Ca2+-independent contraction of uterine smooth muscle

Inhibitory effect of Pyr6 (an Orai channel blocker) on agonist-induced contractions in rat uterus

AIM

Both human and rat myometrium express stromal interaction molecule (STIM) and Orai/ transient receptor potential canonical (TRPC) proteins, which are components of plasma membrane Ca²⁺ store-operated channels. There are reports that these proteins mediate agonist-induced Ca²⁺ influx in cultured myometrial cells. In this study, we aimed to determine the effects of Pyr6, an Orai channel blocker, on different agonist-induced contractions in isolated segments of rat uterus.

MAIN FINDINGS

In Ca²⁺ -free Tyrode's solution, Pyr6 (3 μM) promoted a reduction in both the magnitude and frequency of Ca²⁺ (1 mM)-induced uterine contractions after the addition of carbachol (CCh, 100 μM), but not after the addition of oxytocin (OT, 150 nM). In Ca²⁺ (0.18 mM)-Tyrode's solution, Pyr6 completely relaxed uterine contractions induced by both CCh and cloprostenol (300 nM), but not those induced by either KCI (40-80 mM) or OT. The addition of Pyr6 abolished the oscillatory uterine contractions induced by Ca²⁺ after the addition of cyclopiazonic acid (CPA, 10 μM). When pre-incubated (5 min), Pyr6 reduced the magnitude of both CCh-induced phasic and tonic contractions. The addition of Pyr2 (3 μM), an Orai and TRPC channel blocker, abolished uterine contractions induced by CCh or OT.

CONCLUSION

Considering Pyr6 as an Orai channel blocker and its inhibitory effect on uterine contractions induced by CCh, CPA, and cloprostenol, we suggest that Orai channels are required for the maintenance of contractions induced by these agonists in rat uterus.

Sphingosine 1-phosphate induces cell contraction via calcium-independent/Rho-dependent pathways in undifferentiated skeletal muscle cells

We have previously shown that sphingosine 1-phosphate (S1P) can induce intracellular Ca(2+) mobilization and cell contraction in C2C12 myoblasts and that the two phenomena are temporally unrelated. Although Ca(2+)-independent mechanisms of cell contraction have been the focus of numerous studies on Ca(2+) sensitization of smooth muscle, comparatively less studies have focused on the role that these mechanisms play in the regulation of skeletal muscle contractility. Phosphorylation and activation of myosin by Rho-dependent kinase mediate most of Ca(2+)-independent contractile responses. In the present study, we examined the potential role of Rho/Rho-kinase cascade activation in S1P-induced C2C12 cell contraction. First, we showed that depletion of Ca(2+), by pre-treatment with BAPTA, did not affect S1P-induced myoblastic contractility, whereas it abolished S1P-induced Ca(2+) transients. These results correlated with the absence of troponin C and with the immature cytoskeletal organization of these cells. Experimental evidence demonstrating the involvement of Rho pathway in S1P-stimulated myoblast contraction included: the activation/translocation of RhoA to the membrane in response to agonist-stimulation in cells depleted of Ca(2+) and the inhibition of dynamic changes of the actin cytoskeleton in cells where Rho functions had been inhibited either by overexpression of RhoGDI, a physiological inhibitor of GDP dissociation from Rho proteins, or by pretreatment with Y-27632, a specific Rho kinase inhibitor. Contribution of protein kinase C in this cytoskeletal rearrangement was also evaluated. However, the pretreatment with Gö6976 or rottlerin, specific inhibitors of PKC alpha and PKC delta, respectively, failed to inhibit the agonist-induced myoblastic contraction. Single particle tracking of G-actin fluorescent probe was performed to statistically evaluate actin cytoskeletal dynamics in response to S1P. Stimulation with S1P was also able to increase the phosphorylation level of myosin light chain II. In conclusion, our results strongly suggest that Ca(2+)-independent/Rho-Rho kinase-dependent pathways may exert an important role in S1P-induced myoblastic cell contraction.

