Variable potency of nitrergic-nitrovasodilator relaxations of the mouse anococcygeus against different forms of induced tone

The role of STIM1 and SOCE in smooth muscle contractility

Contraction is a central feature for skeletal, cardiac and smooth muscle; this unique feature is largely dependent on calcium (Ca²⁺) signaling and therefore maintenance of internal Ca²⁺ stores. Stromal interaction molecule 1 (STIM1) is a single-pass transmembrane protein that functions as a Ca²⁺ sensor for the activation store-operated calcium channels (SOCCs) on the plasma membrane in response to depleted internal sarco(endo)plasmic (S/ER) reticulum Ca²⁺ stores. STIM1 was initially characterized in non-excitable cells; however, evidence from both animal models and human mutations suggests a role for STIM1 in modulating Ca²⁺ homeostasis in excitable tissues as well. STIM1-dependent SOCE is particularly important in tissues undergoing sustained contraction, leading us to believe STIM1 may play a role in smooth muscle contraction. To date, the role of STIM1 in smooth muscle is unknown. In this review, we provide a brief overview of the role of STIM1-dependent SOCE in striated muscle and build off that knowledge to investigate whether STIM1 contributes to smooth muscle contractility. We conclude by discussing the translational implications of targeting STIM1 in the treatment of smooth muscle disorders.

Influence of cinaciguat on gastrointestinal motility in apo-sGC mice

BACKGROUND

Cinaciguat (BAY 58-2667), an NO- and heme-independent sGC activator, was shown to be more effective when the heme-group of sGC is oxidized in vascular tissue. In apo-sGC mice (sGCβ1 (His105Phe) knockin) both sGC isoforms (sGCα1 β1 and sGCα2 β1 ) are heme-deficient and can no longer be activated by NO; these mice, showing decreased gastrointestinal nitrergic relaxation and decreased gastric emptying, can be considered as a model to study the consequence of heme-oxidation in sGC. Our aim was to compare the influence of cinaciguat, on in vitro muscle tone of gastrointestinal tissues, and on gastric emptying in WT and apo-sGC mice.

METHODS

Gastrointestinal smooth muscle strips were mounted in organ baths for isometric force recording and cGMP levels were determined by enzyme immunoassay. Protein levels of sGC subunits were assessed by immunoblotting. Gastric emptying was determined by phenol red recovery.

KEY RESULTS

Although protein levels of the sGC subunits were lower in gastrointestinal tissues of apo-sGC mice, cinaciguat induced concentration-dependent relaxations and increased cGMP levels in apo-sGC fundus and colon to a similar or greater extent than in WT mice. The sGC inhibitor ODQ increased cinaciguat-induced relaxations and cGMP levels in WT fundus and colon. In apo-sGC antrum, pylorus and jejunum, cinaciguat was not able to induce relaxations. Cinaciguat did not improve delayed gastric emptying in apo-sGC mice.

CONCLUSIONS & INFERENCES

Cinaciguat relaxes the fundus and colon efficiently when sGC is in the heme-free condition; the non-effect of cinaciguat in pylorus explains its inability to improve the delayed gastric emptying in apo-sGC mice.

Store-operated calcium entry in vascular smooth muscle

In non-excitable cells, activation of G-protein-coupled phospholipase C (PLC)-linked receptors causes the release of Ca(2+) from intracellular stores, which is followed by transmembrane Ca(2+) entry. This Ca(2+) entry underlies a small and sustained phase of the cellular [Ca(2+)](i) increases and is important for several cellular functions including gene expression, secretion and cell proliferation. This form of transmembrane Ca(2+) entry is supported by agonist-activated Ca(2+)-permeable ion channels that are activated by store depletion and is referred to as store-operated Ca(2+) entry (SOCE) and represents a major pathway for agonist-induced Ca(2+) entry. In excitable cells such as smooth muscle cells, Ca(2+) entry mechanisms responsible for sustained cellular activation are normally considered to be mediated via either voltage-operated or receptor-operated Ca(2+) channels. Although SOCE occurs following agonist activation of smooth muscle, this was thought to be more important in replenishing Ca(2+) stores rather than acting as a source of activator Ca(2+) for the contractile process. This review summarizes our current knowledge of SOCE as a regulator of vascular smooth muscle tone and discusses its possible role in the cardiovascular function and disease. We propose a possible hypothesis for its activation and suggest that SOCE may represent a novel target for pharmacological therapeutic intervention.

Involvement of non-selective Ca2+ channels in the contraction induced by alkalinization of rat anococcygeus muscle cells

Intracellular pH is a modulator of cellular functions such as smooth muscle contraction. Changes in cytosolic Ca(2+) concentration ([Ca(2+)](c)) associated with contraction are brought about by Ca(2+) influx and release from the sarcoplasmic reticulum, and alterations in the intracellular pH can affect both processes. In this work, therefore, we have investigated the Ca(2+) influx pathway that contributes to the contraction induced by the alkalinizing agent NH(4)Cl in the rat anococcygeus smooth muscle. For this purpose, we measured the isometric tension in muscle preparations, and [Ca(2+)](c) was measured on isolated cells loaded with 5 micromol/l FURA2/AM by using the ratio 340/380 nm. NH(4)Cl (10 mmol/l) induced a larger increase in [Ca(2+)](c) (100%) when compared with the [Ca(2+)](c) increase induced by 0.1 micromol/l phenylephrine (57.0+/-12.3% n=4). Incubation of the muscle preparations for 1 min in Ca(2+)-free medium reduced the contractions induced by 10 mmol/l NH(4)Cl to 11.5+/-5.1% (n=5), when compared with the contractions induced in 2.5 mmol/l Ca(2+) solution (100%). After 3 min in Ca(2+) free medium, contractions stimulated with NH(4)Cl were almost abolished (0.6+/-0.4%, n=5). In the same way, incubation with 10 micromol/l 1-[beta-[3[(4-methoxyphenyl)propoxyl]-4-methoxy-phenetyl]-1H-imidazole hydrochloride (SKF96365), a non-selective Ca(2+) channels, reduced the contractions stimulated with NH(4)Cl to 47.6+/-6.7% (n=7). On the other hand, 1 micromol/l verapamil, a voltage-operated Ca(2+) channel blocker and 0.05 micromol/l calphostin C, a protein kinase-C inhibitor, did not alter the contractions induced by NH(4)Cl. On isolated cells, [Ca(2+)](c) was reduced to 72.2+/-1.7% (n=4) by 10 micromol/l SKF96365. Taken together, our results suggest that NH(4)Cl induces contraction of rat anococcygeus smooth muscle cells, as well as [Ca(2+)](c) increase due to Ca(2+) influx through non-selective Ca(2+) channels.

