EXPRESSION OF THE INDUCIBLE NITRIC-OXIDE SYNTHASE GENE IN MOUSE UTERINE LEUKOCYTES AND POTENTIAL RELATIONSHIPS WITH UTERINE FUNCTION DURINGPREGNANCY

Nitric oxide (NO), a potent and versatile free radical, is synthesized in leukocytes by the inducible form of NO synthase (iNOS). In this st udy, leukocytes in pregnant mouse uterus were investigated for express ion of the iNOS gene. Inducible NOS mRNA, which was identified by reve rse transcriptase polymerase chain reaction, was high relative to an i nvariant mRNA, glyceraldehyde-3-phosphate dehydrogenase, in midgestati on uteri (gestation days [g.d.] 10, 12, and 14) but was low in late-ge station uteri (g.d. 16 and 18). Inducible NOS protein, identified immu nohistochemically in paraformaldehyde-fixed uteri taken from g.d. 6 th rough 18 using rabbit antibodies generated to mouse carboxyl terminus iNOS peptides, was prominent in a few myometrial mast cells at early s tages and was strongly expressed from g.d. 6 through g.d. 14 in myomet rial macrophage-like cells. Inducible NOS protein was first detected i n uterine (u) natural killer (NK) cells at g.d. 8. Signals peaked in t his lineage at g.d. 10 and declined thereafter. Uterine leukocytes cul tured in vitro expressed the iNOS gene; a hybridoma cell line derived from mouse uNK cells (GWM1-2) contained iNOS mRNA, and cells migrating from mouse metrial gland explants included iNOS+ leukocytes. Large, g ranular iNOS+ uNK cells were absent from the uteri of homologously mat ed pregnant TgE26 mice, an NK cell-deficient transgenic mouse strain, but immunoreactive iNOS was detectable in trophoblast, a cell lineage that did not contain immunoreactive iNOS in NK cell-competent Swiss-We bster mice. In TgE26 mothers gestating normal embryos, the same patter n was observed. Collectively, the results of this study demonstrate th at iNOS is present in mouse uterine leukocytes including mast cells, m acrophage-like cells, and uNK cells, and suggest that in the absence o f uNK cells, the placenta synthesizes iNOS. These findings are consist ent with the postulate that leukocyte NO contributes importantly to ev ents associated with successful pregnancy that are likely to include r elaxation of vascular smooth muscle.