Excitation-contraction coupling and the ultrastructure of smooth muscle

Endoplasmic Reticulum-Plasma Membrane Junctions as Sites of Depolarization-Induced Ca2+ Signaling in Excitable Cells

Membrane contact sites between endoplasmic reticulum (ER) and plasma membrane (PM), or ER-PM junctions, are found in all eukaryotic cells. In excitable cells they play unique roles in organizing diverse forms of Ca2+ signaling as triggered by membrane depolarization. ER-PM junctions underlie crucial physiological processes such as excitation-contraction coupling, smooth muscle contraction and relaxation, and various forms of activity-dependent signaling and plasticity in neurons. In many cases the structure and molecular composition of ER-PM junctions in excitable cells comprise important regulatory feedback loops linking depolarization-induced Ca2+ signaling at these sites to the regulation of membrane potential. Here, we describe recent findings on physiological roles and molecular composition of native ER-PM junctions in excitable cells. We focus on recent studies that provide new insights into canonical forms of depolarization-induced Ca2+ signaling occurring at junctional triads and dyads of striated muscle, as well as the diversity of ER-PM junctions in these cells and in smooth muscle and neurons.

Intracellular Calcium Homeostasis and Kidney Disease

Kidney disease is a serious health problem that burdens our healthcare system. It is crucial to find the accurate pathogenesis of various types of kidney disease to provide guidance for precise therapies for patients suffering from these diseases. However, the exact molecular mechanisms underlying these diseases have not been fully understood. Disturbance of calcium homeostasis in renal cells plays a fundamental role in the development of various types of kidney disease, such as primary glomerular disease, diabetic nephropathy, acute kidney injury and polycystic kidney disease, through promoting cell proliferation, stimulating extracellular matrix accumulation, aggravating podocyte injury, disrupting cellular energetics as well as dysregulating cell survival and death dynamics. As a result, preventing the disturbance of calcium homeostasis in specific renal cells (such as tubular cells, podocytes and mesangial cells) is becoming one of the most promising therapeutic strategies in the treatment of kidney disease. The endoplasmic reticulum and mitochondria are two vital organelles in this process. Calcium ions cycle between the endoplasmic reticulum and mitochondria at the conjugation of these two organelles known as the mitochondria-associated endoplasmic reticulum membrane, maintaining calcium homeostasis. The pharmacologic modulation of cellular calcium homeostasis can be viewed as a novel therapeutic method for renal diseases. Here, we will introduce calcium homeostasis under physiological conditions and the disturbance of calcium homeostasis in kidney diseases. We will focus on the calcium homeostasis regulation in renal cells (including tubular cells, podocytes and mesangial cells), especially in the mitochondria- associated endoplasmic reticulum membranes of these renal cells.

Elucidation of Cellular Mechanisms That Regulate the Sustained Contraction and Relaxation of the Mammalian Iris

Purpose

In mammals, pupil constriction and dilation form the pupillary light reflex (PLR), which is mediated by both brain-regulated (parasympathetic) and local iris-driven reflexes. To better understand the cellular mechanisms that regulate pupil physiological dynamics via central and local photoreception, we have examined the regulation of the PLR via parasympathetic and local activation, respectively.

Methods

In this study, the PLR was examined in mouse enucleated eyes ex vivo in real-time under different ionic conditions in response to acetylcholine and/or blue light (480 nm). The use of pupillometry recordings captured the relaxation, contraction, and pupil escape (redilation) processes for 10 minutes up to 1 hour.

Results

Among others, our results show that ryanodine receptor channels are the main driver for iridal stimulation-contraction coupling, in which extracellular influx of Ca2+ is required for amplification of pupil constriction. Both local and parasympathetic iridal activations are necessary, but not sufficient for sustained pupil constriction. Moreover, the degree of membrane potential repolarization in the dark is correlated with the latency and velocity of iridal constriction. Furthermore, pupil escape is driven by membrane potential hyperpolarization where voltage-gated potassium channels play a crucial role.

Conclusions

Together, this study presents new mechanisms regulating synchronized pupil dilation and contraction, sustained pupil constriction, iridal stimulation-contraction coupling, and pupil escape.

Calcium Channels in Vascular Smooth Muscle

Calcium (Ca²⁺) plays a central role in excitation, contraction, transcription, and proliferation of vascular smooth muscle cells (VSMs). Precise regulation of intracellular Ca²⁺ concentration ([Ca²⁺]_i) is crucial for proper physiological VSM function. Studies over the last several decades have revealed that VSMs express a variety of Ca²⁺-permeable channels that orchestrate a dynamic, yet finely tuned regulation of [Ca²⁺]_i. In this review, we discuss the major Ca²⁺-permeable channels expressed in VSM and their contribution to vascular physiology and pathology.

Intracellular renin increases the inward calcium current in smooth muscle cells of mesenteric artery of SHR. Implications for hypertension and vascular remodeling

The influence of intracellular renin on the inward calcium current in isolated smooth muscle cells from SHR mesenteric arteries was investigated. Measurements of calcium current were performed using the whole cell configuration of pCLAMP. The results indicated that: 1) renin (100nM) dialyzed into smooth muscle cells, increased the inward calcium current; 2) verapamil (10-9M) administered to the bath inhibited the effect of renin on the inward calcium current; 3) concurrently with the increase of calcium current a depolarization of 6.8+/-2.1mV (n=16)(P<0.05) was found in cells dialyzed with renin; 4) intracellular dialysis of renin (100nM) into smooth muscle cells isolated from mesenteric arteries of normal Wystar Kyoto rats showed no significant change on calcium current; 5) aliskiren (10-9M) dialyzed into the cell together with renin (100nM) abolished the effect of the enzyme on the calcium current in SHR; 6) Ang II (100nM) dialyzed into the smooth muscle cell from mesenteric artery of SHR in absence of renin, decreased the calcium current-an effect greatly reduced by valsartan (10-9M) added to the cytosol; 7) administration of renin (100nM) plus angiotensinogen (100nM) into the cytosol of muscles cells from SHR rats reduced the inward calcium current; 8) extracellular administration of Ang II (100nM) increased the inward calcium current in mesenteric arteries of SHR.

CONCLUSIONS

intracellular renin in vascular resistance vessels from SHR due to internalization or expression, contributes to the regulation of vascular tone and control of peripheral resistance-an effect independently of Ang II. Implications for hypertension and vascular remodeling are discussed.

Intracellular angiotensin II as a regulator of muscle tone in vascular resistance vessels. Pathophysiological implications

The influence of intracellular angiotensin II on the regulation of potassium current and membrane potential of smooth muscle cells of mesenteric arteries and its relevance for the regulation of vascular tone was reviewed. The presence of components of the renin angiotensin system (RAS) in different cells of the cardiovascular system, was discussed including their presence in the nuclei and mitochondria. Emphasis was given to the opposite effects of intracellular and extracellular angiotensin II (Ang II) on the regulation of potassium current, membrane potential and contractility of vascular resistance vessels and its implication to vascular physiology and pathology and the possible role of epigenetic factors on the expression of angiotensin II (Ang II) and renin in vascular resistance vessels as well as its possible pathophysiological role in hypertension and other cardiovascular diseases.

Arterial Smooth Muscle Mitochondria Amplify Hydrogen Peroxide Microdomains Functionally Coupled to L-Type Calcium Channels

RATIONALE

Mitochondria are key integrators of convergent intracellular signaling pathways. Two important second messengers modulated by mitochondria are calcium and reactive oxygen species. To date, coherent mechanisms describing mitochondrial integration of calcium and oxidative signaling in arterial smooth muscle are incomplete.

OBJECTIVE

To address and add clarity to this issue, we tested the hypothesis that mitochondria regulate subplasmalemmal calcium and hydrogen peroxide microdomain signaling in cerebral arterial smooth muscle.

