CHANGES IN THE NITRIC-OXIDE SYSTEM OF RAT MYOMETRIUM DURING MIDGESTATION AND DELIVERY AT TERM

The aims of this study were to investigate whether a nitric oxide (NO) system is present in the pregnant rat myometrium and whether it diffe rentially inhibits myometrial contractility during midgestation and de livery at term. Myometrial strips were obtained from eight rats at mid gestation (day 16) and eight rats during delivery at term, and mounted in an organ bath for assessment of isometric tension. There were scar ce spontaneous contractions with gradually increased amplitude that ce ased after short periods at midgestation, whereas the frequency of spo ntaneous contractions increased and were regular, and the amplitude of contractions became stable and long lasting during delivery. Spontane ous and carbachol-induced contractions were decreased by L-arginine an d increased by N-G-nitro-L-arginine methyl ester (L-NAME) at midgestat ion and delivery, but much higher concentrations of L-arginine and L-N AME were required to obtain this effect at delivery. These results sho w: (1) that there is basal and carbachol-stimulated production of NO i n pregnant rat myometrium; (2) that the L-arginine-NO system inhibits myometrial contractility; (3) that the effect of the L-arginine-NO sys tem is elevated at midgestation and decreased during delivery; and (4) that the L-arginine-NO system may contribute to uterine quiescence du ring gestation and the initiation of labour at term. (C) 1998 The Ital ian Pharmacological Society.