Localization of nicotinamide adenine dinucleotide phosphate diaphorase (NADPH-d) activity in the gastrointestinal sphincters in the guinea pig.

The distribution of nicotinamide adenine dinucleotide phosphate-diaphorase (NADPH-d), a marker for nitric oxide synthase (NOS), in the nerves of gastrointestinal sphincters in guinea pigs was investigated to throw some light on the role of nitric oxide (NO) in these sphincteric regions. The nerve fibres with NADPH-d activity were observed chiefly in the circular muscle layer of the wall of gastrointestinal sphincters. Compared with the adjacent non-sphincteric regions, the proportions of NADPH-d positive myenteric neurons in the lower esophageal sphincter (LES), pyloric sphincter (PS) and internal anal sphincter (IAS) were higher (P < 0.05). The densities of NADPH-d-positive fibres in the circular muscle of above sphincteric regions were also higher (in LES P < 0.01; in PS and IAS P < 0.05). Within the circular muscle of LES, some discrete NADPH-d-positive intrinsic nerve cell bodies, usually smaller than their outlying myenteric neurons, were also observed. The dense distribution of NADPH-d-positive fibres within the circular muscles of gastrointestinal sphincteric regions suggests that NO, as a non-adrenergic non-cholinergic (NANC) inhibitory neurotransmitter, might play an important role in the neuronal regulation of the guinea-pig gastrointestinal sphincters.