Effects of cyclopiazonic acid on rhythmic contractions in uterine smooth muscle bundles of the rat

Review Article: Safety Assessment of the Mycotoxin Cyclopiazonic Acid

Cyclopiazonic acid (CPA) is an indol-tetramic acid mycotoxin and is produced by the nearly ubiquitous molds, Aspergillus and Penicillium. CPA produced by these molds has been identified in a number of food sources (including, but not limited to, grain, legumes, meat, milk, and cheese) and from parasitic infections of man and other animals. Few incidents of CPA mycotoxicoses have been reported because of the benign nature of the intoxication, the small amounts present, and its effects may be disguised with concurrent aflatoxicosis (some toxicity data may have been generated using aflatoxin-contaminated CPA). CPA is absorbed in the gastrointestinal tract and following oral administration; it has a half-life of approximately 30 hours and is excreted largely unchanged in the urine and feces. Cyclopiazonic acid is not considered to be a potent acute toxin as its oral LD50 in rodents is in the range of 30 to 70 mg/kg. Multiple dose studies also show a range of effects in several species and among mammalian models, the pig appears to be the most sensitive with a no-observable-effect level (NOEL) in the range of 1.0 mg/kg/day. The preponderance of evidence from the rat and other test animals supports this dose as a defensible estimate of a no effect level. The target organs of CPA toxicity appear to be muscle, hepatic tissue, and spleen, with a localization in the former, although a more apparent toxic change in the latter two. The toxicity and symptoms of CPA poisoning can be attributed to its ability to alter normal intracellular calcium flux through its inhibition of the reticular form of the Ca2+-ATPase pump. CPA was not teratogenic in mice. CPA is not considered a carcinogen and the weight of evidence militates against its characterization as a mutagen. Despite CPA-induced pathological changes ascribed to the spleen or bursa of Fabricius, there does not appear to be an effect on the immune system. In vitro studies imply a potential immunomodulatory effect of CPA, but in all of those reports very high concentrations of CPA were required and none of these findings have been supported with in vivo studies. Therefore, based on a NOEL of 1 mg/kg/day and accounting for species variation, an appropriate acceptable daily intake (ADI) would be approximately 10 μg/kg/day or 700 μg/day. In the context of human exposure, if the uppermost limit of CPA found in cheese is 4 μg/g and the average individual consumes 50 g of cheese daily, this allows an intake of 200 μg, less than one third of a traditionally established ADI.

Enhanced Expression of Contractile-Associated Proteins and Ion Channels in Preterm Delivery Model Mice With Chronic Odontogenic Porphyromonas Gingivalis Infection

Inflammation and infection have been reported to induce preterm delivery. We have studied the relationship between inflammation and various ion channels, including the L-type Ca(2+) channel and P2X7 receptor, during acute inflammation of the pregnant rat uterus induced by lipopolysaccharides. Recently, we found that mice with odontogenic Porphyromonas gingivalis (P.g, an important odontogenic pathogen) infection delivered at day 18.3 of gestation (vs. day 20.5 in normal mice). The purpose of this study was to investigate the expression of myometrial contractile-associated proteins inducing contractions and confirm that these mice are useful as a model for preterm delivery induced by chronic inflammation. We examined the expression of the oxytocin receptor, connexin 43, prostaglandin F receptors, L-type Ca(2+) channel, and P2X7 receptor in the myometrium at day 18 of gestation by real-time PCR and western blot analyses. We also measured TNF-α and IL-1β levels in the blood serum, placenta, fetal membrane and myometrium on the same day. mRNA expression of the oxytocin receptor, connexin 43, prostaglandin F receptors, L-type Ca(2+) channel, and P2X7 receptor was elevated by 5.4, 3.2, 2.4, 2.5, and 1.7 fold, respectively, in the P.g-infected mice. Protein levels of the oxytocin receptor and connexin 43 also increased. Serum levels of TNF-α and IL-1β were elevated, showing that systemic inflammation continued during pregnancy. IL-1β levels in the placenta and fetal membrane also increased, suggesting inflammatory reactions were induced. Thus, mice with odontogenic infection may be useful as a model of chronic inflammation-induced preterm delivery.

Effect of inhibiting the sarcoplasmic reticulum on spontaneous and oxytocin-induced contractions of human myometrium

OBJECTIVE

1. To assess the contribution of the sarcoplasmic reticulum calcium store in the generation of uterine smooth muscle contractions; 2. to evaluate the contribution of calcium induced calcium release or ryanodine gated calcium channels to myometrial force production.

