Ca2+ channels, ryanodine receptors and Ca(2+)-activated K+ channels: a functional unit for regulating arterial tone

Function and expression of ryanodine receptors and inositol 1,4,5-trisphosphate receptors in smooth muscle cells of murine feed arteries and arterioles

We tested the hypothesis that vasomotor control is differentially regulated between feed arteries and downstream arterioles from the cremaster muscle of C57BL/6 mice. In isolated pressurized arteries, confocal Ca(2+) imaging of smooth muscle cells (SMCs) revealed Ca(2+) sparks and Ca(2+) waves. Ryanodine receptor (RyR) antagonists (ryanodine and tetracaine) inhibited both sparks and waves but increased global Ca(2+) and myogenic tone. In arterioles, SMCs exhibited only Ca(2+) waves that were insensitive to ryanodine or tetracaine. Pharmacological interventions indicated that RyRs are functionally coupled to large-conductance, Ca(2+)-activated K(+) channels (BK(Ca)) in SMCs of arteries, whereas BK(Ca) appear functionally coupled to voltage-gated Ca2+ channels in SMCs of arterioles. Inositol 1,4,5-trisphosphate receptor (IP3R) antagonists (xestospongin D or 2-aminoethoxydiphenyl borate) or a phospholipase C inhibitor (U73122) attenuated Ca(2+) waves, global Ca(2+) and myogenic tone in arteries and arterioles but had no effect on arterial sparks. Real-time PCR of isolated SMCs revealed RyR2 as the most abundant isoform transcript; arteries expressed twice the RyR2 but only 65% the RyR3 of arterioles and neither vessel expressed RyR1. Immunofluorescent localisation of RyR protein indicated bright, clustered staining of arterial SMCs in contrast to diffuse staining in arteriolar SMCs. Expression of IP(3)R transcripts and protein immunofluorescence were similar in SMCs of both vessels with IP(3)R1>>IP(3)R2>IP(3)R3. Despite similar expression of IP(3)Rs and dependence of Ca(2+) waves on IP(3)Rs, these data illustrate pronounced regional heterogeneity in function and expression of RyRs between SMCs of the same vascular resistance network. We conclude that vasomotor control is differentially regulated in feed arteries vs. downstream arterioles.

Perspective on the role of P2X-purinoceptor activation in human vas deferens contractility

The contractile actions of α,β-methylene ATP (α,β-meATP) and ATP and the effects of K(+) channel blockers in longitudinal and circular muscles of human vas deferens were investigated with a view to clarifying the functional importance of P2X(1)-purinoceptor activation and K(+) channels in modulating contractility of the tissues. The results provide an experiment-based perspective for resolving differing reports on purinergic activation of the tissues and uncertain roles of large-conductance Ca(2+)-activated K(+) (BK(Ca)) and voltage-gated delayed rectifier K(+) (K(V)) channels. α,β-Methylene ATP (3-100 μm) evoked suramin-sensitive contractions of longitudinal muscle but rarely of circular muscle. ATP (0.1-3 mm) less reliably activated only longitudinal muscle contractions. These were enhanced by ARL 67156 (100 μm), but a different ectonucleotidase inhibitor, POM 1, was ineffective. Both muscle types were unresponsive to ADP-βS (100 μm), a P2Y-purinoceptor agonist. Longitudinal muscle contractions in response to α,β-meATP were enhanced by FPL 64176 (1 μm), an L-type Ca(2+) agonist, TEA (1 mm), a non-specific K(+) channel blocker, 4-aminopyridine (0.3 mm), a selective blocker of K(V) channels, and iberiotoxin (0.1 μm), a selective blocker of BK(Ca) channels. Quiescent circular muscles responded to α,β-meATP reliably in the presence of FPL 64176 or iberiotoxin. Apamin (0.1 μm), a selective blocker of small conductance Ca(2+)-activated K(+) (SK(Ca)) channels had no effect in both muscle types. Y-27632 (1-10 μm) reduced longitudinal muscle contractions in response to α,β-meATP, suggesting involvement of Rho-kinase-dependent contractile mechanisms. The results indicate that P2X(1)-purinoceptor stimulation elicits excitatory effects that: (a) lead to longitudinal muscle contraction and secondary activation of 4-aminopyridine-sensitive (K(V)) and iberiotoxin-sensitive (BK(Ca)) K(+) channels; and (b) are subcontractile in circular muscle due to ancillary activation of BK(Ca) channels. The novel finding of differential action by P2X(1)-purinoceptor agonists in the muscle types has functional implication in terms of the purinergic contribution to overall contractile function of human vas deferens. The modulatory effects of K(V) and BK(Ca) channels following P2X(1)-purinoceptor activation may be pivotal in providing the crucial physiological mechanism that ensures temporal co-ordination of longitudinal and circular muscle contractility.

Endogenous cytosolic Ca(2+) buffering is necessary for TRPM4 activity in cerebral artery smooth muscle cells

The melastatin transient receptor potential (TRP) channel, TRPM4, is a critical regulator of smooth muscle membrane potential and arterial tone. Activation of the channel is Ca(2+)-dependent, but prolonged exposures to high global Ca(2+) causes rapid inactivation under conventional whole-cell patch clamp conditions. Using amphotericin B perforated whole cell patch clamp electrophysiology, which minimally disrupts cytosolic Ca(2+) dynamics, we recently showed that Ca(2+) released from 1,2,5-triphosphate receptors (IP(3)R) on the sarcoplasmic reticulum (SR) activates TRPM4 channels, producing sustained transient inward cation currents (TICCs). Thus, Ca(2+)-dependent inactivation of TRPM4 may not be inherent to the channel itself but rather is a result of the recording conditions. We hypothesized that under conventional whole-cell configurations, loss of intrinsic cytosolic Ca(2+) buffering following cell dialysis contributes to inactivation of TRPM4 channels. With the inclusion of the Ca(2+) buffers ethylene glycol-bis(2-aminoethylether)-N,N,N',N'-tetraacetic acid (EGTA, 10mM) or bis-ethane-N,N,N',N'-tetraacetic acid (BAPTA, 0.1mM) in the pipette solution, we mimic endogenous Ca(2+) buffering and record novel, sustained whole-cell TICC activity from freshly-isolated cerebral artery myocytes. Biophysical properties of TICCs recorded under perforated and whole-cell patch clamp were nearly identical. Furthermore, whole-cell TICC activity was reduced by the selective TRPM4 inhibitor, 9-phenanthrol, and by siRNA-mediated knockdown of TRPM4. When a higher concentration (10mM) of BAPTA was included in the pipette solution, TICC activity was disrupted, suggesting that TRPM4 channels on the plasma membrane and IP(3)R on the SR are closely opposed but not physically coupled, and that endogenous Ca(2+) buffer proteins play a critical role in maintaining TRPM4 channel activity in native cerebral artery smooth muscle cells.

Hypoxic pulmonary vasoconstriction

It has been known for more than 60 years, and suspected for over 100, that alveolar hypoxia causes pulmonary vasoconstriction by means of mechanisms local to the lung. For the last 20 years, it has been clear that the essential sensor, transduction, and effector mechanisms responsible for hypoxic pulmonary vasoconstriction (HPV) reside in the pulmonary arterial smooth muscle cell. The main focus of this review is the cellular and molecular work performed to clarify these intrinsic mechanisms and to determine how they are facilitated and inhibited by the extrinsic influences of other cells. Because the interaction of intrinsic and extrinsic mechanisms is likely to shape expression of HPV in vivo, we relate results obtained in cells to HPV in more intact preparations, such as intact and isolated lungs and isolated pulmonary vessels. Finally, we evaluate evidence regarding the contribution of HPV to the physiological and pathophysiological processes involved in the transition from fetal to neonatal life, pulmonary gas exchange, high-altitude pulmonary edema, and pulmonary hypertension. Although understanding of HPV has advanced significantly, major areas of ignorance and uncertainty await resolution.

Large conductance Ca2+-activated K+ channels modulate endothelial cell outward currents and nitric oxide release in the intact rat superior mesenteric artery

Endothelial cells (EC) control vascular smooth muscle cell (VSMC) tone by release of paracrine factors. VSMC may also influence the EC layer, and therefore, the present study hypothesized that the opening of large-conductance Ca(2+) activated K(+) (BK(Ca)) channels may indirectly modulate EC hyperpolarization and nitric oxide (NO) release via myoendothelial gap junctions (MEGJ). To address this hypothesis 'in situ' EC ion current recordings, isolated VSMC patch clamp recordings, and simultaneous measurements of NO concentration and relaxation were conducted using segments of the rat superior mesenteric artery. In arteries constricted by α(1)-adrenoceptor activation, ACh (1 μM) evoked EC outward currents, vasorelaxation, and NO release. In contrast to preincubation with iberiotoxin (IbTx, 100nM) application of IbTx after ACh decreased EC outward currents, NO release and vasorelaxation. Furthermore, in phenylephrine (Phe)-contracted arteries treated with a gap junction uncoupler, cabenoxolone (CBX), IbTx failed to decrease ACh-evoked EC outward currents. In addition, CBX decreased EC outward currents, time constant of the capacitative transients, input capacitance, and increased input resistance. In isolated VSMC CBX did not affect BK(Ca) currents. Immunohistochemistry revealed only BK(Ca) channel positive staining in the VSMC layer. Therefore, the present results suggest that BK(Ca) channels are expressed in the VSMC, and that Phe by activation of VSMC BK(Ca) channels modulates ACh-evoked EC outward currents, NO release and vasorelaxation via MEGJ in rat superior mesenteric artery.

