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Division of Gastroenterology and Hepatology, Department of Medicine, Jefferson Medical College of Thomas Jefferson University, Philadelphia, Pennsylvania 19107
Despite its widespread distribution and
significance in the gut, the role of pituitary adenylate
cyclase-activating peptide (PACAP) in internal anal sphincter (IAS)
relaxation has not been examined. This study examined the role of PACAP
in nonadrenergic noncholinergic (NANC) nerve-mediated relaxation of IAS
smooth muscle. Circular smooth muscle strips from the opossum IAS were prepared for measurement of isometric tension. The influence of PACAP
and vasoactive intestinal peptide (VIP) antagonists and tachyphylaxis
on the neurally mediated IAS relaxation was examined either separately
or in combination. The release of these neuropeptides in response to
NANC nerve stimulation before and after the nitric oxide (NO) synthase
inhibitor
N
-nitro-L-arginine
and NO was also investigated. Both PACAP and VIP antagonists caused
significant attenuation of IAS relaxation by NANC nerve stimulation.
The combination of the antagonists, however, did not have an additive
effect on IAS relaxation. VIP tachyphylaxis caused significant
suppression of IAS relaxation by NANC nerve stimulation. PACAP and VIP
were found to be released by NANC nerve stimulation and exogenous NO.
The data suggest the involvement of PACAP in IAS relaxation primarily
by the activation of PACAP1/VIP
receptor and lack of its independent role in the relaxation.
Furthermore, NO may regulate the presynaptic release of PACAP and VIP.
vasoactive intestinal polypeptide; nonadrenergic noncholinergic; nitric oxide synthase; inhibitory neurotransmitter; smooth muscle
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