Abstract
When afferent nerve endings in the airway detect a foreign substance, centrally mediated reflexes, acting through cholinergic parasympathetic efferents, contract airway smooth muscle. It has been proposed that these efferents also release nitric oxide, which relaxes airway smooth muscle, and that hyperresponsiveness in exercise induced asthma is due to a lack of neuronal nitric oxide release. Thus, we hypothesized that the removal of nitric oxide synthesis in healthy canine airways would cause reflex cholinergic contraction to go unopposed and allow airway smooth muscle to contract more readily.|We measured changes in central airway resistance in a-chloralose anesthetized, artificially ventilated dogs in response to the afferent nerve stimuli hypertonic saline (1ml, into a lobar bronchus) and capsaicin (injected into right atrium).|The first part of our study demonstrated that changes in airway resistance to hypertonic saline and capsaicin are neurally mediated. We compared changes in airway resistance to hypertonic saline and capsaicin before and after administration of the muscarinic receptor antagonist atropine (1mg/kg, i.v.). Before atropine, hypertonic saline increased airway resistance by 1.0±.3 cmH20/L/sec. Similarly, capsaicin increased airway resistance by 2.3±.9 cmH20/L/sec. However, after atropine, airway resistance did not increase to either hypertonic saline or capsaicin (A=0.4±.1 cmH20/L/sec and 0.3±.1 cmH20/L/sec respectively). These results confirm the role of cholinergic parasympathetic efferent nerves in mediating airway smooth muscle contraction to hypertonic saline and capsaicin.|We then established dose response relationships between the afferent nerve stimulation and changes in airway resistance. We compared changes in airway resistance to three concentrations of saline (0.9%, 3.6% and 7.2%) and capsaicin (100pg, 200pg and 300pg). Isotonic saline, which does not stimulate afferent nerves, did not increase airway resistance (A=0.4±.1 cmH20/L/sec). However, 3.6% and 7.2% hypertonic saline injections increased airway resistance in a dose dependent manner (0.9±,2 cmH20/L/sec and 1.6±.4 cmH20/L/sec, respectively). Similarly, airway resistance increased to capsaicin in a dose dependent manner (0.9±.3 cmH20/L/sec to 100pg, 2.0±.7 cmH20/L/sec to 200pg and 3.2±1.2 cmH20/L/sec to 300pg). Therefore, because changes in airway resistance were observed only when afferent nerves were stimulated and only before muscarinic receptor blockade, we conclude that changes in airway resistance to hypertonic saline and capsaicin are neurally mediated.|We determined the role of neurally released nitric oxide on reflex contraction of airway smooth muscle by comparing changes in central airway resistance to hypertonic saline and capsaicin before and after nitric oxide synthesis inhibition in 17 dogs. Before nitric oxide synthesis inhibition, airway resistance increased by 1.4±.2 cmH20/L/sec to hypertonic saline (7.2%, 1 ml) and by 1.6±.5 cmH20/L/sec to capsaicin (200pg). We then blocked nitric oxide synthesis with nitro-L-arginine methyl ester (L-NAME i.v.) and repeated stimuli injections. During L-NAME the changes in airway resistance to hypertonic saline and capsaicin were not increased (0.9±.1 cmH20/L/sec; p=0.04 and 2.1 ±.6 cmH- 20/l_/sec; p=0.12, respectively).|L-NAME increased systemic blood pressure, activating the baroreceptor reflex, which relaxes airway smooth muscle. Therefore, we infused sodium nitroprusside to restore pre-L-NAME arterial pressure and repeated the stimuli injections. After baseline nitric oxide levels were restored, the change in airway resistance was not increased to hypertonic saline (A=0.9±.2 cmH20/L/sec) or capsaicin (A=2.0±.6 cmH20/L/sec).|Hypertonic saline increased airway resistance more in some dogs than in others. It may have been that dogs with larger responses to hypertonic saline were hyperresponsive. If hyperresponsiveness was due to a lack of neuronal nitric oxide release, blockade of nitric oxide synthesis would have no effect on the ‘most responsive’ dogs. The ‘less responsive’ dogs may have had healthy airways that released nitric oxide and thus would show larger increases in resistance after L-NAME. In the ‘less responsive’ dogs (n=8), the response to hypertonic saline was not increased during L-NAME (1.1 ±.2 cmH20/L/sec before L-NAME; 0.6±.1 cmH20/L/sec during L-NAME; p=0.09). Nor was response increased during L-NAME and sodium nitroprusside (A=0.71±.2 cmH20/L/sec).|Similarly, L-NAME had no effect on the change in resistance to hypertonic saline in the ‘most responsive’ dogs (A=1.9±.4 cmH20/L/sec before L-NAME, 1.2±.3 cmH20/L/sec during L-NAME). After administration of sodium nitroprusside the change in airway resistance to hypertonic saline was not increased (1.2±.3 cmH20/Usec).|Therefore, because the change in airway resistance was not increased after blocking nitric oxide synthesis, we concluded that neurally released nitric oxide does not oppose reflex cholinergic contraction of airway smooth muscle in dogs.|The reflex effects of rapidly adapting receptor stimulation on airway smooth muscle are disputed. Therefore to clarify the effects of rapidly adapting receptor stimulation, we compared the change in airway resistance when rapidly adapting receptors and C-fibers are directly stimulated with hypertonic saline to the change in airway resistance when C-fibers are directly stimulated with capsaicin. From this comparison, we deduced the effect of rapidly adapting receptor stimulation on airway smooth muscle. We chose dogs with a similar change in heart rate to 7.2% hypertonic saline and 100pg capsaicin. Stimulation of both rapidly adapting receptors and C-fibers with hypertonic saline increased airway resistance more than stimulation of C-fibers only with capsaicin (A=1.4±.4 cmH20/L/sec to hypertonic saline 0.5±.3 cmH20/L/sec to capsaicin).|Thus, for a similar level of afferent nerve stimulation, activation of both rapidly adapting receptors and C-fibers increases airway resistance more than activation of C-fibers alone. Therefore, stimulation of rapidly adapting receptors may contribute to the bronchoconstrictor response to hypertonic saline.