Roles of potassium channels and nitric oxide in modulation of uterine contractions in rat pregnancy

Effects of corticotropin-releasing hormone on the expression of adenosine triphosphate-sensitive potassium channels (Kir6.1/SUR2B) in human term pregnant myometrium

Objective

Corticotropin-releasing hormone (CRH) is a crucial regulator of human pregnancy and parturition. Adenosine triphosphate (ATP)-sensitive potassium (KATP) channels are important for regulating myometrial quiescence during pregnancy. We investigated regulatory effects of different concentrations of CRH on KATP channel expression in human myometrial smooth muscle cells (HSMCs) in in vitro conditions.

Methods

After treating HSMCs with different concentrations of CRH (1, 10, 10², 10³, 10⁴ pmol/L), mRNA and protein expression of K_ATP channel subunits (Kir6.1 and SUR2B) was analyzed by reverse transcription-polymerase chain reaction and western blot. We investigated which CRH receptor was involved in the reaction and measured the effects of CRH on intracellular Ca²⁺ concentration when oxytocin was administered in HSMCs using Fluo-8 AM ester.

Results

When HSMCs were treated with low (1 pmol/L) and high (10³, 10⁴ pmol/L) CRH concentrations, K_ATP channel expression significantly increased and decreased, respectively. SUR2B mRNA expression at low and high CRH concentrations was significantly antagonized by antalarmin (CRH receptor-1 antagonist) and astressin 2b (CRH receptor-2 antagonist), respectively; however, Kir6.1 mRNA expression was not affected. After oxytocin treatment, the intracellular Ca²⁺ concentration in CRH-treated HSMCs was significantly lowered in low concentration of CRH (1 pmol/L), but not in high concentration of CRH (10³ pmol/L), compared to control.

Conclusion

Our data demonstrated the regulatory effect was different when HSMCs were treated with low (early pregnancy-like) and high (labor-like) CRH concentrations and the KATP channel expression showed significant increase and decrease. This could cause inhibition and activation, respectively, of uterine muscle contraction, demonstrating opposite dual actions of CRH.

Role of NO-cGMP pathway in ovine cervical relaxation induced by Erythroxylum caatingae Plowman

Erythroxylum caatingae Plowman has a myorelaxing effect on smooth muscle tissue. We investigated the effect of the crude ethanolic extract of E. caatingae Plowman (Ec-EtOH) on the contractility of the ovine cervix. In an isometric system, circular strips were subjected to 90mM potassium (K(+)) or 30μM carbamylcholine (CCh)-induced contraction. We then exposed the tissue to cumulative concentrations of Ec-EtOH (1-729 μg/ml). In other bath solutions, the tissues were exposed to l-NG-nitroarginine methyl ester (l-NAME; 100μM), l-NAME (100μM)+l-arginine (300μM), 1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one, ODQ; 5μM), 4-aminopyridine (4-AP; 3mM), tetraethylammonium (TEA; 0.3mM), glybenclamide (1μM), atosiban (10μM) or verapamil (3μM), followed by the addition of Ec-EtOH (1-729 μg/ml). We also evaluated the effect of cervical Ec-EtOH infusion (2mg) on cervical contractility in vivo. Ec-EtOH decreased cervical contractility induced by K(+) or CCh, and 729 μg/ml Ec-EtOH decreased 85.4±5.1% the amplitude of basal contractility in vitro, with an EC50 of 17.9±3.7 μg/ml. This effect of Ec-EtOH was prevented by l-NAME or ODQ. l-arginine impaired the blunting effect of l-NAME on cervical relaxation caused by Ec-EtOH. However, the potassium channel blockers 4-AP, TEA, and glybenclamide did not modify this myorelaxation triggered by Ec-EtOH. Ec-EtOH also decreased acetylcholine-induced contractions in tissue preincubated with verapamil. In addition, Ec-EtOH decreased ovine cervical contractions in vivo. Thus, Ec-EtOH had a relaxant effect on ovine cervical contractions. This may involve the nitric oxide signal, mediated by cGMP cellular transduction, and be related to intracellular calcium sequestration.

