Maintained contractions of rat uterine smooth muscle incubated in a Ca2+-free solution

Selective Inhibition of Calcium Entry Induced by Benzylisoquinolines in Rat Smooth Muscle

The mechanism of relaxant activity of six benzylisoquinolines was examined in order to determine the minimal structural requirements that enable these compounds to have either a non-specific action like papaverine or an inhibitory activity on calcium entry via potential-operated channels. All the alkaloids tested totally or partially relaxed KCl-depolarized rat uterus and inhibited oxytocin-induced rhythmic contractions. Only glaucine and laudanosine inhibited K+-induced uterine contractions more than oxytocin-induced uterine contractions. In Ca+-free medium, sustained contractions induced by oxytocin or vanadate were relaxed by the alkaloids tested except for glaucine and laudanosine indicating no inhibitory effect on intracellular calcium release. Those alkaloids containing an unsaturated heterocyclic ring (papaverine, papaverinol, papaveraldine, N-methylpapaverine and dehydropapaverine) exhibited a more specific activity than those with a tetrahydroisoquinoline ring.

Effects of sodium fluoride on the mechanical activity in mouse gastric preparations

The aim of the present study was to investigate the responses induced by sodium fluoride (NaF) on gastric mechanical activity, using mouse whole-stomach preparations. The mechanical activity was recorded in vitro as changes of intraluminal pressure. In most of the preparations, NaF induced a tetrodotoxin-insensitive biphasic effect characterized by early relaxation followed by slowly developing contractile response. The contraction was dependent on the concentration of NaF, whereas the relaxation was observed at only 10-30 mmol/L NaF. The contractile effect was significantly reduced by nifedipine (an L-type Ca(2+) channel blocker), ryanodine or ruthenium red (inhibitors of Ca(2+) release from sarcoplasmic reticulum), and GF109203X (a protein kinase C inhibitor). Moreover, it was abolished by neomycin (an inhibitor of phospholipase C) and potentiated by SQ22536 (an inhibitor of adenylyl cyclase). All the drugs significantly increased the relaxation, except SQ22536, which abolished it. The present results suggest that NaF causes a complex mechanical response in the whole-stomach, which might explain gastric discomfort after fluoride ingestion. The relaxation appears owing to production of cAMP, while the contractile effects imply activation of phospholipase C, protein kinase C, influx of Ca(2+), and release of Ca(2+) from ryanodine-sensitive intracellular store.

Effect of 14-deoxyandrographolide on calcium-mediated rat uterine smooth muscle contractility

In this study, the effect of 14-deoxyandrographolide (14-DAP) on calcium channel-dependent rat uterine smooth muscle contraction was evaluated. Using a tissue bath preparation, 14-DAP was able to reduce the contractile response to 0.3 and 3.0 mm of CaCl(2), with an IC(50) of 1.24 +/- 0.23 x 10(-5) m and 5.94 +/- 0.29 x 10(-5) m, respectively. 14-DAP shifted the CaCl(2) cumulative dose response curve to the right, increasing the EC(50) from 2.08 +/- 0.20 x 10(-4) m to 4.22 +/- 0.22 x 10(-4) m (5 micrometer 14-DAP) and 2.5 +/- 1.0 x 10(-3) m (50 micrometer 14-DAP). In order to determine if 14-DAP had any effect on intracellular calcium, the relaxant response to 14-DAP following contraction by oxytocin, PGF(2alpha) and vanadate in Ca(+2)-free solution was compared with that of isoproterenol and phenylbutazone. While isoproterenol and phenylbutazone relaxed the smooth muscle in a dose-dependent manner, 14-DAP did not have any effect on either the oxytocin, PGF(2alpha) or vanadate-induced smooth muscle contraction. Based on these data, it appears that 14-DAP is an uterine smooth muscle relaxant which produces a selective blockade of voltage operated calcium channels.

New developments in the management of preterm labor

Current management of preterm labor has not changed the incidence of preterm delivery; therefore, significant research effort has been concentrated on the search for new methods of management. New tocolytics like inhibitors of cyclooxygenase 2 and nitric oxide donors have been tested in animal models and in preliminary clinical trials with promising results. Inhibition of cervical ripening may be one alternative to tocolysis. This new approach has a potential to be a valuable method of management of preterm labor if human studies confirm the promising results reported in animals. Growing evidence suggests that premature delivery may be associated with infection or fetal growth abnormalities, with dire consequences to the fetus. If these associations are to be included in risk and benefit assessment, then inhibition of preterm labor may prove to be detrimental to the fetus.

Andrographis paniculata(Ness) induces relaxation of uterus by blocking voltage operated calcium channels and inhibits Ca+2influx

The possible relaxation of uterine smooth muscle by Andrographis paniculata dried extract via a blockade of voltage operated calcium channels was investigated in rats. Uterine horns pretreated with oestradiol were incubated in Ca(+2) -free Jalon's solution and stimulated with KCl (20-60 mM) in order to produce depolarization of the membrane. The isometric contractile response to 1 mM or cumulative concentrations of CaCl2 were blockaded by 0.2, 0.4 and 0.8 mg/mL of A. paniculata. The maximum contractile response induced by acetylcholine was moderately antagonized by A. paniculata. The possible blockade of Ca(+2) entry by A. paniculata was evaluated with (45)Ca(+2) uptake in uterine rings incubated with free-Ca(+2)-Ringer's solution high in K+ (KCl 40 mM). The influx was completely blockaded with 0.4 mg/mL of A. paniculata. These results strongly suggest that A. paniculata blockades voltage operated calcium channels inhibiting the entry of Ca(+2) from the external medium.

Instrumentation for the diagnosis of term and preterm labour

The problems associated with labor during pregnancy are among the most important health issues facing physicians. Understanding the role of the uterus and cervix in labor and developing methods to control their function is essential to solving problems relating to labor. At the moment, only crude, inaccurate and subjective methods are used to assess changes in the uterus and cervix that occur in preparation for or during labor. In the past several years, we have developed noninvasive methods to quantitatively evaluate the uterus and cervix based respectively on recording of uterine electrical signals from the abdominal surface (uterine EMG) and measurement of light-induced cervical collagen fluorescence (LIF) with an optical device (Collascope). The methods are rapid and allow assessment of uterine contractility and cervical ripening. Studies in rats and humans indicate that uterine and cervical function can be successfully monitored during pregnancy using these approaches and that these techniques might be used in a variety of conditions associated with labor to better define management. The potential benefits of the proposed instrumentation and methods include a reducing the rate of preterm delivery, improving maternal and perinatal outcome, monitoring treatment, decreasing cesarean section rate and improving research methods to understand uterine and cervical function.

Effects of leaf extract of Caesalpinia bonduc (Caesalpiniaceae) on the contractile activity of uterine smooth muscle of pregnant rats

The calcium dependency and the cholinergic effect of the leaf extract of Caesalpinia bonduc Roxb. (Caesalpiniaceae) was studied in isolated pregnant rat myometrium preparations. Isometric contractions were recorded. The extract (Cebo) increased the contractile force in the isolated strips in a concentration-dependent manner. The effects were comparable to those obtained with acetylcholine. Contractions induced by Cebo or acetylcholine were inhibited in the presence of atropine. The stimulating action of Cebo on the contractile responses of isolated myometrium preparations inhibited by atropine may be mediated by cholinergic receptors. In calcium-free solution Cebo induced a tonic contraction (contracture) of the muscle. Moreover, in high-potassium calcium-free solution Cebo caused contracture of the uterine smooth muscle. Cebo was still able to elicit contractions in calcium-free solution containing EDTA or EGTA. These findings suggest the existence of cholinergic receptors sensitive to Cebo which could influence the influx of calcium (phasic contraction) and mobilization of calcium from cellular stores (tonic contraction), both of which are responsible for the increase of contractile activity and development of the contracture of uterine smooth muscle.

