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Abstract
The lower airways of guinea-pigs were analyzed for pituitary adenylate cyclase activating
peptide (PACAP) using immunocytochemistry. In the trachea a moderate supply of PACAP-immunoreactive
nerve fibers occurred around smooth muscle bundles, glands and small blood vessels.
In the lung, PACAP-immunoreactive nerve fibers were distributed around small glands
and bronchi. A rich supply of PACAP immunoreactive nerve fibers was found around blood
vessels in the lungs. PACAP-suppressed smooth muscle responses were analysed using
isolated circular segments of trachea, pulmonary arteries and aorta of guinea-pigs.
In both airways and arteries PACAP caused a concentration-dependent relaxation of
precontracted segments. The maximal relaxation effects were more pronounced in the
airways than in the arteries while the order of potency was aorta greater than pulmonary
artery greater than trachea. The effect of PACAP was compared to those of acetylcholine
(ACh) and vasoactive intestinal peptide (VIP). In the pulmonary artery the vasomotor
responses expressed as maximal dilatation had the order: ACh greater than VIP = PACAP
while the order of potency was PACAP = VIP greater than ACh. In the trachea, PACAP
was slightly more potent than VIP. The relaxatory responses to PACAP in the trachea
and the intrapulmonary arteries were unaffected by pretreatment with atropine, prazosin,
yohimbine, propranolol, mepyramine, cimetidine and Spantide. Removal of the endothelium
abolished PACAP-induced vascular relaxation. Conceivably, PACAP-containing nerve fibers
play a role in the regulation of airway resistance and local blood flow.