The stretch-dependent potassium channel TREK-1 and its function in murine myometrium

Smooth muscle of the uterus stays remarkably quiescent during normal pregnancy to allow sufficient time for development of the fetus. At present the mechanisms leading to uterine quiescence during pregnancy and how the suppression of activity is relieved at term are poorly understood. Myometrial excitability is governed by ion channels, and a major hypothesis regarding the regulation of contractility during pregnancy has been that expression of certain channels is regulated by hormonal influences. We have explored the expression and function of stretch-dependent K+ (SDK) channels, which are likely to be due to TREK channels, in murine myometrial tissues and myocytes using PCR, Western blots, patch clamp, intracellular microelectrode and isometric force measurements. TREK-1 is more highly expressed than TREK-2 in myometrium, and there was no detectable expression of TRAAK. Expression of TREK-1 transcripts and protein was regulated during pregnancy and delivery. SDK channels were activated in response to negative pressure applied to patches. SDK channels were insensitive to a broad-spectrum of K+ channel blockers, including tetraethylammonium and 4-aminopyridine, and insensitive to intracellular Ca2+. SDK channels were activated by stretch and arachidonic acid and inhibited by reagents that block TREK-1 channels, l-methionine and/or methioninol. Our data suggest that uterine excitability and contractility during pregnancy is regulated by the expression of SDK/TREK-1 channels. Up-regulation of these channels stabilizes membrane potential and controls contraction during pregnancy and down-regulation of these channels induces the onset of delivery.

In vitro study of the tocolytic effect of oroxylin A from Scutellaria baicalensis root

Scutellariae Radix is one of the well-known tocolytic Chinese herbs. Oroxylin A is isolated from the root of Scutellaria baicalensis. The main syndrome of preterm birth is caused by uterus contractions from excitatory factors. Administration of tocolytic agents is a strategy to prevent the occurrence of preterm births. The aim of this study was to investigate the effects of oroxylin A on contractions of uterine strips isolated from non-pregnant female Wistar rats (250~350 g). Contractions of the uterus were induced with acetylcholine (Ach) (1 μM), PGF_2α (0.1 μM), oxytocin (10^-3 U/ml), KCl (56.3 mM), tetraethylammonium (TEA; 1 and 10 mM), 4-aminopyridine (4-AP; 5 mM), glipizide (30 μM), a nitric oxide synthase (NOS) inhibitor (LNNA; 10^-3M), a β-receptor blocker (propranolol; 10 μM), and a cyclooxygenase inhibitor (indomethacin; 60 μM). The inhibitory effects of the amplitude and frequency of spontaneous contractions by oroxylin A were antagonized with Ach (IC₅₀ 22.85 μM), PGF_2α (IC₅₀27.28 μM), oxytocin (IC₅₀ 12.34 μM), TEA; 1 and 10 mM (IC₅₀ 52.73 and 76.43 μM), 4-AP (IC₅₀ 67.16 μM), and glipizide (IC₅₀27.53 μM), but oroxylin A was not influenced by Ca²⁺-free medium, LNNA, propranolol, or indomethacin. Otherwise, oroxylin A-mediated relaxation of the rat uterus might occur through opening of uterine calcium-dependent potassium channels or adenosine triphosphate potassium channel activation. This suggests that oroxylin A is the tocolytic principle constituent of Scutellariae Radix, and oroxylin A may provide a lead compound for new tocolytic drug development in the future.

The role of voltage-gated potassium channels in the regulation of mouse uterine contractility

BACKGROUND

Uterine smooth muscle cells exhibit ionic currents that appear to be important in the control of uterine contractility, but how these currents might produce the changes in contractile activity seen in pregnant myometrium has not been established. There are conflicting reports concerning the role of voltage-gated potassium (Kv) channels and large-conductance, calcium-activated potassium (BK) channels in the regulation of uterine contractility. In this study we provide molecular and functional evidence for a role for Kv channels in the regulation of spontaneous contractile activity in mouse myometrium, and also demonstrate a change in Kv channel regulation of contractility in pregnant mouse myometrium.

METHODS

Functional assays which evaluated the effects of channel blockers and various contractile agonists were accomplished by quantifying contractility of isolated uterine smooth muscle obtained from nonpregnant mice as well as mice at various stages of pregnancy. Expression of Kv channel proteins in isolated uterine smooth muscle was evaluated by Western blots.