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.

Relaxant effect of tetrazepam on rat uterine smooth muscle: role of calcium movement

Tetrazepam is a benzodiazepine derivative clinically used as a muscle relaxant. The aim of the present work was to examine its effect on uterine smooth muscle of the rat in estrus. Tetrazepam required micromolar concentrations to relax contractile responses induced by KCl and acetylcholine in Ca2+ solution, but not oxytocin-induced contraction. In Ca(2+)-free solution, tetrazepam inhibited Ca(2+)-induced contractions in depolarized uterus and vanadate-induced contractions. We suggest that tetrazepam relaxes contractile responses induced by activation of voltage-sensitive calcium channels and receptor-operated calcium channels with little selectivity or that it antagonizes the effect of calcium at subsequent steps, possibly intracellular stores sensitive to vanadate but not sensitive to oxytocin. The inhibition of contraction of rat uterus is not related to high-affinity peripheral benzodiazepine binding sites.

Uterotonic effects of hydromethanolic extract of Parquetina nigrescens (Periplocaceae) on spontaneous contractile activity in the isolated myometrium of pregnant rats

Parquetina nigrescens (Afz.) Bullock (Periplocaceae) hydromethanolic extract (PAR) in reference solution exerts a stimulating and spasmogenic action on the pregnant rat myometrium in vitro. This action is characterised by increasing amplitude of spontaneous isometric contractions and a slight elevation of muscular basic tonus. These effects on myometrium may be compared with those provoked by sulprostone (PGE2) in the same experimental conditions. In calcium free solution, Parquetina nigrescens extract could restore partially spontaneous contractile activity. On the contrary, in presence of EDTA PAR induces a complete abolition of the activity. On the other hand, in calcium free solution, sulprostone is able to develop a clear elevation of basic tonus (contracture). These observations support the hypothesis that the extract has an oxytocin-like effect, which is characterised by an extracellular influx of calcium, responsible for the increase of the maximum isometric contraction amplitude. Sulprostone activates both extracellular and intracellular calcium responsible for the increase of contractile activity and development of the contracture of uterine smooth muscle.

Involvement of sodium/calcium exchange in the diclofenac-induced spasmolytic effect on rat uterus

1. The effect of diclofenac (10-100 microM) on rat uterus contraction and its modification by ouabain (0.1 mM), amiloride (0.1 and 1 mM), ouabain (0.1 mM) plus amiloride (1 mM) and the replacement of sodium by choline have been assayed. 2. Diclofenac produces dose-dependent relaxation of vanadate (0.3 mM)-induced contraction (EC50, 17.3 +/- 1.8 microM). This effect is significantly reduced in choline medium (EC50, 49.1 +/- 4.5 microM) and by ouabain in sodium-medium (EC50, 52 +/- 7 microM). 3. Amiloride displaces, in a dose-dependent way, the diclofenac-induced relaxant effect. However, ouabain plus amiloride did not produce a sinergic effect. 4. Our results suggest that diclofenac produces relaxation of vanadate-induced contraction by activation of Na+/Ca(2+)-exchange.

Effects on rat uterine and aorta strip smooth muscle of Thymus leptophyllus extract

The diethylether extract from Thymus leptophyllus was found to be more active on uterine smooth muscle than on aorta strips. Rat uterus experiments with and without extracellular calcium, yielded similar IC50 values. A nonspecific mechanism for the relaxant activity can therefore be postulated. In rat aorta and in the presence of extracellular calcium the extract inhibited the contractile response induced by K+ depolarizing solution and had a less inhibitory effect on noradrenaline (NA) contraction. In a Ca(2+)-free solution the extract strongly reduced the Ca(2+)-release induced by NA, but it did not affect the transient contraction caused by caffeine (CAF).

GRP-preferring bombesin receptors increase generation of inositol phosphates and tension in rat myometrium

In the estrogen-treated rat myometrium, bombesin (Bn) and related agonists triggered contraction and the increased generation of inositol phosphates. The relative order of potencies was identical for both responses: Bn = gastrin releasing peptide (GRP) = litorin = neuromedin C >> neuromedin B. Two specific GRP-preferring receptor antagonists, namely [D-Phe6]Bn-(6-13) methyl ester and [Leu14,psi 13-14]Bn were inhibitory for both Bn-mediated tension and generation of inositol phosphates. [125I-Tyr4]Bn bound to myometrial membranes with high affinity (Kd = 104 pM) to a single class of sites in a saturable and reversible manner. The relative potencies for inhibiting binding were GRP = litorin = [Tyr4]Bn (Ki = 0.4 to 0.6 nM) >> neuromedin B (Ki = 10.3 nM). The high affinity displayed by [D-Phe6]Bn-(6-13) methyl ester (Ki = 2.8 nM) and [Leu14,psi 13-14]Bn (Ki = 35 nM) for competing for [Tyr4]Bn binding supported the involvement of a GRP-preferring Bn receptor. Guanine nucleotides decreased the binding of [125I-Tyr4]Bn and accelerated the rate of ligand dissociation, reflecting the coupling of receptors to guanine nucleotide regulatory proteins (G proteins). The results demonstrate that rat myometrium expresses functional GRP-preferring Bn receptors whose activation stimulates the phospholipase C pathway, pertussis toxin-insensitive event that contributes to Bn-mediated uterine contractions.

Comparative study of the rat uterine smooth muscle relaxant activity of three bisbenzyltetrahydroisoquinolines with tetrandrine

The relaxant activity of three bisbenzyltetrahydroisoquinolines--obaberine, popisonine and lindoldhamine--was examined in rat isolated uterus and their inhibitory potencies were compared with that of tetrandrine. All alkaloids tested relaxed KCl-depolarized rat uterus and totally or partially inhibited oxytocin-induced rhythmic contractions. The degree of methylation of the free phenolic hydroxy groups and the loss of one diarylether bridge influence the potency of relaxant action of these alkaloids. Only alkaloids with absolute configuration 1R,1'S or 1R1'R acted intracellularly, promoting relaxation of contractile responses induced by oxytocin or vanadate in a Ca(2+)-free medium.

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.

External calcium dependence of the uterine contraction induced by prostaglandins E2 and F2 alpha and its antagonism with natural progestins

Prostaglandins (PGs) E2 and F2 alpha are strong inducers of uterine contraction by promoting a Ca2+ increase into the cell through specific receptors coupled with the calcium channels. On the contrary, progesterone and 5 beta-reduced progestins promote smooth muscle relaxation by blocking the ion calcium influx. Thus, this study was designed to emphasize the importance of external calcium in the PGs-induced rat uterus contraction. Likewise, also studied was the antagonism and the interaction between PGs and progestins (progesterone and its 5 alpha and 5 beta-reduced derivatives) in the myometrium. Results showed that uterine contraction induced by PGs depends on external calcium, since verapamil or extracellular calcium depletion abolished the PGs effect. Regarding the PGs-progestins antagonism, it was observed that pregnanedione, pregnanolone and epipregnanolone were quite effective for counteracting of PGs-induced contraction. However, progesterone was effective in a middle range, whereas 5 alpha-reduced progestins (allopregnanedione and allopregnanolone) were almost ineffective. It has been concluded that the participation of PGs and progestins in the modulation of uterine contraction might be achieved through the control of calcium influx by opening (PGs) or blocking (progestins) receptor-operated calcium channels.