Calcium signaling in smooth muscle

Changes in intracellular Ca(2+) are central to the function of smooth muscle, which lines the walls of all hollow organs. These changes take a variety of forms, from sustained, cell-wide increases to temporally varying, localized changes. The nature of the Ca(2+) signal is a reflection of the source of Ca(2+) (extracellular or intracellular) and the molecular entity responsible for generating it. Depending on the specific channel involved and the detection technology employed, extracellular Ca(2+) entry may be detected optically as graded elevations in intracellular Ca(2+), junctional Ca(2+) transients, Ca(2+) flashes, or Ca(2+) sparklets, whereas release of Ca(2+) from intracellular stores may manifest as Ca(2+) sparks, Ca(2+) puffs, or Ca(2+) waves. These diverse Ca(2+) signals collectively regulate a variety of functions. Some functions, such as contractility, are unique to smooth muscle; others are common to other excitable cells (e.g., modulation of membrane potential) and nonexcitable cells (e.g., regulation of gene expression).

Calcium sparks in the intact gerbil spiral modiolar artery

Background

Calcium sparks are ryanodine receptor mediated transient calcium signals that have been shown to hyperpolarize the membrane potential by activating large conductance calcium activated potassium (BK) channels in vascular smooth muscle cells. Along with voltage-dependent calcium channels, they form a signaling unit that has a vasodilatory influence on vascular diameter and regulation of myogenic tone. The existence and role of calcium sparks has hitherto been unexplored in the spiral modiolar artery, the end artery that controls blood flow to the cochlea. The goal of the present study was to determine the presence and properties of calcium sparks in the intact gerbil spiral modiolar artery.

Results

Calcium sparks were recorded from smooth muscle cells of intact arteries loaded with fluo-4 AM. Calcium sparks occurred with a frequency of 2.6 Hz, a rise time of 17 ms and a time to half-decay of 20 ms. Ryanodine reduced spark frequency within 3 min from 2.6 to 0.6 Hz. Caffeine (1 mM) increased spark frequency from 2.3 to 3.3 Hz and prolonged rise and half-decay times from 17 to 19 ms and from 20 to 23 ms, respectively. Elevation of potassium (3.6 to 37.5 mM), presumably via depolarization, increased spark frequency from 2.4 to 3.2 Hz. Neither ryanodine nor depolarization changed rise or decay times.

Conclusions

This is the first characterization of calcium sparks in smooth muscle cells of the spiral modiolar artery. The results suggest that calcium sparks may regulate the diameter of the spiral modiolar artery and cochlear blood flow.

Differential regulation and recovery of intracellular Ca2+ in cerebral and small mesenteric arterial smooth muscle cells of simulated microgravity rat

Background

The differential adaptations of cerebrovasculature and small mesenteric arteries could be one of critical factors in postspaceflight orthostatic intolerance, but the cellular mechanisms remain unknown. We hypothesize that there is a differential regulation of intracellular Ca²⁺ determined by the alterations in the functions of plasma membrane Ca_L channels and ryanodine-sensitive Ca²⁺ releases from sarcoplasmic reticulum (SR) in cerebral and small mesenteric vascular smooth muscle cells (VSMCs) of simulated microgravity rats, respectively.

Methodology/Principal Findings

Sprague-Dawley rats were subjected to 28-day hindlimb unweighting to simulate microgravity. In addition, tail-suspended rats were submitted to a recovery period of 3 or 7 days after removal of suspension. The function of Ca_L channels was evaluated by patch clamp and Western blotting. The function of ryanodine-sensitive Ca²⁺ releases in response to caffeine were assessed by a laser confocal microscope. Our results indicated that simulated microgravity increased the functions of Ca_L channels and ryanodine-sensitive Ca²⁺ releases in cerebral VSMCs, whereas, simulated microgravity decreased the functions of Ca_L channels and ryanodine-sensitive Ca²⁺ releases in small mesenteric VSMCs. In addition, 3- or 7-day recovery after removal of suspension could restore the functions of Ca_L channels and ryanodine-sensitive Ca²⁺ releases to their control levels in cerebral and small mesenteric VSMCs, respectively.

Conclusions

The differential regulation of Ca_L channels and ryanodine-sensitive Ca²⁺ releases in cerebral and small mesenteric VSMCs may be responsible for the differential regulation of intracellular Ca²⁺, which leads to the altered autoregulation of cerebral vasculature and the inability to adequately elevate peripheral vascular resistance in postspaceflight orthostatic intolerance.

N-cadherin and integrin blockade inhibit arteriolar myogenic reactivity but not pressure-induced increases in intracellular Ca

The vascular myogenic response is characterized by arterial constriction in response to an increase in intraluminal pressure and dilatation to a decrease in pressure. This mechanism is important for the regulation of blood flow, capillary pressure, and arterial pressure. The identity of the mechanosensory mechanism(s) for this response is incompletely understood but has been shown to include the integrins as cell-extracellular matrix receptors. The possibility that a cell-cell adhesion receptor is involved has not been studied. Thus, we tested the hypothesis that N-cadherin, a cell-cell adhesion molecule in vascular smooth muscle cells (VSMCs), was important for myogenic responsiveness. The purpose of this study was to investigate: (1) whether cadherin inhibition blocks myogenic responses to increases in intraluminal pressure and (2) the effect of the cadherin or integrin blockade on pressure-induced changes in [Ca(2+)]i. Cadherin blockade was tested in isolated rat cremaster arterioles on myogenic responses to acute pressure steps from 60 to 100 mmHg and changes in VSMC Ca(2+) were measured using fura-2. In the presence of a synthetic cadherin inhibitory peptide or a function-blocking antibody, myogenic responses were inhibited. In contrast, during N-cadherin blockade, pressure-induced changes in [Ca(2+)]i were not altered. Similarly, vessels treated with function-blocking β1- or β3-integrin antibodies maintained pressure-induced [Ca(2+)]i responses despite inhibition of myogenic constriction. Collectively, these data suggest that both cadherins and integrins play a fundamental role in mediating myogenic constriction but argue against their direct involvement in mediating pressure-induced [Ca(2+)]i increases.

Activation of vascular BK channels by docosahexaenoic acid is dependent on cytochrome P450 epoxygenase activity

AIMS

n-3 Polyunsaturated fatty acids (PUFAs) are known to protect the cardiovascular system and improve blood pressure control. These important dietary constituents are converted into bioactive metabolites, but their role in regulation of the cardiovascular system is unclear. In particular, the functions of the cytochrome P450 (CYP) metabolites of n-3 PUFAs remain virtually unexplored. In this study, we examined the effects of docosahexaenoic acid (DHA) on the regulation of large-conductance calcium-activated potassium (BK) channel activities in coronary arterial smooth muscle cells.

METHODS AND RESULTS

Using whole-cell patch-clamp techniques, we found that DHA is a potent activator of BK currents in rat coronary arterial smooth muscle cells with an EC(50) of 0.23 ± 0.03 µM. This effect was abolished by pre-incubation with the CYP epoxygenase inhibitor, SKF525A (10 µM). The effects of DHA on the BK channels were reproduced by 16,17-epoxydocosapentaenoic acid (16,17-EpDPE) with an EC(50) of 19.7 ± 2.8 nM. The physiological role of the CYP metabolites of DHA was confirmed by measuring DHA-mediated vasodilatation in isolated rat coronary arteries. DHA dilated pressurized isolated coronary arteries in a dose-dependent manner, and the DHA effects were abolished after pre-treatment with SKF525A (10 µM) or with iberiotoxin (100 nM). In addition, 16,17-EpDPE directly produced coronary vasodilatation that was iberiotoxin sensitive.

CONCLUSION

These results suggest that DHA-mediated vasodilatation is mediated through CYP epoxygenase metabolites by activation of vascular BK channels.

Activation of vascular BK channels by docosahexaenoic acid is dependent on cytochrome P450 epoxygenase activity

AIMS

n-3 Polyunsaturated fatty acids (PUFAs) are known to protect the cardiovascular system and improve blood pressure control. These important dietary constituents are converted into bioactive metabolites, but their role in regulation of the cardiovascular system is unclear. In particular, the functions of the cytochrome P450 (CYP) metabolites of n-3 PUFAs remain virtually unexplored. In this study, we examined the effects of docosahexaenoic acid (DHA) on the regulation of large-conductance calcium-activated potassium (BK) channel activities in coronary arterial smooth muscle cells.

