Regional changes in calcium underlying contraction of single smooth muscle cells

Regulation of Myosin Light-Chain Phosphatase Activity to Generate Airway Smooth Muscle Hypercontractility

Smooth muscle is a central structure involved in the regulation of airway tone. In addition, it plays an important role in the development of some pathologies generated by alterations in contraction, such as hypercontractility and the airway hyperresponsiveness observed in asthma. The molecular processes associated with smooth muscle contraction are centered around myosin light chain (MLC) phosphorylation, which is controlled by a balance in the activity of myosin light-chain kinase (MLCK) and myosin light-chain phosphatase (MLCP). MLCK activation depends on increasing concentrations of intracellular Ca²⁺, while MLCP activation is independent of Ca²⁺. MLCP contains a phosphatase subunit (PP1c) that is regulated through myosin phosphatase target subunit 1 (MYPT1) and other subunits, such as glycogen-associated regulatory subunit and myosin-binding subunit 85 kDa. Interestingly, MLCP inhibition may contribute to exacerbation of smooth muscle contraction by increasing MLC phosphorylation to induce hypercontractility. Many pathways inhibiting MLCP activity in airway smooth muscle have been proposed and are focused on inhibition of PP1c, inhibitory phosphorylation of MYPT1 and dissociation of the PP1c-MYPT1 complex.

The Effect of a Single dose Dantrolene in Patients with Vasospasm Following Aneurysmal Subarachnoid Hemorrhage

Background:

Cerebral vasospasm is a prolonged, occasionally severe, but reversible narrowing of the cerebral arteries that begins 3 to 5 days after the hemorrhage becomes maximal at 14 days. This study is designed to evaluate the effect of dantrolene on the reduction of vasospasm following aneurysmal subarachnoid hemorrhage (SAH).

Materials and Methods:

This randomized controlled clinical trial was conducted on 32 patients with proven aneurysms in AL-Zahra hospital during 2011-2013. They were randomly divided into two groups. In all patients, daily transcranial Doppler sonography was performed and as soon as the diagnosis of vasospasm onset in the first group, in addition to conventional treatment of vasospasm 2.5 mg/kg Dantrolene infusion within 60 minutes and while the blood pressure and heart rate of patient monitored, and arterial flow velocity changes such as PSV and MFV were measured by transcranial Doppler sonography in 45, 90 and 135 minutes. Data was analyzed by SPSS 22 and Chi-square, Student t, Mann-Whitney and ANOVA tests with repeated observations.

Results:

There was no significant difference in the site of the aneurysm in the two groups. The mean of PSV index prior to treatment and the 45^th minute was not different but at 90^th and 135^th minutes it was significantly lower in the Dantrolene receiving group (P < 0.05). The mean of MFV index prior to intervention and in the 45^th minute was not different between two groups, but at 90^th and 135^th minutes was significantly lower in the target group

Conclusion:

Using dantrolene in patients with artery vasospasm significantly reduced artery spasm and increased the patient recovery.

Dantrolene for cerebral vasospasm after subarachnoid haemorrhage: a randomised double blind placebo-controlled safety trial

BACKGROUND

Dantrolene is neuroprotective in animal models and may attenuate cerebral vasospasm (cVSP) in human aneurysmal subarachnoid haemorrhage (aSAH). We evaluated safety, feasibility and tolerability of intravenous dantrolene (IV-D) in patients with aSAH.

METHODS

In this single-centre, randomised, double blind, placebo-controlled trial, 31 patients with aSAH were randomised to IV-D 1.25 mg every 6 h for 7 days (n=16) or equiosmolar free water/5% mannitol (placebo; n=15). Primary safety end points were incidence of hyponatraemia (sNa≤132 mmol/L) and liver toxicity (proportion of patients alanine transaminase, aspartate aminotransferase and AlkPhos >5× upper-limit-of-normal). Secondary end points included tolerability, systemic hypotension and intracranial hypertension. Efficacy was explored for clinical/radiological cVSP, delayed cerebral ischaemia (DCI), and 3-month functional outcomes. Quantitative analyses of angiograms and daily transcranial Doppler (TCD) were performed.

RESULTS

Between IV-D versus placebo, no differences were observed in the primary outcomes (hyponatremia 44% vs 67% (p=0.29); liver toxicity 6% vs 0% (p=1.0)). Three patients in the IV-D versus two in the placebo group had severe adverse events possibly attributable to infusion and reached stop criteria: one IV-D patient developed liver toxicity; two patients in each group developed brain oedema requiring osmotherapy. The majority of adverse events were not related to infusion (17 vs 5 (RR 2.2; 95% CI 0.7 to 6.7; p=0.16) in IV-D vs placebo). No differences in any categorical cVSP outcomes, DCI, 3-month outcomes or quantitative angiogram and TCD analyses were seen in this small safety trial not powered to detect efficacy.

CONCLUSIONS

In this small trial, IV-D after aSAH was feasible, tolerable and safe.

TRIAL REGISTRATION NUMBER

http://clinicaltrials.gov NCT01024972.

Effects of a single dose of dantrolene in patients with cerebral vasospasm after subarachnoid hemorrhage: a prospective pilot study

BACKGROUND AND PURPOSE

New therapies for cerebral vasospasm after subarachnoid hemorrhage are needed because of its high morbidity and mortality rates. We investigated the feasibility and safety of a single dose of intravenous dantrolene and its effect on transcranial Doppler in cerebral vasospasm after subarachnoid hemorrhage.

METHODS

In a prospective, open-label, single-dose ascending safety trial, 5 patients received intravenous dantrolene 1.25 mg/kg and the next 5 patients received 2.5 mg/kg over the course of 60 minutes. All other infusions were kept steady and hemodynamic parameters were recorded. Transcranial Doppler was performed at 0, 45, 90, and 135 minutes relative to infusion start. Basic chemistries, serum osmolality, arterial blood gas, and liver enzymes were measured before and after.

RESULTS

Laboratory values and hemodynamic parameters remained unchanged except for a decrease in the systolic blood pressure in the low-dose group (-8 mm Hg; 95% CI, -26 to 10 mm Hg; P=0.027). After correcting for this decrease in blood pressure, peak systolic transcranial Doppler velocities decreased significantly (-26 cm/s; 95% CI, -47 to -5 cm/s; P=0.02), with a borderline change in mean velocities in the low-dose group (-16 cm/s; 95% CI, -36 to 4 cm/s; P=0.07) and peak systolic transcranial Doppler velocity in the high-dose group (-26 cm/s; 95% CI, -56 to 5 cm/s; P=0.05).

CONCLUSIONS

In this pilot study, a single dose of intravenous dantrolene in cerebral vasospasm after subarachnoid hemorrhage appears feasible while inhibiting vasoconstriction in the low-dose group, but it may lower blood pressure. Our study provides useful data for the design of larger future studies. Clinical Trial Registration- URL: http://clinicaltrials.gov. Unique identifier: NCT00964548.

Temporal and spatial dynamics underlying capacitative calcium entry in human colonic smooth muscle

Following smooth muscle excitation and contraction, depletion of intracellular Ca(2+) stores activates capacitative Ca(2+) entry (CCE) to replenish stores and sustain cytoplasmic Ca(2+) (Ca(2+)(i)) elevations. The objectives of the present study were to characterize CCE and the Ca(2+)(i) dynamics underlying human colonic smooth muscle contraction by using tension recordings, fluorescent Ca(2+)-indicator dyes, and patch-clamp electrophysiology. The neurotransmitter acetylcholine (ACh) contracted tissue strips and, in freshly isolated colonic smooth muscle cells (SMCs), caused elevation of Ca(2+)(i) as well as activation of nonselective cation currents. To deplete Ca(2+)(i) stores, the sarcoplasmic reticulum Ca(2+)-ATPase (SERCA) inhibitors thapsigargin and cyclopiazonic acid were added to a Ca(2+)-free bathing solution. Under these conditions, addition of extracellular Ca(2+) (3 mM) elicited increased tension that was inhibited by the cation channel blockers SKF-96365 (10 microM) and lanthanum (100 microM), suggestive of CCE. In a separate series of experiments on isolated SMCs, SERCA inhibition generated a gradual and sustained inward current. When combined with high-speed Ca(2+)-imaging techniques, the CCE-evoked rise of Ca(2+)(i) was associated with inward currents carrying Ca(2+) that were inhibited by SKF-96365. Regional specializations in Ca(2+) influx and handling during CCE were observed. Distinct "hotspot" regions of Ca(2+) rise and plateau were evident in 70% of cells, a feature not previously recognized in smooth muscle. We propose that store-operated Ca(2+) entry occurs in hotspots contributing to localized Ca(2+) elevations in human colonic smooth muscle.

