Release and recycling of calcium by the sarcoplasmic reticulum in guinea-pig portal vein smooth muscle

Evolving mechanisms of vascular smooth muscle contraction highlight key targets in vascular disease

Vascular smooth muscle (VSM) plays an important role in the regulation of vascular function. Identifying the mechanisms of VSM contraction has been a major research goal in order to determine the causes of vascular dysfunction and exaggerated vasoconstriction in vascular disease. Major discoveries over several decades have helped to better understand the mechanisms of VSM contraction. Ca2+ has been established as a major regulator of VSM contraction, and its sources, cytosolic levels, homeostatic mechanisms and subcellular distribution have been defined. Biochemical studies have also suggested that stimulation of Gq protein-coupled membrane receptors activates phospholipase C and promotes the hydrolysis of membrane phospholipids into inositol 1,4,5-trisphosphate (IP3) and diacylglycerol (DAG). IP3 stimulates initial Ca2+ release from the sarcoplasmic reticulum, and is buttressed by Ca2+ influx through voltage-dependent, receptor-operated, transient receptor potential and store-operated channels. In order to prevent large increases in cytosolic Ca2+ concentration ([Ca2+]c), Ca2+ removal mechanisms promote Ca2+ extrusion via the plasmalemmal Ca2+ pump and Na+/Ca2+ exchanger, and Ca2+ uptake by the sarcoplasmic reticulum and mitochondria, and the coordinated activities of these Ca2+ handling mechanisms help to create subplasmalemmal Ca2+ domains. Threshold increases in [Ca2+]c form a Ca2+-calmodulin complex, which activates myosin light chain (MLC) kinase, and causes MLC phosphorylation, actin-myosin interaction, and VSM contraction. Dissociations in the relationships between [Ca2+]c, MLC phosphorylation, and force have suggested additional Ca2+ sensitization mechanisms. DAG activates protein kinase C (PKC) isoforms, which directly or indirectly via mitogen-activated protein kinase phosphorylate the actin-binding proteins calponin and caldesmon and thereby enhance the myofilaments force sensitivity to Ca2+. PKC-mediated phosphorylation of PKC-potentiated phosphatase inhibitor protein-17 (CPI-17), and RhoA-mediated activation of Rho-kinase (ROCK) inhibit MLC phosphatase and in turn increase MLC phosphorylation and VSM contraction. Abnormalities in the Ca2+ handling mechanisms and PKC and ROCK activity have been associated with vascular dysfunction in multiple vascular disorders. Modulators of [Ca2+]c, PKC and ROCK activity could be useful in mitigating the increased vasoconstriction associated with vascular disease.

Acidosis dilates brain parenchymal arterioles by conversion of calcium waves to sparks to activate BK channels

RATIONALE

Acidosis is a powerful vasodilator signal in the brain circulation. However, the mechanisms by which this response occurs are not well understood, particularly in the cerebral microcirculation. One important mechanism to dilate cerebral (pial) arteries is by activation of large-conductance, calcium-sensitive potassium (BK(Ca)) channels by local Ca(2+) signals (Ca(2+) sparks) through ryanodine receptors (RyRs). However, the role of this pathway in the brain microcirculation is not known.

OBJECTIVE

The objectives of this study were to determine the mechanism by which acidosis dilates brain parenchymal arterioles (PAs) and to elucidate the roles of RyRs and BK(Ca) channels in this response.

METHODS AND RESULTS

Internal diameter and vascular smooth muscle cell Ca(2+) signals were measured in isolated pressurized murine PAs, using imaging techniques. In physiological pH (7.4), vascular smooth muscle cells exhibited primarily RyR-dependent Ca(2+) waves. Reducing external pH from 7.4 to 7.0 in both normocapnic and hypercapnic conditions decreased Ca(2+) wave activity, and dramatically increased Ca(2+) spark activity. Acidic pH caused a dilation of PAs which was inhibited by about 60% by BK(Ca) channel or RyR blockers, in a nonadditive manner. Similarly, dilator responses to acidosis were reduced by nearly 60% in arterioles from BK(Ca) channel knockout mice. Dilations induced by acidic pH were unaltered by inhibitors of K(ATP) channels or nitric oxide synthase.

CONCLUSIONS

These results support the novel concept that acidification, by converting Ca(2+) waves to sparks, leads to the activation of BK(Ca) channels to induce dilation of cerebral PAs.

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.

A single luminally continuous sarcoplasmic reticulum with apparently separate Ca2+ stores in smooth muscle

Whether or not the sarcoplasmic reticulum (SR) is a continuous, interconnected network surrounding a single lumen or comprises multiple, separate Ca2+ pools was investigated in voltage-clamped single smooth muscle cells using local photolysis of caged compounds and Ca2+ imaging. The entire SR could be depleted or refilled from one small site via either inositol 1,4,5-trisphosphate receptors (IP3R) or ryanodine receptors (RyR) suggesting the SR is luminally continuous and that Ca2+ may diffuse freely throughout. Notwithstanding, regulation of the opening of RyR and IP3R, by the [Ca2+] within the SR, may create several apparent SR elements with various receptor arrangements. IP3R and RyR may appear to exist entirely on a single store, and there may seem to be additional SR elements that express either only RyR or only IP3R. The various SR receptor arrangements and apparently separate Ca2+ storage elements exist in a single luminally continuous SR entity.

Mechanisms of the inhibitory effects of a phenoxazine compound, 2-amino-4,4.ALPHA.-dihydro-4 .ALPHA.-7-dimethyl-3H-phenoxazine-3-one, on the contraction of the smooth muscle of the guinea pig taenia cecum

To elucidate the mechanisms involved in the relaxing effect of 2-amino-4, 4alpha-dihydro-4alpha-7-dimethyl-3H-phenoxazine-3-one (Phx-1) on smooth muscle, we investigated its effects on the contraction of both intact and skinned (cell membrane permeabilized) preparations from the guinea pig taenia cecum. In intact preparations, Phx-1 concentration-dependently suppressed the contraction induced by either acetylcholine (ACh) or high-K(+) with an IC(50) value estimated at around 100 muM. Similar inhibitory actions of Phx-1 on force were observed in intracellular Ca store depleted preparations. In cell membrane depolarized preparations in the absence of extracellular Ca, however, Phx-1 had little effect on either caffeine- or ACh-induced contractions. In skinned preparations, Phx-1 suppressed Ca(2+)-induced contractions at concentrations higher than 100 muM. These results suggest that inhibition of smooth muscle contraction by Phx-1 is due mainly to inhibition of Ca(2+)-influx, although Phx-1 also seems to have direct inhibitory effects on the activities of the contractile apparatus.

In vitro Pharmacological Characterization of Mitemcinal (GM-611), the First Acid-Resistant Non-Peptide Motilin Receptor Agonist, in Smooth Muscle of Rabbit Small Intestine

The pharmacological properties of mitemcinal (GM-611), the first acid-resistant non-peptide motilin agonist, were investigated in the smooth muscle of the rabbit small intestine and compared with porcine motilin (pMTL), erythromycin A (EMA) and its derivatives (EM-523, EM-574 and ABT-229). Mitemcinal, pMTL, EMA, EM-523, EM-574 and ABT-229 produced concentration-dependent contractions with approximately the same maximum contractions in the isolated rabbit duodenum longitudinal strips. The contractile response to mitemcinal or pMTL was competitively inhibited by a selective motilin antagonist, GM-109. The pA(2) values for GM-109 as an antagonist of mitemcinal and pMTL showed approximately the same values. However, the concentration-dependent contractile responses to mitemcinal or pMTL were not affected by pretreatment with atropine, tetrodotoxin, hexamethonium, naloxone or tropisetron. The removal of calcium ions from the medium and pretreatment with verapamil greatly suppressed the contractions induced by mitemcinal and pMTL. The contractile response to mitemcinal was not affected by preincubation in acidic solutions, while those of EM-523, EM-574 and ABT-229 were strongly diminished in the same condition. Mitemcinal as well as other motilin agonists displaced (125)I-pMTL bound to a homogenate of the rabbit duodenum muscle tissue. The displacement curves of all these compounds were parallel. These results indicate that mitemcinal is a selective and full motilin receptor agonist in the smooth muscle of the rabbit small intestine and that this agent has an excellent acid-resistant property.

