NITRIC-OXIDE SYNTHASE IN THE PHARYNX OF THE PLANARIAN DUGESIA-TIGRINA

The distribution of the nicotinamide adenine dinucleotide phosphate-(N ADPH) diaphorase reaction, an indicator of nitric oxide synthase activ ity, was studied in the freshwater planarian Dugesia tigrina (Platyhel minthes). The reaction was restricted to the pharynx. where the inner epithelium was intensely stained and the outer epithelium moderately s tained. Neurons that innervated the pharynx were also stained. The enz yme activity was studied by high pressure liquid chromatographic quant itation of the formed citrulline. The presumed nitric oxide synthase w as dependent on NADPH, whereas no dependency on Ca2+ and calmodulin co uld be detected. Tetrahydrobiopterin increased the activity about five -fold to 218.2+/-23.9 fmol/mg protein per min. N-omega-nitro-L arginin e depressed the enzyme activity by about 80%. The results indicate tha t nitric oxide has a role in the feeding behavior of planarians.