Effects of cGMP on [Ca2+]i, myosin light chain phosphorylation, and contraction in human myometrium

Cyclic GMP-independent effects of nitric oxide on guinea-pig uterine contractility

1. The role of nitric oxide (NO) in the regulation of uterine contractility has yet to be clearly defined. We evaluated the effect of NO (in the form of S-nitroso-cysteine, CysNO) upon uterine contractility and guanosine 3',5'-cyclic monophosphate (cyclic GMP) accumulation in pregnant and nonpregnant guinea-pig myometrium. 2. While CysNO had no effect upon spontaneous contractile activity in either pregnant or nonpregnant uterine tissues, addition of CysNO resulted in an immediate and reversible relaxation of oxytocin- or acetylcholine (ACh)-evoked contractions. 3. Relaxation of agonist-evoked contractions in response to CysNO was associated with significant elevations in intracellular cyclic GMP concentrations ([cyclic GMP]i). 4. Elevations in [cyclic GMP]i were not required for relaxation, as inhibition of guanylyl cyclase by methylene blue prevented [cyclic GMP]i accumulation while having no effect upon the ability of CysNO to relax agonist-evoked contractions. 5. Addition of the cyclic GMP-analogues, 8-Br-cyclic GMP and PET-cyclic GMP, only at high concentrations, produced partial relaxation of agonist-contracted tissues, suggesting the possibility that cyclic GMP may be sufficient but not necessary for myometrial relaxation. 6. Our studies not only provide evidence for a functional role for NO-modulation of agonist-evoked contractions in the pregnant and nonpregnant guinea-pig uterus, but also that these occur by a mechanism which is not dependent upon guanylyl cyclase activity.

Dissociation of P2 purinoceptor-mediated increase in intracellular Ca2+ level from myosin light chain phosphorylation and contraction in rat aorta

1. The effects of P2 agonists, adenosine-5'-triphosphate (ATP), alpha, beta-methylene-adenosine-5'-triphosphate (alpha, beta-me-ATP) and adenosine 5-O-(3-thiotriphosphate) (ATP gamma S), on the intracellular free Ca2+ level ([Ca2+]i), myosin light chain (MLC) phosphorylation and force of contraction were examined in vascular smooth muscle of rat aorta. 2. ATP (0.1 microM-1 mM), alpha, beta-me-ATP (0.1-100 microM) and ATP gamma S (1-100 microM) induced transient increases followed by sustained increase in [Ca2+]i. The effects of these agonists were concentration-dependent. Compared with the effects of a high concentration of KCl (17.5-72.4 mM), the contractions induced by these P2 purinoceptor agonists were smaller at a given [Ca2+]i. 3. In the absence of extracellular Ca2+ (with 0.5 mM EGTA), ATP gamma S (10 microM) induced large transient increase in [Ca2+]i with only small contraction in Ca(2+)-free solution. In contrast, alpha, beta-me-ATP (10 microM) induced only a very small increase in [Ca2+]i and contraction. 4. ATP (1 mM), alpha, beta-me-ATP (10 microM) and ATP gamma S (10 microM), added during stimulation with 0.1 microM noradrenaline, induced additional and transient increases in [Ca2+]i which were also not associated with contraction. 5. High K+ (72.4 mM) increased MLC phosphorylation with a similar time course to that of the increase in [Ca2+]i (peak phosphorylation was 56% when [Ca2+]i increased to 100%). In contrast, the time course of the increase in MLC phosphorylation due to ATP (1 mM) did not coincide with that of the large increases in [Ca2+]i; MLC phosphorylation increased to only 31% when [Ca2+]i increased to 163%. The MLC phosphorylation due to alpha, beta-me-ATP (10 microM) and ATP gamma S (10 microM), measured at peak [Ca2+]i, were only 19% and 14%, respectively, irrespective of a large increase in [Ca2+]i (138% and 188%, respectively). 6. The absence of a clear relationship between P2-purinoceptor-mediated increase in [Ca2+]i (either by Ca2+ influx or Ca2+ release) and MLC phosphorylation or force generation appears to imply that elevation in [Ca2+]i does not contribute to these responses.