METHODS AND RESULTS

Using whole-cell patch-clamp techniques, we found that DHA is a potent activator of BK currents in rat coronary arterial smooth muscle cells with an EC(50) of 0.23 ± 0.03 µM. This effect was abolished by pre-incubation with the CYP epoxygenase inhibitor, SKF525A (10 µM). The effects of DHA on the BK channels were reproduced by 16,17-epoxydocosapentaenoic acid (16,17-EpDPE) with an EC(50) of 19.7 ± 2.8 nM. The physiological role of the CYP metabolites of DHA was confirmed by measuring DHA-mediated vasodilatation in isolated rat coronary arteries. DHA dilated pressurized isolated coronary arteries in a dose-dependent manner, and the DHA effects were abolished after pre-treatment with SKF525A (10 µM) or with iberiotoxin (100 nM). In addition, 16,17-EpDPE directly produced coronary vasodilatation that was iberiotoxin sensitive.

CONCLUSION

These results suggest that DHA-mediated vasodilatation is mediated through CYP epoxygenase metabolites by activation of vascular BK channels.

Antecedent hydrogen sulfide elicits an anti-inflammatory phenotype in postischemic murine small intestine: role of BK channels

The objectives of this study were to determine the role of calcium-activated, small (SK), intermediate (IK), and large (BK) conductance potassium channels in initiating the development of an anti-inflammatory phenotype elicited by preconditioning with an exogenous hydrogen sulfide (H(2)S) donor, sodium hydrosulfide (NaHS). Intravital microscopy was used to visualize rolling and firmly adherent leukocytes in vessels of the small intestine of mice preconditioned with NaHS (in the absence and presence of SK, IK, and BK channel inhibitors, apamin, TRAM-34, and paxilline, respectively) or SK/IK (NS-309) or BK channel activators (NS-1619) 24 h before ischemia-reperfusion (I/R). I/R induced marked increases in leukocyte rolling and adhesion, effects that were largely abolished by preconditioning with NaHS, NS-309, or NS-1619. The postischemic anti-inflammatory effects of NaHS-induced preconditioning were mitigated by BKB channel inhibitor treatment coincident with NaHS, but not by apamin or TRAM-34, 24 h before I/R. Confocal imaging and immunohistochemistry were used to demonstrate the presence of BKα subunit staining in both endothelial and vascular smooth muscle cells of isolated, pressurized mesenteric venules. Using patch-clamp techniques, we found that BK channels in cultured endothelial cells were activated after exposure to NaHS. Bath application of the same concentration of NaHS used in preconditioning protocols led to a rapid increase in a whole cell K(+) current; specifically, the component of K(+) current blocked by the selective BK channel antagonist iberiotoxin. The activation of BK current by NaHS could also be demonstrated in single channel recording mode where it was independent of a change in intracellular Ca(+) concentration. Our data are consistent with the concept that H(2)S induces the development of an anti-adhesive state in I/R in part mediated by a BK channel-dependent mechanism.

Intravascular pressure augments cerebral arterial constriction by inducing voltage-insensitive Ca2+ waves

This study examined whether elevated intravascular pressure stimulates asynchronous Ca(2+) waves in cerebral arterial smooth muscle cells and if their generation contributes to myogenic tone development. The endothelium was removed from rat cerebral arteries, which were then mounted in an arteriograph, pressurized (20-100 mmHg) and examined under a variety of experimental conditions. Diameter and membrane potential (V(M)) were monitored using conventional techniques; Ca(2+) wave generation and myosin light chain (MLC(20))/MYPT1 (myosin phosphatase targeting subunit) phosphorylation were assessed by confocal microscopy and Western blot analysis, respectively. Elevating intravascular pressure increased the proportion of smooth muscle cells firing asynchronous Ca(2+) waves as well as event frequency. Ca(2+) wave augmentation occurred primarily at lower intravascular pressures (<60 mmHg) and ryanodine, a plant alkaloid that depletes the sarcoplasmic reticulum (SR) of Ca(2+), eliminated these events. Ca(2+) wave generation was voltage insensitive as Ca(2+) channel blockade and perturbations in extracellular [K(+)] had little effect on measured parameters. Ryanodine-induced inhibition of Ca(2+) waves attenuated myogenic tone and MLC(20) phosphorylation without altering arterial V(M). Thapsigargin, an SR Ca(2+)-ATPase inhibitor also attenuated Ca(2+) waves, pressure-induced constriction and MLC(20) phosphorylation. The SR-driven component of the myogenic response was proportionally greater at lower intravascular pressures and subsequent MYPT1 phosphorylation measures revealed that SR Ca(2+) waves facilitated pressure-induced MLC(20) phosphorylation through mechanisms that include myosin light chain phosphatase inhibition. Cumulatively, our findings show that mechanical stimuli augment Ca(2+) wave generation in arterial smooth muscle and that these transient events facilitate tone development particularly at lower intravascular pressures by providing a proportion of the Ca(2+) required to directly control MLC(20) phosphorylation.

Subtype identification and functional characterization of ryanodine receptors in rat cerebral artery myocytes

Ryanodine receptors (RyRs) regulate contractility in resistance-size cerebral artery smooth muscle, yet their molecular identity, subcellular location, and phenotype in this tissue remain unknown. Following rat resistance-size cerebral artery myocyte sarcoplasmic reticulum (SR) purification and incorporation into POPE-POPS-POPC (5:3:2; wt/wt) bilayers, unitary conductances of 110 +/- 8, 334 +/- 15, and 441 +/- 27 pS in symmetric 300 mM Cs(+) were usually detected. The most frequent (34/40 bilayers) conductance (334 pS) decreased to Collapse

Key Words

• calcium-release channels
• vascular smooth muscle

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MESH Headings

• Animals
• Cells, Cultured
• Cerebral Arteries/chemistry
• Cerebral Arteries/cytology
• Cerebral Arteries/physiology
• Female
• Male
• Muscle Cells/chemistry
• Muscle Cells/physiology
• Protein Subunits/analysis
• Protein Subunits/physiology
• Rats
• Rats, Sprague-Dawley
• Ryanodine Receptor Calcium Release Channel/analysis
• Ryanodine Receptor Calcium Release Channel/physiology

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Grants

• AA11560 NIAAA NIH HHS
• R01 HL094378 NHLBI NIH HHS
• HL77424 NHLBI NIH HHS
• HL67061 NHLBI NIH HHS
• HL94378 NHLBI NIH HHS

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Affiliation(s)

Thirumalini Vaithianathan Department Pharmacology, University of Tennessee Health Science Center, Memphis, Tennessee 38163, USA
Damodaran Narayanan
Maria T Asuncion-Chin
Loice H Jeyakumar
Jianxi Liu
Sidney Fleischer
Jonathan H Jaggar
Alejandro M Dopico

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Dihydropyridine-insensitive calcium currents contribute to function of small cerebral arteries

Although dihydropyridines are widely used for the treatment of vasospasm, their effectiveness is questionable, suggesting that other voltage-dependent calcium channels (VDCCs) contribute to control of cerebrovascular tone. This study therefore investigated the role of dihydropyridine-insensitive VDCCs in cerebrovascular function. Using quantitative PCR and immunohistochemistry, we found mRNA and protein for L-type (Ca(V)1.2) and T-type (Ca(V)3.1 and Ca(V)3.2) channels in adult rat basilar and middle cerebral arteries and their branches. Immunoelectron microscopy revealed both L- and T-type channels in smooth muscle cell (SMC) membranes. Using patch clamp electrophysiology, we found that a high-voltage-activated calcium current, showing T-type channel kinetics and insensitivity to nifedipine and nimodipine, comprised approximately 20% of current in SMCs of the main arteries and approximately 45% of current in SMCs from branches. Both components were abolished by the T-type antagonists mibefradil, NNC 55-0396, and efonidipine. Although nifedipine completely blocked vasoconstriction in pressurized basilar arteries, a nifedipine-insensitive constriction was found in branches and this increased in magnitude as vessel size decreased. We conclude that a heterogeneous population of VDCCs contributes to cerebrovascular function, with dihydropyridine-insensitive channels having a larger role in smaller vessels. Sensitivity of these currents to nonselective T-type channel antagonists suggests that these drugs may provide a more effective treatment for therapy-refractory cerebrovascular constriction.

Ca2+ release from the sarcoplasmic reticulum is required for sustained TRPM4 activity in cerebral artery smooth muscle cells

The melastatin transient receptor potential (TRP) channel TRPM4 is a critical regulator of vascular smooth muscle cell membrane potential and contractility. Activation of the channel is Ca(2+)-dependent, but prolonged exposure to high (>1 microM) levels of intracellular Ca(2+) causes rapid (within approximately 2 min) desensitization of TRPM4 currents under conventional whole cell and inside-out patch-clamp conditions. The goal of the present study was to establish a novel method to record sustained TRPM4 currents in smooth muscle cells under near-physiological conditions. Using the amphotericin B-perforated patch-clamp technique, we recorded and characterized sustained (up to 30 min) transient inward cation currents (TICCs) in freshly isolated cerebral artery myocytes. In symmetrical cation solutions, TICCs reversed at 0 mV and had an apparent unitary conductance of 25 pS. Replacement of extracellular Na(+) with the nonpermeable cation N-methyl-d-glucamine abolished the current. TICC activity was attenuated by the TRPM4 blockers fluflenamic acid and 9-phenanthrol. Selective silencing of TRPM4 expression using small interfering RNA diminished TICC activity, suggesting that the molecular identity of the responsible ion channel is TRPM4. We used the perforated patch-clamp method to test the hypothesis that TRPM4 is activated by intracellular Ca(2+) signaling events. We found that TICC activity is independent of Ca(2+) influx and ryanodine receptor activity but is attenuated by sarco(endo)plasmic reticulum Ca(2+)-ATPase inhibition and blockade of inositol 1,4,5-trisphosphate receptor-mediated Ca(2+) release from the sarcoplasmic reticulum. Our findings suggest that TRPM4 channels in cerebral artery myocytes are regulated by Ca(2+) release from inositol 1,4,5-trisphosphate receptor on the sarcoplasmic reticulum.

