title: Nitric oxide decreases ammonium release in tadpoles of the clawed frog, Xenopus laevis, Daudin.
abstract: In the present study, we quantified the physiological consequences of nitric oxide (NO) on ammonium release in tadpoles of Xenopus laevis. Tadpoles exposed to S-nitro-N-acetylpenicillamine (SNAP), an NO-donor, or L: -arginine, the substrate of NO synthase (NOS), showed a reversible decrease, whereas animals exposed to the NOS inhibitor Nomega-methyl-L: -arginine (L: -NMMA) exhibited an increase in ammonium release. Release of ammonium may be of physiological relevance during stress response of the animal. Handling of tadpoles as well as exposure to hyposmotic environments increased ammonium release. To localize NO synthesizing cells, we used diaminofluorescein-diacetate (DAF-2DA), an NO-sensitive fluorescent dye, and NADPH-diaphorase histochemistry, an indicator for NOS activity. We observed a fluorescence signal as well as NADPH-diaphorase activity in small, solitary cells in the epidermis. Similarly to NADPH-diaphorase histochemistry, silver nitrate staining and rhodamine labelling, markers for mitochondria-rich cells, showed a strong reaction in these cells. These observations indicate that NO (1) inhibits ammonium release, and (2) is endogenously synthesized in mitochondria-rich cells in Xenopus tadpoles. Based on our histochemical results, we speculate that gill epithelium and epidermis work in parallel to release ammonium as epidermal tissue contains mitochondria-rich and NADPH-diaphorase positive cells.
authors.affiliation: Division of Animal Physiology, Department of Cellular Biology, University of Salzburg, 5020 Salzburg, Austria.
authors.forename: Stefan
authors.initials: S
authors.lastname: Wildling
authors.forename: Hubert H
authors.initials: HH
authors.lastname: Kerschbaum
mesh_terms: D000818:Animals; D001120:Arginine / Q000494:pharmacology; D006152:Cyclic GMP / Q000378:metabolism; D004791:Enzyme Inhibitors / Q000494:pharmacology; D004817:Epidermis / Q000187:drug effects / Q000201:enzymology*; D019793:Fluorescein; D005880:Gills / Q000187:drug effects / Q000201:enzymology*; D006234:Handling, Psychological; D007150:Immunohistochemistry; D007202:Indicators and Reagents; D007814:Larva / Q000378:metabolism; D008928:Mitochondria / Q000378:metabolism; D009252:NADPH Dehydrogenase / Q000378:metabolism; D009569:Nitric Oxide / Q000378:metabolism*; D020030:Nitric Oxide Donors / Q000494:pharmacology; D019001:Nitric Oxide Synthase / Q000037:antagonists & inhibitors / Q000378:metabolism; D000644:Quaternary Ammonium Compounds / Q000378:metabolism*; D026423:S-Nitroso-N-Acetylpenicillamine / Q000494:pharmacology; D013312:Stress, Physiological / Q000378:metabolism; D014982:Xenopus laevis / Q000378:metabolism*; D019323:omega-N-Methylarginine / Q000494:pharmacology
chemical_list: C414689:4,5-diaminofluorescein diacetate; D004791:Enzyme Inhibitors; D007202:Indicators and Reagents; D020030:Nitric Oxide Donors; D000644:Quaternary Ammonium Compounds; D019323:omega-N-Methylarginine; D009569:Nitric Oxide; D026423:S-Nitroso-N-Acetylpenicillamine; D001120:Arginine; D019001:Nitric Oxide Synthase; D009252:NADPH Dehydrogenase; D006152:Cyclic GMP; D019793:Fluorescein