Occurrence of intracellular pH transients during spontaneous contractions in rat uterine smooth muscle

Modeling and experimental approaches for elucidating multi-scale uterine smooth muscle electro- and mechano-physiology: A review

The uterus provides protection and nourishment (via its blood supply) to a developing fetus, and contracts to deliver the baby at an appropriate time, thereby having a critical contribution to the life of every human. However, despite this vital role, it is an under-investigated organ, and gaps remain in our understanding of how contractions are initiated or coordinated. The uterus is a smooth muscle organ that undergoes variations in its contractile function in response to hormonal fluctuations, the extreme instance of this being during pregnancy and labor. Researchers typically use various approaches to studying this organ, such as experiments on uterine muscle cells, tissue samples, or the intact organ, or the employment of mathematical models to simulate the electrical, mechanical and ionic activity. The complexity exhibited in the coordinated contractions of the uterus remains a challenge to understand, requiring coordinated solutions from different research fields. This review investigates differences in the underlying physiology between human and common animal models utilized in experiments, and the experimental interventions and computational models used to assess uterine function. We look to a future of hybrid experimental interventions and modeling techniques that could be employed to improve the understanding of the mechanisms enabling the healthy function of the uterus.

Parturition dysfunction in obesity: time to target the pathobiology

Over a third of women of childbearing age in the United States are obese, and during pregnancy they are at increased risk for delayed labor onset and slow labor progress that often results in unplanned cesarean delivery. The biology behind this dysfunctional parturition is not well understood. Studies of obesity-induced changes in parturition physiology may facilitate approaches to optimize labor in obese women. In this review, we summarize known and proposed biologic effects of obesity on labor preparation, contraction/synchronization, and endurance, drawing on both clinical observation and experimental data. We present evidence from human and animal studies of interactions between obesity and parturition signaling in all elements of the birth process, including: delayed cervical ripening, prostaglandin insensitivity, amniotic membrane strengthening, decreased myometrial oxytocin receptor expression, decreased myocyte action potential initiation and contractility, decreased myocyte gap junction formation, and impaired myocyte neutralization of reactive oxygen species. We found convincing clinical data on the effect of obesity on labor initiation and successful delivery, but few studies on the underlying pathobiology. We suggest research opportunities and therapeutic interventions based on plausible biologic mechanisms.

Physiological increases in lactate inhibit intracellular calcium transients, acidify myocytes and decrease force in term pregnant rat myometrium

Key points

Lactate is increased in myometrial capillary blood from women in slow or non‐progressive labour (dystocia), suggesting that it is detrimental to uterine contractions.

There are no studies of the effect of lactate on the myometrium. In the present study, we have investigated its effects and mechanism of action on myometrial strips from term pregnant rats.

We show that lactate significantly decreased spontaneous contractility. Lactatedecreased pH_i and inhibited Ca²⁺ transients. Nulling the decrease in pH_i abolished the effects of lactate effects. If Ca²⁺ entry was enhanced, the effects of lactate were abolished.

The present study suggests that the accumulation of extracellular lactate could contribute to labour dystocia.

Abstract

Lactate is increased in myometrial capillary blood from women in slow or non‐progressive labour (dystocia), suggesting that it is detrimental to uterine contractions. There are, however, no studies of the effect of lactate on the myometrium. We therefore investigated its effects and mechanism of action on myometrial strips from term pregnant rats. The effects on spontaneous and oxytocin‐induced contractility in response to sodium lactate and other weak acids (1–20 mM) were studied. In some experiments, simultaneous force and intracellular Ca²⁺ or pH (pH_i) were measured with Indo‐1 or Carboxy‐SNARF, respectively. Statistical differences were tested using non‐parametric tests. Lactate significantly decreased spontaneous contractility with an EC₅₀ of 3.9 mM. Propionate, butyrate and pyruvate also reduced contractions with similar potency. The effects of lactate were reduced in the presence of oxytocin but remained significant. Lactate decreased pH_i and nulling the decrease in pH_i abolished its effects. We also show that lactate inhibited Ca²⁺ transients, with these changes mirroring those produced on force. If Ca²⁺ entry was enhanced by depolarization (high KCl) or applying the Ca²⁺ channel agonist, Bay K 4644, the effects of lactate were abolished. Taken together, these data show that lactate in the physiological range potently decreases myometrial contractility as a result of its inhibition of Ca²⁺ transients, which can be attributed to the induced acidification. The present study suggests that the accumulation of extracellular lactate will reduce myometrial contractions and could therefore contribute to labour dystocia.