Stimulation of the adenosine A3 receptor reverses vascular hyporeactivity after hemorrhagic shock in rats

AIM

To investigate whether adenosine A(3) receptors (A(3)AR) stimulation restore vascular reactivity after hemorrhagic shock through a ryanodine receptor (RyR)-mediated and large conductance calcium-activated potassium (BK(Ca)) channel-dependent pathway.

METHODS

Rat hemorrhagic shock model (40 mmHg) and vascular smooth muscle cell (VSMC) hypoxic model were used. The expression of A(3)AR was determined by Western blot and RT-PCR. The effect of A(3)AR stimulation on RyR-mediated Ca(2+) release in VSMCs was analyzed by the Fura-3/AM loading Ca(2+) imaging. The modulation of vascular reactivity to norepinephrine (NE) by A(3)AR stimulation was monitored by an isolated organ tension instrument.

RESULTS

Decrease of A(3)AR expression is consistent with the loss of vasoreactivity to NE in hemorrhagic shock rats. The stimulation of A(3)AR with a selective agonist, IB-MECA, could partly but significantly restore the vasoreactivity in the rats, and this restorative effect could be counteracted by MRS1523, a selective A(3)AR antagonist. In hypoxic VSMCs, RyR activation by caffeine significantly evoked the rise of [Ca(2+)] compared with the control cells, a phenomenon closely associated with the development of vascular hyporeactivity in hemorrhagic shock rats. The stimulation of A(3)AR with IB-MECA significantly blocked this over activation of RyR-mediated Ca(2+) release. RyR activation by caffeine and BK(Ca) channel activation by NS1619 attenuated the restoration of vasoreactivity to NE resulting from A(3)AR stimulation by IB-MECA after hemorrhagic shock; this attenuation effect could be antagonized by a selective BK(Ca) channel blocker.

CONCLUSION

These findings suggest that A(3)AR is involved in the modulation of vasoreactivity after hemorrhagic shock and that stimulation of A(3)AR can restore the decreased vasoreactivity to NE through a RyR-mediated, BK(Ca) channel-dependent signal pathway.

Sarcoplasmic reticulum function in smooth muscle

The sarcoplasmic reticulum (SR) of smooth muscles presents many intriguing facets and questions concerning its roles, especially as these change with development, disease, and modulation of physiological activity. The SR's function was originally perceived to be synthetic and then that of a Ca store for the contractile proteins, acting as a Ca amplification mechanism as it does in striated muscles. Gradually, as investigators have struggled to find a convincing role for Ca-induced Ca release in many smooth muscles, a role in controlling excitability has emerged. This is the Ca spark/spontaneous transient outward current coupling mechanism which reduces excitability and limits contraction. Release of SR Ca occurs in response to inositol 1,4,5-trisphosphate, Ca, and nicotinic acid adenine dinucleotide phosphate, and depletion of SR Ca can initiate Ca entry, the mechanism of which is being investigated but seems to involve Stim and Orai as found in nonexcitable cells. The contribution of the elemental Ca signals from the SR, sparks and puffs, to global Ca signals, i.e., Ca waves and oscillations, is becoming clearer but is far from established. The dynamics of SR Ca release and uptake mechanisms are reviewed along with the control of luminal Ca. We review the growing list of the SR's functions that still includes Ca storage, contraction, and relaxation but has been expanded to encompass Ca homeostasis, generating local and global Ca signals, and contributing to cellular microdomains and signaling in other organelles, including mitochondria, lysosomes, and the nucleus. For an integrated approach, a review of aspects of the SR in health and disease and during development and aging are also included. While the sheer versatility of smooth muscle makes it foolish to have a "one model fits all" approach to this subject, we have tried to synthesize conclusions wherever possible.

Minding the calcium store: Ryanodine receptor activation as a convergent mechanism of PCB toxicity

Chronic low-level polychlorinated biphenyl (PCB) exposures remain a significant public health concern since results from epidemiological studies indicate that PCB burden is associated with immune system dysfunction, cardiovascular disease, and impairment of the developing nervous system. Of these various adverse health effects, developmental neurotoxicity has emerged as a particularly vulnerable endpoint in PCB toxicity. Arguably the most pervasive biological effects of PCBs could be mediated by their ability to alter the spatial and temporal fidelity of Ca2+ signals through one or more receptor-mediated processes. This review will focus on our current knowledge of the structure and function of ryanodine receptors (RyRs) in muscle and nerve cells and how PCBs and related non-coplanar structures alter these functions. The molecular and cellular mechanisms by which non-coplanar PCBs and related structures alter local and global Ca2+ signaling properties and the possible short and long-term consequences of these perturbations on neurodevelopment and neurodegeneration are reviewed.

Developmental regulation of oxygen sensing and ion channels in the pulmonary vasculature

The increase in oxygen tension occurring at birth causes sustained and progressive pulmonary vasodilation. The oxygen-induced perinatal pulmonary vasodilation depends on the production of nitric oxide (NO) from the pulmonary endothelium and activation of various K(+) channels in pulmonary artery smooth muscle cells. This chapter reviews a) the oxygen-sensing mechanism that stimulates endothelial NO production; b) how K(+) channels sense changes in oxygen tension; c) whether hypoxia-inducible factor-1alpha (HIF-1alpha), a well defined hypoxia-sensitive transcription factor in adult, contributes to the regulation of NO production and K(+) channel activation; and d) whether and how dysfunctional K(+) channels contribute to the development of pulmonary hypertension in the newborns.

Mechanisms involved in the regulation of bovine pulmonary vascular tone by the 5-HT1B receptor

BACKGROUND AND PURPOSE

5-HT(1B) receptors may have a role in pulmonary hypertension. Their relationship with the activity of BK(Ca,) a T-type voltage-operated calcium channel (VOCC) and cyclic nucleotide-mediated relaxation was examined.

EXPERIMENTAL APPROACH

Ring segments of bovine pulmonary arteries were mounted in organ baths in modified Krebs-Henseleit buffer (37 degrees C) under a tension of 20 mN and gassed with 95% O(2)/5% CO(2). Isometric recordings were made using Chart 5 software.

KEY RESULTS

Contractile responses to 5-HT (10 nM-300 microM) were inhibited similarly by the 5-HT(1B) receptor antagonist SB216641 (100 nM) and the T-type VOCC blockers mibefradil (10 microM) and NNC550396 (10 microM) with no additive effect between SB216641 and mibefradil. Inhibition by SB216641 was prevented by the potassium channel blocker, charybdotoxin (100 nM). 5-HT(1B) receptor activation and charybdotoxin produced a mibefradil-sensitive potentiation of responses to U46619. Bradykinin (0.1 nM-30 microM), sodium nitroprusside (0.01 nM-3 microM), zaprinast (1 nM-3 microM), isoprenaline (0.1 nM-10 microM) and rolipram (1 nM-3 microM) produced 50% relaxation of arteries constricted with 5-HT (1-3 microM) or U46619 (30-50 nM) in the presence of 5-HT(1B) receptor activation, but full relaxation of arteries constricted with U46619, the 5-HT(2A) receptor agonist 2,5 dimethoxy-4 iodoamphetamine (1 microM) or 5-HT in the presence of 5-HT(1B) receptor antagonism. Enhanced relaxation of 5-HT-constricted arteries by cGMP-dependent pathways, seen in the presence of the 5-HT(1B) receptor antagonist, was reversed by charybdotoxin whereas cAMP-dependent relaxation was only partly reversed by charybdotoxin.

CONCLUSIONS AND IMPLICATIONS

5-HT(1B) receptors couple to inhibition of BK(Ca), thus increasing tissue sensitivity to contractile agonists by activating a T-type VOCC and impairing cGMP-mediated relaxation. Impaired cAMP-mediated relaxation was only partly mediated by inhibition of BK(Ca).

