Induction of nitric oxide synthase in rat gastric smooth muscle preparations

The induction in vitro of inducible nitric oxide synthase (iNOS) in intact gastric circular (CM) and longitudinal (LM) smooth muscle preparations was evaluated 1) pharmacologically, by the appearance of 1 mM L-arginine (L-Arg)-induced relaxation in a precontracted tissue; 2) biochemically, according to the appearance of iNOS mRNA using a reverse transcriptase-polymerase chain reaction; and 3) immunohistochemically, using an iNOS-specific antibody. Functionally, iNOS induction affected the contractile properties of the CM but not the LM preparation. The time course of iNOS induction monitored pharmacologically paralleled exactly the appearance of iNOS mRNA. The relaxant response to L-Arg in iNOS-induced CM tissues was blocked by the iNOS inhibitor aminoguanidine and by the guanylyl cyclase inhibitor LY-83583. The addition of oxyhemoglobin to the organ bath also attenuated the relaxant response, but tetrodotoxin had no effect. The transcriptional inhibitor actinomycin D completely blocked iNOS induction as assessed by both pharmacological and biochemical criteria. In iNOS-induced preparations the iNOS immunoreactivity was not detected in the smooth muscle elements but was localized in a layer of macrophage-related cells that were in apposition to the CM smooth muscle elements. We conclude that the spontaneous induction of iNOS in rat gastric tissue can affect the pharmacomechanical reactivity of the CM elements and that this regulation of the CM contractility is due to the induction of iNOS in a set of macrophage-related cells that are closely apposed to the CM elements so that they selectively affect only the contractility of the CM preparation.