The role of mechanical and hormonal stimuli on uterine involution in the rat

The Myometrium: From Excitation to Contractions and Labour

We start by describing the functions of the uterus, its structure, both gross and fine, innervation and blood supply. It is interesting to note the diversity of the female's reproductive tract between species and to remember it when working with different animal models. Myocytes are the overwhelming cell type of the uterus (>95%) and our focus. Their function is to contract, and they have an intrinsic pacemaker and rhythmicity, which is modified by hormones, stretch, paracrine factors and the extracellular environment. We discuss evidence or not for pacemaker cells in the uterus. We also describe the sarcoplasmic reticulum (SR) in some detail, as it is relevant to calcium signalling and excitability. Ion channels, including store-operated ones, their contributions to excitability and action potentials, are covered. The main pathway to excitation is from depolarisation opening voltage-gated Ca²⁺ channels. Much of what happens downstream of excitability is common to other smooth muscles, with force depending upon the balance of myosin light kinase and phosphatase. Mechanisms of maintaining Ca²⁺ balance within the myocytes are discussed. Metabolism, and how it is intertwined with activity, blood flow and pH, is covered. Growth of the myometrium and changes in contractile proteins with pregnancy and parturition are also detailed. We finish with a description of uterine activity and why it is important, covering progression to labour as well as preterm and dysfunctional labours. We conclude by highlighting progress made and where further efforts are required.

Transient receptor potential c4/5 like channel is involved in stretch-induced spontaneous uterine contraction of pregnant rat

Spontaneous myometrial contraction (SMC) in pregnant uterus is greatly related with gestational age and growing in frequency and amplitude toward the end of gestation to initiate labor. But, an accurate mechanism has not been elucidated. In human and rat uterus, all TRPCs except TRPC2 are expressed in pregnant myometrium and among them, TRPC4 are predominant throughout gestation, suggesting a possible role in regulation of SMC. Therefore, we investigated whether the TRP channel may be involved SMC evoked by mechanical stretch in pregnant myometrial strips of rat using isometric tension measurement and patch-clamp technique. In the present results, hypoosmotic cell swelling activated a potent outward rectifying current in G protein-dependent manner in rat pregnant myocyte. The current was significantly potentiated by 1µM lanthanides (a potent TRPC4/5 stimulator) and suppressed by 10µM 2-APB (TRPC4-7 inhibitor). In addition, in isometric tension experiment, SMC which was evoked by passive stretch was greatly potentiated by lanthanide (1µM) and suppressed by 2-APB (10µM), suggesting a possible involvement of TRPC4/5 channel in regulation of SMC in pregnant myometrium. These results provide a possible cellular mechanism for regulation of SMC during pregnancy and provide basic information for developing a new agent for treatment of premature labor.

Temporal and spatial variations in spontaneous Ca events and mechanical activity in pregnant rat myometrium

OBJECTIVES

The aim of this study was to investigate the temporal and spatial characteristics of spontaneous Ca signals in pregnant rat myometrium.

STUDY DESIGN

Confocal imaging of longitudinal strips of 21-day pregnant rats loaded with the Ca sensitive indicator Fluo-4, was combined with measurements of mechanical activity in uterine smooth muscle cells, in situ and freshly isolated.

RESULTS

Our results show that the Ca transients in pregnant uterine tissue are composed of Ca spikes, which are associated with the spike-like action potentials. There is large variation in the pattern of spontaneous activity in myometrium, ranging from non-propagating Ca spikes confined to individual smooth muscle cells, through to regional and global propagating Ca spikes. Irrespective of the pattern of activity displayed, the Ca signals were always in the form of Ca spikes, singularly or in bursts. These Ca spikes did not show fixed initiations sites, propagated in longitudinal and transverse directions from the initiation regions, and had a variable pattern of propagation in preparations which were not synchronously active. In preparations which showed synchronous activity, Ca spikes singularly or bursts propagated mainly in the transverse direction from the initiation regions. The amplitude of force generated by single spikes was dependent on the number of bundles recruited by the propagating Ca spike within the strip, and was about 30-40% of the maximal force produced by carbachol or high-K stimulation. If Ca spikes appeared in the form of bursts they generated longer lasting fused contractions, the amplitudes of which were dependent on the number and the frequency of Ca spikes in the burst.

CONCLUSIONS

Longitudinal myometrium from pregnant rats generates spontaneous Ca spikes which vary in their initiation sites, spatial spread and frequency and are associated with the spike-like action potentials. They are sensitive to the L-type Ca channel blocker, nifedipine. Contractile activity was dependent on the spatial spread of individual Ca spikes and when fully synchronized, produced single submaximal phasic contraction. The number and frequency of bursts of Ca spikes controlled the amplitude and duration of contraction.