METHODS AND RESULTS

Using an image-based approach, we investigated the impact of mitochondrial regulation of L-type calcium channels on subcellular calcium and reactive oxygen species signaling microdomains in isolated arterial smooth muscle cells. Our single-cell observations were then related experimentally to intact arterial segments and to living animals. We found that subplasmalemmal mitochondrial amplification of hydrogen peroxide microdomain signaling stimulates L-type calcium channels, and that this mechanism strongly impacts the functional capacity of the vasoconstrictor angiotensin II. Importantly, we also found that disrupting this mitochondrial amplification mechanism in vivo normalized arterial function and attenuated the hypertensive response to systemic endothelial dysfunction.

CONCLUSIONS

From these observations, we conclude that mitochondrial amplification of subplasmalemmal calcium and hydrogen peroxide microdomain signaling is a fundamental mechanism regulating arterial smooth muscle function. As the principle components involved are fairly ubiquitous and positioning of mitochondria near the plasma membrane is not restricted to arterial smooth muscle, this mechanism could occur in many cell types and contribute to pathological elevations of intracellular calcium and increased oxidative stress associated with many diseases.

Molecular identification of P2X receptors in vascular smooth muscle cells from rat anterior, posterior, and basilar arteries

BACKGROUND

Purinergic P2X receptors in vascular smooth muscle cells (VSMCs) play an important role in physiological stimulatory responses to the extracellularly released ATP. The aim of this work was to identify molecular P2X receptor subunits in VSMCs isolated from rat anterior, posterior and basilar arteries using a number of contemporary laboratory techniques.

METHODS

P2X mediated ionic currents were recorded using amphotericin B perforated patch clamp method. Gene expression analysis was performed using RT-PCR in manually collected VSMCs. The expression of proteins was confirmed by fluorescent immunocytochemistry.

RESULTS

Under voltage clamp conditions VSMCs stimulated by application of 10 μmol/l selective P2X receptor agonist αβ-meATP, the biphasic currents consisting of rapidly rising rapidly desensitizing and slowly desensitizing components were observed in freshly isolated myocytes from all three arteries. Using RT-PCR, the expression of genes encoding only P2X1 and P2X4 receptor subunits was detected in preparations from all three arteries. The expression of corresponding P2X1 and P2X4 receptor subunit proteins was confirmed in isolated VSMCs.

CONCLUSIONS

Our work therefore identified that in major arteries of rat cerebral circulation VSMCs express only P2X1 and P2X4 receptors subunits. We can propose that these P2X receptor subunits participate in functional P2X receptor structures mediating ATP-evoked stimulatory responses in cerebral vascular myocytes in vivo.

ATP-evoked sustained vasoconstrictions mediated by heteromeric P2X1/4 receptors in cerebral arteries

BACKGROUND AND PURPOSE

Current knowledge states that vasoconstrictor responses to ATP are mediated by rapidly desensitizing ligand-gated P2X1 receptors in vascular smooth muscle cells (VSMCs). However, ATP is implicated in contributing to pathological conditions involving sustained vasoconstrictor response such as cerebral vasospasm. The purpose of this study is to test the hypothesis that the stimulation of VSMC P2XR receptors (P2XRs) contributes to ATP-evoked sustained vasoconstrictions in rat middle cerebral arteries (RMCAs).

METHODS

Reverse transcription- polymerase chain reaction, Western blot, and immunocytochemistry were used to analyze expression of mRNA and proteins in RMCAs VSMCs. Ionic currents and calcium responses were investigated using patch-clamp and confocal imaging techniques, respectively. Functional responses were confirmed using wire myography.

RESULTS

Expression of mRNA and protein for P2X1R and P2X4R subunits was identified in RMCA VSMCs. Confocal imaging in fluo-3-loaded VSMCs showed that ATP and a selective P2XR agonist, αβmeATP, evoked similar dose-dependent increases in [Ca(2+)]i. Patch-clamp experiments identified 2 components of P2XR-mediated currents: consisting of a fast desensitizing phase mediated by homomeric P2X1Rs and a slowly desensitizing phase involving heteromeric P2X1/4Rs. Isometric tension measurements showed that ≈80%:20% of initial ATP-evoked vasoconstriction in RMCA is mediated by homomeric P2X1Rs and heteromeric P2X1/4Rs, respectively. The sustained slowly desensitizing and rapidly recovering from desensitization responses are mediated by heteromeric P2X1/4Rs.

CONCLUSIONS

This study reveals for the first time that apart from rapidly desensitizing homomeric P2X1Rs, heteromeric P2X1/4Rs contribute to the sustained component of the purinergic-mediated vasoconstriction in RMCA. Our study, therefore, identifies possible novel targets for therapeutical intervention in cerebral circulation.

Vascular smooth muscle cells from small human omental arteries express P2X1 and P2X4 receptor subunits

Stimulation of P2X receptors by ATP in vascular smooth muscle cells (VSMCs) is proposed to mediate vascular tone. However, understanding of P2X receptor-mediated actions in human blood vessels is limited, and therefore, the current work investigates the role of P2X receptors in freshly isolated small human gastro-omental arteries (HGOAs). Expression of P2X1 and P2X4 receptor subunit messenger RNA (mRNA) and protein was identified in individual HGOA VSMCs using RT-PCR and immunofluorescent analysis and using Western blot in multi-cellular preparations. ATP of 10 μmol/l and αβ-meATP of 10 μmol/l, a selective P2X receptor agonist, evoked robust increases in [Ca(2+)]i in fluo-3-loaded HGOA VSMCs. Pre-incubation with 1 μmol/l NF279, a selective P2X receptor antagonist, reduced the amplitude of αβ-meATP-induced increase in [Ca(2+)]i by about 70 %. ATP of 10 μmol/l and αβ-meATP of 10 μmol/l produced similar contractile responses in segments of HGOA, and these contractions were greatly reduced by 2 μmol/l NF449, a selective P2X receptor inhibitor. These data suggest that VSMCs from HGOA express P2X1 and P2X4 receptor subunits with homomeric P2X1 receptors likely serving as the predominant target for extracellular ATP.

Regulation of cellular communication by signaling microdomains in the blood vessel wall

It has become increasingly clear that the accumulation of proteins in specific regions of the plasma membrane can facilitate cellular communication. These regions, termed signaling microdomains, are found throughout the blood vessel wall where cellular communication, both within and between cell types, must be tightly regulated to maintain proper vascular function. We will define a cellular signaling microdomain and apply this definition to the plethora of means by which cellular communication has been hypothesized to occur in the blood vessel wall. To that end, we make a case for three broad areas of cellular communication where signaling microdomains could play an important role: 1) paracrine release of free radicals and gaseous molecules such as nitric oxide and reactive oxygen species; 2) role of ion channels including gap junctions and potassium channels, especially those associated with the endothelium-derived hyperpolarization mediated signaling, and lastly, 3) mechanism of exocytosis that has considerable oversight by signaling microdomains, especially those associated with the release of von Willebrand factor. When summed, we believe that it is clear that the organization and regulation of signaling microdomains is an essential component to vessel wall function.

Angiotensin (1-7) increases the potassium current and the resting potential of arterial myocytes from vascular resistance vessels of normal adult rats: Pathophysiological implications

The influence of angiotensin (Ang) (1-7) on potassium current (Kv) and resting potential of smooth muscle cells isolated from mesenteric artery of Sprague Dawley rats was investigated. Measurements of potassium current were performed using the whole cell configuration of pCLAMP. The results indicated that Ang (1-7) (10(-9) M) increased the potassium current by 120% ± 2.6% (P < .05) and the resting potential of smooth muscle cells by 8 ± 2.8 mV (n = 23; P < .05). Ang II (10(-9) M) administered to the bath reduced the potassium current by 35% ± 3.6% (n = 23; P < .05) and depolarized the arterial myocytes by 7.8 ± 2.1 mV (n = 25; P < .05). The effect of the heptapeptide on potassium current was inhibited by a Mas receptor inhibitor (A779; 10(-8) M) as well as by a protein kinase A (PKA) inhibitor (10(-9) M) dialyzed into the cell. Intracellular dialysis of the catalytic subunit of PKA (5 × 10(-8) M) enhanced the potassium current by 38% ± 3.4% (n = 14; P < .05) but did not abolish the effect of Ang (1-7). On the other hand, Bis-1 (10(-9) M), which is a specific inhibitor of PKC, suppressed the effect of Ang (1-7) on potassium current. In conclusion, Ang (1-7) counteracts the effect of Ang II on potassium current and membrane potential of smooth muscle cells from mesenteric arteries, which are resistance vessels involved in the regulation of peripheral resistance and blood pressure. The activation of the cAMP/PKA cascade is essential for the effect of the heptapeptide. Pathophysiological implications are discussed.

