Sympathetic nerve-dependent regulation of mucosal vascular tone modifies airway smooth muscle reactivity

Abstract

The airways contain a dense subepithelial microvascular plexus that is involved in the supply and clearance of substances to and from the airway wall. We set out to test the hypothesis that airway smooth muscle reactivity to bronchoconstricting agents may be dependent on airway mucosal blood flow. Immunohistochemical staining identified vasoconstrictor and vasodilator nerve fibers associated with subepithelial blood vessels in the guinea pig airways. Intravital microscopy of the tracheal mucosal microvasculature in anesthetized guinea pigs revealed that blockade of α-adrenergic receptors increased baseline arteriole diameter by ~40%, whereas the α-adrenergic receptor agonist phenylephrine produced a modest (5%) vasoconstriction in excess of the baseline tone. In subsequent in vivo experiments, tracheal contractions evoked by topically applied histamine were significantly reduced (P < 0.05) and enhanced by α-adrenergic receptor blockade and activation, respectively. α-Adrenergic ligands produced similar significant (P < 0.05) effects on airway smooth muscle contractions evoked by topically administered capsaicin, intravenously administered neurokinin A, inhaled histamine, and topically administered antigen in sensitized animals. These responses were independent of any direct effect of α-adrenergic ligands on the airway smooth muscle tone. The data suggest that changes in blood flow in the vessels supplying the airways regulate the reactivity of the underlying airway smooth muscle to locally released and exogenously administered agents by regulating their clearance. We speculate that changes in mucosal vascular function or changes in neuronal regulation of the airway vasculature may contribute to airways responsiveness in disease.

Fig. 1.

Visualization of tracheal microvasculature. a: Low-power magnification photomicrograph showing an extensive organized subepithelial network of tyrosine hydroxylase immunoreactive nerve fibers (red) in a whole mount preparation of the guinea pig trachea. b: Higher power magnification showing a Y-shaped blood vessel flanked by several tyrosine hydroxylase-positive (red) nerve fibers (arrowheads). This vessel is also flanked by numerous distinct substance P-positive (green) nerve fibers (c, arrowheads). d and e: Representative single subepithelial arteriole indentified in vivo using intravital microscopy (the margins of which are delineated by arrowheads). This vessel is shown before (d) and 10 min after (e) topical application of 1 μM phentolamine. Asterisk shows reference mark (small dark spot) in the field of view which is unchanged in location following phentolamine application. Note the dramatic change in the shape of the dilated blood vessel. Scale bars, 200 μm (in a) and 40 μm (in b–e).

Fig. 2.

Mean changes in tracheal subepithelial arteriole diameter in vivo following topical application of 1 μM phentolamine or 1 μM phenylephrine. Data represent means ± SE of 3–7 individual experiments. Maximum responses were defined in the presence of 0.1 mM papavarine (for vasodilatation) and 1 mM phenylephrine (for vasoconstriction). See text for further details.

Fig. 3.

Effects of phentolamine and phenylephrine on histamine-evoked contractions of the guinea pig trachea in vivo and in vitro. A: representative trace showing in vivo recordings of tracheal tone (Tt), pulmonary insufflation pressure (PIP), and arterial blood pressure (ABP) in a urethane-anesthetized, artificially ventilated guinea pig. Addition of atropine to the tracheal perfusate evokes a rapid and profound reversal of ongoing baseline cholinergic (neural) tone in this preparation. Subsequent addition of histamine (10 μM) restores tone, which is then partially reversed on addition of phentolamine (1 μM) to the perfusate. B: mean data showing the reversal of histamine (0.3 μM–1 mM)-evoked tracheal contractions by 1 μM phentolamine in vivo. C: mean data showing the potentiation of a histamine (1 μM) contraction by 1 μM phenylephrine in vivo. D: mean data showing the lack of effect of phentolamine and phenylephrine on histamine-evoked contractions of guinea pig tracheal strips in vitro. In additional experiments the α-adrenergic antagonist prazosin also significantly reversed the magnitude of histamine-evoked contractions in vivo but was without effect in vitro (see data in text). Each data point represents the mean ± SE of 3–8 experiments. *P < 0.05, significantly different from corresponding control data point.

Fig. 4.

Effects of phentolamine and phenylephrine on capsaicin-evoked contractions of the guinea pig trachea in vivo and in vitro. Time courses of capsaicin evoked tracheal contractions in vitro (A) and in vivo (B) in the presence of either phentolamine or phenylephrine. The mean peak (maximum) capsaicin-evoked contraction (irrespective of the time at which it occurred) was significantly greater (P < 0.05) in vitro compared with in vivo in the presence of phentolamine (60.5 ± 7.3% maximum contraction in vitro vs. 31.5 ± 3.5% maximum contraction in vivo). However, in the presence of phenylephrine, peak in vitro and in vivo contractile responses were comparable (61.4 ± 4.9 vs. 51.4 ± 8.9% maximum contraction, respectively). Data represent means ± SE of 4–7 experiments. *P < 0.05, significant difference between the area under the curves.

Fig. 5.

Effects of phentolamine and phenylephrine on increases in tracheal tone and pulmonary insufflation pressure (PIP) in vivo evoked by antigen, inhaled histamine, and intravenous neurokinin A. A: tracheal superfusion with ovalbumin in actively sensitized guinea pigs evoked a concentration-dependent increase in tracheal tone that was larger in the presence of phenylephrine. B: inhaled histamine evoked a dose-dependent increase in PIP that was potentiated in the presence of phenylephrine. Intravenously administered neurokinin A (NKA) evoked a dose-dependent increase in tracheal tension (C) and PIP (D). NKA-evoked tracheal contractions, but not PIP responses, were potentiated when phentolamine was present in the tracheal perfusate compared with phenylephrine. See text for description of the methods. Each data point represents mean ± SE of 4–7 experiments. *P < 0.05, significant difference between responses in the presence of phentolamine and phenylephrine.