DESIGN

Laboratory scientific study.

METHODS

Myometrial strips were obtained from women undergoing elective prelabour caesarean section at term. These were loaded with the calcium sensitive indicator Indo-1 allowing simultaneous assessment of intracellular calcium concentrations and force production. The effect of exposing the strips to ryanodine (which abolishes calcium induced calcium release), caffeine (which activates calcium induced calcium release) and cyclopiazonic acid (which abolishes the sarcoplasmic reticulum calcium store) was examined.

RESULTS

Exposure to ryanodine had no appreciable effect on either the amplitude or the duration of the myometrial calcium and force transients but did increase the frequency of contractions (139+/-5%). Caffeine did not potentiate force. Cyclopiazonic acid increased frequency, duration and amplitude of both calcium and force transients. The ability of oxytocin to provoke calcium and force transients in the absence of extracellular calcium was abolished by cyclopiazonic acid but not by ryanodine.

CONCLUSIONS

These results demonstrate that calcium induced calcium release does not play a significant role in human myometrium and that no functioning role for the ryanodine receptors in human myometrial tissue could be shown. These data suggest that the sarcoplasmic reticulum may act to limit contractions and act as a calcium sink, rather than to amplify contractions.

Invited review: significance of spatial and temporal heterogeneity of calcium transients in smooth muscle

The multiplicity of mechanisms involved in regulation of intracellular Ca(2+) concentration ([Ca(2+)](i)) in smooth muscle results in both intra- and intercellular heterogeneities in [Ca(2+)](i). Heterogeneity in [Ca(2+)](i) regulation is reflected by the presence of spontaneous, localized [Ca(2+)](i) transients (Ca(2+) sparks) representing Ca(2+) release through ryanodine receptor (RyR) channels. Ca(2+) sparks display variable spatial Ca(2+) distributions with every occurrence within and across cellular regions. Individual sparks are often grouped, and fusion of sparks produces large local elevations in [Ca(2+)](i) that occasionally trigger propagating [Ca(2+)](i) waves. Ca(2+) sparks may modulate membrane potential and thus smooth muscle contractility. Sparks may also be the target of other regulatory factors in smooth muscle. Agonists induce propagating [Ca(2+)](i) oscillations that originate from foci with high spark incidence and also represent Ca(2+) release through RyR channels. With increasing agonist concentration, the peak of regional [Ca(2+)](i) oscillations remains relatively constant, whereas both frequency and propagation velocity increase. In contrast, the global cellular response appears as a concentration-dependent increase in peak as well as mean cellular [Ca(2+)](i), representing a spatial and temporal integration of the oscillations. The significance of agonist-induced [Ca(2+)](i) oscillations lies in the establishment of a global [Ca(2+)](i) level for slower Ca(2+)-dependent physiological processes.

Calcium homeostatic pathways change with gestation in human myometrium

A rise in intracellular calcium is the primary trigger for contractile activity in pregnant human myometrium. It is hypothesized that key proteins involved in myometrial calcium homeostasis are gestationally regulated and play an important role in the preparation for labor. The aims of the study were to investigate the role of sarcoplasmic reticulum Ca ATPases (SERCAs) in regulating spontaneous contractile activity in myometrium, and to determine the expression of SERCA isoforms 2a and 2b, and the plasma membrane Ca ATPase (PMCA), at term and during labor. Western blot analysis demonstrated that the expression of SERCA 2a and 2b significantly increased in myometrium of women in labor compared with those not in labor. The augmentation of contractile activity in laboring myometrium in the presence of a SERCA 2 inhibitor, cyclopiazonic acid (CPA), demonstrated the functional significance of this observation. It is interesting that the application of CPA in the presence of a calcium-activated potassium channel inhibitor to term nonlabor myometrium mimicked the response of myometrium from women in active labor to CPA alone. We conclude that the activity of SERCA isoforms becomes increasingly important in the maintenance of regular contractile activity during labor and may compensate for the functional loss of other calcium control pathways at term.

