Na(+) entry via store-operated channels modulates Ca(2+) signaling in arterial myocytes

Endogenous and exogenous cardiac glycosides and their mechanisms of action

Cardiac glycosides have been used for decades to treat congestive heart failure. The recent identification of cardiotonic steroids such as ouabain, digoxin, marinobufagenin, and telocinobufagin in blood plasma, adrenal glands, and hypothalamus of mammals led to exciting new perspectives in the pathology of heart failure and arterial hypertension. Biosynthesis of ouabain and digoxin occurs in adrenal glands and is under the control of angiotensin II, endothelin, and epinephrine released from cells of the midbrain upon stimulation of brain areas sensing cerebrospinal Na(+) concentration and, apparently, the body's K(+) content. Rapid changes of endogenous ouabain upon physical exercise may favor the economy of the heart by a rise of intracellular Ca(2)(+) levels in cardiac and atrial muscle cells. According to the sodium pump lag hypothesis, this may be accomplished by partial inhibition of the sodium pump and Ca(2+) influx via the Na(+)/Ca(2+) exchanger working in reverse mode or via activation of the Na(+)/K(+)-ATPase signalosome complex, generating intracellular calcium oscillations, reactive oxygen species, and gene activation via nuclear factor-kappaB or extracellular signal-regulated kinases 1 and 2. Elevated concentrations of endogenous ouabain and marinobufagenin in the subnanomolar concentration range were found to stimulate proliferation and differentiation of cardiac and smooth muscle cells. They may have a primary role in the development of cardiac dysfunction and failure because (i) offspring of hypertensive patients evidently inherit elevated plasma concentrations of endogenous ouabain; (ii) such elevated concentrations correlate positively with cardiac dysfunction, hypertrophy, and arterial hypertension; (iii) about 40% of Europeans with uncomplicated essential hypertension show increased concentrations of endogenous ouabain associated with reduced heart rate and cardiac hypertrophy; (iv) in patients with advanced arterial hypertension, circulating levels of endogenous ouabain correlate with BP and total peripheral resistance; (v) among patients with idiopathic dilated cardiomyopathy, high circulating levels of endogenous ouabain and marinobufagenin identify those individuals who are predisposed to progressing more rapidly to heart failure, suggesting that endogenous ouabain (and marinobufagenin) may contribute to toxicity upon digoxin therapy. In contrast to endogenous ouabain, endogenous marinobufagenin may act as a natriuretic substance as well. It shows a higher affinity for the ouabain-insensitive alpha(1) isoform of Na(+)/K(+)-ATPase of rat kidney tubular cells and its levels are increased in volume expansion and pre-eclampsia. Digoxin, which is synthesized in adrenal glands, seems to counteract the hypertensinogenic action of ouabain in rats, as do antibodies against ouabain, for example, (Digibind) and rostafuroxin (PST 2238), a selective ouabain antagonist. It lowers BP in ouabain- and adducin-dependent hypertension in rats and is a promising new class of antihypertensive medication in humans.

Upregulation of Na+/Ca2+ exchanger contributes to the enhanced Ca2+ entry in pulmonary artery smooth muscle cells from patients with idiopathic pulmonary arterial hypertension

A rise in cytosolic Ca(2+) concentration ([Ca(2+)](cyt)) in pulmonary artery smooth muscle cells (PASMC) is a trigger for pulmonary vasoconstriction and a stimulus for PASMC proliferation and migration. Multiple mechanisms are involved in regulating [Ca(2+)](cyt) in human PASMC. The resting [Ca(2+)](cyt) and Ca(2+) entry are both increased in PASMC from patients with idiopathic pulmonary arterial hypertension (IPAH), which is believed to be a critical mechanism for sustained pulmonary vasoconstriction and excessive pulmonary vascular remodeling in these patients. Here we report that protein expression of NCX1, an NCX family member of Na(+)/Ca(2+) exchanger proteins is upregulated in PASMC from IPAH patients compared with PASMC from normal subjects and patients with other cardiopulmonary diseases. The Na(+)/Ca(2+) exchanger operates in a forward (Ca(2+) exit) and reverse (Ca(2+) entry) mode. By activating the reverse mode of Na(+)/Ca(2+) exchange, removal of extracellular Na(+) caused a rapid increase in [Ca(2+)](cyt), which was significantly enhanced in IPAH PASMC compared with normal PASMC. Furthermore, passive depletion of intracellular Ca(2+) stores using cyclopiazonic acid (10 microM) not only caused a rise in [Ca(2+)](cyt) due to Ca(2+) influx through store-operated Ca(2+) channels but also mediated a rise in [Ca(2+)](cyt) via the reverse mode of Na(+)/Ca(2+) exchange. The upregulated NCX1 in IPAH PASMC led to an enhanced Ca(2+) entry via the reverse mode of Na(+)/Ca(2+) exchange, but did not accelerate Ca(2+) extrusion via the forward mode of Na(+)/Ca(2+) exchange. These observations indicate that the upregulated NCX1 and enhanced Ca(2+) entry via the reverse mode of Na(+)/Ca(2+) exchange are an additional mechanism responsible for the elevated [Ca(2+)](cyt) in PASMC from IPAH patients.

Na+ entry via TRPC6 causes Ca2+ entry via NCX reversal in ATP stimulated smooth muscle cells

Reversal of the Na+/Ca2+ -exchanger (NCX) has been shown to mediate Ca2+ influx during activation of G-protein linked receptors. Functional coupling between the reverse-mode NCX and the canonical transient receptor potential channels (TRPCs) has been proposed to mediate Ca2+ influx in HEK-293 cells overexpressing TRPC3. In this communication we present evidence for similar functional coupling of NCX to endogenously expressed TRPC6 in rat aorta smooth muscle cells. Selective inhibition of reverse-mode NCX with KB-R7943 and of non-selective cation-channels with SKF-96365 abolished Ca2+ influx in response to agonist stimulation (ATP). Expression of a dominant negative TRPC6 mutant also reduced the Ca2+ influx in proportion to its transfection efficiency. Calyculin A, which is known to disrupt the junctions of the plasma membrane and sarco/endoplasmic reticulum, increased global Na+ elevations and reduced stimulated Ca2+ influx. Together our data provide evidence that localized Na+ elevations are generated by TRPC6 and drive reversal of NCX to mediate Ca2+ influx.

The reverse mode of the Na(+)/Ca(2+) exchanger provides a source of Ca(2+) for store refilling following agonist-induced Ca(2+) mobilization

Agonist-induced contraction of airway smooth muscle (ASM) can be triggered by an elevation in the intracellular Ca(2+) concentration, primarily through the release of Ca(2+) from the sarcoplasmic reticulum (SR). The refilling of the SR is integral for subsequent contractions. It has been suggested that Ca(2+) entry via store-operated cation (SOC) and receptor-operated cation channels may facilitate refilling of the SR. Indeed, depletion of the SR activates substantial inward SOC currents in ASM that are composed of both Ca(2+) and Na(+). Accumulation of Na(+) within the cell may regulate Ca(2+) handling in ASM by forcing the Na(+)/Ca(2+) exchanger (NCX) into the reverse mode, leading to the influx of Ca(2+) from the extracellular domain. Since depletion of the SR activates substantial inward Na(+) current, it is conceivable that the reverse mode of the NCX may contribute to the intracellular Ca(2+) pool from which the SR is refilled. Indeed, successive contractions of bovine ASM, evoked by various agonists (ACh, histamine, 5-HT, caffeine) were significantly reduced upon removal of extracellular Na(+); whereas contractions evoked by KCl were unchanged by Na(+) depletion. Ouabain, a selective inhibitor of the Na(+)/K(+) pump, had no effect on the reductions observed under normal and zero-Na(+) conditions. KB-R7943, a selective inhibitor of the reverse mode of the NCX, significantly reduced successive contractions induced by all agonists without altering KCl responses. Furthermore, KB-R7943 abolished successive caffeine-induced Ca(2+) transients in single ASM cells. Together, these data suggest a role for the reverse mode of the NCX in refilling the SR in ASM following Ca(2+) mobilization.

Role of Cav1.2 L-type Ca2+ channels in vascular tone: effects of nifedipine and Mg2+

Ca(2+) entry via L-type voltage-gated Ca(2+) channels (LVGCs) is a key factor in generating myogenic tone (MT), as dihydropyridines (DHPs) and other LVGC blockers, including Mg(2+), markedly reduce MT. Recent reports suggest, however, that elevated external Mg(2+) concentration and DHPs may also inhibit other Ca(2+)-entry pathways. Here, we explore the contribution of LVGCs to MT in intact, pressurized mesenteric small arteries using mutant mice (DHP(R/R)) expressing functional but DHP-insensitive Ca(v)1.2 channels. In wild-type (WT), but not DHP(R/R), mouse arteries, nifedipine (0.3-1.0 microM) markedly reduced MT and vasoconstriction induced by high external K(+) concentrations ([K(+)](o)), a measure of LVGC-mediated Ca(2+) entry. Blocking MT and high [K(+)](o)-induced vasoconstriction by <1 microM nifedipine in WT but not in DHP(R/R) arteries implies that Ca(2+) entry via Ca(v)1.2 LVGCs is obligatory for MT and that nifedipine inhibits MT exclusively by blocking LVGCs. We also examined the effects of Mg(2+) on MT and LVGCs. High external Mg(2+) concentration (10 mM) blocked MT, slowed the high [K(+)](o)-induced vasoconstrictions, and decreased their amplitude in WT and DHP(R/R) arteries. To verify that these effects of Mg(2+) are due to block of LVGCs, we characterized the effects of extracellular and intracellular Mg(2+) on LVGC currents in isolated mesenteric artery myocytes. DHP-sensitive LVGC currents are inhibited by both external and internal Mg(2+). The results indicate that Mg(2+) relaxes MT by inhibiting Ca(2+) influx through LVGCs. These data provide new information about the central role of Ca(v)1.2 LVGCs in generating and maintaining MT in mouse mesenteric small arteries.

