Effect of thyrotropin on iodide efflux in FRTL-5 cells mediated by Ca2+.

A functioning rat thyroid cell line (FRTL-5) responds acutely to the addition of TSH, norepinephrine, and the Ca2+ ionophore A23187 by a depression in iodide (I-) uptake levels. The decrease in I- content measured at the steady state depends on the presence of external Ca2+ and can be accounted for by an effect on stimulated I- efflux. As contrasted to the prolonged time (hours and days) involved in the stimulatory effect of TSH on I- uptake, the acute response to TSH is 1) seen within 5 min and maintained for about 20 min, 2) maximum, at a 1 X 10(-7)M concentration of TSH compared with the concentration of 1 X 10(-9)M necessary for the stimulatory effect, 3) independent of whether the cells are growing in the presence or absence of TSH, and 4) not mimicked by the addition of (Bu)2cAMP. The results suggest that TSH and adrenergic stimulation lead to increased membrane permeability to I- which is mediated by an elevation in the intracellular Ca2+ concentration.