Spatial and temporal aspects of ACh-induced [Ca2+]i oscillations in porcine tracheal smooth muscle

This study evaluated the relationship between regional elevation in intracellular calcium concentration ([Ca2+]i) induced by acetylcholine (ACh) and the global cellular responses in porcine tracheal smooth muscle (TSM) cells. Regional (approximately 1.5 microm3) and global (whole cell) changes in [Ca2+]i were measured in fluo-3 loaded TSM cells using real-time confocal microscopy. Regional responses appeared as propagating [Ca2+]i oscillations whereas global responses reflected the spatiotemporal integration of these regional responses. Within a region, [Ca2+]i oscillations were 'biphasic' with initial higher frequencies, followed by slower steady-state oscillations. With increasing ACh concentration, the peak (maximum value relative to 0 nM) of regional [Ca2+]i oscillations remained relatively constant, whereas both frequency and propagation velocity increased. In contrast, the global spatiotemporal integration of the regional oscillatory responses appeared as a concentration-dependent increase in peak as well as mean cellular [Ca2+]i. We conclude that the significance of ACh-induced [Ca2+]i oscillations lies in the establishment of mean [Ca2+]i level for slower Ca2+-dependent physiological processes via modulation of oscillation frequency and propagation velocity.

Dynamic Ca2+ signalling in rat arterial smooth muscle cells under the control of local renin-angiotensin system

1. We visualized the changes in intracellular Ca2+ concentration ([Ca2+]i), using fluo-3 as an indicator, in individual smooth muscle cells within intact rat tail artery preparations. 2. On average in about 45 % of the vascular smooth muscle cells we found spontaneous Ca2+ waves and oscillations ( approximately 0.13 Hz), which we refer to here as Ca2+ ripples because the peak amplitude of [Ca2+]i was about one-seventh of that of Ca2+ oscillations evoked by noradrenaline. 3. We also found another pattern of spontaneous Ca2+ transients often in groups of two to three cells. They were rarely observed and are referred to as Ca2+ flashes because their peak amplitude was nearly twice as large as that in noradrenaline-evoked responses. 4. Sympathetic nerve activity was not considered responsible for the Ca2+ ripples, and they were abolished by inhibitors of either the Ca2+ pump in the sarcoplasmic reticulum (cyclopiazonic acid) or phospholipase C (U-73122). 5. Both angiotensin antagonists ([Sar1,Ile8]-angiotensin II and losartan) and an angiotensin converting enzyme inhibitor (captopril) inhibited the Ca2+ ripples. 6. The extracellular Ca2+-dependent tension borne by unstimulated arterial rings was reduced by the angiotensin antagonist by approximately 50 %. 7. These results indicate that the Ca2+ ripples are generated via inositol 1,4, 5-trisphosphate-induced Ca2+ release from the intracellular Ca2+ stores in response to locally produced angiotensin II, which contributes to the maintenance of vascular tone.

Effect of halothane on intracellular calcium oscillations in porcine tracheal smooth muscle cells

The effect of halothane on intracellular Ca2+ concentration ([Ca2+]i) regulation in porcine tracheal smooth muscle cells was examined with real-time confocal microscopy. Both 1 and 2 minimum alveolar concentration (MAC) halothane increased basal [Ca2+]i when Ca2+ influx and efflux were blocked, suggesting increased sarcoplasmic reticulum (SR) Ca2+ leak and/or decreased reuptake. In beta-escin-permeabilized cells, heparin inhibition of inositol 1,4, 5-trisphosphate-receptor channels blunted the halothane-induced increase in [Ca2+]i. Both 1 and 2 MAC halothane decreased the frequency and amplitude of ACh-induced [Ca2+]i oscillations (which represent SR Ca2+ release through ryanodine-receptor channels), abolishing oscillations in approximately 20% of tracheal smooth muscle cells at 2 MAC. When Ca2+ influx and efflux were blocked, halothane increased the baseline and decreased the frequency and amplitude of [Ca2+]i oscillations, inhibiting oscillations in approximately 70% of cells at 2 MAC. The fall time of [Ca2+]i oscillations and the rate of fall of the [Ca2+]i response to caffeine were both increased by halothane. These results suggest that halothane abolishes agonist-induced [Ca2+]i oscillations by 1) depleting SR Ca2+ via increased Ca2+ leak through inositol 1,4, 5-trisphosphate-receptor channels, 2) decreasing Ca2+ release through ryanodine-receptor channels, and 3) inhibiting reuptake.

