Thiol oxidation causes pulmonary vasodilation by activating K+ channels and inhibiting store-operated Ca2+ channels

Cellular redox change regulates pulmonary vascular tone by affecting function of membrane and cytoplasmic proteins, enzymes, and second messengers. This study was designed to test the hypothesis that functional modulation of ion channels by thiol oxidation contributes to regulation of excitation-contraction coupling in isolated pulmonary artery (PA) rings. Acute treatment with the thiol oxidant diamide produced a dose-dependent relaxation in PA rings; the IC50 was 335 and 58 microM for 40 mM K+ - and 2 microM phenylephrine-induced PA contraction, respectively. The diamide-mediated pulmonary vasodilation was affected by neither functional removal of endothelium nor 8-bromoguanosine-3'-5'-cyclic monophosphate (50 microM) and HA-1004 (30 microM). A rise in extracellular K+ concentration (from 20 to 80 mM) attenuated the thiol oxidant-induced PA relaxation. Passive store depletion by cyclopiazonic acid (50 microM) and active store depletion by phenylephrine (in the absence of external Ca2+ both induced PA contraction due to capacitative Ca2+ entry. Thiol oxidation by diamide significantly attenuated capacitative Ca2+ entry-induced PA contraction due to active and passive store depletion. The PA rings isolated from left and right PA branches appeared to respond differently to store depletion. Although the active tension induced by passive store depletion was comparable, the active tension induced by active store depletion was 3.5-fold greater in right branches than in left branches. These data indicate that thiol oxidation causes pulmonary vasodilation by activating K+ channels and inhibiting store-operated Ca2+ channels, which subsequently attenuate Ca2+ influx and decrease cytosolic free Ca2+ concentration in pulmonary artery smooth muscle cells. The mechanisms involved in thiol oxidation-mediated pulmonary vasodilation or activation of K+ channels and inhibition of store-operated Ca2+ channels appear to be independent of functional endothelium and of the cGMP-dependent protein kinase pathway.     

Enhanced store-operated Ca²+ entry and TRPC channel expression in pulmonary arteries of monocrotaline-induced pulmonary hypertensive rats

Pulmonary hypertension (PH) is associated with profound vascular remodeling and alterations in Ca(2+) homeostasis in pulmonary arterial smooth muscle cells (PASMCs). Previous studies show that canonical transient receptor potential (TRPC) genes are upregulated and store-operated Ca(2+) entry (SOCE) is augmented in PASMCs of chronic hypoxic rats and patients of pulmonary arterial hypertension (PAH). Here we further examine the involvement of TRPC and SOCE in PH with a widely used rat model of monocrotaline (MCT)-induced PAH. Rats developed severe PAH, right ventricular hypertrophy, and significant increase in store-operated TRPC1 and TRPC4 mRNA and protein in endothelium-denuded pulmonary arteries (PAs) 3 wk after MCT injection. Contraction of PA and Ca(2+) influx in PASMC evoked by store depletion using cyclopiazonic acid (CPA) were enhanced dramatically, consistent with augmented SOCE in the MCT-treated group. The time course of increase in CPA-induced contraction corresponded to that of TRPC1 expression. Endothelin-1 (ET-1)-induced vasoconstriction was also potentiated in PAs of MCT-treated rats. The response was partially inhibited by SOCE blockers, including Gd(3+), La(3+), and SKF-96365, as well as the general TRPC inhibitor BTP-2, suggesting that TRPC-dependent SOCE was involved. Moreover, the ET-1-induced contraction and Ca(2+) response in the MCT group were more susceptible to the inhibition caused by the various SOCE blockers. Hence, our study shows that MCT-induced PAH is associated with increased TRPC expression and SOCE, which are involved in the enhanced vascular reactivity to ET-1, and support the hypothesis that TRPC-dependent SOCE is an important pathway for the development of PH.     