Intracellular angiotensin II increases the total potassium current and the resting potential of arterial myocytes from vascular resistance vessels of the rat. Physiological and pathological implications

The influence of intracellular and extracellular administration of angiotensin II (Ang II; 10(-9) M) on total potassium current of arterial myocytes isolated from mesenteric arteries of Sprague Dawley rats was investigated. Measurements of total potassium current were performed using the voltage clamp whole cell configuration while the effect of intracellular Ang II on the resting potential of arterial myocytes was measured using the current clamp configuration of pCLAMP. The results indicated that: 1) intracellular Ang II (10(-9) M) increased the total potassium current by 73% ± 2.6% (n = 22; P < .05) within 5 minutes; 2) concurrently with the increment of potassium current, the resting potential was increased by 7 ± 1.5 mV (n = 23; P < .05); 3) extracellular administration of Ang II (10(-9) M) reduced the total potassium current by 20% ± 1.6% (n = 21; P < .05) within 5 minutes and depolarized the smooth muscle cells by 9 ± 2.3 mV (n = 26; P < .05); 4) the effects of intracellular Ang II on potassium current and membrane potential were inhibited by dialyzing a PKA inhibitor (10(-9) M) inside the cell together with Ang II (10(-9) M; P > .05); 5) valsartan (10(-9) M) dialyzed into the cell together with Ang II (10(-9) M) abolished the effect of the peptide on potassium current and membrane potential. The presence of endogenous or internalized intracellular Ang II in vascular resistance vessels and its effect on potassium current and resting potential indicates that the peptide present inside the arterial myocytes plays an important role on the regulation of vascular tone and consequently on peripheral resistance, which is a determining factor in the regulation of arterial blood pressure.

Reconstructing adhesion structures in tissues by cryo-electron tomography of vitrified frozen sections

Cryo-electron tomography enables three-dimensional insights into the macromolecular architecture of cells in a close-to-life state. However, it is limited to thin specimens, <1.0 μm in thickness, typically restricted to the peripheral areas of intact eukaryotic cells. Analysis of tissue ultrastructure, on the other hand, requires physical sectioning approaches, preferably cryo-sectioning, following which electron tomography (ET) may be performed. Nevertheless, cryo-electron microscopy of vitrified sections is a demanding technique and typically cannot be used to examine thick sections, >80-100 nm, due to surface crevasses. Here, we explore the potential use of cryo-ET of vitrified frozen sections (VFSs) for imaging cell adhesions in chicken smooth muscle and mouse epithelial tissues. By investigating 300-400 nm thick sections, which are collected on the EM grid and re-vitrified, we resolved fine 3D structural details of the membrane-associated dense plaques and flanking caveoli in smooth muscle tissue, and desmosomal adhesions in stratified epithelium. Technically, this method offers a simple approach for reconstructing thick volumes of hydrated frozen sections.

Functional modeling of the shift in cellular calcium dynamics at the onset of synchronization in smooth muscle cells

In the present paper we address the nature of synchronization properties found in populations of mesenteric artery smooth muscle cells. We present a minimal model of the onset of synchronization in the individual smooth muscle cell that is manifested as a transition from calcium waves to whole-cell calcium oscillations. We discuss how different types of ion currents may influence both amplitude and frequency in the regime of whole-cell oscillations. The model may also explain the occurrence of mixed-mode oscillations and chaotic oscillations frequently observed in the experimental system.

The effects of a high-fat, high-cholesterol diet on markers of uterine contractility during parturition in the rat

Increasing levels of obesity within women of reproductive age is a major concern in the UK. Approximately, 13% of women aged <30 and 22% of 31- to 40-year-old women are obese. Obesity increases complications during pregnancy and the risk of caesarean section due to prolonged labour and poor uterine activity. The aim was to investigate whether a high-fat, high-cholesterol (HFHC) diet decreases markers of uterine contractility during parturition in the rat. Female Wistar rats were fed control (CON, n=10) or HFHC (n=10) diets for 6 weeks. Animals were mated and, once pregnant, maintained on their diet throughout gestation. On gestational day 19, rats were monitored continuously and killed at the onset of parturition. Body and fat depot weights were recorded. Myometrial tissue was analysed for cholesterol (CHOL), triglycerides (TAG), and expression of the contractile associated proteins gap junction protein alpha 1 (GJA1; also known as connexin-43, CX-43), prostaglandin-endoperoxide synthase 2 (PTGS2; also known as cyclo-oxygenase-2, COX-2) and caveolin-1 (CAV1) and maternal plasma for prostaglandin F(2)(α) (PGF(2)(α)) and progesterone. HFHC fed rats gained greater weight than CON (P<0.003) with significant increases in peri-renal fat (P<0.01). The HFHC diet increased plasma CHOL, TAG and progesterone, but decreased PGF(2)(α) versus CON (P<0.01, P<0.01, P=0.05 and P<0.02 respectively). Total CHOL and TAG levels of uterine tissue were similar. However, HFHC fed rats showed significant increases in PTGS2 (P<0.037), but decreases in GJA1 and CAV1 (P=0.059). In conclusion, a HFHC diet significantly increases body weight and alters lipid profiles that correlate with decreases in key markers of uterine contractility. Further work is required to ascertain whether these changes have adverse effects on uterine activity.

Inhibition of SERCA pumps induces desynchronized RyR activation in overloaded internal Ca2+ stores in smooth muscle cells

We have previously shown that rapid inhibition of sarcoplasmic reticulum (SR) ATPase (SERCA pumps) decreases the amplitude and rate of rise (synchronization) of caffeine induced-Ca2+ release without producing a reduction of free luminal SR Ca2+ level in smooth muscle cells (Gómez-Viquez L, Guerrero-Serna G, García U, Guerrero-Hernández A. Biophys J 85: 370–380, 2003). Our aim was to investigate the role of luminal SR Ca2+ content in the communication between ryanodine receptors (RyRs) and SERCA pumps. To this end, we studied the effect of SERCA pump inhibition on RyR-mediated Ca2+ release in smooth muscle cells with overloaded SR Ca2+ stores. Under this condition, the amplitude of RyR-mediated Ca2+ release was not affected but the rate of rise was still decreased. In addition, the caffeine-induced Ca2+-dependent K+ outward currents revealed individual events, suggesting that SERCA pump inhibition reduces the coordinated activation of RyRs. Collectively, our results indicate that SERCA pumps facilitate the activation of RyRs by a mechanism that does not involve the regulation of SR Ca2+ content. Importantly, SERCA pumps and RyRs colocalize in smooth muscle cells, suggesting a possible local communication between these two proteins.

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.

Visualizing the temporal effects of vasoconstrictors on PKC translocation and Ca2+ signaling in single resistance arterial smooth muscle cells

Arterial smooth muscle (ASM) contraction plays a critical role in regulating blood distribution and blood pressure. Vasoconstrictors activate cell surface receptors to initiate signaling cascades involving increased intracellular Ca(2+) concentration ([Ca(2+)](i)) and recruitment of protein kinase C (PKC), leading to ASM contraction, though the PKC isoenzymes involved vary between different vasoconstrictors and their actions. Here, we have used confocal microscopy of enhanced green fluorescence protein (eGFP)-labeled PKC isoenzymes to visualize PKC translocation in primary rat mesenteric ASM cells in response to physiological vasoconstrictors, with simultaneous imaging of Ca(2+) signaling. Endothelin-1, angiotensin II, and uridine triphosphate all caused translocation of each of the PKC isoenzymes alpha, delta, and epsilon; however, the kinetics of translocation varied between agonists and PKC isoenzymes. Translocation of eGFP-PKCalpha mirrored the rise in [Ca(2+)](i), while that of eGFP-PKCdelta or -epsilon occurred more slowly. Endothelin-induced translocation of eGFP-PKCepsilon was often sustained for several minutes, while responses to angiotensin II were always transient. In addition, preventing [Ca(2+)](i) increases using 1,2-bis-(o-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid tetra-(acetoxymethyl) ester prevented eGFP-PKCalpha translocation, while eGFP-PKCdelta translocated more rapidly. Our results suggest that PKC isoenzyme specificity of vasoconstrictor actions occurs downstream of PKC recruitment and demonstrate the varied kinetics and complex interplay between Ca(2+) and PKC responses to different vasoconstrictors in ASM.

