Both soluble guanylate cyclase and particulate guanylate cyclase regulate myometrial contractility

Expression of iNOS mRNA and inhibitory effect of NO on uterine contractile activity in rats are determined by local rather than systemic factors of pregnancy

Our purpose was to investigate whether the local or systemic factors of pregnancy are associated with inducible nitric oxide synthase (iNOS) mRNA expression and to determine the inhibitory effects of pharmacological agents that increase cGMP levels in rat myometrium. iNOS mRNA expression was determined in uterine tissues from nonpregnant rats and on day 17 of gestation in the pregnant and non-pregnant uterine horns by RT-PCR. In addition, uterine rings from the pregnant and non-pregnant uterine horns were placed in Krebs-Henseleit solution for isometric recordings of spontaneous contractions. Concentration-inhibition relationships to diethylamine/nitric oxide complex, 8-bromo-cGMP, and the selective phosphodiesterase V inhibitor were obtained. Compared to nonpregnant rats, expression of iNOS mRNA in myometrium increased during pregnancy, which was maximal on day 17, followed by a decrease on day 21 of gestation. Expression of iNOS mRNA at day 17 of gestation was greater in pregnant uterine horns than in nonpregnant ones. Maximal inhibition of phosphodiesterase V and increasing cGMP induced similar inhibition of spontaneous contractions in nonpregnant and pregnant uterine horns, while NO induced less inhibition in the former. The results suggest that the local pregnancy factor is needed for signal transduction from NO to soluble guanylate cyclase at a time when maximal expression of iNOS mRNA is evident.

Up-regulation of nitric oxide synthase and modulation of the guanylate cyclase activity by corticotropin-releasing hormone but not urocortin II or urocortin III in cultured human pregnant myometrial cells

The biological actions of corticotropin-releasing hormone (CRH) in the human myometrium during pregnancy and labor are unknown. We hypothesized that CRH may modulate the nitric oxide system, and influence myometrial relaxation/contractility. Incubation of myometrial cells with CRH, but not urocortin II or urocortin III, for 8-16 h significantly induced mRNA and protein expression of endothelial and brain but not inducible nitric oxide synthase (NOS) isoforms. This action resulted in increased activity of soluble guanylate cyclase (GC(s)), demonstrated by the enhanced cGMP-producing capacity of the NO donor, sodium nitroprusside. CRH also caused acute activation of the membrane-bound GC, shown by increased basal or atrial natriuretic peptide (ANP)-stimulated cGMP production. These effects appeared to be mediated via the R1 receptors because the CRH receptor antagonists, astressin and antalarmin but not anti-sauvagine 30, could block them. The acute effects of CRH were significantly reduced by inhibition of protein kinase A (PKA) activity, suggesting it is partially PKA dependent. Activation of protein kinase C (PKC) resulted in significant inhibition of both ANP-and CRH-stimulated cGMP production, suggesting a direct effect of PKC on membrane-bound GC. In conclusion, CRH appears to have a dual effect on myometrial NOS/GC pathway, a short term effect predominantly mediated by PKA, and a long-term effect increasing constitutive NOS expression, mediated by a PKA-independent mechanism. This mechanism could potentially be active during human pregnancy, and, because cGMP stimulates myometrial relaxation, these findings further suggest that during pregnancy CRH primarily activates intracellular signals that contribute to the maintenance of myometrial quiescence.

Contractility of late pregnant rat myometrium is refractory to activation of soluble but not particulate guanylate cyclase

OBJECTIVE

Our purpose was to compare the effects of agents activating particulate or soluble guanylate cyclases on the spontaneous contractile activity of the isolated pregnant rat uterus.

STUDY DESIGN

Uterine rings from midpregnant (14-day) and late pregnant (21-day) rats were suspended in organ chambers to record spontaneous contractile activity. Concentration-response curves were obtained for the following natriuretic peptides: atrial, brain, and C-type; concentration-response curves were also obtained for diethylamine nitric oxide, 3-morpholino-sydnominine, and authentic nitric oxide.

