Stimulation by thyroid hormone of Na+-H+ exchange activity in cultured opossum kidney cells

Cultured opossum kidney (OK) cells were used to determine whether thyroid hormone has a direct stimulatory effect on Na(+)-H+ exchange activity in an intact cellular preparation. Na+ uptake or intracellular pH recovery was measured in confluent monolayer cells following acid loading with NH4Cl. Triiodo-L-thyronine (T3) had no effect on cell number or protein and DNA contents but stimulated amiloride-sensitive Na+ uptake in a dose- and time-dependent manner. Maximal stimulation of Na+ uptake was observed at 10(-7) M T3 and 10(-6) M L-thyroxine (T4) with half-maximal effects at 10(-9) M T3 and 3 x 10(-8) M T4. The T3 specific binding capacity of OK cells was 96 +/- 15 fmol/mg DNA with a KD of 1.1 +/- 0.2 x 10(-9) M T3. Neither T3 nor T4 had any effect on amiloride-insensitive Na+ uptake. In kinetic studies of Na+ uptake, T3 increased the Vmax from 123 +/- 22 to 157 +/- 24 nmol.mg-1.min-1 without changing the Michaelis-Menten kinetics (Km) for Na+ (21 +/- 1 in control and 22 +/- 4 mM in T3-treated cells). Studies of intracellular pH (pHi) showed that the resting pHi and the buffering capacity were unaffected by T3. However, after an acid load, OK cells treated with T3 exhibited a greater rate of Na(+)-dependent pH recovery than untreated control cells. These results indicate that thyroid hormone can stimulate Na(+)-H+ exchange activity directly in renal cell line without apparent changes in pHi, cellular hypertrophy, or hyperplasia.