Nitric oxide and the myometrium

Nitric oxide (NO) is a potent smooth muscle relaxant in blood vessels, the gastrointestinal tract and the respiratory system. Recent evidence has shown that NO has a relaxant (tocolytic) effect on myometrium. NO is produced within the female genital tract during pregnancy, and a reduction in NO synthesis may be involved in the initiation of parturition. Furthermore, the administration of NO donors may be useful in inhibiting uterine contractions in situations where such activity is unwanted, e.g., in preterm labour. NO is also produced in the myometrium in the nonpregnant state, and has potential roles in the facilitation of implantation and the prevention of dysmenorrhoea. This article aims to examine the evidence suggesting that NO has a physiological role in the maintenance of pregnancy and potential pharmacological use in the treatment of preterm labour.

Preterm labour: a pharmacological challenge

Preterm labour is a major cause of perinatal mortality and morbidity, but its prevention is difficult because most of the available drugs lack uterine selectivity and have potentially serious side-effects for the mother or the foetus. In this article, Andrés López Bernal and colleagues discuss new evidence that shows pregnancy is associated with changes in G protein signalling and second messenger formation in human myometrium. During gestation uterine relaxation is favoured by a pronounced increase in G alpha s levels, thereby facilitating the effect of agonists that increase cAMP formation. The change in G alpha s is reversed in spontaneous labour enabling the uterus to become responsive to contractile agents. Although it is not established that these changes in G protein function are causally related to the spontaneous onset of labour, nevertheless they provide a novel viewpoint towards increased understanding of the cellular mechanisms of uterine contractility, which may result in better drugs for the management of preterm labour.

Myosin phosphorylation and the control of myometrial contraction/relaxation

In summary, phosphorylation of the regulatory light chain of myosin by Ca2+/CaM-dependent MLCK plays an important role in smooth muscle contraction. Although there have been major advances in our understanding of the regulation and physiological functions of contractile proteins in smooth muscle in recent years, very little information exists on the functional status of these proteins in human myometrium during pregnancy. The simple view that contractile force in smooth muscle is proportionate to cytoplasmic Ca2+ concentrations (Ca2+i) and myosin light chain phosphorylation is now more complex as more experiments provide insights into mechanisms of regulation of the contractile elements. MLCK can be phosphorylated, which desensitizes its activation by Ca2+/CaM, and protein phosphatase activity toward myosin may also be regulated. Examples in smooth muscle tissue are sparse, and the different mechanisms by which these processes may be adapted in uterine smooth muscle during pregnancy are not well-defined. Much research is needed to define further the cellular, biochemical, and molecular basis for these physiological processes involved in the regulation of uterine smooth muscle contraction and relaxation.

Volume regulatory responses in frog isolated proximal cells

Cells respond to increase in volume by activating solute efflux pathways, resulting in water loss and restoration of the original cell volume. The solute efflux pathways underlying these volume regulatory decrease (VRD) responses have been relatively well studied. However, the transduction pathways whereby the change in cell volume is converted into an intracellular signal resulting in VRD are much less well understood. We have examined VRD in isolated proximal tubule cells from the frog, with particular attention to the roles of stretch-activated channels, Ca2+ and protein kinases. Cell length was taken as an index of cell volume, and was measured continuously using a photodiode array. VRD was observed in approximately 50% of cells, and was inhibited by Ba2+, Gd3+ and 4,4'-diisothiocyanatostilbene 2,2'-disulphonic acid (DIDS), and removal of extracellular Ca2+. VRD was accelerated by the active phorbol ester, phorbol 12-myristate 13-acetate (PMA), and the phosphatase inhibitor F-; on the other hand, VRD was prolonged by 4 alpha-phorbol 12,13-didecanoate (PDC), an inactive phorbol ester), and inhibited by PMA and Gd3+, PMA and 0 Ca2+, and staurosporine. Volume regulation was unaffected by di-butyryl cAMP and 3-isobutyl-1-methyl-xanthene (IBMX). These data suggest that Ca2+ and PKC, via protein phosphorylation, play a stimulatory role in VRD.