Local subplasma membrane Ca2+ signals detected by a tethered Ca2+ sensor

Accumulating evidence indicates that plasma membrane (PM) microdomains and the subjacent "junctional" sarcoplasmic/endoplasmic reticulum (jS/ER) constitute specialized Ca(2+) signaling complexes in many cell types. We examined the possibility that some Ca(2+) signals arising in the junctional space between the PM and jS/ER may represent cross-talk between the PM and jS/ER. The Ca(2+) sensor protein, GCaMP2, was targeted to different PM domains by constructing genes for fusion proteins with either the alpha1 or alpha2 isoform of the Na(+) pump catalytic (alpha) subunit. These fusion proteins were expressed in primary cultured mouse brain astrocytes and arterial smooth muscle cells. Immunocytochemistry demonstrated that alpha2(f)GCaMP2, like native Na(+) pumps with alpha2-subunits, sorted to PM domains that colocalized with subjacent S/ER; alpha1(f)GCaMP2, like Na(+) pumps with alpha1-subunits, was more uniformly distributed. The GCaMP2 moieties in both constructs were tethered just beneath the PM. Both constructs detected global Ca(2+) signals evoked by serotonin (in arterial smooth muscle cells) and ATP, and by store-operated Ca(2+) channel-mediated Ca(2+) entry after S/ER unloading with cyclopiazonic acid (in Ca(2+)-free medium). When cytosolic Ca(2+) diffusion was markedly restricted with EGTA, however, only alpha2(f)GCaMP2 detected the local, store-operated Ca(2+) channel-mediated Ca(2+) entry signal. Thus, alpha1 Na(+) pumps are apparently excluded from the PM microdomains occupied by alpha2 Na(2+) pumps. The jS/ER and adjacent PM may communicate by Ca(2+) signals that are confined to the tiny junctional space between the two membranes. Similar methods may be useful for studying localized Ca(2+) signals in other subPM microdomains and signals associated with other organelles.

A TRPC-like non-selective cation current activated by α1-adrenoceptors in rat mesenteric artery smooth muscle cells

The TRPC family of non-selective cation channels has been suggested to play a key role in the responses to alpha1-adrenoceptor stimulation of vascular smooth muscle. However, there are still very few reports of non-selective cation currents activated by alpha1-AR in resistance arteries. Here, we examine the expression of TRPC channels and the currents activated by alpha1-adrenoceptors in rat mesenteric resistance artery smooth muscle. Messenger RNA and protein for TRPC1, TRPC3 and TRPC6 were detected within the arteries by RT-PCR and immunoblotting. Endothelial and adventitial layers were found to express the TRPC1, TRPC3 and TRPC6 proteins whereas only TRPC1 and TRPC6 were detected in the arterial smooth muscle by confocal immunofluorescence microscopy. In whole-cell patch-clamp recordings from isolated mesenteric arterial myocytes, an outwardly rectifying non-selective cation current was activated by both the alpha1-adrenoceptor agonist, phenylephrine (10 microM), and the diacylglycerol analogue, 1-oleoyl-2-acetyl-sn-glycerol (100 microM). Responses to 1-oleoyl-2-acetyl-sn-glycerol were not blocked, but increased, following inhibition of protein-kinase-C with either bisindolylmaleimide-I (1 microM) or chelerythrine (1 microM). The currents activated by both phenylephrine and 1-oleoyl-2-acetyl-sn-glycerol were inhibited by Gd3+ (100 microM) but potentiated by flufenamic acid (100 microM). Collectively, these findings demonstrate for the first time the expression of TRPC1 and TRPC6 in rat mesenteric artery smooth muscle and the existence in rat isolated mesenteric arterial myocytes of a TRPC-like non-selective cation current activated by alpha1-adrenoceptor stimulation and 1-oleoyl-2-acetyl-sn-glycerol.

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.

VEGF activates receptor-operated cation channels in human microvascular endothelial cells

OBJECTIVE

Vascular endothelial growth factor (VEGF) exerts many of its effects by stimulating endothelial calcium influx, but little is known about channels mediating VEGF-induced cation entry. The aim of this study was to measure and characterize for the first time the VEGF-activated cation current in human microvascular endothelial cells (HMVECs).

METHODS AND RESULTS

Whole-cell patch-clamp recordings were made from HMVECs. During applied voltage ramps, VEGF activated a current that reversed at 0 mV, was sensitive to gadolinium, and required extracellular cations. Noise analysis yielded a single-channel conductance of 27 pS. The current was not dependent on intracellular calcium stores, and was not blocked by inositol triphosphate (IP3) receptor or serine/threonine kinase inhibition but was partially inhibited by flufenamic acid. A similar current was activated by 1-oleoyl-2-acetyl-sn-glycerol (OAG), a membrane-permeant analog of diacylglycerol (DAG). To determine whether VEGF could activate recombinant ion channels with similar properties, we investigated the effect of VEGF on Chinese hamster ovary cells cotransfected with VEGFR2 and the canonical transient receptor potential (TRPC) channels, TRPC3 or TRPC6. VEGF induced a similar current to that described above in VEGFR2-TRPC3 and VEGFR2-TRPC6 cells but not in cells transfected with either cDNA alone.

CONCLUSIONS

VEGF activates a receptor-operated cation current in HMVECs and OAG can activate directly a similar current in these cells. VEGF is also able to activate heterologously expressed TRPC3/6 channels through VEGFR2.

Novel role for K+-dependent Na+/Ca2+ exchangers in regulation of cytoplasmic free Ca2+ and contractility in arterial smooth muscle

Cytoplasmic free Ca2+ ([Ca2+]cyt) is essential for the contraction and relaxation of blood vessels. The role of plasma membrane Na+/Ca2+ exchange (NCX) activity in the regulation of vascular Ca2+ homeostasis was previously ascribed to the NCX1 protein. However, recent studies suggest that a relatively newly discovered K+-dependent Na+/Ca2+ exchanger, NCKX (gene family SLC24), is also present in vascular smooth muscle. The purpose of the present study was to identify the expression and function of NCKX in arteries. mRNA encoding NCKX3 and NCKX4 was demonstrated by RT-PCR and Northern blot in both rat mesenteric and aortic smooth muscle. NCXK3 and NCKX4 proteins were also demonstrated by immunoblot and immunofluorescence. After voltage-gated Ca2+ channels, store-operated Ca2+ channels, and Na+ pump were pharmacologically blocked, when the extracellular Na+ was replaced with Li+ (0 Na+) to induce reverse mode (Ca2+ entry) activity of Na+/Ca2+ exchangers, a large increase in [Ca2+]cyt signal was observed in primary cultured aortic smooth muscle cells. About one-half of this [Ca2+]cyt signal depended on the extracellular K+. In addition, after the activity of NCX was inhibited by KB-R7943, Na+ replacement-induced Ca2+ entry was absolutely dependent on extracellular K+. In arterial rings denuded of endothelium, a significant fraction of the phenylephrine-induced and nifedipine-resistant aortic or mesenteric contraction could be prevented by removal of extracellular K+. Taken together, these data provide strong evidence for the expression of NCKX proteins in the vascular smooth muscle and their novel role in mediating agonist-stimulated [Ca2+]cyt and thereby vascular tone.

Ouabain modulation of endothelial calcium signaling in descending vasa recta

Using fura 2-loaded vessels, we tested whether ouabain modulates endothelial cytoplasmic calcium concentration ([Ca(2+)](CYT)) in rat descending vasa recta (DVR). Over a broad range between 10(-10) and 10(-4) M, ouabain elicited biphasic peak and plateau [Ca(2+)](CYT) elevations. Blockade of voltage-gated Ca(2+) entry with nifedipine did not affect the response to ouabain mitigating against a role for myo-endothelial gap junctions. Reduction of extracellular Na(+) concentration ([Na(+)](o)) or Na(+)/Ca(2+) exchanger (NCX) inhibition with SEA-0400 (10(-6) M) elevated [Ca(2+)](CYT), supporting a role for NCX in the setting of basal [Ca(2+)](CYT). SEA-0400 abolished the [Ca(2+)](CYT) response to ouabain implicating NCX as a mediator. The transient peak phase of [Ca(2+)](CYT) elevation that followed either ouabain or reduction of [Na(+)](o) was abolished by 2-aminoethoxydiphenyl borate (5 x 10(-5) M). Cation channel blockade with La(3+) (10 muM) or SKF-96365 (10 muM) also attenuated the ouabain-induced [Ca(2+)](CYT) response. Ouabain pretreatment increased the [Ca(2+)](CYT) elevation elicited by bradykinin (10(-7) M). We conclude that inhibition of ouabain-sensitive Na(+)-K(+)-ATPase enhances DVR endothelial Ca(2+) store loading and modulates [Ca(2+)](CYT) signaling through mechanisms that involve NCX, Ca(2+) release, and cation channel activation.