Comparison of human TRPC3 channels in receptor-activated and store-operated modes. Differential sensitivity to channel blockers suggests fundamental differences in channel composition

Capacitative calcium entry or store-operated calcium entry in nonexcitable cells is a process whereby the activation of calcium influx across the plasma membrane is signaled by depletion of intracellular calcium stores. Transient receptor potential (TRP) proteins have been proposed as candidates for store-operated calcium channels. Human TRPC3 (hTRPC3), an extensively studied member of the TRP family, is activated through a phospholipase C-dependent mechanism, not by store depletion, when expressed in HEK293 cells. However, store depletion by thapsigargin is sufficient to activate hTRPC3 channels when expressed in DT40 avian B-lymphocytes. To gain further insights into the differences between hTRPC3 channels generated in these two expression systems and further understand the role of hTRPC3 in capacitative calcium entry, we examined the effect of two well characterized inhibitors of capacitative calcium entry, Gd3+ and 2-aminoethoxydiphenyl borane (2APB). We confirmed that in both DT40 cells and HEK293 cells, 1 microm Gd3+ or 30 microm 2APB completely blocked calcium entry due to receptor activation or store depletion. In HEK293 cells, 1 microm Gd3+ did not block receptor-activated hTRPC3-mediated cation entry, whereas 2APB had a partial (approximately 60%) inhibitory effect. Interestingly, store-operated hTRPC3-mediated cation entry in DT40 cells was also partially inhibited by 2APB, whereas 1 microm Gd3+ completely blocked store-operated hTRPC3 activity in these cells. Furthermore, the sensitivity of store-operated hTRPC3 channels to Gd3+ in DT40 cells was similar to the endogenous store-operated channels, with essentially 100% block of activity at concentrations as low as 0.1 microm. Finally, Gd3+ has a rapid inhibitory effect when added to fully developed hTRPC3-mediated calcium entry, suggesting a direct action of Gd3+ on hTRPC3 channels. The distinct action of these inhibitors on hTRPC3-mediated cation entry in these two cell types may result from their different modes of activation and may also reflect differences in basic channel structure.     

Novel pharmacological TRPC inhibitors block hypoxia-induced vasoconstriction

The Ca(2+)-permeable, nonselective cation channel TRPC6 is gated via phospholipase C-activating receptors and has recently been implicated in hypoxia-induced pulmonary vasoconstriction (HPV), idiopathic pulmonary hypertension and focal segmental glomerulosclerosis (FSGS). Therefore, TRPC6 is a promising target for pharmacological interference. To identify and develop TRPC6-blocking compounds, we screened the Chembionet library, a collection of 16,671 chemically diverse drug-like compounds, for biological activity to prevent the 1-oleoyl-2-acetyl-sn-glycerol-triggered Ca(2+) influx in a stably transfected HEK(TRPC6-YFP) cell line. Hits were validated and characterised by fluorometric and electrophysiological methods. Six compounds displayed inhibitory potency at low micromolar concentrations, lack of cytotoxicity and blocked the receptor-dependent mode of TRPC6 activation. The specificity was tested towards closely (TRPC3 and TRPC7) and more distantly related TRP channels. One of the compounds, 8009-5364, displayed a 2.5-fold TRPC6-selectivity compared to TRPC3, and almost no inhibition of TRPC7 or the other TRP channels tested. Block of native TRPC3/6-like responses was confirmed in dissociated pulmonary artery smooth muscle cells. Two non-polar blockers effectively suppressed the HPV responses in the perfused mouse lung model. We conclude that pharmacological targeting of TRPC6 is feasible and provide a promising concept to treat pulmonary diseases that are characterised by excessive hypoxic vasoconstriction.     