Mediators involved in the rat uterus contraction in calcium-free solution

1. The effect of (Na+ + K+)-ATPase inhibitor ouabain (10(-5)-3 x 10(-4) M), and the (Ca2+ + Mg2+)-ATPase inhibitors vanadate (6 x 10(-6)-6 x 10(-4) M), oxytocin (2 x 10(-9)-4 x 10(-8) M, and prostaglandin F2 alpha (PGF2 alpha, 10(-7)-6 x 10(-6) M) were assayed on rat uterus incubated in Ca-free medium. 2. Vanadate, oxytocin and PGF2 alpha, but not ouabain, induced contractions in a dose-dependent way (ED50: 7.5 +/- 0.03 x 10(-5) M; 6.5 +/- 0.064 x 10(-9) M and 3.8 +/- 0.085 x 10(-7) M). 3. Vanadate (3 x 10(-4) M) and oxytocin (OT, 10 mU/ml = 2 x 10(-8) M)-induced tonic contraction were not modified by nifedipine (10(-10)-10(-6) M), monensin (10(-5)-3 x 10(-4) M) or amiloride (10(-5)-10(-3) M). 4. The intracellular calcium release inhibitors TMB-8 (10(-6)-10(-4) M) and dantrolene (3 x 10(-6)-10(-4) M), and the prostaglandin release inhibitor indomethacin (3 x 10(-8)-6 x 10(-5) M) relaxed the vanadate and OT-induced tonic contractions. 5. The calmodulin inhibitors trifluoperazine (3 x 10(-5)-3 x 10(-4) M), bepridil (10(-8)-3 x 10(-4) M), calmidazolium (10(-7)-10(-4) M) and W-7 (10(-7)-10(-5) M) also relaxed the vanadate and OT-induced tonic contractions. 6. Our results suggest that oxytocin and vanadate-induced contractions on rat uterus in Ca-free medium could be produced by release of prostaglandins and intracellular calcium, and mediated by calmodulin.

Effect of external Cd2+ and other divalent cations on carbachol-activated non-selective cation channels in guinea-pig ileum

1. Inhibitory actions of the external divalent cations, Cd2+, Ni2+, Mn2+ and Co2+ on carbachol (CCh)-induced inward current (Ins,ACh) were investigated in caesium aspartate-loaded single longitudinal smooth muscle cells of guinea-pig ileum, using a rapid solution-switching device, under voltage-clamped conditions. 2. Cd2+ (20-1000 microM), when added to the external solution, reduced Ins,ACh rapidly and reversibly. This effect occurred dose dependently, the relationship being adequately described by a Michaelis-Menten equation with a Hill coefficient (n) of 1.0 and a dissociation constant (Kd) of 98 microM. 3. The inhibitory action of Cd2+ was associated neither with agonist concentration (CCh) nor with changes in reversal potential, and was voltage independent. 4. The appearance and removal of the effect of Cd2+ were both rapid (a few hundred milliseconds), in sharp contrast with a relatively slow time course of atropine or CCh action (of the order of seconds). 5. Other divalent cations (Ni2+, Mn2+, Co2+, Mg2+) applied externally also suppressed Ins,ACh, but less potently. The sequence of apparent potency was Cd2+ (98 microM) greater than or equal to Ni2+ (131 microM) much greater than Co2+ (700 microM) greater than or equal to Mn2+ (1000 microM) much greater than Mg2+ (approximately 10 mM). 6. External Ca2+ increased Ins,ACh dose dependently and antagonized the inhibitory effect of Cd2+. However, this effect may not be a simple competition with Cd2+. 7. These results show that external divalent cations strongly modulate Ins,ACh channels, possibly through direct interaction with the channel protein.

The uterotonic action of the aqueous extract of Bridelia atroviridis in the rat

The effects of the aqueous extract of leaves of Bridelia atroviridis (Bridelia), a small African tree, on the mechanical activity of rat uterus were studied. The aqueous extract of leaves of B atroviridis administered in a concentration-dependent manner (5 x 10(-6)-1.2 x 10(-3) g/ml) induced contractions that were antagonized by various calcium entry blockers (nifedipine, diltiazem, manganese chloride). In absence of external calcium ions, repeated applications of a supramaximal concentration of Bridelia (1.2 x 10(-3) g/ml) evoked sustained and repeated contractions the amplitude of which was congruent to 20% of those obtained in the physiological external calcium concentration. Bridelia-induced contractions in calcium-free medium were inhibited by isoprenaline (8 x 10(-7) M), caffeine (15 x 10(-3) M) and trifluoperazine (10(-5) M). Contractile responses induced by Bridelia in both calcium-containing and calcium-free media were antagonized by prior incubation of uterus with phorbol 12, 13-dibutyrate (6 x 10(-7) M), cholera toxin (6 x 10(-8) M) or pertussis toxin (5 x 10(-7) g/ml). These results show that Bridelia has a potent uterotonic action in the rat. The cellular basis of this action appears to be complex, and involves various mechanisms including calcium mobilization from both intra and extracellular compartments and activation of phospholipase C through a G-protein.

Membrane-associated signal transduction modulates contractile responses to Ca2+ in saponin-skinned coronary smooth muscle

Experiments on saponin-permeabilized bovine and porcine coronary arteries were performed to examine whether fluoride-induced contractile mechanisms are retained after permeabilization and whether they modulate muscle tension without a change in cytoplasmic Ca2+. Fluoride induced reversible contractions in skinned coronary smooth muscle and shifted the apparent Ca2(+)-sensitivity of the contractile system to lower Ca2+ values. Force development to fluoride required exogenous ATP and a minimum Ca2+ concentration (pCa less than 8) of the bathing medium. Force development occurred when the concentration of free Ca2+ available to the contractile apparatus seemed to be sufficiently controlled by a Ca2(+)-EGTA buffering system. The effects of fluoride were enhanced by Al3+ and imitated by both guanylyl-imidodiphosphate and 4 beta-phorbol-12,13-dibutyrate which suggests the involvement of both GTP-binding protein(s) and of the C-kinase pathway. Under conditions of sufficiently controlled Ca2+ buffering, saponin-skinned arteries responded to histamine or carbachol with an increase in muscle tension which was inhibited by specific receptor blocking agents (diphenhydramine, atropine), and which required the presence of both exogenous ATP and a low concentration of Ca2+ (pCa 7) which per se did not result in a contractile response. It is concluded that agents acting on different steps of membrane-associated signal transduction pathways are able to induce contractile responses of saponin-skinned vascular smooth muscle without a change in cytosolic Ca2+ concentration.

Reserpine non-selectively inhibits rat uterus contraction in vitro

1. Reserpine produced a direct in vitro non-selective inhibitory effect on smooth muscle contraction of endometrium-free rat uterus. 2. Reserpine uptake into uterine muscle and its antagonistic effect on contraction had a similar time course. 3. Reserpine had a relaxing effect similar to that of trifluoperazine and different from that of verapamil or papaverine, and also failed to exert any inhibitory effect on 45Ca uptake rate. 4. Both reserpine and trifluoperazine but not verapamil inhibited the acetylcholine-induced contraction when present during the Ca-release from intracellular stores. 5. It is hypothesized that reserpine exerts its inhibitory action intracellularly on the activation of smooth muscle contraction by sarcoplasmic Ca2+.