TRPC1 functions as a store-operated Ca2+ channel in intestinal epithelial cells and regulates early mucosal restitution after wounding

An increase in cytosolic free Ca(2+) concentration ([Ca(2+)](cyt)) results from Ca(2+) release from intracellular stores and extracellular Ca(2+) influx through Ca(2+)-permeable ion channels and is crucial for initiating intestinal epithelial restitution to reseal superficial wounds after mucosal injury. Capacitative Ca(2+) entry (CCE) induced by Ca(2+) store depletion represents a major Ca(2+) influx mechanism, but the exact molecular components constituting this process remain elusive. This study determined whether canonical transient receptor potential (TRPC)1 served as a candidate protein for Ca(2+)-permeable channels mediating CCE in intestinal epithelial cells and played an important role in early epithelial restitution. Normal intestinal epithelial cells (the IEC-6 cell line) expressed TRPC1 and TPRC5 and displayed typical records of whole cell store-operated Ca(2+) currents and CCE generated by Ca(2+) influx after depletion of intracellular stores. Induced TRPC1 expression by stable transfection with the TRPC1 gene increased CCE and enhanced cell migration during restitution. Differentiated IEC-Cdx2L1 cells induced by forced expression of the Cdx2 gene highly expressed endogenous TRPC1 and TRPC5 and exhibited increased CCE and cell migration. Inhibition of TRPC1 expression by small interfering RNA specially targeting TRPC1 not only reduced CCE but also inhibited cell migration after wounding. These findings strongly suggest that TRPC1 functions as store-operated Ca(2+) channels and plays a critical role in intestinal epithelial restitution by regulating CCE and intracellular [Ca(2+)](cyt).

Mechanism of ACh-induced asynchronous calcium waves and tonic contraction in porcine tracheal muscle bundle

Stimulation of the tracheal muscle bundle by acetylcholine (ACh) results in the generation of asynchronous repetitive Ca2+ waves (ACW) in intact tracheal smooth muscle (TSM) cells. We showed previously that ACW underlie cholinergic excitation-contraction coupling in porcine TSM and that Ca2+ entry through the L-type voltage-gated Ca2+ channel (VGCC) contributes partially to maintenance of the ACW. However, the mechanism of the ACW remains undefined. In this study, we pharmacologically characterized the mechanism of ACh-induced ACW in the intact porcine tracheal muscle bundle. We found that inhibition of receptor-operated channels/store-operated channels (ROC/SOC) by SKF-96365 completely abolished the nifedipine-insensitive component of ACh-mediated ACW and tonic contraction. Blockade of Na+/Ca2+ exchange with KB-R7943 or 2′,4′-dichlorobenzamil or removal of extracellular Na+ resulted in nearly complete inhibition of the nifedipine-insensitive component of ACh-mediated ACW and tonic contraction. Inhibition of the sarco(endo)plasmic reticulum Ca2+-ATPase by cyclopiazonic acid abolished the ongoing ACW. Application of 2-aminoethoxydiphenyl borate (2-APB) or xestospongin C to inhibit the inositol 1,4,5-trisphosphate-sensitive sarcoplasmic reticulum (SR) Ca2+ release channels produced no effect on ACh-mediated ACW and tonic contraction. However, pretreatment with caffeine or ryanodine inhibited ACh-induced ACW. Furthermore, application of procaine or tetracaine prevented the generation and abolished the ongoing ACh-mediated ACW and tonic contraction. Collectively, these results indicate that the ACh-stimulated ACW in porcine TSM are produced by repetitive cycles of Ca2+ release from SR through 2-APB- and xestospongin C-insensitive Ca2+ release channels, and plasmalemmal Ca2+ entry involving reverse-mode Na+/Ca2+ exchange, ROC/SOC, and L-type VGCC is required to refill the SR via SERCA to support the ongoing ACW.

Extracellular zinc and ATP-gated P2X receptor calcium entry channels: New zinc receptors as physiological sensors and therapeutic targets

In this review, we focus on two attributes of P2X receptor channel function, one essential and one novel. First, we propose that P2X receptors are extracellular sensors as well as receptors and ion channels. In particular, the large extracellular domain (that comprises 70% of the molecular mass of the receptor channel protein) lends itself to be a cellular sensor. Moreover, its exquisite sensitivity to extracellular pH, ionic strength, and multiple ligands evokes the function of a sensor. Second, we propose that P2X receptors are extracellular zinc receptors as well as receptors for nucleotides. We provide novel data in multiple publications and illustrative data in this invited review to suggest that zinc triggers ATP-independent activation of P2X receptor channel function. In this light, P2X receptors are the cellular site of integration between autocrine and paracrine zinc signaling and autocrine and paracrine purinergic signaling. P2X receptors may sense changes in these ligands as well as in extracellular pH and ionic strength and transduce these sensations via calcium and/or sodium entry and changes in membrane potential.

CIF and other mysteries of the store-operated Ca2+-entry pathway

The molecular mechanism of the store-operated Ca2+-entry (SOCE) pathway remains one of the most intriguing and long lasting mysteries of Ca2+ signaling. The elusive calcium influx factor (CIF) that is produced upon depletion of Ca2+ stores has attracted growing attention, triggered by new discoveries that filled the gap in the chain of reactions leading to activation of store-operated channels and Ca2+ entry. Ca2+-independent phospholipase A2 emerged as a target of CIF, and a major determinant of the SOCE mechanism. Here, we present our viewpoint on CIF and conformational-coupling models of SOCE from a historical perspective, trying to resolve some of the problem areas, and summarizing our present knowledge on how depletion of intracellular Ca2+ stores signals to plasma membrane channels to open and provide Ca2+ influx that is required for many important physiological functions.

A store-operated nonselective cation channel in human lymphocytes

1. Agonist interaction with phospholipase C-linked receptors at the plasma membrane can elicit both Ca2+ and Na+ influxes in lymphocytes. While Ca2+ influx is mediated by Ca2+ release-activated Ca2+ (CRAC) channels, the pathway responsible for Na+ influx is largely unknown. 2. We show that thapsigargin, ionomycin, ADP-ribose and IP3 activated a nonselective cation channel in lymphocytes that had a slightly outwardly rectifying I-V relationship, and a single channel conductance of 23.1 pS. We termed this channel a Ca2+ release-activated nonselective cation (CRANC) channel. 3. On activation in cell-attached configuration, switching to an inside-out configuration abolished CRANC channel activity. 4. Transfection of Jurkat T cells with antisense oligonucleotides for LTRPC2 reduced capacitative Ca2+ entry. 5. These results suggest that CRANC channels are responsible for the Na+ influx as well as a portion of the Ca2+ influx in lymphocytes induced by store depletion, that sustained activation of CRANC channels requires some property of the environment of a cell depleted of its Ca2+ stores; and that LTRPC2 protein is a likely component of the CRANC channel.

Endogenous Cardiac Glycosides: Hormones Using the Sodium Pump as Signal Transducer

The search for an endogenous digitalis has led to the identification of the cardenolides ouabain and digoxin and the bufadienolide marinobufagenin in mammalian tissues and biological fluids. Ouabain's release from adrenal glands is under the control of epinephrine and angiotensin II; hence, its blood concentration changes rapidly on physical exercise. It also is controlled by brain areas sensing cerebrospinal Na+ concentration and apparently the body's K+ content because urinary K+ loss leads to an increase in its plasma concentration as well. Long-term treatment of rats with ouabain results in arterial hypertension, and 50% of Caucasians with low-renin hypertension have increased plasma concentrations of this cardenolide. Levels of digoxin, which is synthesized from acetate in adrenal glands, increase slightly in blood on prolonged exercise. It counteracts the hypertensinogenic action of ouabain in rats, as does the ouabain antagonist PST 2238. The plasma concentration of the bufadienolide marinobufagenin is increased after cardiac infarction. It may show natriuretic properties because it inhibits the alpha1 isoform of Na+/K+-adenosine triphosphatase (ATPase), the main sodium pump isoform of the kidney, much better than other sodium pump isoforms. These effects of endogenous cardiac glycosides are observed at concentrations that do not inhibit the sodium pump. Apparently, Na+/K+-ATPase is used by these steroids as a signal transducer to activate tissue proliferation, heart contractility, arterial hypertension, and natriuresis via various intracellular signaling pathways.

Identification of functional voltage-gated Na(+) channels in cultured human pulmonary artery smooth muscle cells

Electrical excitability, which plays an important role in excitation-contraction coupling in the pulmonary vasculature, is regulated by transmembrane ion flux in pulmonary artery smooth muscle cells (PASMC). This study aimed to characterize the electrophysiological properties and molecular identities of voltage-gated Na(+) channels in cultured human PASMC. We recorded tetrodotoxin (TTX) sensitive and rapidly inactivating Na(+) currents with properties similar to those described in cardiac myocytes. Using RT-PCR, we detected transcripts of seven Na(+) channel alpha genes (SCN2A, 3A, 4A, 7A, 8A, 9A, and 11A), and two beta subunit genes (SCN1B and 2B). Our results demonstrate that human PASMC express TTX-sensitive voltage-gated Na(+) channels. Their physiological functions remain unresolved, although our data suggest that Na(+) channel activity does not directly influence membrane potential, intracellular Ca(2+) release, or proliferation in normal human PASMC. Whether their expression and/or activity are heightened in the pathological state is discussed.

