Nitric oxide (NO) relaxes the internal anal sphincter (IAS), but its enzymatic source(s) remains unknown; neuronal (nNOS) and endothelial (eNOS) NO synthase (NOS) isoforms could be involved. Also, interstitial cells of Cajal (ICC) may be involved in IAS relaxation. We studied the relative roles of nNOS, eNOS, and c-Kit-expressing ICC for IAS relaxation using genetic murine models. The basal IAS tone and the rectoanal inhibitory reflex (RAIR) were assessed in vivo by a purpose-built solid-state manometric probe and by using wild-type, nNOS-deficient (nNOS−/−), eNOS-deficient (eNOS−/−), and W/Wv mice (lacking certain c-Kit-expressing ICC) with or without l-arginine or Nω-nitro-l-arginine methyl ester (l-NAME) treatment. Moreover, the basal tone and response to electrical field stimulation (EFS) were studied in organ bath using wild-type and mutant IAS. In vivo, the basal tone of eNOS−/− was higher and W/Wv was lower than wild-type and nNOS−/− mice. l-Arginine administered rectally, but not intravenously, decreased the basal tone in wild-type, nNOS−/−, and W/Wv mice. However, neither l-arginine nor l-NAME affected basal tone in eNOS−/− mice. In vitro, l-arginine decreased basal tone in wild-type and nNOS−/− IAS but not in eNOS−/− or wild-type IAS without mucosa. The in vivo RAIR was intact in wild-type, eNOS−/−, and W/Wv mice but absent in all nNOS−/− mice. EFS-induced IAS relaxation was also reduced in nNOS−/− IAS. Thus the basal IAS tone is largely controlled by eNOS in the mucosa, whereas the RAIR is controlled by nNOS. c-Kit-expressing ICC may not be essential for the RAIR.
Relationship between interstitial cells of Cajal, fibroblast-like cells and inhibitory motor nerves in the internal anal sphincter