Mechanisms involved in the effects of phenidone, diclofenac and ethacrynic acid in rat uterus in vitro

1. The effects of phenidone (P, 10(-4)-10(-3) M), sodium diclofenac (D, 10(-5)-10(-4) M) and ethacrynic acid (E, 10(-5)-10(-4) M), proposed as inhibitors of eicosanoid synthesis, on the contractions of rat uterus induced by several agonists have been studied. 2. P, D and E inhibit the motility induced by oxytocin (4 mU/ml) (IC50: 4.62 x 10(-4), 2.55 x 10(-4) and 2.98 x 10(-5) M, respectively). 3. P (10(-3) M), D (10(-4) M) and E (10(-4) M) also inhibit the contraction induced by methacholine (10(-5) M), prostaglandin F2a (10(-6) M) and CaCl2 (6 mM), and relaxed, in a dose-dependent way, the tonic component of contraction to KCl (60 mM) (IC50: 5.81 x 10(-4), 6.67 x 10(-5) and 7.55 x 10(-5) M, respectively). 4. The CaCl2 (0.1-10 mM) reverted the relaxation of KCl contraction produced by P, but not by D or E. None of the inhibitions on CaCl2 (6 mM) are reverted by Bay K 8644. 5. D and E also relaxed the tonic contraction to vanadate (10(-4) M) in uterus incubated in calcium free solution P, enhances the vanadate-induced contractions.

Fluoride is a contractile agent of guinea pig airway smooth muscle

1. Guinea pig parenchymal lung strips and tracheal smooth muscle contract potently after NaF-addition. Maximal contractions of lung strips and tracheal rings induced by NaF were 208 +/- 17% (n = 6) and 151 +/- 8% (n = 4) of the maximal histamine response respectively. 2. The -log EC50-value for NaF on lung strips and tracheal rings was 2.38 +/- 0.01 (n = 6) and 2.28 +/- 0.01 (n = 4) respectively. 3. Contractions induced by NaF were augmented after Al3+ pretreatment, suggesting the involvement of a G-protein. NaF responses were not affected by blockade of H1-, muscarinic-, leukotriene C4- or leukotriene D4-receptors, indicating that mast cell degranulation or nerve activation is most probably not implicated. 4. Contractions after NaF-addition were relatively insensitive to removal of extracellular calcium and were reversed via cAMP- and cGMP-mediated pathways. 5. Relaxation studies with (-)isoprenaline and 8-bromo-cGMP on lung strips, precontracted to similar levels with either a H1-agonist, KCl or NaF, showed that the level of relaxation depends on the contractile agent that is used. 6. After precontraction with KCl (-)isoprenaline relaxes lung strips only to 58 +/- 9% (n = 5) of the initial contraction, whereas lung strips precontracted with NaF or a H1-agonist relax 114 +/- 8% (n = 4) and 120 +/- 7% (n = 5) respectively with (-)isoprenaline. 7. Similar results were obtained with relaxation induced with 8-bromo-cGMP. 8. These findings suggest that NaF-induced contractions are elicited via a mechanism, that is probably similar to that of the H1-receptor. The involvement of a G-protein in the observed NaF-responses is therefore likely.

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.

Effects of vanadate, ouabain and amiloride on the contraction of the rat testicular capsule to oxytocin

1. The modification of the contraction of the rat testicular capsule to oxytocin (OT) by vanadate (0.7, 7 and 70 microM), ouabain (0.1 mM), and amiloride (10 microM to 1 mM) have been studied. 2. OT (1 nM-6 microM) and vanadate (10 microM-3 mM) induced contraction of the rat testicular capsule in a dose-dependent way (ED50: 188 +/- 66 nM and 82.8 +/- 7.4 microM, respectively). 3. Vanadate (0.7, 7 and 70 microM) and ouabain (0.1 mM) increases the contractile effect of OT (50 and 200 nM). 4. Amiloride (10 microM-1 mM) inhibit, in a dose-dependent way, the OT-contraction. 5. Amiloride (10 microM or 50 microM) block the ouabain but not the vanadate potentiation to OT.

Contractile responses of rat aorta to phenylephrine and serotonin, and aging

1. The potency (pD2 value) of phenylephrine increased with age from 3 to 10 weeks, but decreased thereafter from 10 to 80 weeks, while the affinity (pKA value) of phenylephrine to alpha 1-adrenoceptors did not alter with aging. 2. The potency (pD2 value) of serotonin did not alter with aging. 3. There is no significant difference between slopes of regression lines between a cytosolic free Ca2+ level ([Ca2+]i) and tension in the presence of phenylephrine in aorta strips from 10- and 60-week-old rats, suggesting that the sensitivity of contractile system to Ca2+ did not alter with aging. 4. Effect of ryanodine on the transient increase of [Ca2+]i and the followed sustained contraction induced by phenylephrine or serotonin in Ca2+ free solution did not alter with aging.

Effects of aging on postsynaptic alpha 1-adrenoceptor mechanisms in rat aorta

1. Effects of aging on alpha 1-adrenoceptor and S2-serotonin receptor mechanisms in rat aorta were studied. 2. In rat aorta, the potency (pD2 value) of norepinephrine or phenylephrine increased with age from 3 to 10 weeks, but decreased thereafter with age from 10 to 80 weeks. The affinity (pKA value) of norepinephrine or phenylephrine and of prazosin (pA2 value) did not alter with aging. 3. In rat vas deferens, the efficacy of norepinephrine and the maximum binding sites of [3H]prazosin increased with age from 3 to 18 weeks, but decreased thereafter with age from 18 to 60 weeks. The affinity (pKA value) of norepinephrine and the dissociation constant (KD value) of prazosin did not alter with aging. 4. In rat aorta, the potency (pD2 value) and affinity (pKA value) of serotonin, and affinity (pA2 value) of ketanserin did not alter with aging. 5. There is no significant difference between slopes of regression lines between a cytosolic free Ca2+ level [( Ca2+]i) and tension in the presence of phenylephrine in aorta strips from 10- and 60-week-old rats. 6. These results suggest that changes in alpha 1-adrenoceptor mechanisms with aging are due to changes in receptor density or receptor reserve, but not to changes in affinity of drugs to alpha 1-adrenoceptor or sensitivity of contractile system to Ca2+ mediated through alpha 1-adrenoceptor, and that S2-serotonin receptor mechanisms in rat aorta do not alter with aging.

Effects of vanadate in testicular capsule of the rat

1. The effects of sodium orthovanadate (vanadate. 10(-5) to 3 x 10(-4) M) on testicular capsule of the rat, and the modifications of these effects by the calcium chelator EGTA (2mM), the calcium entry blockers verapamil (5 X 10(-5) M), nifedipine (10(-5 M) and diltiazem (5 x 10(-5) M), the (Na+ + K+)-ATPase inhibitor ouabain, the Na+/Ca2+ exchange inhibitor amiloride, and the calmodulin antagonists trifluoperazine (10(-4) M) and W-7 (5 x 10(-5) M) have been studied. 2. Vanadate induced contraction of the rat testicular capsule in a dose-dependent way (ED50: 82.8 +/- 7.4 x 10(-6) M). 3. The contraction induced by vanadate (3 and 30 x 10(-5) M) were abolished by EGTA and not modified by verapamil, nifedipine, flunarizine or diltiazem. 4. Amiloride (1 and 5 x 10(-5) M), but not ouabain (5 x 10(-5) and 10(-4) M), inhibit in a dose-dependent way the contraction induced by two doses of vanadate (3 and 30 x 10(-5) M). 5. Trifluoperazine and W-7 significantly inhibit the contraction of testicular capsule to 3 and 30 x 10(-5) M vanadate.

