Chloride conductance of apical membrane in cultured porcine thyroid cells activated by cyclic AMP

Abstract

The thyroid epithelium transports fluid bidirectionally using active transport of Na + ions from apical to basal poles and active transport of Cl - in the reverse direction. In these studies we sought evidence for cyclic AMP activated Cl - channels on the apical membranes of thyroid cells in monolayer culture. A Cl --dependent basal-positive short-circuit current (I sc) was demonstrated in bicameral chambers after blocking Na + transport with phenamil, and responded to prostaglandin (PG) E 2 with a spike of 5–10 min duration followed by a plateau. The onset of the spike coincided with an increase in the conductance of the epithelium. Application of an external Cl - concentration gradient, by replacing the medium in the apical compartment with Cl --free medium, resulted in an increase in I sc after, but not before, addition of PGE 2. Forskolin and thyroid-stimulating hormone (TSH), but not A23187, also stimulated Cl - transport. In conjunction with previous observations that Cl - transport was mediated by a bumetanidesensitive NaKCl 2 symporter on the basal membrane, these observations indicated the presence of a cyclic AMP activated Cl - conductance in the apical membrane of thyroid cells.