Rat myometrial smooth muscle cells express endothelial nitric oxide synthase

We examined if rat myometrial cells in culture generate nitric oxide (NO)and express various isoforms of NO synthase (NOS). Myometrial cellsisolated from rats on day 18 of gestation were incubated with variousstimulators and inhibitors of NOS for 24 and 48 h, and NO production wasevaluated by measuring nitrites in the media and NOS proteins in the celllysates. NO was produced by myometrial cells and its production inhibitedby N(G)-methyl-L-arginine (L-NMMA). This inhibition was reversed byL-arginine (3 mM). Interleukin-1beta (IL- 1beta) significantly stimulatedNO production, in a dose-dependent manner. The IL-1beta-stimulated NOproduction was inhibited by the NOS inhibitor, L-NMMA, whose effects werereversed by L-arginine. Abundant NOS III protein was detectable in freshlyisolated myometrial cells, and this was maintained in culture in thepresence of fetal bovine serum (FBS; 10%). In the absence of FBS, NOS IIIlevels decreased significantly (by 90%) within 24 h. In contrast, NOS I andNOS II proteins were undetectable in freshly isolated muscle cells and incells cultured without IL-1beta. However, NOS II protein in these cells wasinduced by IL-1beta. Thus, NO is produced by myometrial cells through theNOS III isoform, and the myometrial NO may be important in maintaininguterine quiescence during pregnancy.