Modulation of the [Ca2+] sensitivity of myosin phosphorylation in intact swine arterial smooth muscle

1. The [Ca2+] sensitivity of myosin light chain phosphorylation in vascular smooth muscle is dependent on the form of stimulation. Contractile agonist stimulation, when compared to high-KCl depolarization, is associated with an increase in [Ca2+] sensitivity of phosphorylation. I evaluated potential mechanisms for this stimulus-specific response by measuring aequorin-estimated myoplasmic [Ca2+], myosin phosphorylation, and isometric stress in swine carotid media. 2. The relative [Ca2+] sensitivity of phosphorylation depended on the type of stimulus (ranked high to low sensitivity): contractile agonists (histamine, phenylephrine) = endothelin (sustained contraction) = combination of histamine and NaF greater than NaF alone = endothelin (initial contraction) = combination of histamine and depolarization = combination of NaF and depolarization greater than depolarization = Bay K 8644 = combination of depolarization and low-dose phorbol diester. 3. Activation of L-type Ca2+ channels with Bay K 8644 induced a [Ca2+] sensitivity of phosphorylation similar to depolarization, suggesting that any other effects of high KCl (such as cellular swelling) were not responsible for the low [Ca2+] sensitivity of phosphorylation. 4. The addition of either histamine or NaF (an activator of G proteins) to depolarized tissues produced similar increases in the [Ca2+] sensitivity of phosphorylation, suggesting that NaF (possibly by activation of a G protein) can mimic contractile agonist-induced increases in the [Ca2+] sensitivity of phosphorylation. 5. Phorbol dibutyrate enhanced the contractile effect of depolarization, and this enhancement was primarily caused by increases in [Ca2+] rather than an alteration in the [Ca2+] sensitivity of phosphorylation. 6. These data suggest that the [Ca2+] sensitivity of phosphorylation in smooth muscle may be regulated by agonists (possible by G protein activation); however, the role of protein kinase C activation or depolarization induced Ca2+ compartmentalization requires further study.

Ryanodine reveals multiple contractile and relaxant mechanisms in vascular smooth muscle: simultaneous measurements of mechanical activity and of cytoplasmic free Ca2+ level with fura-2

1. The effects of ryanodine on changes in cytoplasmic Ca2+ level ([Ca2+]i) and muscle tension induced by maximum concentrations of phenylephrine (Phe; 1 microM), prostaglandin F2 alpha (PGF2 alpha, 10 microM), caffeine (Caf, 30 mM) and isoprenaline (Iso, 1 microM) were examined in rat aortic strips using fura-2. 2. In normal media, Phe and PGF2 alpha produced a phasic contraction, followed by a tonic one. Caf elicited only a transient contraction. When the preparation was treated with 10 microM ryanodine, an increase in [Ca2+]i was induced accompanied by a nicardipine (1 microM)-resistant contraction which was [Ca2+]o-dependent. 3. In Ca2(+)-free solution, the three stimulants elicited transient increases in [Ca2+]i. Transient contractions to Phe and Caf were accompanied by changes in [Ca2+]i. The transient increase in [Ca2+]i induced by PGF2 alpha was not accompanied by a corresponding contraction. 4. Sustained contractions were induced by Phe and PGF2 alpha in the absence of external Ca2+, while the increase in [Ca2+]i was reduced. A larger maximum contraction was induced by PGF alpha than by Phe. 5. Ryanodine abolished both the Caf- and Phe-induced [Ca2+]i transient increases and the corresponding contractions, but had no substantial effect on the PGF2 alpha-induced [Ca2+]i transient increase. Ryanodine had no influence on the sustained contractions induced by Phe and PGF2 alpha. 6. Iso relaxed both sustained contractions almost completely, without any detectable change in [Ca2+]i. Treatment of the preparation with ryanodine had no effect on the concentration-response curves for Iso in relaxing the 0.1 microM Phe- or 40 mM K(+)-induced precontraction. 7. It is suggested that Phe and Caf mobilize Ca2 + from a ryanodine-sensitive Ca2 + store and that PGF2 alpha. releases Ca2+ from a ryanodine-insensitive Ca2+ store. The former contributes to the transient contraction through a Ca2'-dependent process, while the latter seems not to be directly associated with the contraction. The sustained contraction under Ca2+-free conditions might involve a Ca2 '-independent process or a change in the sensitivity of the contractile filaments to Ca2 + 8. In addition to lowering cytoplasmic Ca2+ concentration, it is suggested that Iso counteracts the apparently Ca2 +-independent process. The ryanodine-sensitive Ca2+ store plays no substantial role in active relaxation by Iso, although it does play a major role in the maintenance of cytoplasmic Ca2+ in a quiescent muscle.

Caffeine acting on pregnant rat myometrium: analysis of its relaxant action and its failure to release Ca2+ from intracellular stores

1. The effect of caffeine on mechanical activity was studied in pregnant rat myometrium. 2. In muscle cells with intact plasmalemmae, caffeine (0.1-50 mM) produced no contraction whatever the experimental conditions. 3. Caffeine (0.1-10 mM) inhibited, in a concentration-dependent manner, contractions induced by electrical stimulation, potassium-rich (60 mM K+) solution, sodium-free solution or oxytocin (22.5 nM). 4. In Ca2(+)-free solution, various substances (oxytocin, sodium orthovanadate and prostaglandin E2) evoked sustained contractions that were suppressed by caffeine (5-10 mM). When caffeine (greater than 5 mM) was applied during Ca2(+)-loading of the tissue (2.1 mM Ca2+, 5 min) in the presence of a K(+)-rich solution, the subsequent transient contraction induced by a short application (10s) of oxytocin (22.5 nM) in Ca-free solution was reduced (63 +/- 3.5% reduction for 20 mM caffeine, n = 4). 5. In saponin-skinned strips, application of caffeine (5-10 mM) during loading of the Ca2(+)-store increased the subsequent contraction induced by myo-inositol 1,4,5 trisphosphate (IP3, 10 microM). Caffeine (10-30 mM) decreased calcium-activated contractions in skinned fibres lacking a functional internal Ca-store. This effect was reduced by the cyclic AMP-dependent protein kinase inhibitor Thr-Thr-Tyr-Ala-Asp-Phe-Ile-Ala-Ser-Gly-Arg-Thr-Gly-Arg-Arg-Asn-Ala-Ile- His-Asp (8 microM). 6. In conclusion, it is suggested that the inability of caffeine to cause spasm of rat myometrium is due to the absence of a caffeine-sensitive calcium-release channel in the sarcoplasmic reticulum. Relaxant effects of caffeine can be explained by mechanisms leading to a decrease in both the cytoplasmic free Ca2+ concentration and the Ca2 +-sensitivity of the contractile machinery.

Evidence for the mechanism of the inhibitory action of jatrophone in the isolated rat uterine muscle

1. Jatrophone (JAT), a diterpene isolated from the plant Jatropha elliptica (1-300 microM), caused a concentration-dependent relaxation effect against acetylcholine (Ach)-oxytocin (Ot)- and KCl-induced uterine sustained contraction. The relative potency order was: Ach greater than Ot greater than KCl. 2. The relaxant effect of JAT was not modified by phorbol ester, forskolin, MIX, TMB-8 and W-7. The increase concentration of calcium (0.2-2 mM) in the medium did not reverse the inhibitory effect caused by JAT. 3. Pre-incubation of the preparations with JAT (16-32 microM) for 20 min, caused a concentration-dependent inhibition of KCl-induced contractile response. At 30 microM, JAT inhibited in an apparently non-competitive manner CaCl2-induced contraction in K+-depolarized preparations. High concentrations of JAT (100 microM) also caused a time-dependent relaxation in CaCl2-induced sustained uterine contraction (T1/2 = approx. 15 min). 4. JAT (30 microM) inhibited the dihydropyridine calcium channel agonist Bay K 8644-induced uterine contraction in an apparently non-competitive fashion, while verapamil (0.1 microM) caused an rightward displacement of Bay K 8644 contraction and marked inhibition of the maximal response.