RESULTS

The Kv channel blocker 4-aminopyridine (4-AP) caused contractions in nonpregnant mouse myometrium (EC50 = 54 micromolar, maximal effect at 300 micromolar) but this effect disappeared in pregnant mice; similarly, the Kv4.2/Kv4.3 blocker phrixotoxin-2 caused contractions in nonpregnant, but not pregnant, myometrium. Contractile responses to 4-AP were not dependent upon nerves, as neither tetrodotoxin nor storage of tissues at room temperature significantly altered these responses, nor were responses dependent upon the presence of the endometrium. Spontaneous contractions and contractions in response to 4-AP did not appear to be mediated by BK, as the BK channel-selective blockers iberiotoxin, verruculogen, or tetraethylammonium failed to affect either spontaneous contractions or 4-AP-elicited responses. A number of different Kv channel alpha subunit proteins were found in isolated myometrium from both nonpregnant and term-pregnant mice, and one of these proteins - Kv4.3 - was found to disappear in term-pregnant tissues.

CONCLUSION

These findings suggest a role for Kv channels in the regulation of uterine contractility, and that changes in the expression and/or function of specific Kv channels may account for the functional changes seen in pregnant myometrium.

Potassium channels and uterine function

Ion channels are effector proteins that mediate uterine excitability throughout gestation. This review will focus primarily on the role of potassium channels in regulating myometrial tone in pregnancy and labor contractions. During gestation, potassium channels maintain the uterus in a state of quiescence by contributing to the resting membrane potential and counteracting contractile stimuli. This review summarizes the current knowledge about the significance of the potassium channels in maintaining a normal gestational period and initiating labor contractions at term.

A role for voltage-gated, but not Ca2+-activated, K+ channels in regulating spontaneous contractile activity in myometrium from virgin and pregnant rats

The roles of voltage-gated (K(V)) and large conductance Ca2+-activated K+ (BK(Ca)) channels in regulating basal contractility in myometrial smooth muscle are unresolved. The aim of this study was to determine the effects of inhibition of these channels on spontaneous rhythmic contraction in myometrial strips from four groups of rats: nonpregnant and during early (day 7), mid- (day 14), and late (day 21) pregnancy. BK(Ca) channels were inhibited using iberiotoxin (1-100 nM), paxilline (1-10 microM) or penitrem A (1-500, or 3000 nM); K(V) channels were inhibited using tetraethylammonium (TEA; 1-10 mM) and/or 4-aminopyridine (4-AP; 1-5 mM). Contractility was assessed as mean integral tension (MIT). Time/vehicle controls were also performed. None of the selective BK(Ca) channel inhibitors significantly affected contractility in myometrial strips from either nonpregnant or pregnant animals. 4-AP caused concentration-dependent increases in MIT in myometrium in all four groups. TEA (5 and 10 mM) significantly increased MIT in myometrium from nonpregnant, and mid- and late pregnant rats, but not in myometrium from early pregnant rats. TEA and 4-AP still caused an increase in MIT following treatment with 3000 nM penitrem A or a combination of propranolol, phentolamine, atropine (all 1 microM) and capsaicin (10 microM) in myometrial strips from nonpregnant rats. These results indicate that whereas BK(Ca) channels play little or no part in controlling basal rhythmicity in rat myometrium, K(V) channels appear to play a crucial role in this regard, especially during mid- and late pregnancy.

Molecular signaling through G-protein-coupled receptors and the control of intracellular calcium in myometrium

