Human airway smooth muscle possesses an inhibitory nonadrenergic noncholinergic neural bronchodilator response mediated by nitric oxide (NO). In guinea pig trachea both endogenous NO and vasoactive intestinal peptide (VIP) modulate cholinergic neural contractile responses. To identify whether endogenous NO or VIP can modulate cholinergic contractile responses in human airways in vitro, we studied the effects of specific NO synthase inhibitors and the peptidase a-chymotrypsin on contractile responses evoked by electrical field stimulation (EFS) at three airway levels. Endogenous NO, but not VIP, was shown to inhibit cholinergic contractile responses at all airway levels but this inhibition was predominantly in trachea and main bronchus and less marked in segmental and subsegmental bronchi. To elucidate the mechanism ofthis modulation we then studied the effects of endogenous NO on acetylcholine (ACh) release evoked by EFS from tracheal smooth muscle strips. We confirmed that release was neural in origin, frequency dependent, and that endogenous NO did not affect ACh release. These findings show that endogenous NO, but not VIP, evoked by EFS can inhibit cholinergic neural responses via functional antagonism of ACh at the airway smooth muscle and that the contribution of this modulation is less marked in lower airways. (J. Clin. Invest. 1993.92:736-743.) Key words:

Aims To establish whether enantioselective metabolism of racemic (rac)-salbutamol occurs in the lungs by determining its enantiomeric disposition following inhalation, in the absence and presence of oral charcoal, compared with that following the oral and intravenous routes. Methods Fifteen healthy subjects (eight male) were randomized into an open design, crossover study. Plasma and urine salbutamol enantiomer concentrations were measured for 24 h following oral (2 mg ) with or without oral charcoal (to block oral absorption), inhaled (MDI; 1200 mg ) with or without oral charcoal and intravenous (500 mg) rac-salbutamol. Systemic exposure ( plasma AUC(0,2) and urinary excretion (Au 24h ) of both enantiomers were calculated, and relative exposure to (R)-salbutamol both in plasma (AUC (R)-/AUC (S)-) and urine (Au (R)-/Au (S)-) was derived for each route. Relative exposure after the inhaled with charcoal and oral routes were compared with the intravenous route. Similar results were found when relative exposure was analysed using Au 24h . Conclusions These results show no evidence of significant enantioselective presystemic metabolism in the lungs, whilst confirming it in the gut and systemic circulation, indicating that the (R)-and (S)-enantiomers are present in similar quantities in the airways following inhaled rac-salbutamol.

We investigated the effect of Ba 679 BR, a novel long-acting antimuscarinic agent, on cholinergic neural responses in guinea pig and human airways. Ba 679 BR, atropine, and ipratropium bromide inhibited electrical field stimulation (EFS)-induced contraction with IC50 values of 0.17, 0.74, and 0.58 nM, respectively, in guinea pig trachea. Ba 679 BR had a slower onset and longer duration of action than atropine or ipratropium bromide (the times required to attain 50% of the maximum response were 34.8, 3.8, and 7.6 min, respectively, and the times required for 50% recovery of the response were 540, 31.6, and 81.2 min, respectively). Ba 679 BR, as well as atropine and ipratropium bromide, facilitated evoked [3H]acetylcholine release (an inhibitory effect on prejunctional muscarinic M2 receptors). The facilitation of acetylcholine release by Ba 679 BR was lost 2 h after washout, however, when there was still complete blockade of cholinergic contractile responses evoked by EFS (an effect on airway smooth muscle M3 receptors), confirming binding studies that suggest that Ba 679 BR shows "kinetic receptor subtype selectivity" for M3 over M2 receptors. The high potency, slow onset, and long duration of action of Ba 679 BR were also observed in human bronchi, suggesting that Ba 679 BR may be a useful drug to provide convenient therapy for patients with obstructive airway disease.

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We compared inhibitory nonadrenergic noncholinergic (i-NANC) neural relaxations, evoked by electrical field stimulation (EFS), at three levels (main [MA], proximal [PA], and distal [DA] airways) of isolated human airways and correlated these with nitric oxide synthase-immunoreactive (NOS-IR) nerves, using antiserum raised to rat cerebellar NOS. Maximal relaxations to papaverine (100 microM) were reduced in PA and DA (MA: 1,712 +/- 219 mg, n = 12; DA: 862 +/- 69 mg, n = 5, P < 0.05 versus MA); hence, subsequent relaxations were expressed as a percentage of the papaverine maximum. EFS elicited frequency-dependent relaxations that were largest in MA and reduced in PA and DA, especially at high stimulation frequencies (10 Hz EFS: MA: 51.6 +/- 3.7%, n = 12; PA: 30.5 +/- 6.0%, n = 6, P < 0.01 versus MA; DA: 17.8 +/- 3.6%, n = 5, P < 0.001 versus MA). The NOS inhibitor L-NG-nitroarginine methyl ester (L-NAME) (100 microM) and tetrodotoxin (3 microM) significantly inhibited i-NANC responses at all frequencies, leaving an L-NAME-resistant non-neural relaxation at frequencies > 5 Hz which was reduced in PA and DA. Cumulative concentration-response studies to sodium nitroprusside (1 nM to 0.1 mM) and the NO donor 3-morpholinosydnonimine (1 nM to 1 mM) were not significantly different in PA and DA, suggesting impaired relaxation is not caused by impaired guanylyl cyclase activity. Total nerve density, shown by protein gene product 9.5 staining, was not significantly different in PA and DA; however, NOS-IR nerve density was reduced in PA and DA (NOS-IR [intercepts/mm2]: MA: 705 +/- 98, n = 6; DA: 284 +/- 32, n = 6, P < 0.01 versus MA). These studies demonstrate that i NANC neural relaxations are reduced in DA, apparently due to a decrease in the density of nitrergic innervation.