A TSH dibutyryl cAMP activated Cl-/I- channel in FRTL-5 cells

An iodide (I) and chloride (Cl) channel has been identified in the continuo usly cultured FRTL-5 thyroid cell line using a cell attached patch clamp te chnique. The channel is activated by TSH and dibutyryladenosine cyclic mono phosphate (Bt(2)-cAMP) but not by phorbol 12-myristate 13-acetate (TPA). Gl uconate can not replace chloride or iodide and the channel is impermeable t o Na+,K+ and tetraethylammonium ions. The current-voltage relationship demo nstrates that the single channel current is a Linear function of the clamp voltage. Single channel currents reversed at a pipette potential close to 0 mV. The mean single channel conductance was 60 pS for Cl- and 50 pS for I- . From the I-V relationship there was a strong outward rectification with C l-, and a complete block with I-, in the single channel current above +40 m V. The feature of the channel is manifested in the single channel records b y four distinct, equally spaced conductance levels. We suggest the channel is important for the transport of I and Cl ions across the apical membrane into the colloid space and is important for hormone synthesis and follicle formation. (C) 1999 Academic Press.