Regulation of nuclear calcium concentration

Transient increases in nuclear calcium concentration have been shown to activate gene expression and other nuclear processes. It has been suggested that nuclear calcium signals are controlled by a mechanism that is independent of calcium signalling in the cytosol. This would be possible if calcium diffusion is slow and a separate calcium release mechanism is localized to the nuclear region. Alternatively, the nuclear envelope could act as a diffusion barrier for calcium ions released either inside or outside the nucleus. It has also been proposed that inositol 1,4,5-trisphosphate (InsP3) can be generated inside the nucleus and that there are calcium release channels in the inner membrane of the nuclear envelope. Most of the experimental evidence supporting these hypotheses is based on the calibration of nuclear and cytosolic calcium concentrations. However, recent studies suggest that the local calibration of calcium indicators may not be accurate. We propose that nuclear calcium signals can be investigated by a different approach that does not rely on accurate calibration of indicators. We have developed calcium indicators that minimize facilitated calcium diffusion and are localized to either the nucleus or the cytosol. Using the diffusion coefficient of calcium ions, and measuring the delay between cytosolic and nuclear calcium increases, we show that the nuclear envelope is not a substantial barrier for calcium ions in PC12 (phaeochromocytoma) cells. This suggests that nuclear and cytosolic calcium signals equilibrate rapidly in these cells.

Modification of smooth muscle Ca2+-sparks by tetracaine: evidence for sequential RyR activation

Spontaneous Ca(2+)-sparks were imaged using confocal line scans of fluo-4 loaded myocytes in retinal arterioles. Tetracaine produced concentration-dependent decreases in spark frequency, and modified the spatiotemporal characteristics of residual sparks. Tetracaine (10 microM) reduced the rate of rise but prolonged the average rise time so that average spark amplitude was unaltered. The mean half-time of spark decay was also unaffected, suggesting that spark termination, although delayed, remained well synchronized. Sparks spread transversely across the myocytes in these vessels, and the speed of spread within individual sparks was slowed by approximately 60% in 10 microM tetracaine, as expected if the spark was propagated across the cell but the average P(o) for RyRs was reduced. Staining of isolated vessels with BODIPY-ryanodine and di-4-ANEPPS showed that RyRs were located both peripherally, adjacent to the plasma membrane, and in transverse extensions of the SR from one side of the cell to the other. Immuno-labelling of retinal flat mounts demonstrated the presence RyR(2) in arteriole smooth muscle but not RyR(1). We conclude that Ca(2+)-sparks in smooth muscle can result from sequential activation of RyRs distributed over an area of several microm(2), rather than from tightly clustered channels as in striated muscle.

Effects of Phytoestrogens on Sarcoplasmic/Endoplasmic Reticulum Calcium ATPase 2a and Ca2+Uptake into Cardiac Sarcoplasmic Reticulum

Phytoestrogens are naturally occurring estrogenic compounds found in plants and plant products. These compounds are also known to exert cellular effects independent of their interactions with estrogen receptors. We studied the effects of the phytoestrogens phloretin, phloridzin, genistein, and biochanin A on Ca(2+) uptake into the cardiac muscle sarcoplasmic reticulum (SR). Genistein and biochanin A did not affect SR Ca(2+) uptake. On the other hand, phloretin and phloridzin decreased the maximum velocity of SR Ca(2+) uptake but did not affect the Hill coefficient or the Ca(2+) sensitivity of uptake. Measurements of the ATPase activity of the cardiac SR Ca(2+) pump (SERCA2a) revealed direct inhibitory effects of phloretin and phloridzin on SERCA2a. Neither compound induced a detectable change in the permeability of the SR membrane to Ca(2+). These results indicate that phloretin and phloridzin inhibit cardiac SR Ca(2+) uptake by directly inhibiting SERCA2a.

Ion channels in smooth muscle: regulators of intracellular calcium and contractility

Smooth muscle (SM) is essential to all aspects of human physiology and, therefore, key to the maintenance of life. Ion channels expressed within SM cells regulate the membrane potential, intracellular Ca2+ concentration, and contractility of SM. Excitatory ion channels function to depolarize the membrane potential. These include nonselective cation channels that allow Na+ and Ca2+ to permeate into SM cells. The nonselective cation channel family includes tonically active channels (Icat), as well as channels activated by agonists, pressure-stretch, and intracellular Ca2+ store depletion. Cl--selective channels, activated by intracellular Ca2+ or stretch, also mediate SM depolarization. Plasma membrane depolarization in SM activates voltage-dependent Ca2+ channels that demonstrate a high Ca2+ selectivity and provide influx of contractile Ca2+. Ca2+ is also released from SM intracellular Ca2+ stores of the sarcoplasmic reticulum (SR) through ryanodine and inositol trisphosphate receptor Ca2+ channels. This is part of a negative feedback mechanism limiting contraction that occurs by the Ca2+-dependent activation of large-conductance K+ channels, which hyper polarize the plasma membrane. Unlike the well-defined contractile role of SR-released Ca2+ in skeletal and cardiac muscle, the literature suggests that in SM Ca2+ released from the SR functions to limit contractility. Depolarization-activated K+ chan nels, ATP-sensitive K+ channels, and inward rectifier K+ channels also hyperpolarize SM, favouring relaxation. The expression pattern, density, and biophysical properties of ion channels vary among SM types and are key determinants of electrical activity, contractility, and SM function.

Enhanced [Ca2+]i in renal arterial smooth muscle cells of pregnant rats with reduced uterine perfusion pressure

Reduction of uterine perfusion pressure (RUPP) during late pregnancy has been suggested to trigger increases in renal vascular resistance and lead to hypertension of pregnancy. We investigated whether the increased renal vascular resistance associated with RUPP in late pregnancy reflects increases in intracellular Ca(2+) concentration ([Ca(2+)](i)) and contraction of renal arterial smooth muscle. Single smooth muscle cells were isolated from renal interlobular arteries of normal pregnant Sprague-Dawley rats and a rat model of RUPP during late pregnancy. The cells were loaded with fura 2 and both cell length and [Ca(2+)](i) were measured. In cells of normal pregnant rats incubated in Hanks' solution (1 mM Ca(2+)), ANG II (10(-7) M) caused an initial increase in [Ca(2+)](i) to 414 +/- 13 nM, a maintained increase to 149 +/- 8 nM, and 21 +/- 1% cell contraction. In RUPP rats, the initial ANG II-induced [Ca(2+)](i) (431 +/- 18 nM) was not different from pregnant rats, but both the maintained [Ca(2+)](i) (225 +/- 9 nM) and cell contraction (48 +/- 2%) were increased. Membrane depolarization by 51 mM KCl and the Ca(2+) channel agonist BAY K 8644 (10(-6) M), which stimulate Ca(2+) entry from the extracellular space, caused maintained increases in [Ca(2+)](i) and cell contraction that were greater in RUPP rats than control pregnant rats. In Ca(2+)-free (2 mM EGTA) Hanks' solution, the ANG II- and caffeine (10 mM)-induced [Ca(2+)](i) transient and cell contraction were not different between normal pregnant and RUPP rats, suggesting no difference in Ca(2+) release from the intracellular stores. The enhanced maintained ANG II-, KCl- and BAY K 8644-induced [Ca(2+)](i) and cell contraction in RUPP rats compared with normal pregnant rats suggest enhanced Ca(2+) entry mechanisms of smooth muscle contraction in resistance renal arteries and may explain the increased renal vascular resistance associated with hypertension of pregnancy.

Activation of smooth muscle myosin light chain kinase by calmodulin. Role of LYS(30) and GLY(40)

Calmodulin (CaM)-dependent myosin light chain kinase (MLCK) plays a key role in activation of smooth muscle contraction. A soybean isoform of CaM, SCaM-4 (77% identical to human CaM) fails to activate MLCK, whereas SCaM-1 (90.5% identical to human CaM) is as effective as CaM. We exploited this difference to gain insights into the structural requirements in CaM for activation of MLCK. A chimera (domain I of SCaM-4 and domains II-IV of SCaM-1) behaved like SCaM4, and analysis of site-specific mutants of SCaM-1 indicated that K30E and G40D mutations were responsible for the reduction in activation of MLCK. Competition experiments showed that SCaM-4 binds to the CaM-binding site of MLCK with high affinity. Replacement of CaM in skinned smooth muscle by exogenous CaM or SCaM-1, but not SCaM-4, restored Ca(2+)-dependent contraction. K30E/M36I/G40D SCaM-1 was a poor activator of contraction, but site-specific mutants, K30E, M36I and G40D, each restored Ca(2+)-induced contraction to CaM-depleted skinned smooth muscle, consistent with their capacity to activate MLCK. Interpretation of these results in light of the high-resolution structures of (Ca(2+))(4)-CaM, free and complexed with the CaM-binding domain of MLCK, indicates that a surface domain containing Lys(30) and Gly(40) and residues from the C-terminal domain is created upon binding to MLCK, formation of which is required for activation of MLCK. Interactions between this activation domain and a region of MLCK distinct from the known CaM-binding domain are required for removal of the autoinhibitory domain from the active site, i.e., activation of MLCK, or this domain may be required to stabilize the conformation of (Ca(2+))(4)-CaM necessary for MLCK activation.