Calcium signaling phenomena in heart diseases: a perspective

Ca(2+) is a major intracellular messenger and nature has evolved multiple mechanisms to regulate free intracellular (Ca(2+))(i) level in situ. The Ca(2+) signal inducing contraction in cardiac muscle originates from two sources. Ca(2+) enters the cell through voltage dependent Ca(2+) channels. This Ca(2+) binds to and activates Ca(2+) release channels (ryanodine receptors) of the sarcoplasmic reticulum (SR) through a Ca(2+) induced Ca(2+) release (CICR) process. Entry of Ca(2+) with each contraction requires an equal amount of Ca(2+) extrusion within a single heartbeat to maintain Ca(2+) homeostasis and to ensure relaxation. Cardiac Ca(2+) extrusion mechanisms are mainly contributed by Na(+)/Ca(2+) exchanger and ATP dependent Ca(2+) pump (Ca(2+)-ATPase). These transport systems are important determinants of (Ca(2+))(i) level and cardiac contractility. Altered intracellular Ca(2+) handling importantly contributes to impaired contractility in heart failure. Chronic hyperactivity of the beta-adrenergic signaling pathway results in PKA-hyperphosphorylation of the cardiac RyR/intracellular Ca(2+) release channels. Numerous signaling molecules have been implicated in the development of hypertrophy and failure, including the beta-adrenergic receptor, protein kinase C, Gq, and the down stream effectors such as mitogen activated protein kinases pathways, and the Ca(2+) regulated phosphatase calcineurin. A number of signaling pathways have now been identified that may be key regulators of changes in myocardial structure and function in response to mutations in structural components of the cardiomyocytes. Myocardial structure and signal transduction are now merging into a common field of research that will lead to a more complete understanding of the molecular mechanisms that underlie heart diseases. Recent progress in molecular cardiology makes it possible to envision a new therapeutic approach to heart failure (HF), targeting key molecules involved in intracellular Ca(2+) handling such as RyR, SERCA2a, and PLN. Controlling these molecular functions by different agents have been found to be beneficial in some experimental conditions.

Biphasic neurogenic vasodilatation in the bovine intraocular long posterior ciliary artery: involvement of nitric oxide and an additional unidentified neurotransmitter

We have investigated the neurogenic factors inducing relaxation in the intraocular segment of the bovine long posterior ciliary artery. In precontracted vessels, electrical field stimulation (EFS, 0.5-128 Hz, 10 s trains) in the presence of guanethidine (30 microM) evoked biphasic relaxation: optimal relaxation for the first and second components occurred at 10 and 50 s, respectively. The first component, but not the second, was abolished by L-NAME (100 microM) or ODQ (3 microM). Relaxation to exogenous CGRP (0.1-300 nM) was inhibited by the CGRP antagonist, CGRP(8-37) (1-5 microM), but neither component of neurogenic relaxation was affected. Preincubation with the sensory nerve excitotoxin, capsaicin (1 microM), had no effect on either the first or second components of neurogenic relaxation. Substance P (0.1 nM-0.1 microM) induced relaxation, but rapid and complete desensitisation occurred within minutes. Neither desensitisation to substance P (0.1 microM) nor incubation with the NK(1) antagonist, L-733,060 (0.3 microM), had any effect on the first or second components of neurogenic relaxation.VIP (0.1 nM-0.3 microM) induced relaxation and this was followed by substantial desensitisation. Neither desensitisation to VIP (0.6 microM) nor treatment with the protease, alpha-chymotrypsin (10 U ml(-1)), had any effect on the first or second components of neurogenic relaxation. The results indicate that nitric oxide mediates the first component of neurogenic relaxation in the bovine intraocular ciliary artery. The neurotransmitter mediating the second component remains to be determined but is unlikely to be CGRP, substance P or VIP.

Nitrergic relaxation in rat gastric fundus: influence of mechanism of induced tone and possible role of sarcoplasmic/endoplasmic reticulum Ca2+ ATPase

The aim of this study was to investigate the influence of the mechanism of induced tone and the role of sarcoplasmic/endoplasmic reticulum Ca2+ ATPase (SERCA) in nitrergic relaxation of rat gastric fundus. Prostaglandin F(2alpha) (PGF(2alpha)), thapsigargin (TSG) and cyclopiazonic acid (CPA) were used in concentrations that induced a similar contraction (20 g force/g tissue). Nifedipine (3 x 10(-7) M) completely relaxed PGF(2alpha)-contracted tissues and relaxed tissues contracted by TSG and CPA by 20 +/- 6% and 56 +/- 12% respectively; contraction induced by the three contractile agents was fully reversed by a general Ca2+ entry blocker 1-[2-(4-methoxyphenyl)-2-[3-(4-metoxyphenyl)propoxy]ethyl-1H-imidazole HCl (SKF 96365; 10(-5) M). In the presence of nifedipine (3 x 10(-7) M) or verapamil (10(-5) M), PGF(2alpha) and CPA-induced contractions were still approximately 50% relaxed by SKF 96365. This suggests that contractions induced by PGF(2alpha) are related to Ca2+ entry through L-type voltage-operated Ca2+ channels and that contractions by TSG are mainly related to Ca2+ entry through store-operated Ca2+ channels. Relaxant responses to exogenous nitric oxide (NO), to endogenous NO released by electrical field stimulation, and to vasoactive intestinal polypeptide (VIP) were studied in tissues contracted by TSG and CPA and compared to responses in tissues contracted by PGF(2alpha). Responses to exogenous and endogenous NO were greatly attenuated in TSG-contracted tissues, but not in CPA-contracted tissues. When contraction was induced by CPA in the presence of nifedipine or verapamil, relaxations to exogenous and endogenous NO were also significantly reduced. Relaxation induced by VIP was reduced in tissues contracted by either TSG or CPA in the presence of nifedipine or verapamil. These results suggest that the ability of the nitrergic neurotransmitter to induce relaxation of rat gastric fundus is influenced by the mechanism used to induce tone and are indicative for a role for SERCA in nitrergic relaxation. However, activation of SERCA appears to not be unique for nitrergic relaxation, but might also be used by VIP, a co-transmitter of NO in this tissue.

Receptor-independent activation of Rho-kinase-mediated calcium sensitisation in smooth muscle

1. The aim of this work was to determine whether Rho-kinase-mediated calcium sensitisation contributes to contractions of the mouse anococcygeus smooth muscle and, if so, whether the process was activated by receptor-dependent or receptor-independent mechanisms. 2. The Rho-kinase inhibitor Y27632 produced concentration-dependent decreases in tone raised by either the muscarinic receptor agonist carbachol (CCh), or the sarco-endoplasmic reticulum calcium ATPase inhibitor thapsigargin (Tg) (EC(50) values against CCh and Tg of 8.4+/-3.3 (n=6) and 6.1+/-2.1 (n=7) micro M, respectively). Pretreatment of tissues with Y27632 also inhibited contractions produced by 65 mM external potassium (69+/-7% (n=4) inhibition using 10 micro M Y27632). Y27632 had no effect on contractions produced by the inhibitor of smooth muscle myosin light-chain phosphatase, calyculin-A. 3. In beta-escin-permeabilised preparations, both CCh and Tg produced significant increases in tone over-and-above that produced by a combination of calcium (1 micro M) and GTP (100 micro M). These responses to CCh and Tg were inhibited by Y27632 (10 micro M). 4. Western blot analysis of fractionated tissue samples probed for RhoA immunoreactivity, indicated that both CCh and Tg were able to induce translocation of RhoA from the cytosol to the membrane. 5. These findings suggest that Rho-kinase-mediated calcium sensitisation is activated by both receptor-dependent and receptor-independent mechanisms in the mouse anococcygeus.