Cellular mechanisms underlying magnesium sulfate inhibition of phasic myometrial contractions

These studies sought to test the hypothesis that magnesium inhibits extracellular calcium entry, thereby inhibiting intracellular calcium release and cytosolic calcium oscillations in myometrial smooth muscle. In vitro contraction studies were performed using oxytocin and other uterotonic agonists with and without the addition of magnesium in the presence and absence of extracellular calcium. Cytosolic calcium studies were performed using myometrial strips loaded with Fura 2. Oxytocin produced cytosolic calcium oscillations and simultaneous phasic contractions; both were inhibited by magnesium. The other uterotonic agonists tested also produced phasic contractions which were significantly inhibited by magnesium. The magnesium effect was reversible with washout and counteracted by Bay K 8644 (a calcium channel agonist). In the absence of extracellular calcium, intracellular calcium release in response to oxytocin was inhibited by magnesium. In summary, magnesium inhibited extracellular calcium entry, intracellular calcium release, cytosolic calcium oscillations, and phasic contractions of myometrial smooth muscle.

Phosphatidylinositol-specific phospholipase C isoform expression in pregnant and nonpregnant rat myometrial tissue

OBJECTIVE

Activation of the phosphatidylinositol signaling pathway plays a significant role during the intracellular signal transduction events activated during agonist-stimulated phasic myometrial contractions. Phospholipase C is an essential molecular component of this signaling pathway. These studies sought to characterize the expression of phospholipase C isoform messenger ribonucleic acid in both pregnant and nonpregnant rat myometrium.

STUDY DESIGN

Total cellular ribonucleic acid was isolated from myometrial tissue collected from Sprague-Dawley rats by use of the acidic guanidinium thiocyanate-phenol-chloroform extraction technique. After deoxyribonuclease treatment to ensure removal of genomic deoxyribonucleic acid, as well as resolution on formaldehyde-1% agarose horizontal slab gels to rule out degradation, the ribonucleic acid was used for semiquantitative competitive reverse transcriptase-polymerase chain reaction studies to evaluate the expression of five of the reported phospholipase C isoforms. These studies were performed with isoform-specific 20-mer primers and the inclusion of internal standard heterologous deoxyribonucleic acid sequences designed with ends homologous to the isoform-specific primers. The identity of the polymerase chain reaction products was confirmed with restriction endonuclease digestions and homology analysis of the sequenced polymerase chain reaction product deoxyribonucleic acid.

RESULTS

These reverse transcriptase-polymerase chain reaction studies have confirmed expression of the phospholipase C-beta1a, phospholipase C-beta3, phospholipase C-gamma1, phospholipase C-beta2, and phospholipase C-delta1 isoforms in rat myometrial tissue. During pregnancy the levels of expression of the phospholipase C-beta3, phospholipase C-gamma1, and phospholipase C-delta1 isoforms were increased compared with the levels of expression in myometrium from nonpregnant rats. In myometrium from both pregnant and nonpregnant animals the phospholipase C-beta1 a isoform was expressed at the highest level, the phospholipase C-beta3, phospholipase C-gamma1, and phospholipase C-gamma2 isoforms at an intermediate level, and the phospholipase C-delta1 isoform was expressed at the lowest levels.

CONCLUSIONS

These studies have confirmed at the messenger ribonucleic acid level significant expression of several isoforms of phospholipase C in both pregnant and nonpregnant myometrial tissue. These observations provide additional support for the hypothesis that the phosphatidylinositol signaling pathway plays an important role in uterine smooth muscle.

Intracellular mechanisms underlying prostaglandin F2alpha-stimulated phasic myometrial contractions

These studies sought to test the hypothesis that prostaglandin F2alpha (PGF2alpha)-stimulated phasic myometrial contractions are characterized by the activation of the phosphatidylinositol-signaling pathway resulting in the generation of cytosolic calcium oscillations. For the experiments described in this report rat myometrial tissue was used, after the tissue was loaded with fura 2, to perform cytosolic calcium imaging studies and to perform computer-digitalized in vitro isometric contraction studies. Consistent with the above hypothesis, the cytosolic calcium-imaging studies demonstrated PGF2alpha-stimulated cytosolic calcium oscillations occurring simultaneously with phasic contractions. The in vitro isometric contraction studies confirmed that previously reported inhibitors of the phosphatidylinositol-signaling pathway and cytosolic calcium oscillation mechanisms resulted in significant inhibition of PGF2alpha-stimulated phasic myometrial contractions. In summary, these studies have provided substantial support for the hypothesis that PGF2alpha-stimulated phasic myometrial contractions are generated by intracellular signaling mechanisms involving activation of the phosphatidylinositol-signaling pathway and the production of cytosolic calcium oscillation-like phenomena.