Cardamonin is a bifunctional vasodilator that inhibits Ca(v)1.2 current and stimulates K(Ca)1.1 current in rat tail artery myocytes

An in-depth analysis of the effects of cardamonin, 2',4'-dihydroxy-6'-methoxychalcone, on rat tail artery preparations was performed by means of whole-cell patch-clamp recordings of Ca(v)1.2 Ca(2+) [I(Ca(L))] or Ba(2+) [I(Ba(L))] current as well as K(Ca)1.1 currents in single myocytes and by measuring contractile responses in endothelium-denuded isolated rings. At a holding potential (V(h)) of -80 mV, cardamonin decreased both I(Ba(L)) and I(Ca(L)) in a concentration-dependent manner with similar pIC(50) values. The maximum of the I(Ba(L))-voltage relationship was shifted by 10 mV in the hyperpolarizing direction, but threshold remained unaffected. Cardamonin modified both the activation and the inactivation kinetics of I(Ba(L)) and shifted the voltage dependence of both inactivation and activation curves to more negative potentials by 19 and 7 mV, respectively, thus markedly decreasing the Ba(2+) window current. Block of I(Ba(L)) was frequency-dependent, and rate of recovery from inactivation was slowed. Cardamonin increased K(Ca)1.1 currents in a concentration-dependent manner; this stimulation was iberiotoxin- and BAPTA [1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid]-sensitive. On the contrary, iberiotoxin did not modify cardamonin-induced relaxation of rings precontracted either with phenylephrine or with (S)-(-)-methyl-1,4-dihydro-2,6-dimethyl-3-nitro-4-(2-trifluoromethylphenyl)pyridine-5-carboxylate [(S)-(-)-Bay K 8644]. The overall effects of cardamonin were incompletely reversed by washout. In conclusion, cardamonin is a naturally occurring, bifunctional vasodilator that, by simultaneously inhibiting I(Ca(L)) and stimulating K(Ca)1.1 current, may represent a scaffold for the design of novel drugs of potential interest for treatment of systemic hypertension.

Doxorubicin-induced vasomotion and [Ca(2+)](i) elevation in vascular smooth muscle cells from C57BL/6 mice

AIM

To explore the action of doxorubicin on vascular smooth muscle cells.

METHODS

Isometric tension of denuded or intact thoracic aortic vessels was recorded and [Ca(2+)](i) in isolated aortic smooth muscle cells was measured by using Fluo-3.

RESULTS

Doxorubicin induced phasic and tonic contractions in denuded vessels and increased levels of [Ca(2+)](i) in single muscle cells. Treatment with 10 micromol/L ryanodine had no effect on basal tension, but it did abolish doxorubicin-induced phasic contraction. Treatment with 10 mmol/L caffeine induced a transient phasic contraction only, and the effect was not significantly altered by ryanodine, the omission of extracellular Ca(2+) or both. Phenylephrine induced rhythmic contraction (RC) in intact vessels. Treatment with 100 micromol/L doxorubicin enhanced RC amplitude, but 1 mmol/L doxorubicin abolished RC, with an increase in maximal tension. Caffeine at 100 micromol/L increased the frequency of the RC only. In the presence of 100 micromol/L caffeine, however, 100 micromol/L doxorubicin abolished the RC and decreased its maximal tension. Treatment with 10 micromol/L ryanodine abolished the RC, with an increase in the maximal tension. In Ca(2+)-free solution, doxorubicin induced a transient [Ca(2+)](i) increase that could be abolished by ryanodine pretreatment in single muscle cells. The doxorubicin-induced increase in [Ca(2+)](i) was suppressed by nifedipine and potentiated by ryanodine and charybdotoxin.

CONCLUSION

Doxorubicin not only releases Ca(2+) from the sarcoplasmic reticulum but also promotes the entry of extracellular Ca(2+) into vascular smooth muscle cells.

Therapeutic potential of pharmacologically targeting arteriolar myogenic tone

The arteriolar myogenic response, which is defined as vasoconstriction to increases in intraluminal pressure and, conversely, dilation to a reduction in pressure, is key in the setting of vascular resistance, local control of microvascular blood flow through autoregulation, and in the control of capillary hydrostatic pressure. Although considerable progress has been made in the quest for understanding the underlying sensory apparatus and cellular mechanisms, fundamental questions remain - particularly if this pathway is to be considered as a target for novel strategies of pharmacological intervention. We propose that an ability to 're-set' myogenic tone would enable modification of systemic vascular resistance and pressure while at the same time preserving existing interactions with neurohumoral regulatory mechanisms. The challenge, therefore, is to identify steps unique to the myogenic signaling pathway to enable specific pharmacological targeting.

Heterogeneity in function of small artery smooth muscle BKCa: involvement of the beta1-subunit

Arteriolar myogenic vasoconstriction occurs when increased stretch or membrane tension leads to smooth muscle cell depolarization and opening of voltage-gated Ca2+ channels. To prevent positive feedback and excessive pressure-induced vasoconstriction, studies in cerebral artery smooth muscle have suggested that activation of large conductance, Ca2+-activated K+ channels (BKCa) provides an opposing hyperpolarizing influence reducing Ca2+ channel activity. We have hypothesized that this mechanism may not equally apply to all vascular beds. To establish the existence of such heterogeneity in vascular reactivity, studies were performed on rat vascular smooth muscle (VSM) cells from cremaster muscle arterioles and cerebral arteries. Whole cell K+ currents were determined at pipette [Ca2+] of 100 nM or 5 microM in the presence and absence of the BKCa inhibitor, iberiotoxin (IBTX; 0.1 microM). Similar outward current densities were observed for the two cell preparations at the lower pipette Ca2+ levels. At 5 microM Ca2+, cremaster VSM showed a significantly (P < 0.05) lower current density compared to cerebral VSM (34.5 +/- 1.9 vs 45.5 +/- 1.7 pA pF(-1) at +70 mV). Studies with IBTX suggested that the differences in K+ conductance at 5 microM intracellular [Ca2+] were largely due to activity of BKCa. 17beta-Oestradiol (1 microM), reported to potentiate BKCa current via the channel's beta-subunit, caused a greater effect on whole cell K+ currents in cerebral vessel smooth muscle cells (SMCs) compared to those of cremaster muscle. In contrast, the alpha-subunit-selective BKCa opener, NS-1619 (20 microM), exerted a similar effect in both preparations. Spontaneously transient outward currents (STOCs) were more apparent (frequency and amplitude) and occurred at more negative membrane potentials in cerebral compared to cremaster SMCs. Also consistent with decreased STOC activity in cremaster SMCs was an absence of detectable Ca2+ sparks (0 of 76 cells) compared to that in cerebral SMCs (76 of 105 cells). Quantitative PCR showed decreased mRNA expression for the beta1 subunit and a decrease in the beta1:alpha ratio in cremaster arterioles compared to cerebral vessels. Similarly, cremaster arterioles showed a decrease in total BKCa protein and the beta1:alpha-subunit ratio. The data support vascular heterogeneity with respect to the activity of BKCa in terms of both beta-subunit regulation and interaction with SR-mediated Ca2+ signalling.

Vascular control in humans: focus on the coronary microcirculation

Myocardial perfusion is regulated by a variety of factors that influence arteriolar vasomotor tone. An understanding of the physiological and pathophysiological factors that modulate coronary blood flow provides the basis for the judicious use of medications for the treatment of patients with coronary artery disease. Vasomotor properties of the coronary circulation vary among species. This review highlights the results of recent studies that examine the mechanisms by which the human coronary microcirculation is regulated in normal and disease states, focusing on diabetes. Multiple pathways responsible for myogenic constriction and flow-mediated dilation in human coronary arterioles are addressed. The important role of endothelium-derived hyperpolarizing factors, their interactions in mediating dilation, as well as speculation regarding the clinical significance are emphasized. Unique properties of coronary arterioles in human vs. other species are discussed.

Structural determinants of monohydroxylated bile acids to activate beta 1 subunit-containing BK channels

Lithocholate (LC) (10-300 microM) in physiological solution is sensed by vascular myocyte large conductance, calcium- and voltage-gated potassium (BK) channel beta(1) accessory subunits, leading to channel activation and arterial dilation. However, the structural features in steroid and target that determine LC action are unknown. We tested LC and close analogs on BK channel (pore-forming cbv1+beta(1) subunits) activity using the product of the number of functional ion channels in the membrane patch (N) and the open channel probability (Po). LC (5beta-cholanic acid-3alpha-ol), 5alpha-cholanic acid-3alpha-ol, and 5beta-cholanic acid-3beta-ol increased NPo (EC(50) approximately 45 microM). At maximal increase in NPo, LC increased NPo by 180%, whereas 5alpha-cholanic acid-3alpha-ol and 5beta-cholanic acid-3beta-ol raised NPo by 40%. Thus, the alpha-hydroxyl and the cis A-B ring junction are both required for robust channel potentiation. Lacking both features, 5alpha-cholanic acid-3beta-ol and 5-cholenic acid-3beta-ol were inactive. Three-dimensional structures show that only LC displays a bean shape with clear-cut convex and concave hemispheres; 5alpha-cholanic acid-3alpha-ol and 5beta-cholanic acid-3beta-ol partially matched LC shape, and 5alpha-cholanic acid-3beta-ol and 5-cholenic acid-3beta-ol did not. Increasing polarity in steroid rings (5beta-cholanic acid-3alpha-sulfate) or reducing polarity in lateral chain (5beta-cholanic acid 3alpha-ol methyl ester) rendered poorly active compounds, consistent with steroid insertion between beta(1) and bilayer lipids, with the steroid-charged tail near the aqueous phase. Molecular dynamics identified two regions in beta(1) transmembrane domain 2 that meet unique requirements for bonding with the LC concave hemisphere, where the steroid functional groups are located.