Oxytocin-induced phosphorylation of myosin light chain is mediated by extracellular calcium influx in pregnant rat myometrium

Studies of oxytocin-induced phosphorylation of myosin light chain (MLC), resulting in myometrial contraction, suggest that extracellular Ca(2+) influx is involved in its signal transduction. To explore the possibility that intracellular Ca(2+) mobilization by oxytocin may also contribute to MLC phosphorylation, we investigated the relative contributions of these Ca(2+) sources to oxytocin signal transduction in myometrium of pregnant rat. In pregnant rat myometrium, oxytocin-induced Ca(2+) influx occurs via an L-type voltage-dependent Ca(2+) channel. Treatment with verapamil, an antagonist specific for these channels, significantly diminished MLC phosphorylation observed in response to oxytocin administration without affecting the release of Ca(2+) from intracellular Ca(2+) stores. Furthermore, oxytocin-induced MLC phosphorylation was not observed when extracellular Ca(2+) was not present. Our results clearly indicate that extracellular Ca(2+) influx, rather than release from Ca(2+) storage sites, is essential for oxytocin-induced MLC phosphorylation.

Pharmacomechanical coupling in rat vas deferens: effects of agents that modulate intracellular release of calcium and protein kinase C activation

The effects of agents that modulate intracellular release of calcium and protein kinase C (PKC) activation on noradrenaline (NA)-induced contractions of epididymal vas deferens in calcium-free/EGTA (1 mM) medium were investigated. NA (100 microM) or methoxamine (100 microM) evoked repeatable contractions. Clonidine (100-300 microM) was ineffective. The contractions to NA were reduced by procaine (1-10 mM) but not by thapsigargin (0.1-30 microM), ryanodine (1-30 microM) or TMB-8 (1-30 microM). Contractions to cumulative additions of NA (1-100 microM) were enhanced in the presence of cyclopiazonic acid (10 & 30 microM) but not ryanodine (10 & 30 microM). Sequential contractions to NA were not blocked by PKC inhibitors, calphostin C (1 microM) or Ro 31-8220 (1-30 microM) but were reduced by H-7 (1-30 microM), a broad spectrum protein kinase inhibitor. Although RT-PCR experiments detected mRNA for some Ca2+-dependent/DAG-activated and Ca2+-independent/DAG-activated PKC isoforms in epididymal vas deferens, the PKC activators, phorbol 12, 13-dibutyrate (100 microM) or phorbol 12-myristate 13-acetate (100 microM) failed to activate the tissues in calcium-free medium but enhanced subsequent contractions to NA. These results indicate a limited role for intracellular calcium stores and phorbol ester/DAG-sensitive PKC isoforms in NA-induced contraction of epididymal rat vas deferens in calcium-free medium. The results suggest that pharmacomechanical coupling triggered by NA may involve the sensitization of contractile myofilaments to Ca2+ or a Ca2+-independent mechanism. The possible involvement of Ca2+-independent/DAG-insensitive PKC isoforms and agonist-dependent but PKC-independent sensitization pathway is discussed.

Mitogen-activated protein kinase cascade is involved in endothelin-1-induced rat puerperal uterine contraction

The regulation of mitogen-activated protein (MAP) kinase by endothelin-1 (ET-1) in cultured rat puerperal uterine myometrial cells was investigated. ET-1 caused the rapid stimulation of MAP kinase activity. ET-1-induced MAP kinase activation is neither extracellular Ca2+- nor intracellular Ca2+-dependent. ET-1 stimulation also led to an increase in phosphorylation of son-of-sevenless (SOS), and transfection of dominant negative SOS attenuated the ET-1-induced MAP kinase activity. Phorbol-12-myristate 13-acetate (PMA) also induced the MAP kinase activity, but pretreatment of the cultured cells with PMA, to down-regulate protein kinase C (PKC), did not abolish the activation of MAP kinase by ET-1. In addition, down-regulation of PKC had no effect on ET-1-induced SOS phosphorylation. Pertussis toxin, which inactivates Gi/Go proteins, blocked the ET-1-induced MAP kinase activation but not the PMA-induced MAP kinase activation. The results suggested that MAP kinase is acutely activated by ET-1 through a pertussis toxin-sensitive G protein and SOS, not through the PMA-sensitive PKC. In addition, although reverse-transcriptase PCR assays detected messenger RNA for both ET- 1 receptor subtypes in cultured rat puerperal uterine myometrial cells, ET-1-induced MAP kinase activity and uterine contraction were blocked by treatment with BQ485, an antagonist selective for an ET type A receptor (but not by BQ788, an ET type B receptor antagonist). Ritodrine, which is known to relax uterine muscle contraction, attenuated ET-1-induced MAP kinase activity. We further examined the role of MAP kinase pathway in uterine contraction using an inhibitor of MEK activity, PD098059. This inhibitor completely inhibited the ET-1-induced MAP kinase activation and partially, but significantly, inhibited the ET-1-induced uterine contraction. These results indicate that ET-1-induced MAP kinase signaling cascade may play an important role in the ET-1-induced uterine contraction.

