Cholinergic Control of Human Airways<i>In Vitro</i>following Extrinsic Denervation of the Human Respiratory Tract by Heart-Lung Transplantation

In heart-lung transplantation (HLT), the airways have been assumed to be denervated since cholinergic, adrenergic, and sensory nerves are severed. Challenge studies of such patients suggest that there is an increase in airway responsiveness to inhaled cholinergic agonists, which may be explained by denervation hypersensitivity of muscarinic receptors on airway smooth muscle. We have studied the cholinergic control of airways from lungs removed from five patients (8 to 31 yr of age) undergoing retransplantation because of rejection-related bronchiolitis, with time since transplantation ranging from 12 to 32 months. These airways were compared with airways obtained from eight heart donors (24 to 42 yr of age) and from five patients undergoing surgical lobectomy for bronchial carcinoma (54 to 72 yr of age). Bronchial rings (distal lobar and subsegmental) were mounted in organ baths and isometric contractile responses measured. Contractile responses to acetylcholine (ACh, 10 nM to 10 mM) and to electrical field stimulation (EFS) (40 V, 0.5 ms, 1 to 64 Hz for 15 s) were determined. Transplant and control airways showed the same response to ACh, with mean EC50 values of 61.0 ± 0.32 µM for HLT patients, 57.6 ± 0.24 µM for donor patients, and 48.7 ± 1.2 µM for lobectomy patients, suggesting no denervation hypersensitivity of muscarinic receptors. EFS, which activates postganglionic cholinergic nerves, caused similar frequency responses in both transplant and control airways, suggesting that postganglionic nerves are intact. [3H]quinuclidinyl benzilate binding to membranes prepared from lung was performed to determine muscarinic binding characteristics. There was a small but insignificant increase in receptor density (Bmax = 61.4 ± 7.5 in HLT and 46.1 ± 7.1 fmol/mg protein in control patients) and no difference in the binding affinity (Kd = 0.48 ± 0.08 in HLT and 0.37 ± 0.06 nM in control patients), indicating no alteration in muscarinic receptors with denervation. We conclude that surgically denervated human airways from lung transplant patients have normally functioning postganglionic nerves and no change in cholinergic responsiveness or receptors, presumably because parasympathetic neurons survive. Denervation hypersensitivity cannot account for the increased cholinergic responsiveness observed in these patients.