Chloride channels in the apical membrane of thyroid epithelial cells are regulated by cyclic AMP

Porcine thyroid epithelial cells cultured as a monolayer with their apical membranes facing the medium are known to absorb Na+ and to secrete the anions Cl- and HCO3-. Chloride channels were found in the apical membrane, and displayed a reversal potential close to the resting membrane potential, linear current-voltage relationships, a conductance at physiological temperature of 6.5 pS, and a small but significant permeability to HCO3-. Stimulation of ion transport with prostaglandin E2 or 8-(4-chlorophenylthio) adenosine 3':5'-cyclic monophosphate promoted activation of Cl- channels in cell-attached patches, and excised patches were reactivated by exposure of their cytoplasmic surface to protein kinase A and ATP. Physiological temperatures were necessary for activation of Cl- channels in cell-attached patches. The channels exhibited sub-states with a conductance exactly half that of the full unit conductance, suggesting a dual-barrelled channel structure. It is concluded that the apical membrane of thyroid epithelial cells contains cyclic AMP-activated Cl- channels controlling anion transport.