Characterization of the myometrial transcriptome in women with an arrest of dilatation during labor

OBJECTIVE

The molecular basis of failure to progress in labor is poorly understood. This study was undertaken to characterize the myometrial transcriptome of patients with an arrest of dilatation (AODIL).

STUDY DESIGN

Human myometrium was prospectively collected from women in the following groups: (1) spontaneous term labor (TL; n=29) and (2) arrest of dilatation (AODIL; n=14). Gene expression was characterized using Illumina® HumanHT-12 microarrays. A moderated Student's t-test and false discovery rate adjustment were used for analysis. Quantitative reverse transcription-polymerase chain reaction (qRT-PCR) of selected genes was performed in an independent sample set. Pathway analysis was performed on the Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway database using Pathway Analysis with Down-weighting of Overlapping Genes (PADOG). The MetaCore knowledge base was also searched for pathway analysis.

RESULTS

(1) Forty-two differentially expressed genes were identified in women with an AODIL; (2) gene ontology analysis indicated enrichment of biological processes, which included regulation of angiogenesis, response to hypoxia, inflammatory response, and chemokine-mediated signaling pathway. Enriched molecular functions included transcription repressor activity, heat shock protein (Hsp) 90 binding, and nitric oxide synthase (NOS) activity; (3) MetaCore analysis identified immune response chemokine (C-C motif) ligand 2 (CCL2) signaling, muscle contraction regulation of endothelial nitric oxide synthase (eNOS) activity in endothelial cells, and triiodothyronine and thyroxine signaling as significantly overrepresented (false discovery rate <0.05); (4) qRT-PCR confirmed the overexpression of Nitric oxide synthase 3 (NOS3); hypoxic ischemic factor 1A (HIF1A); Chemokine (C-C motif) ligand 2 (CCL2); angiopoietin-like 4 (ANGPTL4); ADAM metallopeptidase with thrombospondin type 1, motif 9 (ADAMTS9); G protein-coupled receptor 4 (GPR4); metallothionein 1A (MT1A); MT2A; and selectin E (SELE) in an AODIL.

CONCLUSION

The myometrium of women with AODIL has a stereotypic transcriptome profile. This disorder has been associated with a pattern of gene expression involved in muscle contraction, an inflammatory response, and hypoxia. This is the first comprehensive and unbiased examination of the molecular basis of an AODIL.

A molecular signature of an arrest of descent in human parturition

OBJECTIVE

This study was undertaken to identify the molecular basis of an arrest of descent.

STUDY DESIGN

Human myometrium was obtained from women in term labor (TL; n = 29) and arrest of descent (AODes; n = 21). Gene expression was characterized using Illumina HumanHT-12 microarrays. A moderated Student t test and false discovery rate adjustment were applied for analysis. Confirmatory quantitative reverse transcription-polymerase chain reaction and immunoblot were performed in an independent sample set.

RESULTS

Four hundred genes were differentially expressed between women with an AODes compared with those with TL. Gene Ontology analysis indicated enrichment of biological processes and molecular functions related to inflammation and muscle function. Impacted pathways included inflammation and the actin cytoskeleton. Overexpression of hypoxia inducible factor-1a, interleukin -6, and prostaglandin-endoperoxide synthase 2 in AODes was confirmed.

CONCLUSION

We have identified a stereotypic pattern of gene expression in the myometrium of women with an arrest of descent. This represents the first study examining the molecular basis of an arrest of descent using a genome-wide approach.

