NITRIC-OXIDE SYNTHASE-I IMMUNOREACTIVITY AND NOS-ASSOCIATED NADPHD HISTOCHEMISTRY IN THE VISCERAL EPITHELIAL-CELLS OF THE INTRAPLACENTAL MOUSE YOLK-SAC

In the course of our studies on the local blood flow modulation in the NMRI-mouse placenta we have focussed on regulatory pathways involving recently appreciated gaseous messenger molecules nitric oxide (NO) an d carbon monoxide (GO), which are generated by NO synthase (NOS) and h eme oxygenase (HO)-2, respectively. The distribution of NOS was invest igated by immunohistochemistry using an antiserum to the neuronal isof orm (NOS-I) and by NADPH diaphorase (NADPHd) histochemistry, supplemen ted with procedures (permanganate and formaldehyde method) serving to enhance the specificity of the enzyme histochemical method for NOS vis ualization. HO-2 was demonstrated immunohistochemically. In addition, cyclic guanosine monophosphate (cGMP)-forming soluble guanylate cyclas e (sGC) and dehydrogenases generating the NOS co-substrate NADPH were analysed either by immunohistochemistry or enzyme histochemistry. NOS- I immunostaining was observed in the intraplacental visceral yolk sac epithelial cells but not in the placenta and extraplacental visceral e pithelial yolk sac cells. Go-localization of NOS-I immunolabeling and NOS-associated NADPHd was exclusively found in the intraplacental visc eral epithelial cells, while NADPHd activity not associated to NOS was present in other placental and extraplacental cells additionally anal ysed for control reasons. HO-2 and sGC immunoreactivity could not be d etected in the placenta including the intraplacental visceral epitheli al cells but were expressed in several extraplacental cells. Dehydroge nases producing the NOS co-substrate NADPH were present in the intrapl acental visceral epithelium as well as in other placental and extrapla cental cells. Since the intraplacental visceral epithelial yolk sac la yer closely accompanies large fetal blood vessels entering the placent al labyrinth from the chorionic plate it may be assumed that NO, gener ated by the NADPH-consuming NOS-I in the intraplacental yolk sac epith elium, acts to regulate the blood flow by relaxing smooth muscle cells in the wall of these fetal vessels. The lack of immunoreactivity to t he NO-effector molecule sGC may be due to methodological reasons. The absence of the HO-2/CO system suggests its insignificant role as a pot ential gas signaling pathway in the vascular smooth muscle system of t he intraplacental visceral yolk sac of mice.