Introduction:

The CFTR plays a crucial role in airway fluid homeostasis since mutations are associated with cystic fibrosis lung disease. Luminal acetylcholine (ACh) has been identified to activate Cl^- secretion in airway epithelia although the underlying mechanisms and targets are not identified. The present study investigates whether or not the CFTR is a putative target of luminal ACh.

Methods:

Porcine tracheal preparations were mounted in Ussing-chambers for short-circuit-current (I_SC) recordings. ACh (100 µM) was applied from the luminal side. CFTR-dependent Cl^- secretion was assessed by application of IBMX/forskolin (100/10 µM, luminal) in the presence of amiloride (10 µM, luminal). GlyH-101 (10 or 50 µM, luminal) was used to block CFTR currents and DIDS (100 µM, luminal) to block Ca²⁺-dependent Cl^- channels.

Results:

Application of luminal ACh significantly increased I_SC. The ACh effect was decreased by amiloride but not affected by DIDS. In the presence of amiloride the IBMX/forskolin-activated current (?I_I/F) was 32.4±6.4 µA/cm² (n=6). GlyH-101 reduced ?I_I/F to 13.1±3.7 µA/cm² (n=6) demonstrating that a substantial portion of that current is CFTR dependent. Interestingly, GlyH-101 also attenuated the effect of luminal ACh (?I_ACh: 10.6±2.7 (control); 6.7±2 (GlyH-101); n=11) indicating that ACh, among other effects, activates a CFTR-sensitive conductance.

Conclusions:

From these results it can be concluded that luminal ACh activates ion transport processes in porcine airway epithelium. This includes ENaC-dependent Na⁺ reabsorption as well as CFTR-dependent Cl^- secretion. Therefore, luminal ACh can be considered as non-neuronal para-/autocrine mediator for airway fluid homeostasis.