Rat Uterine Cells Research Articles

Docosahexaenoic Acid Is a Potent Inhibitor of Rat Uterine Stromal Cell Proliferation

The effect of different families of fatty acids on the proliferation of rat uterine stromal cells (U III) was studied. Docosahexaenoic acid (DHA) exerted a strong and dose-dependent inhibitory effect (IC 50 ∼ 2 μM), whereas arachidonic acid (AA) stimulated U III cell proliferation at the optimal concentration of 10 μM. Oleic, linoleic and linolenic acids were ineffective from 0.1 to 10 μM. The inhibitory effect of DHA was independent of the eicosanoid biosynthesis and lipid peroxidation, since it was not reversed by the addition of the antioxidant BHT and no significant production of oxidized species from DHA occurred in our culture conditions.

Serotonin regulation of interleukin-1 messenger RNA in rat uterine smooth muscle cells. Relationship to the production of interstitial collagenase.

Previous studies have shown that the production of interstitial collagenase by rat myometrial smooth muscle cells is dependent on serotonin. These cells fail to produce collagenase early in culture, however, and produce the enzyme only 8-12 days after confluence. During the early quiescent period, collagenase production can be induced by low concentrations of bacterial endotoxin. Under these conditions, interleukin (IL)-1 alpha and IL-1 beta mRNAs increase coincident with collagenase and collagenase mRNA. Serotonin removal decreases IL-1 alpha and IL-1 beta mRNAs, and effect that is rapidly reversed upon readdition of serotonin. Conversely, serotonin-dependent increases in IL-1 mRNAs are blocked by progesterone. Experiments with 5-HT2 receptor agonists and antagonists indicate that induction is mediated by the 5-HT2 receptor subtype. In serotonin-treated cells late in culture, IL-1 mRNAs increase coincident with the production of collagenase. Similarly, exogenous IL-1 fully substitutes for lipopolysaccharide in stimulating myometrial cells to produce collagenase early in culture. Cells treated with IL-1 receptor antagonist fail to make IL-1 mRNAs or collagenase but produce collagenase and IL-1 mRNAs following antagonist removal. These results indicate that serotonin-dependent IL-1 production by the myometrial cell is required for collagenase production and that IL-1 participates in its own production via an autocrine mechanism.

Progesterone regulation of heparin-binding epidermal growth factor-like growth factor gene expression during sensitization and decidualization in the rat uterus: effects of the antiprogestin, ZK 98.299.

The ability of uterine stromal cells to respond to embryo attachment or an artificial decidual stimulus and the development and maintenance of decidual cells are primarily regulated by progesterone (P), yet few P-induced genes have been described. Previously, we reported that P stimulated the expression of heparin-binding epidermal growth factor-like growth factor (HB-EGF) messenger RNA (mRNA) in rat uterine stromal cells. We now present evidence that the effects of P on HB-EGF expression are blocked by treatment with ZK98299 (onapristone), indicating that they are receptor mediated. Additionally, animals treated with ZK98299 alone or together with P failed to respond to a decidual stimulus, suggesting that stromal cell sensitivity was inhibited. These results provide further evidence that HB-EGF plays a role in the development of stromal cell sensitivity to decidual stimuli. Expression of HB-EGF mRNA was also investigated in differentiating stromal cells after an artificial decidual stimulus. HB-EGF mRNA levels increased from 8-48 h after stimulation, the same interval in which stromal cells exhibit maximal mitotic activity. In situ hybridization revealed that stromal and decidual cells were the major sources of HB-EGF mRNA during this period. These results suggest that HB-EGF acts as an autocrine/paracrine factor in stimulating stromal cell proliferation after the induction of decidualization.

Progesterone regulation of heparin-binding epidermal growth factor-like growth factor gene expression during sensitization and decidualization in the rat uterus: effects of the antiprogestin, ZK 98.299

The ability of uterine stromal cells to respond to embryo attachment or an artificial decidual stimulus and the development and maintenance of decidual cells are primarily regulated by progesterone (P), yet few P-induced genes have been described. Previously, we reported that P stimulated the expression of heparin-binding epidermal growth factor-like growth factor (HB-EGF) messenger RNA (mRNA) in rat uterine stromal cells. We now present evidence that the effects of P on HB-EGF expression are blocked by treatment with ZK98299 (onapristone), indicating that they are receptor mediated. Additionally, animals treated with ZK98299 alone or together with P failed to respond to a decidual stimulus, suggesting that stromal cell sensitivity was inhibited. These results provide further evidence that HB-EGF plays a role in the development of stromal cell sensitivity to decidual stimuli. Expression of HB-EGF mRNA was also investigated in differentiating stromal cells after an artificial decidual stimulus. HB-EGF mRNA levels increased from 8-48 h after stimulation, the same interval in which stromal cells exhibit maximal mitotic activity. In situ hybridization revealed that stromal and decidual cells were the major sources of HB-EGF mRNA during this period. These results suggest that HB-EGF acts as an autocrine/paracrine factor in stimulating stromal cell proliferation after the induction of decidualization.