Na(+)/Ca(2+) exchange regulates Ca(2+)-dependent duodenal mucosal ion transport and HCO(3)(-) secretion in mice

Stimulation of muscarinic receptors in duodenal mucosa raises intracellular Ca(2+), which regulates ion transport, including HCO(3)(-) secretion. However, the underlying Ca(2+) handling mechanisms are poorly understood. The aim of the present study was to determine whether Na(+)/Ca(2+) exchanger (NCX) plays a role in the regulation of duodenal mucosal ion transport and HCO(3)(-) secretion by controlling Ca(2+) homeostasis. Mouse duodenal mucosa was mounted in Ussing chambers. Net ion transport was assessed as short-circuit current (I(sc)), and HCO(3)(-) secretion was determined by pH-stat. Expression of NCX in duodenal mucosae was analyzed by Western blot, and cytosolic Ca(2+) in duodenocytes was measured by fura 2. Carbachol (100 muM) increased I(sc) in a biphasic manner: an initial transient peak within 2 min and a later sustained plateau starting at 10 min. Carbachol-induced HCO(3)(-) secretion peaked at 10 min. 2-Aminoethoxydiphenylborate (2-APB, 100 muM) or LiCl (30 mM) significantly reduced the initial peak in I(sc) by 51 or 47%, respectively, and abolished the plateau phase of I(sc) without affecting HCO(3)(-) secretion induced by carbachol. Ryanodine (100 muM), caffeine (10 mM), and nifedipine (10 muM) had no effect on either response to carbachol. In contrast, nickel (5 mM) and KB-R7943 (10-30 muM) significantly inhibited carbachol-induced increases in duodenal mucosal I(sc) and HCO(3)(-) secretion. Western blot analysis showed expression of NCX1 proteins in duodenal mucosae, and functional NCX in duodenocytes was demonstrated in Ca(2+) imaging experiments where Na(+) depletion elicited Ca(2+) entry via the reversed mode of NCX. These results indicate that NCX contributes to the regulation of Ca(2+)-dependent duodenal mucosal ion transport and HCO(3)(-) secretion that results from stimulation of muscarinic receptors.

Calcium, mitochondria and oxygen sensing in the pulmonary circulation

A key event in hypoxic pulmonary vasoconstriction (HPV) is the elevation in smooth muscle intracellular Ca2+ concentration. However, there is controversy concerning the source of this Ca2+, the signal transduction pathways involved, and the identity of the oxygen sensor. Although there is wide support for the hypothesis that hypoxia elicits depolarisation via inhibition of K+ channels, and thus promotes Ca2+ entry through L-type channels, a significant number of studies are inconsistent with this mechanism being either the sole or even major means by which Ca2+ is elevated during HPV. There is strong evidence that intracellular Ca2+ stores play a critical role, and voltage-independent Ca2+ entry mechanisms including capacitative Ca2+ entry (CCE) have also been implicated. There is renewed interest in the role of mitochondria in HPV, both in terms of modulators of Ca2+ homeostasis per se and as oxygen sensors. There is however considerable uncertainty concerning the mechanisms involved in the latter, with proposals for changes in redox couples and both an increase and decrease in mitochondrial production of reactive oxygen species (ROS). In this article we review the evidence for and against involvement of such mechanisms in HPV, and propose a model for the regulation of intracellular [Ca2+] in pulmonary artery during hypoxia in which the mitochondria play a central role.

Calyculin-A disrupts subplasmalemmal junction and recurring Ca2+ waves in vascular smooth muscle

Excitation-contraction coupling (E-C coupling) in phenylephrine(PE)-stimulated rabbit inferior vena cava (IVC) depends on the generation of asynchronous recurring Ca2+ waves in the in situ vascular smooth muscle cells (VSMC). Previous studies by our group have implicated a putative non-selective cationic store-operated channel and the reverse-mode Na+-Ca2+ exchange in refilling of the intracellular Ca2+ store via the sarco/endoplasmic reticulum ATPase (SERCA) and the maintenance of the recurring Ca2+ waves. We hypothesize that for the proper functioning of these three Ca2+ translocators in the process of SR refilling, the plasma membrane (PM) and the underlying superficial sarcoplasmic reticulum (SR) form specialized PM-SR junctions, which are essential for the maintenance of the recurring Ca2+ waves. In order to test this hypothesis, calyculin-A, a serine/threonine phosphatase inhibitor that has been demonstrated to result in the disruption of the PM-SR junctions was used. In the control rabbit IVC, electron microscopy of the in situ VSMC indicates that 14.2+/-0.7% of the PM is closely apposed by the prominent superficial SR network, forming numerous flattened PM-SR junctional cytoplasmic spaces. In the control IVC stimulation with 5 microM PE resulted in sustained recurring Ca2+ waves with a frequency of 0.42+/-0.02 Hz. In calyculin-A treated rabbit IVC, a concentration-dependent dissociation of the superficial SR and loss of PM-SR junctions was observed. This progressive loss of the PM-SR junctions occurs over the same concentration range as the inhibition of PE-induced recurring Ca2+ waves. These findings offer support for the hypothesis that the presence of the PM-SR junctions is required for the generation of asynchronous recurring Ca2+ waves, which underlie excitation-contraction coupling in the VSMC of the rabbit IVC.

Role of Na+/Ca2+ exchange in regulating cytosolic Ca2+ in cultured human pulmonary artery smooth muscle cells

A rise in cytosolic Ca2+ concentration ([Ca2+]cyt) in pulmonary artery smooth muscle cells (PASMC) is an important stimulus for cell contraction, migration, and proliferation. Depletion of intracellular Ca2+ stores opens store-operated Ca2+ channels (SOC) and causes Ca2+ entry. Transient receptor potential (TRP) cation channels that are permeable to Na+ and Ca2+ are believed to form functional SOC. Because sarcolemmal Na+/Ca2+ exchanger has also been implicated in regulating [Ca2+]cyt, this study was designed to test the hypothesis that the Na+/Ca2+ exchanger (NCX) in cultured human PASMC is functionally involved in regulating [Ca2+]cyt by contributing to store depletion-mediated Ca2+ entry. RT-PCR and Western blot analyses revealed mRNA and protein expression for NCX1 and NCKX3 in cultured human PASMC. Removal of extracellular Na+, which switches the Na+/Ca2+ exchanger from the forward (Ca2+ exit) to reverse (Ca2+ entry) mode, significantly increased [Ca2+]cyt, whereas inhibition of the Na+/Ca2+ exchanger with KB-R7943 (10 microM) markedly attenuated the increase in [Ca2+]cyt via the reverse mode of Na+/Ca2+ exchange. Store depletion also induced a rise in [Ca2+]cyt via the reverse mode of Na+/Ca2+ exchange. Removal of extracellular Na+ or inhibition of the Na+/Ca2+ exchanger with KB-R7943 attenuated the store depletion-mediated Ca2+ entry. Furthermore, treatment of human PASMC with KB-R7943 also inhibited cell proliferation in the presence of serum and growth factors. These results suggest that NCX is functionally expressed in cultured human PASMC, that Ca2+ entry via the reverse mode of Na+/Ca2+ exchange contributes to store depletion-mediated increase in [Ca2+]cyt, and that blockade of the Na+/Ca2+ exchanger in its reverse mode may serve as a potential therapeutic approach for treatment of pulmonary hypertension.

Non-selective cationic channels of smooth muscle and the mammalian homologues of Drosophila TRP

Throughout the body there are smooth muscle cells controlling a myriad of tubes and reservoirs. The cells show enormous diversity and complexity compounded by a plasticity that is critical in physiology and disease. Over the past quarter of a century we have seen that smooth muscle cells contain--as part of a gamut of ion-handling mechanisms--a family of cationic channels with significant permeability to calcium, potassium and sodium. Several of these channels are sensors of calcium store depletion, G-protein-coupled receptor activation, membrane stretch, intracellular Ca2+, pH, phospholipid signals and other factors. Progress in understanding the channels has, however, been hampered by a paucity of specific pharmacological agents and difficulty in identifying the underlying genes. In this review we summarize current knowledge of these smooth muscle cationic channels and evaluate the hypothesis that the underlying genes are homologues of Drosophila TRP (transient receptor potential). Direct evidence exists for roles of TRPC1, TRPC4/5, TRPC6, TRPV2, TRPP1 and TRPP2, and more are likely to be added soon. Some of these TRP proteins respond to a multiplicity of activation signals--promiscuity of gating that could enable a variety of context-dependent functions. We would seem to be witnessing the first phase of the molecular delineation of these cationic channels, something that should prove a leap forward for strategies aimed at developing new selective pharmacological agents and understanding the activation mechanisms and functions of these channels in physiological systems.

Non-selective cationic channels (NSCC) in smooth muscle cells from human umbilical arteries

The aim of our work was to investigate the presence of non-selective cation channels (NSCC) in freshly isolated smooth muscle cells from the human umbilical artery (HUA), one of the vessels involved in fetal-placental circulation. We studied the electrophysiological properties of NSCC using the patch-clamp technique in whole-cell configuration, and their possible role in the contractile state of intact vessels' rings. Recording with a high intracellular Cs(+) solution and a near physiological extracellular saline solution, we found a Gd(3+)-sensitive current (IC(50) = 1.05 microM) with a linear current-voltage relationship showing a reversal potential (E(rev)) of -2.1 +/- 1.2 mV (n =15 cells). La(3+) (100 microM) and Mg(2+) (5 mM) also blocked this current. In such conditions, inward currents were carried by Na(+) and Ca(2+); hence, a Na(+)-free solution inhibited only inward current (-67.3 +/- 11.4%, at -40 mV, n = 7, p < 0.05) and a Ca(2+)-Na(+)-free solution decreased the current even further with respect to values obtained in Na(+)-free solution (-69.8 +/- 8.8% at -40 mV, n = 9, p < 0.05). The permeability ratios (P(X)/P(Cs(+))) for monovalent and divalent cations were 1, 0.9, 0.7, 0.7, 0.7, and 0.5 where X = Cs(+), Na(+), Li(+), Ca(2+), Ba(2+) and Tris(+), respectively. In intact tissue, a 0 Ca(2+) extracellular solution, Gd(3+) (100-250 microM), La(3+) (200 microM) and Mg(2+) (5 mM) induced vasorelaxation in non-stimulated HUA rings.