The roles of pH in regulation of uterine contraction in the laying hens

In the laying hens, the uterus (shell gland) plays essential roles in calcium transfer for calcification of the eggshell and expulsion of the egg through the vagina for oviposition. Much is known about the effects of pH changes on eggshell production of the uterus. However, very little is understood about the effects of pH changes on uterine contractility. We investigated the effects of pH changes on uterine contraction in the laying hens. The laying hens were humanely killed, and strips of uterine smooth muscles were isolated. Isometric force was measured and the effects of intracellular and extracellular pH changes studied. The results show that alterations of pH clearly have marked effects on force in the hen uterus. Both intracellular and extracellular acidifications significantly decreased uterine activity, whether it arises spontaneously or in the presence of agonists such as prostaglandin F(2alpha) and arachidonic acid. Alkalinization produced the opposite effects. Thus, changes in pH can regulate uterine contraction. This insight into pH regulation of the uterine activity provides a focus for egg production management directed at physiological and pathological oviposition in the laying hens.

Insights into the uterus

A better understanding of the mechanisms that generate and modulate uterine contractility is needed if progress is to be made in the prevention or treatment of problems in labour. Dysfunctional labour describes the condition when uterine contractility is too poor to dilate the cervix, and it is the leading cause of emergency Caesarean sections. Recently, insight has been gained into a possible causal mechanism for dysfunctional labour. Study of the physiological mechanisms that produce excitation in the uterus, the subsequent Ca(2)(+) signals and biochemical pathway leading to contraction has underpinned this progress. In this review, I give an account of excitation-contraction signalling in the myometrium and explore the implications of recent findings concerning lipid rafts for these processes. I also discuss how changes of pH are fundamentally enmeshed in uterine activity and biochemistry and explore the effect that pH changes will have on human myometrium. Finally, I present the evidence that acidification of the myometrium is correlated with dysfunctional labour and suggest the processes by which it is occurring. It is only by gaining a better understanding of uterine physiology and pathophysiology that progress will be made and research findings translated into clinical benefit for women and their families.

Depletion of membrane cholesterol eliminates the Ca2+-activated component of outward potassium current and decreases membrane capacitance in rat uterine myocytes

Changes in membrane cholesterol content have potent effects on cell signalling and contractility in rat myometrium and other smooth muscles. We have previously shown that depletion of cholesterol with methyl-beta-cyclodextrin (MCD) disrupts caveolar microdomains. The aim of this work was to determine the mechanism underlying the increase in Ca(2+) signalling and contractility occurring in the myometrium with MCD. Patch clamp data obtained on freshly isolated myocytes from the uterus of day 19-21 rats showed that outward K(+) current was significantly reduced by MCD. Membrane capacitance was also reduced. Cholesterol-saturated MCD had no effect on the amplitude of outward current suggesting that the reduction in the outward current was due to cholesterol depletion induced by MCD rather than a direct inhibitory action of MCD on the K(+) channels. Confocal visualization of the membrane bound indicator Calcium Green C18, revealed internalization of the surface membrane with MCD treatment. Large conductance, Ca(2+)-sensitive K(+) channel proteins have been shown to localize to caveolae. When these channels were blocked by iberiotoxin outward current was significantly reduced in the uterine myocytes; MCD treatment reduced the density of outward current. Following reduction of outward current by MCD pretreatment, iberiotoxin was unable to produce any additional decrease in the current, suggesting a common target. MCD treatment also increased the amplitude and frequency of spontaneous rises in cytosolic Ca(2+) level ([Ca(2+)](i) transients) in isolated myocytes. In intact rat myometrium, MCD treatment increased Ca(2+) signalling and contractility, consistent with previous findings, and this effect was also found to be reduced by BK channel inhibition. These data suggest that (1) disruption of cholesterol-rich microdomains and caveolae by MCD leads to a decrease in the BK channel current thus increasing cell excitability, and (2) the changes in membrane excitability produced by MCD underlie the changes found in Ca(2+) signalling and uterine contractility.

The effects of pH change on Ca(++) signaling and force in pregnant human myometrium

OBJECTIVE

This study was designed to determine the effects of both intracellular and extracellular pH change on contractile activity and intracellular Ca(++) during spontaneous contractions, oxytocin, and depolarization-induced stimulation of human myometrium.