RESULTS

All 3 natriuretic peptides inhibited spontaneous uterine contractions equally at midgestation and late gestation. The inhibitory effects of the nitric oxide donors diethylamine nitric oxide, 3-morpholino-sydnominine, and authentic nitric oxide were attenuated in uterine tissues from animals in late stages of pregnancy.

CONCLUSION

Agents activating either soluble or particulate guanylate cyclase inhibit contractions of uterine rings from midgestation rats, whereas the effects of soluble guanylate cyclase are attenuated at late pregnancy. Thus spontaneous uterine contractions are under the control of both soluble and particulate guanylate cyclases; the former is dependent on gestational age but the latter is not.

Molecular mechanism of cGMP-mediated smooth muscle relaxation

Contraction and relaxation of smooth muscle is a tightly regulated process involving numerous endogenous substances and their intracellular second messengers. We examine the key role of cyclic guanosine monophosphate (cGMP) in mediating smooth muscle relaxation. We briefly review the current art regarding cGMP generation and degradation, while focusing on the recent identification of the molecular mechanisms underlying cGMP-mediated smooth muscle relaxation. cGMP-induced SM relaxation is mediated mainly by cGMP-dependent protein kinase activation. It involves several molecular events culminating in a reduction in intracellular Ca(2+) concentration and a decrease in the sensitivity of the contractile system to Ca(2+). We propose that the cGMP-induced decrease in Ca(2+) sensitivity is a strategic way to achieve "active relaxation" of the smooth muscle. In summary, we present compelling evidence supporting a key role for cGMP as a mediator of smooth muscle relaxation in physiological and pharmacological settings.

Mechanism of mifepristone-induced spasmolytic effect on isolated rat uterus

Mifepristone, a synthetic 19-norsteroid, relaxed the KCl-induced tonic contraction in isolated rat uterus in a concentration-dependent way and CaCl2 (0.1 to 10 mM) counteracted it. This effect was similar to other steroids although the mechanisms involved are unclear. Before adding the contracturant, tissue was incubated with actinomycin D (10 microM), cycloheximide (300 microM), TPCK (3 and 10 microM), Rp-cAMPS (30 microM), DDA (100 microM) and H-7 (1 microM). None of these modified the relaxing effect of mifepristone. Incubation with drugs that interfere with cGMP such as a nucleotide analogue DDG (100 microM), a soluble guanylyl cyclase inhibitor ODQ (1 microM) and an inhibitor of protein kinase G 8pCPTcGMPS (1 microM) significantly modified the effect of mifepristone, increasing its IC50.

The role of cyclic nucleotides in the spontaneous contractility and responsiveness to nitric oxide of the rat uterus at midgestation and term

OBJECTIVES

The aim of this investigation was to study the effects of pharmacologic manipulation of cyclic nucleotide levels on the uterine spontaneous contractile activity and responsiveness to nitric oxide in pregnant rats at midgestation and term.

STUDY DESIGN

Uterine rings from pregnant rats at midgestation and term were placed in organ chambers for isometric tension recording. Concentration-response curves were obtained for phosphodiesterase and guanylate cyclase inhibitors, membrane-permeable cyclic nucleotide analogs, and forskolin. In addition, the effects of minimal inhibitory concentrations of these agents on the concentration-response relationships for diethylamine nitric oxide (a nitric oxide donor) were studied.

RESULTS

Nonselective phosphodiesterase inhibitors induced more inhibition of contractions of uterine rings from pregnant rats at term than at midgestation and zaprinast induced less. Inhibitors of guanylate cyclase and membrane-permeable analogs of cyclic guanosine monophosphate were equally effective in tissues from pregnant rats at midgestation and term. All agents attenuated the inhibitory effect of diethylamine nitric oxide at midgestation; however, dibutyryl cyclic adenosine monophosphate and papaverine increased the inhibitory effect of diethylamine nitric oxide in tissues from pregnant animals at term.

CONCLUSIONS

Cyclic nucleotides modulate both spontaneous and nitric oxide-induced changes in uterine contraction during pregnancy. Application of nonselective inhibitors of phosphodiesterase, as well as membrane-permeable analogs of cyclic adenosine monophosphate, may counteract refractoriness to nitric oxide at term.