Increased contractility of rat uterine smooth muscle at the end of pregnancy

Upon stimulation with high K+, oxytocin, prostaglandin E2, prostaglandin F2 alpha or carbachol, myometrium isolated from pregnant rats (21 days after pregnancy) developed 2-3 times greater isometric force than that from non-pregnant rats (estrus). High K+ increased the level of myosin light chain (MLC) phosphorylation to a similar extent in these tissues, and therefore pregnant myometrium developed greater contraction than non-pregnant myometrium at a given MLC phosphorylation. In the permeabilized muscle with alpha-toxin, Ca2+ (0.1-10 microM) induced greater contraction in pregnant myometrium than in non-pregnant myometrium. Ca2+ sensitivity was not altered after pregnancy. MLC kinase and phosphatase activities did not differ significantly between pregnant and non-pregnant myometria. Stimulation with 10 microM Ca2+ and 1 microM calyculin-A elicited similar magnitudes of contractions in the permeabilized muscles isolated from non-pregnant and pregnant rats. SDS-PAGE showed that the percentage of the content of MLC was not altered between these preparations, although actin content increased after pregnancy. These results suggest that the stress generating capacity of myometrium is increased after pregnancy without changing the MLC phosphorylation step. The equal capacity of force generation after the maximum phosphorylation by Ca2+ and phosphatase inhibitor suggests that a MLC phosphorylation-independent mechanism is responsible for the development of greater force in the pregnant myometrium.

Contribution of sarcoplasmic reticular calcium to smooth muscle contractile activation: gestational dependence in isolated rat uterus

1. The contribution of Ca2+ released from the sarcoplasmic reticulum (SR) to smooth muscle contractile activation remains poorly understood. By simultaneously monitoring cytosolic [Ca2+] ([Ca2+]i) and force in isolated rat uterine smooth muscle, we report the influence of SR Ca2+ release on contractility during conditions (a) of altered SR Ca2+ homeostasis and (b) where the only source of activating Ca2+ was derived from the SR. 2. In myometria of non-pregnant rats, ryanodine (1-50 microM), a modulator of calcium-induced calcium release (CICR), had no effect on the spontaneous [Ca2+]i or force transients. However, depletion of SR Ca2+ by inhibiting the SR Ca2+-ATPase (with cyclopiazonic acid (CPA), 20 microM) resulted in an enhancement of spontaneous [Ca2+]i and force transients. 3. In myometria of pregnant rats, although ryanodine had no effect in 40% of tissues studied it produced a small but significant enhancement of the integrated spontaneous [Ca2+]i and force transient in 60% of cases. The potentiating effects of CPA were enhanced in myometria of pregnant rats compared with non-pregnant rats, often resulting in maintained [Ca2+]i increases and contraction. 4. In zero external Ca2+, agonist-induced SR Ca2+ release resulted in transient increases in [Ca2+]i and force. The magnitude of these agonist-induced [Ca2+]i and force changes were significantly enhanced in myometria of pregnant rats. No evidence for agonist-induced Ca2+-independent force production was observed. 5. These results indicate that CICR plays little role in SR Ca2+ release from the myometrium, and that there are gestational-dependent alterations in the ability of SR Ca2+ mobilization to contribute to contractile activation. The implications of these findings for the co-ordination of myometrial [Ca2+]i signalling and contractility are discussed.

Calcium independent contraction of bladder smooth muscle

BACKGROUND

Recently, it has been suggested that in vascular smooth muscle a Ca(2+)-independent mechanism or Ca(2+)-sensitization of contractile elements may participate in smooth muscle contraction. In this study, we evaluate this mechanism in detrusor muscle.

METHODS

Strips of smooth muscle from rabbit aorta, rabbit bladder and human bladder were evaluated by in vitro contraction studies.