Store-operated Ca2+-permeable non-selective cation channels in smooth muscle cells

Over twenty years ago it was shown that depletion of the intracellular Ca2+ store in smooth muscle triggered a Ca2+ influx mechanism. The purpose of this review it to describe recent electrophysiological data which indicate that Ca2+ influx occurs through discrete ion channels in the plasmalemma of smooth muscle cells. The effect of external Ca2+ on the amplitude and reversal potential of whole-cell and single channel currents suggests that there are at least two, and probably more, distinct store-operated channels (SOCs) which have markedly different permeabilities to Ca2+ ions. Two activation mechanisms have been identified which involve Ca2+ influx factor and protein kinase C (PKC) activation via diacylglycerol. In addition, in rabbit portal vein cells there is evidence that stimulation of alpha-adrenoceptors can stimulate SOC opening via PKC in a store-independent manner. There is at present little knowledge on the molecular identity of SOCs but it has been proposed that TRPC1 may be a component of the functional channel. We also summarise the data showing that SOCs may be involved in contraction and cell proliferation of smooth muscle. Finally, we highlight the similarities and differences of SOCs and receptor-operated cation channels that are present in native rabbit portal vein myocytes.

Effect of butanolic fraction of Desmodium adscendens on the anococcygeus of the rat

The chemical composition of plants can vary according to factors such as soil and time of collection. Desmodium adscendens (Sw.) D.C. var. adscendens (Papillionaceae) is a plant employed in the treatment of asthma in Ghana, Africa. Studies have shown that butanolic extract inhibits contraction of the ileum and trachea in guinea pigs. In Mato Grosso, this plant is used only in the treatment of ovarian inflammation. The objective of this work was to verify if the plant found in Mato Grosso also relaxes smooth muscle and to understand better its action. The cumulative application of the butanolic fraction relaxed the contraction maintained in the isolated anococcygeus of a rat, induced by high potassium, but not that induced by phenylephrine. Relaxation was not altered by methylene blue. The butanolic fraction reduced in a concentration-dependent way the maximum response of concentration-response curve to calcium in the anococcygeus muscle. The results suggest that the butanolic fraction acts, at least partly, through the blockade of voltage-sensitive Ca+2 channels.

Mechanical and electrophysiological effects of endothelin-1 on guinea-pig isolated lower oesophageal sphincter circular smooth muscle

1. The effects of endothelin-1 (ET-1) on guinea-pig lower oesophageal sphincter (LOS) circular smooth muscle were investigated by using intracellular microelectrodes and isometric tension recording techniques. 2. ET-1 produced biphasic mechanical responses; an initial transient relaxation followed by a sustained contraction. The initial relaxation was not inhibited by either tetrodotoxin (TTX, 1 microM) or L-N(G)-nitroarginine (L-NOARG, 100 microM). The sustained contraction was greatly attenuated by nifedipine (1 microM). 3. ET-1 (1 - 30 nM) induced a concentration-dependent hyperpolarisation that was unaffected by TTX or L-NOARG. The ET(A) receptor antagonist, BQ123 (0.3 microM) abolished the ET-1-induced hyperpolarisation, whereas the ET(B) receptor antagonist, BQ788 (0.3 microM) had no detectable effect. Sarafotoxin S6c (10 nM) did not change the membrane potential. 4. The ET-1-induced hyperpolarisation was abolished by apamin (0.1 microM). Interestingly, apamin abolished the ET-1-induced transient relaxation but potentiated the sustained contraction. 5. In Ca(2+)-free Krebs solution, the ET-1-induced hyperpolarisation was greatly attenuated and returned to the control value when the tissue was reperfused with Krebs solution containing Ca(2+). The ET-1-induced hyperpolarisation was insensitive to nifedipine but was attenuated by SK&F 96365 (1 - [beta-[3-(4 - methoxy - phenyl)propoxy] - 4 - methoxyphenethyl] - 1H-imidazole hydrochloride, 50 microM), an inhibitor of receptor-mediated Ca(2+) entry. The residual component of the ET-1-induced hyperpolarisation was sensitive to thapsigargin (1 microM). 6. These results demonstrate that, in guinea-pig LOS circular smooth muscle, ET-1 hyperpolarizes the membrane by activating apamin-sensitive K(+) channels, mainly as a result of receptor-mediated Ca(2+) entry and partly by Ca(2+) release from intracellular stores. The hyperpolarisation triggers the initial transient relaxation, which acts to oppose the sustained contraction.

The developing relationship between receptor-operated and store-operated calcium channels in smooth muscle

Contraction of smooth muscle is initiated, and to a lesser extent maintained, by a rise in the concentration of free calcium in the cell cytoplasm ([Ca(2+)](i)). This activator calcium can originate from two intimately linked sources--the extracellular space and intracellular stores, most notably the sarcoplasmic reticulum. Smooth muscle contraction activated by excitatory neurotransmitters or hormones usually involves a combination of calcium release and calcium entry. The latter occurs through a variety of calcium permeable ion channels in the sarcolemma membrane. The best-characterized calcium entry pathway utilizes voltage-operated calcium channels (VOCCs). However, also present are several types of calcium-permeable channels which are non-voltage-gated, including the so-called receptor-operated calcium channels (ROCCs), activated by agonists acting on a range of G-protein-coupled receptors, and store-operated calcium channels (SOCCs), activated by depletion of the calcium stores within the sarcoplasmic reticulum. In this article we will review the electrophysiological, functional and pharmacological properties of ROCCs and SOCCs in smooth muscle and highlight emerging evidence that suggests that the two channel types may be closely related, being formed from proteins of the Transient Receptor Potential Channel (TRPC) family.

Biology of the anococcygeus muscle

The anococcygeus is a smooth muscle tissue of the urogenital tract which, in the male, runs on to form the retractor penis. The motor innervation is classically sympathetic with noradrenaline as transmitter, but the relaxant parasympathetic transmitter has only recently been identified as nitric oxide. Indeed, the anococcygeus has provided an extremely useful model with which to probe the mechanisms underlying this novel nitrergic system, including the importance of physiological antioxidants in maintaining the potency of nitric oxide as a neurotransmitter. The cellular mechanisms of contraction and relaxation are slowly being clarified, with particular interest in the contribution of capacitative calcium entry and the guanylyl cyclase/cyclic GMP system. Many questions remain unanswered, however, including the precise physiological role of the muscle, the identity of substances released from subcellular vesicles of nitrergic nerves, the unusual sensitivity of the tissue to certain peptides (oxytocin and urotensin II), and the nature of store-operated channels through which calcium enters the cell to maintain contraction.

