-type voltage-gated Ca(2+) channel and the sarco(endo)plasmic reticulum Ca(2+) ATPase. Furthermore, 2-APB had no effect on the ryanodine-sensitive Ca(2+) release channel and the store-operated channel (SOC) in the IVC. These results indicate that the putative NSCC involved in refilling the sarcoplasmic reticulum (SR) and maintaining the tonic contraction is most likely an SOC-type channel because it appears to be activated by IP(3)R-channel-mediated SR Ca(2+) release or store depletion. This is in accordance with its sensitivity to Ni(2+) and La(3+) (SOC blockers). More interestingly, RT-PCR analysis indicates that transient receptor potential (Trp1) mRNA is strongly expressed in the rabbit IVC. The Trp1 gene is known to encode a component of the store-operated NSCC. These new data suggest that the activation of both the IP(3)R channels and the SOC are required for PE-mediated [Ca(2+)](i) oscillations and constriction of the rabbit IVC.

Regulation of capacitative and noncapacitative receptor-operated Ca2+ entry by rho-kinase in tracheal smooth muscle

To determine the mechanisms of Ca2+ mobilization induced by receptor agonists, we examined the role of Rho-kinase on the sarcoplasmic reticulum (SR) Ca2+ stores-dependent and -independent Ca2+ influx in guinea pig tracheal smooth muscle (TSM). Isometric tension and intracellular Ca2+ concentration ([Ca2+]i) were simultaneously measured using fura-2-loaded tissues. Depletion of the SR Ca2+ stores by thapsigargin caused an increase in [Ca2+]i and contraction, demonstrating capacitative Ca2+ entry (CCE). Because CCE was not inhibited by nifedipine, voltage-operated Ca2+ channels are not involved in CCE. Under the condition that CCE is fully activated, methacholine (MCh) and histamine caused further increases in [Ca2+]i and tension, demonstrating noncapacitative receptor-operated Ca2+ entry (non-CCE). The Ca2+ influx and contraction via non-CCE was inhibited by Y-27632, a Rho-kinase inhibitor, in a concentration-dependent fashion. In contrast, Y-27632 did not affect thapsigargin-induced CCE. Cytochalasin D, which disrupts actin cytoskeleton, inhibited contraction induced by CCE or MCh with no change in [Ca2+]i. Our results indicate that not only CCE but also non-CCE exist in TSM and that the latter is regulated by Rho-kinase, independent of actin cytoskeleton. In conclusion, Ca2+ influx regulated by the RhoA/Rho-kinase pathway may play a functional role in contraction by agonists.

The developing relationship between receptor-operated and store-operated calcium channels in smooth muscle

Contraction of smooth muscle is initiated, and to a lesser extent maintained, by a rise in the concentration of free calcium in the cell cytoplasm ([Ca(2+)](i)). This activator calcium can originate from two intimately linked sources--the extracellular space and intracellular stores, most notably the sarcoplasmic reticulum. Smooth muscle contraction activated by excitatory neurotransmitters or hormones usually involves a combination of calcium release and calcium entry. The latter occurs through a variety of calcium permeable ion channels in the sarcolemma membrane. The best-characterized calcium entry pathway utilizes voltage-operated calcium channels (VOCCs). However, also present are several types of calcium-permeable channels which are non-voltage-gated, including the so-called receptor-operated calcium channels (ROCCs), activated by agonists acting on a range of G-protein-coupled receptors, and store-operated calcium channels (SOCCs), activated by depletion of the calcium stores within the sarcoplasmic reticulum. In this article we will review the electrophysiological, functional and pharmacological properties of ROCCs and SOCCs in smooth muscle and highlight emerging evidence that suggests that the two channel types may be closely related, being formed from proteins of the Transient Receptor Potential Channel (TRPC) family.

Store-operated channels mediate Ca(2+) influx and contraction in rat pulmonary artery

Cation channels activated by Ca(2+) store depletion have been proposed to mediate Ca(2+) influx in vascular smooth muscle cells. The aim of this study was to determine if store-operated channels have a functional role in pulmonary artery smooth muscle cells (PASMCs). In intact rat pulmonary artery rings, cyclopiazonic acid (CPA) produced a sustained contraction that was resistant to inhibition by nifedipine, but abolished in Ca(2+)-free solution and 50% blocked in the presence of 6 micromol/L Cd(2+), 10 micromol/L Ni(2+), 600 micromol/L La(3+), and 7 micromol/L SKF96365. In freshly isolated PASMCs loaded with fura-2, CPA increased the intracellular Ca(2+) concentration by stimulating dihydropyridine-resistant Ca(2+) influx, which was approximately 50% blocked by 10 micromol/L Ni(2+) and 7 micromol/L SKF96365. In perforated-patch recordings, CPA activated a sustained inward current at negative membrane potentials, which persisted in cells dialyzed with BAPTA, showed a near linear dependence on membrane potential when Cs(+) was the main intracellular cation, and was blocked by Ni(2+), Cd(2+), and SKF96365 at concentrations preventing contraction. The current showed a bimodal dependence on extracellular Ca(2+), being enhanced 2-fold in the absence of Ca(2+) and around 10-fold on reducing Ca from 1.8 to 0.2 mmol/L. RT-PCR revealed the expression of Trp1, Trp3, Trp4, Trp5, and Trp6 mRNA, whereas immunostaining identified Trp1, Trp3, Trp4, and Trp6 channel proteins in isolated PASMCs. At least one of these subunits may contribute to cation channels in PASMCs, which are activated by store depletion to bring about Ca(2+) influx and contraction.