Sodium-calcium exchanger contributes to membrane hyperpolarization of intact endothelial cells from rat aorta during acetylcholine stimulation

1. The role of sodium-calcium exchanger in acetylcholine (Ach)-induced hyperpolarization of intact endothelial cells was studied in excised rat aorta. The membrane potential was recorded using perforated patch-clamp technique. 2. The mean resting potential of endothelial cells was -44.1+/-1.4 mV. A selective inhibitor of sodium-calcium exchanger benzamil (100 microm) had no significant effect on resting membrane potential, but reversibly decreased the amplitude of sustained Ach-induced endothelial hyperpolarization from 20.9+/-1.4 to 5.7+/-1.1 mV when applied during the plateau phase. 3. The blocker of reversed mode of the exchanger KB-R7943 (2-[2-[4-(4-nitrobenzyloxy)phenyl]ethyl]isothiourea methanesulfonate, 20 microm) reversibly decreased the amplitude of sustained Ach-induced hyperpolarization from 20.5+/-2.9 to 7.5+/-1.8 mV. 4. Introduction of tetraethylammonium (10 mm) in the continuous presence of Ach decreased the sustained phase of hyperpolarization from 17.9+/-1.5 by 12.9+/-0.9 mV. Subsequent addition of 20 microm KB-R7943 further depolarized endothelial cells by 4.8+/-1.1 mV. 5. Substituting external sodium with N-methyl d-glucamine during the plateau phase of Ach-evoked hyperpolarization reversibly decreased the hyperpolarization from -61.8+/-2.7 to -54.2+/-1.9 mV. In the majority of preparations, the initial response to removal of external sodium was a transient further rise in the membrane potential of several mV. Sodium ionophore monensin hyperpolarized endothelium by 10.3+/-0.7 mV. 6. The inhibitory effect of benzamil on Ach-induced endothelial sustained hyperpolarization was observed in endothelium mechanically isolated from smooth muscle. 7. These results suggest that the sodium-calcium exchanger of intact endothelial cells is able to operate in reverse following stimulation by Ach, contributing to sustained hyperpolarization. Myoendothelial electrical communications do not mediate the effect of blockers of sodium-calcium exchanger.

Magnesium-inhibited, TRPM6/7-like channel in cardiac myocytes: permeation of divalent cations and pH-mediated regulation

Cardiac tissue expresses several TRP proteins as well as a Mg2+ -inhibited, non-selective cation current (IMIC) that bears many characteristics of TRP channel currents. We used the whole-cell voltage clamp technique in pig and rat ventricular myocytes to characterize the permeation, blockage properties and regulation of the cardiac IMIC channels in order to compare them with TRP channels, in particular with Mg2+ -sensitive TRPM6 and TRPM7. We show that removing extracellular divalent cations unmasks large inward and outward monovalent currents, which can be inhibited by intracellular Mg2+. Inward currents are suppressed upon replacing extracellular Na+ by NMDG+. Divalent cations block monovalent IMIC and, at 10-20 mm, carry measurable currents. Their efficacy sequence in decreasing outward IMIC (Ni2+ = Mg2+ > Ca2+ > Ba2+) and in inducing inward IMIC (Ni2+ >> Mg2+ = Ca2+ approximately Ba2+), and their permeabilities calculated from reversal potentials are similar to those of TRPM6 and TRPM7 channels. The trivalent cations Gd3+ and Dy3+ also block IMIC in a voltage-dependent manner (delta = 0.4-0.5). In addition they inhibit the inward current carried by divalent cations. IMIC is regulated by pH. Decreasing or increasing extracellular pH decreased and increased IMIC, respectively (pH0.5 = 6.9, nH = 0.98). Qualitatively similar results were obtained on IMIC in rat basophilic leukaemia cells. These effects in cardiac myocytes were absent in the presence of high intracellular buffering by 40 mm Hepes. Our results suggest that IMIC in cardiac cells is due to TRPM channels, most probably to TRPM6 or TRPM7 channels or to their heteromultimeres.

Store-operated channels: diversity and activation mechanisms

This perspective addresses two questions: How many store-operated channels (SOCs) are there, and how many mechanisms can account for SOC activation by depleted stores? Accumulating evidence suggests that the SOC family is not limited to the calcium-selective SOC that is responsible for ICRAC (Ca2+-SOC), but includes poorly selective cation SOCs (cat-SOCs) that may satisfy physiological needs in diverse excitable and nonexcitable cells. A growing number of studies in different cell types support the idea that all the members of SOC family (Ca2+-SOC and cat-SOC) may be activated by depletion of the stores through the same mechanism, which is mediated by calcium influx factor (CIF) and calcium-independent phospholipase A2 (iPLA2). A conformational coupling model is also discussed. To account for the most recent findings, we propose that two distinct classes of calcium-conducting channels may exist in plasma membrane, which respond to different signals: SOCs, which are activated by depletion of calcium stores through the CIF-iPLA2 mechanism [no inositol triphosphate (IP3) needed]; and IP3 receptor-operated channels (IP3ROCs), which are activated by IP3 receptor through a direct coupling mechanism (no store depletion is needed). This model, with two separate mechanisms linked to different channels, may resolve many conflicting findings and interpretations and may give a new perspective on the diversity of calcium influx pathways.

Chronic hypoxia-induced upregulation of store-operated and receptor-operated Ca2+ channels in pulmonary arterial smooth muscle cells: a novel mechanism of hypoxic pulmonary hypertension

Chronic hypoxic pulmonary hypertension is associated with profound vascular remodeling and alterations in Ca(2+) homeostasis in pulmonary arterial smooth muscle cells (PASMCs). Recent studies show that transient receptor potential (TRPC) genes, which encode store-operated and receptor-operated cation channels, play important roles in Ca(2+) regulation and cell proliferation. However, the influence of chronic hypoxia on TRPC channels has not been determined. Here we compared TRPC expression, and store- and receptor-operated Ca(2+) entries in PASMCs of normoxic and chronic hypoxic rats. Reverse-transcription polymerase chain reaction (RT-PCR), Western blot, and immunostaining showed consistently that TRPC1, TRPC3, and TRPC6 were expressed in intralobar pulmonary arteries (PAs) and PASMCs. Application of 1-oleoyl-2-acetyl-sn-glycerol (OAG) to directly activate receptor-operated channels, or thapsigargin to deplete Ca(2+) stores, caused dramatic increase in cation entry measured by Mn(2+) quenching of fura-2 and by Ca(2+) transients. OAG-induced responses were approximately 700-fold more resistant to La(3+) inhibition than thapsigargin-induced responses. siRNA knockdown of TRPC1 and TRPC6 specifically attenuated thapsigargin- and OAG-induced cation entries, respectively, indicating that TRPC1 mediates store-operated entry and TRPC6 mediates receptor-operated entry. In hypoxic PAs, there were 2- to 3-fold increases in TRPC1 and TRPC6 expression. They were accompanied by significant increases in basal, OAG-induced, and thapsigargin-induced cation entries in hypoxic PASMCs. Moreover, removal of Ca(2+) or inhibition of store-operated Ca(2+) entry with La(3+) and SK&F-96365 reversed the elevated basal [Ca(2+)](i) in PASMCs and vascular tone in PAs of chronic hypoxic animals, but nifedipine had minimal effects. Our results for the first time to our knowledge show that both store- and receptor-operated channels of PASMCs are upregulated by chronic hypoxia and contribute to the enhanced vascular tone in hypoxic pulmonary hypertension.

Alpha1-adrenergic signaling mechanisms in contraction of resistance arteries

Our goal in this review is to provide a comprehensive, integrated view of the numerous signaling pathways that are activated by alpha(1)-adrenoceptors and control actin-myosin interactions (i.e., crossbridge cycling and force generation) in mammalian arterial smooth muscle. These signaling pathways may be categorized broadly as leading either to thick (myosin) filament regulation or to thin (actin) filament regulation. Thick filament regulation encompasses both "Ca(2+) activation" and "Ca(2+)-sensitization" as it involves both activation of myosin light chain kinase (MLCK) by Ca(2+)-calmodulin and regulation of myosin light chain phosphatase (MLCP) activity. With respect to Ca(2+) activation, adrenergically induced Ca(2+) transients in individual smooth muscle cells of intact arteries are now being shown by high resolution imaging to be sarcoplasmic reticulum-dependent asynchronous propagating Ca(2+) waves. These waves differ from the spatially uniform increases in [Ca(2+)] previously assumed. Similarly, imaging during adrenergic activation has revealed the dynamic translocation, to membranes and other subcellular sites, of protein kinases (e.g., Ca(2+)-activated protein kinases, PKCs) that are involved in regulation of MLCP and thus in "Ca(2+) sensitization" of contraction. Thin filament regulation includes the possible disinhibition of actin-myosin interactions by phosphorylation of CaD, possibly by mitogen-activated protein (MAP) kinases that are also translocated during adrenergic activation. An hypothesis for the mechanisms of adrenergic activation of small arteries is advanced. This involves asynchronous Ca(2+) waves in individual SMC, synchronous Ca(2+) oscillations (at high levels of adrenergic activation), Ca(2+) sparks, "Ca(2+)-sensitization" by PKC and Rho-associated kinase (ROK), and thin filament mechanisms.

