Thyrotropin regulation of basolateral Cl − and I − effluxes in thyroid follicles in culture

Abstract

This report describes chloride and iodide effluxes across the basolateral membrane of porcine thyroid follicles reconstituted in culture. Basolateral chloride efflux is activated by thyrotropin (TSH). TSH (10 mU/ml) induces a twofold increase in the initial rate of chloride efflux. Forskolin (FSK, 5μM) which increases intracellular cAMP also stimulates the initial rate of chloride efflux 3.5-fold, whereas an increase in the free cytosolic Ca 2 + with the ionophore A23187 or thapsigargin, fails to mimic the TSH effect The chloride channel blocker 5-nitro-2(3-phenylpropylamino)benzoic acid (NPPB) dose dependently inhibits chloride efflux rates with the maximal and half maximal effects observed for 100μM and 30μM, respectively. Basolateral chloride efflux rates are also inhibited in the presence of the organic anion transporter blocker probenecid (5 mM) or the Cl −/HCO 3 − exchanger blocker 4-acetamido-4'-isothiocyanostilbene-2,2'-disulfonic acid (SITS, 250μM), respectively, by 60% and 40%, whereas it is not affected by ClO 4 (100μM). The initial rate of iodide efflux is weakly activated (1.4-fold) by TSH (10 mU/ml). TSH effect could be reproduced by agents known to activate Ca 2+-dependent processes as A23187, ionomycin (1 μM), phorbol 12-myristate 13-acetate (TPA 0.1 μM) and epidermal growth factor (EGF 0.1 μM) which increase the initial rate of iodide efflux from 1.2- to 1.8-fold, whereas FSK is without effect. The chloride channel blocker NPPB (500 μM)is required to significantly inhibit the initial rate of iodide efflux by 30%. The initial rate of iodide efflux is also reduced by 30% in the presence of SITS (250 μM) or probenecid (5 mM) whereas it is activated by ClO 4 (100 μM). We conclude that basolateral chloride and iodide effluxes are both regulated by TSH, using two different transduction pathways. Chloride efflux regulation may involve a cAMP transduction signal, whereas the regulation of iodide efflux may involve a Ca 2+ signal. Furthermore, as the sensitivities of chloride and iodide effluxes for the anion transporter blockers (especially NPPB) are different, it seems likely that chloride and iodide use two different transport pathways.