RESULTS

The results show that (1) in Ca(2+)-free solution containing ethyleneglycol bis (-aminothylether)-N,N,-tetraacetic acid (EGTA), carbachol and phorbol ester produced sustained contractions in detrusor muscle (Ca(2+)-free contraction); (2) depletion of Ca2+ stores by caffeine did not affect Ca(2+)-free contraction induced by carbachol; and (3) W-7 (calmodulin inhibitor) and ML-9 (myosin light chain kinase [MLCK] inhibitor) did not show inhibitory effects on Ca(2+)-free contraction, while H-7 (protein kinase C. [PKC] inhibitor) abolished this contraction.

CONCLUSIONS

These results suggest that neither stored Ca2+ nor the Ca(2+)-calmodulin-MLCK system is involved in the carbachol-induced Ca(2+)-free contraction of detrusor muscle. This Ca(2+)-independent contraction seems to be mediated by the activation of PKC coupled with agonist stimulation of the muscarinic receptor.

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).

Oxytocin induced a biphasic increase in the intracellular Ca2+ concentration of porcine myometrial cells: participation of a pertussis toxin-insensitive G-protein, inositol 1,4,5-trisphosphate-sensitive Ca2+ pool, and Ca2+ channels

This study investigated the underlying mechanisms of oxytocin (OT)-induced increases in intracellular Ca2+ concentrations ([Ca2+]i) in acutely dispersed myometrial cells from prepartum sows. A dose-dependent increase in [Ca2+]i was induced by OT (0.1 nM to 1 microM) in the presence and absence of extracellular Ca2+ ([Ca2+]e). [Ca2+]i was elevated by OT in a biphasic pattern, with a spike followed by a sustained plateau in the presence of [Ca2+]e. However, in the absence of [Ca2+]e, the [Ca2+]i response to OT became monophasic with a lower amplitude and no plateau, and this monophasic increase was abolished by pretreatment with ionomycin, a Ca2+ ionophore. Administration of OT (1 microM) for 15 sec increased inositol 1,4,5-trisphosphate (IP3) formation by 61%. Pretreatment with pertussis toxin (PTX, 1 microgram/ml) for 2 hr failed to alter the OT-induced increase in [Ca2+]i and IP3 formation. U-73122 (30 nM to 3 microM), a phospholipase C (PLC) inhibitor, depressed the rise in [Ca2+]i by OT dose dependently. U-73122 (3 microM) also abolished the OT-induced IP3 formation. Thapsigargin (2 microM), an inhibitor of Ca(2+)-ATPase in the endoplasmic reticulum, did not increase [Ca2+]i. However, it did time-dependently inhibit the OT-induced increase in [Ca2+]i. Nimodipine (1 microM), a voltage-dependent Ca2+ channel (VDCC) blocker, inhibited the OT-induced plateau by 26%. La3+ (1 mM), a nonspecific Ca2+ channel blocker, abrogated the OT-induced plateau. In whole-cell patch-clamp studies used to evaluate VDCC activities, OT (0.1 microM) increased Ca2+ current (ICa) by 40% with no apparent changes in the current-voltage relationship.(ABSTRACT TRUNCATED AT 250 WORDS)

Alternating nitroglycerin and syntocinon to facilitate uterine exploration and removal of an adherent placenta

Nitroglycerin (NTG) has been demonstrated to provide uterine relaxation in the management of various obstetric complications. A 32-yr-old woman presented 40 min postpartum for manual removal of a retained placenta. Repeated, alternating doses of NTG 250 micrograms and syntocinon (SYN) 10U iv were used over 15 min to produce periods of uterine relaxation and contraction respectively for uterine exploration. Multiple attempts to extract the placenta failed and a diagnosis of placenta accreta was made. There were no major side effects from this combination of drugs apart from a transient 20% decrease in blood pressure after NTG, which responded to ephedrine 10-15 mg iv. The rapid change in uterine tone was believed to be due not only to the short duration of action of NTG and SYN, but also to the possible physiological antagonism between the two drugs. The mechanism of interaction may involve calcium mobilization and myosin light chain phosphorylation. We conclude that NTG and SYN can be used to produce alternating periods of uterine relaxation and contraction rapidly and consistently with little sustained effects from either agents.

