Na⁺-H⁺ antiport and monensin effects on cytosolic pH and iodide transport in FRTL-5 rat thyroid cells

Na⁺-H⁺ exchange may proceed via an endogenous antiporter or by exposure to the Na⁺ ionophore monensin. We investigated the characteristics of Na⁺- H⁺ exchange induced by antiporter stimulation and by monensin in FRTL-5 rat thyroid cells. We also examined the effects of intracellular pH (pH(i)) changes on iodide uptake and efflux. pH(i) was determined using 2',7'-bis(2- carboxyethyl)-5(6)-carboxyfluorescein. The resting pH(i) was 7.33 ± 0.02 units; this level correlated directly with extracellular pH. In acid-loaded cells, K(m) for external Na⁺ activation of the antiporter was 7.1 mM and maximum velocity was 0.3801 ΔpH units/min. Dimethylamiloride was 42 times more potent than amiloride in inhibiting sodium-dependent recovery in acidified cells. Metabolic inhibition reduced the initial alkalinization rate. Monensin increased pH(i), and this response was dependent on extracellular Na⁺ and HCO₃/^- but not on antiporter function. Low-dose monensin (1 μM) and 1 mM NH₄Cl enhanced ¹²⁵I uptake. High-dose monensin (100 μM), but not NH₄Cl, reduced iodide uptake. Neither NH₄Cl nor monensin altered ¹²⁵I efflux. Thus FRTL-5 cells possess an amiloride-sensitive Na⁺-H⁺ exchanger, which is not essential for maintaining basal pH(i) but is affected by ATP depletion. Monensin also alkalinizes these cells but independently of the antiporter. Iodide uptake, but not efflux, is affected by changes in intracellular Na⁺ and H⁺ levels.