Bidirectional ion transport in thyroid: secretion of anions by monolayer cultures that absorb sodium.

Cultured porcine thyroid cell monolayers transport Na+ in an apical-to-basal direction, resulting in the development of a basal-positive transepithelial potential difference (TEP) and the formation of domes (fluid-filled elevations of the cell layer above the culture dish substrate). Stimulation by prostaglandin E2 (PGE2) increases the magnitude of the TEP, the short-circuit current (Isc) measured in Transwell Ussing chambers, and the height of domes in cultures grown on impermeable substrates. A phenamil-resistant, PGE2-stimulated component of the Isc in Transwells and of the TEP in monolayers in conventional culture dishes was inhibitable by bumetanide, a diuretic drug that blocks NaKCl2 symporters, mediating active transport of Cl-. The rate of decrease in height of domes in cultures after addition of phenamil, presumably indicative of transport of fluid in a basal-to-apical direction, was also reduced by bumetanide. Studies with Transwells in Cl(-)-free, HCO(3-)-free or Cl(-)- and HCO(3-)-free media indicated that thyroid cells transported HCO3- as well as Cl- in a basal-to-apical direction. It was concluded that the thyroid epithelium is both sodium absorbing and anion secreting.