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Thyroid Hormone Export Varies among Primary Cells and Appears to Differ from Hormone Uptake

Department of Pharmaceutical Sciences, University of Brasília (F.A.R.N., L.A.S., R.C.J.R.), Brasília, DF 70910-900, Brazil; Veterans Affairs Medical Center (R.R.C.) and Metabolic Research Unit (D.G.G., J.D.B.), University of California, San Francisco, California 94143; Chiron Corp. (B.F.S.), Emeryville, California 94608; and Department of Biology, University of Cergy-Pontoise (N.L.), Paris, France

Address all correspondence and requests for reprints to: Dr. Ralff C. J. Ribeiro, Campus Universitário Darcy Ribeiro, Asa Norte, Department of Pharmaceutical Sciences, University of Brasília, Brasília, DF 70910-900, Brazil. E-mail: ralff{at}unb.br

We characterized T₃ efflux in primary cultures of cells derived from human placenta, neonatal rat cardiac myocytes, and rat inner medullary collecting ducts (IMCD). The T₃ efflux rate was highest in placenta cells, followed by ventriculocytes, atriocytes, and IMCD cells. Verapamil reversibly blocked [¹²⁵I]T₃ efflux in these cells in a manner that correlated with their T₃ efflux rate. Thus, verapamil inhibition of [¹²⁵I]T₃ efflux in placenta cells led to a 432% increase in the [¹²⁵I]T₃ content compared with 33% increase in IMCD cells. Several unlabeled iodothyronines, but not TRIAC, differentially blocked [¹²⁵I]T₃ efflux such as (T₄ > T₃ > rT₃ = D-T₃ > D-T₄) in placenta cells and (T₄ > rT₃ = D-T₄ = T₃ > D-T₃) in ventriculocytes, suggesting tissue-specific differences in the carriers/transporters responsible for T₃ efflux. This hypothesis draws further support from the fact that D-T₃ inhibited [¹²⁵I]T₃ efflux in placenta cells, but not in ventriculocytes. TRIAC did not affect T₃ efflux in ventriculocytes or placenta cells, but it greatly inhibited [¹²⁵I]T₃ uptake in these cells, suggesting that [¹²⁵I]T₃ uptake and efflux mechanisms are distinct and appear to be mediated by distinct carrier/transporter proteins. Collectively, these data suggest that differences in thyroid hormone transport in target cells may provide an important mechanism for regulating hormone action in a tissue-specific fashion.

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