Is Reverse Triiodothyronine a Physiological Nonactive Competitor for the Action of Triiodothyronine upon the Electrical Properties of GH ₃ Cells?

Abstract

The electrical properties of GH₃ cells, a transformed line from the rat pituitary, were studied with a current-clamp technique in the presence of triiodothyronine (T₃) and reverse triiodothyronine (rT₃) and compared with the known effects of T₃. rT₃ had an effect only in a concentration of 1 µ M; action potentials were suppressed, and changes in the slope conductance were similar to those seen in response to T₃. Membrane resistance R_I decreased, and the resting membrane potential V_M hyperpolarized. The action of rT₃ is a 1,000 times less potent than that of T₃ and is unlikely to have a physiological significance. When a cell was beforehand superfused with rT₃ (10 n M), the effect of T₃ was prevented even in a concentration of 10 n M. The specificity of T₃, the prevention of the effect of T₃ by rT₃, and the small dose needed suggest a binding site for T₃. The short latency of its action implies an extranuclear binding site, probably in or near the plasma membrane of the GH₃ cell.