Regulation of uterine collagenase gene expression: interactions between serotonin and progesterone

This report seeks to further define the requirements for the previously established induction of collagenase gene expression by serotonin and inhibition by progesterone in primary cultures of rat uterine smooth muscle cells. Detectable increases in collagenase production were observed after as little as 3 h exposure of cells to 5 μM serotonin, with maximal induction occurring after approximately 8 h of exposure. The apparent half-life of collagenase mRNA upon removal of serotonin was estimated to be approximately 12 h, and was not dependent on the duration of induction. Inhibition by either cycloheximide or progesterone showed similar half lives for collagenase mRNA, however a much shorter half-life (6 h) was obtained in the presence of actinomycin D. These experiments suggest that neither serotonin induction nor progesterone inhibition of collagenase synthesis represents a primary effect on collagenase gene transcription. Rather they appear to be secondary to changes that occur at one or more primary intermediate genes whose induction or decay must occur prior to changes in collagenase transcription. The progesterone receptor antagonist, RU-486, abrogates the ability of progesterone to inhibit serotonin-induced collagenase gene expression, indicating that the effects of progestins in this system likely are receptor-mediated. Finally, the present studies demonstrate that pretreatment of cells for times as long as 5 days with medroxyprogesterone in the absence of serotonin is unable to prevent subsequent serotonin-induced collagenase mRNA increases. These data suggest the possibility of a unique interaction between the molecular pathways of inducer and inhibitor, one in which serotonin may help mediate the progesteronedependent repression of the levels of collagenase mRNA.

Differentially regulated immediate early genes in the rat uterus

Estrogen stimulates cellular proliferation in the luminal epithelium, stroma, and smooth muscle of immature rat uterus. Progesterone administered concurrently with estrogen blocks the stimulatory effect of estrogen specifically in the epithelium, whereas progesterone administered alone stimulates proliferation in the endometrial stroma and myometrium. The present studies determined the effects of estrogen and progesterone on expression of the growth-associated, immediate early genes c-fos, c-jun, and jun-B in the luminal epithelium of the uterus. Hormonal effects were quantitated by Northern analysis of RNA extracted directly from the uterine luminal epithelium. Estrogen stimulated c-fos and jun-B expression, but repressed c-jun mRNA levels in the epithelium. In contrast, when whole organ RNA extracts were analyzed, estrogen increased mRNA levels for all three genes. Although progesterone administered alone showed no effect on mRNA levels in either epithelial or whole uterus extracts, it did attenuate the estrogen-induced increase in c-fos mRNA by 50% in whole uterus extracts and by 23% in epithelial extracts. The estrogen-induced increase in epithelial jun-B mRNA was not affected by progesterone pretreatment. Thus, in the immature rat uterus, no simple correlation exists between cellular proliferation and increased expression of the genes studied. However, progesterone completely blocked the repressive effect of estrogen on epithelial c-jun, suggesting a link between decreased c-jun expression and induction of cell proliferation in the uterine luminal epithelium. Estrogen repression of epithelial c-jun expression was hormone specific and sensitive to antiestrogen blockade. After estrogen treatment, epithelial c-jun mRNA decreased with a rate similar to its half-life, as determined in primary cultures of rat uterine cells. These results suggest that estrogen, acting through its receptor, directly represses transcription of c-jun in the uterine epithelium. Differences in hormonal regulation of immediate early genes between epithelial and nonepithelial uterine tissues probably results from tissue-specific transactivating factors that control the expression of these genes.

Heparin-binding epidermal growth factor-like growth factor is differentially regulated by progesterone and estradiol in rat uterine epithelial and stromal cells.

