The aim of this study was to determine if nitric oxide (NO) production and nitric oxide synthase (NOS) isoforms change within the uterus and cervix during pregnancy and labour either at term or preterm. NO production was compared in the rat uterus and cervix of non-pregnant and pregnant rats on days 18-22 prior to labour, day 22 during delivery, 1 day post-partum and after treatment with either 10 mg onapristone or progesterone. Uterine NO synthesis, reflected in nitrite production, increased during gestation (194.2 +/- 22.6 nmol/g on day 19) compared with the non-pregnant state (76.2 +/- 18.4 nmol/g, P < 0.05) and decreased during term labour and post-partum. Furthermore, injection of lipopolysaccharide (LPS) (100 micrograms/rat i.p.) on day 20 of gestation resulted in a significant increase in NO synthesis after 6 h. Conversely, cervical NO synthesis and nitrite production was low in the non-pregnant (65.1 +/- 9.2 nmol/g) and pregnant animals on days 18-22 of gestation (53.2 +/- 9.0 nmol/g on day 22, P > 0.05), but markedly increased during term labour (139 +/- 28.6 nmol/g, P < 0.05). Treatment with the antiprogestin onapristone suppressed uterine NO production and increased cervical production while continuous administration of progesterone from day 19 had the opposite effect. LPS produced a significant increase in cervical NO production in both the pregnant (8-fold) and non-pregnant (4-fold) states. All three known NOS isoforms (i.e., iNOS, nNOS and eNOS) were detected in the cervical samples but only two were present in the uterus (iNOs and eNOS). An increase in the presence of iNOS occurred during labour at term compared with cervices collected from day 19. This was contrary to the measurements of the isoform in the uterus. Also, there was a similar increase of nNOS in the cervix during labour. This isoform seemed absent in the uterus during gestation. No significant changes occurred in the abundance of eNOS in the cervix during labour at term compared with day 19. During preterm labour after onapristone, iNOS concentrations increased significantly in the cervix. In order to examine whether the NO pathway plays a role in cervical ripening, the effects of the nitric oxide synthesis inhibitor L-nitro-arginine methylester (L-NAME) on the duration of delivery and on cervical extensibility were also investigated. The duration of delivery was significantly prolonged in L-NAME-treated rats compared with the control group (2.4-fold). Moreover, cervical extensibility decreased significantly (1.7-fold) after in-vitro incubation with L-NAME (P < 0.005). We conclude that the NO system may have an active role in the cascade of processes involved in preparing the uterus and cervix for parturition.

Myometrial connexin-43 gap junctions are scarce throughout gestation but appear in large numbers at term to facilitate contractions during labor. The mechanisms that regulate this process are incompletely characterized. This report investigates the effects of protein kinase C activation on the regulation of connexin-43 gene transcription in human uterine smooth muscle cells. In primary myometrial cells treated with phorbol ester, transient increases in c-Fos and c-Jun protein levels were observed at 2-4 h, followed by significant increases in connexin-43 protein levels at 6-8 h. Nuclear run-on transcription analysis showed an increase in connexin-43 transcription 3 h after phorbol ester treatment. AP-1 sites were identified in the sequence of the 5'-flanking promoter region of the human connexin-43 gene at 44 and 1000 base pairs upstream of transcription start. Transcription from a reporter plasmid containing the proximal human connexin-43 promoter was increased in transfected primary cultures treated with phorbol ester. Mutation of the proximal AP-1 site in the promoter abolished the phorbol ester-dependent transactivation. This work provides evidence that transcription of the human connexin-43 gene is induced through protein kinase C activation in uterine smooth muscle cells, and that the induction involves up-regulation and activation of c-Jun and c-Fos.