Selective inhibition of thapsigargin-induced contraction and capacitative calcium entry in mouse anococcygeus by trifluoromethylphenylimidazole (TRIM)

This study examined the effects of trifluoromethylphenylimidazole (TRIM) on tone, and calcium entry, in mouse anococcygeus stimulated by either thapsigargin (Tg; 100 nM) which activates capacitative calcium entry (CCE), or high K (60 mM) which activates voltage-operated calcium channels. TRIM (1 - 333 microM) produced concentration-related relaxation of Tg-induced tone (EC(50), 42 microM) but was much less effective against high K. In single smooth muscle cells loaded with FURA-2, TRIM reduced the increase in fluorescence ratio produced by Tg but had no effect on that produced by high K. The relaxations of Tg-induced tone, and reduction in fluorescence ratio, were obtained in the presence of L-N(G)-nitroarginine and were thus independent of nitric oxide synthase inhibition; further, TRIM had no discernible effect on nitrergic responses. TRIM provides a novel drug for the selective inhibition of CCE and a template for the development of more potent inhibitors.

Inhibition of capacitative calcium entry is not obligatory for relaxation of the mouse anococcygeus by the NO/cyclic GMP signalling pathway

1. The object of this study was to determine whether inhibition of capacitative calcium entry is essential for relaxation of the mouse anococcygeus via the NO/cyclic GMP signalling pathway. 2. In intact muscles, thapsigargin (Tg; 100 nM)-induced tone was relaxed by NO, sodium nitroprusside (SNP), 8-Br-cyclic GMP, and nitrergic field stimulation. The relaxations were similar in magnitude to those observed against carbachol (50 microM) tone and, with the exception of those to 8-Br-cyclic GMP, were reduced by the soluble guanylyl cyclase inhibitor 1H-[1,2,4]oxodiazolo[4,3-a]quinoxalin-1-one (ODQ, 5 microM). 3. In single smooth muscle cells, loaded with Fura-2, both carbachol and Tg produced sustained elevations in cytoplasmic calcium levels ([Ca2+]i). SNP inhibited the rise in [Ca2+]i produced by carbachol, an effect attenuated by ODQ. In contrast, neither SNP nor 8-Br-cyclic GMP reduced the elevated [Ca2+]i associated with Tg. 4. In beta-escin skinned preparations, NO had no effect on tone induced by calcium (1 microM in the presence of 100 microM GTP). Carbachol and Tg produced further increases in calcium/GTP-induced tone and, in both cases, this additional tone was relaxed by NO and 8-Br-cyclic GMP. 5. The results support the hypothesis that the NO/cyclic GMP pathway inhibits capacitative calcium entry by refilling the internal stores, since reduction in [Ca2+]i was not observed in the presence of Tg. However, as muscle relaxation was still observed, impairment of capacitative calcium entry cannot be considered obligatory for relaxation. Results from skinned tissues suggest that inhibition of calcium sensitization processes, perhaps associated with store-depletion, may be an important mechanism of NO/cyclic GMP-induced relaxation.

Investigation of the interaction between nitric oxide and vasoactive intestinal polypeptide in the guinea-pig gastric fundus

The interaction between nitric oxide (NO) and vasoactive intestinal polypeptide (VIP) was investigated in isolated circular smooth muscle cells and strips of the guinea-pig gastric fundus. VIP induced a concentration-dependent inhibition of carbachol-induced contraction in smooth muscle cells with a maximum at 10(-6) M. The relaxation by 10(-6) M VIP was inhibited for 79.1+/-5.8% (mean+/-s.e. mean) by the NO-synthase (NOS) inhibitor L-N(G)-nitroarginine (L-NOARG; 10(-4) M) in a L-arginine reversible way. Also the inducible NOS (iNOS) selective inhibitor N-(3-(acetaminomethyl)-benzyl)acetamide (1400 W; 10(-6) M) inhibited the VIP-induced relaxation, but its inhibitory effect was not reversed by L-arginine. When cells were incubated with the guanylyl cyclase inhibitor 1H-(1,2,4)oxadiazolo(4,3-a)quinoxalin-1-one (ODQ, 10(-6) M), the protein kinase A-inhibitor (R)-p-cyclic adenosine-3', 5'-monophosphothioate ((R)-p-cAMPS, 10(-6) M) and the glucocorticoid dexamethasone (10(-5) M), the relaxant effect of VIP was decreased by respectively 80.9+/-7.6, 77.0+/-11.6 and 87.1+/-4.5%. In circular smooth muscle strips of the guinea-pig gastric fundus, the VIP (10(-9) - 10(-7) M)-induced relaxations were not significantly influenced by 10(-4) M L-NOARG, 10(-6) M 1400 W, 10(-6) M ODQ and 10(-5) M dexamethasone. These results suggest that iNOS, possibly induced by the procedure to prepare the smooth muscle cells, is involved in the relaxant effect of VIP in isolated smooth muscle cells but not in smooth muscle strips of the guinea-pig gastric fundus. This study illustrates the importance of the experimental method when studying the influence of NOS inhibitors on the relaxation induced by VIP in gastrointestinal smooth muscle preparations.

Correlation between store-operated cation current and capacitative Ca2+ influx in smooth muscle cells from mouse anococcygeus

In mouse anococcygeus cells, simultaneous measurements of membrane currents and changes in intracellular Ca2+ were obtained using "perforated-patch" whole-cell recordings and Fura-2 microfluorimetry. Carbachol (50 microM) and cyclopiazonic acid (10 microM) produced a biphasic inward current; a transient Ca2+-dependent chloride current (I(ClCa)), followed by a smaller, sustained current (I(DOC)) This response was mirrored by a biphasic increase in the intracellular Ca2+ concentration. SKF96365 (1-{beta-[3-(4-methoxyphenyl) propoxyl]-4-methoxyphenethyl}-1H-imidazole; 10 microM) and Cd2+ (100 microM) inhibited both I(DOC) and the sustained increase in intracellular Ca2+; La3+ (400 microM) inhibited neither response. The results confirm that the non-selective cation current I(DOC) underlies capacitative Ca2+ influx supporting sustained contractions in this tonic smooth muscle.

Excitation-contraction coupling in gastrointestinal and other smooth muscles

The main contributors to increases in [Ca2+]i and tension are the entry of Ca2+ through voltage-dependent channels opened by depolarization or during action potential (AP) or slow-wave discharge, and Ca2+ release from store sites in the cell by the action of IP3 or by Ca(2+)-induced Ca(2+)-release (CICR). The entry of Ca2+ during an AP triggers CICR from up to 20 or more subplasmalemmal store sites (seen as hot spots, using fluorescent indicators); Ca2+ waves then spread from these hot spots, which results in a rise in [Ca2+]i throughout the cell. Spontaneous transient releases of store Ca2+, previously detected as spontaneous transient outward currents (STOCs), are seen as sparks when fluorescent indicators are used. Sparks occur at certain preferred locations--frequent discharge sites (FDSs)--and these and hot spots may represent aggregations of sarcoplasmic reticulum scattered throughout the cytoplasm. Activation of receptors for excitatory signal molecules generally depolarizes the cell while it increases the production of IP3 (causing calcium store release) and diacylglycerols (which activate protein kinases). Activation of receptors for inhibitory signal molecules increases the activity of protein kinases through increases in cAMP or cGMP and often hyperpolarizes the cell. Other receptors link to tyrosine kinases, which trigger signal cascades interacting with trimeric G-protein systems.