Heparin-binding epidermal growth factor-like growth factor (HB-EGF) cDNA was isolated from a subtracted cDNA library that selected for progesterone-induced transcripts from rat uterine stromal cells. In this study, the effects of progesterone and estradiol on the expression of HB-EGF in mature rat uterine epithelial and stromal cells have been examined. RNase protection assays and in situ hybridization demonstrated that progesterone stimulated expression of HB-EGF in rat uterine stromal cells, but repressed levels of HB-EGF mRNA in luminal and glandular epithelial cells. In contrast, estradiol treatment strongly enhanced HB-EGF expression in epithelial cells, but had no effect on mRNA levels for this growth factor in stromal cells. Progesterone treatment followed by estradiol injection stimulated HB-EGF expression in stromal cells and repressed expression in luminal and glandular epithelium. Stimulation of HB-EGF expression in stromal cells by progesterone was not inhibited by treatment with cycloheximide, demonstrating that HB-EGF mRNA expression is a primary response of stromal cells to progesterone. These results reveal that expression of HB-EGF is stimulated in epithelial and stromal cells in vivo under the same hormonal conditions that induce cell proliferation in each of these cell types and strongly suggest that HB-EGF may mediate the mitogenic effects of steroid hormones in the rat uterus.

Heparin-binding epidermal growth factor-like growth factor is differentially regulated by progesterone and estradiol in rat uterine epithelial and stromal cells

Heparin-binding epidermal growth factor-like growth factor (HB-EGF) cDNA was isolated from a subtracted cDNA library that selected for progesterone-induced transcripts from rat uterine stromal cells. In this study, the effects of progesterone and estradiol on the expression of HB-EGF in mature rat uterine epithelial and stromal cells have been examined. RNase protection assays and in situ hybridization demonstrated that progesterone stimulated expression of HB-EGF in rat uterine stromal cells, but repressed levels of HB-EGF mRNA in luminal and glandular epithelial cells. In contrast, estradiol treatment strongly enhanced HB-EGF expression in epithelial cells, but had no effect on mRNA levels for this growth factor in stromal cells. Progesterone treatment followed by estradiol injection stimulated HB-EGF expression in stromal cells and repressed expression in luminal and glandular epithelium. Stimulation of HB-EGF expression in stromal cells by progesterone was not inhibited by treatment with cycloheximide, demonstrating that HB-EGF mRNA expression is a primary response of stromal cells to progesterone. These results reveal that expression of HB-EGF is stimulated in epithelial and stromal cells in vivo under the same hormonal conditions that induce cell proliferation in each of these cell types and strongly suggest that HB-EGF may mediate the mitogenic effects of steroid hormones in the rat uterus.

Phospholipase A 2 inhibitors regulate the proliferation of normal uterine cells

The effects of inhibitors of phospholipase A 2, cyclooxygenase, lipoxygenase and cytochrome P450 activity on the proliferation of normal rat uterine stromal cells (U III) were studied. At non-cytotoxic doses, inhibitors of cyclooxygenase, lipoxygenase and cytochrome P450 activity had no effect; U III cells did not lose their ability to synthesize and secrete arachidonic acid metabolites, mainly prostaglandin I 2 and prostaglandin E 2, after successive passages. Inhibition of prostaglandin production did not affect their proliferation. In contract, phospholipase A 2 inhibitors significantly reduced U III cell proliferation in a reversible and dose-dependent manner. Aristolochic acid was the most potent inhibitor withan C 50 of 0.3 μmol/l on day 7 of culture. Moreover, low doses of arachidonic acid stimulated U III cell proliferation. Thus, the proliferation of normal uterine stromal cells appears to be independent of arachidonic acid oxygenated metabolites, contrary to what is observed in tumor cells, but requires an intact phospholipase A 2 pathway.

The effect of progesterone on the lipid composition of blood plasma and of plasma membranes of rat uterine cells

The effect of progesterone on the lipid composition of blood plasma and of plasma membranes of rat uterine cells

Forskolin inhibition of K + current in pregnant rat uterine smooth muscle cells

Two kinds of outward K + currents were examined in single smooth muscle cells from pregnant rat uterus, using whole-cell voltage clamp. The first and faster component was more sensitive to 4-aminopyridine (4-AP), whereas the second and slower (delayed) component was more sensitive to tetraethylammonium (TEA). A possible third K + component (Ca activated K + current) was not recorded as the pipette solution included EGTA. Forskolin inhibited the outward current in a concentration-dependent manner (50% inhibition occurred at about 30 μM); it affected the delayed component rather than the fast component. 8-Bromo-cAMP did not alter the outward current. In addition, inhibitors of protein kinase A and GDP-βS and GTP-γS did not affect the forskolin-induced inhibition. These results indicate that forskolin inhibition of the delayed component of the outward current is independent of cAMP generation in the pregnant rat myometrial cells. Therefore, forskolin seems to directly inhibit specific K + channels, as was reported for several other cell types.