Direct regulation of BK channels by phosphatidylinositol 4,5-bisphosphate as a novel signaling pathway

Large conductance, calcium- and voltage-gated potassium (BK) channels are ubiquitous and critical for neuronal function, immunity, and smooth muscle contractility. BK channels are thought to be regulated by phosphatidylinositol 4,5-bisphosphate (PIP(2)) only through phospholipase C (PLC)-generated PIP(2) metabolites that target Ca(2+) stores and protein kinase C and, eventually, the BK channel. Here, we report that PIP(2) activates BK channels independently of PIP(2) metabolites. PIP(2) enhances Ca(2+)-driven gating and alters both open and closed channel distributions without affecting voltage gating and unitary conductance. Recovery from activation was strongly dependent on PIP(2) acyl chain length, with channels exposed to water-soluble diC4 and diC8 showing much faster recovery than those exposed to PIP(2) (diC16). The PIP(2)-channel interaction requires negative charge and the inositol moiety in the phospholipid headgroup, and the sequence RKK in the S6-S7 cytosolic linker of the BK channel-forming (cbv1) subunit. PIP(2)-induced activation is drastically potentiated by accessory beta(1) (but not beta(4)) channel subunits. Moreover, PIP(2) robustly activates BK channels in vascular myocytes, where beta(1) subunits are abundantly expressed, but not in skeletal myocytes, where these subunits are barely detectable. These data demonstrate that the final PIP(2) effect is determined by channel accessory subunits, and such mechanism is subunit specific. In HEK293 cells, cotransfection of cbv1+beta(1) and PI4-kinaseIIalpha robustly activates BK channels, suggesting a role for endogenous PIP(2) in modulating channel activity. Indeed, in membrane patches excised from vascular myocytes, BK channel activity runs down and Mg-ATP recovers it, this recovery being abolished by PIP(2) antibodies applied to the cytosolic membrane surface. Moreover, in intact arterial myocytes under physiological conditions, PLC inhibition on top of blockade of downstream signaling leads to drastic BK channel activation. Finally, pharmacological treatment that raises PIP(2) levels and activates BK channels dilates de-endothelized arteries that regulate cerebral blood flow. These data indicate that endogenous PIP(2) directly activates vascular myocyte BK channels to control vascular tone.

Nitric oxide mediates stretch-induced Ca2+ release via activation of phosphatidylinositol 3-kinase-Akt pathway in smooth muscle

BACKGROUND

Hollow smooth muscle organs such as the bladder undergo significant changes in wall tension associated with filling and distension, with attendant changes in muscle tone. Our previous study indicated that stretch induces Ca(2+) release occurs in the form of Ca(2+) sparks and Ca(2+) waves in urinary bladder myocytes. While, the mechanism underlying stretch-induced Ca2+ release in smooth muscle is unknown.

METHODOLOGY/PRINCIPAL FINDINGS

We examined the transduction mechanism linking cell stretch to Ca(2+) release. The probability and frequency of Ca(2+) sparks induced by stretch were closely related to the extent of cell extension and the time that the stretch was maintained. Experiments in tissues and single myocytes indicated that mechanical stretch significantly increases the production of nitric oxide (NO) and the amplitude and duration of muscle contraction. Stretch-induced Ca(2+) sparks and contractility increases were abrogated by the NO inhibitor L-NAME and were also absent in eNOS knockout mice. Furthermore, exposure of eNOS null mice to exogenously generated NO induced Ca(2+) sparks. The soluble guanylyl cyclase inhibitor ODQ did not inhibit SICR, but this process was effectively blocked by the PI3 kinase inhibitors LY494002 and wortmannin; the phosphorylation of Akt and eNOS were up-regulated by 204+/-28.6% and 258+/-36.8% by stretch, respectively. Moreover, stretch significantly increased the eNOS protein expression level.

CONCLUSIONS/SIGNIFICANCE

Taking together, these results suggest that stretch-induced Ca2+ release is NO dependent, resulting from the activation of PI3K/Akt pathway in smooth muscle.

Role of endothelium-derived hyperpolarizing factor in the regulation of radial artery basal diameter and endothelium-dependent dilatation in vivo

1. The role of the balance between nitric oxide (NO) and endothelium-derived hyperpolarizing factor (EDHF), synthesized by cytochrome epoxygenase and acting through calcium-activated potassium channels, in the regulation of basal diameter and endothelium-dependent flow-mediated dilatation of conduit arteries has been poorly assessed in humans. 2. Radial artery diameter and flow (echotracking coupled to Doppler) were measured in healthy volunteers under basal conditions and during flow-mediated dilatation induced by hand skin heating, in the presence of saline and inhibitors of NO-synthase, N(G)-monomethyl-L-arginine (L-NMMA), calcium-activated potassium channels, tetraethylammonium (TEA) and cytochrome epoxygenases, fluconazole, infused alone and in combination. Mean wall shear stress, the flow-mediated dilatation stimulus, was calculated and taken as cofactor into statistical analysis. 3. Under basal conditions, the radial artery diameter was not affected by L-NMMA and fluconazole infused alone but was decreased by TEA, the combinations of L-NMMA + fluconazole and, to a greater extent, L-NMMA + TEA. During heating, radial artery diameter increased with temperature in all cases. This increase in diameter, compared with saline, was reduced by L-NMMA, TEA, fluconazole and to a greater extent, by L-NMMA + TEA and L-NMMA + fluconazole. 4. These data show that EDHF is involved in balance with NO in the regulation of basal diameter and endothelium-dependent dilatation of human peripheral conduit arteries. The alteration of this balance could play a major role in the physiopathology of the endothelial dysfunction, in particular during essential hypertension.

Design and synthesis of hydroxy-alkynoic acids and their methyl esters as novel activators of BK channels

Physiological and pharmacological agents that activate large conductance, voltage-, and calcium-gated potassium (BK) channels located in the smooth muscle are effective vasodilators. Thus, activators of smooth muscle BK channels may be potential therapeutic tools to treat cardiovascular disease associated with vasoconstriction and/or impaired dilation, such as cerebrovascular spasm and constriction. We previously showed that lithocholic acid (LC) and other cholane derivatives activated smooth muscle BK channels and, thus, caused endothelium-independent cerebral artery dilation. However, clinical use of these cholane derivatives could be limited by the actions of these steroids, such as elevation of intracellular calcium and induction of apoptosis. Using LC as template, we designed and synthesized a series of hydroxy-alkynoic acids and corresponding methyl esters, as putative, non-steroid BK channel activators. Indeed, the newly synthesized compounds effectively and reversibly activated rat cerebrovascular myocyte BK channel at concentrations similar to those found effective with LC. Among all the novel compounds tested, C-10 hydroxy-alkynoic acid methyl ester appears to be the most effective activator of vascular myocyte BK channels.

The non-excitable smooth muscle: calcium signaling and phenotypic switching during vascular disease

Calcium (Ca(2+)) is a highly versatile second messenger that controls vascular smooth muscle cell (VSMC) contraction, proliferation, and migration. By means of Ca(2+) permeable channels, Ca(2+) pumps and channels conducting other ions such as potassium and chloride, VSMC keep intracellular Ca(2+) levels under tight control. In healthy quiescent contractile VSMC, two important components of the Ca(2+) signaling pathways that regulate VSMC contraction are the plasma membrane voltage-operated Ca(2+) channel of the high voltage-activated type (L-type) and the sarcoplasmic reticulum Ca(2+) release channel, Ryanodine Receptor (RyR). Injury to the vessel wall is accompanied by VSMC phenotype switch from a contractile quiescent to a proliferative motile phenotype (synthetic phenotype) and by alteration of many components of VSMC Ca(2+) signaling pathways. Specifically, this switch that culminates in a VSMC phenotype reminiscent of a non-excitable cell is characterized by loss of L-type channels expression and increased expression of the low voltage-activated (T-type) Ca(2+) channels and the canonical transient receptor potential (TRPC) channels. The expression levels of intracellular Ca(2+) release channels, pumps and Ca(2+)-activated proteins are also altered: the proliferative VSMC lose the RyR3 and the sarcoplasmic/endoplasmic reticulum Ca(2+) ATPase isoform 2a pump and reciprocally regulate isoforms of the ca(2+)/calmodulin-dependent protein kinase II. This review focuses on the changes in expression of Ca(2+) signaling proteins associated with VSMC proliferation both in vitro and in vivo. The physiological implications of the altered expression of these Ca(2+) signaling molecules, their contribution to VSMC dysfunction during vascular disease and their potential as targets for drug therapy will be discussed.

