Polyspecific cation transporters mediate luminal release of acetylcholine from bronchial epithelium

Abstract

In airway epithelia, non-neuronal cholinergic regulations have been described; however, the route for acetylcholine (ACh) release has not been verified. To investigate whether organic cation transporters (OCTs) serve this function, we studied the expression of OCTs in airway epithelia and their capability to translocate ACh. Using immunohistochemistry in rats and humans, OCT1, OCT2, and OCT3 were localized to the luminal membrane of ciliated epithelial cells. In humans, OCT2 showed the strongest expression in the luminal membrane. We expressed the OCT isoforms in oocytes of Xenopus laevis and measured uptake and efflux of ACh. Tracer flux measurements showed that ACh is transported by OCT1 and OCT2 but not by OCT3. Two-electrode-voltage-clamp measurements revealed that OCT2 mediates electrogenic uptake and efflux of ACh. For ACh uptake by human OCT2, a K(M) value of approximately 0.15 mM was determined. At -50 mV, ACh efflux by human OCT2 was trans-inhibited by micromolar concentrations of the inhalational glucocorticoid budesonide, which is used in treatment of asthma (K(i) approximately 2.7 microM). The data show that OCT1 and OCT2 mediate luminal ACh release in human airways and suggest that ACh release is blocked after inhalation of budesonide.