Estrogen induces expression of c-jun and jun-B protooncogenes in specific rat uterine cells.

Expression of the protooncogene c-jun is induced in the uteri of ovariectomized rats in response to treatment with 17 beta-estradiol (E2-17 beta). E2 also specifically induces the uterine expression of jun-B-encoding mRNA. Medroxyprogesterone acetate and testosterone propionate treatment had no effect on the expression of c-jun- and jun-B-encoding mRNAs. Dexamethasone treatment, however, induced expression of c-jun mRNA, although less than that observed in response to E2. Cycloheximide treatment failed to block the E2-induced expression of c-jun and jun-B mRNAs, indicating that these were immediate early responses. E2-16 alpha, a short-acting estrogen, also induced c-jun and jun-B mRNA expression. The expression of c-Jun protein was examined by immunohistological methods and detected in all uterine cell types in response to treatment with estrogen. The Jun-B protein, however, was localized in uterine epithelial cells. The results of these experiments suggest that the cell type-specific expression of members of the jun family of protooncogenes may be an important regulatory event in the response of the uterus to estrogen.

Estrogen induces expression of c-jun and jun-B protooncogenes in specific rat uterine cells

Estrogen induces expression of c-jun and jun-B protooncogenes in specific rat uterine cells

Stimulation of estrogen receptor-mediated transcription and alteration in the phosphorylation state of the rat uterine estrogen receptor by estrogen, cyclic adenosine monophosphate, and insulin-like growth factor-I.

We have shown previously that exposure of rat uterine cells in primary culture to estradiol (E2), insulin-like growth factor-I (IGF-I), or agents which alter intracellular cAMP levels, such as cholera toxin plus isobutylmethylxanthine (CT + IBMX) and 8-Br-cAMP, results in the up-regulation of cellular levels of the progesterone receptor, an effect believed to be mediated through the activation of estrogen receptor (ER) and phosphorylation pathways. We have therefore undertaken studies using transient transfection of these uterine cell cultures with a simple estrogen-responsive reporter gene in order to determine the ability of these agents to stimulate ER-mediated gene transcription directly. We also compared the ability of these same agents to alter the phosphorylation state of the endogenous uterine ER protein. Plasmid DNA containing two tandem estrogen responsive elements and a TATA box linked to the chloramphenicol acetyl transferase (CAT) gene was introduced into immature rat uterine cells grown in primary culture. Treatment of transfected cells with 10(-9) M E2, CT (1 micrograms/ml) + IBMX (10(-4) M), 8-Br-cAMP (10(-4) M), or IGF-I (20 ng/ml) resulted in an 8- to 10-fold induction of CAT activity. CAT activity stimulated by all agents was nearly completely suppressed by coincubation with the antiestrogen ICI 164,384 (ICI) or the protein kinase (PK) inhibitor H8. CAT activity induced by 8-Br-cAMP was more readily suppressed by ICI than that induced by E2, indicating that ER in cells exposed to 8-Br-cAMP is either unoccupied or minimally occupied by ligand. The level of ER phosphorylation in uterine cells was increased 3- to 5-fold upon exposure to E2, CT + IBMX, 8-Br-cAMP, or IGF-I. Of interest, the antiestrogen ICI also elicited a similar increase in overall ER phosphorylation. The PK inhibitors H8 and PKI suppressed the increase in overall ER phosphorylation stimulated by these agents by 50-75%. The results of our study indicate that E2, IGF-I, and agents which raise intracellular cAMP are able to stimulate ER-mediated trans-activation and ER phosphorylation. The fact that antiestrogen (ICI) evokes a similar increase in ER phosphorylation without a similar increase in transcription activation indicates that an increase in overall ER phosphorylation does not necessarily result in increased transcriptional activity.(ABSTRACT TRUNCATED AT 400 WORDS)

Properties of large-conductance K+ channels in human myometrium during pregnancy and labour.