Effects of tyrosine kinase inhibitor, genistein, and phosphotyrosine-phosphatase inhibitor, orthovanadate, on Ca(2+)-free contraction of uterine smooth muscle of the rat

1. The effects of a protein-tyrosine kinase inhibitor, genistein, and a protein-tyrosine phosphatase inhibitor, orthovanadate, were tested on the Ca(2+)-free contraction of the estrogen-dominated rat, which has been proved to be induced mainly via protein kinase C entirely independently of Ca2+. 2. Genistein (30 microM) significantly inhibited the contraction indicating participation of tyrosine kinase activity in the contraction. 3. Orthovanadate caused contraction concentration-dependently and augmented the Ca(2+)-free contraction at concentrations of more than 1 microM. The contraction by orthovanadate was not inhibited so significantly by genistein (30 microM). 4. Possible participation of tyrosine kinase activity in Ca(2+)-free contraction is discussed in addition to the formerly reported participation of protein kinase C.

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

1. U46619 (thromboxane A2 receptors; 0.002-1 microM), carbachol (muscarinic M3 receptors; 0.1-100 microM), cyclopiazonic acid (CPA; Ca(2+)-ATPase inhibitor; 0.1-30 microM) and K+ (5-100 mM) produced concentration-dependent contractions of the mouse isolated anococcygeus muscle. Equi-effective, submaximal concentrations of each agent were used in further experiments (40 nM U46619; 5 microM carbachol; 5 microM CPA; 70 mM K+). 2. Nifedipine (1 microM) totally abolished contractile responses to K+; those to U46619, carbachol and CPA were reduced by only 20-30% in the presence of nifedipine, but were greatly reduced (> 90%) by a combination of nifedipine and SKF 96365 (0.1-40 microM). 3. In Ca(2+)-free medium, contractions to K+ and CPA were abolished. Small residual responses remained to both carbachol and U46619; those to carbachol were transient, could not be repeated in the continued absence of Ca2+ and were prevented by pre-incubation with CPA, but unaffected by SKF 96365; those to U46619 were sustained, could be repeated in the absence of Ca2+, and were resistant to CPA and SKF 96365. 4. Tone induced by all four agents could be relaxed by sodium nitroprusside (SNP), but with a clear order of potency. SNP (pIC40) was most effective against U46619 (7.92), less so against carbachol (6.80) and CPA (6.68), and least potent against K+ (5.94). A similar order of potency was observed with 8Br-cyclic GMP (50 microM) and nitrergic field stimulation (1-20 Hz). 5. The relaxant potency of SNP was similar in normal Krebs solution and in high K+ (70 mM) Krebs containing 1 microM nifedipine. 6. Inclusion of SNP (0.01-1 microM) or 8Br-cyclic GMP (50 microM) in the Ca2+-free medium inhibited the transient residual response to carbachol. Inclusion of similar concentrations of SNP or 8Br-cyclic GMP,during Ca2+ re-loading, increased the subsequent residual contraction to carbachol in Ca2+-free medium.7. At higher concentrations, SNP (0.1-10 microM) produced a partial relaxation of the sustained contraction to U46619 in Ca2+-free medium.8. Thus, the relaxant potency of the nitrergic stimuli was dependent on the agent and mechanism used to induce tone in the preparation. Examination of the contractile/relaxant interactions suggests that altered Ca2+ sequestration and inhibition of contractile protein function may underlie nitrergic relaxations of this tissue.

Ca(2+)-inhibition of Ca(2+)-induced small contraction of rat uterine smooth muscle

Ca2+ has been reported to exert an inhibitory effect on various kinds of smooth muscle. The physiological role of this inhibition is unclear. We investigated the inhibitory action of Ca2+ on the uterine smooth muscle of the rat in estrus, which shows a prominent Ca(2+)-induced inhibition. At concentrations of 0.1-30 microM Ca2+ inhibited the Ca(2+)-independent contraction of this muscle induced by oxytocin in Ca(2+)-free medium. We then investigated the inhibitory action of Ca2+ at various concentrations of Ca2+ in the bathing medium and found that Ca2+ at 1.0-10 microM also inhibited Ca(2+)-dependent contractions, which appeared phasically upon the onset of contractions. The magnitude of these phasic contractions was inversely proportional to the concentration of Ca2+ (between 1-10 microM). At 30 microM Ca2+, however, this inhibition was overcome and large pendular contractions began. Thus, the inhibition may regulate the initiation of smooth muscle contractions. The mechanism of this Ca(2+)-induced inhibition is also discussed with regard to an effect on actin.