The RCK1 high-affinity Ca2+ sensor confers carbon monoxide sensitivity to Slo1 BK channels

Carbon monoxide (CO) is a lethal gas, but it is also increasingly recognized as a physiological signaling molecule capable of regulating a variety of proteins. Among them, large-conductance Ca(2+)- and voltage-gated K(+) (Slo1 BK) channels, important in vasodilation and neuronal firing, have been suggested to be directly stimulated by CO. However, the molecular mechanism of the stimulatory action of CO on the Slo1 BK channel has not been clearly elucidated. We report here that CO reliably and repeatedly activates Slo1 BK channels in excised membrane patches in the absence of Ca(2+) in a voltage-sensor-independent manner. The stimulatory action of CO on the Slo1 BK channel requires an aspartic acid and two histidine residues located in the cytoplasmic RCK1 domain, and the effect persists under the conditions known to inhibit the conventional interaction between CO and heme in other proteins. We propose that CO acts as a partial agonist for the high-affinity divalent cation sensor in the RCK1 domain of the Slo1 BK channel.

BK channels are linked to inositol 1,4,5-triphosphate receptors via lipid rafts: a novel mechanism for coupling [Ca(2+)](i) to ion channel activation

Glioma cells prominently express a unique splice variant of a large conductance, calcium-activated potassium channel (BK channel). These channels transduce changes in intracellular calcium to changes of K(+) conductance in the cells and have been implicated in growth control of normal and malignant cells. The Ca(2+) increase that facilitates channel activation is thought to occur via activation of intracellular calcium release pathways or influx of calcium through Ca(2+)-permeable ion channels. We show here that BK channel activation involves the activation of inositol 1,4,5-triphosphate receptors (IP(3)R), which localize near BK channels in specialized membrane domains called lipid rafts. Disruption of lipid rafts with methyl-beta-cyclodextrin disrupts the functional association of BK channel and calcium source resulting in a >50% reduction in K(+) conductance mediated by BK channels. The reduction of BK current by lipid raft disruption was overcome by the global elevation of intracellular calcium through inclusion of 750 nm Ca(2+) in the pipette solution, indicating that neither the calcium sensitivity of the channel nor their overall number was altered. Additionally, pretreatment of glioma cells with 2-aminoethoxydiphenyl borate to inhibit IP(3)Rs negated the effect of methyl-beta-cyclodextrin, providing further support that IP(3)Rs are the calcium source for BK channels. Taken together, these data suggest a privileged association of BK channels in lipid raft domains and provide evidence for a novel coupling of these Ca(2+)-sensitive channels to their second messenger source.

Integrin-mediated mechanotransduction in renal vascular smooth muscle cells: activation of calcium sparks

Integrins are transmembrane heterodimeric proteins that link extracellular matrix (ECM) to cytoskeleton and have been shown to function as mechanotransducers in nonmuscle cells. Synthetic integrin-binding peptide triggers Ca2+ mobilization and contraction in vascular smooth muscle cells (VSMCs) of rat afferent arteriole, indicating that interactions between the ECM and integrins modulate vascular tone. To examine whether integrins transduce extracellular mechanical stress into intracellular Ca2+ signaling events in VSMCs, unidirectional mechanical force was applied to freshly isolated renal VSMCs through paramagnetic beads coated with fibronectin (natural ligand of α5β1-integrin in VSMCs). Pulling of fibronectin-coated beads with an electromagnet triggered Ca2+ sparks, followed by global Ca2+ mobilization. Paramagnetic beads coated with low-density lipoprotein, whose receptors are not linked to cytoskeleton, were minimally effective in triggering Ca2+ sparks and global Ca2+ mobilization. Preincubation with ryanodine, cytochalasin-D, or colchicine substantially reduced the occurrence of Ca2+ sparks triggered by fibronectin-coated beads. Binding of VSMCs with antibodies specific to the extracellular domains of α5- and β1-integrins triggered Ca2+ sparks simulating the effects of fibronectin-coated beads. Preincubation of microperfused afferent arterioles with ryanodine or integrin-specific binding peptide inhibited pressure-induced myogenic constriction. In conclusion, integrins transduce mechanical force into intracellular Ca2+ signaling events in renal VSMCs. Integrin-mediated mechanotransduction is probably involved in myogenic response of afferent arterioles.

Inhibition of ryanodine receptors by 4-(2-aminopropyl)-3,5-dichloro-N,N-dimethylaniline (FLA 365) in canine pulmonary arterial smooth muscle cells

Ryanodine is a selective ryanodine receptor (RyR) blocker, with binding dependent on RyR opening. In whole-cell studies, ryanodine binding can lock the RyR in an open-conductance state, short-circuiting the sarcoplasmic reticulum, which restricts studies of inositol-1,4,5-trisphosphate receptor (InsP3R) activity. Other RyR blockers have nonselective effects that also limit their utility. 4-(2-aminopropyl)-3,5-dichloro-N,N-dimethylaniline (FLA 365) blocks RyR-elicited Ca2+ increases in skeletal and cardiac muscle; yet, its actions on smooth muscle are unknown. Canine pulmonary arterial smooth muscle cells (PASMCs) express both RyRs and InsP3Rs; thus, we tested the ability of FLA 365 to block RyR- and serotonin-mediated InsP3R-elicited Ca2+ release by imaging fura-2-loaded PASMCs. Acute exposure to 10 mM caffeine, a selective RyR activator, induced Ca2+ increases that were reversibly reduced by FLA 365, with an estimated IC50 of approximately 1 to 1.5 microM, and inhibited by 10 microM ryanodine or 10 microM cyclopiazonic acid. FLA 365 also blocked L-type Ca2+ channel activity, with 10 microM reducing Ba2+ current amplitude in patch voltage-clamp studies to 54 +/- 6% of control and 100 microM FLA 365 reducing membrane current to 21 +/- 6%. InsP3R-mediated Ca2+ responses elicited by 10 microM 5-hydroxytryptamine (serotonin) in canine PASMCs and 100 microM carbachol in human embryonic kidney (HEK)-293 cells were not reduced by 2 microM FLA 365, but they were reduced by 20 microM FLA 365 to 76 +/- 9% of control in canine PASMCs and 52 +/- 1% in HEK-293 cells. Thus, FLA 365 preferentially blocks RyRs with limited inhibition of L-type Ca2+ channels or InsP3R in canine PASMCs.

New technologies for dissecting the arteriolar myogenic response

The arteriolar myogenic response is crucial for the setting of vascular resistance and for providing a level of tone upon which vasodilators can act. Despite its physiological importance, questions remain regarding the underlying signaling mechanisms of the arteriolar myogenic response. Does an increase in pressure within an arteriole exert its effects via the extracellular matrix, an action on cell membranes and/or deformation of cytoskeletal structures? Recent advances in methodology, particularly involving sophisticated imaging approaches, are enabling the study of forces at single-cell and even subcellular levels. Atomic force microscopy (AFM) not only enables detection of cell morphology and stiffness but also allows discrete forces to be applied to single smooth muscle cells and subsequent responses to be observed. Importantly, the repertoire of approaches involving AFM can be expanded by using it in combination with other imaging approaches - including fluorescence imaging for cellular signals such as Ca(2+), and total internal reflectance fluorescence, fluorescence resonance energy transfer and confocal microscopy for probing cellular contact function. Combinations of these advanced imaging and nanomechanical approaches will be instructive to studies of intact vessels and the circulatory system in general.

KCa3.1 Ca2+ activated K+ channels regulate human airway smooth muscle proliferation

Airway smooth muscle cell hyperplasia contributes to airway remodeling and hyperreactivity characteristic of asthma. Changes to potassium channel activity in proliferating human airway smooth muscle (HASM) cells have been described, but no regulatory role in proliferation has been attributed to them. We sought to investigate the expression of the intermediate conductance calcium-activated potassium channel K(Ca)3.1 in HASM cells and investigate its role in proliferation. Smooth muscle cells derived from human airways were grown in vitro and K(Ca)3.1 channel expression was measured using Western blot, RT-PCR, and patch clamp electrophysiology. Pharmacologic inhibitors of the channel were used in assays of cellular proliferation, and flow cytometry was used to identify cell cycle regulation. HASM cells expressed K(Ca)3.1 channel mRNA, protein, and activity with up-regulation evident after transforming growth factor-beta stimulation. Pharmacologic inhibition of K(Ca)3.1 led to growth arrest in cells stimulated to proliferate with mitogens. These inhibitors did not cause cellular toxicity or induce apoptosis. We have demonstrated, for the first time, the expression of K(Ca)3.1 channels in HASM cells. In addition, we have shown that K(Ca)3.1 channels are important in HASM cell proliferation, making these channels a potential therapeutic target in airway remodeling.