Cellular mechanisms regulating myometrial intracellular free calcium (Ca2+(i)) are addressed in this review, with emphasis on G-protein-coupled receptor pathways. An increase in myometrial Ca2+(i) results in phosphorylation of myosin light chain, an increase in myosin adenosine monophosphatase (ATPase) activity and contraction. Dephosphorylation of myosin light chain and a decline in Ca2+(i) are associated with relaxation. Increases in Ca2+(i) are controlled by multiple signaling pathways, including receptor-mediated activation of phospholipase Cbeta (PLCbeta), leading to release of Ca2+ from intracellular stores. Ca2+ also enters myometrial cells through plasma membrane Ca2+ channels. Conversely, adenosine triphosphate (ATP)-dependent Ca2+ pumps lower Ca2+(i) concentrations and potassium channels promote hyperpolarization that can decrease Ca2+ entry. Receptor-coupled pathways that promote uterine relaxation primarily involve activation of cyclic adenosine monophosphate (cAMP)- or cyclic guanosine monophosphate (cGMP)-stimulated protein kinases that phosphorylate proteins regulating Ca2+ homeostasis. cAMP has inhibitory effects on myometrial contractile activity, agonist-stimulated phosphatidylinositide turnover and increases in Ca2+(i). Some of these effects require association of protein kinase A (PKA) with a plasma membrane-associated A-kinase-anchoring-protein (AKAP). Near term in the rat, there is a decline in the plasma membrane localization of PKA associated with this anchoring protein. This correlates with changes in the regulation of signaling pathways controlling Ca2+(i). L-type voltage-operated Ca2+ entry is an important regulator of myometrial contraction. In addition, putative signal-regulated or capacitative Ca2+ channel proteins, TrpCs, are expressed in myometrium, and signal-regulated Ca2+ entry is observed in human myometrial cells. This Ca2+ entry mechanism may play a significant role in the control of myometrial Ca2+(i) dynamics and myometrial contraction. The regulation of myometrial Ca2+(i) is complex. Understanding the mechanisms involved may lead to design of tocolytics that target multiple pathways and achieve improved suppression of premature labor.

Nitric oxide donor-induced inhibition of pregnant rat uterine spontaneous contractile activity and release of nitric oxide from uterus measured by microdialysis

Our aim was to study whether nitric oxide (NO) donor-induced inhibition of pregnant rat myometrium contractility correlates with the release of NO. Uterine rings from mid-pregnant and late pregnant Sprague-Dawley rats were used for isometric tension recording. Concentration-response relationships to sodium nitroprusside (SNP), nitroglycerine (NTG) and diethylamine (DEA)/NO were assessed. The time course of NO release after addition to the organ chambers of the 3 NO-donors was assessed by the detection of NO products NOx (NO3+NO2) using the microdialysis probe by a HPLC-NO detector system. DEA/ NO induced greater inhibition of the spontaneous contractile activity of uterine rings from mid-pregnant rats than SNP or NTG. In uterine rings from late pregnant rats, however, the maximal inhibition of the contractility by all 3 NO-donors were significantly less. The NOx levels measured in the uterine ring walls from either mid-pregnant or late pregnant rats significantly increased after DEA/ NO as compared to the basal levels or the levels after NTG or SNP. The decrease of NO-donor-induced inhibition of rat myometrium contractility, with unchanged formation of NOx, at term, suggests that the changes in NO signaling are responsible for gestational age-dependent attenuation of the inhibitory effect.

Expression of inducible nitric oxide synthase messenger RNA, but not guanylate cyclase messenger RNA, depends on gestational age in rat myometrium

We studied the expression of inducible nitric oxide (NO) synthase (iNOS) and soluble guanylate cyclase (sGC) mRNAs in pregnant rat myometrium. Expression of iNOS and sGC alpha1, beta1 and beta2 mRNA was analyzed in non-pregnant and pregnant (days 10, 14, 17 and 21) Wistar rats by reverse transcription-polymerase chain reaction. Expression of iNOS mRNA increased during pregnancy but decreased on day 21 of gestation. Expression of GC alpha1 mRNA was greater than GC beta1 mRNA at all time points. Expression of uterine GC alpha1 and GC beta1 mRNA did not change significantly during pregnancy and did not differ significantly from non-pregnant levels. The values of sGC beta2 mRNA were below the limit of detection. In conclusion, the expression of iNOS mRNA increased during pregnancy in the myometrium and decreased at term, while the expression of sGC mRNA was not affected by pregnancy. Thus, it is the changes in NO production, rather than changes in its target, that are responsible for uterine quiescence during pregnancy and initiation of labor.

