NITRINERGIC NERVES CONTROLLING PACEMAKER ACTIVITIES OF THE INNER SUBLAYER (P-LAYER) IN THE CANINE PROXIMAL COLON CIRCULAR MUSCLES

The inner sublayer (P-layer) of the circular muscle coat in the canine proximal colon has been known to produce spontaneous mechanical contr actions associated with characteristic electrical activities called sl ow waves. We recorded the mechanical activities of tissue preparations from this P-layer. Normal Krebs solution (K+; 6 mM) was used as the p erfusate. Elevation of extracellular K+ concentrations in the range of 12 mM and 36 mM induced intensified phasic contractions. Administrati on of an NO-synthase inhibitor, N omega-nitro-arginine methyl ester (L -NAME, 50 mu M), enhanced both the spontaneous mechanical rhythms and high extracellular K+-induced contractions. Administration of the subs trate for NO synthases, L-arginine (400 mu M) remarkably suppressed th e 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 di nucleotide phosphate)-diaphorase technique and by the immunohistochemi stry of NO-synthases, since NO-producing (nitrinergic) nerves usually, if not always, show a histochemical NADPH-diaphorase positive reactio n in formaldehyde-fixed specimens, and since features of ganglia and n erve strands in the outer subdivision of the submucosal plexus (plexus submucosus externus; or so-called Henle's plexus) together with the d elicate network of nerve terminal varicosities within the P-layer were clearly visualized by this method. The topographical arrangement of n itrinergic nerves supported the view that they produce nitric oxide (N O), being one of the major chemical mediators of the neural control of the spontaneous rhythms in the P-layer.