Postnatal maturation attenuates pressure-evoked myogenic tone and stretch-induced increases in Ca2+ in rat cerebral arteries

Although postnatal maturation potently modulates agonist-induced cerebrovascular contractility, its effects on the mechanisms mediating cerebrovascular myogenic tone remain poorly understood. Because the regulation of calcium influx and myofilament calcium sensitivity change markedly during early postnatal life, the present study tested the general hypothesis that early postnatal maturation increases the pressure sensitivity of cerebrovascular myogenic tone via age-dependent enhancement of pressure-induced calcium mobilization and myofilament calcium sensitivity. Pressure-induced myogenic tone and changes in artery wall intracellular calcium concentrations ([Ca(2+)](i)) were measured simultaneously in endothelium-denuded, fura-2-loaded middle cerebral arteries (MCA) from pup [postnatal day 14 (P14)] and adult (6-mo-old) Sprague-Dawley rats. Increases in pressure from 20 to 80 mmHg enhanced myogenic tone in MCA from both pups and adults although the normalized magnitudes of these increases were significantly greater in pup than adult MCA. At each pressure step, vascular wall [Ca(2+)](i) was also significantly greater in pup than in adult MCA. Nifedipine significantly attenuated pressure-evoked constrictions in pup MCA and essentially eliminated all responses to pressure in the adult MCA. Both pup and adult MCA exhibited pressure-dependent increases in calcium sensitivity, as estimated by changes in the ratio of pressure-induced myogenic tone to wall [Ca(2+)](i). However, there were no differences in the magnitudes of these increases between pup and adult MCA. The results support the view that regardless of postnatal age, changes in both calcium influx and myofilament calcium sensitivity contribute to the regulation of cerebral artery myogenic tone. The greater cerebral myogenic response in P14 compared with adult MCA appears to be due to greater pressure-induced increases in [Ca(2+)](i), rather than enhanced augmentation of myofilament calcium sensitivity.

Cellular and molecular mechanisms regulating vascular tone. Part 1: basic mechanisms controlling cytosolic Ca2+ concentration and the Ca2+-dependent regulation of vascular tone

General anesthetics cause hemodynamic instability and alter blood flow to various organs. There is mounting evidence that most general anesthetics, at clinical concentrations, influence a wide variety of cellular and molecular mechanisms regulating the contractile state of vascular smooth muscle cells (i.e., vascular tone). In addition, in current anesthetic practice, various types of vasoactive agents are often used to control vascular reactivity and to sustain tissue blood flow in high-risk surgical patients with impaired vital organ function and/or hemodynamic instability. Understanding the physiological mechanisms involved in the regulation of vascular tone thus would be beneficial for anesthesiologists. This review, in two parts, provides an overview of current knowledge about the cellular and molecular mechanisms regulating vascular tone-i.e., targets for general anesthetics, as well as for vasoactive drugs that are used in intraoperative circulatory management. This first part of the two-part review focuses on basic mechanisms regulating cytosolic Ca2+ concentration and the Ca2+-dependent regulation of vascular tone.

Activation of a cGMP-sensitive calcium-dependent chloride channel may cause transition from calcium waves to whole cell oscillations in smooth muscle cells

In vitro, alpha-adrenoreceptor stimulation of rat mesenteric small arteries often leads to a rhythmic change in wall tension, i.e., vasomotion. Within the individual smooth muscle cells of the vascular wall, vasomotion is often preceded by a period of asynchronous calcium waves. Abruptly, these low-frequency waves may transform into high-frequency whole cell calcium oscillations. Simultaneously, multiple cells synchronize, leading to rhythmic generation of tension. We present a mathematical model of vascular smooth muscle cells that aims at characterizing this sudden transition. Simulations show calcium waves sweeping through the cytoplasm when the sarcoplasmic reticulum (SR) is stimulated to release calcium. A rise in cGMP leads to the experimentally observed transition from waves to whole cell calcium oscillations. At the same time, membrane potential starts to oscillate and the frequency approximately doubles. In this transition, the simulated results point to a key role for a recently discovered cGMP-sensitive calcium-dependent chloride channel. This channel depolarizes the membrane in response to calcium released from the SR. In turn, depolarization causes a uniform opening of L-type calcium channels on the cell surface, stimulating a synchronized release of SR calcium and inducing the shift from waves to whole cell oscillations. The effect of the channel is therefore to couple the processes of the SR with those of the membrane. We hypothesize that the shift in oscillatory mode and the associated onset of oscillations in membrane potential within the individual cell may underlie sudden intercellular synchronization and the appearance of vasomotion.

Ion channels and calcium signaling in cerebral arteries following subarachnoid hemorrhage

Entry of Ca(2+) through voltage-dependent calcium channels (VDCCs) is critical to the regulation of intracellular free calcium concentration ([Ca(2+)](i)) in vascular smooth muscle and thus the control of cerebral artery diameter. Increased VDCC activity in cerebral artery myocytes may contribute to decreased cerebral blood flow and the accompanying neurological deficits associated with subarachnoid hemorrhage (SAH). This review will focus on the impact of SAH on VDCCs and K(+)-selective ion channels, two important classes of ion channels located in the plasma membrane of cerebral artery myocytes. SAH may act through a variety of direct and indirect mechanisms to increase the activity of VDCCs promoting cerebral artery constriction and reduced cerebral blood flow. Further, SAH may lead to suppression of K(+) channel activity to cause membrane potential depolarization to enhance VDCC activity. The ability of VDCC blockers or K(+) channel activators to alleviate SAH-induced vasospasm will also be examined.

Analysis of calcium channels by conditional mutagenesis

Ca2+ influx through various ion channels is an important determinant of the cytosolic Ca2+ concentration, which plays a pivotal role in countless cellular processes. The cardiac L-type Ca2+ channel, Ca(v)1.2, represents a major pathway for Ca2+ entry and is in many cells expressed together with other high- and low-voltage-activated Ca2+ channels. This article will focus on the use of conditional transgenic mouse models to clarify the roles of Ca2+ channels in several biological systems. The phenotypes of conditional Ca2+ channel transgenic mice have provided novel, and often unexpected, insights into the in vivo function of L-type and T-type Ca2+ channels as mediators of signaling between cell membrane and intracellular processes in blood pressure regulation, smooth muscle contractility, insulin secretion, cardiac function, sleep, learning, and memory.

Intracellular cAMP: the "switch" that triggers on "spontaneous transient outward currents" generation in freshly isolated myocytes from thoracic aorta

Spontaneous transient outward currents (STOCs) have been reported in resistance and small arteries but have not yet been found in thoracic aorta. Do thoracic aorta myocytes possess cellular machinery that generates STOCs? It was found that the majority of aortic myocytes do not generate STOCs. STOCs were generated in 8.7% of freshly isolated aortic myocytes. Myocytes that did not generate STOCs we have called "silent" myocytes and myocytes with STOCs have been called "active." STOCs recorded in active myocytes were voltage dependent and were inhibited by ryanodine, caffeine, and charybdotoxin. Forskolin was reported to increase STOCs frequency in myocytes isolated from resistance arteries. Forskolin (10 microM) triggered STOCs generation in 35.1% of silent aortic myocytes. In 36.8% percent of silent myocytes, forskolin did not trigger STOCs but increased the amplitude of charybdotoxin-sensitive outward net current to 136.1 +/- 8.5% at 0 mV. Membrane-permeable 8BrcAMP triggered STOCs generation in 38.7% of silent myocytes. Forskolin- or 8BrcAMP-triggered STOCs were inhibited by charybdotoxin. 8BrcAMP also increased open probability of BK(Ca) channels in BAPTA-AM-pretreated cells. Our data demonstrate that, in contrast to resistance arteries, STOCs are present just in the minority of myocytes in the thoracic aorta. However, cellular machinery that generates STOCs can be "switched" on by cAMP. Such an inactive cellular mechanism could modulate the contractility of the thoracic aorta in response to physiological demand.

(+/-)-Naringenin as large conductance Ca(2+)-activated K+ (BKCa) channel opener in vascular smooth muscle cells

BACKGROUND AND PURPOSE. The aim of this study was to investigate, in vascular smooth muscle cells, the mechanical and electrophysiological effects of (+/-)-naringenin.

EXPERIMENTAL APPROACH

Aorta ring preparations and single tail artery myocytes were employed for functional and patch-clamp experiments, respectively.

KEY RESULTS

(+/-)-Naringenin induced concentration-dependent relaxation in endothelium-denuded rat aortic rings pre-contracted with either 20 mM KCl or noradrenaline (pIC(50) values of 4.74 and 4.68, respectively). Tetraethylammonium, iberiotoxin, 4-aminopyridine and 60 mM KCl antagonised (+/-)-naringenin-induced vasorelaxation, while glibenclamide did not produce any significant antagonism. Naringin [(+/-)-naringenin 7-beta-neohesperidoside] caused a concentration-dependent relaxation of rings pre-contracted with 20 mM KCl, although its potency and efficacy were significantly lower than those of (+/-)-naringenin. In rat tail artery myocytes, (+/-)-naringenin increased large conductance Ca(2+)-activated K(+) (BK(Ca)) currents in a concentration-dependent manner; this stimulation was iberiotoxin-sensitive and fully reversible upon drug wash-out. (+/-)-Naringenin accelerated the activation kinetics of BK(Ca) current, shifted, by 22 mV, the voltage dependence of the activation curve to more negative potentials, and decreased the slope of activation. (+/-)-Naringenin-induced stimulation of BK(Ca) current was insensitive either to changes in the intracellular Ca(2+) concentration or to the presence, in the pipette solution, of the fast Ca(2+) chelator BAPTA. However, such stimulation was diminished when the K(+) gradient across the membrane was reduced.

CONCLUSIONS AND IMPLICATIONS

The vasorelaxant effect of the naturally-occurring flavonoid (+/-)-naringenin on endothelium-denuded vessels was due to the activation of BK(Ca) channels in myocytes.

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.