Ca2+ activation of smooth muscle contraction: evidence for the involvement of calmodulin that is bound to the triton insoluble fraction even in the absence of Ca2+

Smooth muscle contraction is activated by phosphorylation of the 20-kDa light chains of myosin catalyzed by Ca(2+)/calmodulin (CaM)-dependent myosin light chain kinase (MLCK). According to popular current theory, the CaM involved in MLCK regulation is Ca(2+)-free and dissociated from the kinase at resting cytosolic free Ca(2+) concentration ([Ca(2+)](i)). An increase in [Ca(2+)](i) saturates the four Ca(2+)-binding sites of CaM, which then binds to and activates actin-bound MLCK. The results of this study indicate that this theory requires revision. Sufficient CaM was retained after skinning (demembranation) of rat tail arterial smooth muscle in the presence of EGTA to support Ca(2+)-evoked contraction, as observed previously with other smooth muscle tissues. This tightly bound CaM was released by the CaM antagonist trifluoperazine (TFP) in the presence of Ca(2+). Following removal of the (Ca(2+))(4)-CaM-TFP(2) complex, Ca(2+) no longer induced contraction. The addition of exogenous CaM to TFP-treated tissue at a [Ca(2+)] subthreshold for contraction or even in the absence of Ca(2+) (presence of 5 mm EGTA), followed by washout of unbound CaM, restored Ca(2+)-induced contraction; this required MLCK activation, since it was blocked by the MLCK inhibitor ML-9. The data suggest, therefore, that a specific pool of cellular CaM, tightly bound to myofilaments at resting [Ca(2+)](i), or even in the absence of Ca(2+), is responsible for activation of contraction following a local increase in [Ca(2+)]. This mechanism would allow for localized changes in [Ca(2+)] in regions of the cell distant from the myofilaments to regulate distinct Ca(2+)-dependent processes without triggering a contractile response. Immobilized CaM, therefore, resembles troponin C, the Ca(2+)-binding regulatory protein of striated muscle, which is also bound to the thin filament in a Ca(2+)-independent manner.

Calcium flux measurements in apoptosis

Early studies in apoptosis implicated an increase in cytosolic Ca2+ as a direct mediator of DNA fragmentation. However, efforts to delineate targets for this increase in Ca2+ have been slow in evolving. Several previous studies have implicated ER Ca2+ pool depletion in the initiation of apoptosis. Our own preliminary studies confirm that many (but not all) apoptotic stimuli empty the ER store via a mechanism that is blocked by BCL-2 expression. Furthermore, ER pool depletion is not affected by broad spectrum caspase inhibitors, indicating that it occurs via a caspase-independent mechanism. Finally, our data demonstrate that ER pool depletion occurs prior to release of cytochrome c from mitochondria. Given previous work demonstrating close coordination of ER and mitochondrial Ca2+ levels, we speculate that ER-dependent changes in mitochondrial Ca2+ serve as important signals for cytochrome c release. Alternative mechanisms include activation of caspase-12 and/or the JNK pathway, both of which can be directly stimulated by depletion of the ER Ca2+ pool. Although substantial improvements in intracellular Ca2+ imaging have emerged, compelling answers to many of the present questions related to the role of Ca2+ in apoptosis await future technical improvements. The development of organelle-specific, recombinant Ca2+ probes (targeted aequorins and cameleons) certainly should facilitate some of this work, although the target cell of interest must be amenable to molecular manipulation (transfection), which precludes straightforward analysis of primary cells. Pharmacological tools (i.e., thapsigargin and DBHQ) can provide conclusive data on ER pool status without requiring an overly sophisticated image analysis system. However, confocal microscopy allows for the effective analysis of Ca2+ pools as long as dye localization is homogeneous and properly controlled. However, current techniques should be considered semiquantitative at best and will remain so until specific organelle-targeted fluorescent dyes are developed and widely available.

Dependency of detrusor contractions on calcium sensitization and calcium entry through LOE-908-sensitive channels

1. The subcellular mechanisms regulating stimulus-contraction coupling in detrusor remain to be determined. We used Ca(2+)-free solutions, Ca(2+) channel blockers, cyclopiazonic acid (CPA), and RhoA kinase (ROK) inhibitors to test the hypothesis that Ca(2+) influx and Ca(2+) sensitization play primary roles. 2. In rabbit detrusor, peak bethanechol (BE)-induced force was inhibited 90% by incubation for 3 min in a Ca(2+)-free solution. By comparison, a 20 min incubation of rabbit femoral artery in a Ca(2+)-free solution reduced receptor-induced force by only 5%. 3. In detrusor, inhibition of sarcoplasmic reticular (SR) Ca(2+) release by 2APB, or depletion of SR Ca(2+) by CPA, inhibited BE-induced force by only 27%. The CPA-insensitive force was abolished by LaCl3. By comparison, 2APB inhibited receptor-induced force in rabbit femoral artery by 71%. 4. In the presence of the non-selective cation channel (NSCC) inhibitor, LOE-908, BE did not produce an increase in [Ca(2+)]i but did produce weak increases in myosin phosphorylation and force. 5. Inhibitors of ROK-induced Ca(2+) sensitization, HA-1077 and Y-27632, inhibited BE-induced force by approximately 50%, and in combination with LOE-908, nearly abolished force. 6. These data suggest that two principal muscarinic receptor-stimulated detrusor contractile mechanisms include NSCC activation, that elevates [Ca(2+)]i and ROK activation, that sensitizes cross bridges to Ca(2+).

Myosin light chain kinase- and PKC-dependent contraction of LES and esophageal smooth muscle

In smooth muscle cells enzymatically isolated from circular muscle of the esophagus (ESO) and lower esophageal sphincter (LES), ACh-induced contraction and myosin light chain (MLC) phosphorylation were similar. Contraction and phosphorylation induced by purified MLC kinase (MLCK) were significantly greater in LES than ESO. ACh-induced contraction and MLC phosphorylation were inhibited by calmodulin and MLCK inhibitors in LES and by protein kinase C (PKC) inhibitors in ESO. Contraction of LES and ESO induced by the PKC agonist 1,2-dioctanoylglycerol (DG) was unaffected by MLCK inhibitors. Caldesmon and calponin concentration-dependently inhibited ACh-induced contraction of ESO and not LES. In ESO, caldesmon antagonist GS17C reversed caldesmon- but not calponin-induced ACh inhibition. GS17C caused contraction of permeabilized ESO but had much less effect on LES. GS17C-induced contraction was not affected by MLCK inhibitors, suggesting that MLCK may not regulate caldesmon-mediated contraction. DG-induced contraction of ESO and LES was inhibited by caldesmon and calponinin, suggesting that these proteins may regulate PKC-dependent contraction. We conclude that calmodulin and MLCK play a role in ACh-induced LES contraction, whereas the classical MLCK may not be the major kinase responsible for contraction and phosphorylation of MLC in ESO. ESO contraction is PKC dependent. Caldesmon and/or calponin may play a role in PKC-dependent contraction.

Ca2+-independent smooth muscle contraction. a novel function for integrin-linked kinase

Smooth muscle contraction follows an increase in cytosolic Ca(2+) concentration, activation of myosin light chain kinase, and phosphorylation of the 20-kDa light chain of myosin at Ser(19). Several agonists acting via G protein-coupled receptors elicit a contraction without a change in [Ca(2+)](i) via inhibition of myosin light chain phosphatase and increased myosin phosphorylation. We showed that microcystin (phosphatase inhibitor)-induced contraction of skinned smooth muscle occurred in the absence of Ca(2+) and correlated with phosphorylation of myosin light chain at Ser(19) and Thr(18) by a kinase distinct from myosin light chain kinase. In this study, we identify this kinase as integrin-linked kinase. Chicken gizzard integrin-linked kinase cDNA was cloned, sequenced, expressed in E. coli, and shown to phosphorylate myosin light chain in the absence of Ca(2+) at Ser(19) and Thr(18). Subcellular fractionation revealed two distinct populations of integrin-linked kinase, including a Triton X-100-insoluble component that phosphorylates myosin in a Ca(2+)-independent manner. These results suggest a novel function for integrin-linked kinase in the regulation of smooth muscle contraction via Ca(2+)-independent phosphorylation of myosin, raise the possibility that integrin-linked kinase may also play a role in regulation of nonmuscle motility, and confirm that integrin-linked kinase is indeed a functional protein-serine/threonine kinase.

