INDUCTION OF NITRIC-OXIDE SYNTHASE IN RAT GASTRIC SMOOTH-MUSCLE PREPARATIONS

The induction in vitro of inducible nitric oxide synthase (iNOS) in in tact gastric circular (CM) and longitudinal (LM) smooth muscle prepara tions was evaluated 1) pharmacologically, by the appearance of 1 mM L- arginine (L-Arg)-induced relaxation in a precontracted tissue; 2) bioc hemically, according to the appearance of iNOS mRNA using a reverse tr anscriptase-polymerase chain reaction; and 3) immunohistochemically, u sing an iNOS-specific antibody. Functionally, iNOS induction affected the contractile properties of the CM but not the LM preparation. The t ime course of iNOS induction monitored pharmacologically paralleled ex actly the appearance of iNOS mRNA. The relaxant response to L-Arg in i NOS-induced CM tissues was blocked by the iNOS inhibitor aminoguanidin e and by the guanylyl cyclase inhibitor LY-83583. The addition of oxyh emoglobin to the organ bath also attenuated the relaxant response, but tetrodotoxin had no effect. The transcriptional inhibitor actinomycin D completely blacked iNOS induction as assessed by both pharmacologic al and biochemical criteria. In iNOS-induced preparations the iNOS imm unoreactivity was not detected in the smooth muscle elements but was l ocalized in a layer of macrophage-related cells that were in appositio n to the CM smooth muscle elements. We conclude that the spontaneous i nduction of iNOS in rat gastric tissue can affect the pharmacomechanic al reactivity of the CM elements and that this regulation of the CM co ntractility is due to the induction of iNOS in a set of macrophage-rel ated cells that are closely apposed to the CM elements so that they se lectively affect only the contractility of the CM preparation.