STUDY DESIGN

Human myometrial tissue was obtained at elective caesarean delivery at term (37-41 completed weeks of gestation). Longitudinal strips were dissected and loaded with the calcium sensitive indicator Indo-1. Statistical significance was tested with the Student t test.

RESULTS

Both intracellular and extracellular acidification significantly reduces or even abolishes phasic activity, whether it arises spontaneously or in the presence of oxytocin. These contractile changes can be accounted for by the changes in intracellular Ca(++). Alkalinization produced the opposite effects. However, baseline or maintained tension changes could not be accounted for by changes in intracellular Ca(++).

CONCLUSION

We suggest that the effects on phasic activity are due to the inhibition of L-type calcium entry and that, during maintained or baseline activity, pH-sensitive Ca(++) release, possibly from the sarcoplasmic reticulum occurs; but it is insufficient to overcome the inhibitory effects at the myofilaments. We conclude that alterations of pH significantly affect calcium signaling and force production in the human myometrium and may contribute to dysfunction in labor.

A novel model for the in vivo monitoring of uterine microcirculation and intracellular free calcium changes in rat

The aim of this work was to develop a model to study the microcirculation and relative levels of intracellular free calcium in the myometrium of pregnant rats. On Day 21 of gestation a lobe of uterus was prepared free, flipped over, and mounted in a superfusion chamber leaving the radix and thereby the innervation and circulation intact. RBC velocity and arteriolar diameters were determined by means of intravital video microscopy before and after stimulation (norepinephrine). To study intracellular free calcium changes, the fluorescent dye Indo-1 AM was added to the superfusate in the chamber. Fluorescence images were recorded and ratios of the images collected at 400 and 506 nm were calculated and changes thereof were assumed to represent intracellular free calcium changes. RBC velocity and arteriolar diameter did not change for at least 1 h, while the response to norepinephrine was similar at the beginning of the experiment and after 120 min. In four separate interventions, the uterus was challenged with 5 x 10(-4) IU/ml oxytocin, 4.5 mM calcium, 5 x 10(-4) IU/ml oxytocin with 4.5 mM calcium, and 5 microM ionomycin, resulting in an increase of the 400/506 nm ratio of 27, 31, 76, and 103%, respectively, representing a relative increase in intracellular free calcium. This novel in vivo model is suitable for monitoring intracellular free calcium changes and to record RBC velocities and blood vessel diameters in the myometrium of pregnant rats.

In vivo pH and metabolite changes during a single contraction in rat uterine smooth muscle

1. We have used 31P NMR spectroscopy to measure metabolites and pHi at three periods during a phasic contraction of the uterus, in vivo, to determine whether they change as a consequence of contraction. The regular uterine contractions were recorded via a balloon catheter in the uterine lumen. Each phasic contraction was divided into three parts: the period between contractions (rest), the development of force (up) and the relaxation of force (down). The NMR data were summed separately from each of these three periods over 20-40 successive contractions. 2. Significant changes in ATP, phosphocreatine (PCr) and inorganic phosphate (Pi) occurred during the contraction. [ATP] fell from 2.0 to 1.6 mM and [PCr] from 2.6 to 2.0 mM during the up period, while [Pi] increased from 2.2 to 2.8 mM. Recovery of ATP and PCr occurred during the relaxation part of the contraction, whereas Pi did not fully recover until the contraction was complete. 3. Significant acidification from pH 7.28 +/- 0.02 at rest to 7.16 +/- 0.02, occurred with contraction. This acidification is greater than that previously reported for in vitro uterine preparations. Measurements of uterine blood flow show that it decreased with contraction. Therefore, ischaemia, in addition to the metabolic consequences of contraction, may account for the larger acidification observed in vivo. 4. Lowering pHi in an in vitro uterine preparation by a similar level to that found in vivo produced a significant reduction of the phasic contractions. Thus we propose that these changes, especially the fall in pHi during force development, feed back negatively on the contraction to limit its strength, and may help prevent uterine ischaemia and fetal hypoxia during labour.