Extracellular Zinc and ATP Restore Chloride Secretion across Cystic Fibrosis Airway Epithelia by Triggering Calcium Entry

Cystic fibrosis (CF) is caused by defective cyclic AMP-dependent cystic fibrosis transmembrane conductance regulator Cl(-) channels. Thus, CF epithelia fail to transport Cl(-) and water. A postulated therapeutic avenue in CF is activation of alternative Ca(2+)-dependent Cl(-) channels. We hypothesized that stimulation of Ca(2+) entry from the extracellular space could trigger a sustained Ca(2+) signal to activate Ca(2+)-dependent Cl(-) channels. Cytosolic [Ca(2+)](i) was measured in non-polarized human CF (IB3-1) and non-CF (16HBE14o(-)) airway epithelial cells. Primary human CF and non-CF airway epithelial monolayers as well as Calu-3 monolayers were used to assess anion secretion. In vivo nasal potential difference measurements were performed in non-CF and two different CF mouse (DeltaF508 homozygous and bitransgenic gut-corrected but lung-null) models. Zinc and ATP induced a sustained, reversible, and reproducible increase in cytosolic Ca(2+) in CF and non-CF cells with chemistry and pharmacology most consistent with activation of P2X purinergic receptor channels. P2X purinergic receptor channel-mediated Ca(2+) entry stimulated sustained Cl(-) and HCO(3)(-) secretion in CF and non-CF epithelial monolayers. In non-CF mice, zinc and ATP induced a significant Cl(-) secretory response similar to the effects of agonists that increase intracellular cAMP levels. More importantly, in both CF mouse models, Cl(-) permeability of nasal epithelia was restored in a sustained manner by zinc and ATP. These effects were reversible and reacquirable upon removal and readdition of agonists. Our data suggest that activation of P2X calcium entry channels may have profound therapeutic benefit for CF that is independent of cystic fibrosis transmembrane conductance regulator genotype.

Mechanism of acidic pH-induced contraction in spontaneously hypertensive rat aorta: role of Ca2+release from the sarcoplasmic reticulum

AIM

This study was conducted to investigate the mechanism of acidic pH-induced contraction (APIC) with regard to Ca2+ handling using isometric tension recording experiments.

RESULTS

Decreasing extracellular pH from 7.4 to 6.5 produced a marked and sustained contraction of spontaneously hypertensive rat (SHR) aorta, that was 128.7 +/- 2.0% of the 64.8 mm KCl-induced contraction. Verapamil, an inhibitor of voltage-dependent Ca2+ channels (VDCC) significantly inhibited the APIC. In Ca2+-deficient solution, sustained contraction induced by acidic pH was abolished completely, while a transient contraction was still observed suggesting the release of Ca2+ from intracellular site. Ryanodine (1 microm), a ryanodine receptor blocker, and 10 microm cyclopiazonic acid (CPA; a sarco/endoplasmic reticulum Ca2+ ATPase inhibitor) abolished the transient contraction induced by acidosis. In normal Ca2+-containing solution, ryanodine significantly decreased the rate of rise as well as maximum level of APIC. Interestingly, ryanodine and CPA showed an additive inhibitory effect with verapamil and the combined treatment of ryanodine or CPA with verapamil nearly abolished the APIC.

CONCLUSIONS

It is concluded that acidic pH induces Ca2+ release from ryanodine/CPA-sensitive store of sarcoplasmic reticulum in SHR aorta. This Ca2+ plays an important role in the facilitation of the rate of rise of APIC, as well as contributing to the sustained contraction via a mechanism which is independent of Ca2+ influx through VDCC.

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.

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.

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.

Sustained calcium entry through P2X nucleotide receptor channels in human airway epithelial cells

Purinergic receptor stimulation has potential therapeutic effects for cystic fibrosis (CF). Thus, we explored roles for P2Y and P2X receptors in stably increasing [Ca(2+)](i) in human CF (IB3-1) and non-CF (16HBE14o(-)) airway epithelial cells. Cytosolic Ca(2+) was measured by fluorospectrometry using the fluorescent dye Fura-2/AM. Expression of P2X receptor (P2XR) subtypes was assessed by immunoblotting and biotinylation. In IB3-1 cells, ATP and other P2Y agonists caused only a transient increase in [Ca(2+)](i) derived from intracellular stores in a Na(+)-rich environment. In contrast, ATP induced an increase in [Ca(2+)](i) that had transient and sustained components in a Na(+)-free medium; the sustained plateau was potentiated by zinc or increasing extracellular pH. Benzoyl-benzoyl-ATP, a P2XR-selective agonist, increased [Ca(2+)](i) only in Na(+)-free medium, suggesting competition between Na(+) and Ca(2+) through P2XRs. Biochemical evidence showed that the P2X(4) receptor is the major subtype shared by these airway epithelial cells. A role for store-operated Ca(2+) channels, voltage-dependent Ca(2+) channels, or Na(+)/Ca(2+) exchanger in the ATP-induced sustained Ca(2+) signal was ruled out. In conclusion, these data show that epithelial P2X(4) receptors serve as ATP-gated calcium entry channels that induce a sustained increase in [Ca(2+)](i). In airway epithelia, a P2XR-mediated Ca(2+) signal may have therapeutic benefit for CF.

Role of Na+/H+ exchanger in acetylcholine-mediated pulmonary artery contraction of spontaneously hypertensive rats

Compared to sympathetic nervous system, the role of parasympathetic innervation on tone development, especially under diseased conditions, of the pulmonary artery is relatively unknown. In this study, the contractile effect of acetylcholine and the type(s) of muscarinic (M) receptor involved in the pulmonary artery (1st intralobar branch; endothelium-denuded, under resting tension) of the normotensive Wistar-Kyoto (WKY) and age-matched (male, 22-26 weeks old) Spontaneously hypertensive rats (SHR) were investigated. Cumulative administration of acetylcholine (> or =0.1 microM) caused a concentration-dependent increase in tension (antagonised by p-fluoro-hexahydro-sila-difenidol and 4-diphenylacetoxy-N-methylpiperidine, both are selective muscarinic M(3) receptor antagonists) and the magnitude of maximum contraction (expressed as % of 50 mM [K(+)](o)-induced contraction) was markedly enhanced in the presence of neostigmine (10 microM, an anti-cholinesterase) (acetylcholine 30 microM, SHR: 72% vs. 35%; WKY: 32% vs. 20%). In SHR only, acetylcholine-elicited contraction was suppressed by 1-[beta-[3-(4-Methoxyphenyl)-propoxyl]-4-methoxyphenethyl]-1H-imidazole (SK&F 96365, 1 microM), amiloride (500 microM), ethyl-isopropyl-amiloride (EIPA, 10 microM), 2-[2-[4-(4-Nitrobenzyloxy)phenyl]ethyl]isothiourea (KB-R 7943, 5 microM), 2,4-dichlorobenzamil (10 microM), and an equal molar substitution of [Na(+)](o) (< or =30 mM) with choline or N-methyl-D-glucamine. In nominally [Ca(2+)](o)-free, EGTA (0.5 mM)-containing Krebs' solution, acetylcholine (> or =3 microM) only elicited a small contraction. In conclusion, muscarinic M(3) receptor activation is responsible for the pulmonary artery contraction induced by acetylcholine, with a greater magnitude observed in SHR. The exaggerated contraction in SHR is probably due to an influx of [Na(+)](o) through the Na(+)/H(+) exchanger and the store-operated channels (SOC) into smooth muscle cells. Elevation of cytosolic [Na(+)](i) subsequently leads to an influx of [Ca(2+)](o) through the reverse mode of the Na(+)/Ca(2+) exchanger seems to play a permissive role in mediating the exaggerated contractile response of acetylcholine recorded in the SHR.

Na+ pump alpha 2-subunit expression modulates Ca2+ signaling

The role of the Na(+) pump alpha(2)-subunit in Ca(2+) signaling was examined in primary cultured astrocytes from wild-type (alpha(2)+/+ = WT) mouse fetuses and those with a null mutation in one [alpha(2)+/- = heterozygote (Het)] or both [alpha(2)-/- = knockout (KO)] alpha(2) genes. Na(+) pump catalytic (alpha) subunit expression was measured by immunoblot; cytosol [Na(+)] ([Na(+)](cyt)) and [Ca(2+)] ([Ca(2+)](cyt)) were measured with sodium-binding benzofuran isophthalate and fura 2 by using digital imaging. Astrocytes express Na(+) pumps with both alpha(1)- ( approximately 80% of total alpha) and alpha(2)- ( approximately 20% of total alpha) subunits. Het astrocytes express approximately 50% of normal alpha(2); those from KO express none. Expression of alpha(1) is normal in both Het and KO cells. Resting [Na(+)](cyt) = 6.5 mM in WT, 6.8 mM in Het (P > 0.05 vs. WT), and 8.0 mM in KO cells (P < 0.001); 500 nM ouabain (inhibits only alpha(2)) equalized [Na(+)](cyt) at 8 mM in all three cell types. Resting [Ca(2+)](cyt) = 132 nM in WT, 162 nM in Het, and 196 nM in KO cells (both P < 0.001 vs. WT). Cyclopiazonic acid (CPA), which inhibits endoplasmic reticulum (ER) Ca(2+) pumps and unloads the ER, induces transient (in Ca(2+)-free media) or sustained (in Ca(2+)-replete media) elevation of [Ca(2+)](cyt). These Ca(2+) responses to 10 microM CPA were augmented in Het as well as KO cells. When CPA was applied in Ca(2+)-free media, the reintroduction of Ca(2+) induced significantly larger transient rises in [Ca(2+)](cyt) (due to Ca(2+) entry through store-operated channels) in Het and KO cells than in WT cells. These results correlate with published evidence that alpha(2) Na(+) pumps and Na(+)/Ca(2+) exchangers are confined to plasma membrane microdomains that overlie the ER. The data suggest that selective reduction of alpha(2) Na(+) pump activity can elevate local [Na(+)] and, via Na(+)/Ca(2+) exchange, [Ca(2+)] in the tiny volume of cytosol between the plasma membrane and ER. This, in turn, augments adjacent ER Ca(2+) stores and thereby amplifies Ca(2+) signaling without elevating bulk [Na(+)](cyt).