Pharmacological modulation of sarcoplasmic reticulum function in smooth muscle

The sarco/endoplasmic reticulum (SR/ER) is the primary storage and release site of intracellular calcium (Ca2+) in many excitable cells. The SR is a tubular network, which in smooth muscle (SM) cells distributes close to cellular periphery (superficial SR) and in deeper aspects of the cell (deep SR). Recent attention has focused on the regulation of cell function by the superficial SR, which can act as a buffer and also as a regulator of membrane channels and transporters. Ca2+ is released from the SR via two types of ionic channels [ryanodine- and inositol 1,4,5-trisphosphate-gated], whereas accumulation from thecytoplasm occurs exclusively by an energy-dependent sarco-endoplasmic reticulum Ca2+-ATPase pump (SERCA). Within the SR, Ca2+ is bound to various storage proteins. Emerging evidence also suggests that the perinuclear portion of the SR may play an important role in nuclear transcription. In this review, we detail the pharmacology of agents that alter the functions of Ca2+ release channels and of SERCA. We describe their use and selectivity and indicate the concentrations used in investigating various SM preparations. Important aspects of cell regulation and excitation-contractile activity coupling in SM have been uncovered through the use of such activators and inhibitors of processes that determine SR function. Likewise, they were instrumental in the recent finding of an interaction of the SR with other cellular organelles such as mitochondria. Thus, an appreciation of the pharmacology and selectivity of agents that interfere with SR function in SM has greatly assisted in unveiling the multifaceted nature of the SR.

Organization of Ca2+ release units in excitable smooth muscle of the guinea-pig urinary bladder

Ca(2+) release from internal stores (sarcoplasmic reticulum or SR) in smooth muscles is initiated either via pharmaco-mechanical coupling due to the action of an agonist and involving IP3 receptors, or via excitation-contraction coupling, mostly involving L-type calcium channels in the plasmalemma (DHPRs), and ryanodine receptors (RyRs), or Ca(2+) release channels of the SR. This work focuses attention on the structural basis for the coupling between DHPRs and RyRs in phasic smooth muscle cells of the guinea-pig urinary bladder. Immunolabeling shows that two proteins of the SR: calsequestrin and the RyR, and one protein the plasmalemma, the L-type channel or DHPR, are colocalized with each other within numerous, peripherally located sites located within the caveolar domains. Electron microscopy images from thin sections and freeze-fracture replicas identify feet in small peripherally located SR vesicles containing calsequestrin and distinctive large particles clustered within small membrane areas. Both feet and particle clusters are located within caveolar domains. Correspondence between the location of feet and particle clusters and of RyR- and DHPR-positive foci allows the conclusion that calsequestrin, RyRs, and L-type Ca(2+) channels are associated with peripheral couplings, or Ca(2+) release units, constituting the key machinery involved in excitation-contraction coupling. Structural analogies between smooth and cardiac muscle excitation-contraction coupling complexes suggest a common basic mechanism of action.

Smooth muscle cells and interstitial cells of blood vessels

A rise in intracellular ionised calcium concentration ([Ca(2+)](i)) at sites adjacent to the contractile proteins is a primary signal for contraction in all types of muscles. Recent progress in the development of imaging techniques with special accent on the fluorescence confocal microscopy and new achievements in the synthesis of organelle- and ion-specific fluorochromes provide an experimental basis for study of the relationship between the structural organisation of the living smooth muscle myocyte and the features of calcium signalling at subcellular level. Applying fluorescent confocal microscopy and tight-seal recording of transmembrane ion currents to freshly isolated vascular myocytes we have demonstrated that: (1) Ca(2+) sparks originate from clustered opening of ryanodine receptors (RyRs) and build up a cell-wide increase in [Ca(2+)](i) upon myocyte excitation; (2) spontaneous Ca(2+) sparks occurred at the highest rate at certain preferred locations, frequent discharge sites (FDS), which are associated with a prominent portion of the sarcoplasmic reticulum (SR) located close to the cell membrane; (3) Ca(2+)-dependent K(+) and Cl(-) channels sense the local changes in [Ca(2+)](i) during a calcium spark and thereby couple changes in [Ca(2+)](i) within a microdomain to changes in the membrane potential, thus affecting excitability of the cell; (4) an intercommunication between RyRs and inositol trisphosphate receptors (IP(3)Rs) is one of the important determinants of intracellular calcium dynamics that, in turn, can modulate the cell membrane potential through differential targeting of calcium dependent membrane ion channels. Furthermore, using immunohystochemical approaches in combination with confocal imaging we identified non-contractile cells closely resembling interstitial cells (ICs) of Cajal (which are considered to be pacemaker cells in the gut) in the wall of portal vein and mesenteric artery. Using electron microscopy, tight-seal recording and fluorescence confocal imaging we obtained information on the morphology of ICs and their possible coupling to smooth muscle cells (SMCs), calcium signalling in ICs and their electrophysiological properties. The functions of these cells are not yet fully understood; in portal vein they may act as pacemakers driving the spontaneous activity of the muscle; in artery they may have other a yet unsuspected functions.

Endoplasmic reticulum of animal cells and its organization into structural and functional domains

The endoplasmic reticulum (ER) in animal cells is an extensive, morphologically continuous network of membrane tubules and flattened cisternae. The ER is a multifunctional organelle; the synthesis of membrane lipids, membrane and secretory proteins, and the regulation of intracellular calcium are prominent among its array of functions. Many of these functions are not homogeneously distributed throughout the ER but rather are confined to distinct ER subregions or domains. This review describes the structural and functional organization of the ER and highlights the dynamic properties of the ER network and the mechanisms that support the positioning of ER membranes within the cell. Furthermore, we outline processes involved in the establishment and maintenance of an anisotropic distribution of ER-resident proteins and, thus, in the organization of the ER into functionally and morphologically different subregions.

Phospholamban regulation of bladder contractility: evidence from gene-altered mouse models

1. Phospholamban (PLB) is an inhibitor of the sarcoplasmic reticulum (SR) Ca2+-ATPase (SERCA). Its presence and/or functional significance in contractility of bladder, a smooth muscle tissue particularly dependent on SR function, is unknown. We investigated this by measuring the effects of carbachol (CCh) on force and [Ca2+]i in bladder from mice in which the PLB gene was ablated (PLB-KO mice). In the PLB-KO bladder, the maximum increases in [Ca2+]i and force were significantly decreased (41.5 and 47.4 % of WT), and the EC50 values increased. 2. Inhibition of SERCA with cyclopiazonic acid (CPA) abolished these differences between WT and PLB-KO bladder, localizing the effects to the SR. 3. To determine whether these effects were specific to PLB, we generated mice with smooth-muscle-specific expression of PLB (PLB-SMOE mice), using the SMP8 alpha-actin promoter. Western blot analysis of PLB-SMOE mice showed approximately an eightfold overexpression of PLB while SERCA was downregulated 12-fold. 4. In PLB-SMOE bladders, in contrast, the response of [Ca2+]i and force to CCh was significantly increased and the EC50 values were decreased. CPA had little affect on the CCh-induced increases in [Ca2+]i and force in PLB-SMOE bladder. 5. These results show that alteration of the PLB:SERCA ratio can significantly modulate smooth muscle [Ca2+]i. Importantly, our data show that PLB can play a major role in modulation of bladder contractility.

Direct visualization of sarcoplasmic reticulum regions discharging Ca(2+)sparks in vascular myocytes

Localized Ca(2+)-release events, Ca(2+)sparks, have been suggested to be the 'elementary building blocks' of the calcium signalling system in all types of muscles. In striated muscles these occur at regular intervals along the fibre corresponding to the sarcomeric structures which do not exist in smooth muscle. We showed previously that in visceral and vascular myocytes Ca(2+)sparks occurred much more frequently at certain sites (frequent discharge sites [FDSs]). In this paper, we have related the position of FDSs to the distribution of the sarcoplasmic reticulum in the same living myocyte. The three-dimensional distribution of the SR in freshly isolated rabbit portal vein myocytes was visualized by means of high-resolution confocal imaging after staining with DiOC(6)and/or BODIPY TR-X ryanodine. Both fluorochromes revealed a similar staining pattern indicating a helical arrangement of well-developed superficial SR which occupied about 6% of the cell volume. Computing the frequency of spontaneous Ca(2+)sparks detected by means of fluo-4 fluorescence revealed that in about 70% of myocytes there was only one major FDS located on a prominent portion of superficial SR network usually within 1-2 microm of the nuclear envelope, although a few sparks occurred at other sites scattered generally in superficial locations throughout the cell. Polarized mitochondria were readily identified by accumulation of tetramethylrhodamine ethyl ester (TMRE). These were closely associated with the SR network in extra-nuclear regions. TMRE staining, however, failed to reveal any mitochondria near the FDS-related SR element. When observed, propagating [Ca(2+)](i)waves and associated myocyte contractions were initiated at FDSs. This study provide first insight into the three-dimensional arrangement of the SR in living smooth muscle cells and relates the peculiarity of the structural organization of the myocyte to the features of Ca(2+)signalling at subcellular level.