Oxytocin-induced phasic and tonic contractions are modulated by the contractile machinery rather than the quantity of oxytocin receptor

To investigate the relationship between the oxytocin (OT) receptor (OTR) quantity and the contractile features systematically, we measured the mRNA expression levels of OTR and L-type Ca(2+) channel alpha(1C)-subunit (alpha(1C)) and examined the regulatory mechanisms of OT-induced phasic or tonic contractions of the longitudinal smooth muscles in mouse uteri. The mRNA expression of OTR in 19.0 G (19.0 days of gestation) was greater than those in nonpregnant phases, and that of alpha(1C) in estrus and 19.0 G was higher than in diestrus. OT-induced contractions sparsely occurred in diestrus. The OT-induced all-or-none-type phasic contractions at low concentrations were abolished by verapamil in both estrus and 19.0 G. OT-induced tonic contractions had similar pD(2) values in both estrus and 19.0 G. However, the magnitude in 19.0 G was much greater than that in estrus. The large tonic contractions also occurred in PGF(2alpha) receptor (FP) knockout mice in 19.0 G despite a small amount of OTR. Verapamil and Y-27632 partially inhibited the tonic contractions in 19.0 G. Cyclopiazonic acid-induced tonic contractions were reciprocally decreased with the increase in the OT-induced ones in 19.0 G. These results indicate that the phasic contractions are dependent on alpha(1C). The tonic contractions in 19.0 G are dependent on both Ca(2+) influxes via L-type Ca(2+) channels and store-operated Ca(2+) channels, and the force is augmented by the Rho signal pathway, which increases the Ca(2+) sensitivity. Thus the uterine contractions are mainly controlled by the modification of contractile signal machinery rather than simply by the OTR quantity.

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.

Glucose reduces endothelin inhibition of voltage-gated potassium channels in rat arterial smooth muscle cells

Prolonged hyperglycaemia impairs vascular reactivity and inhibits voltage-activated K(+) (Kv) channels. We examined acute effects of altering glucose concentration on the activity and inhibition by endothelin-1 (ET-1) of Kv currents of freshly isolated rat arterial myocytes. Peak Kv currents recorded in glucose-free solution were reversibly reduced within 200 s by increasing extracellular glucose to 4 mm. This inhibitory effect of glucose was abolished by protein kinase C inhibitor peptide (PKC-IP), and Kv currents were further reduced in 10 mm glucose. In current-clamped cells, membrane potentials were more negative in 4 than in 10 mm glucose. In 4 mm d-glucose, 10 nm ET-1 decreased peak Kv current amplitude at +60 mV from 23.5 +/- 3.3 to 12.1 +/- 3.1 pA pF(-1) (n = 6, P < 0.001) and increased the rate of inactivation, decreasing the time constant around fourfold. Inhibition by ET-1 was prevented by PKC-IP. When d-glucose was increased to 10 mm, ET-1 no longer inhibited Kv current (n = 6). Glucose metabolism was required for prevention of ET-1 inhibition of Kv currents, since fructose mimicked the effects of d-glucose, while l-glucose, sucrose or mannitol were without effect. Endothelin receptors were still functional in 10 mm d-glucose, since pinacidil-activated ATP-dependent K(+) (K(ATP)) currents were reduced by 10 nm ET-1. This inhibition was nearly abolished by PKC-IP, indicating that endothelin receptors could still activate PKC in 10 mm d-glucose. These results indicate that changes in extracellular glucose concentration within the physiological range can reduce Kv current amplitude and can have major effects on Kv channel modulation by vasoconstrictors.

Mitochondria buffer NCX-mediated Ca2+-entry and limit its diffusion into vascular smooth muscle cells

The reverse-mode of the Na(+)/Ca(2+)-exchanger (NCX) mediates Ca(2+)-entry in agonist-stimulated vascular smooth muscle (VSM) and plays a central role in salt-sensitive hypertension. We investigated buffering of Ca(2+)-entry by peripheral mitochondria upon NCX reversal in rat aortic smooth muscle cells (RASMC). [Ca(2+)] was measured in mitochondria ([Ca(2+)](MT)) and the sub-plasmalemmal space ([Ca(2+)](subPM)) with targeted aequorins and in the bulk cytosol ([Ca(2+)](i)) with fura-2. Substitution of extracellular Na(+) by N-methyl-d-glucamine transiently increased [Ca(2+)](MT) ( approximately 2microM) and [Ca(2+)](subPM) ( approximately 1.3microM), which then decreased to sustained plateaus. In contrast, Na(+)-substitution caused a delayed and tonic increase in [Ca(2+)](i) (<100nM). Inhibition of Ca(2+)-uptake by the sarcoplasmic reticulum (SR) (30microM cyclopiazonic acid) or mitochondria (2microM FCCP or 2microM ruthenium red) enhanced the elevation of [Ca(2+)](subPM). These treatments also abolished the delay in the [Ca(2+)](i) response to 0Na(+) and increased its amplitude. Extracellular ATP (1mM) caused a peak and plateau in [Ca(2+)](i), and only the plateau was inhibited by KB-R7943 (10microM), a selective blocker of reverse-mode NCX. Evidence for ATP-mediated NCX-reversal was also found in changes in [Na(+)](i). Mitochondria normally exhibited a transient elevation of [Ca(2+)] in response to ATP, but inhibiting the mitochondrial NCX with CGP-37157 (10microM) unmasked an agonist-induced increase in mitochondrial Ca(2+)-flux. This flux was blocked by KB-R7943. In summary, mitochondria and the sarcoplasmic reticulum co-operate to buffer changes in [Ca(2+)](i) due to agonist-induced NCX reversal.

Roles of CaM kinase II and phospholamban in SNP-induced relaxation of murine gastric fundus smooth muscles

The mechanisms by which nitric oxide (NO) relaxes smooth muscles are unclear. The NO donor sodium nitroprusside (SNP) has been reported to increase the Ca2+ release frequency (Ca2+ sparks) through ryanodine receptors (RyRs) and activate spontaneous transient outward currents (STOCs), resulting in smooth muscle relaxation. Our findings that caffeine relaxes and hyperpolarizes murine gastric fundus smooth muscles and increases phospholamban (PLB) phosphorylation by Ca2+/calmodulin (CaM)-dependent protein kinase II (CaM kinase II) suggest that PLB phosphorylation by CaM kinase II participates in smooth muscle relaxation by increasing sarcoplasmic reticulum (SR) Ca2+ uptake and the frequencies of SR Ca2+ release events and STOCs. Thus, in the present study, we investigated the roles of CaM kinase II and PLB in SNP-induced relaxation of murine gastric fundus smooth muscles. SNP hyperpolarized and relaxed gastric fundus circular smooth muscles and activated CaM kinase II. SNP-induced CaM kinase II activation was prevented by KN-93. Ryanodine, tetracaine, 2-aminoethoxydiphenylborate, and cyclopiazonic acid inhibited SNP-induced fundus smooth muscle relaxation and CaM kinase II activation. The Ca2+-activated K+ channel blockers iberiotoxin and apamin inhibited SNP-induced hyperpolarization and relaxation. The soluble guanylate cyclase inhibitor 1H-[1,2,4]oxadiazolo-[4,3-alpha]quinoxalin-1-one inhibited SNP-induced relaxation and CaM kinase II activation. The membrane-permeable cGMP analog 8-bromo-cGMP relaxed gastric fundus smooth muscles and activated CaM kinase II. SNP increased phosphorylation of PLB at Ser16 and Thr17. Thr17 phosphorylation of PLB was inhibited by cyclopiazonic acid and KN-93. Ser16 and Thr17 phosphorylation of PLB was sensitive to 1H-[1,2,4]oxadiazolo-[4,3-alpha]quinoxalin-1-one. These results demonstrate a novel pathway linking the NO-soluble guanylyl cyclase-cGMP pathway, SR Ca2+ release, PLB, and CaM kinase II to relaxation in gastric fundus smooth muscles.