Epidermal growth factor induces tyrosine phosphorylation, membrane insertion, and activation of transient receptor potential channel 4

Various members of the canonical family of transient receptor potential channels (TRPCs) exhibit increased cation influx following receptor stimulation or Ca(2+) store depletion. Tyrosine phosphorylation of TRP family members also results in increased channel activity; however, the link between the two events is unclear. We report that two tyrosine residues in the C terminus of human TRPC4 (hTRPC4), Tyr-959 and Tyr-972, are phosphorylated following epidermal growth factor (EGF) receptor stimulation of COS-7 cells. This phosphorylation was mediated by Src family tyrosine kinases (STKs), with Fyn appearing to be the dominant kinase. In addition, EGF receptor stimulation induced the exocytotic insertion of hTRPC4 into the plasma membrane dependent on the activity of STKs and was accompanied by a phosphorylation-dependent increase in the association of hTRPC4 with Na(+)/H(+) exchanger regulatory factor. Furthermore, this translocation and association was defective upon mutation of Tyr-959 and Tyr-972 to phenylalanine. Significantly, inhibition of STKs was concomitant with a reduction in Ca(2+) influx in both native COS-7 cells and hTRPC4-expressing HEK293 cells, with cells expressing the Y959F/Y972F mutant exhibiting a reduced EGF response. These findings represent the first demonstration of a mechanism for phosphorylation to modulate TRPC channel function.     

On the endothelial cell I(SOC)

Ca2+ store depletion activates both Ca2+ selective and non-selective currents in endothelial cells. Recently, considerable progress has been made in understanding the molecular make-up and regulation of an endothelial cell thapsigargin-activated Ca2+ selective current, I(SOC). Indeed, I(SOC) is a relatively small inward Ca2+ current that exhibits an approximate +40mV reversal potential and is strongly inwardly rectifying. This current is sensitive to organization of the actin-based cytoskeleton. Transient receptor potential (TRP) proteins 1 and 4 (TRPC1 and TRPC4, respectively) each contribute to the molecular basis of I(SOC), although it is TRPC4 that appears to be tethered to the cytoskeleton through a dynamic interaction with protein 4.1. Activation of I(SOC) requires association between protein 4.1 and the actin-based cytoskeleton (mediated through spectrin), suggesting protein 4.1 mediates the physical communication between Ca2+ store depletion and channel activation. Thus, at present findings indicate a TRPC4-protein 4.1 physical linkage regulates I(SOC) activation following Ca2+ store depletion.     

Role of lipid rafts in the interaction between hTRPC1, Orai1 and STIM1

Store-operated Ca2+ entry (SOCE) is a mechanism regulated by the filling state of the intracellular Ca2+ stores that requires the participation of the Ca2+ sensor STIM1, which communicates the Ca2+ content of the stores to the plasma membrane Ca2+-permeable channels. We have recently reported that Orai1 mediates the communication between STIM1 and the Ca2+ channel hTRPC1. This event is important to confer hTRPC1 store depletion sensitivity, thus supporting the functional role of the STIM1-Orai1-hTRPC1 complex in the activation of SOCE. Here we have explored the relevance of lipid rafts in the formation of the STIM1-Orai1-hTRPC1 complex and the activation of SOCE. Disturbance of lipid raft domains, using methyl-beta-cyclodextrin, reduces the interaction between endogenously expressed Orai1 and both STIM1 and hTRPC1 upon depletion of the intracellular Ca2+ stores and attenuates thapsigargin-evoked Ca2+ entry. These findings suggest that TRPC1, Orai1 and STIM1 form a heteromultimer associated with lipid raft domains and regulated by the intracellular Ca2+ stores.     

5,6-EET-induced contraction of intralobar pulmonary arteries depends on the activation of Rho-kinase.