Regulation of matrilysin in the rat uterus

Matrilysin was first discovered in the involuting rat uterus; it has also been known as uterine metalloproteinase, putative metalloproteinase (Pump-1), and matrix metalloproteinase 7 (MMP-7). It is the smallest member (28 kDa) of a family of 15 MMPs that together are able to degrade most of the macromolecules of the extracellular matrix. This family is briefly reviewed; all members are zinc metalloproteinases that occur in zymogen form with the active site zinc blocked by cysteine. Matrilysin can degrade a wide range of gelatins, proteoglycans, and glycoproteins of the matrix and can activate several other MMPs including collagenase. With respect to the uterus, matrilysin is localized to epithelial cells and varies in amount with the estrus cycle and is found in high levels during postpartum involution. There is evidence for a role in the last stage of cervical ripening and immediately postpartum. Induction of premature delivery by onapristone and prostaglandin E2 advances these changes in matrilysin. Regulation of the enzyme levels in the uterus are considered from four viewpoints: control of protein synthesis (particularly in response to hormones), activation of the proenzyme to functional protease, retention of enzyme by binding to matrix components such as heparan sulfate, and inhibition by natural inhibitors such as tissue inhibitor of metalloproteinases (TIMPs) and alpha 2-macroglobulin.

Cell death and proliferation and its relation to collagen degradation in uterine involution of rat

Collagen concentration, procollagenase localization, and their association with cell proliferation and apoptosis during postpartum involution, were investigated biochemically and histochemically in postpartum day 1, 3, 5, and 7 rat uterine tissues. In control animals, uterine wet weight, soluble protein, and collagen decreased rapidly during days 1 to 3 postpartum, and the DNA concentration in the uterine horn rapidly decreased, as noted by others. Simultaneously, both apoptosis and cell proliferation were observed in these tissues. These processes were highest in smooth muscle cells on day 3 postpartum. Procollagenase was found in the cell cytoplasm through days 1 to 3 postpartum, was highest on the third day postpartum, and appeared to gradually diminish by day 5 postpartum. Disorganization of collagen fibers was observed, under polarized microscopy by a strong birefringence of collagen fibers of the circular smooth muscle cell layers. However, this disorganization of the uterine collagen diminished progressively from day 3 to day 7. Treatment with estradiol or a combination of estradiol and progesterone suppressed cellular turnover and attenuated the changes in DNA, total amino acids, and collagen on day 3 postpartum. In this study, cellular turnover and biochemical and morphological changes appeared to be closely associated. Gonadal steroid hormones appear to influence these changes and retard uterine involution. This study suggests that a dynamic turnover of the cellular population takes place during uterine involution. It is possible that other factors, in addition to steroid hormones, contribute to uterine involution. It is to be postulated that these factors either are themselves decreased or, alternatively, may increase the inhibition of other unknown factors by an indirect mechanism.

Expression of matrix-degrading enzymes in pulmonary vascular remodeling in the rat

Exposure of rats to hypoxia causes pulmonary arterial remodeling, which is partly reversible after return to air. We hypothesized that degradation of excess collagen in remodeled pulmonary arteries in the posthypoxic period is mediated by endogenous matrix metalloproteinases (MMPs). Total proteolytic, collagenolytic, and gelatinolytic activities, levels of stromelysin-1 and tissue inhibitor of metalloprotease-1 (TIMP-1), and immunolocalization of stromelysin-1 in main pulmonary arteries were determined after exposure of rats to 10% O2 for 10 days followed by normoxia. We observed transient increases in total proteolytic, collagenolytic, and gelatinolytic activities and expression of approximately 72-, 68-, and 60-kDa gelatinases by zymography within 3 days of cessation of hypoxic exposure. The level of TIMP-1 increased as the stromelysin-1 level increased. Immunoreactive stromelysin-1 was localized predominantly in the luminal region of normal and hypertensive pulmonary arteries. These results are consistent with the notion that endogenous MMPs may mediate the breakdown of excess collagen in remodeled pulmonary arteries during the early posthypoxic period.