Effect of nitric oxide on contractions of uterine and cervical tissues from pregnant rats

Our objective was to evaluate the role of the nitric oxide (NO)-cyclic guanosine monophosphate (cGMP) pathway in rat uterine and cervical contractility at mid- and late gestation. Rings of uterus and cervix from Sprague Dawley rats on day 14 of pregnancy (mid-) and day 21 of pregnancy (late) were equilibrated at 2 g passive tension in organ chambers filled with Krebs-Henseleit solution and bubbled with 5% CO2 in air (37 degrees C, pH approximately 7.4). Rings were treated with an inhibitor of outward potassium current, tetraethylammonium, to activate phasic contractions, and the concentration-response relationships to diethylamine/NO and 8-bromo-cGMP (8-br-cGMP) were assessed. Baseline spontaneous activity was significantly higher at term gestation in both uterine and cervical rings compared with mid-gestation. Spontaneous contractile activity was not apparent in cervical rings from rats in mid-gestation, but was persistent after treatment with tetraethylammonium. Oxyhemoglobin did not affect NO-induced inhibition of activation by tetraethylammonium contractile activity in either cervical or uterine tissues in mid- or late gestation. The 8-br-cGMP concentration-dependently inhibited tetraethylammonium-activated contractions that were more pronounced in uterine tissues compared with cervical tissues in both mid- and late gestation. We concluded that activation of the NO-cGMP pathway inhibits both uterine and cervical smooth muscle contractility. Both tissues demonstrated refractoriness to NO at term.

Dissociation of cGMP accumulation and relaxation in myometrial smooth muscle: effects of S-nitroso-N-acetylpenicillamine and 3-morpholinosyndonimine

In guinea pig, primate and man, nitric oxide (NO)-induced regulation of myometrial smooth muscle contraction is distinct from other smooth muscles because cyclic guanosine 3',5'-cyclic monophosphate (cGMP) accumulation is neither necessary nor sufficient to relax the tissue. To further our understanding of the mechanism of action of NO in myometrium, we employed the NO donors, S-nitroso-N-acetylpenicillamine (SNAP), and 3-morpholinosyndonimine (SIN-1) proposed to relax airway smooth muscle by disparate mechanisms involving elevation in intracellular calcium ([Ca(2+)](i)) or cGMP accumulation, respectively. Treatment of guinea pig myometrial smooth muscle with either NO donor at concentrations thought to produce maximal relaxation of smooth muscles resulted in significant elevations in cGMP that were accompanied by phosphorylation of the cGMP-dependent protein kinase substrate vasodilator-stimulated phosphoprotein (VASP), shown here for the first time to be present and phosphorylated in myometrium. Stimulation of myometrial strips with oxytocin (OT, 1 microM) produced an immediate increase in contractile force that persisted in the continued presence of the agonist. Addition of SNAP (100 microM) in the presence of OT relaxed the tissue completely as might be expected of an NO donor. SIN-1 failed to relax the myometrium at any concentration tested up to 300 microM. In Fura-2 loaded myometrial cells prepared from guinea pig, addition of SNAP (100 microM) in the absence of other agonists caused a significant, reproducible elevation of intracellular calcium while SIN-1 employed under the same conditions did not. Our data further support the notion that NO action in myometrium is distinct from that in other smooth muscles and underscores the possibility that discrete regional changes in [Ca(2+)](i), rather than cGMP, signal NO-induced relaxation of the muscle.

Apamin inhibits NO-induced relaxation of the spontaneous contractile activity of the myometrium from non-pregnant women

There is now considerable evidence for the involvement of K+ channels in nitric oxide (NO) induced relaxation of smooth muscles including the myometrium. In order to assess whether apamin-sensitive K+ channels play a role in NO - induced relaxation of the human uterus, we have studied the effect of specific blockers of these channels on the relaxation of myometrium from non-pregnant women. In vitro isometric contractions were recorded in uterine tissues from non-pregnant premenopausal women who had undergone hysterectomy. Apamin (10 nM) and scyllatoxin (10 nM) did not alter spontaneous myometrial contractions. However, 15-min pretreatment of the myometrium strips with apamin completely inhibited relaxation caused by diethylamine-nitric oxide (DEA/NO). The pretreatment with scyllatoxin significantly reduced (about 2.6 times) maximum relaxation of the strips induced by DEA/NO (p < 0.05). These results strongly suggest that, beside Ca2+ and voltage dependent charybdotoxin-sensitive (CTX-sensitive) K+ channels, apamin-sensitive K+ channels are also present in the human non-pregnant myometrium. These channels offer an additional target in the development of new tocolytic agents.