TPA- and agonist-induced force development in myometrium from pregnant and non-pregnant rats

In myometrium from pregnant rats, 100 nM-TPA elevated resting tension and initially slightly enhanced the contraction induced by 138 mM-KCl. After 20 min this force development significantly declined. In saponin-treated skinned myometrial cells from pregnant rats, 100 nM-TPA enhanced the contraction induced by 0.3 microM-Ca2+, but reduced that induced by 1 microM-Ca2+. These findings suggest that the excitatory and inhibitory actions of TPA on the myometrium are probably due to its action on the contractile proteins. In myometrium from non-pregnant rats, TPA affected neither the resting tension, nor the amplitude of the evoked contractions, nor the Ca2+-induced contractions in skinned myometrium. While TPA only affected tension development in pregnant rats, both 1 mM-carbachol and 90 nM-oxytocin induced a tonic contraction in Ca-free solution independently of the hormonal status of the rats. The latter finding makes it unlikely that activation of protein kinase C is involved in the agonist-induced tonic force development in Ca-free solution.

Properties of intracellular calcium stores in pregnant rat myometrium

1. The properties of the Ca2+ stores in myometrium of 21-day pregnant rats were studied by recording the contractile responses of saponin-treated skinned muscles. 2. After accumulation of Ca2+ into the stores in the presence of 5 mM NaN3, inositol 1,4,5-trisphosphate (InsP3) at concentrations exceeding 3 microM produced a contraction. The amplitude of this contraction was maximal at about 20 microM. A second application of 20 microM InsP3 produced a smaller contraction than the first one. However after reloading the stores with Ca2+, 20 microM InsP3 produced a contraction of the same amplitude as the initial one. 3. After application of 20 microM InsP3, 1 microM A23187 still evoked a large contraction. If A23187 was applied first, the subsequent application of InsP3 or A23187 no longer induced a contraction, even after Ca2+ loading. 4. Guanosine triphosphate (GTP) or arachidonic acid, both 100 microM neither evoked a contraction nor enhanced the subsequent contraction elicited by 20 microM InsP3. 5. Caffeine 25 mM did not induce a contraction nor did it affect the contraction elicited by 20 microM InsP3. 6. The results indicate that in pregnant rat myometrium InsP3 releases Ca2+ from intracellular stores as has been proposed in vascular smooth muscles.

Gestational changes in the utilization of intracellularly stored calcium in the myometrium of guinea-pigs

1. The ability of oxytocin and prostaglandin F2 alpha (PGF) to induce contraction in guinea-pig myometrium in calcium-free solution was studied in an attempt to assess the extent to which intracellular calcium stores could be released by these two agonists. Both longitudinal and circular muscle layers were studied separately and the effects of gestational age were also examined. 2. In longitudinal strips, the responses to oxytocin and PGF in the absence of external calcium decreased progressively throughout gestation. Responses of circular strips to both agonists were unchanged throughout pregnancy, until day 64, when no response to PGF could be elicited. 3. Pre-treatment with high potassium (and normal calcium) increased the responses to the agonists in calcium-free medium while pre-treatment with beta-adrenoceptor agonists had no effect on responses to oxytocin or PGF. 4. Responses to both agonists decreased with time in calcium-free solution suggesting a loss of calcium from stores with a half-time of 3 min. The rate of the decline in the responses was the same in both muscle layers and did not change with gestational age. 5. In the presence of lanthanum contractions evoked by oxytocin, but not PGF, were augmented 2-3-fold. This potentiation of the response to oxytocin occurred in both muscle layers and throughout gestation. 6. Each agonist evoked only one response in calcium-free solution containing EGTA. The response to PGF in longitudinal strips following a challenge with oxytocin was reduced, compared with the response to PGF when applied first while the response to oxytocin in these strips was unchanged following exposure to PGF. In circular strips neither oxytocin- nor PGF-induced contractions were altered following prior exposure to the other agonist. 7. It is concluded that oxytocin and PGF operate via two distinct mechanisms to release intracellularly stored calcium in both longitudinal and circular components of the guinea-pig myometrium and a hypothesis to explain the results is presented.

Caffeine does not contract skinned uterine fibers with a functional Ca store

Chemically skinned fibers were prepared from pregnant rat myometrium. In the presence of a low EGTA concentration (0.1 mM), caffeine (0.1-50 mM) produced no contraction whereas both inositol-1,4,5,-triphosphate and the ionophore A23187 evoked contractile responses insensitive to cyanide and azide. Furthermore, caffeine antagonized the maximal contractions induced by pCa = 5 (in the presence of 4 mM EGTA). It is concluded that caffeine inhibits some mechanism of the contraction in rat myometrium and thus that caffeine is not a good tool for the study of intracellular Ca stores in this smooth muscle.

Extra- and intracellular calcium in vanadate-induced contraction of vascular smooth muscle

The effects of extracellular Ca and Ca antagonists on vanadate-induced contractions of vascular smooth muscles of aortae and mesenteric arteries from rabbits, guinea pigs, and Wistar-Kyoto (WKY) and stroke-prone spontaneously hypertensive rats (SHRSP) were studied. Vanadate-induced contractions of aortae were greatly diminished by extracellular Ca removal; the size of the remaining contraction was variable. Vanadate-induced contractions of mesenteric arteries, which were only observed in the presence of elevated K, were suppressed by the removal of Ca. Verapamil and nifedipine depressed vanadate-induced contractions of aortae from WKY and SHRSP, whereas they produced no or only slight inhibition of responses in guinea pig and rabbit aortae. Ca uptake into smooth muscle cell increased in the presence of vanadate, but the increase was much less than that induced by high K. In saponin-skinned smooth muscle, vanadate depressed the Ca-induced contraction. It is concluded that the vanadate-induced contraction utilizes both extracellular and intracellularly bound Ca, the relative contribution of which varies from preparation to preparation.

Contractile properties of chemically skinned fibers from pregnant rat myometrium: existence of an internal Ca-store

Smooth muscle fibers isolated from pregnant rat myometrium were skinned by saponin treatment. Properties of the contractile system and the involvement of an intracellular source of activator calcium were studied. (1) In the presence of 4 mM total EGTA, skinned fibers contracted in a concentration-dependent manner to micromolar applications of calcium ions. Rat myometrium exhibited a high calcium sensitivity (80% of the maximum contraction was achieved in the presence of 10(-6) M Ca2+). (2) Several divalent cations induced concentration-dependent contractile responses in skinned uterine muscle. The rank order of potency was: Ca2+ greater than Mn2+ greater than Sr2+ greater than Ba2+. All these divalent cation-activated contractions were antagonized by trifluoperazine in a concentration-dependent manner. (3) Pretreatment of skinned fibers with cAMP (5 X 10(-5)-5 X 10(-4) M) depressed cation-activated contractions. This effect was dependent on the free cation concentration. (4) In the presence of a low EGTA concentration (0.1 mM) the pCa-tension curve was shifted to the right with a 2.5-fold increase in the Ca-concentration required to induce half-maximum contraction. (5) After Ca-loading (10(-6) M Ca2+ for 3 min in low EGTA-containing solution), total replacement of K+ ions by choline induced a small and tonic contraction. In these conditions, the ionophore A23287 (5 X 10(-8) M-5 X 10(-5) M) and inositol 1,4,5-triphosphate (InsP3; 2 X 10(-6) M-2 X 10(-5) M) also produced contractions of skinned uterine fibers even in the presence of NaN3 (5 mM) and of NaCN (5 mM).(ABSTRACT TRUNCATED AT 250 WORDS)

