Nitrinergic Nerves Controlling Pacemaker Activities of the Inner Sublayer (P-layer) in the Canine Proximal Colon Circular Muscles

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The inner sublayer (P-layer) of the circular muscle coat in the canine proximal colon has been known to produce spontaneous mechanical contractions associated with characteristic electrical activities called slow waves. We recorded the mechanical activities of tissue preparations from this P-layer. Normal Krebs solution (K⁺; 6mM) was used as the perfusate. Elevation of extracellular K⁺ concentrations in the range of 12mM and 36mM induced intensified phasic contractions. Administration of an NO-synthase inhibitor, N_ω-nitro-arginine methyl ester (L-NAME, 50μM), enhanced both the spontaneous mechanical rhythms and high extracellular K⁺-induced contractions. Administration of the substrate for NO synthases, L-arginine (400μM) remarkably suppressed the effects of L-NAME on the amplitude of the spontaneous rhythms and on responses to extracellular high K⁺.
Histological structures of nerves in the P-layer were investigated by an NADPH (nicotinamide adenine dinucleotide phosphate)-diaphorase technique and by the immunohistochemistry of NO-synthases, since NO-producing (nitrinergic) nerves usually, if not always, show a histochemical NADPH-diaphorase positive reaction in formaldehyde-fixed specimens, and since features of ganglia and nerve strands in the outer subdivision of the submucosal plexus (plexus submucosus externus; or so-called Henle's plexus) together with the delicate network of nerve terminal varicosities within the P-layer were clearly visualized by this method. The topographical arrangement of nitrinergic nerves supported the view that they produce nitric oxide (NO), being one of the major chemical mediators of the neural control of the spontaneous rhythms in the P-layer.