Changes in expression of the nitric oxide synthase isoforms in rat uterus and cervix during pregnancy and parturition

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.