Effects of salbutamol on intracellular calcium oscillations in porcine airway smooth muscle

Relaxation of airway smooth muscle (ASM) by beta-adrenoceptor agonists involves reduction of intracellular Ca2+ concentration ([Ca2+]i). In porcine ASM cells, acetylcholine induces [Ca2+]i oscillations that display frequency modulation by agonist concentration and basal [Ca2+]i. We used real-time confocal microscopy to examine the effect of salbutamol (1 nM to 1 microM), a beta 2-adrenoceptor agonist, on [Ca2+]i oscillations in freshly dissociated porcine ASM cells. Salbutamol decreased the frequency of [Ca2+]i oscillations in a concentration-dependent fashion, completely inhibiting the oscillations at 1 microM. These effects were mimicked by a cell-permeant analog of adenosine 3',5'-cyclic monophosphate. The inhibitory effect of salbutamol was partially reversed by BAY K 8644. Salbutamol reduced [Ca2+]i even when sarcoplasmic reticulum (SR) Ca2+ reuptake and Ca2+ influx were blocked. Lanthanum blockade of Ca2+ efflux attenuated the inhibitory effect of salbutamol on [Ca2+]i. The [Ca2+]i response to caffeine was unaffected by salbutamol. On the basis of these results, we conclude that beta 2-adrenoceptor agonists have little effect on SR Ca2+ release in ASM cells but reduce [Ca2+]i by inhibiting Ca2+ influx through voltage-gated channels and by enhancing Ca2+ efflux.

Effects of Mn2+ on the responses induced by different spasmogens in the oestrogen-primed rat uterus

We investigated the effect of Mn2+ on the mechanical responses evoked by high K+ (60 mM) or low Na+ (25 mM) solutions, oxytocin and neurokinin A in the oestrogen-primed rat uterus. In a Ca2+-free, Mn2+ (0.54 mM)-containing solution, high K+ or low Na+ solutions produced contractions of smaller amplitude than those observed in a normal Ca2+ (0.54 mM) solution, which were abolished by nifedipine (1 microM). Oxytocin (1 microM) and neurokinin A (1 microM, in the presence of phosphoramidon 1 microM) evoked nifedipine-insensitive contractile responses similar to (oxytocin) or smaller (neurokinin A) in amplitude than those observed in Ca2+ (0.54 mM)-containing solution. In strips loaded with Ca2+ (2.16 mM) for 10 min and then exposed to a Ca2+- and Mn2+-free, EGTA (3 mM)-containing medium for 4 min, both oxytocin and neurokinin A induced transient contraction followed by a small sustained response. The transient component of the response was abolished by cyclopiazonic acid (10 microM). When preparations were loaded with Mn2+ (2.16 mM) for 10 min, only the small, tonic contraction was observed. In Ca2+-containing solution, Mn2+ (0.01-10 mM) inhibited in a concentration-dependent manner the rhythmic contractions developed either spontaneously or by electrical stimulation as well as high K+- and neurokinin A-induced contractions. Mn2+ also abolished the rhythmic, but not the tonic component of the response to oxytocin, and the preparation remained maximally contracted. These data suggest that in the oestrogen-primed rat uterus, Mn2+ acts as an antagonist of Ca2+ influx through L-type voltage-operated Ca2+ channels. In addition, Mn2+ enters the cell mainly through nifedipine-insensitive receptor-operated channels and, to a lesser degree, through L-type Ca2+ channels to produce contraction by directly activating the contractile machinery.

