The thyroid cell monolayer in culture

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Abstract
When cultured on collagen coated nitrocellulose filters, thyroid epithelial cells form morphologically and functionally polarized monolayers. The bioelectric parameters of these monolayers were measured after mounting in Ussing chambers; transepithelial potential (Vab), short circuit current (Isc) and transepithelial resistance were respectively 12±1 mV (apical side negative), 3.8±0.2 μA cm−2 and 3250±214 Ω cm2 (mean±SEM,n=75). Eighty two percent of the short circuit current was related to sodium absorption as shown by inhibition by apical amiloride (Km=0.2 μM) and by basal ouabain (K1/2=0.3 μM). Amphotericin B (5–25 μg/ml) added to the apical bath increasedIsc suggesting an apical rate-limiting step. Step by step replacement of choline by Na+ in a Na+-free medium resulted in a progressive increase inVab andIsc with half maximal effect at 20±1 mM Na+. Thyrotropin (TSH) increasedIsc andVab in a biphasic way with a transient maximum after 5 min and a plateau after 20 min (about four times the basal level at 100 μU/ml TSH). This increase in sodium transport was also inhibited by apical amiloride. Thus, in culture, the thyroid cell monolayer behaves as a tight sodium absorbing epithelium controlled by TSH, with a rate limiting apical sodium channel as the entry mechanism and a basolateral Na+, K+-ATPase as the electromotive force.