[Ca(2+)](i) signaling in renal arterial smooth muscle cells of pregnant rat is enhanced during inhibition of NOS

Vascular resistance and arterial pressure are reduced during normal pregnancy, but dangerously elevated during pregnancy-induced hypertension (PIH), and changes in nitric oxide (NO) synthesis have been hypothesized as one potential cause. In support of this hypothesis, chronic inhibition of NO synthesis in pregnant rats has been shown to cause significant increases in renal vascular resistance and hypertension; however, the cellular mechanisms involved are unclear. We tested the hypothesis that the pregnancy-associated changes in renal vascular resistance reflect changes in contractility and intracellular Ca(2+) concentration ([Ca(2+)](i)) of renal arterial smooth muscle. Smooth muscle cells were isolated from renal interlobular arteries of virgin and pregnant Sprague-Dawley rats untreated or treated with the NO synthase inhibitor nitro-L-arginine methyl ester (L-NAME; 4 mg. kg(-1). day(-1) for 5 days), then loaded with fura 2. In cells of virgin rats incubated in Hanks' solution (1 mM Ca(2+)), the basal [Ca(2+)](i) was 86 +/- 6 nM. Phenylephrine (Phe, 10(-5) M) caused a transient increase in [Ca(2+)](i) to 417 +/- 11 nM and maintained an increase to 183 +/- 8 nM and 32 +/- 3% cell contraction. Membrane depolarization by 51 mM KCl, which stimulates Ca(2+) entry from the extracellular space, caused maintained increase in [Ca(2+)](i) to 292 +/- 12 nM and 31 +/- 2% contraction. The maintained Phe- and KCl-induced [Ca(2+)](i) and contractions were reduced in pregnant rats but significantly enhanced in pregnant rats treated with L-NAME. Phe- and KCl-induced contraction and [Ca(2+)](i) were not significantly different between untreated and L-NAME-treated virgin rats or between untreated and L-NAME + L-arginine treated pregnant rats. In Ca(2+)-free Hanks', application of Phe or caffeine (10 mM), to stimulate Ca(2+) release from the intracellular stores, caused a transient increase in [Ca(2+)](i) and a small cell contraction that were not significantly different among the different groups. Thus renal interlobular smooth muscle of normal pregnant rats exhibits reduction in [Ca(2+)](i) signaling that involves Ca(2+) entry from the extracellular space but not Ca(2+) release from the intracellular stores. The reduced renal smooth muscle cell contraction and [Ca(2+)](i) in pregnant rats may explain the decreased renal vascular resistance associated with normal pregnancy, whereas the enhanced cell contraction and [Ca(2+)](i) during inhibition of NO synthesis in pregnant rats may, in part, explain the increased renal vascular resistance associated with PIH.

Genetically targeted calcium sensors enhance the study of organelle function in living cells

1. Understanding the regulation of calcium (Ca2+), the most common of the mineral ions within the human body, has always been of extreme interest to physiologists. While the importance of Ca2+ in contributing to physiological events through regulation of levels has been significantly established, seldom is consideration given to the intricacies of this ion and its mechanics in producing such diverse physiological responses in different regions of the cell. 2. The present review will summarize new methodologies used in our laboratories for the study of two major intracellular organelles, mitochondria and the nucleus. These techniques are based predominantly on the use of molecular biological approaches to both create and then target protein-based sensor molecules to specific intracellular locations. 3. The regulation of Ca2+ in the mitochondria and nucleus is of particular interest to us because of the central involvement of these organelles in: (i) cardiac cell responses during ischaemia/reperfusion; and (ii) the control of gene expression, respectively.

Nucleoplasmic Ca(2+)loading is regulated by mobilization of perinuclear Ca(2+)

Regulation of nucleoplasmic calcium (Ca(2+)) concentration may occur by the mobilization of perinuclear luminal Ca(2+)pools involving specific Ca(2+)pumps and channels of both inner and outer perinuclear membranes. To determine the role of perinuclear luminal Ca(2+), we examined freshly cultured 10 day-old embryonic chick ventricular cardiomyocytes. We obtained evidence suggesting the existence of the molecular machinery required for the bi-directional Ca(2+)fluxes using confocal imaging techniques. Embryonic cardiomyocytes were probed with antibodies specific for ryanodine-sensitive Ca(2+)channels (RyR2), sarco/endoplasmic reticulum Ca(2+)ATPase (SERCA2)-pumps, and fluorescent BODIPY derivatives of ryanodine and thapsigargin. Using immunocytochemistry techniques, confocal imaging showed the presence of RyR2 Ca(2+)channels and SERCA2-pumps highly localized to regions surrounding the nucleus, referable to the nuclear envelope. Results obtained from Fluo-3, AM loaded ionomycin-perforated embryonic cardiomyocytes demonstrated that gradual increases of extranuclear Ca(2+)from 100 to 1600 nM Ca(2+)was localized to the nucleus. SERCA2-pump inhibitors thapsigargin and cyclopiazonic acid showed a concentration-dependent inhibition of nuclear Ca(2+)loading. Furthermore, ryanodine demonstrated a biphasic concentration-dependence upon active nuclear Ca(2+)loading. The concomitant addition of thapsigargin or cyclopiazonic acid with ryanodine at inhibitory concentrations caused an significant increase in nuclear Ca(2+)loading at low concentrations of extranuclear added Ca(2+). Our results show that the perinuclear lumen in embryonic chick ventricular cardiomyocytes is capable of autonomously regulating nucleoplasmic Ca(2+)fluxes.

Gender-specific reduction in contractility and [Ca(2+)](i) in vascular smooth muscle cells of female rat

The hypothesis that vascular protection in females and its absence in males reflects gender differences in [Ca(2+)](i) and Ca(2+) mobilization mechanisms of vascular smooth muscle contraction was tested in fura 2-loaded aortic smooth muscle cells isolated from intact and gonadectomized male and female Wistar-Kyoto (WKY) and spontaneously hypertensive (SHR) rats. In WKY cells incubated in Hanks' solution (1 mM Ca(2+)), the resting length and [Ca(2+)](i) were significantly different in intact males (64.5 +/- 1.2 microm and 83 +/- 3 nM) than in intact females (76.5 +/- 1.5 microm and 64 +/- 7 nM). In intact male WKY, phenylephrine (Phe, 10(-5) M) caused transient increase in [Ca(2+)](i) to 428 +/- 13 nM followed by maintained increase to 201 +/- 8 nM and 32% cell contraction. In intact female WKY, the Phe-induced [Ca(2+)](i) transient was not significantly different, but the maintained [Ca(2+)](i) (159 +/- 7 nM) and cell contraction (26%) were significantly less than in intact male WKY. In Ca(2+)-free (2 mM EGTA) Hanks', Phe and caffeine (10 mM) caused transient increases in [Ca(2+)](i) and contraction that were not significantly different between males and females. Membrane depolarization by 51 mM KCl caused 31% cell contraction and increased [Ca(2+)](i) to 259 +/- 9 nM in intact male WKY, which were significantly greater than a 24% contraction and 214 +/- 8 nM [Ca(2+)](i) in intact female WKY. Maintained Phe- and KCl-stimulated cell contraction and [Ca(2+)](i) were significantly greater in SHR than WKY in all groups of rats. Reduction in cell contraction and [Ca(2+)](i) in intact females compared with intact males was significantly greater in SHR ( approximately 30%) than WKY ( approximately 20%). No significant differences in cell contraction or [Ca(2+)](i) were observed between castrated males, ovariectomized (OVX) females, and intact males, or between OVX females with 17beta-estradiol implants and intact females. Exogenous application of 17beta-estradiol (10(-8) M) to cells from OVX females caused greater reduction in Phe- and KCl-induced contraction and [Ca(2+)](i) in SHR than WKY. Thus the basal, maintained Phe- and depolarization-induced [Ca(2+)](i) and contraction of vascular smooth muscle triggered by Ca(2+) entry from the extracellular space exhibit differences depending on gender and the presence or absence of female gonads. Cell contraction and [Ca(2+)](i) due to Ca(2+) release from the intracellular stores are not affected by gender or gonadectomy. Gender-specific reduction in contractility and [Ca(2+)](i) in vascular smooth muscle of female rats is greater in SHR than WKY rats.

