Relationship between the thyroid hormone transport system and the Na(+)-H+ exchanger in cultured rat brain astrocytes

The entry of T3 and T4 into rat cultured astrocytes is mediated by a sterospecific saturable transport system. This study examines the effect of inhibiting the Na(+)-H+ exchanger and intracellular acidification on the initial velocity of [125I]T3 and [125I]T4 uptake. The resting intracellular pH (pHi) was approximately 7.15 in astrocytes exposed to CO2/HCO3(-)-free medium buffered with HEPES at pH 7.40 at 22 C. Isoosmotic replacement of extracellular sodium by mannitol or choline decreased the pHi by 0.15 pH unit and reduced uptake by about 20%. Replacing sodium with lithium had no effect on uptake. Amiloride, a specific blocker of the Na(+)-H+ exchanger, reduced pHi, as described above, and inhibited T3 and T4 uptake by about 35%. Acid loading the cells with a NH4+ pulse decreased the pHi by up to 1.2 pH units and the uptake of T3 and T4 by up to 50%. The maximum velocity of uptake was decreased, whereas the Km was unchanged. An isoosmotic increase in the extracellular K+ concentration to 50 mM had no effect on T3 uptake. The initial velocity of T3 uptake by acid-loaded cells was gradually restored by increasing the extracellular Na+ concentration. These results indicate that thyroid hormone transport into rat cultured astrocytes involves a mechanism linked to the activity of the Na(+)-H+ exchanger and the H+ concentration inside the cells.