Actin-severing and Ca(2+)-induced reversal of smooth muscle contraction that is independent of Ca2+

1. Intracellular actin filament organization of gastric smooth muscle cells of the guinea pig in primary culture was examined with rhodamine-labelled phalloidin using a confocal laser fluorescence microscope. 2. The resting cells, both in the presence and absence of Ca2+, showed an even distribution of microfilamentous actin fibers. 3. The characteristic image of the stimulated cells with 10 microM acetylcholine in the presence of 1.8 mM Ca2+ was that the actin filaments were located only on the periphery of the cell. 4. The characteristic image of the cells stimulated as above, but in the absence of Ca2+, was that the actin filaments were unevenly distributed in the cell. 5. The characteristic image of the cells stimulated in the presence of 1 microM Ca2+, which inhibits the above contraction, was pultaceous with the actin filaments absent, indicating severing of actin filaments by a Ca(2+)-activated system, such as gelsolin.

[Ca2+]i signaling in pregnant human myometrium

The purpose of the present study was to determine the changes in intracellular ionized calcium concentration ([Ca2+]i) or [Ca2+]i sensitivity accompanying spontaneous and agonist-induced contraction of human myometrium at term pregnancy, as well as to quantify the response to three prototypical agonists: 1) oxytocin, 2) vasopressin, and 3) phenylephrine. Uterine biopsies were obtained at the time of cesarean section from patients who delivered at or near full-term gestation. These preparations were used to measure isometric force development and [Ca2+]i levels with the luminescent calcium indicator aequorin. Concentration-response relationships were determined with respect to isometric force development in the presence of the agonist. [Ca2+]i-force relationships were determined with respect to spontaneous phasic contractions, as well as agonist-induced phasic and tonic contractions. The results provide evidence that the phasic nature of term human myometrium is due to 1) the resting [Ca2+]i level being less than the calcium threshold for contractions and 2) the inability of the tissue to maintain high [Ca2+]i levels for prolonged periods of time. In addition, calcium-independent mechanisms of regulation were suggested by the relatively minor calcium sensitizing action of oxytocin and the observation that relaxation of tonic contractions preceded the fall in [Ca2+]i levels.

Contraction of the sheep middle cerebral, pulmonary and coronary arteries initiated by release of intracellular calcium

The aim of the study was to compare contraction initiated by intracellular Ca2+ release in the middle cerebral, coronary and pulmonary arteries of the sheep. With all three arteries from the sheep, incubation in Ca(2+)-free physiological salt solution (PSS) reduced agonist-induced contraction much more than occurred with the rabbit aorta. The intracellular Ca2+ store appeared to be of limited capacity, since contraction was transient in Ca(2+)-free conditions with most agonists. In the middle cerebral artery, contraction in Ca(2+)-free conditions was much reduced if a previous contraction had been obtained (for 5-hydroxytryptamine, 5-HT, from 11 +/- 4 to 1 +/- 0.5% of control contraction in 2.5 mM Ca2+), suggesting that the previous contraction had partly discharged the intracellular Ca2+ store. Contraction was less affected in the pulmonary artery and almost unaffected in the coronary artery (for 5-HT, from 15 +/- 1 to 11 +/- 1%) by a previous contraction in Ca(2+)-free conditions. Rings prepared from small branches of the pulmonary and coronary arteries were affected by Ca2+ deprivation in a similar manner to large diameter pulmonary and coronary artery rings. In Ca(2+)-free PSS, contraction induced by prostaglandin E2 was almost eliminated (3 +/- 1% of control contraction in 2.5 mM Ca2+), contractions induced by 5-HT and noradrenaline were reduced, and contraction induced by the thromboxane mimetic U46619 was least affected (up to 73 +/- 8%). Increasing agonist concentration from EC50 to the maximally effective concentration raised the percentage contraction remaining in the middle cerebral artery (for noradrenaline from 7 +/- 2% to 12 +/- 3%) but not in the pulmonary artery (for noradrenaline from 22 +/- 2% to 24 +/- 6%). The present study has revealed notable differences, in coupling to intracellular Ca2+ release between the three vascular territories studied.