Regulation of intracellular calcium oscillations in porcine tracheal smooth muscle cells

Using real-time confocal microscopy, we examined the dynamic intracellular Ca2+ concentration ([Ca2+]i) response of porcine tracheal smooth muscle (TSM) cells to acetylcholine (ACh). Exposure to ACh caused regenerative, propagating [Ca2+]i oscillations. The amplitude and fall time of the [Ca2+]i oscillations were inversely correlated to basal [Ca2+]i, whereas the frequency and rise time were directly correlated to basal [Ca2+]i. ACh-induced [Ca2+]i oscillations were initiated in the absence of extracellular Ca2+ and after membrane depolarization with KCl, suggesting that 1) [Ca2+]i oscillations primarily arise by release from internal stores such as the sarcoplasmic reticulum (SR), and 2) Ca2+ influx is necessary for maintenance of oscillations. Exposure to both caffeine and ryanodine inhibited ongoing ACh-induced [Ca2+]i oscillations, suggesting a role for caffeine-sensitive ryanodine receptor (RyR) SR Ca2+ channels. Inhibition of SR Ca2+ reuptake by thapsigargin increased basal [Ca2+]i and decreased [Ca2+]i oscillation amplitude, suggesting that Ca2+ reuptake is also essential. The present results suggest that [Ca2+]i oscillations in porcine TSM cells involve repetitive Ca2+ release and reuptake from RyR channels, perhaps through a Ca2+ -induced Ca2+ release mechanism.

Inhibitory effects of okadaic acid on rat uterine contractile responses to different spasmogens

In the present study, we examined the effects of okadaic acid, a selective inhibitor of type 1 and 2A protein phosphatases, on the mechanical responses evoked by oxytocin, K(+)- and Na(+)-modified solutions and ouabain in estrogen-primed rat myometrium. Oxytocin elicited a rapid, phasic contraction followed by rhythmic oscillations. The phasic response was partially resistant to the absence of external Ca2+. Okadaic acid (1 microM) and the L-type calcium channel blocker nifedipine (1 microM) abolished the oscillatory component and reduced the initial, phasic response to about 80% of the control response. High K+ (60 mM) solution, ouabain (1 mM), K(+)-free medium and low Na+ (25 mM) solution induced extracellular Ca(2+)-dependent biphasic responses composed by an early rapid (KCl, ouabain and K(+)-free solution) or slower developed (25 mM Na+ solution) phasic contraction followed by a sustained increase in tension. Okadaic acid and nifedipine, alone or in combination, abolished or decreased similarly the contractile response evoked by these stimulants. The okadaic acid- and nifedipine-insensitive responses to ouabain, K(+)-free and low Na+ solution were enhanced by increasing the extracellular concentration of Ca2+ in the medium and were inhibited in a dose-dependent manner by amiloride (0.05-0.5 mM). These data suggest that, in estrogen-primed rat uterus, dephosphorylating mechanisms by OA-sensitive protein phosphatases play an important role in regulating myometrial contractions elicited by Ca2+ entry through voltage-sensitive Ca2+ channels.

Ca(2+)-independent contraction induced by hyperosmolar K(+)-rich solutions in rat uterus

The present experiments were designed to investigate the mechanisms involved in the contractile responses evoked by KCl, added either isoosmotically or hyperosmotically, in the rat uterus. Exposure of uterine strips to a Ca(2+)-free, 3 mM EGTA-containing solution abolished the responses induced by isoosmotic KCl solutions. Conversely, addition of hyperosmolar KCl induced concentration-dependent tonic responses in a Ca(2+)-free, 3 mM EGTA-containing solution. The maximum increase in tension was reached with 210 mM K+. The response to hyperosmotic K+ was unaffected by previous depletion of intracellular Ca2+ stores with oxytocin (1 microM), by inhibition of refilling of the intracellular Ca2+ stores using cyclopiazonic acid (10 microM) or by increasing the concentration of EGTA in the medium to 10 mM. Sucrose and mannitol (60-420 mM) induced concentration-dependent sustained contractions which were not reproducible and were significantly smaller in size than those evoked by the maximally effective concentration of hyperosmotic K+ (210 mM). The contraction induced by hyperosmotic K+ in Ca(2+)-free solution was not altered by the calmodulin inhibitor N-(6-aminohexyl)-5-chloro-1-naphthalenesulfonamide hydrochloride (W-7, 100 microM), the Ca2+/calmodulin protein kinase II inhibitor 1-[N,O-bis(1,5-isoquinolinesulphonyl)-N-methyl-L-tyrosyl]-4-phenyl piperazine (KN-62, 10 microM) or the tyrosine kinase inhibitor genistein (10 microM). The protein kinase C inhibitor calphostin C (1-3 microM) failed to modify the K(+)-effect curve, which was however partially inhibited in the presence of the non-selective protein kinase inhibitor 1-(5-isoquinolinylsulphonyl)-2 methylpiperazine dihydrochloride (H-7, 3-100 microM). The protein kinase inhibitor staurosporine (30-300 nM) depressed the contraction induced by hyperosmolar K+ in a concentration-dependent manner. The contraction induced by sucrose in Ca(2+)-free solution was unaffected by W-7 (100 microM) and KN-62 (10 microM) but was partially reduced by calphostin C (1 microM), H-7 (30 microM), staurosporine (100 nM) and genistein (10 microM). These results suggest that different mechanisms are involved in the responses evoked by isoosmotic and hyperosmotic KCl in the rat uterus. A component of the contraction induced by hypertonic KCl seems mainly independent of both external and internal Ca2+ and of hyperosmolar stress. This contraction is not mediated by protein kinase C, Ca2+/calmodulin-dependent kinases or protein tyrosine kinases but involves activation of other, at the present unknown, staurosporine-sensitive protein kinase(s).