Contribution of sarcoplasmic reticulum calcium uptake and L-type calcium channels to altered vascular responsiveness in the aorta of renal hypertensive rats

This study examined whether alterations in intracellular or extracellular Ca2+ mobilization were related to differences in caffeine and phenylephrine (PHE)-induced contractions between two-kidney. one-clip hypertensive (2K-1C) and normotensive (2K) rat aortas. After depletion and reloading of intracellular Ca2+ stores, caffeine and PHE-induced contractions in Ca2+-free solution were increased in 2K-1C. Thapsigargin reduced the contraction to caffeine in 2K-1C and 2K with similar sensitivity. PHE-induced contraction in 1.6-mM Ca2+ solution was decreased in 2K-1C, and nifedipine was less effective in lowering this response. The responsiveness to extracellular Ca2+ was decreased in 2K-1C hypertensive rat aortas. Our results indicate an increased intracellular Ca2+ stores that are not related to alteration in Ca2+-ATPase function and a lower contribution of L-type channels to the contraction of 2K-1C aortas.

Endothelium-Independent Relaxation of Vascular Smooth Muscle by 17beta-Estradiol

BACKGROUND: Estrogens directly dilate arteries, and this acute relaxation of vascular smooth muscle (VSM) may contribute to the cardioprotective effect of this important hormone. However, the mechanism by which estrogens relax VSM is not clear. METHODS AND RESULTS: Based on observations in isolated smooth muscle cells, we hypothesized that 17beta-estradiol (E(2)) causes dilation through receptor-mediated activation of K(+) channels in VSM cells. To test this hypothesis, E(2)-relaxation was studied in arteries from male Sprague-Dawley rats. We observed that the estrogen receptor antagonist, tamoxifen (3 µmol) attenuated E(2) relaxation, suggesting that at least a portion of the relaxation depends on activation of E(2) receptors. The nitric oxide synthase inhibitor, Nomega-nitro-larginine (100 µmol) did not affect E(2) relaxation in either denuded or endothelium-intact arterial strips. Furthermore, inhibition of guanylyl cyclase with LY83583 (10 µmol) had no effect on the relaxation, suggesting that nitric oxide does not contribute to this relaxation. Vascular segments contracted with 90 mmol KCl to disrupt the K(+) gradient had a similar E(2) relaxation does not require K(+)-channel activation. Finally, E(2) pretreatment inhibited contraction of arterial segments depleted of intracellular calcium but in the presence of extracellular calcium. However, E(2) did not affect contraction of strips in calcium-free solution. CONCLUSIONS: These final experiments suggest that E(2) inhibits Ca(2+) influx but not intracellular calcium release. Together, these studies establish that E(2) causes receptor-mediated relaxation of peripheral resistance arteries through inhibition of calcium entry independent of nitric oxide production, guanylyl cyclase stimulation, and K(+)-channel activation.

Pharmacomechanical coupling: the role of calcium, G-proteins, kinases and phosphatases

The concept of pharmacomechanical coupling, introduced 30 years ago to account for physiological mechanisms that can regulate contraction of smooth muscle independently of the membrane potential, has since been transformed from a definition into what we now recognize as a complex of well-defined, molecular mechanisms. The release of Ca2+ from the SR by a chemical messenger, InsP3, is well known to be initiated not by depolarization, but by agonist-receptor interaction. Furthermore, this G-protein-coupled phosphatidylinositol cascade, one of many processes covered by the umbrella of pharmacomechanical coupling, is part of complex and general signal transduction mechanisms also operating in many non-muscle cells of diverse organisms. It is also clear that, although the major contractile regulatory mechanism of smooth muscle, phosphorylation/dephosphorylation of MLC20, is [Ca2+]-dependent, the activity of both the kinase and the phosphatase can also be modulated independently of [Ca2+]i. Sensitization to Ca2+ is attributed to inhibition of SMPP-1M, a process most likely dominated by activation of the monomeric GTP-binding protein RhoA that, in turn, activates Rho-kinase that phosphorylates the regulatory subunit of SMPP-1M and inhibits its myosin phosphatase activity. It is likely that the tonic phase of contraction activated by a variety of excitatory agonists is, at least in part, mediated by this Ca(2+)-sensitizing mechanism. Desensitization to Ca2+ can occur either through inhibitory phosphorylation of MLCK by other kinases or autophosphorylation and by activation of SMPP-1M by cyclic nucleotide-activated kinases, probably involving phosphorylation of a phosphatase activator. Based on our current understanding of the complexity of the many cross-talking signal transduction mechanisms that operate in cells, it is likely that, in the future, our current concepts will be refined, additional mechanisms of pharmacomechanical coupling will be recognized, and those contributing to the pathologenesis diseases, such as hypertension and asthma, will be identified.

Differences in contractile properties of anorectal smooth muscle and the effects of calcium channel blockade

BACKGROUND

Pharmacological manipulation of the anal sphincter is hampered by a lack of specificity. This study aimed to determine differences in the role of intracellular and extracellular calcium in the development of tone and agonist-induced contractions between internal anal sphincter (IAS) and rectal circular muscle which might allow targeted manipulation.

METHODS

Smooth muscle strips from the IAS and rectal circular muscle of 24 Large White pigs were mounted for isometric tension recording in a superfusion organ bath in the presence of different perfusates.

RESULTS

IAS developed tone and spontaneous activity that were abolished by nifedipine, which also reduced contractions to noradrenaline to 72 per cent of control values. Rectal smooth muscle developed spontaneous activity but no tone. Nifedipine abolished the activity and reduced contractions to carbachol to 17 per cent of control. Contractile activity was abolished in both tissues in calcium-free solution. Transient exposure to a high calcium concentration reloaded the stores, and the ability of agonists to release stored calcium was tested after 3 min in calcium-free solution. In IAS, noradrenaline contraction was 76 per cent of control and in rectal circular muscle carbachol contraction was 57 per cent of control. Store loading was prevented by nifedipine in rectal smooth muscle but not IAS. Cyclopiazonic acid reduced store filling in both tissues.

CONCLUSION

Agonist-induced contraction of IAS is largely due to release of stored calcium and L-type calcium channels are not needed for store filling. Rectal circular smooth muscle depends more on extracellular calcium and uses L-type calcium channels for agonist-induced contraction and store filling. These differences suggest that targeted manipulation may be possible in patients with anorectal disorders.

Myorelaxant activity of 2-t-butyl-4-methoxyphenol (BHA) in guinea pig gastric fundus

This study investigates the mechanism whereby the antioxidant 2-t-butyl-4-methoxyphenol (BHA) relaxes guinea pig gastric fundus smooth muscle. In circular smooth muscle strips, 10 microM cyclopiazonic acid, a specific inhibitor of sarcoplasmic reticulum Ca2+-ATPase, induced a prolonged rise in tension which depended on the presence of extracellular Ca2+. BHA (pIC50 = 5.83), sodium nitroprusside (6.85), isoproterenol (7.69) and nifedipine (8.02), but not 2,6-di-t-butyl-4-methoxyphenol (DTBHA) (up to 30 microM), relaxed muscle strips contracted with cyclopiazonic acid. Methyl-1,4-dihydro-2,6-dimethyl-3-nitro-4-(2-trifluoromethylphenyl)-pyri dine-5-carboxylate (Bay K 8644) (1 microM) antagonised the nifedipine- but not the BHA-induced relaxation. Nifedipine and isoproterenol (10 microM) caused a decrease in spontaneous tone, but did not counteract the subsequent rise in tension elicited by 10 microM cyclopiazonic acid. Conversely, 100 microM BHA and 100 microM sodium nitroprusside not only significantly reduced spontaneous tone but also markedly impaired the response of the muscles to cyclopiazonic acid. DTBHA failed to show either effect. When added to preparations completely relaxed by 100 microM BHA, 10 mM tetraethylammonium still elicited nifedipine-sensitive tonic and phasic contractions in the presence or absence of 10 microM cyclopiazonic acid. BHA and DTBHA inhibited, in a concentration-dependent manner, the Ca2+-promoted contraction of strips depolarised by 10 mM tetraethylammonium. The BHA antagonism showed a non-competitive profile while that of DTBHA was competitive. In muscle strips at rest, 10 microM BHA caused a significant increase in tissue cAMP concentration, leaving cGMP unmodified. To conclude, the myorelaxant action of BHA on gastric fundus smooth muscle appears to be mediated partly by an increase in cAMP levels and partly by inhibition of Ca2+ influx from the extracellular space.

