Neuronal nitric oxide synthase in the gill of the killifish, Fundulus heteroclitus.

Neuronal NOS (nNOS) is a constitutively expressed enzyme that catalyzes the oxidation of L-arginine and water to L-citrulline and the gas nitric oxide (NO). Nitric oxide is involved in regulation of a variety of processes, including: vascular tone, neurotransmission, and ion balance in mammals and fishes. In this study, we have cloned and characterized a putative NOS homologue from the brain of the euryhaline killifish, Fundulus heteroclitus. Killifish NOS has 75% amino acid identity to human nNOS, and phylogenetic analysis groups the killifish sequence with the mammalian nNOS, suggesting that it is a mammalian orthologue. Relative quantitative reverse transcriptase-PCR demonstrated that killifish nNOS mRNA is highly expressed in the brain and gill followed by the stomach, kidney, opercular epithelium, intestine and heart. Immunohistochemistry localized nNOS to nerve fibers and epithelial cells adjacent to mitochondrion-rich cells (ion transporting cell) in the gill, suggesting that nNOS production of NO may contribute to regulation of vascular tone and/or MRC function in the teleost gill.