EXPRESSION OF NITRIC-OXIDE SYNTHASE IMMUNOREACTIVITY BY INTERSTITIAL-CELLS OF THE CANINE PROXIMAL COLON

A subpopulation of interstitial cells (ICs) are interposed between ner ve terminals and smooth muscle cells in the gastrointestinal tract and may participate in neuromuscular transmission. These cells appear to be targets for NO released from enteric inhibitory nerves and respond to exogenous NO with: (i) an elevation in cGMP levels; (ii) an increas e in intracellular Ca2+; (iii) and release of a diffusible substance t hat has tentatively been identified as NO. For the latter to be possib le, ICs must express a constitutive isoform of NOS. This study charact erized the expression of NOS-like immunoreactivity (NOS-LI) in ICs of the canine colon using 3 antibodies raised against the 2 known constit utive forms of NOS (i.e., neural (nNOS) and endothelial (eNOS) isoform s). Antibodies raised against eNOS and an antibody raised against rat cerebellar nNOS labeled ICs along the submucosal surface of the circul ar muscle layer (IC-SM), along the surface of septa that separate the circular muscle into fiber bundles (IC-SM), and in the myenteric regio n between the circular and longitudinal muscle layers (IC-MY). Another antibody raised against rat cerebellar nNOS failed to label ICs. Cult ured IC-SM also expressed NOS-LI, suggesting that this feature of the IC phenotype survives culture conditions. Arteriolar endothelial cells in the canine colon were labeled with the same 2 antibodies that labe led ICs, suggesting there are significant structural similarities betw een NO synthases in ICs and endothelial cells. The data suggest that I C-SM and IC-MY express a constitutive form of NOS. Synthesis of NO by ICs may influence electrical rhythmicity and may serve to amplify and even propagate enteric inhibitory neurotransmission.