GTP-binding protein amplifies contractile responses of alpha-toxin-permeabilized uterine smooth muscle to Ca2+

1. alpha-toxin of Staphylococcus aureus readily permeabilized rat uterine smooth muscle after incubation for a short time. 2. The permeabilized muscle responded to Ca2+ dose-dependently and repeatedly in the same manner. 3. The threshold concentration of Ca2+ for contraction was 0.1-0.3 microM and the maximal contraction was achieved with 1 or 3 microM Ca2+. 4. GTP gamma S or GTP augmented the contractile response to Ca2+. 5. GDP beta S or GDP suppressed the contraction. 6. The role of GTP-binding protein in sensitization of Ca(2+)-induced contractile response of smooth muscle is discussed.

Protein kinase C of smooth muscle

The primary mechanism of regulation of smooth muscle contraction involves the phosphorylation of myosin catalyzed by Ca2+/calmodulin-dependent myosin light chain kinase. However, additional mechanisms, both Ca(2+)-dependent and Ca(2+)-independent, can modulate the contractile state of smooth muscle. Protein kinase C was first implicated in the regulation of smooth muscle contraction with the observation that phorbol esters induce slowly developing, sustained contractions. Protein kinase C occurs in at least four Ca(2+)-dependent (alpha, beta I, beta II, and gamma) and four Ca(2+)-independent (delta, epsilon, zeta, and eta) isoenzymes. Only the alpha, beta, epsilon, and zeta isoenzymes have been identified in smooth muscle. Both classes of isoenzymes have been implicated in the regulation of smooth muscle contraction. However, the physiologically important protein substrates of protein kinase C have not yet been identified. Specific isoenzymes may be activated by different contractile agonists, and individual isoenzymes exhibit some degree of substrate specificity. Prolonged activation of protein kinase C can result in its proteolysis to the constitutively active catalytic fragment protein kinase M, which would dissociate from the sarcolemma and phosphorylate proteins such as myosin that are inaccessible to membrane-bound protein kinase C. Protein kinase M induces relaxation of demembranated smooth muscle fibers contracted at submaximal Ca2+ concentrations. We suggest that protein kinase C plays two distinct roles in regulating smooth muscle contractility. Stimuli triggering phosphoinositide turnover or phosphatidylcholine hydrolysis induce translocation of protein kinase C (probably specific isoenzymes) to the sarcolemma, phosphorylation of protein, and a slow contraction. Prolonged association of the kinase with the membrane may lead to proteolysis and release into the cytosol of protein kinase M, resulting in myosin phosphorylation and relaxation.

Ca(2+)-independent contraction of uterine smooth muscle induced by vanadate and its inhibition by Ca2+

Vanadate, 30 microM, contracts uterine smooth muscle of estrogen-dominated non-pregnant rats in Ca(2+)-free medium after preincubation with 3 mM EGTA. In spite of the phosphorylation of the myosin light chain during this contraction, studies with fura-2 suggested that this contraction was not accompanied by an increase in the cytosolic Ca2+ level. Inhibitors of the myosin light chain kinase and protein kinase C partly inhibited this contraction. Vanadate seems to enter the cell through anion channels to inhibit phosphatases, resulting in phosphorylation via basal activities of the myosin light chain kinase and protein kinase C. An increase in the cytosolic free Ca2+ level resulted in relaxation of the contracting muscle in the same manner as in the oxytocin-induced Ca(2+)-free contraction.

Ca(2+)-dependent inhibition of Ca(2+)-independent contraction in uterine smooth muscle

Uterine smooth muscle of the rat shows Ca(2+)-independent contraction in response to oxytocin in Ca(2+)-free medium. Micromolar Ca2+ inhibits this contraction. We now tested whether Ca2+ itself is the cause of this inhibition. The ratio of fura-2 fluorescence, the indicator of the intracellular level of Ca2+, was increased in parallel with the degree of inhibition by Ca2+. When inhibition was elicited by Ca2+, EGTA released the inhibition. Comparison of the dose-response curve for oxytocin in Ca(2+)-free solution and that in the medium with 1 microM Ca2+ showed that the inhibition by Ca2+ is non-competitive. EGTA chelation of the intracellular Ca2+ by loading of EGTA as its acetoxymethylester resulted in diminution of inhibition by Ca2+. EGTA suppressed the Ca(2+)-induced contraction but did not affect Ca(2+)-independent contraction. It is concluded that the inhibition is induced by intracellular Ca2+ itself.