Preconditioning of coronary artery against vasoconstriction by endothelin-1 and prostaglandin F2alpha during repeated downregulation of epsilon-protein kinase C

The cellular mechanisms of coronary vasospasm are unclear, and a role for protein kinase C (PKC) activation by the endogenous vasoconstrictors endothelin-1 (ET-1) and prostaglandin F2alpha (PGF2alpha) has been suggested. In this study, we developed a phorbol ester-induced PKC downregulation protocol to investigate the relation between the amount and activity of specific PKC isoforms in coronary arterial smooth muscle and coronary vasoconstriction by ET-1 and PGF2alpha. Isometric tension was measured in deendothelialized porcine coronary artery strips, [Ca2+]i was monitored in single coronary smooth muscle cells loaded with fura-2, and the whole tissue, cytosolic, and particulate fractions were examined for PKC activity and reactivity with isoform-specific anti-PKC antibodies using Western blot analysis. In Ca(2+)-free (2 mM EGTA) Krebs solution, ET-1 (10(-7) M), PGF2alpha (10(-5) M) and PKC activator phorbol 12,13-dibutyrate (PDBu) (10(-6) M) caused significant contractions that were completely inhibited by the PKC inhibitors staurosporine and calphostin C, no significant change in [Ca2+]i, and significant activation and translocation of the Ca(2+)-independent epsilon-PKC but not the Ca(2+)-dependent alpha-PKC. In Ca(2+)-free Krebs, a single application of PDBu produced maximal contraction and PKC activity after 30 min, which declined to basal levels in 3 h and remained steady for 24 h, but did not prevent subsequent increases in contraction and PKC activity with a new addition of PDBu and did not significantly decrease the amount of alpha- or epsilon-PKC. Repeated (five to eight) applications of PDBu in Ca(2+)-free Krebs at 3-h intervals completely inhibited subsequent increases in contraction and PKC activity to PDBu, ET-1, or PGF2alpha, and significantly decreased the amount of epsilon-PKC but not that of alpha-PKC. These results provide evidence that a Ca(2+)-independent coronary vasoconstriction induced by ET-1 and PGF2alpha is associated with activation of the epsilon-PKC isoform. The results suggest that, in coronary artery smooth muscle, downregulation of PKC is isoform specific and is more dependent on the frequency rather than the duration of PKC activation. The results also suggest that repeated downregulation of epsilon-PKC might play a role in preconditioning of the coronary artery against vasoconstriction by ET-1 and PGF2alpha.

Pathways of cadmium influx in mammalian neurons

The influx of the toxic cation Cd2+ was studied in fura 2-loaded rat cerebellar granule neurons. In cells depolarized with Ca2(+)-free, high-KCI solutions, the fluorescence emission ratio (R) increased in the presence of 100 microM Cd2(+). This increase was fully reversed by the Cd2+ chelator tetrakis(2-pyridylmethyl)ethylenediamine, indicating a cadmium influx into the cell. The rate of increase, dR/dt, was greatly reduced (67+/-5%) by 1 microM nimodipine and enhanced by 1 microM Bay K 8644. Concurrent application of nimodipine and omega-agatoxin IVA (200 nM) blocked Cd2+ permeation almost completely (88+/-5%), whereas omega-conotoxin MVIIC (2 microM) reduced dR/dt by 24+/-8%. These results indicate a primary role of voltage-dependent calcium channels in Cd2+ permeation. Stimulation with glutamate or NMDA and glycine also caused a rise of R in external Cd2+. Simultaneous application of nimodipine and omega-agatoxin IVA moderately reduced dR/dt (25+/-3%). NMDA-driven Cd2(+) entry was almost completely prevented by 1 mM Mg2+, 50 microM memantine, and 10 microM 5,7-dichlorokynurenic acid, suggesting a major contribution of NMDA-gated channels in glutamate-stimulated Cd2+ influx. Moreover, perfusion with alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate caused a slow increase of R. These results suggest that Cd2+ permeates the cell membrane mainly through the same pathways of Ca2+ influx.

Mercuric chloride induces increases in both cytoplasmic and nuclear free calcium ions through a protein phosphorylation-linked mechanism

The mechanism of the lymphocyte stimulatory action of sulfhydryl group-reactive mercuric ions was studied with respect to its potential ability to induce a protein tyrosine phosphorylation-linked signal for mobilization of free Ca2+ into cytoplasm and nucleus of the cell. Exposure of human leukamic T cell line (Jurkat) cells to high (1 mM) and low (0.01 mM) concentrations of HgCl2 induced tyrosine phosphorylation of multiple proteins in a concentration-dependent manner. Confocal microscopy directly visualized the time course localization of Ca2+ inside the cells after exposure to HgCl2. The onset and level of Ca2+ mobilization following HgCl2 exposure were in parallel to those of protein tyrosine phosphorylation. Interestingly, by either concentration of HgCl2, Ca2+ was mobilized in both cytoplasm and nucleus almost simultaneously, and the level of Ca2+ mobilization in the nucleus was more than that in the cytoplasm. All the HgCl2-mediated Ca2+ mobilization was prevented by addition of protein kinase inhibitor staurosporin prior to HgCl2. These results suggest that heavy metal stress triggers a protein tyrosine phosphorylation-linked signal that leads to a nuclear event-dominant Ca2+ mobilization.

Protein kinase C: modulation of vasopressin-induced calcium influx and release in A7r5 vascular smooth muscle cells

This study was guided by the hypothesis that specific isoforms of protein kinase C may participate in modulating increases in intracellular Ca2+ that are induced by stimulation of vascular smooth muscle cells with vasopressin. Immunoblot analysis revealed that A7r5 vascular smooth muscle cells expressed conventional (alpha), novel (delta and epsilon), and atypical (iota/lambda and mu) isoforms of protein kinase C. Stimulation of fura-2-loaded cells with 20 nM vasopressin induced a rapid transient increase in the intracellular concentration of calcium that was followed by a slowly declining component which was above baseline throughout the period of observation. Cell fractionation studies showed that the calcium response was associated with (a) transient translocation of the alpha and delta isoforms of protein kinase C from the cytosolic fraction to the particulate-membrane fraction, (b) sustained translocation of the epsilon isoform, and (c) no translocation of iota/lambda or mu isoforms. Ratiometric and isobestic fluorescence analysis showed that vasopressin-induced Ca2+ influx and release were markedly inhibited in cells that were preincubated with either 1 microM phorbol 12-myristate 13-acetate, or 10 microM 1, 2 dioctanoyl-sn-glycerol, two structurally different activators of protein kinase C. In contrast, vasopressin-induced increases in intracellular Ca2+ were not significantly altered following preincubation with either 1 microM 4alpha-phorbol or 4alpha-phorbol 12,13-didecanoate, analogs of phorbol 12-myristate 13-acetate that do not activate protein kinase C. Moreover, the inhibitory effects of phorbol 12-myristate 13-acetate were prevented by treatment with 1 microM GF109203X, a potent inhibitor of protein kinase C. Taken together, these results show that direct activation of protein kinase C can negatively modulate vasopressin-induced Ca2+ influx and release in cultured vascular smooth muscle cells. They also show that stimulation with vasopressin induces translocation of specific isoforms of protein kinase C, an observation suggesting that one or more of these isoforms may participate in modulation of vasopressin-induced increases in intracellular Ca2+.

Hydrogen peroxide-induced DNA damage is independent of nuclear calcium but dependent on redox-active ions

In cells undergoing oxidative stress, DNA damage may result from attack by .OH radicals produced by the Fenton reaction, and/or by nucleases activated by nuclear calcium. In the present study, the participation of these two mechanisms was investigated in HeLa cells. Nuclear-targeted aequorin was used for selectively monitoring Ca2+ concentrations within the nuclei ([Ca2+]n), in conjunction with the cell-permeant calcium chelator bis-(o-aminophenoxy)ethane-N,N,N', N'-tetraacetic acid acetoxymethyl ester (BAPTA/AM), the lipid-soluble broad-spectrum metal chelator with low affinity for Ca2+ and Mg2+ N,N,N',N'-tetrakis(2-pyridylmethyl)ethylenediamine (TPEN), and the high-affinity iron/copper chelator 1, 10-phenanthroline (PHE). In Ca2+-containing medium, H2O2 induced extensive DNA strand breaks and an increase in [Ca2+]n that was almost identical to that observed in the cytosol ([Ca2+]c). In cells bathed in Ca2+-free/EGTA medium, in which the increases in [Ca2+]n and [Ca2+]c due to H2O2 were significantly reduced, similar levels of DNA fragmentation also occurred. In cells preloaded with BAPTA/AM or TPEN, the small increase of [Ca2+]n normally elicited by H2O2 in Ca2+-free medium was completely buffered, and DNA damage was largely prevented. On the other hand, pretreatment with PHE did not affect the calcium response in the nuclei, but completely prevented DNA strand breakage induced by H2O2. Re-addition of 100 microM CuSO4 and 100 microM FeSO4 to TPEN- and PHE-treated cells prior to H2O2 challenge reversed the effect of TPEN and PHE, whereas 1 mM was necessary to negate the effect of BAPTA/AM. The levels of DNA strand breakage observed, however, did not correlate with the amounts of 8-hydroxy 2'-deoxyguanosine (8-OHdG): H2O2 did not produce 8-OHdG, whereas PHE alone slightly increased 8-OHdG levels. CuSO4 and FeSO4 enhanced the effects of PHE, particularly in the presence of H2O2. Exposure of cells to a mixture of CuSO4/FeSO4 also resulted in a significant increase in 8-OHdG levels, which was prevented in cells preloaded with BAPTA/AM. Similar results were obtained in a cell-free system using isolated calf thymus DNA exposed to CuSO4/FeSO4, regardless of whether H2O2 was present or not. These results suggest that BAPTA/AM prevents H2O2-induced DNA damage by acting as an iron/copper chelator. These data also indicate that caution must be exercised in using Ca2+ chelating agents as evidence for a role in cellular Ca2+ levels in experimental conditions in which transition-metal-ion-mediated oxidant production is also occurring.