The effect of nifedipine on spontaneous, drug-induced and reflexly-activated contractions of the rat urinary bladder: evidence for the participation of an intracellular calcium store to micturition contraction

1. The effect of nifedipine on spontaneous and stimulated motility of the rat urinary bladder has been investigated in vitro (isolated detrusor strips) and in vivo (micturition reflex). 2. Nifedipine inhibited tone and spontaneous activity of the isolated rat bladder, its effect being greater in indomethacin-treated preparations. Nifedipine suppressed the KCl induced phasic and tonic contraction and inhibited by 60-80% the carbachol- or ATP- induced contractions. Nifedipine reduced by about 70% amplitude of the nerve-mediated bladder contractions. 3. Exposure to Ca free medium containing EDTA suppressed tone and spontaneous activity of the rat bladder. In these conditions the response to KCl or ATP was rapidly abolished while a response to carbachol was still evident even after a long exposure to the Ca free medium. 4. In vivo, nifedipine affected reflex micturition e.g. increased volume threshold and slightly reduced amplitude of micturition contraction. In addition, nifedipine reduced voiding efficiency e.g. increased residual volume after micturition. These effects were evident following ligation of the ureters because in normal conditions nifedipine induced a marked diuresis which masked its effect on volume threshold. 5. These findings indicate that in the rat urinary bladder Ca from both intra- and extracellular pools is mobilized during spontaneous or stimulated contractions. Mobilization of an intracellular Ca pool by cholinomimetics or other neurotransmitter(s) may be responsible for the nifedipine-resistant component of the voiding contraction in vivo.

Effects of low extracellular Ca2+ on prostaglandins F2 alpha and E2 action in longitudinal muscle of guinea-pig caecum

1. Effects of low extracellular Ca2+ on the action of prostaglandin F2 alpha (PGF2 alpha) and prostaglandin E2 (PGE2) in longitudinal muscle of guinea-pig caecum were evaluated by the changes in the membrane potential, action potential frequency and contractions. 2. At 2.5 mmol extracellular Ca2+ the EC50 values for the stimulant effects of PGF2 alpha and PGE2 on the membrane potential, the frequency of action potential discharge and contractions were higher for PGF2 alpha than the PGE2. 3. The lowering of extracellular Ca2+ from 2.5 mmol to 0.4 mmol did not affect essentially the PGF2 alpha and PGE2 effects. 4. In Ca2+-free solution containing EGTA the stimulant effects of PGF2 alpha and PGE2 significantly decreased but the action potentials and contractions were not completely abolished. D-600 pretreatment inhibited in a dose-dependent manner the PGF2 alpha- and PGE2-evoked action potential discharges and contractions. 5. It is concluded that the mechanisms limiting Ca2+ influx at 2.5 mmol extracellular Ca2+ are more sensitive to PGE2 than to PGF2 alpha. 6. At low extracellular Ca2+ and in Ca2+-free solution containing EGTA the effects of PGF2 alpha and PGE2 on the action potential generation and the corresponding contractions might be determined by Ca2+-independent processes.

Calcium release in smooth muscle

In smooth muscle, maintenance of the contractile response is due to Ca2+ influx through two types of Ca2+ channel, a voltage-dependent Ca2+ channel and a receptor-linked Ca2+ channel. However, a more transient contraction can be obtained by release of Ca2+ from a cellular store, possibly the sarcoplasmic reticulum. In spike generating smooth muscle (e.g., guinea-pig taenia caeci), spike discharges may trigger the release of cellular Ca2+ by activating a Ca2+-induced Ca2+ release mechanism. Caffeine directly activates this mechanism in the absence of a triggered Ca2+ influx. In contrast to this, maintained depolarization may not only release but also refill the Ca2+ store. Drug-receptor interactions also release Ca2+ from a cellular store. This release may be elicited with inositol trisphosphate produced by receptor-linked phosphoinositide turnover. In non-spike generating smooth muscle (e.g., rabbit thoracic aorta), maintained membrane depolarization does not release but, instead, fills the Ca2+ store. However, caffeine and receptor-agonists release the Ca2+ store - possibly by activating the Ca2+-induced Ca2+ release mechanism and phosphoinositide turnover, respectively. The Ca2+ store in smooth muscle is filled by Ca2+ entry through voltage dependent Ca2+ channels and also by resting Ca2+ influx in the absence of receptor-agonists. The Ca2+ entering the cells through these pathways may be accumulated by the Ca2+ store and may activate the contractile filaments.

A comparison of uterine contraction induced by PGE1 and oxytocin in Ca-free solution

The contraction of the rat uterus incubated in Ca-free EDTA-containing solution in response to PGE1, oxytocin and vanadate has been investigated in order to examine the mechanism of the release of Ca from intracellular stores. The results obtained show that PGE1 evoked a sustained contraction the magnitude of which diminishes slightly after successive additions of PGE1 but not after long exposure to Ca-free medium. Oxytocin induced two different contractions: one of them was transient and observed only after incubating for 5 min in Ca-free solution; the other remained constant during prolonged incubation in Ca-free medium. Vanadate, an inhibitor of Ca-ATPase, induced sustained contraction after prolonged exposure to Ca-free medium, and isoprenaline, which stimulates Ca re-uptake by intracellular organelles, counteracted the sustained contractile response induced by the three agonists.

Stimulant action of pethidine on the pregnant rat uterus in-vitro

Pethidine's stimulant action on the 22-day pregnant rat isolated uterus does not involve receptors sensitive to methysergide and is unlikely to involve the synthesis and release of endogenous prostaglandins. The sensitivity of pethidine-induced contractions to verapamil suggests that mobilization of extracellular calcium is necessary for pethidine's action.

Effects of magnesium chloride on the contractile response of uterus to several agonists in Ca-free solution

The effects of MgCl2 on the oestrogen-dominated rat uterus have been examined. Tissues were preincubated in a Ca2+- and Mg2+-free medium containing 3 mM EDTA. Most experiments were subsequently performed in a similar medium containing either no EDTA or EDTA (1 mM). When MgCl2 was added cumulatively (1-32 mM) no contractile responses were obtained in Ca,Mg-free medium or in Ca,Mg-free high K+ solution. When 2 mM CaCl2 was added, a sustained contraction was obtained. Subsequent addition of cumulative concentrations of MgCl2 caused concentration-dependent relaxation. Oxytocin, 2 microM, produced a small and sustained contraction in a Ca,Mg-free medium. Addition of MgCl2, 2 mM, increased this contraction. In a Ca,Mg-free medium vanadate (8 X 10(-5)M) did not evoke spasm of uterine smooth muscle. After addition of MgCl2 in cumulative amounts (1-32 mM) in the presence of vanadate, a concentration-dependent contraction was obtained. The present work shows that in Ca-free solution, maintained contractions induced by oxytocin and vanadate are augmented by Mg2+.