Calcium antagonistic properties of the cyclooxygenase-2 inhibitor nimesulide in human myometrial myocytes

The non-steroidal anti-inflammatory drug nimesulide is a selective inhibitor of cyclooxygenase-2 which relaxes spontaneously contracting human myometrium in vivo and is potentially a useful tocolytic drug. Part of the relaxant action of nimesulide may be via block of myometrial Ca2+ channels. Here, we describe the Ca2+ channel blocking properties of nimesulide in freshly dispersed human term-pregnant myometrial smooth muscle cells (HMSMCs). Both L- and T-components of the whole cell Ca2+ channel current were inhibited by 100 microM nimesulide (38+/-3 and 35+/-1% block, respectively). At physiological pH inside and outside the cell (pHo/pHi = 7.4/7.2), this block did not depend on the holding or test potential, although a degree of use-dependence was observed during high frequency stimulation at a higher concentration of drug (300 microM). At pHo/pHi = 6.8, under which condition the concentration of the non-ionized form of the drug is increased 3 fold compared to pH 7.4, nimesulide blocked the L-type current more potently (58+/-3% inhibition at 100 microM, P<0.01) compared to physiological pH. Nimesulide caused a 7 mV leftward shift in the availability curve of the current at pH 6.8, suggesting that the affinity of the drug for the inactivated channel is approximately 4 fold higher than its affinity for the closed channel. We speculate that acidification and depolarization of the myometrium during the intense and prolonged contractions of labour might increase the potency of nimesulide as a Ca2+ channel antagonist, promoting its action as a tocolytic agent.

Interaction of talin with actin: sensitive modulation of filament crosslinking activity

Talin is an adhesion plaque protein believed important in linking actin filaments to the plasma membrane. The nature of a direct talin-actin interaction, however, is complex and has remained unclear. We have systematically characterized the effects of pH, ionic strength, temperature, and protein molar ratio on the interaction between highly purified talin and actin. The ability of talin to increase viscosity of F-actin at 25 degrees C and low ionic strength increased with decreasing pH from 7.3 to 6.4 and increasing molar ratio of talin to actin. At pH 6.4 and low ionic strength, talin could extensively crosslink actin filaments into ordered bundles as shown by negative staining and could cosediment with F-actin at molar ratios as high as one talin to two actin monomers. Talin crosslinked prepolymerized actin filaments to a similar extent as actin filaments polymerized in its presence. The 190-kDa calpain-generated proteolytic fragment of talin bound poorly to actin under conditions favorable for intact talin, but was able to crosslink actin filaments at a lower pH. Increasing the ionic strength within a relatively narrow range significantly decreased ability of talin to bind to actin, regardless of pH. The effects of pH and ionic strength on the talin-actin interaction were rapid and reversible. Low-shear-viscosity studies revealed a strong temperature dependence in the talin-actin interaction with significant crosslinking activity at physiological-like ionic conditions and temperature (37 degrees C). Our results consistently demonstrated that talin crosslinks actin filaments and that this direct interaction is highly sensitive to, and dependent upon, ionic conditions and temperature.

The role of the sarcolemmal Ca(2+)-ATPase in the pH transients associated with contraction in rat smooth muscle

1. We have investigated the origin of the intracellular acid pH transients that accompany myometrial contraction. Intra- and extracellular pH were measured with SNARF and intracellular Ca2+ concentration ([Ca2+]i) with indo-1. 2. An intracellular acidification accompanied spontaneous contractions and those elicited by KCl depolarization or the addition of the agonists carbachol or prostaglandin F2 alpha. The size of the acidification increased with the magnitude of the contraction. 3. The intracellular acidification was accompanied by an extracellular alkalinization, showing that it results from proton movement across the surface membrane. Furthermore, it was decreased either by addition of Cd2+ (20 nM, an inhibitor of the sarcolemmal Ca(2+)-ATPase) or by elevating [Ca2+]o. 4. Extracellular alkalinization increased the magnitude of the rise of [Ca2+]i and force produced by KCl. 5. An intracellular acidification was also associated with contraction in the portal vein and ureter. 6. We conclude that the sarcolemmal Ca(2+)-ATPase produces a significant intracellular acidification while removing Ca2+. Both the acidification and decrease of [Ca2+]i will promote relaxation. Since Ca2+ and protons have opposite effects on many cellular processes, this dual regulation by these two ions may be of general importance.