Chloride channel blockers inhibit Ca2+ uptake by the smooth muscle sarcoplasmic reticulum

Despite the fact that Ca2+ transport into the sarcoplasmic reticulum (SR) of muscle cells is electrogenic, a potential difference is not maintained across the SR membrane. To achieve electroneutrality, compensatory charge movement must occur during Ca2+ uptake. To examine the role of Cl- in this charge movement in smooth muscle cells, Ca2+ transport into the SR of saponin-permeabilized smooth muscle cells was measured in the presence of various Cl- channel blockers or when I-, Br-, or SO42- was substituted for Cl-. Calcium uptake was inhibited in a dose-dependent manner by 5-nitro-2-(3-phenylpropylamino) benzoic acid (NPPB) and by indanyloxyacetic acid 94 (R(+)-IAA-94), but not by niflumic acid or 4,4'-dinitrostilbene-2,2'-disulfonic acid (DNDS). Smooth muscle SR Ca2+ uptake was also partially inhibited by the substitution of SO42- for Cl-, but not when Cl- was replaced by I- or Br-. Neither NPPB nor R(+)-IAA-94 inhibited Ca2+ uptake into cardiac muscle SR vesicles at concentrations that maximally inhibited uptake in smooth muscle cells. These results indicate that Cl- movement is important for charge compensation in smooth muscle cells and that the Cl- channel or channels involved are different in smooth and cardiac muscle cells.

Physiological features of visceral smooth muscle cells, with special reference to receptors and ion channels

Visceral smooth muscle cells (VSMC) play an essential role, through changes in their contraction-relaxation cycle, in the maintenance of homeostasis in biological systems. The features of these cells differ markedly by tissue and by species; moreover, there are often regional differences within a given tissue. The biophysical features used to investigate ion channels in VSMC have progressed from the original extracellular recording methods (large electrode, single or double sucrose gap methods), to the intracellular (microelectrode) recording method, and then to methods for recording from membrane fractions (patch-clamp, including cell-attached patch-clamp, methods). Remarkable advances are now being made thanks to the application of these more modern biophysical procedures and to the development of techniques in molecular biology. Even so, we still have much to learn about the physiological features of these channels and about their contribution to the activity of both cell and tissue. In this review, we take a detailed look at ion channels in VSMC and at receptor-operated ion channels in particular; we look at their interaction with the contraction-relaxation cycle in individual VSMC and especially at the way in which their activity is related to Ca2+ movements and Ca2+ homeostasis in the cell. In sections II and III, we discuss research findings mainly derived from the use of the microelectrode, although we also introduce work done using the patch-clamp procedure. These sections cover work on the electrical activity of VSMC membranes (sect. II) and on neuromuscular transmission (sect. III). In sections IV and V, we discuss work done, using the patch-clamp procedure, on individual ion channels (Na+, Ca2+, K+, and Cl-; sect. IV) and on various types of receptor-operated ion channels (with or without coupled GTP-binding proteins and voltage dependent and independent; sect. V). In sect. VI, we look at work done on the role of Ca2+ in VSMC using the patch-clamp procedure, biochemical procedures, measurements of Ca2+ transients, and Ca2+ sensitivity of contractile proteins of VSMC. We discuss the way in which Ca2+ mobilization occurs after membrane activation (Ca2+ influx and efflux through the surface membrane, Ca2+ release from and uptake into the sarcoplasmic reticulum, and dynamic changes in Ca2+ within the cytosol). In this article, we make only limited reference to vascular smooth muscle research, since we reviewed the features of ion channels in vascular tissues only recently.

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

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

Anti-phospholamban and protein kinase A alter the Ca2+ sensitivity and maximum velocity of Ca2+ uptake by the cardiac sarcoplasmic reticulum

The activity of the SERCA2a Ca2+ pump in the sarcoplasmic reticulum (SR) of cardiac muscle is inhibited by phospholamban. When phospholamban is phosphorylated by cyclic-AMP-dependent protein kinase (PKA) this inhibition is relieved. It is generally agreed that this results in an increase in the Ca2+ sensitivity of the SR Ca2+ pump; however, some investigators have also reported an increase in the maximum velocity of the pump. We have used a sensitive fluorescence method to measure net Ca2+ uptake by native cardiac SR vesicles and compared the effects of a constitutively active subunit of PKA (cPKA) with those of a monoclonal antibody (A1) that binds to phospholamban and is thought to mimic the effect of phosphorylation. Both the Ca2+ sensitivity and the maximum velocity of uptake were increased by cPKA and by A1. The effects of cPKA and A1 on uptake velocity were only slightly additive. No changes in uptake were detected with denatured cPKA or denatured A1. These results indicate that the functional effect of phospholamban phosphorylation is to increase both the Ca2+ sensitivity and the maximum velocity of net Ca2+ uptake into the SR.

Modulation of high- and low-voltage-activated calcium currents in smooth muscle by calcium

Ca2+ currents (ICa) and cytoplasmic Ca2+ concentration ([Ca2+]c) were measured in isolated gastric myocytes from Bufo marinus using whole cell voltage clamp and fura 2, respectively. After a conditioning train of depolarizing pulses, high-voltage-activated ICa (test potential of +10 mV) was increased, returning to control values after approximately 85 s. This enhancement was [Ca2+]c dependent, with a maximal increase at approximately 600 nM [Ca2+]c. During the conditioning train, ICa measured at 70 ms, which provides a measure of high-voltage-activated current, initially decreased with each successive pulse to a minimum of 56 +/- 5% of the first pulse in the train. Thereafter, the 70-ms current showed considerable recovery. Blockade of calmodulin activity with a peptide (RS20) or calmidazolium did not affect the early inhibition but did abolish current recovery. A peptide inhibitor of calmodulin-dependent protein kinase II (CK3AA) had similar effects. Substraction of currents measured in the presence and absence of RS20 revealed a 2-s delay between the start of the train and the onset of current enhancement. It was also observed that low-voltage-activated current (test potential of -17 mV) was reduced to 76 +/- 7% of control 5 s after the conditioning train; this inhibition recovered to 92 +/- 4% after 35 s and was not dependent on [Ca2+]c elevation.

Genistein sensitivity of calcium transport pathways in serotonin-activated vascular smooth muscle cells

Recent studies showed that serotonin-activated increases in intracellular Ca2+ in vascular smooth muscle cells are associated with enhanced protein tyrosine phosphorylation. These responses were blocked by inhibition of tyrosine kinase activity with genistein, suggesting that the increases in Ca2+ and tyrosine phosphorylation are functionally coupled. Therefore, we sought to characterize genistein-sensitive Ca2+ transport pathways in rat aortic A10 cells loaded with fura-2. In the presence of extracellular Ca2+, serotonin evoked a transient increase in [Ca2+]i that was followed by a smaller sustained increase. The transient was inhibited 25-40% by L-type Ca2+ channel antagonists and inhibited 90-95% by genistein. The sustained response was unaffected by L-channel antagonists and only slightly inhibited by genistein. In the absence of extracellular Ca2+, the transient was reduced by 50%, while the sustained component was virtually abolished. These results suggest that influx and release pathways are major contributors to the transient component, whereas the lower sustained component is largely limited to influx pathways. The influx pathway during the transient probably involves an L-type Ca2+ channel that is regulated by tyrosine kinase activity. The pathways that participate in the sustained response are different because they are insensitive to l-channel antagonists and only slightly inhibited by genistein. The transient evoked in Ca2+-free media was blocked by genistein, inhibited by caffeine, and prevented by thapsigargin. Ionomycin-induced release of Ca2+ was unaffected by genistein, reduced by caffeine, and essentially eliminated by thapsigargin. Therefore, thapsigargin-mediated suppression of serotonin-activated release probably reflects depletion of Ca2+ from the sarcoplasmic reticulum, whereas genistein-mediated suppression probably reflects inhibition of tyrosine kinase linked release. Caffeine-mediated suppression appears to involve both partial depletion of Ca2+ and interference with release. Each A10 cell expressed at least two different ryanodine receptors and two different receptors for inositol 1,4,5-trisphosphate.