The conversion of the electrically silent pregnant uterus to highly excitable at term represents a dramatic physiological event which is poorly understood. Here we provide the first description, from single-channel recordings, of a large conductance (212 pS) calcium-activated potassium channel (BKCa) in human pregnant myometrium which, in labour tissue, is either absent or has been considerably altered in its physiological and pharmacological properties. In the latter, the K+ channels have an identical conductance (221 pS) and K+ selectivity to BKCa channels but exhibit no Ca2+ or voltage sensitivity. We have termed these BK channels. Furthermore, the activity of the BKCa channel from pregnant tissue is inhibited by internal application of Ba2+ but not tetraethylammonium (TEA), whereas the activity of the BK channel is sensitive to internal TEA but not Ba2+. The role of the BKCa channel may be to suppress myometrial activity during gestation whereas BK channel activity may be important in providing a Ca(2+)-independent K+ conductance which would allow cytoplasmic Ca2+ levels to rise without activating a counteracting Ca(2+)-dependent outward current, normally provided by the BKCa channels which, by its very nature, would tend to oppose depolarization. The findings suggest that K+ channels may have an important role in determining the functional activity of the myometrium.

Oxytocin actions on voltage-dependent ionic channels in pregnant rat uterine smooth muscle cells.

The effects of oxytocin, a uterotonic polypeptide hormone, on the voltage-dependent slow calcium, fast sodium, and potassium channel currents were studied using whole-cell voltage clamp of freshly isolated cells from late pregnant (18-21 day) rat myometrium. The calcium current was rapidly inhibited by oxytocin (about 25% inhibition at 20 nM) in a dose-dependent manner, and this inhibitory effect was completely reversible by washout. However, inhibition was not observed when barium was used as the charge carrier. Sodium current and potassium current were not modified by oxytocin, thus sodium and potassium currents may not play important roles in oxytocin-induced augmentation of uterine contraction. It is concluded that oxytocin stimulates uterine contraction by mechanisms other than augmentation of the voltage-dependent calcium current, e.g., by release of Ca from sarcoplasmic reticulum (by inositol triphosphate) or by activation of a receptor-operated Ca channel. The inhibition of the slow calcium current may be induced by the elevation of [Ca]i.

Proliferative and morphogenic changes induced by the coculture of rat uterine and peritoneal cells: a cell culture model for endometriosis

To evaluate the proliferative and morphogenic effects induced by the coculture of uterine and peritoneal cells to establish a cell culture model for endometriosis. Uterine epithelial and stromal cells and peritoneal mesothelial and subserosal cells were cocultured with homologous cell types, heterologous cell types, or as isolated populations using a bicameral chamber design. Department of Obstetrics and Gynecology at the University of Missouri, Columbia, Missouri. Cells isolated and purified from five mature female Sprague Dawley rats of normal reproductive status were used to establish cell cultures. Cell proliferation (deoxyribonucleic acid synthesis) was measured by the incorporation of 3H-thymidine, and cell morphology was assessed using inverted phase-contrast microscopy. Peritoneal mesothelial cells augmented proliferation and induced cellular aggregation of uterine stromal cell monolayers. Peritoneal subserosal cells amplified proliferation and induced an irregular, compacted morphology in uterine epithelial cells. The proliferation and morphology of the two peritoneal cell types was not altered by uterine cell coculture. The coculture of uterine and peritoneal cells in bicameral chambers provides a tool to study the paracrine interactions of cells that comprise the endometriotic lesion. The altered proliferation and morphology of the uterine cells may be related to the histologic and biochemical asynchrony observed between uterine endometrium and ectopic endometriotic tissue in vivo and offers insight into possible mechanisms of the histogenesis of endometriosis.

Regulation of collagenase gene expression by serotonin and progesterone in rat uterine smooth muscle cells.

The regulation of collagenase gene expression by serotonin and progesterone was investigated in primary cultures of rat uterine smooth muscle cells. Northern blot analysis demonstrates that serotonin (5-hydroxytryptamine (5-HT)), when administered to cells in serotonin-depleted medium, causes 6-8-fold increases in levels of collagenase mRNA. Selective serotonin 5-HT2 receptor agonists were able to mimic the effect of the natural hormone, while the induction by serotonin could be blocked by 5-HT2 receptor antagonists. Addition of phorbol ester (PMA) to 5-HT-depleted cultures fully mimicked the effect of 5-HT on collagenase mRNA induction. Treatment with progesterone analogs caused a decrease in collagenase mRNA, even in the presence of saturating levels of serotonin or PMA. In all experiments, levels of secreted collagenase were observed to correspond to levels of collagenase mRNA. Experiments with cycloheximide demonstrate that serotonin- and PMA-induced increases in collagenase mRNA are dependent on protein synthesis. Furthermore, nuclear run-on analysis shows that mRNA increases are accompanied by increases in initiation of transcripts. These data indicate that transcription of collagenase mRNA in myometrial smooth muscle cells is stimulated by serotonin, possibly via activation of protein kinase C, but is in some way prevented by the negative influence of progesterone.