Evidence that inhibitory neurotransmission differs between the proximal and distal segments of guinea-pig taenia caeci

The effect of atropine (1 microM) and N(G)-nitro-L-arginine (L-NOARG, 10 microM) on electrical field stimulation induced relaxation in proximal and distal segments of guinea-pig taenia caeci in the presence of guanethidine (4 microM) was studied. The frequency-dependent relaxations were lower in proximal than in distal segments both in the presence and in the absence of atropine. The effect of L-NOARG (an inhibitor of nitric oxide (NO) synthase) on relaxation in the presence of atropine depended on the frequency of electrical stimulation and the segment used; the effect of L-NOARG was greater in proximal segments than in distal segments. In the absence of atropine, the inhibitory effect of L-NOARG was the same in both segments at all frequencies tested. This study demonstrates differences between the opposite extremes of guinea-pig taenia caeci in relaxations induced by electrical stimulation. Our data also show a role of NO that is dependent on the integrity of cholinergic transmission.

A comparison of the effects of capsaicin with inhibitory nerve stimulation in the rat anococcygeus muscle in vitro

Capsaicin was used to test whether centrifugal activation of sensory fibres in the rat anococcygeus muscle can contribute to non-adrenergic non-cholinergic (NANC) relaxation of the muscle. In a solution containing 0.5 mM Ca2+ and in the presence of carbachol (10 microM) capsaicin evoked a fast concentration-dependent relaxation of the muscle that was usually followed by a smaller, slower, relaxant response. The fast relaxant response was reduced when extracellular Ca2+ was raised to 2.5 mM, desensitized after a single application of capsaicin and was blocked by tetrodotoxin (1 microM) or ruthenium red (10 microM). The fast response was greatly reduced by haemoglobin, by cold storage of the muscles or by N-monomethyl-L-arginine (100 microM) in the absence but not in the presence of L-arginine (100 microM). It is concluded that centrifugal activation of sensory fibres evokes a nitric oxide-mediated relaxation of the anococcygeus muscles that probably contributes to electrically evoked NANC relaxation.

Comparison of contractions produced by carbachol, thapsigargin and cyclopiazonic acid in the guinea-pig tracheal muscle

1. Thapsigargin (TPG, 3 microM) and cyclopiazonic acid (CPA, 10 microM) slowly increased muscle tone in the guinea-pig isolated tracheal muscle. A large sustained contraction was produced when 2.4 mM Ca2+ was readmitted after 10 min exposure to Ca2+-free solution following 30 min treatment with TPG or CPA. 2. The sustained contraction after Ca2+ readmission was partially inhibited by nifedipine (3 microM) and highly dependent on external Ca2+. The TPG- and CPA-induced sustained contractions were 75% and 67%, respectively, of the sustained contraction produced by carbachol (Cch, 1 microM, EC80) in the presence of nifedipine. 3. The contractions produced by Cch, TPG and CPA were all inhibited by isoprenaline (ISO) and sodium nitroprusside (SNP). In the presence of nifedipine, the IC50 of ISO was 11, 17, and 23 nM and that of SNP was 0.5, 1, 0.8 microM for Cch-, TPG-, and CPA-induced contractions, respectively. The contraction produced by 60 mM K+ was only weakly inhibited by ISO and SNP. As with ISO and SNP, the Cch-, TPG- and CPA-induced contractions were also similarly inhibited by SKF 96365 (100 microM) and cadmium (Cd2+, 100 microM). 4. It was concluded that TPG and CPA increased Ca2+ influx probably via a mechanism activated by Ca2+ depletion of the sarcoplasmic reticulum. The susceptibility of the contraction produced by TPG, CPA and Cch to inhibition by ISO and SNP and also by SKF-96365 and Cd2+ suggests that the contractions use common pathways for increasing intracellular Ca2+, and that the contractions produced by K+ involve a different mechanism.

Role of sarcoplasmic reticulum in the myorelaxant activity of nitric oxide donors in guinea pig gastric fundus

The relaxant effect of two nitric oxide (NO) donors: sodium nitroprusside and 3-morpholino-sydnonimine (SIN-1) on circular smooth muscle strips isolated from guinea pig gastric fundus was studied with the view to elucidating the mechanism, which underlies the NO-induced relaxation of this tissue. Both sodium nitroprusside (10(-9)-10(-5) M) and SIN-1 (10(-9)-10(-4) M) suppressed the spontaneous fundus tone and hyperpolarized the muscle cells by about 5 mV. They antagonized the acetylcholine (10(-6) M)-induced tone and exerted their relaxant effects even when Ca2+ influx into the cells was triggered through the Na+/Ca2+ exchanger. Sodium nitroprusside and SIN-1 antagonized the contraction induced by cyclopiazonic acid (10(-5) M), a specific inhibitor of the sarcoplasmic reticulum Ca2+-ATPase. In the presence of high concentrations of sodium nitroprusside or SIN-1, cyclopiazonic acid (10(-5) M) exerted only a slight if any contractile effect. After the complete relaxation induced by sodium nitroprusside or SIN-1, the K+-channel blockers, tetraethylammonium, apamin and charybdotoxin, as well as the Ca2+ ionophore, A 23187, induced high-amplitude contractions, suggesting that the Ca2+ sensitivity of the contractile myofilaments was not affected. The results suggest that NO, released from NO donors increases the sarcoplasmic reticulum Ca2+ uptake thereby enhancing the vectorial sarcoplasmic reticulum Ca2+ release toward the plasmalemma to elicit membrane hyperpolarization and relaxation in guinea pig gastric fundus.