Localization of ryanodine receptors in smooth muscle

The ryanodine receptor (RyR) in aortic and vas deferens smooth muscle was localized using immunofluorescence confocal microscopy and immunoelectron microscopy. Indirect immunofluorescent labeling of aortic smooth muscle with anti-RyR antibodies showed a patchy network-like staining pattern throughout the cell cytoplasm, excluding nuclei, in aortic smooth muscle and localized predominantly to the cell periphery in the vas deferens. This distribution is consistent with that of the sarcoplasmic reticulum (SR) network, as demonstrated by electron micrographs of osmium ferrocyanide-stained SR in the two smooth muscles. Immunoelectron microscopy of vas deferens smooth muscle showed anti-RyR antibodies localized to both the sparse central and predominant peripheral SR elements. We conclude that RyR-Ca2+-release channels are present in both the peripheral and central SR in aortic and vas deferens smooth muscle. This distribution is consistent with the possibility that both regions are release sites, as indicated by results of electron probe analysis, which show a decrease in the Ca2+ content of both peripheral and internal SR in stimulated smooth muscles. The complex distribution of inositol 1,4,5-trisphosphate and ryanodine receptors (present study) is compatible with their proposed roles as agonist-induced Ca2+-release channels and origins of Ca2+ sparks, Ca2+ oscillations, and Ca2+ waves.

Actions of 4-chloro-3-ethyl phenol on internal Ca2+ stores in vascular smooth muscle and endothelial cells

1. Recently, 4-chloro-3-ethyl phenol (CEP) has been shown to cause the release of internally stored Ca2+ apparently through ryanodine-sensitive Ca2+ channels, in fractionated skeletal muscle terminal cisternae and in a variety of non-excitable cell types. Its action on smooth muscle is unknown. In this study, we characterized the actions of CEP on vascular contraction in endothelium-denuded dog mesenteric artery. We also determined its ability to release Ca2+, by use of Ca2+ imaging techniques, on dog isolated mesenteric artery smooth muscle cells and on bovine cultured pulmonary artery endothelial cells. 2. After phenylephrine-(PE, 10 microM) sensitive Ca2+ stores were depleted by maximal PE stimulation in Ca2+-free medium, the action of CEP on refilling of the emptied PE stores was tested, by first pre-incubating the endothelium-denuded artery in CEP for 15 min before Ca2+ was restored for a 30 min refilling period. At the end of this period, Ca2+ and CEP were removed, and the arterial ring was tested again with PE to assess the degree of refilling of the internal Ca2+ store. 3. In a concentration-dependent manner (30, 100 and 300 microM), CEP significantly reduced the size of the post-refilling PE contraction (49.4, 28.9 and 5.7% of control, respectively) in Ca2+-free media. This suggests that Ca2+ levels are reduced in the internal stores by CEP treatment. CEP alone did not cause any contraction either in Ca2+-containing or Ca2+-free Krebs solution. 4. Restoring Ca2+ in the presence of PE caused a large contraction, which reflects PE-induced influx of extracellular Ca2+. The contraction of tissues pretreated with 300 microM CEP was significantly less compared with controls. However, tissues pretreated with 30 and 100 microM CEP were unaffected. Washout of CEP over 30 min produced complete recovery of responses to PE in Ca2+-free and Ca2+-containing medium suggesting a rapid reversal of CEP effects. 5. Concentration-response curves were constructed for PE, 5-hydroxytryptamine (5-HT) and K+ in the absence of and after 30 min pre-incubation with 30, 100 and 300 microM CEP. In all cases, CEP caused a concentration-dependent depression of the maximum response to PE (84.8, 43.4 and 11.6% of control), 5-HT (65.4, 25.7 and 6.9% of control) and K+ (77.6, 41.1 and 10.8% of control). 6. Some arterial rings were pre-incubated with ryanodine (30 microM) for 30 min before the construction of PE concentration-response curves. In Ca2+-free Krebs solution, ryanodine alone did not cause any contraction. However, 58% (11 out of 19) of the tissues tested with ryanodine developed contraction (6.9+/-1.2% of 100 mM K+ contraction, n=11) in the presence of external Ca2+. EC50 values for PE in ryanodine-treated tissues (1.7+/-0.25 microM, n=16) were not significantly different from controls (2.5+/-0.41 microM, n=22). Maximum contractions to PE (118.5+/-4.4% of 100 mM K+ contraction, n=16) were also unaffected by ryanodine when compared to controls (129+/-4.2%, n=23). 7. When fura-2 loaded smooth muscle cells (n=13) and endothelial cells (n=27) were imaged for Ca2+ distribution, it was observed that 100 and 300 microM CEP in Ca2+-free medium caused Ca2+ release in both cell types. Smooth muscle cells showed a small decrease in cell length. Addition of EGTA (5 mM) reversed the effect of CEP on intracellular Ca2+ to control values. 8. These data show, for the first time in vascular smooth muscle and endothelial cells, that CEP releases Ca2+ more rapidly than ryanodine. Unlike ryanodine, CEP caused no basal contraction but depressed contractions to PE, 5-HT and K+. The lack of basal contraction may result from altered responsiveness of the contractile system to intracellular Ca2+ elevation.

Vasoconstrictor effect of Cissus sicyoides on guinea-pig aortic rings

1. The effect of the aqueous extract of Cissus sicyoides (CS) on isolated guinea pig aortic rings was studied. CS contracts the smooth muscle of the aorta in a dose-response relation. 2. The extract of CS increases the norepinephrine contraction in normal calcium and in solutions without calcium. 3. Lanthanum inhibits the contraction induced by CS. 4. The vasoconstrictor effect of CS was increased in solutions without calcium or with low calcium, which is an inverse calcium-dependent contraction. 5. Prolonged exposure to calcium-free solution did not abolish CS contraction. These contractions can be elicited repeatedly even after 6 hr of continuous exposure to calcium-free solutions. 6. Caffeine reduces contractile response induced by CS in normal calcium, as well as in solutions without calcium. 7. Our results support the idea that the aqueous extract of CS acts at the membrane level, increasing the calcium entry through the membrane as well as acting on the internal calcium deposits, possibly on the sarcoplasmic reticulum.

Measurement of changes in sarcoplasmic reticulum [Ca2+] in rat tail artery with targeted apoaequorin delivered by an adenoviral vector

The physiologic relevance of Ca2+ release from the sarcoplasmic reticulum in arterial smooth muscle contraction is controversial. Therefore, we sought to measure changes in sarcoplasmic reticulum free [Ca2+] (i.e. [Ca2+]sr) in the intact rat tail artery. We exploited a novel technique to measure [Ca2+]sr with genetically targeted apoaequorin acting as a pseudo-luciferase rather than as classic aequorin. Intact rat tail arteries were infected with a replication deficient adenoviral vector (RAdER) containing the apoaequorin gene targeted to the sarcoplasmic reticulum. Addition of apoaequorin's substrate, coelenterazine, to the perfusate increased light production in a [Ca2+] dependent manner, consistent with apoaequorin action on coelenterazine. Within the limits of the photon counting system, imaging of infected rat tail artery segments revealed light production from the whole thickness of the vascular wall. Phenylephrine stimulation decreased apoaequorin generated light and induced a contraction. Washout of phenylephrine relaxed the tissues and increased light indicating refilling of the sarcoplasmic reticulum with Ca2+. Incubation in 10 microM cyclopiazonic acid, a SERCA inhibitor, did not alter apoaequorin generated light or induce a contraction. In the presence of cyclopiazonic acid, phenylephrine contractions were enhanced and apoaequorin generated light decreased further than that observed in the absence of cyclopiazonic acid. Cyclopiazonic acid also prevented the increase in apoaequorin generated light upon washout of phenylephrine, consistent with its inhibition of sarcoplasmic reticulum refilling. These results suggest that light production from targeted apoaequorin, delivered by a replication deficient adenovirus, is a valid measure of changes in [Ca2+]sr in the intact arterial wall. There appeared to be a correlation between Ca2+ release and contraction in these lightly loaded arteries.

The amount of acetylcholine mobilisable Ca2+ in single smooth muscle cells measured with the exogenous cytoplasmic Ca2+ buffer, Indo-1

Single smooth muscle cells from guinea pig urinary bladder were voltage clamped with patch electrodes containing 1 mM Indo-1. As Indo-1 entered the cell, delta[Ca2+]i in response to Ca2+ influx with ICa (1 s steps to -10 mV) was progressively decreased. delta F410 was used as a measure of the Ca2+ amount bound to Indo-1. Within less than 2 min after establishment of the whole-cell configuration, the fraction of Ca2+ entering the cell with ICa which binds to Indo-1 became constant, suggesting that Indo-1 completely overrides the endogenous Ca2+ buffers. Under these conditions, delta F410 was satisfactorily fitted with the time integral of ICa during 1 s long steps. Acetylcholine (ACh, 50 microM) was rapidly applied to Indo-1 loaded cells to induce IP3-induced Ca2+ release (IICR), which peaked within about 1 s. From delta F410 in response to ICa and ACh and from the time integral of ICa the amount of Ca2+ released during IICR was estimated to be 680 attomole (680 x 10(-18) mole), corresponding to 230 microM for 3 pl of accessible cytoplasmic volume.