The mechanism mediating epoxyeicosatrienoic acid (EET)-induced contraction of intralobar pulmonary arteries (PA) is currently unknown. EET-induced contraction of PA has been reported to require intact endothelium and activation of the thromboxane/endoperoxide (TP) receptor. Because TP receptor occupation with the thromboxane mimetic U-46619 contracts pulmonary artery via Rho-kinase activation, we examined the hypothesis that 5,6-EET-induced contraction of intralobar rabbit pulmonary arteries is mediated by a Rho-kinase-dependent signaling pathway. In isolated rings of second-order intralobar PA (1-2 mm OD) at basal tension, 5,6-EET (0.3-10 microM) induced increases in active tension that were inhibited by Y-27632 (1 microM) and HA-1077 (10 microM), selective inhibitors of Rho-kinase activity. In PA in which smooth muscle intracellular Ca(2+) concentration ([Ca(2+)](i)) was increased with KCl (25 mM) to produce a submaximal contraction, 5,6-EET (1 microM) induced a contraction that was 7.0 +/- 1.6 times greater than without KCl. 5,6-EET (10 microM) also contracted beta-escin permeabilized PA in which [Ca(2+)](i) was clamped at a concentration resulting in a submaximal contraction. Y-27632 inhibited the 5,6-EET-induced contraction in permeabilized PA. 5,6-EET (10 microM) increased phosphorylation of myosin light chain (MLC), increasing the ratio of phosphorylated MLC/total MLC from 0.10 +/- 0.03 to 0.30 +/- 0.02. Y-27632 prevented this increase in MLC phosphorylation. These data suggest that 5,6-EET induces contraction in intralobar PA by increasing Rho-kinase activity, phosphorylating MLC, and increasing the Ca(2+) sensitivity of the contractile apparatus.     

Differences in STIM1 and TRPC expression in proximal and distal pulmonary arterial smooth muscle are associated with differences in Ca2+ responses to hypoxia

Hypoxic pulmonary vasoconstriction (HPV) requires Ca(2+) influx through store-operated Ca(2+) channels (SOCC) in pulmonary arterial smooth muscle cells (PASMC) and is greater in distal than proximal pulmonary arteries (PA). SOCC may be composed of canonical transient receptor potential (TRPC) proteins and activated by stromal interacting molecule 1 (STIM1). To assess the possibility that HPV is greater in distal PA because store-operated Ca(2+) entry (SOCE) is greater in distal PASMC, we measured intracellular Ca(2+) concentration ([Ca(2+)](i)) and SOCE in primary cultures of PASMC using fluorescent microscopy and the Ca(2+)-sensitive dye fura 2. Both hypoxia (4% O(2)) and KCl (60 mM) increased [Ca(2+)](i). Responses to hypoxia, but not KCl, were greater in distal cells. We measured SOCE in PASMC perfused with Ca(2+)-free solutions containing cyclopiazonic acid to deplete Ca(2+) stores in sarcoplasmic reticulum and nifedipine to prevent Ca(2+) entry through L-type voltage-operated Ca(2+) channels. Under these conditions, the increase in [Ca(2+)](i) caused by restoration of extracellular Ca(2+) and the decrease in fura 2 fluorescence caused by Mn(2+) were greater in distal PASMC, indicating greater SOCE. Moreover, the increase in SOCE caused by hypoxia was also greater in distal cells. Real-time quantitative polymerase chain reaction analysis of PASMC and freshly isolated deendothelialized PA tissue demonstrated expression of STIM1 and five of seven known TRPC isoforms (TRPC1 > TRPC6 > TRPC4 > TRPC3 approximately TRPC5). For both protein, as measured by Western blotting, and mRNA, expression of STIM1, TRPC1, TRPC6, and TRPC4 was greater in distal than proximal PASMC and PA. These results provide further support for the importance of SOCE in HPV and suggest that HPV is greater in distal than proximal PA because greater numbers and activation of SOCC in distal PASMC generate bigger increases in [Ca(2+)](i).     