The relationship between oxytocin, phosphoinositide-specific phospholipase C, and phasic myometrial contractions

OBJECTIVES

Oxytocin-stimulated uterine contractions are associated with repetitive cycles of elevated cytosolic calcium, i.e., cytosolic calcium oscillations. The studies in this report were performed to test the hypothesis that phosphoinositide-specific phospholipase C (PI-PLC) is an important component of the myometrial intracellular oscillator.

METHODS

In vitro isometric contraction studies were performed using longitudinal strips of myometrium from nonpregnant, adult Sprague-Dawley rats. Cumulative dose-response studies were performed using oxytocin and aluminum fluoride with and without 2-nitro-4-carboxyphenyl-N, N-diphenylcarbamate (NCDC), an inhibitor of PI-PLC.

RESULTS

Stimulation of G-proteins coupled to PI-PLC with aluminum fluoride resulted in a significant increase in phasic myometrial contractions comparable to those produced by oxytocin. Inhibition of PI-PLC with NCDC resulted in significant suppression of oxytocin- and aluminum fluoride-stimulated myometrial contractions. In contrast, doses that suppressed agonist-stimulated contractions had only a minimal effect on KCl-stimulated tonic myometrial contractions.

CONCLUSIONS

These studies provide significant support for the novel hypothesis that PI-PLC is an important component of the agonist-stimulated cytosolic calcium oscillator that generates phasic myometrial contractions.

An L-arginine-nitric oxide-cyclic guanosine monophosphate system exists in the uterus and inhibits contractility during pregnancy

OBJECTIVE

Nitric oxide is synthesized from L-arginine and it causes relaxation of smooth muscle by elevating cyclic guanosine monophosphate levels. We hypothesized that an L-arginine-nitric oxide-cGMP system is present in the uterus and modulates contractility.

STUDY DESIGN

Isometric tension of the uterus was measured in vitro from pregnant rats in response to various agents that modulate nitric oxide-cyclic guanosine monophosphate production or action.

RESULTS

Major findings are as follows: (1) The substrate and a donor of nitric oxide produced uterine relaxation; (2) inhibitors of the nitric oxide-cyclic guanosine monophosphate pathway blocked the relaxation responses; (3) nitric oxide synthase was localized to several uterine cell types; (4) nitric oxide was produced by the uterus during periods when L-arginine was consumed and citrulline levels increased; (5) effects of nitric oxide substrate on relaxation were mimicked by cyclic guanosine monophosphate; (6) nitric oxide-cyclic guanosine monophosphate responses were decreased during delivery; (7) L-arginine responses were increased by progesterone, and antiprogesterone treatment decreased cyclic guanosine monophosphate-induced relaxations.

CONCLUSION

An L-arginine-nitric oxide-cyclic guanosine monophosphate system is present in the uterus and it may regulate relaxation during pregnancy. The inhibitory action of L-arginine and 8-bromo-cyclic guanosine monophosphate was considerably lower during delivery and post partum, indicating that the nitric oxide system may contribute to the maintenance of uterine quiescence during pregnancy, when progesterone levels are elevated, but not during delivery.

Effects of cyclic GMP on smooth muscle relaxation

Cyclic GMP levels within smooth muscle are affected then by a number of different pathways. Physiologically NO and ANF are probably the two most important regulators for smooth muscle function, but a variety of other mediators and pharmacological agents may also influence this system. Because of the important role that cyclic GMP plays in the control of smooth muscle tone, which clearly includes vascular smooth muscle, it is now and will continue to be in the future an important physiological and biochemical target for research and a pharmacological target for therapeutic agents.

Gestational changes in L-arginine-induced relaxation of pregnant rat and human myometrial smooth muscle

OBJECTIVE

We intended to demonstrate the presence of an L-arginine-nitric oxide system in human myometrium and to clarify the mechanisms of action of nitric oxide on rat myometrium during gestation.

