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 ap ical membranes facing the medium are known to absorb Na+ and to secret e the anions Cl- and HCO3-. Chloride channels were found in the apical membrane, and displayed a reversal potential close to the resting mem brane potential, linear current-voltage relationships, a conductance a t physiological temperature of 6.5 pS, and a small. but significant pe rmeability to HCO3-. Stimulation of ion transport with prostaglandin E (2) or 8-(4-chlorophenylthio) adenosine 3':5'-cyclic monophosphate pro moted 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 fo r activation of Cl- channels in cell-attached patches. The channels ex hibited sub-states with a conductance exactly half that of the full un it conductance, suggesting a dual-barrelled channel structure. It is c oncluded that the apical membrane of thyroid epithelial cells contains cyclic AMP-activated Cl- channels controlling anion transport.