A functional link between store-operated and TRPC channels revealed by the 3,5-bis(trifluoromethyl)pyrazole derivative, BTP2

The coupling between receptor-mediated Ca2+ store release and the activation of "store-operated" Ca2+ entry channels is an important but so far poorly understood mechanism. The transient receptor potential (TRP) superfamily of channels contains several members that may serve the function of store-operated channels (SOCs). The 3,5-bis(trifluoromethyl)pyrazole derivative, BTP2, is a recently described inhibitor of SOC activity in T-lymphocytes. We compared its action on SOC activation in a number of cell types and evaluated its modification of three specific TRP channels, canonical transient receptor potential 3 (TRPC3), TRPC5, and TRPV6, to throw light on any link between SOC and TRP channel function. Using HEK293 cells, DT40 B cells, and A7r5 smooth muscle cells, BTP2 blocked store-operated Ca2+ entry within 10 min with an IC50 of 0.1-0.3 microM. Store-operated Ca2+ entry induced by Ca2+ pump blockade or in response to muscarinic or B cell receptor activation was similarly sensitive to BTP2. Using the T3-65 clonal HEK293 cell line stably expressing TRPC3 channels, TRPC3-mediated Sr2+ entry activated by muscarinic receptors was also blocked by BTP2 with an IC50 of <0.3 microM. Importantly, direct activation of TRPC3 channels by diacylglycerol was also blocked by BTP2 (IC50 approximately 0.3 microM). BTP2 still blocked TRPC3 in medium with N-methyl-D-glucamine-chloride replacing Na+, indicating BTP2 did not block divalent cation entry by depolarization induced by activating monovalent cation entry channels. Whereas whole-cell carbachol-induced TRPC3 current was blocked by 3 microM BTP2, single TRPC3 channel recordings revealed persistent short openings suggesting BTP2 reduces the open probability of the channel rather than its pore properties. TRPC5 channels transiently expressed in HEK293 cells were blocked by BTP2 in the same range as TRPC3. However, function of the highly Ca(2+)-selective TRPV6 channel, with many channel properties akin to SOCs, was entirely unaffected by BTP2. The results indicate a strong functional link between the operation of expressed TRPC channels and endogenous SOC activity.     

Differential inducible nitric oxide synthase expression in systemic and pulmonary vessels after endotoxin

Inducible nitric oxide synthase (iNOS) is associated with vascular hypocontractility in systemic vessels after endotoxin lipopolysaccharide (LPS) administration. Although lung iNOS is increased after LPS, its role in the pulmonary circulation is unclear. We hypothesized that whereas iNOS upregulation is responsible for LPS-induced vascular dysfunction in systemic vessels, iNOS does not play a significant role in the pulmonary artery (PA). Using isolated aorta (AO) and PA rings, we examined the effect of nonselective NOS inhibition [N(G)-monomethyl-L-arginine (L-NMMA); 100 micromol/l] and selective iNOS inhibition (aminoguanidine, AG; 100 micromol/l) on alpha(1)-adrenergic-mediated vasoconstriction (phenylephrine; 10(-9) to 10(-3) M) after LPS (Salmonella typhimurium, 20 mg/kg ip). We also determined the presence of iNOS using Western blot and immunohistochemistry. LPS markedly impaired AO contractility (maximal control tension 1,076 +/- 33 mg vs. LPS 412 +/- 39 mg, P < 0.05), but PA contractility was unchanged (control 466 +/- 29 mg vs. LPS 455 +/- 27 mg, P > 0.05). Selective iNOS inhibition restored the AO's response to vasoconstriction (LPS + AG 1,135 +/- 54 mg, P > 0.05 vs. control and P < 0.05 vs. LPS), but had no effect on the PA (LPS + AG 422 +/- 38 mg, P > 0.05 vs. control and LPS). Western blot and immunohistochemistry revealed increased iNOS expression in the AO after LPS, but iNOS was not detected in the PA. Our results suggest that differential iNOS expression after LPS in systemic and pulmonary vessels contributes to the phenomenon of sepsis/endotoxemia-induced systemic hypotension and pulmonary hypertension.     