Quantitative contribution of the acid production to the intracellular acidification in human neutrophils stimulated by N-formyl-methionyl-leucyl-phenylalanine

A chemotactic peptide, N-formyl-methionyl-leucyl-phenylalanine (fMLP), induced an acidification of cytosol by about 0.05 pH units in 30 sec followed by an alkalinization in human neutrophils. The quantitative contribution of acid production to the acidification was studied. The superoxide (O2-) production stimulated by fMLP was not involved in the acidification because the production of acids in neutrophils from patients with chronic granulomatous disease who do not produce O2-, was the same as that in normal neutrophils. The intracellular acidification was completely inhibited by deoxyglucose, suggesting that energy metabolism enhanced upon stimulation by fMLP might be the main source of the acidification. Although enhancement of the lactate formation by fMLP was 0.8 nmol/10(6) cells, which could lower intracellular pH by 0.08 pH units, the lactate production could not explain the initial acidification because the production of lactate started at 1 min after the stimulation while the intracellular acidification began immediately after the stimulation. Mitochondrial respiratory inhibitors such as KCN and rotenone had no effects on the fMLP-induced intracellular acidification. The fMLP-induced production of CO2 in 30 sec through the hexose monophosphate shunt was only 2.6 pmol/10(6) cells, which was calculated to decrease intracellular pH by only 0.0014. Thus, changes of energy metabolism induced by fMLP does not explain the acidification.

Changes in intracellular pH close to term and their possible significance to labour

The contractile pattern of uterine smooth muscle is markedly altered by even modest manipulations of intracellular pH (pHi); an acidification can abolish contractions, while alkalinization increases contractility. As at the end of gestation the uterus changes from being relatively quiescent to highly contractile, we investigated whether pHi changes with pregnancy in women. Isolated tissue was loaded with the pH-sensitive fluorophore, carboxy-SNARF. We found a significant alkalinization over the last few weeks of pregnancy. Such an increase in pHi will increase contractility of the uterus, and may therefore contribute to the mechanisms ensuring strong and efficient contractions occur during labour.

Mechanisms of intracellular Mg2+ regulation affected by amiloride and ouabain in the guinea-pig taenia caeci

1. The effects of amiloride and ouabain on the regulation of the intracellular, free Mg2+ concentration ([Mg2+]i) were investigated in the taenia isolated from the guinea-pig caecum, using nuclear magnetic resonance (NMR) techniques. 2. [Mg2+]i were mainly estimated from the separation of the alpha- and beta-ATP peaks observed in 31P NMR spectra. In normal (physiological) and nominally Ca(2+)-free solutions, [Mg2+]i was approximately 0.3-0.4 mM. Application of either amiloride or ouabain in Ca(2+)-free solutions significantly increased [Mg2+]i, with only a small change in ATP content. Washout of the drugs reversed the changes in [Mg2+]i. 3. Changes in pHi were estimated from: (1) the chemical shift of phosphoethanolamine, and (2) solving two relational equations of pHi and [Mg2+]i obtained from the beta- and gamma-ATP peaks. Both estimations revealed some intracellular alkalosis during application of these two drugs. After correction for pHi, a significant increase in [Mg2+]i was still obtained 150 min after application of either drug. 4. In the presence of amiloride, simultaneous removal of extracellular Mg2+ and Ca2+ significantly depleted intracellular Mg2+. This result suggests the presence of an amiloride-insensitive (or less sensitive) pathway which passively transports Mg2+ across the plasma membrane. 5. The intracellular Rb+ concentration was monitored as an index of Na(+)-K+ pump activity, using 87Rb NMR. In Ca(2+)-free solutions containing 5 mM Rb+, the intracellular Rb+ concentration was hardly changed by amiloride, but was depleted by additional applications of ouabain. Wash-out of ouabain restored the intracellular Rb+ in the presence of amiloride. 6. These results are consistent with the presence of Na(+)-Mg2+ exchange as an effective Mg(2+)-extruding mechanism in smooth muscle. Although many other factors may cause changes in [Mg2+]i, it seems likely that amiloride directly inhibits the Na(+)-Mg2+ exchanger, whilst ouabain does so indirectly through reduction of the Na+ gradient across the plasma membrane.