No evidence for a significant non-nitrergic, hyperpolarising factor contribution to field stimulation-induced relaxation of the mouse anococcygeus

1. The aim of the study was to determine whether a nerve-derived hyperpolarizing factor (NDHF) might contribute to non-adrenergic, non-cholinergic (NANC) relaxations of the mouse anococcygeus when low concentrations of contractile agent are used to raise tone and low frequencies of field stimulation applied; such a non-nitrergic NDHF has been proposed to contribute to NANC relaxations of the rat anococcygeus and guinea-pig taenia coli. 2. Phenylephrine (0.1-100 microM) produced concentration-related contractions of the mouse isolated anococcygeus muscle; 0.2 microM phenylephrine (EC26) was used to raise tone in subsequent experiments. 3. Field stimulation (0.5, 1.0 and 5.0 Hz) produced frequency-dependent relaxations of phenylephrine-induced tone. In the presence of the nitric oxide synthase inhibitor L-NG-nitro-arginine (L-NOARG; 100 microM), the soluble guanylate cyclase inhibitor 1H-[1,2,4]oxodiazolo[4,3-a]quinoxalin-1-one (ODQ; 5 microM), or a combination of these two drugs, relaxations to field stimulation were abolished at all frequencies studied. Relaxations to sodium nitroprusside (0.01-5 microM) were unaffected by L-NOARG but strongly inhibited by ODQ; neither enzyme inhibitor affected relaxations to 8-Br-cyclic GMP (10 microM). 4. Nifedipine (1 microM) reduced the contractile response to 0.2 microM phenylephrine by 38%; however, it had no effect on NANC relaxations. 5. It is concluded that NANC relaxations of the mouse anococcygeus are purely nitrergic and that there is no significant contribution from a putative NDHF.

Comparison of relaxations evoked by photoactivation of NO-containing compounds and nitrergic nerve stimulation in 5-hydroxytryptamine- and potassium-contracted rat gastric fundus

1. The aim of the present study was to further investigate our earlier proposal of liberation of nitric oxide (NO) by photoactivation of molecules containing NO or NO2, which in turn relaxes gastric smooth muscle, and to determine whether presynaptic- and/or postsynaptic NO-mediated relaxation is affected differently by the degree of membrane depolarization in rat gastric fundus smooth muscle. 2. During contraction of rat gastric fundus with 5-hydroxytryptamine (5-HT, 10 microM), low (K+, 25 mM) and high potassium (K+, 65.4 mM), relaxation responses to nitrergic nerve stimulation, photo-activation of caged NO compounds (streptozotocin [STZ], N omega-nitro-L-arginine-methylacetate [L-NAME], N omega-nitro-D-arginine-methylacetate [D-NAME]), and sodium nitroprusside (SNP) were compared. 3. Nitrergic nerve (presynaptic) stimulation and photoactivation (postsynaptic) of all caged NO compounds produced rapid, transient and reversible relaxation of 5-HT and low-K(+)-contracted tissues. However, when contractions were induced by high K+, the relaxation induced by nerve stimulation was abolished, whereas relaxations induced by photoactivated NO compounds were significantly (P < 0.01) reduced. 4. The relaxation induced by sodium nitroprusside (SNP), but not papaverine, was also diminished in high-K(+)-contracted tissues. The magnitude of photoactivated NO-induced relaxation was related to the amount of NO release, light intensity and concentration of compounds. 5. The evidence that photoactivated NO-induced relaxation is mediated by cGMP comes from the observation that zaprinast, but not forskolin, potentiated the relaxation. 6. It is concluded that rat gastric smooth muscle relaxes to photoactivation of NO or NO2-carrying molecules via NO, and it appears that degree of membrane depolarization may be a critical factor in dissociating the response to presynaptic- and postsynaptic NO-mediated relaxation in this muscle.

Physiological role of Ca2+-permeable nonselective cation channel in endothelin-1-induced contraction of rabbit aorta

We previously showed a role for a nonselective cation channel (NSCC) in the ETA-dependent action of endothelin-1 in mouse fibroblast and rabbit aortic smooth-muscle cell. To clarify the physiological significance of NSCCs in endothelin-1 (ET-1)-induced vasocontraction, we examined the effects of NSCC blockers such as mefenamic acid and SK&F 96365 on the contractions of deendothelialized rabbit aortic rings induced by a low (10[-10] M) or high (10[-8] M) concentration of ET-1. Mefenamic acid (< or =10[-3] M) had little effect on the contraction induced by 45 x 10(-3) M K+ or 1 x 10(-6) M Bay K-8644 in combination with 15 x 10(-3) M K+, indicating that it does not affect voltage-operated calcium channels (VOCs) and contractile mechanisms. The contraction by a low concentration of ET-1 was abolished after removal of extracellular Ca2+, but it was reduced only to 50% by a maximally effective concentration (10[-5] M) of nifedipine, an inhibitor of L-type VOCs (L-VOC). Mefenamic acid and SK&F 96365 inhibited the ET-1-induced contraction with 50% inhibitory concentration (IC50) values of 10(-4) M and 2 x 10(-5) M, respectively, and abolished it at 10(-3) M and 10(-4) M. By contrast, nifedipine, mefenamic acid, or SK&F 96365 had little effect on the contraction by a high concentration of ET-1. The contraction induced by a low or high concentration of ET-1 was abolished by an ETA antagonist, BQ-123, but not by an ETB antagonist, BQ-788. These results demonstrate that the contraction induced by ET-1 is totally mediated exclusively by ETA, but that Ca2+ entry through NSCCs in addition to L-VOCs plays an important role in contractions induced by low concentrations of ET-1, whereas it plays only a minor role in contractions induced by high concentrations of ET-1.

Cellular mechanisms underlying carbachol-induced oscillations of calcium-dependent membrane current in smooth muscle cells from mouse anococcygeus

1. At a holding potential of -40 mV, carbachol (50 microM) produced a complex pattern of inward currents in single smooth muscle cells freshly isolated from the mouse anococcygeus. Membrane currents were monitored by the whole-cell configuration of the patch-clamp technique. Previous work has identified the first, transient component as a calcium-activated chloride current (ICl(Ca)) and the second sustained component as a store depletion-operated non-selective cation current (I(DOC)). The object of the present study was to examine the cellular mechanisms underlying the third component, a series of inward current oscillations (I(oscil)) superimposed on I(DOC). 2. Carbachol-induced I(oscil) (amplitude 97 +/- 11 pA; frequency 0.26 +/- 0.02 Hz) was inhibited by the chloride channel blocker anthracene-9-carboxylic acid (A-9-C; 1 mM), and by inclusion of 1 mM EGTA in the patch-pipette filling solution. 3. In calcium-free extracellular medium (plus 1 mM EGTA), carbachol produced an initial burst of oscillatory current which lasted 94 s before decaying to zero; I(oscil) could be restored by re-admission of calcium. The frequency, but not the amplitude, of I(oscil) increased with increasing concentrations of extracellular calcium (0.5-10 mM). 4. Inclusion of the inositol triphosphate (IP3) receptor antagonist heparin (5 mg ml(-1) in the patch-pipette filling solution, or pretreatment of cells with the sarcoplasmic reticulum (SR) calcium ATPase inhibitor cyclopiazonic acid (CPA; 10 microM), prevented the activation of I(oscil) by carbachol. Caffeine (10 mM) activated both ICl(Ca) and I(DOC) and prevented the induction of I(oscil) by carbachol. Caffeine and CPA also abolished I(oscil) in the presence of carbachol, as did both a low (3 microM) and a high (30 microM) concentration of ryanodine. 5. Carbachol-induced I(oscil) was abolished by the general calcium entry blocker SKF 96365 (10 MM) and by Cd2+ (100 microM), but was unaffected by La3+ (400 microM). As found previously, I(DOC) was also blocked by SKF 96365 and Cd2+, but not La3+; the inhibition of I(DOC) preceded the abolition of I(oscil) by 27 s with SKF 96365 and by 30 s with Cd2+. Nifedipine (1 microM) produced a partial inhibition of the carbachol-induced I(oscil) frequency at holding potentials of -20 mV and -60 mV and, in addition, reduced I(DOC) at -60 mV by 18%. 6. It is concluded that carbachol-induced inward current oscillations in mouse anococcygeus cells are due to a calcium-activated chloride current, and reflect oscillatory changes in cytoplasmic calcium ion concentration. These calcium oscillations are derived primarily from the SR stores, but entry of calcium into the cell is necessary for store replenishment and maintenance of the oscillations. Capacitative calcium entry (via I(DOC) appears to be important not only for sustained contraction of this tissue, but also as a route for re-filling of the SR and, therefore, represents an important target for the development of novel and selective drugs.