Parallel changes in nuclear and cytosolic calcium in mouse pancreatic beta-cells

In the neuroendocrine pancreatic beta-cell, elevations in intracellular Ca2+ lead to insulin secretion and the initiation of gene transcription. However, the relationship between cytosolic and nuclear Ca2+ in these cells is unknown. The Ca2+ permeability of the nuclear membrane would therefore determine if Ca2+ could play a direct role in Ca2+-dependent nuclear processes. Using confocal fluorescence microscopy with the ratiometric Ca2+ indicator indo-1 and carefully correcting for compartmentalized indicator, we now demonstrate that there is no difference between the nuclear Ca2+ concentration and the cytosolic Ca2+ concentration ([Ca2+]c) in the resting beta-cell. Slow Ca2+ oscillations induced by glucose, fast oscillations induced by glucagon-like peptide-1 and responses to potassium and carbachol all indicate that changes in cytosolic Ca2+ are reflected within the nucleus. We conclude that there are no restrictions on Ca2+ entry into the nucleus of the pancreatic beta-cell subsequent to increases in [Ca2+]c. This implies that any signal involved in increasing [Ca2+]c, and thereby insulin release, may also promote nuclear Ca2+-induced gene transcription.

Inositol 1,4,5-trisphosphate receptor is located to the inner nuclear membrane vindicating regulation of nuclear calcium signaling by inositol 1,4,5-trisphosphate. Discrete distribution of inositol phosphate receptors to inner and outer nuclear membranes

Transient rise in nuclear calcium concentration is implicated in the regulation of events controlling gene expression. Mechanism by which calcium is transported to the nucleus is vehemently debated. Inositol 1,4,5-trisphosphate (InsP3) and inositol-1,3,4,5-tetrakisphosphate (InsP4) receptors have been located to the nucleus and their role in nuclear calcium signaling has been proposed. Outer nuclear membrane was separated from the inner membrane. The two membrane preparations were, as best as possible, devoid of cross contamination as attested by marker enzyme activity, Western blotting with antilamin antibody, and electron microscopy. InsP4 receptor and Ca(2+)-ATPase were located to the outer nuclear membrane. InsP3 receptor was located to the inner nuclear membrane. ATP or InsP4 induced nuclear calcium uptake. External free calcium concentration, in the medium bathing the nuclei, determined the choice for ATP or InsP4-mediated calcium transport. We present a mechanistic model for nuclear calcium transport. According to this model, calcium can reach the nucleus envelope either by the action of ATP or InsP4. However, the calcium release from the nucleus envelope to the nucleoplasm is mediated by InsP3 through the activation of InsP3 receptor, which is located to the inner nuclear membrane. The action of InsP3 in this process was instantaneous and transient and was sensitive to heparin.

Nuclear pH gradient in mammalian cells revealed by laser microspectrofluorimetry

Intracellular pH has been measured by laser microspectrofluorimetry, using the pH-sensitive dyes SNARF-1, SNARF-calcein and SNARF-1-dextran. By this technique it was possible to accurately determine pH in volumes as small as 0.5 × 0.5 × 1 microns 3. The probes were loaded into the cells either by diffusion of their acetoxymethylester derivatives (SNARF-1-AM, SNARF-calcein-AM) or by microinjection (SNARF-1-dextran). When the five types of cells were studied in RPMI medium, the nuclear pH was consistently found to be 0.3 to 0.5 units above that of the cytosol. Although the presence of pores in the nuclear membrane has been taken as evidence that free diffusion of ions and small molecules can occur in and out the nucleus, we conclude that the nuclear membrane of these cells presents a permeability barrier to H+. The pH gradient was not observed in cells suspended in PBS.

Maintenance of Fura-2 fluorescence in glial cells and neurons of the leech central nervous system

Identified glial cells and neurones of the leech central nervous system (CNS) were injected iontophoretically with the calcium indicator dye Fura-2 to measure intracellular Ca2+, while simultaneously recording the membrane potential using a double-barrelled theta-type microelectrode. Both glial cells and neurones responded with Ni(2+)-sensitive Ca2+ transients upon membrane depolarization, indicating Ca2+ influx through voltage-gated Ca2+ channels. In contrast to neurones, the glial cells showed a rapid loss of fluorescence with a half-time of 6.3 +/- 1.8 min (n = 6) after dye injection. Both kinetics and amplitudes of the stimulus-induced Ca2+ transients were affected by this rapid dye loss. The anion exchange inhibitor probenicid (2 mM) significantly reduced, but did not prevent, the loss of Fura-2 fluorescence, suggesting that some dye left the glial cell via an anion exchanger. In order to compensate this fluorescence loss, we injected Fura-2 throughout the experiment. Under this condition, similar Ca2+ transients could be elicited repeatedly for more than 1 h. In Retzius neurones single injections of Fura-2 yielded enough intracellularly trapped dye to allow measurement of intracellular Ca2+ for up to 30 min after the end of injection without large decrease in absolute fluorescence.

Changes in either cytosolic or nucleoplasmic inositol 1,4,5-trisphosphate levels can control nuclear Ca2+ concentration

The free nucleoplasmic Ca2+ concentration ([Ca2+]n) may regulate many nuclear events, such as gene transcription. Since the nucleus may possess the enzymes necessary to generate the second messenger inositol 1,4,5-trisphosphate (Ins(1,4,5)P3), and because the nuclear envelope may enclose an Ins(1,4,5)P3-releasable Ca2+ store, we tested the hypothesis that nuclear and/or cytosolic levels of Ins(1,4,5)P3 can control [Ca2+]n. To assay [Ca2+]n, we measured the fluorescence of the Ca2+ indicator fluo 3 in the nucleus of Xenopus oocytes by confocal microscopy. When we injected Ins(1,4,5)P3 into the cytosol, [Ca2+]n increased. This increase in [Ca2]n still occurred when heparin was present in the nucleus, but was abolished when heparin was present in the cytosol, indicating that cytosolic Ins(1,4,5)P3 levels could control [Ca2+]n. When we injected Ins(1,4,5)P3 directly into the nucleus, [Ca2+]n increased, even when heparin was present in the cytosol, indicating that Ins(1,4,5)P3 could control [Ca2+]n from within the nucleus. These results provide functional evidence for Ins(1,4,5)P3 receptors facing the nucleoplasm and raise the possibility that a phosphoinositide cycle situated at the nuclear membranes can control Ca(2+)-dependent nuclear functions.

Excitation-transcription coupling mediated by zinc influx through voltage-dependent calcium channels

Electrical activity initiates a program of selective gene expression in excitable cells. Although such transcriptional activation is commonly attributed to depolarization-induced changes in intracellular Ca2+, zinc represents a viable alternative given its prominent role as a cofactor in DNA-binding proteins coupled with evidence that Zn2+ can enter excitable cells in a voltage-dependent manner. Here it is shown that Zn2+ entry into heart cells depends upon electrical stimulation and occurs via dihydropyridine-sensitive Ca2+ channels. The addition of extracellular Zn2+ to spontaneously depolarizing GH3 pituitary tumor cells induced the expression of a reporter gene driven by the metallothionein promoter, an effect that was prevented by exposure to dihydropyridine Ca2+ channel blockers. Thus, Zn2+ influx through L-type Ca2+ channels can mediate voltage-dependent gene expression.

Confocal fluorescence microscopy for studying thapsigargin-induced bivalent-cation entry into B cells

We studied thapsigargin-induced bivalent-cation entry into antigen-specific B cells (TP67.21) with a confocal fluorescence microscope. Confocal fluorescence images of fluo-3-loaded B cells showed that thapsigargin-stimulated Ca2+ signals were transferred not only to the cytoplasm but also to the nucleus. In the absence of external Ca2+ ions, the free Ca2+ concentrations both in the cytosol and in the nucleus declined to basal levels by 5 min after addition of thapsigargin. However, subsequent addition of Ca2+ in the external medium made the fluo-3 (fura-2) fluorescence intensity rise, reflecting the fact that Ca2+ accumulated again in the nucleus as well as in the cytoplasm. Then, we added Ba2+ and Mn2+ instead of Ca2+, because Ba2+ and Mn2+ are known to enter via Ca2+ channels. The addition of Ba2+ and Mn2+ in the external medium quenched the fluo-3 fluorescence both in the nucleus and in the cytoplasm of B cells. This suggested the possibility that the increase in intranuclear Ca2+ after thapsigargin stimulation may come from the cytoplasm, not from the nuclear stores.