Inositol 1,4,5-trisphosphate binding to porcine tracheal smooth muscle aldolase

A cytoskeletal fraction of porcine tracheal smooth muscle (PTSM) was found to contain > 90% of total cellular aldolase (fructose 1,6-bisphosphate aldolase, EC 4.1.2.13) activity. PTSM aldolase was purified by DEAE and inositol 1,4,5-trisphosphate (Ins(1,4,5)P3) affinity chromatography and found to react with an antibody directed against human aldolase C, but not anti-aldolase A and B. The molecular mass of native aldolase was about 138 kDa (on Sephacryl S-300); SDS-denatured enzyme was 35 kDa (comigrated with rabbit skeletal muscle aldolase). Total cellular aldolase tetramer (aldolase4) content was 34.5 pmol/100 nmol lipid P(i). Ins(1,4,5)P3) binding activity coeluted with aldolase during Sephacryl 300, DEAE, and Ins(1,4,5)P3 affinity chromatography. Ins(1,4,5)P3 bound to purified aldolase (at 0 degree C) in a dose-dependent manner over the range [Ins(1,4,5)P3] 20 nM to 20 microM, with maximal binding of 1 mol of Ins(1,4,5)P3/mol aldolase4 and a Kd of 12-14 microM. Fru(1,6)P2 and Fru(2,6)P2 displaced bound Ins(1,4,5)P3) with a 50% inhibition at 30 and 170 microM, respectively. Ins(1,3,4)P3 (20 microM) and glyceraldehyde 3-phosphate (2 mM) were also potent inhibitors of Ins(1,4,5)P3 binding, but not inositol 4-phosphate or inositol 1,4-bisphosphate (20 microM each). Aldolase-bound Ins(1,4,5)P3 may play a role in phospholipase C-independent increases in free [Ins(1,4,5)P3].

The effect of metabolic inhibition on rat uterine intracellular pH and its role in contractile failure

Although intracellular pH (pHi) is expected to change during inhibition of oxidative phosphorylation and to affect force, there have been no simultaneous measurements of its effect on pHi and force in smooth muscle. Therefore, we have investigated the relationship between force and pHi in strips of longitudinal rat myometrium, loaded with the pH-sensitive indicator carboxy-SNARF and simultaneously measured tension. The application of cyanide produced an abolition of spontaneous contractions and a rapid initial fall in pHi. In 9/19 preparations pHi then started to return to resting values but there was no corresponding restoration of force. Cyanide and weak base were simultaneously added to uterine preparations to prevent any acidification; force still fell. Addition of cyanide to depolarized preparations also produced an acidification and a fall in force. Depolarization of preparations in which spontaneous force had been abolished by cyanide often produced a transient rise in force, despite further acidification of the cytoplasm. Cyanide produced an acidification in zero Ca(2+)-containing solution, similar to that in the presence of Ca2+ indicating little role for changes in [Ca2+] in producing the acidification. It is concluded that cyanide decreases pHi and force in the uterus, but that there is not a simple relationship between the two.

Abolition of contractions in the myometrium by acidification in vitro

Uterine dystocia, often of unknown cause, remains one of the commonest causes of emergency caesarean section. We have tested the hypothesis that small acidic changes in pH, that can occur during labour contractions, can decrease contractions and contribute to dystocia. We simultaneously recorded intracellular pH (pHi) and force in human myometrium: acidification abolished contractions. Such effects of pHi on uterine contractile activity may be of clinical significance. Blood flow reduces during each uterine contraction; the resulting fall in pHi could lead to inefficient uterine action in some labours and failure to progress (ie, dystocia).