Ultrastructural features of smooth muscle and endothelial cells of isolated isobaric human placental and maternal arteries

The ability of a blood vessel to develop tone is dependent upon morphological parameters of the smooth muscle cells (SMC), including density, relationship with the endothelium and subcellular distribution of myofilaments and intracellular organelles. Consequently, wall ultrastructure of isolated human placental chorionic plate arteries (n=12), fixed when pressurised to mimic their in vivo geometry, was examined qualitatively using electron microscopy, and compared with maternal arteries (omental, n=10, myometrial, n=6). Arteries from women with uncomplicated pregnancy were tested for contractile viability before fixing, with some vessels post-fixed in osmium-ferricyanide for sarcoplasmic reticulum (SR) identification. In contrast to maternal arteries, placental arteries had no internal elastic lamina but exhibited considerable extracellular matrix separating circularly orientated SMC. Human SMC contained tightly packed arrays of myofilaments running parallel to the plasma membrane, enveloping cellular organelles. Synthetic SMC, with few myofilaments and much rough SR, were observed in placental arteries only. SR in SMC from maternal arteries was located centrally, often encircling mitochondria, and also near the plasma membrane associated with caveolae. Positive SR staining was rarely observed in SMC of placental arteries. This study highlights ultrastructural differences between placental and maternal arteries that may underlie specialised mechanisms of regulating vascular tone in the placenta.

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

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

Emerging role of cyclic ADP-ribose (cADPR) in smooth muscle

Cyclic adenosine diphosphate ribose (cADPR) is a naturally occurring cyclic nucleotide and represents a novel class of endogenous Ca(2+) messengers implicated in the regulation of the gating properties of ryanodine receptors (RyRs). This action of cADPR occurs independently from the inositol-1,4,5-trisphosphate (IP(3)) receptor. The regulation of intracellular Ca(2+) release is a fundamental element of cellular Ca(2+) homeostasis since a number of smooth muscle functions (tone, proliferation, apoptosis, and gene expression) are modulated by intracellular Ca(2+) concentration ([Ca(2+)](i)). There has been a surge in the efforts aimed at understanding the mechanisms of cADPR-mediated Ca(2+) mobilization and its impact on smooth muscle function. This review summarizes the proposed roles of cADPR in the regulation of smooth muscle tone.

Functional architecture of the SR calcium store in uterine smooth muscle

Sarcoplasmic reticulum (SR) is abundant in uterine smooth muscle cells. The functional role of this organelle in the regulation of uterine myocytes is not fully understood. The data available in the literature suggest that SR plays a dual role: as a source of calcium and as a calcium sink shaping calcium transients produced by membrane depolarisation and uterotonic agonists. Advances in digital imaging techniques including confocal microscopy of isolated living cells, and the development of methods for direct measurement of intraluminal calcium, has triggered a substantial increase in the number of publications elucidating the role of intracellular stores in calcium signalling. In this paper we review the literature and our own work on the SR calcium store in uterine smooth muscle cells.

SERCA pump optimizes Ca2+ release by a mechanism independent of store filling in smooth muscle cells

Thapsigargin-sensitive sarco/endoplasmic reticulum Ca(2+) pumps (SERCAs) are involved in maintaining and replenishing agonist-sensitive internal stores. Although it has been assumed that release channels act independently of SERCA pumps, there are data suggesting the opposite. Our aim was to study the relationship between SERCA pumps and the release channels in smooth muscle cells. To this end, we have rapidly blocked SERCA pumps with thapsigargin, to avoid depletion of the internal Ca(2+) stores, and induced Ca(2+) release with either caffeine, to open ryanodine receptors, or acetylcholine, to open inositol 1,4,5-trisphosphate receptors. Blocking SERCA pumps produced smaller and slower agonist-induced [Ca(2+)](i) responses. We determined the Ca(2+) level of the internal stores both indirectly, measuring the frequency of spontaneous transient outward currents, and directly, using Mag-Fura-2, and demonstrated that the inhibition of SERCA pumps did not produce a reduction of the sarco/endoplasmic reticulum Ca(2+) levels to explain the decrease in the agonist-induced Ca(2+) responses. It appears that SERCA pumps are involved in sustaining agonist-induced Ca(2+) release by a mechanism that involves the modulation of Ca(2+) availability in the lumen of the internal stores.

Adenosine affects the calcium dynamics of rat portal vein

1. The present study was carried out to characterize the effect of adenosine on calcium dynamics in the rat portal vein. Isolated portal vein of male albino rats was used as the experimental model as it exhibits autorhythmicity. 2. Adenosine and its analogues 2-CAD, N6-CHA and NECA were used to characterize the type of adenosine receptor involved and 2-CAD was used along with adenosine throughout the other part of study to characterize the effect of adenosine on Ca2+ dynamics. Adenosine and its analogues were found to inhibit the spontaneous contractions of rat portal vein in a concentration-related manner. The order of potency was NECA > 2-CAD > N6-CHA > adenosine. Incubation of the tissue with CGS-15943A, an adenosine receptor antagonist, had a per se enhancing effect on autorhythmicity. Adenosine and 2-CAD failed to reverse the contractile response produced by hypertonic KCl (80 or 30 mM). Whereas adenosine and 2-CAD effectively relaxed the tissues contracted with phenylephrine (10(-5) m). 3. Preincubation of the tissue with 2-CAD (10(-4), 10(-5) or 10(-6) m) for 5 min raised the threshold concentration of CaCl2 to evoke contractile response and also significantly increased the mean EC50 values of CaCl2. Nifedipine was found to be more potent than 2-CAD on Ca2+ channels. 4. The results of the present study suggest that the endogenous adenosine plays a significant role in producing vascular relaxation through the participation of A2 receptor subtype. This effect may be due to its inhibitory effect on release of Ca2+ from the intracellular stores. Further to this effect, 2-CAD had a major inhibitory effect on voltage-operated Ca2+ channels compared with receptor-operated Ca2+ channels. 5. It can be concluded that adenosine through its A2 receptor produces vasorelaxant effect by interfering with the release of Ca2+ from the intracellular stores coupled with influx of Ca2+ from the extracellular sources.

Functional role of ryanodine-sensitive Ca2+ stores in acidic pH-induced contraction in Wistar Kyoto rat aorta

Acidic pH induced a contraction in the isolated aorta from Wistar Kyoto rat. The magnitude of contraction was dependent upon the degree of extracellular acidification. The maximum level of contraction observed at pH 6.5 was 84.6 +/- 3.4% of the 64.8 mM KCl-induced contraction. To investigate the role of extracellular as well as intracellular Ca(2+) in acidic pH-induced contraction (APIC), we changed the extracellular pH in the presence of EGTA. Sustained contraction induced by acidic pH in the presence of extracellular Ca(2+) was completely abolished in the presence of EGTA, while a transient but significant contraction was still observed. Ryanodine, a selective ryanodine receptor blocker and cyclopiazonic acid (CPA), an inhibitor of sarco-/endoplasmic reticulum Ca(2+) ATPase, abolished the transient contraction, when pH was decreased in Ca(2+)-free solution. On the other hand, neither xestospongin C, a selective inositol-1,4,5-trisphosphate receptor antagonist nor U-73122, a phospholipase C inhibitor showed this effect. These results suggest the involvement of Ca(2+) release from ryanodine-/CPA-sensitive store of sarcoplasmic reticulum (SR). In normal Ca(2+)-containing solution, ryanodine and CPA did not alter the maximum level of APIC. However, they significantly decreased the rate of rise of APIC. U-73122, suppressed the maximum contraction induced by acidic pH without affecting the rate of rise of APIC, while xestospongin C and U-73343, an inactive analogue of U-73122, had no effect on both parameters of APIC. From these results, it is concluded that acidic pH induces Ca(2+) release from the ryanodine-/CPA-sensitive store of SR and that release provides supportive effect on initiating rapid transient contraction, but not on the sustained contraction, which is entirely due to Ca(2+) influx.

