Investigation of the potential modulatory effect of biliverdin, carbon monoxide and bilirubin on nitrergic neurotransmission in the pig gastric fundus.

In porcine gastric fundus, we have investigated the colocalization of the bile pigment biosynthetic enzymes heme oxygenase-2 and biliverdin reductase with neuronal nitric oxide synthase (nNOS), the effect of carbon monoxide (CO) on fundic circular smooth muscle and the possible modulatory effect of the bile pigments biliverdin and bilirubin on CO-mediated relaxations and on nitrergic relaxation. Heme oxygenase-2 and biliverdin reductase immunoreactivity was present in all nNOS containing myenteric neurons. CO induced a concentration-dependent relaxation of fundic circular smooth muscle strips, which was completely blocked by the specific guanylate cyclase inhibitor 1H-(1,2,4)oxadiazolo(4,3-a)quinoxalin-1-one (ODQ). 3-(5'-hydroxymethyl-2'-furyl)-1-benzylindazole (YC-1), biliverdin and bilirubin strongly enhanced the amplitude of the CO-induced relaxation. Tin protoporphyrin had no effect on electrically induced nitrergic relaxation, but spectrophotometric analysis learned that incubation of porcine gastric fundus circular muscle strips with tin protoporphyrin did not influence heme oxygenase activity. In conclusion, our data suggest that nitrergic neurons in the pig gastric fundus are able to produce biliverdin and bilirubin, and that these agents potentiate the relaxant effect of CO, which is formed concomitantly with biliverdin by heme oxygenase-2.