Distribution of sodium in cellular and luminal compartments of turtle thyroid glands

Sodium distribution in luminal and cellular compartments of turtle thyroid was studied by measurements of radiosodium uptakes into whole thyroid and luminal fluid, sampled with micropipettes, and of Na+ activities in the luminal and cellular fluids of thyroid follicles with a liquid ion-exchange microelectrode. Thyroid water and electrolyte contents were also determined chemically. Radiosodium uptake by both the whole thyroid and by the luminal fluid of control and thyrotropin (TSH)-treated turtles did not reach maximal levels until 16-24h after injection of the radioisotope. In control animals, the volumes of distribution of radiosodium in the whole thyroid and the luminal fluid were much smaller than those calculated from the chemical analysis data for the corresponding tissues. TSH increased the radiosodium spaces to values almost equal to those of the chemically determined sodium spaces. Luminal fluid sodium concentration increased after TSH and decreased after ouabain and furosemide treatments. Intracellular sodium concentration markedly increased after ouabain and decreased after TSH and furosemide. These and our previous results suggest a Na+-I- cotransport mechanism in the thyroid.