STUDY DESIGN

By examining very small myometrial muscle strips (approximately 750 muscle cells), characteristic features of contraction of rat longitudinal muscle at the midstage of gestation (day 16) and during delivery at term were determined.

RESULTS

Spontaneous contractions were significantly different during delivery compared with the midstage of gestation of rat myometrium. L-Arginine relaxed spontaneous and carbachol-induced, but not potassium chloride-evoked, contractions at both stages. However, much higher concentrations of L-arginine were required during delivery, 8-Bromo-cyclic guanosine monophosphate inhibited spontaneous contractions from concentrations of 1 nmol/L in the midstage of gestation and from 0.1 mmol/L during delivery. In human myometrial tissues L-arginine also inhibited contractions during the late stages of gestation.

CONCLUSION

(1) The experimental model is sufficient to compare properties of longitudinal myometrial strips during gestation. (2) In rat and human myometrium an L-arginine-nitric oxide system has an important role in inhibiting uterine contractility and possibly maintaining pregnancy. (3) The relaxing effect of the nitric oxide system is largely because of the voltage-independent action of cyclic guanosine monophosphate systems.

Biochemistry and physiology of preterm labour and delivery

Human parturition is associated with profound changes in uterine connective tissue affecting mainly the cervix, but the endocrine control of cervical ripening remains obscure. Connective tissue changes are also implicated in premature rupture of the membranes, a problem often associated with preterm delivery, and it is believed that local inflammatory infiltration may play a role in both this condition and cervical ripening, but it is difficult to define which changes precede parturition and which are a consequence of the trauma of labour. Chorioamnionitis can cause preterm labour by provoking the release of inflammatory mediators in the decidua/fetal membranes area and it is likely that activation of prostaglandin release by decidual macrophages is involved in triggering labour. However, the role of macrophages and other bone marrow derived cells in normal labour and in labour associated with chorioamnionitis needs to be defined. It is likely that treatment with a combination of antibiotics and prostaglandin synthase inhibitors and/or other anti-inflammatory drugs is the most appropriate therapeutic approach. Idiopathic preterm labour and spontaneous labour at term are probably due to changes in the sensitivity of the myometrium to endogenous agonists. Recent progress in cell signalling pathways, such as the characterization of regulatory G proteins and the cloning of hormone receptors, should clarify the mechanism of action of relaxing and contracting agents on myometrial cells and should provide the means for the development of new therapeutic agents of high effectiveness and selectivity. This approach should result in better management of both term and preterm labour.

Contractile elements and myosin light chain phosphorylation in myometrial tissue from nonpregnant and pregnant women

Smooth muscle contraction is initiated primarily by an increase in intracellular Ca2+, Ca(2+)-dependent activation of myosin light chain kinase, and phosphorylation of myosin light chain. In this investigation, we identified pregnancy-associated alterations in myosin light chain phosphorylation, force of contraction, and content of contractile proteins in human myometrium. Steady-state levels of myosin light chain phosphorylation and contractile stress were correlated positively in both tissues, but the myometrial strips from pregnant women developed more stress at any given level of myosin light chain phosphorylation. During spontaneous contractions and during conditions that favor maximal generation of stress, the rate and extent of myosin light chain phosphorylation were attenuated in myometrial strips from pregnant women. The content of myosin and actin per milligram of protein and per tissue cross-sectional area was similar between myometrium of nonpregnant and pregnant women. Although cell size was significantly increased in tissues obtained from pregnant women, the amounts of contractile proteins per cellular cross-sectional area were similar. In addition, myosin light chain kinase and phosphatase activities were similar in the two tissues. The content of caldesmon was significantly increased in myometrium of pregnant women, whereas that of calponin (a smooth muscle-specific protein associated with the thin filaments) was not different. We conclude that adaptations of human myometrium during pregnancy include (a) cellular mechanisms that preclude the development of high levels of myosin light chain phosphorylation during contraction and (b) an increase in the stress generating capacity for any given level of myosin light chain phosphorylation.