EFFECTS OF NITRIC OXIDE DONORS ON NON-PREGNANT AND PREGNANT RAT UTERINE AND AORTIC SMOOTH MUSCLE

OBJECTIVE

To compare the effects of nitric oxide (NO) donors, diethylamine/nitric oxide (DEA/NO) and nitroglycerin (NTG), on isolated uterine and aortic tissues from non-pregnant, mid and late pregnant rats.

METHODS

The uterus and thoracic aorta were obtained from non-pregnant (estrous cycle) and pregnant Sprague-Dawley rats on day 14 and day 21. The uterine and aortic rings were incubated in organ chambers filled with Krebs-Henseleit solution bubbled with 5% CO2 in air for isometric tension recordings. Cumulative concentration-response relationships to DEA/NO and NTG were obtained in the aortic rings contracted with phenylephrine and in spontaneously contracting uterine rings.

RESULTS

The sensitivity and the maximal inhibitory effects of DEA/NO did not differ in aortic tissues of any group. DEA/NO-induced Maximal inhibition of spontaneous contractions of uterine tissues from mid-pregnant rats was greater (although not significantly) than in the tissues from non-pregnant animals (with similar sensitivity), but it was significantly depressed in tissues from late pregnant rats. The sensitivity to and maximal inhibitory effects of NTG were less in aortic tissues from late pregnant versus mid-pregnant and non-pregnant rats. In uterine tissues from late pregnant rats the effect of NTG was negligible. The inhibitory action of both NO donors was much more pronounced in aortic versus uterine tissues.

CONCLUSIONS

Uterine smooth muscle is less sensitive than vascular smooth muscle to NO. Uterine smooth muscle from late pregnant animals demonstrates refractoriness to both DEA/NO and NTG, while vascular smooth muscle from late pregnant animals demonstrates refractoriness to NTG, but not to DEA/NO.

Prostaglandin-induced activation of uterine contractility in pregnant rats does not involve potassium channels

OBJECTIVE

The uterus is a target for prostaglandins, especially at the end of gestation. Whether potassium channels are involved in the effect of prostaglandins is not clear. The aim of this study was to find out.

STUDY DESIGN

Concentration-response relationships to prostaglandins (prostaglandin F2alpha, prostaglandin E2, and prostaglandin I2 [carbacyclin]; 10(-10) mol/L-10(-4) mol/L) were studied in isolated uterine rings from mid pregnancy (day 14) and late pregnancy (day 21) rats (Krebs solution, 5% CO2 in air, 37 degrees C; pH, 7.4). Rings were incubated for 30 minutes with either solvent or adenosine triphosphate-sensitive potassium channel inhibitor or opener glibenclamide and levcromakalim or with calcium-sensitive potassium channel inhibitor or opener NS1619 and iberiotoxin, respectively. The changes in integral activity were compared after each concentration of the agent and were expressed as a percent of the basal integral activity.

RESULTS

The increases in spontaneous contractile activity induced by prostaglandin E2 and carbacyclin, but not prostaglandin F2alpha, were statistically significantly higher in tissues from late pregnancy versus mid pregnancy rats and were not affected by any of the K-channel openers or inhibitors.

CONCLUSION

Adenosine triphosphate-sensitive and calcium-sensitive potassium channels are not involved in the effect of prostaglandin F2alpha, prostaglandin E2, and prostaglandin I2 on pregnant rat uterus.

New developments in the management of preterm labor

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

Oestrous cycle and pregnancy alter the reactivity of the rat uterine vasculature

Isolated uterine vascular beds from virgin and pregnant rats were used to assess vascular reactivity and the ability of nitric oxide (NO), prostanoids and endothelium-derived hyperpolarizing factor (EDHF) to modulate these responses. One uterine horn from female rats in each oestrous cycle day and gestation day 17 was removed and perfused with physiological saline solution. Tone was induced with cirazoline (1 micromol/l), and concentration-response curves to acetylcholine (ACh) generated. Responsiveness to ACh was tested in the presence of N-nitro-L-arginine (L-NA), ibuprofen (IBU) and tetrabutylammonium (TBA), to inhibit NO synthase, cyclo-oxygenase and K+ channels respectively. Cirazoline-induced tone was smaller in the pregnant compared with the proestrous group. Sensitivity to ACh was cycle day and pregnancy dependent with pregnant > dioestrous day-1 > dioestrous day-2 > proestrous and oestrous. L-NA shifted the curve to the right in all groups except dioestrous day-1. IBU inhibited the ACh response in the pregnant group only. TBA virtually abolished the response in all groups. These results suggest that in the uterine vascular bed from pregnant rats, EDHF, along with NO and a dilator prostanoid mediate ACh-induced dilatation. In contrast, in the dioestrous day-1 group, only EDHF seems to be released by ACh in this vascular bed. In the oestrous, dioestrous day-2 and proestrous groups, ACh releases both EDHF and NO.