Source of nuclear calcium signals

Transient increases of Ca2+ concentration in the nucleus regulate gene expression and other nuclear processes. We investigated whether nuclear Ca2+ signals could be regulated independently of the cytoplasm or were controlled by cytoplasmic Ca2+ signals. A fluorescent Ca2+ indicator that is targeted to the nucleus was synthesized by coupling a nuclear localization peptide to Calcium Green dextran, a 70-kDa Ca2+ indicator. Stimulation of rat basophilic leukemia cells by antigen or by photolytic uncaging of inositol 1,4,5-trisphosphate induced transient increases in nuclear and cytosolic Ca2+ concentrations. Elevations in the nuclear Ca2+ concentration followed those in the nearby perinuclear cytosol within 200 ms. Heparin-dextran, an inhibitor of the inositol 1,4,5-trisphosphate receptor that is excluded from the nucleus, was synthesized to specifically block the release of Ca2+ from cytosolic stores. Addition of this inhibitor suppressed Ca2+ transients in the nucleus and the cytosol. We conclude that the Ca2+ level in the nucleus is not independently controlled. Rather, nuclear Ca2+ increases follow cytosolic Ca2+ increases with a short delay most likely due to Ca2+ diffusion from the cytosol through the nuclear pores.

Nuclear calcium transport and the role of calcium in apoptosis

The last decade has been the rapid development of research investigating the molecular mechanisms whereby hormones, peptide growth factors and cytokines regulate cell metabolism, differentiation and proliferation. One general signalling mechanism used to transfer the information delivered by agonists into appropriate intracellular compartments involves the rapid Ca2+ redistribution throughout the cell, which results in transient elevations of the cytosolic free Ca2+ concentration. Ca2+ signals are required for a number of cellular functions, including the activation of nuclear processes such as gene transcription and cell cycle events. The latter requires that appropriate Ca2+ signals elicited in response to agonists be transduced across the nuclear envelope. It has generally been assumed that small molecules, metabolites and ions could diffuse freely across the nuclear envelope. Nevertheless, several findings during the past few years have suggested that nuclear pore permeability can be regulated and that ion transport systems and ion-selective channels may exist in the nuclear membranes and regulate intranuclear processes. Intranuclear Ca2+ fluctuations can affect chromatin organization, induce gene expression and also activate cleavage of nuclear DNA by nucleases during programmed cell death or apoptosis. The possible mechanisms involved in nuclear Ca2+ transport and the regulation of nuclear Ca(2+)-dependent enzymes in apoptosis are discussed in the following sections.

Propagation of cytosolic calcium waves into the nuclei of hepatocytes

The temporal and spatial organization of [Ca2+] changes within the nucleus of Fura-2 loaded hepatocytes maintained in primary culture has been investigated. Vasopressin stimulation induced oscillatory waves of cytosolic free [Ca2+] increase, which propagated freely through the nuclear region. Based on the amplitude of the Fura-2 signals from this region, the morphology of the hepatocyte nucleus and the rapid penetration of the nucleus by injected Fura-2, it can be concluded that the nuclear Ca2+ responses reflect changes occurring within the nucleoplasm. Intranuclear Ca2+ increases occurred as waves that appear to be directed by the Ca2+ waves passing through the surrounding cytoplasm. The apparent velocity of Ca2+ waves was higher in the nucleoplasm than in the cytoplasm (19.5 +/- 2.9 versus 11.0 +/- 1.1 microns/s). The nucleoplasm does not contain vesicular Ca2+ stores that might be released by Ins(1,4,5)P3. However, the nuclear envelope functions as a Ca2+ store that is sensitive to mobilization by Ins(1,4,5)P3. We conclude that the [Ca2+] in the nucleoplasm of the hepatocyte is close to equilibrium with the cytosolic [Ca2+] and that oscillatory waves of cytosolic [Ca2+] are closely paralleled by similar [Ca2+] changes in the nucleoplasm. The nuclear envelope is a component of the intracellular Ins(1,4,5)P3-sensitive Ca2+ storage pool and may serve as a reservoir for [Ca2+] elevations within the nucleus.

Methods for determining cardiac sarcoplasmic reticulum Ca2+ pump kinetics from fura 2 measurements

We explored the use of four methods for analyzing real and simulated fura 2 measurements of Ca2+ uptake by membrane vesicles derived from the sarcoplasmic reticulum (SR) of cardiac muscle. Uptake velocity was calculated 1) directly from the raw data, 2) after segmenting the raw data and averaging the data points in each segment, 3) after smoothing of the raw data by moving-window averaging, and 4) by Savitsky-Golay convolution. Methods 2, 3, and 4 could be used to determine maximum uptake velocity, the Hill coefficient, and the Ca2+ concentration at half-maximal pump velocity from Ca2+ concentration vs. time and velocity curves that were too noisy to analyze directly. Data analysis using these methods should have general applicability to biological experiments, especially those in which large numbers of measurements are made. The fluorometric method we describe for measuring Ca2+ uptake by cardiac SR vesicles opens up the possibility of studying SR function from very small starting tissue samples.

Relationship between [Ca2+] changes in nucleus and cytosol

The free calcium concentration in nucleus ([Ca2+]n) and in cytoplasm ([Ca2+]c) of single cells were estimated by confocal laser microscopy using the Ca(2+)-indicator Indo-1. It is shown that in various cell types a nucleo-cytosolic Ca(2+)-gradient is present at rest and during stimulation. The direction and the extent of the nucleo-cytosolic Ca(2+)-gradient may vary with the cell type, differentiation status, phosphorylation conditions and also with the type of agonist. Evidence is given for the role of extra- and intranuclear storage sites as well as for Ca(2+)-influx. Finally potential artefactual interference with the measurements is discussed.

Calmodulin and the regulation of smooth muscle contraction

Calmodulin, the ubiquitous and multifunctional Ca(2+)-binding protein, mediates many of the regulatory effects of Ca2+, including the contractile state of smooth muscle. The principal function of calmodulin in smooth muscle is to activate crossbridge cycling and the development of force in response to a [Ca2+]i transient via the activation of myosin light-chain kinase and phosphorylation of myosin. A distinct calmodulin-dependent kinase, Ca2+/calmodulin-dependent protein kinase II, has been implicated in modulation of smooth-muscle contraction. This kinase phosphorylates myosin light-chain kinase, resulting in an increase in the calmodulin concentration required for half-maximal activation of myosin light-chain kinase, and may account for desensitization of the contractile response to Ca2+. In addition, the thin filament-associated proteins, caldesmon and calponin, which inhibit the actin-activated MgATPase activity of smooth-muscle myosin (the cross-bridge cycling rate), appear to be regulated by calmodulin, either by the direct binding of Ca2+/calmodulin or indirectly by phosphorylation catalysed by Ca2+/calmodulin-dependent protein kinase II. Another level at which calmodulin can regulate smooth-muscle contraction involves proteins which control the movement of Ca2+ across the sarcolemmal and sarcoplasmic reticulum membranes and which are regulated by Ca2+/calmodulin, e.g. the sarcolemmal Ca2+ pump and the ryanodine receptor/Ca2+ release channel, and other proteins which indirectly regulate [Ca2+]i via cyclic nucleotide synthesis and breakdown, e.g. NO synthase and cyclic nucleotide phosphodiesterase. The interplay of such regulatory mechanisms provides the flexibility and adaptability required for the normal functioning of smooth-muscle tissues.

Nuclear Ca2+: physiological regulation and role in apoptosis

The last decade has seen the rapid development of research investigating the molecular mechanisms whereby hormones, peptide growth factors and cytokines regulate cell metabolism, differentiation and proliferation. One general signalling mechanism used to transfer the information delivered by agonists into appropriate intracellular compartments involves the rapid Ca2+ redistribution throughout the cell, which results in transient elevations of the cytosolic free Ca2+ concentration. Ca2+ signals are required for a number of cellular processes including the activation of nuclear processes such as gene transcription and cell cycle events. The latter require that appropriate Ca2+ signals elicited in response to agonists be transduced across the nuclear envelope. It has generally been assumed that small molecules, metabolites and ions could freely diffuse across the nuclear envelope. Nevertheless several findings during the past few years have suggested that nuclear pore permeability can be regulated and that ion transport systems and ion-selective channels may exist on the nuclear membranes and regulate intranuclear processes. Intranuclear Ca2+ fluctuations can affect chromatin organization, induce gene expression and also activate cleavage of nuclear DNA by nucleases during programmed cell death or apoptosis. The possible mechanisms involved in nuclear Ca2+ transport and the control of nuclear Ca(2+)-dependent enzymes in apoptosis is discussed below.