Oestradiol enhances in vitro the histamine release induced by embryonic histamine-releasing factor (EHRF) from uterine mast cells.

The relationship between maternal hormones and factors secreted by the implanting embryo is still controversial. We have analysed the in-vitro effect of oestradiol and human embryo-derived histamine-releasing factor (EHRF) on histamine release from rat uterine mast cells. Rat uterine mast cells which were preincubated with oestradiol and then challenged with human EHRF gave histamine release values two- to threefold higher than those without preincubation. The enhancement observed was time- and temperature-dependent. A similar enhancement was obtained with human sensitized basophils but not with rat peritoneal mast cells. Oestradiol, used as a direct challenge, did not induce any histamine release from either rat uterine or peritoneal mast cells, or from human sensitized basophils. Oestradiol preincubation also enhanced the histamine release induced by anti-IgE but did not enhance the histamine release induced by substance P or compound 48/80, two secretagogues that are not mediated by IgE. Moreover, uterine fragments derived from rats at various oestrus phases, with different amounts of endogenous oestrogen, were challenged in vitro with EHRF. The release of histamine by mast cells was higher at the proestrus and preimplantation phases than at dioestrus. All these findings suggest that the interaction of oestradiol with rat uterine mast cells was capable of enhancing in vitro the histamine releasing effect of EHRF.

Fast Na+ channels and slow Ca2+ current in smooth muscle from pregnant rat uterus

Smooth muscle cells normally do not possess fast Na+ channels, but inward current is carried through two types of Ca2+ channels: slow (L-type) Ca2+ channels and fast (T-type) Ca2+ channels. Using whole-cell voltage clamp of single smooth muscle cells isolated from the longitudinal layer of 18-day pregnant rat uterus, depolarizing pulses, applied from a holding potential of -90 mV, evoked two types of inward current, fast and slow [8]. The fast inward current decayed within 30 ms, depended on [Na]o, and was inhibited by TTX (K0.5 = 27 nM). The slow inward current decayed slowly, was dependent on [Ca]o, and was inhibited by nifedipine. These results suggest that the fast inward current is a fast Na+ channel current, and that the slow inward current is a Ca2+ slow channel current. A fast-inactivating Ca2+ channel current was not evident. Thus, the ion channels which generate inward currents in pregnant rat uterine cells are TTX-sensitive fast Na+ channels and dihydropyridine-sensitive slow Ca2+ channels. The number of fast Na+ channels increased during gestation. The averaged current density increased from 0 on day 5, to 0.19 on day 9, to 0.56 on day 14, to 0.90 on day 18, and to 0.86 pA/pF on day 21. This almost linear increase occurs because of an increase in the fraction of cells which possess fast Na+ channels, and it is suggested that the fast Na+ current may be involved in spread of excitation. The Ca2+ channel current density also was higher during the latter half of gestation. These results indicate that the fast Na+ channels and Ca2+ slow channels in myometrium become more numerous as term approaches, and may facilitate parturition. Isoproterenol (beta-agonist) did not affect either ICa(s) or INa(f), whereas Mg2+ (K0.5 of 12 mM) and nifedipine (K0.5 of 3.3 nM) depressed ICa(s). Oxytocin had no effect on INa(f) and actually depressed ICa(s) to a small extent. Therefore, the tocolytic action of beta-agonists cannot be explained by an inhibition of ICa(s), whereas that of Mg2+ can be so explained. The stimulating action of oxytocin on uterine contractions is not due to stimulation of ICa(s). Figure 11 summarizes the possible mechanisms by which uterine contractility can be modulated. In contrast to vascular smooth muscle, neither ISO nor adenosine, which produce elevation of cyclic AMP, affected ICa and INa. Therefore, no arrow can be drawn between cA-PK/cG-PK and the Ca2+ slow channel.(ABSTRACT TRUNCATED AT 400 WORDS)