Spontaneous transient outward currents in smooth muscle cells

Spontaneous transient outward currents (STOCs) lasting about 100 ms occur in single smooth muscle cells and represent the simultaneous opening of up to a hundred calcium-activated potassium (BK) channels. The recent observation of brief focal releases of sarcoplasmic reticulum (SR) calcium ('sparks') in smooth muscle cells has provided support for the original suggestion that STOCs arise due to the spontaneous releases of calcium from the SR close to the sarcolemma. However, it is possible that such releases occur in a region of close apposition of SR membrane and sarcolemma about 0.1 microns wide ('junctional space') in which case they would be detectable by endogenous calcium-sensitive molecules such as BK channels but, using present confocal microscopy technique, not by calcium-indicator dyes introduced into the cell; should calcium escape from the junctional space then it may be visualised as 'sparks' by the fluorescent emission from calcium-indicator dyes using confocal microscopy. Some STOCs seem too large to represent the effect of a single 'spark' and some form of calcium-induced calcium release or 'macrospark' may be involved in their generation. Depletion of calcium stores by caffeine, ryanodine, or by activation of receptors linked to the phospholipase C/inositol trisphosphate system abolishes STOCs. However, low concentrations of caffeine or inositol trisphosphate accelerate STOC discharge by an unknown mechanism and often decrease STOC size presumably by depleting store calcium; similar effects are produced by agents such as cyclopiazonic acid and thapsigargin which inhibit calcium storage mechanisms (largely the SR calcium pump).

The effects of cisapride on elemental content in neonatal mouse small intestine in vivo

The effects of cisapride on elemental content of enterocytes, smooth muscle cells and lamina propria in neonatal mouse jejunum were studied using electron probe X-ray microanalysis. One hour after injection with cisapride (1.7 or 0.017 mg/kg body weight i.p.), Cl content was significantly reduced in villus base, crypt and smooth muscle cells and Na content decreased in muscle cells. No changes were observed in Na or Cl within villus tip cells. Total Ca content did not change significantly in any cell type following treatment with cisapride. These results confirm that cisapride induces net Cl- secretion in neonatal mouse jejunum.

Relative contributions of extracellular Ca2+ and Ca2+ stores to smooth muscle contraction in arteries and arterioles of rat, guinea-pig, dog and rabbit

1. These studies describe the functional effects of modulation of the sarcoplasmic reticulum (SR) Ca2+ stores at three levels of the vasculature: (i) large arteries (rat and guinea-pig aorta); (ii) small resistance arteries (rat tail artery, rabbit mesenteric artery, dog mesenteric artery); and (iii) arterioles (guinea-pig submucosal arterioles of the small intestine). 2. All tissues responded to phenylephrine (PE; 10 mumol/L) with a transient contraction in Ca(2+)-free Krebs', reflecting Ca2+ release from PE-sensitive Ca2+ stores. After pretreatment with cyclopiazonic acid (CPA; 30 mumol/L) or thapsigargin (TSG; 1 mumol/L), putative SR Ca2+ pump inhibitors, the PE-induced contraction in a Ca(2+)-free medium was significantly inhibited in arterial tissues at all levels of the vasculature. Similarly, ryanodine (RYA; 30 mumol/L), an agonist that enhances Ca2+ release from the SR, also reduced the PE contraction in a Ca(2+)-free solution. 3. CPA or TSG alone in the presence of extracellular Ca2+, caused marked and sustained contraction in the rat and guinea-pig aorta and marked but transient or no contraction in the resistance arteries. In the rat and guinea-pig aorta, RYA caused a slowly developing tension. Little increase in basal tension was produced by RYA in resistance arteries and arterioles. 4. The findings show that an agonist-releasable Ca2+ pool is present at all levels of the vasculature that is independent of the size of the vessels and suggest that under normal physiological conditions there is an intimate balance between the roles of the plasma membrane and of the SR in the maintenance of vascular contractility. It appears that the role of the SR diminishes as the arteries become smaller, while Ca2+ fluxes across the plasma membrane predominates.

Dissociation of subsarcolemmal from global cytosolic [Ca2+] in myocytes from guinea-pig coronary artery

1. Changes in cytosolic Ca2+ concentration (delta[Ca2+]c) were measured by global indo-1 fluorescence and compared with changes in subsarcolemmal Ca2+ concentration (delta[Ca2+]sl) indicated by Ca(2+)-activated K+ currents (IK(Ca)). 2. At -50 mV holding potential, 10mM caffeine increased both IK(Ca) and [Ca2+]c without measurable delay. While IK(Ca) peaked within 0.3 +/- 0.16 s (mean +/- S.D.) and decayed to 50% within 0.4 +/- 0.2 s, delta[Ca2+]c peaked within 1.5 +/- 0.5 s and decayed to 50% within 5.2 +/- 1.0 s. The different time courses support the idea that [Ca2+]sl and [Ca2+]c deviate. 3. When 10 mM caffeine was applied 20 s after an initial 2 s caffeine application, IK(Ca) was suppressed to 22 +/- 5% and delta [Ca2+]c to 40 +/- 4%. During the following 1 min caffeine-free period, IK(Ca) recovered to 61 +/- 7% while delta [Ca2+]c remained at 40 +/- 3%. The differences between IK(Ca) and delta[Ca2+]c suggest that Ca2+ deprivation and Ca2+ refilling is faster in peripheral than in central sarcoplasmic reticulum (SR). 4. During the loading period of indo-1, a spontaneous delta[Ca2+]c of 30-80 nM appeared both at -50 mV and at more positive potentials. The amplitude of spontaneous delta[Ca2+]c increased with the amplitude, the frequency or the fusion of spontaneous transient outward currents (STOCs). 5. Block of sarcolemmal Ca2+ fluxes by 1 mM La3+ increased [Ca2+]c by 250 +/- 100 nM and suppressed the spontaneous delta[Ca2+]c. However, La3+ did not significantly retard the rate of decay of STOCs which may therefore be limited by Ca2+ diffusion into the cytosol and not by Ca2+ extrusion. 6. The dissociation of IK(Ca) (or STOCs) and delta[Ca2+]c may indicate a Ca2+ concentration gradient during Ca2+ release directed from the sarcolemma towards the centre of the cell, which later reverses direction.

Intracellular Ca2+ release in flow-induced contraction of venous smooth muscle

We designed the present study to determine whether Ca2+ release from intracellular stores contributes to flow-induced contraction. We carried out experiments on segments of rabbit facial vein under isometric conditions. Intraluminal flow by perfusion of physiological salt solution (10 to 80 microL/min) caused contraction in this vessel, which was significantly inhibited by (1) 30-minute pretreatment with 10 mumol/L ryanodine, the sarcoplasmic reticulum Ca2+ channel opener, and (2) 30-minute pretreatment with concomitant application of 20 mmol/L caffeine and 1 mumol/L cyclopiazonic acid in Ca(2+)-free medium to deplete the sarcoplasmic reticulum. In comparison, contraction initiated by 300 nmol/L histamine was significantly attenuated by the same interventions. K+ (25 mmol/L)-induced contraction was unaffected by ryanodine but was reduced after depletion of the sarcoplasmic reticulum. The phospholipase C inhibitor 2-nitro-4-carboxyphenyl-N,N-diphenylcarbamate (10 mumol/L) inhibited contractions induced by flow and histamine but not by K+. These findings indicate that Ca2+ release from intracellular stores, presumably via the phosphatidylinositol pathway, contributes to flow- and histamine- but not raised K(+)-induced contractions in this vessel.

Major difference between rat and guinea-pig ureter in the ability of agonists and caffeine to release Ca2+ and influence force

1. We have investigated the internal Ca2+ store and its ability to affect contraction by simultaneously measuring force and Ca2+ in the ureter from guinea-pig and rat. Both species responded in a similar manner to electrical stimulation and depolarization with high-K+, generating plateau-type action potentials and increasing intracellular calcium ([Ca2+]i) and force. 2. In the guinea-pig, carbachol had no effect on [Ca2+]i and force in the resting ureter. In contrast, resting rat ureter always responded with a large [Ca2+]i rise and maintained force to carbachol in Ca(2+)-containing solution, and in Ca(2+)-free solution it showed a transient increase in [Ca2+]i and force. This Ca2+ release and force development was also present in both polarized and high-K(+)-depolarized preparations and was insensitive to nifedipine, suggesting the presence of a receptor-coupled pathway of Ca2+ release in rat ureter. 3. Caffeine was able to produce a release of Ca2+ from the internal store of guinea-pig ureter and elicit contraction. However, rat ureter failed to respond to caffeine. In the presence of La3+, the caffeine response in the guinea-pig ureter and carbachol response in the rat ureter, elicited in Ca(2+)-free solutions, were always increased and prolonged and could be repeatedly evoked, suggesting similarity in Ca2+ uptake behaviour of the store in both species. 4. Ryanodine blocked the caffeine responses of the guinea-pig ureter elicited both in Ca(2+)-containing and Ca(2+)-free solutions, both in the absence and presence of La3+. However, ryanodine failed to prevent the rat ureter responding to carbachol, suggesting that carbachol was releasing Ca2+ from a ryanodine-insensitive channel in the sarcoplasmic reticulum (SR). 5. Cyclopiazonic acid, which inhibits the SR Ca(2+)-ATPase, abolished the effects of both caffeine and carbachol in Ca(2+)-free solutions in guinea-pig and rat, respectively. 6. We conclude that there is a major difference in the mechanisms of Ca2+ release in the internal Ca2+ store of smooth muscle from guinea-pig and rat ureter. The data suggest that the guinea-pig store is purely a calcium-induced calcium release (CICR)-type store and that the rat store is a pure receptor-operated Ca2+ store.