Heparin and heparan sulfate disaccharides bind to the exchanger inhibitor peptide region of Na+/Ca2+ exchanger and reduce the cytosolic calcium of smooth muscle cell lines. Requirement of C4-C5 unsaturation and 1--> 4 glycosidic linkage for activity

Heparin and heparan sulfate fragments, obtained by bacterial heparinase and heparitinases, bearing an unsaturation at C4-C5 of the uronic acid moiety, are able to produce up to 80% reduction of the cytosolic calcium of smooth muscle cell lines. Unsaturated disaccharides from chondroitin sulfate, dermatan sulfate, and hyaluronic acid are inactive, indicating that, besides the unsaturation of the uronic acid, a vicinal 1 --> 4 glycosidic linkage is needed. An inverse correlation between the molecular weight and activity is observed. Thus, the ED(50) of the N-acetylated disaccharide derived from heparan sulfate (430 Da) is 88 microm compared with 250 microm of the trisulfated disaccharide (650 Da) derived from heparin. Except for enoxaparin (which contains an unsaturation at the non-reducing end and 1 --> 4 glycosidic linkage), other low molecular weight heparins and native heparin are practically inactive in reducing the cytosolic calcium levels. Thapsigargin (sarcoplasmic reticulum Ca(2+)-ATPase inhibitor), vanadate (cytoplasmic membrane Ca(2+)-ATPase inhibitor), and nifedipine and verapamil (Ca(2+) channel antagonists) do not interfere with the effect of the trisulfated disaccharide upon the decrease of the intracellular calcium. A significant decrease of the activity of the trisulfated disaccharide is observed by reducing extracellular sodium, suggesting that the fragments might act upon the Na(+)/Ca(2+) exchanger promoting the extrusion of Ca(2+). This was further substantiated by binding experiments and circular dichroism analysis with the exchanger inhibitor peptide.

Ca2+-dependent actin remodeling in the contracting A7r5 cell

Previous work has shown that stimulation of contraction in A7r5 smooth muscle cells with phorbol ester (PDBu) results in the disassembly and remodeling of the alpha-actin component of the cytoskeleton (Fultz et al., 2000, J Mus Res Cell Motil 21: 775-781). In the present study, we evaluated the effect of increasing intracellular calcium ion concentration [Ca2+]i by A23187 and thapsigargin on alpha- and beta-actin remodeling. The effects of A23187 and thapsigargin on cell contraction and actin remodeling were effectively identical. The two compounds caused contraction of A7r5 cells that was earlier in onset and more quickly completed than PDBu-induced contractions. Both the alpha- and beta-actin isoforms were incorporated into stress cables in the resting cell. During the interval of contraction, beta-actin cables shortened without evidence of disassembly. By comparison, the increase of [Ca2+]i resulted in partial or complete dissolution of alpha-actin cables without further remodeling. In addition, PDBu-mediated alpha-actin remodeling was blocked in the presence of A23187. Increased [Ca2+]i also caused dispersal of alpha-actinin but had no effect on the cellular distribution of talin suggesting the effect was selective for alpha-actin cytoskeletal structure. The incubation of cells in calcium-free media prevented alpha-actin dissolution by A23187/thapsigargin and also blocked PDBu-mediated remodeling. Finally, of six kinase inhibitors investigated, only ML-7 partially blocked the dissolution of alpha-actin cables by increased [Ca2+]i. The results suggest that the sustained elevation of [Ca2+]i beyond a threshold level initiates depolymerization of alpha-actin but not beta-actin. It further appears that PDBu-induced alpha-actin remodeling requires Ca2+ but increases of [Ca2+]i beyond a threshold level may inhibit this activity. The finding that ML-7 partially inhibits alpha-actin dissolution in the presence of A23187/thapsigargin may be suggesting that myosin light chain kinase (MLCK) plays a role in destabilizing alpha-actin structure in the activated cell.

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

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

Functionally separate intracellular Ca2+ stores in smooth muscle

In smooth muscle, release via the inositol 1,4,5-trisphosphate (Ins(1,4,5)P(3)R) and ryanodine receptors (RyR) on the sarcoplasmic reticulum (SR) controls oscillatory and steady-state cytosolic Ca(2+) concentrations ([Ca(2+)](c)). The interplay between the two receptors, itself determined by their organization on the SR, establishes the time course and spatial arrangement of the Ca(2+) signal. Whether or not the receptors are co-localized or distanced from each other on the same store or whether they exist on separate stores will significantly affect the Ca(2+) signal produced by the SR. To date these matters remain unresolved. The functional arrangement of the RyR and Ins(1,4,5)P(3)R on the SR has now been examined in isolated single voltage-clamped colonic myocytes. Depletion of the ryanodine-sensitive store, by repeated application of caffeine, in the presence of ryanodine, abolished the response to Ins(1,4,5)P(3), suggesting that Ins(1,4,5)P(3)R and RyR share a common Ca(2+) store. Ca(2+) release from the Ins(1,4,5)P(3)R did not activate Ca(2+)-induced Ca(2+) release at the RyR. Depletion of the Ins(1,4,5)P(3)-sensitive store, by the removal of external Ca(2+), on the other hand, caused only a small decrease ( approximately 26%) in caffeine-evoked Ca(2+) transients, suggesting that not all RyR exist on the common store shared with Ins(1,4,5)P(3)R. Dependence of the stores on external Ca(2+) for replenishment also differed; removal of external Ca(2+) depleted the Ins(1,4,5)P(3)-sensitive store but caused only a slight reduction in caffeine-evoked transients mediated at RyR. Different mechanisms are presumably responsible for the refilling of each store. Refilling of both Ins(1,4,5)P(3)-sensitive and caffeine-sensitive Ca(2+) stores was inhibited by each of the SR Ca(2+) ATPase inhibitors thapsigargin and cyclopiazonic acid. These results may be explained by the existence of two functionally distinct Ca(2+) stores; the first expressing only RyR and refilled from [Ca(2+)](c), the second expressing both Ins(1,4,5)P(3)R and RyR and dependent upon external Ca(2+) for refilling.

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

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

Smooth muscle excitation-contraction coupling: a role for caveolae and caveolins?

Agonist stimulation of smooth muscle contractility involves integration of many signal-transducing events from the plasma membrane to myofilaments in the cytoplasm. Recent evidence suggests an important role for membranous invaginations termed caveolae, and their integral protein components caveolins, in the coordination of extracellular contractile stimuli and intracellular effectors in smooth muscle.

Angiotensin II inhibits rat arterial KATP channels by inhibiting steady-state protein kinase A activity and activating protein kinase Ce

We used whole-cell patch clamp to investigate steady-state activation of ATP-sensitive K+ channels (KATP) of rat arterial smooth muscle by protein kinase A (PKA) and the pathway by which angiotensin II (Ang II) inhibits these channels. Rp-cAMPS, an inhibitor of PKA, did not affect KATP currents activated by pinacidil when the intracellular solution contained 0.1 mM ATP. However, when ATP was increased to 1.0 mM, inhibition of PKA reduced KATP current, while the phosphatase inhibitor calyculin A caused a small increase in current. Ang II (100 nM) inhibited KATP current activated by the K+ channel opener pinacidil. The degree of inhibition was greater with 1.0 mM than with 0.1 mM intracellular ATP. The effect of Ang II was abolished by the AT1 receptor antagonist losartan. The inhibition of KATP currents by Ang II was abolished by a combination of PKA inhibitor peptide 5-24 (5 microM) and PKC inhibitor peptide 19-27 (100 microM), while either alone caused only partial block of the effect. In the presence of PKA inhibitor peptide, the inhibitory effect of Ang II was unaffected by the PKC inhibitor Go 6976, which is selective for Ca2+-dependent isoforms of PKC, but was abolished by a selective peptide inhibitor of the translocation of the epsilon isoform of PKC. Our results indicate that KATP channels are activated by steady-state phosphorylation by PKA at normal intracellular ATP levels, and that Ang II inhibits the channels both through activation of PKCepsilon and inhibition of PKA.

