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1 Department of Veterans Affairs Medical Center, Center of Swallowing and Motility Disorders, West Roxbury, MA, USA; Brigham and Women's Hospital, Harvard Medical School, Boston, MA, USA
* To whom correspondence should be addressed. E-mail: hiroshi_mashimo{at}hms.harvard.edu.
Nitric oxide (NO) relaxes the internal anal sphincter (IAS), but its enzymatic
source(s) remains unknown; neuronal (nNOS) and endothelial (eNOS) 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 N
-nitro-L-arginine
methyl ester 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 IV, 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, while the RAIR is controlled by nNOS. c-Kit-expressing ICC
may not be essential for the RAIR.
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