Regenerative caffeine-induced responses in native rabbit aortic endothelial cells

1. Single native aortic endothelial cells obtained by enzymatic dispersion of the rabbit aortic endothelium were held under voltage clamp using patch pipette and whole-cell membrane currents were measured. In parallel experiments performed on cells from the same batches, the free internal calcium concentration, [Ca2+]i, in the cell was estimated by use of the Ca(2+)-sensitive fluorescent dye, fura-2. 2. Caffeine (20 mM) applied to the cell evoked an outward current and an initial peak in [Ca2+]i followed by a lower sustained rise (plateau). Ca(2+)-free, EGTA-containing solution applied outside the cells did not reduce these responses. 3. Following caffeine stimulation there was a biphasic rising phase of outward current both in the presence and absence of extracellular Ca2+. 4. Application of graded doses of caffeine revealed all-or-none type responses of both the outward current and the rise in [Ca2+]i. 5. Preincubation with lower doses of caffeine reduced the magnitude of both the outward current and the [Ca2+]i transient evoked by 20 mM caffeine. 6. Tetraethylammonium (3 mM) applied to the bathing solution blocked unitary and spontaneous transient outward currents (STOCs) stimulated by Ca(2+)-free solution, but only reduced the outward current evoked by caffeine (20 mM). 7. In conclusion, our results reveal the all-or-none nature of Ca2+ release from the endoplasmic reticulum (ER) in native aortic endothelial cells. Lower concentrations of caffeine (0.4-0.5 mM) may deplete intracellular Ca2+ stores. Extracellular Ca2+ is not necessary for maintaining the activity of spontaneous and caffeine-induced outward currents in native aortic endothelial cells. Spontaneous outward currents are believed to represent the sporadic release of calcium from store sites independent of both extracellular Ca2+ and the caffeine-sensitive Ca2+ stores which stimulate the outward current.

17 beta-Estradiol inhibits Ca2+ influx and Ca2+ release induced by thromboxane A2 in porcine coronary artery

BACKGROUND

We wished to investigate the possible mechanism of the protective effect of estrogen replacement on coronary atherosclerosis observed in postmenopausal women.

METHODS AND RESULTS

Cytosolic Ca2+ concentration ([Ca2+]i) and contraction were measured simultaneously in fura 2-loaded porcine coronary arterial strips stimulated by the thromboxane A2 analogue U46619 and high-K+ depolarization in the presence and absence of 17 beta-estradiol. Pretreatment with 17 beta-estradiol (30 nmol/L to 30 mumol/L) inhibited the sustained elevation of [Ca2+]i and the sustained contraction induced by 300 nmol/L U46619. Higher concentrations of 17 beta-estradiol (1 to 100 mumol/L) also inhibited the U46619-induced transient increase in [Ca2+]i and contraction in the absence of extracellular Ca2+. In the strips precontracted by 90 mmol/L K+, 17 beta-estradiol (30 mumol/L) inhibited the increases in [Ca2+]i and contraction to resting levels. In contrast, 30 mumol/L 17 beta-estradiol only partially inhibited the U46619-induced sustained contraction, despite complete inhibition of the sustained increase in [Ca2+]i. Verapamil (10 mumol/L) also strongly inhibited the sustained increase in [Ca2+]i induced by 300 nmol/L U46619, with a partial inhibition of the U46619-induced sustained contraction. A subsequent addition of 30 mumol/L 17 beta-estradiol did not show an additional inhibitory effect on either the [Ca2+]i or the tension after the addition of verapamil. 17 beta-Estradiol (10 mumol/L) also inhibited the increase in [Ca2+]i and the contraction induced by cumulative addition of Ca2+ in the strips pretreated with 90 mmol/L K+. However, 17 beta-estradiol did not change the slope of the [Ca2+]i-tension curves. 17 beta-Estradiol (10 mumol/L) had no effect on the levels of cAMP and cGMP in the coronary strips.

CONCLUSIONS

17 beta-Estradiol inhibits the contraction of coronary vascular smooth muscle mainly inhibiting Ca2+ influx without changing Ca2+ sensitivity of contractile elements. The Ca2+ channel blocker-like action of 17 beta-estradiol may explain at least a part of the antiatherosclerotic effect of estrogen.

Efficacy of peak Ca2+ currents (ICa) as trigger of sarcoplasmic reticulum Ca2+ release in myocytes from the guinea-pig coronary artery

1. Increments in cytosolic Ca2+ concentration (delta[Ca2+]c) were measured in single smooth muscle cells from guinea-pig coronary artery together with the density of peak Ca2+ currents (ICa) in response to clamp steps from -50 to 0 mV. The comparison of depolarization- with caffeine-induced delta[Ca2+]c was used to define the efficacy by which ICa can trigger Ca2+ release from the sarcoplasmic reticulum (SR). 2. At 2.5 mM extracellular calcium concentration ([Ca2+]o), depolarization induced a rapid rise of delta[Ca2+]c followed by a slow creep. Peak [Ca2+]c occurred within ca 30 s and could be followed by an undershoot and a second rise in [Ca2+]c. The creep was blocked by ryanodine but was insensitive to block of InsP3 receptors with heparin. The creep was not observed in Cs(+)-filled cells. After disappearance of the creep, a tonic delta[Ca2+]c became unmasked. 3. At 2.5 mM [Ca2+]o, peak ICa was -0.80 +/- 0.17 microA cm-2. delta[Ca2+] peaked at the end of the 6 s pulse at 202 +/- 98 nM while caffeine-induced delta[Ca2+]c peaked at 1330 +/- 410 nM. The ratio of depolarization- to caffeine-induced delta[Ca2+]c was 10 +/- 6%. 4. In media containing 10 mM [Ca2+]o plus 1 microM Bay K 8644, peak ICa was -2.6 +/- 1.1 microA cm-2 and delta[Ca2+]c peaked within 2.5 s at 451 +/- 194 nM. Paired measurements yielded the ratio of depolarization- to caffeine induced delta[Ca2+]c as 30 +/- 10%. Depolarization-induced delta[Ca2+]c was nearly blocked by caffeine and reduced by ryanodine to 30%, suggesting the contribution of Ca2+ release from caffeine- and ryanodine-sensitive Ca2+ stores. 5. Trypsin (1 mg ml-1) in the electrode solution (10 mM [Ca2+]o plus 1 microM Bay K 8644) increased peak ICa up to 12.5 microA cm-2. ICa induced a delta[Ca2+]c of 990 +/- 210 nM and was accompanied by a 'hump' of IK,Ca. When applied briefly after peak delta[Ca2+]c, caffeine increased [Ca2+]c only moderately. The results suggest that a peak ICa can trigger a synchronized whole-cell Ca2+ release only if ICa is strongly augmented. 6. Amplitude and rate of rise of delta[Ca2+]c were graded by test step potentials along a bell-shaped voltage-dependent curve, similar to that of L-type ICa. Steps to +80 mV induced no delta[Ca2+]c when the electrode solution contained 10 mM Na+. However, with 150 mM intrapipette Na+, pulses to +80 mV induced delta[Ca2+]c.(ABSTRACT TRUNCATED AT 400 WORDS)

Phosphatidylinositol hydrolysis is involved in production of Ca(2+)-dependent currents, but not non-selective cation currents, by muscarine in chromaffin cells

Whether phosphatidylinositol hydrolysis and a subsequent Ca2+ mobilization are responsible for muscarine-induced transient outward currents (IO) and non-selective cation currents (INS) in the guinea-pig chromaffin cell was investigated using the perforated patch method. IO, but not INS, failed to be reproduced in Ca(2+)-free solution and was markedly reduced by prior exposure to caffeine under Ca(2+)-free conditions or by addition to normal solution of cyclopiazonic acid (CPA), a Ca2+ ATPase inhibitor. Application of CPA in Ca(2+)-free solution, however, suppressed INS by about 50% in 73% of the cells tested. Bath application of 1.5 mM neomycin, a phospholipase C inhibitor, induced the time-dependent decline of IO with near abolition at 20 min or less, whereas it produced a time-independent decrease of INS and an inwardly rectifying K+ current. INS in the presence or absence of neomycin was well fitted to rectangular hyperbolas with the same ED50 of 2.17 microM, but with a 33% smaller maximum amplitude in the former, indicating a non-competitive inhibition by neomycin. We conclude that, while phosphatidylinositol hydrolysis mediates the production of IO, it does not mediate that of INS by muscarine.