Mast cell collagenase correlates with regression of pulmonary vascular remodeling in the rat

Pulmonary vascular remodeling, produced by cell hypertrophy and extracellular matrix protein synthesis in response to hemodynamic stress, regresses after reduction of blood pressure, possibly by proteolysis of structural proteins. To test this postulate, we assessed the breakdown of extracellular matrix proteins and expression of collagenase and elastase in pulmonary arteries of rats exposed to hypoxia (10% O2 for 10 d) followed by normoxia. During hypoxia, contents of collagen and elastin increased in pulmonary arteries and latent rat interstitial collagenase was expressed without increased collagenolytic activity or mRNA levels. At 3 days after normoxia, collagen and elastin contents decreased coincident with the new appearance of activated collagenase and transient increases in collagenolytic and elastolytic activities. The amount of immunoreactive collagenase, localized predominately in connective tissue-type mast cells, was increased in the adventitia and media of hypertensive vessels. We conclude that mast cells containing latent collagenase are recruited into the outer walls of pulmonary arteries during remodeling. It is possible that mast cell-derived collagenase contributes to collagen breakdown in pulmonary arteries during early recovery from hypoxia and plays a role in restoration of vascular architecture.

Prevention of retained placenta by injection of collagenase into umbilical arteries of calves delivered by cesarean section: A tolerance study

In the cow, cesarean section delivery is often followed by retention of fetal membranes. Hypothetically, the retention of fetal membranes could be prevented by intraplacental injections of the enzyme collagenase. However, the infusion of this potent proteolytic enzyme into a uterus traumatized by surgery can lead to uterine damage, including perforation. Thus, the objective of this research was to evaluate tolerance of intraplacental treatment of bacterial collagenase. A cesarean section was performed on 10 experimental cows undergoing induced delivery or diagnosed with dystocia. During the surgical procedure, 200,000 units of bacterial collagenase in 1 L of saline were infused via the umbilical arteries. A cesarean section was also performed on control cows (n = 25) affected by dystocia, but these received no collagenase. The collagenase-treated cows showed no clinical or laboratory signs of abnormality over a 3- to 4-wk observation period post treatment. When membrane retention time was set at 36 h post surgery, 20% of the experimental cows and 60% of the control cows had retained the fetal membranes. It was concluded that intraplacental administration of collagenase during cesarean section is safe. However, treatment effectiveness and economic benefits for commercial application need further study.

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

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

Expulsion of the placenta from the uterus is the principal initiator for collagen degradation in mouse uterus

In unilaterally pregnant mice, collagen degradation in the non-pregnant uterine horn was not initiated by removal of the fetus only but by removal of both the fetus and placenta. The results indicate that expulsion of the placenta from the uterus is a principal factor in the initiation of the process of collagen degradation simultaneously in the whole uterus.

Mechanogenetic regulation of transcription

In many biological systems mechanical forces regulate gene expression: in bacteria changes in turgor pressure cause a deformation of the membrane and induce the expression of osmoregulatory genes; in plants gravity regulates cell growth ('geotropism'); in mammals stretching a muscle induces hypertrophy which is accompanied by qualitative changes in protein synthesis. Consequently, the term 'mechanogenetic control' seems to be a suitable common name for all these processes. The mechanism by which mechanical factors modulate transcriptional activity is still unknown. The purpose of this review is to bring together data from different fields in order to obtain a better understanding of the mechanogenetic control of cell growth.

Effect of loss of mechanical distension on collagen degradation in the mouse uterus

In unilaterally pregnant mice, loss of mechanical distension induced by artificial removal of fetuses from the pregnant uterus was followed by oedema in the endometrium. No endometrial oedema was found in the non-pregnant uterine horn, which did not experience the loss of mechanical distension at parturition. Ovariectomy on the last day of pregnancy did not prevent the appearance of endometrial oedema on the day of parturition. These histological results indicate that collagen degradation following the loss of mechanical distension of the uterine wall is attributable to formation of oedema in the endometrium.

The effects of pregnancy and parturition on phosphorus metabolites in rat uterus studied by 31P nuclear magnetic resonance