Regulation of the Ca2+-sensitive domains of the maxi-K channel in the mouse myometrium during gestation

Large conductance Ca(2+)-activated K(+) channels (maxi-K channels) are known to modulate uterine activity during gestation. Electrophysiological recordings demonstrate that myometrial maxi-K current is suppressed in term-pregnant compared to non-pregnant mice. We sought to determine whether maxi-K current suppression is due to reduction of maxi-K channel protein or differential expression of maxi-K channel isoforms that vary in their Ca(2+) and voltage sensitivities. Immunoblot analyses show an increase of maxi-K channel protein throughout gestation. Polymerase chain reaction of mouse myometrial cDNA identified four alternatively spliced sites within the maxi-K transcript and three within the Ca(2+)-sensitive "tail" domain. Ribonuclease protection analyses demonstrate that total channel transcript levels mimic protein expression; however transcript levels of alternatively spliced regions of regulatory domains that alter sensitivity to voltage and Ca(2+) differ in their gestational expression. An insert that increases the maxi-K channel sensitivity to voltage and Ca(2+) is present at steady levels throughout gestation, differing from total channel transcript regulation. The insert-less form of this transcript, which reduces the channel voltage and Ca(2+) sensitivity, is not detected until midterm pregnancy. These findings verify that multiple isoforms of the maxi-K channel are present in the mouse myometrium and are regulated differentially during gestation, which is a likely mechanism for modulation of myometrial excitability during pregnancy.

Role of nucleotide cyclases in the inhibition of pregnant rat uterine contractions by the openers of potassium channels

OBJECTIVE

Our objective was to study the involvement of adenylate and guanylate cylases in spontaneous uterine contractions and inhibition induced by the opening of potassium channels.

STUDY DESIGN

Uterine rings from rats at mid and term gestation and from rats at term gestation in labor were suspended in organ chambers for isometric tension recording. Concentration-response relationships to an opener of adenosine triphosphate-dependent potassium channels, levcromakalim, or to an opener of calcium-dependent potassium channels, NS 1619, were studied in the absence and presence of inhibitors of adenylate cyclase (MDL 12330 A, 2 x 10(-5) mol/L; SQ 22536, 10(-4) mol/L) or guanylate cyclase (LY 83583,3 x 10(-6) mol/L).

RESULTS

MDL 12330 A and SQ 22536 accentuated contractions in rings from rats at mid gestation but not at term gestation or at term gestation in labor. LY 83583 inhibited contractions in the rings from all 3 groups. Levcromakalim was equally effective in inhibiting contractions of rings from all 3 groups. MDL 12330 A, but not SQ 22536, decreased sensitivity and maximal inhibition induced by levcromakalim (term gestation greater than mid gestation greater than term gestation in labor). LY 83583 decreased the sensitivity to and maximal inhibition induced by levcromakalim in rings from pregnant rats at mid gestation. NS 1619 attenuated contraction of rings from rats at mid gestation and, to a lesser extent, at term gestation but accentuated contractions in rings from animals at term gestation in labor. MDL 12330 A, but not SQ 22536 or LY 83583, attenuated the changes induced by NS 1619 in rings from all 3 groups.

CONCLUSIONS

(1) The influence of nucleotide cyclases on basal uterine contractility depends on gestational age. (2) The inhibition of uterine contractions that results from the opening of calcium-dependent potassium channels depends on adenylate cyclase, whereas that of adenosine triphosphate-dependent potassium channels depends on both adenylate and guanylate cyclases. 3. Activation of adenosine triphosphate-dependent potassium channels is more efficient than activation of calcium-dependent potassium channels. 4. The inhibition induced by calcium-dependent potassium channel openers, but not adenosine triphosphate-dependent potassium channel openers, decreases as pregnancy progresses, and at delivery the activation of spontaneous contractile activity is evident.