E-2 and not P-4 increases NO release from NANC nerves of the gastrointestinal tract: implications in pregnancy

In women, during pregnancy, there is decreased motility of the gastrointest inal tract leading to a delay in gastric emptying and an increase in coloni c transit time. Whether the rise in estradiol (E-2) or progesterone (P-4) i s responsible for this effect is controversial. As the nitrergic component of the nonadrenergic, noncholinergic (NANC) nerves is responsible for modul ating gastrointestinal motility in vivo, the purpose of this study was to e valuate whether the increased release of nitric oxide (NO) from the nitrerg ic component of the NANC nerves innervating the gastric fundus and colon th at occurs during late pregnancy in rats is mediated by E-2 or P-4. Ovariect omized rats treated with E-2 or P-4 alone or in combination were used for o ur studies. We also wanted to assess the cellular and molecular mechanisms involved. The NANC activity was studied by assessing changes in tone after application of electric field stimulation (EFS). The role of NO was determi ned by observing the effects of EFS in the presence and absence of the NO s ynthase (NOS) inhibitor N-G-nitro-L-arginine methyl ester (L-NAME) and the reversibility of the effects of L-NAME by L-arginine. Our studies indicated that there was increased magnitude of relaxation of isolated strips of rat gastric fundus and rat colon after application of EFS to tissues obtained from animals treated with E2 alone or a combination of E-2 + P-4 but not fr om those treated with P-4 alone. L-NAME attenuated relaxation responses in E-2- and E-2 + P-4 treated animals. To elucidate whether the increased NO r elease may be due to an increase in neuronal NOS (nNOS) protein, we used bo th Western blot analysis and immunohistochemistry. We also used RT-PCR to d etermine whether there was an increase in nNOS mRNA after treatment with se x steroids. In nonpregnant animals, nNOS was detected by Western blot in th e fundus and the colon and was barely detectable in the ileum. In pregnancy , there was an increase in nNOS in both the gastric fundus and the colon. T he nNOS protein was also increased in ovariectomized animals treated with e ither E-2 alone or E-2 + P-4 but not P-4 alone when compared with ovariecto mized animals receiving vehicle. Our results indicated that there was an in crease in nNOS protein that was localized to the neurons of the myenteric p lexus in the gastric fundus and colon in E-2- and E-2 + P-4-treated animals , but this increase was not observed in animals treated with P-4 alone. Thi s increase in nNOS protein was accompanied by an increase in nNOS mRNA. The se results suggest the possibility that E-2, rather than P-4, may be respon sible for the delay in gastric emptying and increase in colonic transit tim e observed in pregnancy.