Concentrations of phosphorus metabolites and intracellular pH have been measured in non-pregnant, late-pregnant and post-partum rat uterus using 31P nuclear magnetic resonance (31P n.m.r.). Intact uterine tissue was superfused with oxygenated de-Jalon solution at 4, 20 or 37 degrees C while inside the n.m.r. spectrometer. The phosphocreatine concentration [PCr], was higher and the inorganic phosphate concentration [Pi], lower than values determined by chemical analysis of extracts from both pregnant and non-pregnant rat uterus. [PCr] was 1.4-fold greater in late-pregnant than in non-pregnant rat uterus. Following parturition, large changes were observed in [PCr], [Pi] and in an unidentified metabolite in the phosphomonoester (PME) region of the n.m.r. spectrum. The time course of the recovery of these metabolites to prepregnant values was determined. The [PCr] remained below the non-pregnant value for at least 1 week post-partum and the [Pi] was elevated, compared to the non-pregnant value, during this period. More rapid changes were seen in the [PME], which doubled on day 0 post-partum but almost returned to its non-pregnant value on day 1 post-partum. No significant difference was observed between intracellular pH values in late-pregnant and non-pregnant rat uterus; however, there was a large acid shift following parturition. Intracellular pH depended upon the temperature at which the tissue was maintained. The effect of muscular work during parturition was investigated by comparing Caesarian-sectioned uteri with uteri which had undergone normal parturition. Uteri examined 1 day after Caesarian operation showed no differences in metabolite levels from normal, 1 day post-partum uteri. We conclude that concentrations of phosphorus metabolites depend upon the physiological state of the uterus. We suggest that the changes following parturition are not a consequence of the mechanical work performed by the uterus, but must be caused by some other event associated with parturition such as hormonal changes.

Hypertrophic smooth muscle. V. Collagen and other extracellular materials. Vascularization

In adult guinea-pigs, oral to a partial obstruction to the flow of ingesta in the ileum there is a marked increase in the diameter of the intestine and a hypertrophy of the muscle coat. The features of the intramuscular blood vessels and of the extracellular materials were studied by electron microscopy. There is a small increase in the amount of intercellular space measured morphometrically. The basal lamina surrounding the hypertrophic muscle cells is more prominent than in controls. In the intercellular space between muscle cells, in addition to collagen fibrils, there is abundant amorphous material of medium electron density and streak-like, electron-dense material often similar to thickened basal laminae. The total amount of stroma (intercellular materials) present in a unit length of intestine is greatly increased in hypertrophy; a role of the muscle cells in the production of new collagen and other extracellular elements is suggested by the present observations. Many new intramuscular blood vessels (mainly capillaries, some of which are fenestrated) are formed during hypertrophy of the intestinal wall, so that the circular muscle layer remains as well vascularized in the hypertrophic intestine as in the controls. Blood vessels are not formed within the longitudinal muscle layer.

The collagen fibrils in the collapsed and the chronically stretched intestinal wall

A partial and progressive obstruction of the ileum (stenosis) was produced by surgery in adult rats and guinea pigs. Oral to the stenosis the accumulation of ingesta imposed a condition of chronic stretch on the intestinal wall: the lumen was much distended and the wall increased in thickness, mainly by hypertrophy of the mucosa and muscle coat. The submucosa too increased in volume and its collagen fibrils showed marked ultrastructural changes: in the rat the collagen fibrils (which were of rather uniform diameter in the control submucosa, mean 87 nm) varied greatly in size and formed two distinct populations: large fibrils (similar to those of controls) and small fibrils (4-5 nm in diameter). The latter were probably newly formed fibrils and their number increased in the more advanced stages of hypertrophy. The small fibrils were usually gathered into groups of 15-60 but were found throughout the entire thickness of the collagen bundles. Aboral to the stenosis the intestine was collapsed. In the submucosa there was a widening of the range of fibril sizes, a small increase in the average size and the occurrence of very large and irregular fibrils. Similar changes occurred in the guinea pig; however, the size of the control collagen fibrils (57 nm) was smaller than in the rat, and in the condition of chronic stretch small and large fibrils could not be clearly separated into distinct populations.

The effect of pregnancy and lactation on the mesometrium of the rat

Weight and collagen content of the mesometrium, that part of the reproductive tract which lies between the uterus and the body wall, were measured in the rat, throughout pregnancy and post-partum. Both the weight and the collagen content increased significantly during pregnancy and decreased rapidly after parturition, the collagen content halving in around four days. The changes were almost entirely in the part of the mesometrium between the main longitudinal blood vessels and the uterus. At any given stage of pregnancy, the larger the uterus, the larger were both the weight and collagen content of the mesometrium. In rats pregnant in only one uterine horn the mesometrium associated with the barren horn showed no changes in weight or collagen content during pregnancy. Removal of fetuses from one uterine horn reversed the growth of the mesometrium associated with that horn, but not the growth of the mesometrium associated with the other (pregnant) horn. Cutting the attachment to the body wall prevented mesometrial growth during pregnancy although the tissue remained attached to the uterus, with its blood supply and the metrial gland intact. These data suggest that mesometrial growth in pregnancy is stimulated by stretch associated with the pregnant uterine horn.