Ryanodine-sensitive intracellular Ca2+ stores in isolated rabbit penile erectile tissue

Norepinephrine release from adrenergic nerve terminals leads to a rise in intracellular Ca2+, which promotes penile smooth muscle contraction and detumescence. Ca2+ sources are the extracellular space and sarcoplasmic Ca2+ stores. To elucidate the role of intracellular stores strips from rabbit erectile tissue were investigated in an organ bath study. Contractions were elicited by phenylephrine (PE) and electrical stimulation. Incubation in Ca(2+)-free solution as well as exposure to nifedipine did not abolish electrical or PE-induced contraction. Ryanodine (10(-5) mol/l), a functional blocker of sarcoplasmic Ca2+ channels, significantly reduced PE response. In the presence of caffeine (10(-3) mol/l) the effect was significantly enhanced. Addition of nifedipine nearly abolished the contraction. These results provide evidence for intracellular Ca2+ pools in cavernosal tissue and indicate that the alpha 1-adrenoceptor-induced contraction requires the opening of voltage-gated Ca2+ channels and the release of Ca2+ from intracellular stores.

Immunogold localization of inositol 1,4,5-trisphosphate receptors and characterization of ultrastructural features of the sarcoplasmic reticulum in phasic and tonic smooth muscle

Although agonist stimulation leads to an increase in inositol 1,4,5-trisphosphate (InsP3) and decreased calcium in peripherally and centrally located sarcoplasmic reticulum in smooth muscle, the distribution of InsP3 receptors is unknown. InsP3 receptor and the calcium binding protein, calsequestrin were localized by immunolabelling in a tonic and a phasic smooth muscle. InsP3 receptor labelling was predominantly localized at the cell periphery, where most of the sarcoplasmic reticulum is localized in vas deferens (phasic muscle). Elements of central sarcoplasmic reticulum, where present, were also labelled. Distribution of calsequestrin in vas deferens was similar to that of the InsP3 receptor. In aorta (tonic muscle) the InsP3 receptor labelling was proportional to sarcoplasmic reticulum distribution: predominantly central. No labelling of sections or immunoblots was observed with the anti-calsequestrin antibody in aorta. InsP3 and caffeine, but not cyclic ADP-ribose, released intracellular Ca2+ in permeabilized vas deferens and aorta. The ultrastructure of the sarcoplasmic reticulum, investigated in stereo views of semi-thick and thin sections of osmium ferricyanide stained tissue, is shown to have several distinctive features, such as fenestrated sheets (single or in stacks), as well as numerous regions of continuity between central and peripheral sarcoplasmic reticulum, suggesting a single compartment within the smooth muscle cell. Regions of the sarcoplasmic reticulum were closely apposed to and often ensheathed mitochondria. We conclude that InsP3 receptors are present in both the central and the peripheral sarcoplasmic reticulum of tonic and phasic smooth muscle, consistent with electron probe analysis results showing calcium release from both regions.

Effects of cyclopiazonic acid on [Ca2+]i and contraction in rat urinary bladder smooth muscle

Cyclopiazonic acid (CPA) has been reported to inhibit the Ca(2+)-ATPase of the sarcoplasmic reticulum (SR) in skeletal and smooth muscle. In the present study the effect of CPA on [Ca2+]i and force in rat urinary bladder smooth muscle was examined. The fluorescent Ca2+ indicator Fura-2 was used to monitor intracellular Ca2+, simultaneously with isometric force production. Addition of CPA to unstimulated muscles bathed in 2.5 mM Ca2+ containing Krebs solution resulted in a significant and sustained increase in [Ca2+]i from 99 +/- 7 to 273 +/- 51 nM. This increase in [Ca2+]i was dependent upon the presence of extracellular Ca2+ since when CPA was added to muscles in Ca(2+)-free media it produced only a small, transient increase in [Ca2+]i that was not sustained. Peak force levels produced by transmural stimulation, carbachol and high KCl solution were not altered by the presence of CPA, however, the increase in [Ca2+]i associated with these contractions was larger when CPA was present. In response to transmural stimulation, the times taken for both force and [Ca2+]i to rise to 50% of their peak values were attenuated in the presence of CPA. Conversely, there was no effect of CPA on the times taken for force or [Ca2+]i to fall to 50% of their stimulated values upon the cessation of stimulation. Under control conditions both carbachol and high KCl could initiate transient increases in [Ca2+]i and force in the absence of extracellular Ca2+. In the presence of CPA, the response to carbachol was virtually completely inhibited, however, the response to high KCl was only partially inhibited. The ability of CPA to inhibit the carbachol response in Ca(2+)-free media suggests that this response is due to release of Ca2+ from the SR. The incomplete inhibition of the response to KCl indicates other Ca2+ storage sites may also be mobilised by sarcolemmal depolarisation. Although the mechanism whereby CPA induces a large, sustained rise in [Ca2+]i remains unknown, the data lend support to the suggestion that depletion of intracellular Ca2+ storage sites may activate a Ca2+ entry pathway across the sarcolemma.

Comparative localization of inositol 1,4,5-trisphosphate and ryanodine receptors in intestinal smooth muscle: an analytical subfractionation study

[3H]Ins(1,4,5)P3- and [3H]ryanodine-binding sites were characterized in membrane fractions from guinea-pig intestinal smooth muscle (longitudinal layer) and their subcellular localization was investigated by analytical cell-fractionation techniques. Fractions collected at low centrifugal fields (N and M fractions) contained predominantly low-affinity [3H]Ins(1,4,5)P3-binding sites (KD 80 nM), whereas microsomal (P) fractions contained only high-affinity binding sites (KD 5 nM). Total sedimentable high-affinity binding sites of [3H]Ins(1,4,5)P3 were 9-10-fold more numerous than those of [3H]ryanodine. Both high-affinity binding sites were purified in microsomal fractions, and their sub-microsomal distribution patterns after isopycnic density-gradient centrifugation were similar to those of presumed endoplasmic reticulum (ER) constituents, indicating that Ins(1,4,5)P3 and ryanodine receptors were localized primarily in ER and probably associated with rough as well as smooth ER. However, the stoichiometric ratio of Ins(1,4,5)P3 to ryanodine receptors was distinctly higher in high-density RNA-rich subfractions than in low-density RNA-poor subfractions, suggesting that Ins(1,4,5)P3 receptors were somewhat concentrated in the ribosome-coated portions of ER. The low overall stoichiometric ratio of ryanodine to Ins(1,4,5)P3 receptors in intestinal smooth muscle (1:9-10) might explain, at least partly, the existence of a Ca(2+)-storage compartment devoid of ryanodine-sensitive Ca2+ channels, but equipped with Ins(1,4,5)P3-sensitive channels, in saponin-permeabilized smooth-muscle cells [Iino, Kobayashi and Endo (1988) Biochem. Biophys. Res. Commun. 152, 417-422].

New transmitters and new targets in the autonomic nervous system

Several recent findings have made research into the autonomic nervous system even more exciting, such as the revelation that nitric oxide is a major neurotransmitter, the delineation of the physiological roles for purines and vasoactive intestinal peptide, and the discovery that the interstitial cells of Cajal are major target cells for enteric innervation. Nitric oxide is probably the major neurotransmitter evoking inhibitory junction potentials in smooth muscle. ATP is a mediator of non-adrenergic non-cholinergic enteric innervation, as well as being a fast neurotransmitter in peripheral and autonomic neuro-neuronal synapses. The interactions between enteric nerves and both immune cells and interstitial cells of Cajal (as pacemaker cells of gut smooth muscle) are forcing a rethink of many aspects of gut physiology.

Effect of 2,3-butanedione monoxime on smooth-muscle contraction of guinea-pig portal vein

Effects of 2,3-butanedione-2-monoxime (BDM) on the contraction of intact and skinned smooth muscles from guinea-pig portal vein were examined. In intact preparations loaded with fura-2, 5-10 mM BDM markedly suppressed Ca2+ transients and force developments induced by 154 mM potassium and by phenylephrine (0.1 mM). On the other hand, in Ca(2+)-free depolarizing solution, BDM did not suppress phenylephrine (0.1 mM)-induced Ca2+ transient and force development. In skinned preparations obtained with Staphylococcus aureus alpha-toxin treatment, BDM did not markedly affect active force development. The above results indicate that BDM suppresses contraction of the portal vein mainly by the inhibition of voltage-dependent cytosolic Ca2+ transients. An additional result suggests that BDM suppresses the force-enhancing effect of alpha 1-adrenergic agents on the contractile elements.