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.

Na/Ca exchanger and PMCA localization in neurons and astrocytes: functional implications

Immunocytochemistry reveals that the Na/Ca exchanger (NCX) in neuronal somata and astrocytes is confined to plasma membrane (PM) microdomains that overlie sub-PM (junctional) endoplasmic reticulum (jER). By contrast, the PM Ca(2+) pump (PMCA) is more uniformly distributed in the PM. At presynaptic nerve terminals, the NCX distribution is consistent with that observed in the neuronal somata, but the PMCA is clustered at the active zones. Thus, the PMCA, with high affinity for Ca(2+) (K(d) congruent with 100 nM), may keep active zone Ca(2+) very low and thereby "reprime" the vesicular release mechanism following activity. NCX, with lower affinity for Ca(2+) (K(d) congruent with 1,000 nM), on the other hand, may extrude Ca(2+) that has diffused away from the active zones and been temporarily sequestered in the endoplasmic reticulum. The PL microdomains that contain the NCX also contain Na(+) pump high ouabain affinity alpha2 (astrocytes) or alpha 3 (neurons) subunit isoforms (IC(50) congruent with 5-50 nM ouabain). In contrast, the alpha1 isoform (low ouabain affinity in rodents; IC(50) >10,000 nM), like the PMCA, is more uniformly distributed in these cells. The sub-PM endoplasmic reticulum in neurons (and probably glia and other cell types as well) and the adjacent PM form junctions that resemble cardiac muscle dyads. We suggest that the PM microdomains containing NCX and alpha 2/alpha 3 Na(+) pumps, the underlying jER, and the intervening tiny volume of cytosol (<10(-18) l) form functional units (PLasmERosomes); diffusion of Na(+) and Ca(2+) between these cytosolic compartments and "bulk" cytosol may be markedly restricted. The activity of the Na(+) pumps with alpha 2/alpha 3 subunits may thus regulate NCX activity and jER Ca(2+) content. This view is supported by studies in mice with genetically reduced (by congruent with 50%) alpha 2 Na(+) pumps: evoked Ca(2+) transients were augmented in these cells despite normal cytosolic Na(+) and resting Ca(2+) concentrations ([Na(+)](CYT) and [Ca(2+)](CYT)). We conclude that alpha 2/alpha 3 Na(+) pumps control PLasmERosome (local) [Na(+)](CYT). This, in turn, via NCX, modulates local [Ca(2+)](CYT), jER Ca(2+) storage, Ca(2+) signaling, and cell responses.

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.

Physiologic increases in extracellular sodium salt enhance coronary vasoconstriction and Ca2+ entry

High dietary sodium salt has been suggested to increase the risk of coronary vasospasm and coronary artery disease. However, whether high-sodium salt directly affects the mechanisms of coronary artery contraction is unclear. This study investigated whether physiologic and supraphysiologic increases in extracellular concentrations of sodium chloride ([NaCl]e) enhance the Ca2+ handling mechanisms of coronary smooth muscle contraction. Isometric contraction and 45Ca2+ influx were measured in endothelium-denuded porcine coronary artery strips incubated in Krebs solution (2.5 mM Ca2+) containing increasing [NaCl]e (120, 121, 123, 126, 130, 140, and 150 mM). Increasing [NaCl]e for 30 min did not increase the resting coronary tone or 45Ca2+ influx. 5-Hydroxytryptamine (5-HT) caused concentration-dependent increases in contraction and 45Ca2+ influx. Preincubation of coronary strips in increasing [NaCl]e for 30 min did not change the median effective dose of 5-HT. However, the magnitude of the 5-HT contraction and 45Ca2+ influx was significantly increased at 121-126 mM [NaCl]e. Preincubation with 2,4-dichlorobenzamil (10-5 M), inhibitor of the Na+/Ca2+ exchanger, or KB-R7943 (10-5 M), selective inhibitor of the reverse mode of the Na+/Ca2+ exchanger, abolished the increases in 5-HT contraction and 45Ca2+ influx at 121-126 mM [NaCl]e. Preincubation in Krebs solution containing 120 mM NaCl plus 1-6 mM LiCl or N-methyl-d-glucamine did not increase 5-HT contraction or 45Ca2+ influx. Higher [NaCl]e (140-150 mM) increased 5-HT-induced 45Ca2+ influx but inhibited 5-HT contraction. 5-HT (10-5 M)- and caffeine (25 mM)-induced contraction in Ca2+-free (2 mM EGTA) solution, a measure of Ca2+ release from the intracellular stores, was not affected by small increases in [NaCl]e (121-126 mM) but was inhibited at higher [NaCl]e (130-150 mM). Thus increases in [NaCl]e within the physiologic range enhance coronary smooth muscle contraction to 5-HT by a mechanism possibly involving Ca2+ entry via the reverse mode of the Na+/Ca2+ exchanger, but not Ca2+ release from the intracellular stores. The reduction of coronary contraction with supraphysiologic [NaCl]e in both Ca2+-containing and Ca2+-free Krebs could be related to excessive increases in ionic strength and may mask significant coronary vasoconstrictor effects of physiologic increases in [NaCl]e.

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).

Ca(2+) oscillations, gradients, and homeostasis in vascular smooth muscle

Vascular smooth muscle shows both plasticity and heterogeneity with respect to Ca(2+) signaling. Physiological perturbations in cytoplasmic Ca(2+) concentration ([Ca(2+)](i)) may take the form of a uniform maintained rise, a transient uniform [Ca(2+)](i) elevation, a transient localized rise in [Ca(2+)](i) (also known as spark and puff), a transient propagated wave of localized [Ca(2+)](i) elevation (Ca(2+) wave), recurring asynchronous Ca(2+) waves, or recurring synchronized Ca(2+) waves dependent on the type of blood vessel and the nature of stimulation. In this overview, evidence is presented which demonstrates that interactions of ion transporters located in the membranes of the cell, sarcoplasmic reticulum, and mitochondria form the basis of this plasticity of Ca(2+) signaling. We focus in particular on how the junctional complexes of plasmalemma and superficial sarcoplasmic reticulum, through the generation of local cytoplasmic Ca(2+) gradients, maintain [Ca(2+)](i) oscillations, couple these to either contraction or relaxation, and promote Ca(2+) cycling during homeostasis.

Gender differences in vascular smooth muscle reactivity to increases in extracellular sodium salt

Hypertension is more common in men and postmenopausal women than in premenopausal women, and gender differences in sensitivity to high dietary Na(+) salt have been suggested; however, the vascular mechanisms involved are unclear. We investigated whether increases in the extracellular concentration of Na(+) ([Na(+)](e)) enhance the mechanisms of vascular smooth muscle contraction and whether the vascular effects of [Na(+)](e) exhibit gender differences. Isometric contraction and (45)Ca(2+) influx were measured in endothelium-denuded aortic strips that were isolated from intact male, intact female, castrated male, and ovariectomized (OVX) female Sprague-Dawley rats and incubated in Krebs' solution (2.5 mmol/L Ca(2+)) containing increasing [Na(+)](e) by the addition of 1, 3, 6, 10, 20, and 30 mmol/L NaCl. Increasing [Na(+)](e) for 30 minutes did not increase the resting tone or (45)Ca(2+) influx in any group of rats. Phenylephrine (Phe) caused concentration-dependent increases in contraction and (45)Ca(2+) influx. In vascular strips from intact males, increasing [Na(+)](e) by the addition of 1 to 6 mmol/L NaCl significantly increased the magnitude of Phe contraction and (45)Ca(2+) influx. Further increases in [Na(+)](e) by the addition of 10, 20, and 30 mmol/L NaCl increased Phe-induced (45)Ca(2+) influx but inhibited Phe contraction, possibly because of excessive increases in ionic strength. Preincubation with 2,4-dichlorobenzamil (10(-5) mol/L), inhibitor of the Na(+)-Ca(2+) exchanger, or KB-R7943 (10(-5) mol/L), selective inhibitor of the reverse mode of the Na(+)-Ca(2+) exchanger, abolished the increases in Phe contraction and (45)Ca(2+) influx at increasing [Na(+)](e) obtained by the addition of 1 to 6 mmol/L NaCl. Preincubation in Krebs' solution containing control [Na(+)](e) plus 1 to 6 mmol/L LiCl or N-methyl-D-glucamine did not increase Phe contraction. In intact females, the Phe contraction and Ca(2+) influx were less than those in intact males and were not enhanced with increases in [Na(+)](e). The enhancement of Phe contraction and Ca(2+) influx with increases in [Na(+)](e) were not significantly different between castrated male rats and intact male rats but were greater in OVX female rats than intact female rats. In OVX female rats or castrated male rats treated with 17beta-estradiol (but not 17alpha-estradiol) subcutaneous implants, no significant changes in Phe contraction or Ca(2+) influx with increases in [Na(+)](e) were observed. In OVX female or castrated male rats simultaneously treated with 17beta-estradiol plus the estrogen receptor antagonist ICI 182,780, the Phe contraction and Ca(2+) influx were enhanced with increases in [Na(+)](e). Thus, in intact male rats, small physiological increases in [Na(+)](e) enhance smooth muscle contraction to Phe by a mechanism involving Ca(2+) entry, possibly via the reverse mode of the Na(+)-Ca(2+) exchanger. This mechanism appears to be reduced in the presence of endogenous or exogenous estrogen and thereby protects female rats against excessive increases in vascular reactivity during high dietary Na(+) intake.

