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

Abstract

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.

Thyroid hormone transporters

“…Similar to hepatocytes, apparent Km values for the high-affinity uptake of T 3 in other cell types are mostly in the nanomolar range. Some authors(22,73),…”

Thyroxine transfer from cerebrospinal fluid into choroid plexus and brain is affected by brefeldin A, low sodium, BCH, and phloretin, in ventriculo-cisternal perfused rabbits

“…Moreover, in hepatic cells, uptake of TH has been induced by the Na + /TA transporting polypeptide (NTCP) and OATP1 (Friesema et al, 1999 ), which are both found to be expressed at low levels in the rat CP (Choudhuri et al, 2003 ). Furthermore, the sodium-dependent mechanism of TH has been demonstrated in rat skeletal muscle (Centanni and Robbins, 1987 ), human glioma cells (Goncalves et al, 1990 ), rat glial cells (Francon et al, 1989 ), astrocytes (Beslin et al, 1995 ), and cerebrocortical neurons (Chantoux et al, 1995 ). Finally, as a consequence of low sodium effect in this study and results of another study (Kassem et al, 2006 ), the abundant presence of 125 I-T 4 in CSF suggests a partial rapid diffusion of 125 I-T 4 into tissue that is driven by a bulk flow of CSF and receptor-mediated endocytosis (Kassem et al, 2006 ).…”