Stretch-induced enhancement of contractions in uterine smooth muscle of rats

1. We studied the effect of servo-controlled stretch of smooth muscle strips from rat uterus on tension and intracellular Ca2+ concentration ([Ca2+]i, using fura-2 as an indicator) at 30 degrees C. 2. When quiescent uterine muscle strips were stretched at a ramp time of 0.5 s by multiples of 5% of the resting muscle length (L0) up to 40%, forty-two out of sixty muscle strips responded with a transient active contraction and a [Ca2+]i increase. The minimum excursion of stretch required for contraction was 26.3 +/- 7.5% of L0 (mean +/- S.D.). The peak response had an all-or-none property and was almost independent of the duration of stretch. 3. Stretches of 30 or 35% of L0 induced contraction in most cases when rapidly applied in 0.2-0.5 s, but slowly applied stretch (ramp duration of 5-10 s) rarely induced contraction. 4. The stretch-induced response was inhibited by the removal of extracellular Ca2+ or by the addition of 10 nM nicardipine. However, it was unaffected by 1 microM tetrodotoxin, 1 microM atropine or by 10 microM cyclopiazonic acid, an inhibitor of Ca2+-ATPase in intracellular Ca2+ stores. 5. When a stretch of 15-35% of L0 was applied during the relaxation phase of 10 nM oxytocin-induced rhythmic contractions, the first contraction after the stretch occurred earlier than that expected from the control rhythm. However, the frequency of the subsequent rhythm returned to almost the control level even during continued application of stretch, although the half-width of rhythmic contractions was increased during stretch. 6. The present study demonstrates that stretch of uterine muscle induces a transient contraction due to Ca2+ influx, which is myogenic and dependent on the excursion and velocity of stretch. The all-or-none property of the stretch-induced contractions suggests initiation of Ca2+ spikes. Furthermore, stretch modulates the oxytocin-induced rhythmic contractions.

Ca2+ entry activated by emptying of intracellular Ca2+ stores in ileal smooth muscle of the rat

1. The effects of depletion of intracellular Ca2+ stores on muscle tension and the intracellular Ca2+ concentration ([Ca2+])i were studied in fura-2 loaded longitudinal smooth muscle cells of the rat ileum. 2. After exposure to a Ca(2+)-free solution, application of Ca2+ caused a small contraction and a rise in [Ca2+]i, both of which were potentiated when the muscle was challenged with carbachol or caffeine before the addition of Ca2+. 3. Cyclopiazonic acid (CPA), a specific inhibitor of sarcoplasmic reticulum Ca(2+)-ATPase, dose-dependently decreased tension development and the rises in [Ca2+]i induced by carbachol and caffeine in the Ca(2+)-free solution, but conversely increased the Ca(2+)-induced responses even in the presence of the voltage-dependent Ca2+ channel blockers, methoxyverapamil and nifedipine. 4. The contraction and rise in [Ca2+]i evoked by Ca2+ gradually declined with time after removal of CPA, while the reverse was the case for the responses to carbachol and caffeine. 5. The Ca(2+)-induced contraction and rise in [Ca2+]i in the presence of CPA were inhibited by the replacement of Na+ with K+ or Cs+, and by the addition of Cd2+, Ba2+, Ni2+ or La3+. 6. The influx of Mn2+ was much greater in extent in the presence of CPA than in its absence. 7. These results suggest that the emptying of intracellular Ca2+ stores may activate Ca2+ influx not associated with voltage-dependent Ca2+ channels in the rat ileal smooth muscle.