Heterogeneity of calcium stores and elementary release events in canine pulmonary arterial smooth muscle cells

To examine the nature of inositol 1,4,5-trisphosphate (IP(3))-sensitive and ryanodine (Ryn)-sensitive Ca(2+) stores in isolated canine pulmonary arterial smooth cells (PASMC), agonist-induced changes in global intracellular Ca(2+) concentration ([Ca(2+)](i)) were measured using fura 2-AM fluorescence. Properties of elementary local Ca(2+) release events were characterized using fluo 3-AM or fluo 4-AM, in combination with confocal laser scanning microscopy. In PASMC, depletion of sarcoplasmic reticulum Ca(2+) stores with Ryn (300 microM) and caffeine (Caf; 10 mM) eliminated subsequent Caf-induced intracellular Ca(2+) transients but had little or no effect on the initial IP(3)-mediated intracellular Ca(2+) transient induced by ANG II (1 microM). Cyclopiazonic acid (CPA; 10 microM) abolished IP(3)-induced intracellular Ca(2+) transients but failed to attenuate the initial Caf-induced intracellular Ca(2+) transient. These results suggest that in canine PASMC, IP(3)-, and Ryn-sensitive Ca(2+) stores are organized into spatially distinct compartments while similar experiments in canine renal arterial smooth muscle cells (RASMC) reveal that these Ca(2+) stores are spatially conjoined. In PASMC, spontaneous local intracellular Ca(2+) transients sensitive to modulation by Caf and Ryn were detected, exhibiting spatial-temporal characteristics similar to those previously described for "Ca(2+) sparks" in cardiac and other types of smooth muscle cells. After depletion of Ryn-sensitive Ca(2+) stores, ANG II (8 nM) induced slow, sustained [Ca(2+)](i) increases originating at sites near the cell surface, which were abolished by depleting IP(3) stores. Discrete quantal-like events expected due to the coordinated opening of IP(3) receptor clusters ("Ca(2+) puffs") were not observed. These data provide new information regarding the functional properties and organization of intracellular Ca(2+) stores and elementary Ca(2+) release events in isolated PASMC.

Paradoxical effects of temperature on vascular tone

Temperature may have significant influence on vascular tone in such cases as organ preservation, coronary bypass surgery, and extracorporeal circulation. The aim of this research was to study the direct effect of temperature variation on vascular tone in an attempt to elucidate the mechanisms involved. In a first series of experiments, the isometric tension of two different vessels (rat thoracic aorta and pig renal branch artery) was studied at different temperatures. To study the role of calcium in this response, a second series of experiments was performed. In this the vessels were incubated with the intracellular chelator BAPTA/AM. Further experiments were performed to test the effect of cold storage. Our results show that changes in temperature lead to different results in pig renal artery and rat aorta. A decrease in temperature induced a highly reproducible relaxation in rat aorta, whereas pig renal artery presented cooling-induced contraction. Moreover, whereas calcium depletion failed to inhibit cooling-induced relaxation in rat aorta, it did not provoke cooling-induced contraction in pig renal artery. Similar responses were obtained with cold storage and calcium depletion. We intend to demonstrate that, just as the effect of temperature variation on pig renal artery is due to a metabolic mechanism, its effect on rat aorta may be due to structural factors. This hypothesis is supported by the result of histological studies which demonstrate a higher proportion of elastin fibres in rat aorta than in pig renal artery.

Estimating oxygen transport resistance of the microvascular wall

The problem of diffusion of O(2) across the endothelial surface in precapillary vessels and its utilization in the vascular wall remains unresolved. To establish a relationship between precapillary release of O(2) and vascular wall consumption, we estimated the intravascular flux of O(2) on the basis of published in vivo measurements. To interpret the data, we utilized a diffusion model of the vascular wall and computed possible physiological ranges for O(2) consumption. We found that many flux values were not consistent with the diffusion model. We estimated the mitochondrial-based maximum O(2) consumption of the vascular wall (M(mt)) and a possible contribution to O(2) consumption of nitric oxide production by endothelial cells (M(NO)). Many values of O(2) consumption predicted from the diffusion model exceeded M(mt) + M(NO). In contrast, reported values of O(2) consumption for endothelial and smooth muscle cell suspensions and vascular strips in vitro do not exceed M(mt). We conjecture that most of the reported values of intravascular O(2) flux are overestimated, and the likely source is in the experimental estimates of convective O(2) transport at upstream and downstream points of unbranched vascular segments.

The effects of anti-hypertensive therapy on the structural, mechanical and metabolic properties of the rat aorta

The vascular system exhibits altered growth, calcium responses and metabolism during hypertension. To relate such changes, we compared histological, tension and metabolic responses in the aorta from 32-week-old spontaneously hypertensive rats (SHRs), normotensive Wistar-Kyoto (WKY) rats, and SHRs treated with Verapamil (V) and ACE-inhibitor, Trandolapril (T) as well as a combination of the two treatments (C). Vascular hypertrophy was apparent in the SHRs. Contractile responses induced by 50 mmol/1 KCl and 2.5 mmol/1 Ca2+ were significantly lower in the SHR (64.4 mN/mm2 vs. 49.2 mN/mm2), but an associated increase in Ca2+ -sensitivity (EC50 of extracellular Ca2+ (mumol/1): SHR, 456 vs. WKY, 616) normalised tension generating ability. All treatments led to significant decreases in blood pressure, although only T and C treated animals became normotensive with concomitant normalisation of vascular hypertrophy. An increase in oxygen consumption was apparent in the SHR aorta, which was associated with significant differences in the activities of key metabolic enzymes. Anti-hypertensive treatment normalised many of the metabolic parameters, with the C therapy being the most efficacious. We conclude that the treatment of hypertension by combined therapy leads to a better normalisation of structural, contractile, and metabolic parameters in the SHR, than either treatment alone and that metabolic changes with the pathology are resolved with appropriate therapy.

F-actin stabilization increases tension cost during contraction of permeabilized airway smooth muscle in dogs

1. Dynamic actin reorganization involving actin polymerization and depolymerization may play an important functional role in smooth muscle. 2. This study tested the hypothesis that F-actin stabilization by phalloidin increases tension cost (i.e. ATP hydrolysis rate per unit of isometric force) during Ca2+-induced activation of Triton X-100-permeabilized canine tracheal smooth muscle. 3. Adenosine 5'-triphosphate (ATP) hydrolysis rate was quantified using an enzyme-coupled NADH fluorometric technique, regulatory myosin light chain (rMLC) phosphorylation was measured by Western blot analysis, and maximum unloaded shortening velocity (Vmax) was estimated by interpolation of the force-velocity relationship to zero load during isotonic loading. 4. Maximal activation with 10 microM free Ca2+ induced sustained increases in isometric force, stiffness, and rMLC phosphorylation. However, the increase in ATP hydrolysis rate initially reached peak values, but then declined to steady-state levels above that of the unstimulated muscle. Thus, tension cost decreased throughout steady-state isometric force. 5. Following incubation of permeabilized strips with 50 microM phalloidin for 1 h, the increases in isometric force and stiffness were not sustained despite a sustained increase in rMLC phosphorylation. Also, after an initial decline, tension cost increased throughout activation. Phalloidin had no effect on Vmax during steady-state isometric force or on rMLC phosphorylation. 6. These findings suggest that dynamic reorganization of actin is necessary for optimal energy utilization during contraction of permeabilized airway smooth muscle.

