ROLE OF NITRIC-OXIDE IN EICOSANOID SYNTHESIS AND UTERINE MOTILITY IN ESTROGEN-TREATED RAT UTERI

Cholinergic stimulation of vascular endothelin activates NO synthase ( NOS), leading to generation of NO from arginine. This NO diffuses to t he overlying vascular smooth muscle and causes vasodilatation. NOS has also been found in the central and peripheral nervous systems and it is clear now that NO plays an important role as a neurotransmitter. He re we investigate the role of NO in controlling contraction of uterine smooth muscle. Our previous work showed that NO activates the cycloox ygenase enzyme in the hypothalamus, leading to production of prostagla ndin E(2) (PGE(2)). We began by determining whether NO was involved in production of arachidonic acid metabolites in the uterus, Uteri were removed from female rats that had been treated with estrogen (17 beta estradiol). Control animals were similarly injected with diluent. Tiss ues were incubated in vitro in the presence of [C-14]arachidonic acid for 60 min. Synthesis of PGs and thromboxane B-2 (TXB(2)) was markedly stimulated by sodium nitroprusside (NP), the releaser of NO. The effe ct was greatest on TXB(2); there were no significant differences in in creases of different PGs. The response to NP was completely prevented by Hb, a scavenger of NO. The inhibitor of NOS, N-G-monomethyl L-argin ine (NMMA), significantly decreased synthesis of PGE(2) but not the ot her prostanoids (6-keto-PGF(1 alpha) and PGF(2 alpha)). Addition of Hb to scavenge the spontaneously released NO inhibited synthesis of 6-ke to-PGF(1 alpha), PGE(2 alpha) and PGF(2 alpha), but not TXB(2). There was a much lesser effect on products of lipoxygenase, such that only 5 -hydroxy-5,8,11,14 eicosatetraenoic acid (5-HETE) synthesis was increa sed by NP, an effect that was blocked by Hb; there was no effect of NM MA or Hb on basal production of 5-HETE. Thus, NO stimulates release of the various prostanoids and 5-HETE; blockade of NOS blocked only PGE( 2) release, whereas Hb to scavenge the NO released also blocked synthe sis of 6-keto-PFG(1 alpha), PGE(2 alpha), and PGF(2 alpha), indicating that basal NO release is involved in synthesis of all these PGs, espe cially PGE(2). Presumably, NMMA did not block NOS completely, whereas Hb completely removed released NO. This may explain the different resp onses of the various prostanoids to NMMA and Hb. To determine the role of these prostanoids and NO in control of spontaneous in vitro uterin e contractility in the estrogen treated uterus, the effect of blocking NOS with NMMA and of scavenging NO produced by Hb on the time course of spontaneous uterine contractility was studied. Surprisingly, blocka de of NOS or removal of NO by Hb prevented the spontaneous decline in uterine motility that occurs over 40 min of incubation. We interpret t his to mean that NO was released in the preparation and activated guan ylate cyclase in the smooth muscle, resulting in production of cGMP, w hich reduces motility and induces relaxation, When the motility had de clined to minimal levels, the effect of increased NO provided by NP wa s evaluated; apparently by stimulating the release of prostanoids, a r apid increase in motility that persisted for 10 min was produced. This effect was completely blocked by Hb. The action of NP was also blocke d by indomethacin, indicating that it was acting via release of PGs. A pparently, when motility is low, activation of PG synthesis by NO to a ctivate the cyclooxygenase enzyme causes a rapid induction of contract ions, whereas, when motility is declining, NO acts primarily via guany late cyclase to activate cGMP release; the action of the prostanoids r eleased at this time is in some manner blocked.