Intracellular Ca2+ transients in HT29 cells induced by hypotonic cell swelling

Cell swelling induced by hypotonic solution led to an osmolality-dependent increase in intracellular Ca2+ activity ([Ca2+]i) in HT29 cells. At moderate reductions in osmolality from 290 to 240 or 225 mosmol/l in most cases only a small monophasic increase of [Ca2+]i to a stable plateau of 10-20 nmol/l above resting [Ca2+]i was observed. Lower osmolalities resulted in a triphasic increase of [Ca2+]i to a peak value. In a first phase after the volume change, lasting 20-40 s, [Ca2+]i increased slowly by about 30 nmol/l. Thereafter [Ca2+]i increased more rapidly within 20-30 s to a peak value. This peak was 189 +/- 45 nmol/l (190 mosmol/l, n = 9) and 243 +/- 41 nmol/l (160 mosmol/l, n = 20) above resting [Ca2+]i. The peak was then followed by a decline of [Ca2+]i over the next 2-3 min to a stable plateau value of 28 +/- 6 (n = 6) and 32 +/- 11 nmol/l (n = 11) above resting [Ca2+]i at 190 and 160 mosmol/l, respectively. The plateau lasted as long as the hypotonic solution was present. Under Ca(2+)-free bath conditions the peak value for the cell-swelling-induced [Ca2+]i transient was reached significantly later (60-100 s, compared to 40-60 s under control conditions). The peak values under Ca(2+)-free conditions were not significantly lower. This indicates that the [Ca2+]i peak was mostly of intracellular origin. No [Ca2+]i plateau phase was observed under Ca(2+)-free bath conditions.(ABSTRACT TRUNCATED AT 250 WORDS)

Regulatory volume decrease in Ehrlich ascites tumor cells is not mediated by a rise in intracellular calcium

Ehrlich ascites tumor cells suspended in hyposmotic solution initially swell and then shrink back towards normal volume, a process known as regulatory volume decrease (RVD). RVD is characterized by a specific loss of KCl, although the mechanism for this is currently unknown. The hypothesis that a rise in intracellular calcium ([Ca2+]i) activates calcium-sensitive ion conductances to initiate RVD was investigated. The results indicate that in the Ehrlich cell no rise in [Ca2+]i occurs when the extracellular osmolality is reduced from 300 mosM to 180 mosM. These findings were substantiated by the lack of sensitivity of RVD to the Ca(2+)-sensitive K+ channel blockers charybdotoxin (CTX) and nifedipine. In contrast, the ionophore ionomycin induced a cell shrinkage that was sensitive to CTX and nifedipine indicating that a rise in [Ca2+]i could play a role in cell volume reduction but that this occurred by a mechanism different from that observed in RVD. The conclusion from these experiments is that Ca2+ does not act as a second messenger for RVD in the Ehrlich cell.

Cell volume regulation in cultured cerebellar granule neurons

Cultured rat cerebellar granule neurons exposed to solutions of reduced osmolarity, responded initially by swelling followed by a regulatory volume decrease (RVD) which is completed within 15 min. Increasing external osmolarity lead to cell shrinking but no evidence of volume regulation was observed within 1 hr. Replacing Na+ by choline did not affect RVD whereas N-methyl-D-glucamine accelerated the volume recovery and K+ suppressed it completely. The blockade of RVD in high extracellular K+ was only observed when chloride and nitrate but not sulfate or gluconate were the accompanying anions. Replacing intracellular Cl-, by long incubations with gluconate, markedly inhibited RVD. Removal of extracellular Ca2+ or addition of dantrolene which blocks Ca2+ released from intracellular stores had no effect on RVD. Increasing extracellular taurine prevented RVD. These results indicate that membrane permeability to K+, Cl-, and taurine is increased by hyposmolarity and suggest the involvement of these molecules in RVD in granule neurons.