Effect of lysophosphatidic acid on the ovum transport in mouse oviducts

The effects of lysophosphatidic acid (LPA) on ovum transport in mouse oviducts were studied. When excised oviducts were incubated at 37 degrees C under 5% CO2 in humidified air for 24 hours, addition of LPA at 10 microM to the medium significantly accelerated the rate of ovum transport, and 1 microM LPA slightly increased the ovum transport rate. These increases were not inhibited by 10 microM indomethacin, a cyclooxygense inhibitor, but were suppressed by 260 ng/ml of pertussis toxin or 10 microM verapamil, a voltage-sensitive calcium channel blocker. These data suggested that LPA stimulates mouse ovum transport by contracting oviductual smooth muscle via a voltage-sensitive calcium channel mediated by a pertussis toxin-sensitive G-protein-linked receptor.

Sarcoplasmic reticulum in vascular cells in hypertension and during proliferation

1. Multiple sarco(endo)plasmic reticulum Ca(2+)-ATPase (SERCA) and two types of sarcoplasmic reticulum Ca2+ channels, the ryanodine receptor and the inositol 1,4,5 triphosphate (IP3) receptor are expressed. The heterogeneity of the Ca2+ pumps and Ca2+ channels in vascular cells will be discussed. 2. An age-related change in expression of the SERCA isoforms is observed in smooth muscle cells. 3. The sarcoplasmic reticulum Ca(2+)-uptake rate and the level of SERCA 2 mRNA are different in thoracic than in abdominal aortas and in aortas from spontaneously hypertensive rats than from normotensive rats. 4. Proliferation of vascular smooth muscle cells is associated with major changes in intracellular Ca(2+)-handling mechanisms.

Role of Ca(2+) channels in NE-induced increase in [Ca(2+)](i) and tension in fetal and adult cerebral arteries

In vascular smooth muscle, elevation of agonist-induced intracellular Ca(2+) concentration ([Ca(2+)](i)) occurs via both Ca(2+) release from intracellular stores and Ca(2+) influx across the plasma membrane. In the cerebral vasculature of the fetus and adult the relative roles of these mechanisms have not been defined. To test the hypothesis that plasma membrane L-type and receptor-operated Ca(2+) channels play a key role in NE-induced vasoconstriction via alterations in plasma membrane Ca(2+) flux and that this may change with developmental age, we performed the following study. In main branch middle cerebral arteries (MCA) from near-term fetal ( approximately 140 days) and nonpregnant adult sheep, we quantified NE-induced responses of vascular tension and [Ca(2+)](i) (by use of fura 2) under standard conditions in response to several Ca(2+) channel blockers and in response to zero extracellular Ca(2+). In fetal and adult MCA, maximal NE-induced tensions (g) were 0.91 +/- 0.12 (n = 10) and 1.61 +/- 0.13 (n = 12), respectively. The pD(2) values for NE-induced tension were both 6.0 +/- 0.1, whereas the fetal and adult maximum responses (%K(max)) were 107 +/- 16 and 119 +/- 7, respectively. The fetal and adult pD(2) values for NE-induced increase of [Ca(2+)](i) were 6.2 +/- 0.1 and 6.4 +/- 0.1, respectively, whereas maximum [Ca(2+)](i) responses were 81 +/- 9 and 103 +/- 15% of K(max), respectively. After 10(-5) M NE-induced contraction, nifedipine resulted in dose-dependent decrease in vessel tone and [Ca(2+)](i) with pIC(50) values for fetal and adult tensions of 7.3 +/- 0.1 and 6.6 +/- 0.1, respectively (P < 0.01; n = 4 each), whereas pIC(50) for [Ca(2+)](i) responses were 7.2 +/- 0.1 and 6.9 +/- 0.1, respectively. The pIC(50) values for tension for diltiazem and verapamil were somewhat lower but showed a similar relationship. The receptor-operated Ca(2+) channel blocker 2-nitro-4 carboxyphenyl-N,N-diphenyl carbamate showed little effect on NE-induced vessel contractility or [Ca(2+)](i). In the absence of extracellular Ca(2+) for 2 min, 10(-5) M NE resulted in markedly attenuated responses of adult MCA tension and [Ca(2+)](i) to 39 +/- 7 and 73 +/- 8% of control values (n = 4). For fetal MCA, exposure to extracellular Ca(2+) concentration resulted in essentially no contractile or [Ca(2+)](i) response (n = 4). Similar blunting of NE-induced tension and [Ca(2+)](i) was seen in response to 10(-3) M lanthanum ion. These findings provide evidence to suggest that especially in fetal, but also in adult, ovine MCA, Ca(2+) flux via L-type calcium channels plays a key role in NE-induced contraction. In contrast, Ca(2+) flux via receptor-operated Ca(2+) channels is of less importance. This developmental difference in the role of cerebrovascular plasma membrane Ca(2+) channels may be an important association with increased Ca(2+) sensitivity of the fetal vessels.

Functional coupling of ryanodine receptors to KCa channels in smooth muscle cells from rat cerebral arteries

The relationship between Ca2+ release ("Ca2+ sparks") through ryanodine-sensitive Ca2+ release channels in the sarcoplasmic reticulum and KCa channels was examined in smooth muscle cells from rat cerebral arteries. Whole cell potassium currents at physiological membrane potentials (-40 mV) and intracellular Ca2+ were measured simultaneously, using the perforated patch clamp technique and a laser two-dimensional (x-y) scanning confocal microscope and the fluorescent Ca2+ indicator, fluo-3. Virtually all (96%) detectable Ca2+ sparks were associated with the activation of a spontaneous transient outward current (STOC) through KCa channels. A small number of sparks (5 of 128) were associated with currents smaller than 6 pA (mean amplitude, 4.7 pA, at -40 mV). Approximately 41% of STOCs occurred without a detectable Ca2+ spark. The amplitudes of the Ca2+ sparks correlated with the amplitudes of the STOCs (regression coefficient 0.8; P < 0.05). The half time of decay of Ca2+ sparks (56 ms) was longer than the associated STOCs (9 ms). The mean amplitude of the STOCs, which were associated with Ca2+ sparks, was 33 pA at -40 mV. The mean amplitude of the "sparkless" STOCs was smaller, 16 pA. The very significant increase in KCa channel open probability (>10(4)-fold) during a Ca2+ spark is consistent with local Ca2+ during a spark being in the order of 1-100 microM. Therefore, the increase in fractional fluorescence (F/Fo) measured during a Ca2+ spark (mean 2.04 F/Fo or approximately 310 nM Ca2+) appears to significantly underestimate the local Ca2+ that activates KCa channels. These results indicate that the majority of ryanodine receptors that cause Ca2+ sparks are functionally coupled to KCa channels in the surface membrane, providing direct support for the idea that Ca2+ sparks cause STOCs.

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

Local calcium transients ('Ca2+ sparks') are thought to be elementary Ca2+ signals in heart, skeletal and smooth muscle cells. Ca2+ sparks result from the opening of a single, or the coordinated opening of many, tightly clustered ryanodine receptor (RyR) channels in the sarcoplasmic reticulum (SR). In arterial smooth muscle, Ca2+ sparks appear to be involved in opposing the tonic contraction of the blood vessel. Intravascular pressure causes a graded membrane potential depolarization to approximately -40 mV, an elevation of arterial wall [Ca2+]i and contraction ('myogenic tone') of arteries. Ca2+ sparks activate calcium-sensitive K+ (KCa) channels in the sarcolemmal membrane to cause membrane hyperpolarization, which opposes the pressure induced depolarization. Thus, inhibition of Ca2+ sparks by ryanodine, or of KCa channels by iberiotoxin, leads to membrane depolarization, activation of L-type voltage-gated Ca2+ channels, and vasoconstriction. Conversely, activation of Ca2+ sparks can lead to vasodilation through activation of KCa channels. Our recent work is aimed at studying the properties and roles of Ca2+ sparks in the regulation of arterial smooth muscle function. The modulation of Ca2+ spark frequency and amplitude by membrane potential, cyclic nucleotides and protein kinase C will be explored. The role of local Ca2+ entry through voltage-dependent Ca2+ channels in the regulation of Ca2+ spark properties will also be examined. Finally, using functional evidence from cardiac myocytes, and histological evidence from smooth muscle, we shall explore whether Ca2+ channels, RyR channels, and KCa channels function as a coupled unit, through Ca2+ and voltage, to regulate arterial smooth muscle membrane potential and vascular tone.