Evidence that NO acts as a redundant NANC inhibitory neurotransmitter in the guinea-pig isolated taenia coli

1. The relative contribution of the putative transmitters, nitric oxide (NO) and an apamin-sensitive factor, possibly ATP, to inhibitory responses evoked by electrical field stimulation (EFS; 0.2-5 Hz, 0.2 ms duration, supra-maximal voltage for 10 s) of non-adrenergic, non-cholinergic (NANC) nerves was investigated in the guinea-pig isolated taenia coli contracted with histamine (1 microM). 2. Peak relaxations to EFS (0.2-5 Hz) were tetrodotoxin (1 microM)-sensitive, maximal at 0.2 Hz and completely resistant to the nitric oxide synthase inhibitor, NG-nitro-L-arginine (L-NOARG; 100 microM) in either the presence or absence of atropine (1 microM). Furthermore, the specific inhibitor of soluble guanylyl cyclase, 1H-[1,2,4] oxadiazolo [4,3-a] quinoxaline-1-one (ODQ; 10 microM), the cytochrome P450 inhibitor and free radical generator, 7-ethoxyresorufin (7-ER; 10 microM) and the NO scavenger, oxyhaemoglobin (HbO; 30 microM) had no effect on EFS-induced relaxations alone and in combination with L-NOARG (100 microM). 3. Maximum relaxation to the NO donor, sodium nitroprusside (SNP; 1 microM) was significantly reduced by HbO (30 microM), abolished by 7-ER (10 microM) and ODQ (10 microM) but was unaffected by apamin (0.1 microM), an inhibitor of small conductance Ca(2+)-activated K+ channels. 4. The relaxation to EFS at 0.2 Hz was resistant to apamin but those to 0.5 and 5 Hz were significantly reduced. EFS (0.2-5 Hz)-evoked relaxations that persisted in the presence of apamin were further significantly inhibited by L-NOARG (100 microM) or ODQ (10 microM), but not by HbO (30 microM) or 7-ER (10 microM). 5. ATP (1-30 microM) produced concentration-dependent relaxations that were abolished by apamin (0.1 microM), unaffected by ODQ (10 microM) but only significantly reduced by L-NOARG (100 microM) at the lowest concentration of ATP (1 microM) used. 6. Nifedipine (0.3 microM), abolished contractions to 67 mM KCl, histamine (10 microM), endothelin-1 (0.03 microM), 5-hydroxytryptamine (5-HT; 10 microM) and the thromboxane-mimetic, 9-11-dideoxy-9 alpha, 11 alpha-methano-epoxy-prostaglandin F2 alpha (U46619; 0.1 microM). 7. The findings of the present study suggest that NO is released during NANC nerve stimulation, but plays no role in NANC relaxations in the guinea-pig taenia coli unless the effects of another apamin-sensitive, nerve-derived hyperpolarizing factor (NDHF) are blocked. Thus, we propose that in this tissue, NO acts as a 'backup' or redundant NANC nerve inhibitory transmitter and like NDHF mediates relaxation via hyperpolarization.

Cyclopiazonic acid-induced changes in contractile activity of smooth muscle strips isolated from cat and guinea-pig stomach

The effects of cyclopiazonic acid (CPA), a specific inhibitor of sarcoplasmic reticulum Ca(2+)-ATPase, on contractile activity of circular smooth muscle strips isolated from the antrum, corpus and fundus regions of the cat and guinea-pig stomach were studied. Contractile activity was recorded under isometric conditions, in organ baths. CPA, concentration dependently (3 x 10(-7)-3 x 10(-5) M) increased the tone of the cat and guinea-pig gastric fundus and corpus as well as the amplitude of the phasic contractions of the cat corpus and antrum, affecting their frequency. CPA had a dual action on the phasic contractions of the guinea-pig antrum: an increase at low concentrations (up to 10(-6) M) and inhibition at high concentrations (10(-6)-3 x 10(-5) M). Tetrodotoxin (10(-6) M), atropine (10(-6) M) and N omega-nitro-L-arginine (10(-4) M) did not change significantly the effects of CPA. Nifedipine completely inhibited the CPA-induced phasic contractions and partly inhibited the CPA-induced tonic contractions. The nitric oxide-releasing agents, sodium nitroprusside (10(-3) M) and 3-morpholino-sydnonimine (10(-3) M), completely inhibited the CPA-induced tonic and phasic contractions. CPA induced tonic contractions in the cat and guinea-pig gastric fundus precontracted by acetylcholine (10(-5) M) and inhibited the acetylcholine (10(-6) M)-induced phasic contractions in the guinea-pig gastric antrum and corpus. The results suggest multiple roles for sarcoplasmic reticulum Ca2+ stores and sarcoplasmic reticulum Ca(2+)-ATPase in the shaping of spontaneous and evoked tonic and phasic contractions of the stomach, and highlight important species and tissue differences.

Inhibition by sodium nitroprusside of a calcium store depletion-activated non-selective cation current in smooth muscle cells of the mouse anococcygeus

