Airway hyperresponsiveness to methacholine in subjects with spinal cord injury

Previously, we found that never-smokers with quadriplegia were hyperresponsive to aerosolized methacholine. To further explore the phenomenon, we compared responsiveness to methacholine in never-smokers with that of smokers and ex-smokers. We also evaluated responsiveness in subjects with high paraplegia (lesions at T-1 to T-6) or low paraplegia (lesions at T-7 and below). We found that smokers and ex-smokers with quadriplegia were hyperresponsive to methacholine (provocative concentration causing a 20% fall in FEV1 = 1.9 mg/mL), and that the response was comparable to that found in never-smokers, revealing that hyperresponsiveness among never-smokers cannot be attributed to preinjury airway hyperreactivity that precluded cigarette use. In contrast, subjects with low paraplegia were not hyperresponsive to methacholine. Among subjects with high paraplegia, the three subjects demonstrating airway hyperresponsiveness had significantly lower FEV1 (percent predicted). The findings support the hypothesis that airway hyperresponsiveness in subjects with quadriplegia represents loss of sympathetic innervation of the lung, thereby leaving intact unopposed bronchoconstrictor cholinergic activity. However, reduced lung volumes in these subjects also suggest the possibility that airway hyperresponsiveness is due to loss of ability to stretch airway smooth muscle by deep breathing.

Augmentation of neurally evoked cholinergic bronchoconstrictor responses by prejunctional NK2 receptors in the guinea-pig

1. We examined the effect of exogenously administered tachykinins, neurokinin A (NKA), substance P (SP) and neurokinin B (NKB) on neurally mediated cholinergic bronchoconstrictor responses in guinea-pigs. 2. Electrical stimulation of regions in the dorsal medulla oblongata produced a cholinergic bronchospasm that was not affected by depletion of endogenous tachykinins with capsaicin pretreatment (50 mg kg-1, s.c., 1 week earlier) or by pretreatment with the neutral endopeptidase inhibitor, phosphoramidon (3 mg kg-1, i.v.). 3. Infusion of NKA (0.03-0.1 microgram kg-1 min-1), SP (1 microgram kg-1 min-1) or NKB (1 microgram kg-1 min-1) potentiated the bronchoconstrictor response to electrical stimulation of the dorsal medulla. The doses of tachykinins tested were subthreshold for direct activation of airway smooth muscle, because they were devoid of direct bronchoconstrictor effects. The relative rank order potency for augmentation of centrally induced bronchospasm was NKA > NKB approximately SP, suggesting activation of the NK2 receptor subtype. 4. Infusion of NKA, SP and NKB had no effect on bronchoconstrictor responses to i.v. methacholine (1 microgram kg-1) indicating that a prejunctional neural mechanism of action was responsible for the effects on CNS stimulation-induced bronchospasm. 5. Potentiation of the bronchoconstrictor response to dorsal medullary stimulation produced by infusion of NKA was blocked by pretreatment with the NK2 antagonist SR 48968 (1 mg kg-1, i.v.) but not by the NK1 antagoinst CP 96,345 (1 mg kg-1, i.v.). 6. The potentiation of CNS-induced bronchospasm produced by infusion of SP was partially inhibited by CP 96,345 (1 mg kg-1, i.v.) but not by SR 48968 (1 mg kg-1, i.v.). Treatment with combined SR 48968 (1 mg kg-1, i.v.) and CP 96,345 (1 mg kg-1, i.v.) completely blocked the SP-induced potentiation of CNS-stimulated bronchospasm. 7. These results identify an important modulatory role for NK2 receptors, located at prejunctional sites on parasympathetic nerves, on cholinergic bronchoconstrictor responses in guinea-pigs. 8. It is proposed that substances that release tachykinins from airway sensory nerves, e.g. inflammatory mediators or irritants, may induce hyperresponsiveness of cholinergic bronchomotor responses by activation of NK2-receptors on parasympathetic airway nerves. Furthermore, these studies indicate that endogenous tachykinins are not involved in the maintenance of basal cholinergic bronchomotor tone in the intact guinea-pig.

Bronchodilatory effects of ipratropium bromide in patients with tetraplegia

Airway hyperresponsiveness was recently described in patients with chronic cervical spinal cord injury (tetraplegia). The response was attributed to unopposed cholinergic broncho-constrictor activity due to loss of sympathetic innervation of the airway. To determine if the administration of a cholinergic antagonist alters resting airway tone in these patients, ipratropium bromide (72 micrograms) was administered by aerosol to 25 tetraplegic patients. We found that 12 of 25 patients (48%) had significant improvement (defined as > or = 12%) in forced expired volume in 1 s (FEV1) and/or forced vital capacity (FVC). A significant correlation between airway responsiveness and complaints of dyspnea at rest, completeness of injury (sensory), or smoking history was not found. These findings of improved airflow after the use of an anticholinergic bronchodilator agent provides further evidence that transection of the cervical cord results in unopposed parasympathetic activity and a resultant increase in resting airway tone.

Evan's blue dye blocks capsaicin-induced cough and bronchospasm in the guinea pig

The influence of Evan's blue dye on capsaicin-induced bronchoconstrictor and cough responses was investigated in the guinea pig. Evan's blue (30 mg kg-1 i.v.) pretreatment shifted the bronchoconstrictor dose-response to capsaicin (0.3-100 micrograms kg-1 i.v.) to the right by 10-fold, but had no effect on the bronchospasm elicited by neurokinin A (0.3-10 micrograms kg-1 i.v.). Evan's blue (0.3-30 mg kg-1 s.c.) also inhibited capsaicin-induced cough in a dose-dependent manner. Evan's blue blocked capsaicin responses by the intravenous, subcutaneous, or inhaled routes of administration. We conclude bronchoconstrictor responses and cough in vivo.