Endogenous estrogen attenuates pulmonary artery vasoreactivity and acute hypoxic pulmonary vasoconstriction: the effects of sex and menstrual cycle

Sex differences exist in a variety of cardiovascular disorders. Sex hormones have been shown to mediate pulmonary artery (PA) vasodilation. However, the effects of fluctuations in physiological sex hormone levels due to sex and menstrual cycle on PA vasoreactivity have not been clearly established yet. We hypothesized that sex and menstrual cycle affect PA vasoconstriction under both normoxic and hypoxic conditions. Isometric force displacement was measured in isolated PA rings from proestrus females (PF), estrus and diestrus females (E/DF), and male (M) Sprague-Dawley rats. The vasoconstrictor response under normoxic conditions (organ bath bubbled with 95% O(2)-5% CO(2)) was measured after stimulation with 80 mmol/l KCl and 1 mumol/l phenylephrine. Hypoxia was generated by changing the gas to 95% N(2)-5% CO(2). PA rings from PF demonstrated an attenuated vasoconstrictor response to KCl compared with rings from E/DF (75.58 +/- 3.2% vs. 92.43 +/- 4.24%, P < 0.01). Rings from M also exhibited attenuated KCl-induced vasoconstriction compared with E/DF (79.34 +/- 3.2% vs. 92.43 +/- 4.24%, P < 0.05). PA rings from PF exhibited an attenuated vasoconstrictor response to phenylephrine compared with E/DF (59.61 +/- 2.98% vs. 70.03 +/- 4.61%, P < 0.05). While the maximum PA vasodilation during hypoxia did not differ between PF, E/DF, and M, phase II of hypoxic pulmonary vasoconstriction was markedly diminished in the PA from PF (64.10 +/- 7.10% vs. 83.91 +/- 5.97% in M, P < 0.05). We conclude that sex and menstrual cycle affect PA vasoconstriction in isolated PA rings. Even physiological increases in circulating estrogen levels attenuate PA vasoconstriction under both normoxic and hypoxic conditions.     

p75 neurotrophin receptor regulates agonist-induced pulmonary vasoconstriction

A member of the TNF receptor family, the p75 neurotrophin receptor (p75(NTR)) has been previously shown to play a role in the regulation of fibrin deposition in the lung. However, the role of p75(NTR) in the regulation of pulmonary vascular tone in the lung is unknown. In the present study, we evaluated the expression of p75(NTR) in mouse pulmonary arteries and the putative role of p75(NTR) in modulating pulmonary vascular tone and agonist responsiveness using wild-type (WT) and p75(NTR) knockout (p75(-/-)) mice. Our data indicated that p75(NTR) is expressed in both smooth muscle and endothelial cells within the pulmonary vascular wall in WT mice. Pulmonary artery rings from p75(-/-) mice exhibited significantly elevated active tension due to endothelin-1-mediated Ca(2+) influx. Furthermore, the contraction due to capacitative Ca(2+) entry (CCE) in response to phenylephrine-mediated active depletion of intracellular Ca(2+) stores was significantly enhanced compared with WT rings. The contraction due to CCE induced by passive store depletion, however, was comparable between WT and p75(-/-) rings. Active tension induced by serotonin, U-46619 (a thromboxane A(2) analog), thrombin, 4-aminopyridine (a K(+) channel blocker), and high extracellular K(+) in p75(-/-) rings was similar to that in WT rings. Deletion of p75(NTR) did not alter pulmonary vasodilation to sodium nitroprusside (a nitric oxide donor). These data suggest that intact p75(NTR) signaling may play a role in modulating pulmonary vasoconstriction induced by endothelin-1 and by active store depletion.     

Overexpression of TRPC3 increases apoptosis but not necrosis in response to ischemia-reperfusion in adult mouse cardiomyocytes