1. The effects of sodium nitroprusside (SNP) on the non-selective cation current activated in response to intracellular calcium store depletion were studied using the whole-cell patch-clamp technique in single smooth muscle cells isolated from the mouse anococcygeus. Voltage-dependent calcium currents were blocked with extracellular nifedipine, and caesium and tetraethylammonium chloride were used to block voltage-dependent potassium currents. Calcium stores were depleted with caffeine (10 mM), carbachol (50 microM) or cyclopiazonic acid (CPA 10 microM; an inhibitor of the sarcoplasmic reticulum [SR] calcium-ATPase). 2. At a holding potential of -40 mV, both CPA and caffeine activated inward currents which consisted of two clearly distinguishable components; an initial transient current followed by a smaller sustained current. In the case of CPA, the amplitudes of the transient and sustained components were 19.7 +/- 2.1 pA and 3.5 +/- 0.3 pA respectively, whilst the equivalent values for caffeine were 188 +/- 21 and 4.8 +/- 0.3 pA. As described previously, the transient current results from activation of a calcium-dependent chloride conductance whilst the sustained current is a non-selective cation current, activated following intracellular calcium store depletion. 3. The muscarinic receptor agonist, carbachol, also activated a transient followed by a sustained current with amplitudes of 238 +/- 55 and 4.7 +/- 0.5 pA respectively. Superimposed on the sustained current were regular, oscillations of calcium-activated chloride current. 4. Both the transient and the sustained currents activated by CPA were absent in cells pretreated with SNP (10 microM). Application of SNP to a cell following activation of the sustained current by CPA inhibited the current by 88.6 +/- 3.8%. SNP (10 microM) did not inhibit the transient current activated by caffeine but abolished the sustained current. 5. SNP (10 microM) had no effect on the initial transient current activated by carbachol (50 microM). However, it did inhibit the oscillations in the inward current. In recordings from cells bathed in extracellular solution containing the chloride channel blocker, anthracene-9-carboxylic acid (A-9-C; 1 mM), carbachol activated only a sustained current. This current was inhibited by 88.1 +/- 6.5% by a concomitant application of SNP (10 microM) and was absent in cells pretreated with the nitrovasodilator. 6. The effects of SNP on the currents activated by caffeine (10 mM) were mimicked by 8-bromo-cyclic GMP (200 microM); thus the nucleotide had no effect on the transient current activated by caffeine but abolished the sustained current. The effects of SNP, but not those of 8-bromo-cyclic GMP, were inhibited by the nitric oxide-sensitive guanylyl cyclase inhibitor, 1H-[1, 2, 4]oxadiazolo[4, 3-a]quinoxaline-1-one (ODQ; 1 microM). ODQ alone produced a significant increase in the size of the sustained current activated by caffeine (7.8 +/- 0.7 pA). 7. These findings suggest that SNP activates guanylyl cyclase to inhibit the non-selective cation current activated as a result of intracellular calcium store depletion in mouse anococcygeus cells. Since the non-selective cation current appears to underlie the calcium entry process responsible for maintaining the sustained contractions to agonists in this tissue, this action of SNP may represent an important mechanism by which nitrates relax non-vascular smooth muscle.

The role of sarcoplasmic reticulum and sarcoplasmic reticulum Ca2+-ATPase in the smooth muscle tone of the cat gastric fundus

Circular smooth muscle strips isolated from cat gastric fundus were studied in order to understand whether the sarcoplasmic reticulum (SR) and SR Ca2+-ATPase could play a role in the regulation of the muscle tone. Cyclopiazonic acid (CPA), a specific inhibitor of SR Ca2+-ATPase, caused a significant and sustained increase in muscle tone, depending on the presence of extracellular Ca2+. Nifedipine and cinnarizin only partially suppressed the CPA-induced tonic contraction. Bay K 8644 antagonized the relaxant effect of nifedipine in CPA-contracted fundus. Nitric-oxide-releasing agents sodium nitroprusside and 3-morpholino-sydnonimine completely suppressed the CPA-induced tonic contraction. The blockers of Ca2+-activated K+ channels, tetraethylammonium, charybdotoxin and/or apamin, decreased the contractile effect of CPA. Vanadate increased the tone but did not change significantly the effect of CPA. CPA exerted its contractile effect even when Ca2+ influx was triggered through the Na+/Ca2+ exchanger and the other Ca2+ entry pathways were blocked. Thapsigargin, another specific SR Ca2+-ATPase inhibitor, also increased the muscle tone. The effect of thapsigargin was completely suppressed by sodium nitroprusside and 3-morpholino-sydnonimine and partially by nifedipine. In conclusion, under conditions when the SR Ca2+-ATPase is inhibited, the tissue develops a strong tonic contraction and a large part of this is mediated by Ca2+ influx presumably via nifedipine-sensitive Ca2+ channels. This study suggests the important role of SR Ca2+-ATPase in the modulation of the muscle tone and the function of SR as a "buffer barrier" to Ca2+ entry in the cat gastric fundus smooth muscle.

Two distinct membrane currents activated by cyclopiazonic acid-induced calcium store depletion in single smooth muscle cells of the mouse anococcygeus

1. By use of the whole-cell configuration of the patch-clamp technique, membrane currents induced by cyclopiazonic acid (CPA; an inhibitor of the sarcoplasmic reticulum (SR) calcium-ATPase) were investigated in single smooth muscle cells freshly dispersed from the mouse anococcygeus. Voltage-dependent calcium currents were blocked with extracellular nifedipine and caesium and tetraethylammonium chloride were used to block voltage-dependent potassium currents. 2. At a holding potential of -40 mV, CPA (10 microM) activated an inward current that consisted of two distinct components. The first was an initial transient current with an amplitude of 19.6 +/- 1.9 pA while the second was sustained and had an amplitude of 3.5 +/- 0.3 pA. 3. The current-voltage (I-V) relationship for the transient current showed marked outward rectification. The current had a reversal potential of 9.1 +/- 1.1 mV which was shifted to 29.0 +/- 4.2 mV when the extracellular chloride concentration was lowered from 148.4 to 58.4 mM. The sustained current had a near-linear I-V relationship and a reversal potential of 31.0 +/- 2.7 mV. Removal of extracellular calcium had no effect on the transient current, but shifted the reversal potential of the sustained current to 18.2 +/- 5.7 mV. 3. The initial transient current was abolished in cells bathed in extracellular solutions containing the chloride channel blockers, 4,4' diisothiocyanato-stilbene-2,2'-disulphonic acid (DIDS; 1 mM) or anthracene-9-carboxylic acid (A-9-C; 1 mM), and was absent in cells containing the calcium buffers EGTA (1 to 5 mM) or BAPTA (10 mM). The second sustained current was unaffected by either the chloride channel blockers or the intracellular calcium buffers. 4. Treatment of the cells with caffeine (10 mM) produced similar inward currents to those produced by CPA. In the presence of caffeine, CPA (10 microM) induced no further inward current. 5. In organ bath studies, CPA (10 microM)-induced contractions of the mouse anococcygeus were inhibited by cadmium and nickel (both 50-400 microM) and the general calcium entry blocker, SKF 96365 (10 microM); lanthanum and gadolinium had no effect at concentrations up to 400 microM. The pharmacology of the CPA-induced non-selective cation current mirrored that of the CPA-induced whole muscle contraction being reversed by cadmium (100 microM) and SKF 96365 (10 microM), but unaffected by lanthanum (400 microM). The initial chloride conductance was unaffected by cadmium, SKF 96365 or lanthanum. 6. It is concluded that CPA activates a transient calcium-dependent chloride current as a consequence of calcium release from intracellular stores; this current would result in depolarization and opening of voltage-operated calcium channels, which mediate the nifedipine-sensitive component of muscle contraction. In addition, as a result of emptying the SR, CPA activates a non-selective cation conductance which may underlie the nifedipine-insensitive calcium entry process utilised during sustained contraction.