RELATIONSHIP BETWEEN THE THYROID-HORMONE TRANSPORT-SYSTEM AND THE NA-H+ EXCHANGER IN CULTURED RAT-BRAIN ASTROCYTES()

The entry of T-3 and T-4 into rat cultured astrocytes is mediated by a stereospecific saturable transport system. This study examines the ef fect of inhibiting the Na+-H+ exchanger and intracellular acidificatio n on the initial velocity of[I-125]T-3 and [I-125]T-4 uptake. The rest ing intracellular pH (pH,) was approximately 7.15 in astrocytes expose d to CO2/HCO(3)(-)free medium buffered with HEPES at pH 7.40 at 22 C. Isoosmotic replacement of extracellular sodium by mannitol or choline decreased the pH(i) by 0.15 pH unit and reduced uptake by about 20%. R eplacing sodium with lithium had no effect on uptake. Amiloride, a spe cific blocker of the Na+-H+ exchanger, reduced pH(i), as described abo ve, and inhibited T-3 and T-4 uptake by about 35%. Acid loading the ce lls with a NH4+ pulse decreased the pH(i) by up to 1.2 pH units and th e uptake of T-3 and T-4 by up to 50%. The maximum velocity of uptake w as decreased, whereas the K-m was unchanged. An isoos motic increase i n the extracellular K+ concentration to 59 mM had no effect on T-3 upt ake. The initial velocity of T-3 uptake by acid-loaded cells was gradu ally restored by increasing the extracellular Na+ con centration. Thes e results indicate that thyroid hormone transport into rat cultured as trocytes involves a mechanism linked to the activity of the Na+-H+ exc hanger and the H+ concentration inside the cells.