An increase in cytosolic Ca2+ via a capacitative calcium entry (CCE)-mediated pathway, attributed to members of the transient receptor potential (TRP) superfamily, TRPC1 and TRPC3, has been reported to play an important role in regulating cardiomyocyte hypertrophy. Increased cytosolic Ca2+ also plays a critical role in mediating cell death in response to ischemia-reperfusion (I/R). Therefore, we tested the hypothesis that overexpression of TRPC3 in cardiomyocytes will increase sensitivity to I/R injury. Adult cardiomyocytes isolated from wild-type (WT) mice and from mice overexpressing TRPC3 in the heart were subjected to 90 min of ischemia and 3 h of reperfusion. After I/R, viability was 51 +/- 1% in WT mice and 42 +/- 5% in transgenic mice (P < 0.05). Apoptosis assessed by annexin V was significantly increased in the TRPC3 group compared with WT (32 +/- 1% vs. 21 +/- 3%; P < 0.05); however, there was no significant difference in necrosis between groups. Treatment of TRPC3 cells with the CCE inhibitor SKF-96365 (0.5 microM) significantly improved cellular viability (54 +/- 4%) and decreased apoptosis (15 +/- 4%); in contrast, the L-type Ca2+ channel inhibitor verapamil (10 microM) had no effect. Calpain-mediated cleavage of alpha-fodrin was increased approximately threefold in the transgenic group following I/R compared with WT (P < 0.05); this was significantly attenuated by SKF-96365. The calpain inhibitor PD-150606 (25 microM) attenuated the increase in both alpha-fodrin cleavage and apoptosis in the TPRC3 group. Increased TRPC3 expression also increased sensitivity to Ca2+ overload stress, but it did not affect the response to TNF-alpha-induced apoptosis. These results suggest that CCE mediated via TRPC may play a role in cardiomyocyte apoptosis following I/R due, at least in part, to increased calpain activation.     

Effect of changes in pH on wall tension in isolated rat pulmonary artery: role of the RhoA/Rho-kinase pathway

Pulmonary arteries (PA) are resistant to the vasodilator effects of extracellular acidosis in systemic vessels; the mechanism underlying this difference between systemic and pulmonary circulations has not been elucidated. We hypothesized that RhoA/Rho-kinase-mediated Ca2+ sensitization pathway played a greater role in tension development in pulmonary than in systemic vascular smooth muscle and that this pathway was insensitive to acidosis. In arterial rings contracted with the alpha1-agonist phenylephrine (PE), the Rho-kinase inhibitor Y-27632 (< or =3 microM) induced greater relaxation in precontracted PA rings than in aortic rings. In PA rings stimulated by PE, the activation of RhoA was greater than in aorta. Normocapnic acidosis (NA) induced a smaller relaxation in precontracted PA than in aorta. However, in the presence of nifedipine and thapsigargin, when PE-induced contraction was predominantly mediated by Rho-kinase, the relaxant effect of NA was reduced and similar in both vessel types. Furthermore, in the presence of Y-27632, NA induced a greater relaxation in both PA and aorta, which was similar in both vessels. Finally, in alpha-toxin-permeabilized smooth muscle, PE-induced contraction at constant Ca2+ activity was inhibited by Y-27632 and unaffected by acidosis. These results indicate that Ca2+ sensitization induced by the RhoA/Rho-kinase pathway played a greater role in agonist-induced vascular smooth muscle contraction in PA than in aorta and that tension mediated by this pathway was insensitive to acidosis. The predominant role of the RhoA/Rho-kinase pathway in the pulmonary vasculature may account for the resistance of this circulation to the vasodilator effect of acidosis observed in the systemic circulation.     

Role of capacitative Ca2+ entry in bronchial contraction and remodeling.