Nitric oxide (NO) is considered to be an important local mediator that suppresses uterine contractility in rats and rabbits during pregnancy until term. The aim of this study was to investigate the mRNA concentrations for the three isoforms of nitric oxide synthase (NOS) in rat uterus and cervix and to determine whether alterations occur in association with labour at term or preterm. RNA was isolated from full thickness uterine and cervical tissues from pregnant rats at various times during gestation, during labour at term or preterm and post partum. RNA was analysed using reverse transcription-polymerase chain reaction (RT-PCR) with a single set of amplimers specifically designed to detect all three isoforms of NOS. Three distinct PCR products were detected which corresponded to the expected sizes for endothelial (e)NOS, neuronal (b)NOS and inducible (i)NOS products (805, 521 and 428 bp respectively). In all tissues, the 428 bp product predominated and sequence analysis revealed this to be iNOS mRNA with a very close homology (97%) to the published sequence of rat iNOS. Densitometric analysis showed that uterine iNOS mRNA was increased during pregnancy, decreased on day 22 before labour and decreased further during labour at term. In contrast, cervical iNOS mRNA was low until delivery (day 22) when it increased and was dramatically elevated during labour. Similarly, 3 h after injection with the antiprogestin onapristone, iNOS mRNA was significantly decreased in the uterus (approximately 45%) and increased in the cervix (approximately 245%) when compared with controls. The mRNAs to bNOS and eNOS (corresponding to the 521 and 805 bp bands) were generally greatly reduced in quantity compared with the 428 bp product. The changes in these constitutive isoforms during gestation were minor compared with those in the inducible isoform. We conclude that the iNOS transcript is the most abundant NOS mRNA in the uterus as well as in the cervix and this probably indicates that the inducible NOS is the main isoform present in these tissues. The changes in iNOS mRNA at the end of pregnancy may play a role in the initiation of term labour and cervical ripening. Furthermore, the changes in expression of iNOS can be mimicked during preterm labour following antiprogesterone treatment, and may suggest that progesterone differentially controls the expression of iNOS in the uterus and cervix.

Parturition is preceded by a large increase in gap junctions between myometrial smooth muscle cells. Connexin 43 is the major structural protein of myometrial gap junctions. To explore transcriptional regulation of the myometrial Cx43 gene, we used DNase I footprinting, electrophoretic mobility shift and transient transfection assays to examine a 312 bp promoter region (-164 to +148) of the gene, utilizing human myometrial cell cultures and nuclear extracts. The DNase I studies showed four regions of nucleoprotein interactions. Protection of region 1 (-80 to -31) encompassed an Activator Protein 1 (AP1) (-44 to -36) and two Specificity Protein 1 (Sp1) (-77 to -69 and -59 to -48) consensus sequences. Regions 2 to 4 included the transcription initiation site (-10 to +25), an Ets/NF-kB consensus sequence (+47 to +74) and a TA-rich region (+81 to +101) respectively. Gel mobility shift and supershift assays demonstrated c-Jun and Sp1 binding at the AP1 and Sp1 sites respectively. Promoter mutagenesis and transient transfection analyses combined with Sp1 and c-Jun/c-Fos over-expression studies indicate that both Sp1 and c-Jun are required for maximal promoter activity and, therefore, may positively regulate transcription of myometrial Cx43 during the initiation of labour.

Extracellular Ca2+ is normally required for myometrial cells to contract. Ca2+ enters muscle cells mainly through voltage-dependent Ca2+ channels (VDCCs) that open in response to action potentials. The synthesis of myometrial VDCCs may change during pregnancy to alter excitation-contraction coupling. We investigated the mRNA levels for the alpha 1- and beta-subunits of the L-type VDCC in rat myometrium to determine whether alterations are associated with term or preterm labor. RNA isolated from myometrial tissues was analyzed by reverse transcription-polymerase chain reaction (PCR) using specific primers designed according to the published sequences of the VDCC subunits. From pregnant rat myometrium, two distinct PCR products were obtained for the alpha 1-subunit: one of the expected size at 372 bp and a smaller at 339 bp. Sequence analysis of the larger product revealed a 99.5 or 88% sequence homology between rat myometrium and rat aorta or rabbit heart, respectively, and the smaller product had an identical sequence to a 33-bp deletion. The two alpha 1-products followed the same trend throughout pregnancy. VDCC alpha 1-mRNA levels increased gradually to 6.9-fold just before labor on day 22 but decreased during labor. However, the beta-subunit mRNA level increased sharply on day 22 and then also declined during labor. Progesterone treatment from day 19 to day 22 inhibited term delivery and prevented the significant increase in alpha 1-mRNA levels. In contrast, antiprogesterone (onapristone, ZK-98.299) treatment on day 17 caused a statistically significant increase in the alpha 1- and beta-VDCC subunit mRNA after 8 and 15 h, respectively, then a decrease during preterm labor at 24 h. We conclude that mRNA levels for the VDCC subunits increase before term and preterm labor but decline during periods when VDCCs are likely at their peaks. The increase in levels of mRNA for VDCC likely reflects changes in expression of VDCCs during periods of term and preterm labor that may facilitate uterine contractility required for this process. Progesterone withdrawal or blockade appears to be responsible for regulating levels of mRNA for VDCC in the myometrium in preparation for labor.