Nitric oxide synthase in the pharynx of the planarian Dugesia tigrina

The distribution of the nicotinamide adenine dinucleotide phosphate-(NADPH) diaphorase reaction, an indicator of nitric oxide synthase activity, was studied in the freshwater planarian Dugesia tigrina (Platyhelminthes). The reaction was restricted to the pharynx, where the inner epithelium was intensely stained and the outer epithelium moderately stained. Neurons that innervated the pharynx were also stained. The enzyme activity was studied by high pressure liquid chromatographic quantitation of the formed citrulline. The presumed nitric oxide synthase was dependent on NADPH, whereas no dependency on Ca2+ and calmodulin could be detected. Tetrahydrobiopterin increased the activity about fivefold to 218.2+/-24.9 fmol/mg protein per min. Nomega-nitro-l-arginine depressed the enzyme activity by about 80%. The results indicate that nitric oxide has a role in the feeding behavior of planarians.