Acetylcholine-induced contraction in human isolated bronchial smooth muscle: role of an intracellular calcium store

The aim of this study was to investigate the role played by an intracellular calcium store in human bronchoconstriction. Human isolated bronchial smooth muscle strips (5-6 mm long, 0.4-0.5 mm wide) were taken from lung specimens during pneumonectomy. Isometric contraction was recorded after stimulation by 10(-4) M acetylcholine in physiological solutions. The amplitude of acetylcholine-induced contractions was measured in the presence and absence of calcium ions in the perfusing medium. When the perfusing medium was switched to a calcium-free solution the amplitude of the acetylcholine-induced contraction was measured with respect to the duration of calcium-free perfusion. The amplitude of the contraction was 82.1 +/- 11.3%, 77.2 +/- 15.4%, 63 +/- 10.6% of the maximum contraction after the strips were perfused in calcium-free solutions for 1, 3 and 5 min respectively. Several successive contractions could be elicited, and even after 20 min of calcium-free perfusion, acetylcholine was still able to elicit contractions. These results suggest that an intracellular calcium store may be involved in human bronchoconstriction. This finding may help further our understanding of the effects of calcium antagonists on human airways.

Actions of some prostaglandins and leukotrienes on rat cerebral and mesenteric arteries

The effects of some prostaglandins (PG's) and leukotrienes (LT's) on rat middle cerebral, basilar and mesenteric arteries were evaluated in vitro. The order of potency of some prostanoids with respect to their contractile effects in basilar arteries was: U44069 greater than PGF2 alpha greater than PGI2 approximately equal to PGE2 greater than 6-keto-PGE1 greater than 6-keto-PGF1 alpha, whereas 6,15-diketo-PGF1 alpha was inactive. Middle cerebral and basilar arteries were 3-5 times more sensitive than mesenteric arteries to PGF2 alpha. LTD4 and LTC4 were inactive in all three vessel types. PGI2 produced a concentration-related relaxation of similar potency in all three arteries contracted by PGF2 alpha. Arteries preactivated by other agents (K+, noradrenaline, 5-hydroxytryptamine) either failed to relax or inconsistently relaxed after PGI2 application. Among the PGI2 metabolites (6-keto-PGF1 alpha, 6,15-diketo-PGF1 alpha, 6-keto-PGE1), only 6-keto-PGE1 elicited relaxation in the PGF2 alpha-contracted basilar artery. However, the drug potency was significantly smaller than that of PGI2. Nifedipine inhibited the PGF2 alpha-induced contraction by 68% in middle cerebral arteries and by 80% in mesenteric arteries. Exposure to Ca2+-free medium for a time period which almost completely abolished the contractile response to K+ (less than 5% left), reduced the PGF2 alpha-induced contraction by 54, 61 and 85% in middle cerebral, basilar and mesenteric arteries, respectively. The PGF2 alpha-induced contraction of cerebral arteries in Ca2+-free medium was usually composed of a rapidly developing first phase, which levelled off after 1-2 min, and a second slowly developing tonic phase.(ABSTRACT TRUNCATED AT 250 WORDS)

Calcium channel, Ca++ mobilization, and mechanical reactivity of estrogen- and progesterone-treated rat uterus

Properties of [3H]nitrendipine binding, high K+- and Ca++-induced contractions and the inhibition of high K+-induced contractions by verapamil were investigated in the uterine preparations isolated from rats treated with estrogen or progesterone or both. In [3H]nitrendipine binding experiments using crude membrane fractions, treatment with estrogen alone or estrogen+progesterone significantly lowered the KD; There was very little change in the Bmax. In the Ca++-depleted, high K+-containing medium, only the progesterone-, and estrogen----progesterone-treated uteri produced contractions. The estrogen-, estrogen----progesterone-, and estrogen+progesterone-treated uteri showed decreases in concentrations of Ca++ required for the maximal contractions. In the estrogen- and estrogen+progesterone-treated uteri, the dose-response curves by verapamil were shifted to the left in a parallel manner. These findings suggest that estrogen appeared to increase the affinity of calcium channels and increase transmembrane influx of Ca++, leading to enhancement of contractions, whereas progesterone might increase the Ca++ storage in the intracellular sites.

Carbachol-induced contraction in the circular muscle of guinea-pig stomach in calcium-free solution

In the circular muscle of the fundic part of the guinea-pig stomach, a small tonic contraction could be repeatedly produced by carbachol in Ca-free solution containing 2 mM EGTA. The carbachol-induced response was gradually increased during prolonged exposure to Ca-free solution for 50 h, whereas a short treatment with 0.1-0.2 mM Ca suppressed the subsequent carbachol response in Ca-free solution. The response was not essentially modified by increasing the external K+ concentration to 40 mM. Calmodulin antagonists, N-(6-aminohexyl)-5-chloro-1-naphthalene sulphonamide (W-7) and trifluoperazine selectively suppressed the carbachol response in the presence of Ca (0.05 mM) and the contraction induced by Ca (0.1 mM), but they had little effect on the response to carbachol in Ca-free solution at a concentration of less than 10 microM. A vasodilator agent, N-(2-guanidinoethyl)-5-isoquinoline sulphonamide (HA-1004), inhibited the carbachol response both in the presence and absence of Ca, as well as the Ca-induced contraction, to a similar extent, provided that the external Ca concentration was less than 0.1 mM. These results led us to propose that the contraction evoked by carbachol in the absence of external Ca is mediated by a process independent of the Ca-calmodulin system.

Effects of calcium entry blockers on tension development and calcium influx in rat uterus

Spontaneous and potassium chloride (KCl)-induced tension development of strips of whole uterus from the day-22 pregnant rat was reduced when the tissues were incubated in a calcium ion (Ca2+)-free medium. Strips of whole uterus, in an initially Ca2+-free medium, responded to the cumulative addition of Ca2+ with graded phasic tension development and associated rapid electrical discharges. The spasms were inhibited by gallopamil (100 nM) and diltiazem (1 microM). Strips of whole uterus in a depolarizing (40 mM K+) medium, which was initially Ca2+-free, responded to the cumulative addition of Ca2+ with graded tonic tension development without associated electrical discharges. These spasms were inhibited by calcium entry blockers with a rank order of potency of nifedipine = gallopamil greater than diltiazem greater than cinnarizine. KCl-induced tension development in endometrium-free uterine strips was antagonized by calcium entry blockers with a rank order of potency of nifedipine greater than gallopamil greater than diltiazem greater than cinnarizine. Ca2+ influx into endometrium-free uterine strips was assessed by means of the 'lanthanum method'. KCl induced a concentration-dependent increase in 45Ca2+ influx which was suppressed or abolished by nifedipine (2.5 nM), gallopamil (100 nM), diltiazem (500 nM) or cinnarizine (5 microM). It is concluded that spontaneous and KCl-induced tension development of rat uterus involves Ca2+ influx from the extracellular medium into the myometrial cell. These results support the hypothesis that nifedipine, gallopamil, diltiazem and cinnarizine inhibit Ca2+- and KCl-induced tension development of rat uterus by reduction of Ca2+ influx.

Calcium requirement of uterine contraction induced by PGE1: importance of intracellular calcium stores

The contraction of the rat uterus in response to PGE1 in high K+ medium and in Ca-free solution which contained EDTA has been investigated in order to examine whether excitation-contraction coupling involves the release of Ca from an intracellular store. In uterus maximally contracted by K+, cumulative concentrations of PGE1 (1.25 - 20 ng/ml) caused maintained concentration-dependent contraction. PGE1 induced sustained contraction of rat uterus in Ca-free medium after incubation with 3mM EDTA for 50 min. In these conditions the involvement of extracellular Ca is highly unlikely. The PGE1-induced contraction could be repeated without exposure to external Ca ions and with only slight reduction in magnitude. The PGE1 concentrations required to elicit uterine contraction in Ca-free solution were about 1000 times higher than the effective doses in KCl-depolarized uterus. In conclusion, the present investigation shows that Ca influx is not essential for PGE1-induced contraction of rat uterus, although extracellular Ca enhances it presumably by increasing the free Ca levels in the cytosol.