Intracellular Ca2+ signaling induced by vasopressin, ATP, and epidermal growth factor in epithelial LLC-PK1 cells

The free calcium concentrations in nucleus ([Ca2+]n) and in cytoplasm ([Ca2+]c) of cultured renal LLC-PK1 epithelial cells were estimated by confocal laser microscopy. No difference between the resting mean [Ca2+]n and [Ca2+]c was found. During stimulation with maximal effective concentrations of arginine vasopressin (AVP) or the purinergic agonist ATP, the transient Ca2+ rise was followed mostly by a decline to basal levels. A differential rise could be observed when the increase in [Ca2+]n attained higher values than [Ca2+]c. In 50-60% of the cells, epidermal growth factor (EGF) also induced a transient Ca2+ rise, and a differential increase ([Ca2+]n > [Ca2+]c) was found. The G protein-linked stimuli AVP and ATP were however quantitatively much more efficacious at stimulating the [Ca2+]n and [Ca2+]c increases than was EGF. To investigate whether AVP, ATP, and EGF released Ca2+ from distinct or overlapping stores, the agonists were sequentially added. AVP and ATP applied after EGF in Ca(2+)-free medium elicited an increase in [Ca2+]n and [Ca2+]c that was not significantly lower than the release of Ca2+ in control cells without EGF prestimulation. Similarly, the amplitude of the Ca2+ responses attained by EGF in cells prestimulated by ATP or AVP was comparable to the response in naive cells. Neither EGF, ATP, nor AVP evoked a Ca2+ signal after thapsigargin treatment, indicating that the intracellular Ca2+ pools stimulated by all these agonists are part of the thapsigargin-sensitive Ca2+ pools. In contrast, when ATP was applied after AVP in Ca(2+)-containing as well as in Ca(2+)-free solutions, the Ca2+ transients were lower as compared with the response without preincubation. No differential rise could be found in Ca(2+)-free conditions. An explanation could be the use of different phospholipase C isozymes by the different receptor types, which possibly gives rise to the mobilization of different Ca2+ pools.

Role of intracellular Ca2+ stores in smooth muscle contractions of the guinea pig vas deferens

Guinea pig vas deferens was used as an animal model for alpha-1 adrenoceptor (alpha 1-receptor) mediated contractions in human hyperplastic prostatic tissue. The selective alpha 1-receptor agonist, phenylephrine (PE), induced fully reversible, dose-dependent contractions antagonized by increasing concentrations of the alpha 1-receptor blockers prazosin (1-100 nM) and YM 617 (0.1-10 nM). Removal of extracellular Ca2+ reduced PE-evoked contractions in a time-dependent manner. Nifedipine (1-1000 nM), a blocker of voltage-dependent L-type Ca2+ channels (VDCC), inhibited the PE-induced response by up to 65%. Removal of extracellular Ca2+ abolished the alpha 1-agonist reactivity in a time-dependent fashion. To elucidate the participation of intracellular Ca2+ stores in alpha 1-receptor-mediated contractions, the tissue was pretreated with ryanodine (10 microM) or thapsigargin (0.1 microM), established inhibitors of Ca2+ release from intracellular pools. Both substances reduced the PE contractions by up to 80%. Nifedipine suppressed the remaining contractions completely. This provides evidence that Ca2+ influx through VDCC and Ca2+ release from intracellular stores contribute to alpha 1-receptor-mediated contractions in the guinea pig vas deferens and may be important in obstructive benign prostatic hyperplasia.

The effect of caffeine on the contractile response of the rabbit urinary bladder to field stimulation

1. Smooth muscle contraction is mediated by an increase in the concentration of cytoplasmic free calcium. 2. Low concentrations of caffeine can induce transient contraction of smooth muscle through the release of intracellular calcium from intracellular storage sites. 3. The current study determined the effect of caffeine (0.4-100 microM) on the response of the in vitro whole bladder preparation to field stimulation (FS) at 0.6, 1.8, and 5.4 mM extracellular calcium. 4. The following parameters were determined: basal pressure (BP), peak pressure response (PP), maximal rate of pressure generation (PR), maximal rate of bladder emptying (RE), and percent of volume emptied (VE). 5. The results demonstrated that in the absence of caffeine, BP, PP, and PR were increased progressively as a function of both frequency of stimulation and the concentration of calcium. 6. In addition, caffeine significantly increased the rate of pressure generation at 0.6 and 1.8 mm calcium.

Effect of TMB-8 on alpha-adrenoceptor agonist and KC1 induced-contractions in isolated rabbit aorta

1. TMB-8 (10(-6) M-10(-4) M) depresses the contractile effect of the selective alpha 1-adrenoceptor agonists methoxamine and phenylephrine in the isolated rabbit aorta. 2. TMB-8 also depresses contractions evoked by 80 mM KCl in this tissue when used at similar concentrations. 3. The calcium antagonist nifedipine potentiates the inhibitory effect of TMB-8 on the alpha 1-contractions. 4. In preparations mounted in Ca-free solution containing 0.5 mM EGTA, 10(-4) M TMB-8 markedly depressed the contractions caused by both alpha 1-adrenoceptor agonists. 5. The Ca2+ agonist BAY K 8644 (10(-6) M) partially prevented the inhibitory effect of TMB-8 on 80 mM KCl contractions.

The endoplasmic-sarcoplasmic reticulum of smooth muscle: immunocytochemistry of vas deferens fibers reveals specialized subcompartments differently equipped for the control of Ca2+ homeostasis

Cryosection immunofluorescence and immunogold labeling with antibodies against specific markers were used in rat vas deferens smooth muscle fibers to reveal the molecular arrangement of the endomembrane system (referred to variously in the text as ER or sarcoplasmic reticulum [SR]; S-ER or ER/SR) known to participate in the control of Ca2+ homeostasis. The lumenal ER chaperon, immunoglobulin binding protein (BiP), as well as protein disulfide isomerase, and calreticulin, a Ca2+ binding protein expressed by most eukaryotic cells, appeared to be evenly distributed throughout the entire system (i.e., within [a] the nuclear envelope and the few rough-surfaced cisternae clustered near the nucleus; [b] single elements scattered around in the contractile cytoplasm; and [c] numerous, heterogeneous, mainly smooth-surfaced elements concentrated in the peripheral cytoplasm, part of which is in close apposition to the plasmalemma). All other structures, including nuclei, mitochondria, Golgi complex, and surface caveolae were unlabeled. An even distribution throughout the endomembrane system appeared also for the proteins recognized by anti-ER membrane antibodies. In contrast, calsequestrin (the protein that in striated muscles is believed to be the main actor of the rapidly exchanging Ca2+ storage within the lumen of the sarcoplasmic reticulum) was found preferentially clustered at discrete lumenal sites, most often within peripheral smooth-surfaced elements of moderate electron density. Within these elements dual labeling revealed intermixing of calsequestrin with the other lumenal ER proteins. Moreover, the calsequestrin-rich elements were enriched also in the receptor for inositol 1,4,5-trisphosphate, the second messenger that induces Ca2+ release from intracellular stores. These results document the previously hypothesized molecular heterogeneity of the smooth muscle endomembrane system, particularly in relation to the rapid storage and release of Ca2+.

A model of calcium regulation in smooth muscle cell

A model of smooth muscle cell is proposed to describe the mechanisms by which intracellular calcium (Cai) is regulated when the cell is stimulated by noradrenaline (NA)/agonist (Ag) or depolarized. The intracellular Ca2+ store is comprised of two calcium pools: A-store which is sensitive to inositol 1,4,5 trisphosphate (IP3) and C-store, insensitive to IP3 but sensitive to Cai. The A-store is refilled by Ca2+ from C-store and reuptake of Cai. The uptake rate is dependent on the degree of filling in the A-store. The C-store, activated by Cai, is replenished by extracellular Ca2+ (Cao). IP3 generated by NA or Ag changes the A-store permeability, discharging its Ca2+ into the cytosol. Depolarization activates Ca2+ current through the L-type channels. The following events are simulated and compared with experimental observations: (a) Cai transient stimulated by high K+; (b) Cai transient and Ca2+ efflux induced by shortlasting and longlasting NA in the absence of Cao; (c) effect of lanthanum on NA-induced Cai and Ca2+ efflux; and (d) effects of nifedipine, ryanodine, BAY K 8644 and cyclopiazonic acid on Cai.