Asthma is characterized by airway inflammation, bronchial hyperresponsiveness, and airway obstruction by bronchospasm and bronchial wall thickening due to smooth muscle hypertrophy. A rise in cytosolic free Ca2+ concentration ([Ca2+]cyt) may serve as a shared signal transduction element that causes bronchial constriction and bronchial wall thickening in asthma. In this study, we examined whether capacitative Ca2+ entry (CCE) induced by depletion of intracellular Ca2+ stores was involved in agonist-mediated bronchial constriction and bronchial smooth muscle cell (BSMC) proliferation. In isolated bronchial rings, acetylcholine (ACh) induced a transient contraction in the absence of extracellular Ca2+ because of Ca2+ release from intracellular Ca2+ stores. Restoration of extracellular Ca2+ in the presence of atropine, an M-receptor blocker, induced a further contraction that was apparently caused by a rise in [Ca2+]cyt due to CCE. In single BSMC, amplitudes of the store depletion-activated currents (I(SOC)) and CCE were both enhanced when the cells proliferate, whereas chelation of extracellular Ca2+ with EGTA significantly inhibited the cell growth in the presence of serum. Furthermore, the mRNA expression of TRPC1, a transient receptor potential channel gene, was much greater in proliferating BSMC than in growth-arrested cells. Blockade of the store-operated Ca2+ channels by Ni2+ decreased I(SOC) and CCE and markedly attenuated BSMC proliferation. These results suggest that upregulated TRPC1 expression, increased I(SOC), enhanced CCE, and elevated [Ca2+]cyt may play important roles in mediating bronchial constriction and BSMC proliferation.     

Capacitative calcium entry and TRPC channel proteins are expressed in rat distal pulmonary arterial smooth muscle

Mammalian homologs of transient receptor potential (TRP) genes in Drosophila encode TRPC proteins, which make up cation channels that play several putative roles, including Ca2+ entry triggered by depletion of Ca2+ stores in endoplasmic reticulum (ER). This capacitative calcium entry (CCE) is thought to replenish Ca2+ stores and contribute to signaling in many tissues, including smooth muscle cells from main pulmonary artery (PASMCs); however, the roles of CCE and TRPC proteins in PASMCs from distal pulmonary arteries, which are thought to be the major site of pulmonary vasoreactivity, remain uncertain. As an initial test of the possibility that TRPC channels contribute to CCE and Ca2+ signaling in distal PASMCs, we measured [Ca2+]i by fura-2 fluorescence in primary cultures of myocytes isolated from rat intrapulmonary arteries (>4th generation). In cells perfused with Ca2+-free media containing cyclopiazonic acid (10 microM) and nifedipine (5 microM) to deplete ER Ca2+ stores and block voltage-dependent Ca2+ channels, restoration of extracellular Ca2+ (2.5 mM) caused marked increases in [Ca2+]i whereas MnCl2 (200 microM) quenched fura-2 fluorescence, indicating CCE. SKF-96365, LaCl3, and NiCl2, blocked CCE at concentrations that did not alter Ca2+ responses to 60 mM KCl (IC50 6.3, 40.4, and 191 microM, respectively). RT-PCR and Western blotting performed on RNA and protein isolated from distal intrapulmonary arteries and PASMCs revealed mRNA and protein expression for TRPC1, -4, and -6, but not TRPC2, -3, -5, or -7. Our results suggest that CCE through TRPC-encoded Ca2+ channels could contribute to Ca2+ signaling in myocytes from distal intrapulmonary arteries.     

A role for hTRPC1 and lipid raft domains in store-mediated calcium entry in human platelets

We have previously suggested that store-mediated Ca2+ entry (SMCE) in human platelets may be activated by a secretion-like coupling model, involving de novo coupling of the type II inositol 1,4,5-trisphosphate receptor (IP(3)RII) to the putative Ca2+ entry channel, hTRPC1. In other cells, hTRPC1 has been reported to be associated with cholesterol-rich lipid raft domains (LRDs) in the plasma membrane. Here we have shown that hTRPC1 is largely associated with detergent-resistant platelet membranes, from which it is partially released when the cells are depleted of cholesterol by treatment with methyl-beta-cyclodextrin (MBCD). MBCD treatment inhibited thapsigargin (TG)-evoked SMCE in a concentration-dependent manner, reducing it to 38.1+/-4.1% at a concentration of 10mM. Similarly, the Ca2+ entry evoked by thrombin (1unit/ml) was reduced to 48.2+/-4.5% of control following MBCD (10mM) treatment. Thrombin- and TG-evoked coupling between IP(3)RII and hTRPC1 was also reduced following cholesterol depletion. These results suggest that hTRPC1 is associated with LRDs in human platelets and that these domains are important for its participation in SMCE.