Pharmacological evidence for capacitative Ca(2+) entry in cannulated and pressurized skeletal muscle arterioles

Arteriolar myogenic tone shows a marked dependency on extracellular Ca(2+). The contribution played by mechanisms such as intracellular Ca(2+) release and capacitative entry, however, are less certain. The present studies aimed to demonstrate functional evidence for involvement of such mechanisms in myogenic tone and reactivity. Single cremaster arterioles were denuded of endothelium, pressurized under no-flow conditions and loaded with fura 2-AM for measurement of changes in intracellular Ca(2+) [Ca(2+)](i). The cell permeable, putative, IP(3) receptor antagonist 2APB (2 aminoethoxydiphenyl borate) was used to determine the possible role of IP(3) receptor-mediated mechanisms in arteriolar myogenic tone and reactivity. Arterioles dilated in response to increasing concentrations of 2APB (1 - 300 microM) without a concomitant change in global [Ca(2+)](i). Also 2APB (50 microM) completely inhibited the myogenic constriction in response to a step change in luminal pressure (50 - 120 mmHg) with no apparent effect on pressure-mediated increases in [Ca(2+)](i). 2APB markedly attenuated the constrictor response and [Ca(2+)](i) increase stimulated by phenylephrine but not KCl. Capacitative Ca(2+) influx in arterioles was demonstrated either by re-addition of extracellular [Ca(2+)] following pre-treatment with 1 or 10 microM nifedipine in Ca(2+) free buffer or exposure of vessels to thapsigargin (1 microM) to induce store depletion. In both cases 2APB inhibited the increase in [Ca(2+)](i). Capacitative Ca(2+) entry showed an inverse relationship with intraluminal pressure over the range 10 - 120 mmHg. Consistent with an effect on a Ca(2+) entry pathway, 2APB had no effect on intracellular (caffeine releasable) Ca(2+) stores while decreasing the rate of Mn(2+) quench of fura 2 fluorescence. The results provide functional evidence for capacitative Ca(2+) entry in intact arteriolar smooth muscle. The effectiveness of 2APB in inhibiting both non-voltage gated Ca(2+) entry and responsiveness to an acute pressure step is consistent with the involvement of an axis involving IP(3)-mediated and or capacitative Ca(2+) entry mechanisms in myogenic reactivity. Given the lack of effect of 2APB on pressure-induced changes in global [Ca(2+)](i) it is suggested that such mechanisms participate on a localized level to couple the myogenic stimulus to contraction.

The mechanism of phenylephrine-mediated [Ca(2+)](i) oscillations underlying tonic contraction in the rabbit inferior vena cava

1. We characterized the mechanisms in vascular smooth muscle cells (VSMCs) that produce asynchronous, wave-like Ca(2+) oscillations in response to phenylephrine (PE). Confocal imaging was used to observe [Ca(2+)](i) in individual VSMCs of intact inferior vena cava (IVC) from rabbits. 2. It was found that the Ca(2+) waves were initiated by Ca(2+) release from the sarcoplasmic reticulum (SR) via inositol 1,4,5-trisphosphate-sensitive SR Ca(2+) release channels (IP(3)R channels) and that refilling of the SR Ca(2+) store through the sarcoplasmic-endoplasmic reticulum Ca(2+)-ATPase (SERCA) was required for maintained generation of the repetitive Ca(2+) waves. 3. Blockade of L-type voltage-gated Ca(2+) channels (L-type VGCCs) with nifedipine reduced the frequency of PE-stimulated [Ca(2+)](i) oscillations, while additional blockade of receptor-operated channels/store-operated channels (ROCs/SOCs) with SKF96365 abolished the remaining oscillations. Parallel force measurements showed that nifedipine inhibited PE-induced tonic contraction by 27 % while SKF96365 abolished it. This indicates that stimulated Ca(2+) entry refills the SR to support the recurrent waves of SR Ca(2+) release and that both L-type VGCCs and ROCs/SOCs contribute to this process. 4. Application of the Na(+)-Ca(2+) exchanger (NCX) inhibitors 2',4'-dichlorobenzamil (forward- and reverse-mode inhibitor) and KB-R7943 (reverse-mode inhibitor) completely abolished the nifedipine-resistant component of [Ca(2+)](i) oscillations and markedly reduced PE-induced tone. 5. Thus, we conclude that each Ca(2+) wave depends on initial SR Ca(2+) release via IP(3)R channels followed by SR Ca(2+) refilling through SERCA. Na(+) entry through ROCs/SOCs facilitates Ca(2+) entry through the NCX operating in the reverse mode, which refills the SR and maintains PE-induced [Ca(2+)](i) oscillations. In addition some Ca(2+) entry through L-type VGCCs and ROCs/SOCs serves to modulate the frequency of the oscillations and the magnitude of force development.

Effect of cyclic stretch on α-subunit mRNA expression of Na+-K+-ATPase in aortic smooth muscle cells

We previously demonstrated that protein expression of both α1- and α2-catalytic subunits of the Na+-K+-ATPase is elevated after a 2- to 4-day chronic cyclic stretch regimen in cultured aortic smooth muscle cells (ASMC). In the present study, we investigated whether cyclic stretch affects mRNA expression of the α-isoforms of the Na+-K+-ATPase. Using a stretch apparatus, rat ASMC were cyclically stretched 10 or 20% of their length for 1, 3, or 6 h. α-Isoform mRNA levels were measured using Northern analysis. A 3-h 10% stretch had no significant affect on mRNA expression for either isoform, but a 20% stretch increased mRNA of both isoforms approximately twofold. Whereas a 6-h 20% stretch increased α1 mRNA by 3.3-fold, α2 was not affected any further. Actinomycin D blocked the stretch-induced stimulation of mRNA expression of both α-subunits. In conclusion, cyclic stretch stimulates the mRNA expression of both α1- and α2-subunits of Na+-K+-ATPase. The sensitivity of the two genes to the degree and duration of stretch is different. The stretch-induced increase of mRNA may be a result of increased transcription.

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

Agonist-induced increases in cytosolic Ca(2+) concentration ([Ca(2+)](cyt)) in pulmonary artery (PA) smooth muscle cells (SMCs) consist of a transient Ca(2+) release from intracellular stores followed by a sustained Ca(2+) influx. Depletion of intracellular Ca(2+) stores triggers capacitative Ca(2+) entry (CCE), which contributes to the sustained increase in [Ca(2+)](cyt) and the refilling of Ca(2+) into the stores. In isolated PAs superfused with Ca(2+)-free solution, phenylephrine induced a transient contraction, apparently by a rise in [Ca(2+)](cyt) due to Ca(2+) release from the intracellular stores. The transient contraction lasted for 3-4 min until the Ca(2+) store was depleted. Restoration of extracellular Ca(2+) in the presence of phentolamine produced a contraction potentially due to a rise in [Ca(2+)](cyt) via CCE. The store-operated Ca(2+) channel blocker Ni(2+) reduced the store depletion-activated Ca(2+) currents, decreased CCE, and inhibited the CCE-mediated contraction. In single PASMCs, we identified, using RT-PCR, five transient receptor potential gene transcripts. These results suggest that CCE, potentially through transient receptor potential-encoded Ca(2+) channels, plays an important role in agonist-mediated PA contraction.

Properties of a native cation channel activated by Ca2+ store depletion in vascular smooth muscle cells

Depletion of intracellular Ca(2+) stores activates capacitative Ca(2+) influx in smooth muscle cells, but the native store-operated channels that mediate such influx remain unidentified. Recently we demonstrated that calcium influx factor produced by yeast and human platelets with depleted Ca(2+) stores activates small conductance cation channels in excised membrane patches from vascular smooth muscle cells (SMC). Here we characterize these channels in intact cells and present evidence that they belong to the class of store-operated channels, which are activated upon passive depletion of Ca(2+) stores. Application of thapsigargin (TG), an inhibitor of sarco-endoplasmic reticulum Ca(2+) ATPase, to individual SMC activated single 3-pS cation channels in cell-attached membrane patches. Channels remained active when inside-out membrane patches were excised from the cells. Excision of membrane patches from resting SMC did not by itself activate the channels. Loading SMC with BAPTA (1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid), which slowly depletes Ca(2+) stores without a rise in intracellular Ca(2+), activated the same 3-pS channels in cell-attached membrane patches as well as whole cell nonselective cation currents in SMC. TG- and BAPTA-activated 3-pS channels were cation-selective but poorly discriminated among Ca(2+), Sr(2+), Ba(2+), Na(+), K(+), and Cs(+). Open channel probability did not change at negative membrane potentials but increased significantly at high positive potentials. Activation of 3-pS channels did not depend on intracellular Ca(2+) concentration. Neither TG nor a variety of second messengers (including Ca(2+), InsP3, InsP4, GTPgammaS, cyclic AMP, cyclic GMP, ATP, and ADP) activated 3-pS channels in inside-out membrane patches. Thus, 3-pS nonselective cation channels are present and activated by TG or BAPTA-induced depletion of intracellular Ca(2+) stores in intact SMC. These native store-operated cation channels can account for capacitative Ca(2+) influx in SMC and can play an important role in regulation of vascular tone.