Calcium ion itself inhibits Ca-free contraction of various smooth muscles in response to adrenergic and cholinergic stimulations and to TPA in the absence of Ca channel blocker and EGTA

1. Smooth muscles such as thoracic aorta, stomach and vas deferens contract tonically in the absence of Ca ion (Ca) in response to various agonists such as norepinephrine, carbachol and TPA. 2. Addition of Ca alone to the Ca-free medium relaxes the muscle that has been contracting in response to the agonist in the Ca-free medium. 3. Ca itself has an inhibitory action on the contraction. 4. Acidification of the medium relaxes the muscle as Ca does, but weakly.

Involvement of protein kinase C in Ca(2+)-independent contraction of rat uterine smooth muscle

The contribution of protein kinase C to the contraction by oxytocin of rat uterine longitudinal smooth muscle in Ca(2+)-free solution was investigated. Immunological analysis revealed that type II (beta) and III (alpha) protein kinase C subspecies were present in rat uterine smooth muscle. The pretreatment of a diacylglycerol kinase inhibitor R59022 to accumulate diacylglycerol potentiated the Ca(2+)-independent contraction. The contractile activity was diminished with the depletion of protein kinase C, when the contraction was evoked repeatedly by oxytocin during the prolonged exposure to a tumor-promoting phorbol ester 12-O-tetradecanoylphorbol 13-acetate. These results suggested the involvement of protein kinase C in oxytocin-induced contraction in Ca(2+)-free solution.

Oxytocin contracts rat uterine smooth muscle in Ca2(+)-free medium without any phosphorylation of myosin light chain

Contraction of rat uterine smooth muscle related to phosphorylation state of myosin light chain under various conditions was investigated. In the Ca2(+)-containing medium, both high K+ and oxytocin induced marked contraction of the muscle accompanied by pronounced phosphorylation of myosin light chain. In the Ca2(+)-free medium, although both vanadate and oxytocin induced slight contraction, phosphorylation of myosin light chain was only evident for vanadate but not for oxytocin. It was suggested that another mechanism distinct from myosin light chain phosphorylation might be involved in Ca2(+)-independent contraction of uterine smooth muscle elicited by oxytocin.

Calcium reversal, relaxation by calcium ion of various smooth muscles contracted by carbachol, norepinephrine or a phorbol ester in calcium-ion free medium

1. Ca reversal was reported on the longitudinal smooth muscle of rat uterus and the fundic part of the circular smooth muscle of guinea pig stomach; that is, addition of Ca ion to the Ca-free bathing solution in which the muscle is contracting to oxytocin or carbachol, respectively, relaxes the muscle. 2. We tested whether this inhibitory action of Ca ion is seen in other smooth muscles including vascular, air way, gastric and genital smooth muscles. 3. We found that Ca reversal was observed in the smooth muscle of rat thoracic aorta contracted by norepinephrine or by a phorbol ester, TPA, guinea pig tracheal smooth muscle contracted to carbachol, fundic smooth muscle of rat stomach to carbachol, corporal and antral smooth muscle of guinea pig stomach to carbachol and rat vas deferens to norepinephrine. 4. Ca reversal was observed not only when the muscle contracted by an agonist of the surface receptor such as oxytocin, norepinephrine or carbachol but also by a phorbol ester that is believed to cause contraction in the cell by activating phosphorylation. 5. Thus, we conclude that Ca reversal is a universal phenomenon in smooth muscles.

Inhibitory effects of protein kinase inhibitors and cytoskeletal inhibitors on Ca2(+)-free contraction of rat uterus

In rat uterine smooth muscle, sustained Ca2(+)-free contraction was observed by oxytocin in Ca2(+)-free solution. This Ca2(+)-free contraction was effectively inhibited by protein kinase inhibitors and cytoskeletal inhibitors but myosin-light chain kinase (MLCK) inhibitors were not so effective. Simultaneous addition of a protein kinase inhibitor and a cytoskeletal inhibitor caused synergistic inhibition. These results suggest that the mechanism for Ca2(+)-free contraction involves some protein kinase and cytoskeletal elements rather than MLCK.