Effects of neuropeptides and capsaicin on the canine tracheal vasculature in vivo

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Abstract
1 The nonadrenergic, noncholinergic nervous system may control the airway vasculature via various neuropeptides. We have perfused the cranial tracheal arteries of the anaesthetized dog and investigated the effects of neuropeptides and capsaicin (which is supposed to release neuropeptides from sensory nerve endings) on the tracheal vasculature by injecting them locally into the perfusion system. 2 Neurokinin A (NKA, 0.02–20 pmol), calcitonin gene-related peptide (CGRP, 2–200 pmol) and peptide histidine isoleucine (PHI, 0.02–2 nmol) dose-dependently decreased tracheal vascular resistance (Rtv). NKA was 10 and 100 times more potent than CGRP and PHI, respectively. The duration of the response to CGRP was greatly prolonged with larger doses. Galanin (0.2–2 nmol) had no appreciable effect on Rtv. 3 Neuropeptide Y (NPY 0.02–2 nmol) and bombesin (0.02–10 nmol) dose-dependently increased Rtv. However, the dose-response curve for bombesin was bell-shaped suggesting the development of tachyphylaxis with larger doses. In smaller doses, bombesin was twice as potent as NPY. The duration of the response to NPY was prolonged with larger doses. 4 With the exception of PHI no neuropeptide altered tracheal smooth muscle tone; PHI (1 and 2 nmol) caused small dilatations of the trachea. 5 The effects of capsaicin (2–100 nmol) were complex. Usually, the vascular response had two dose-dependent phases: a rapid vasoconstriction followed by a small, longer-lasting vasodilatation. The tracheal smooth muscle response was usually biphasic, a contraction followed by a relaxation. 6 According to previous and present data, the order of potency of the neuropeptides on the canine tracheal vasculature is for the vasodilators: NKA > vasoactive intestinal peptide (VIP) > CGRP ≥ substance P > PHI, and for the vasoconstrictors: bombesin > NPY. The longer-acting neuropeptides (VIP, CGRP and NPY) may be more important than the shorter-acting neuropeptides (substance P, NKA, PHI and bombesin) as regulators of the airway wall blood flow.