Thyrotropin-stimulated adenosine triphosphate in isolated thyroid cell membranes

The electrophysiological properties of the resting thyroid follicular cell

Glass microelectrodes have been useful in the study of the electrical properties of the resting thyroid follicular cell membrane. The resting transmembrane potential (RMP) has probably been underestimated in earlier work, possible as a result of leak artefacts, and it is clear that in most species the RMP is certainly greater than -60 mV. The ratio of membrane Na+ permeability to K+ permeability (PNa/PK) is of the order of 0.07 to 0.08, and Cl- is possibly (although not definitely) distributed in a passive fashion across the cell membrane, indicating that the transmembrane K+ gradient is the most important factor in the generation of the RMP. The existence of an electrogenic sodium pump in the follicular cell membrane has been demonstrated: the pump contributes about -2 mV to the RMP under control conditions. Follicular cells are completely electrically coupled, the basic coupled cellular unit probably being equivalent to the individual thyroid follicle, and the specific membrane resistance and specific membrane capacitance have been calculated to be 5 k omega. cm2 and 3.6 microF/cm2 respectively.

Effects of sodium on iodide transport in primary cultures of turtle thyroid cells

Iodide uptake by primary cultures of turtle thyroid cells decreased linearly with reduction of Na+ concentration in the medium, but changes in medium Cl- concentration did not affect iodide uptake. Ouabain, furosemide, monensin, and perchlorate all decreased 125I-uptake by cultured thyroid cells, whereas amiloride and triamterene did not. Ouabain, monensin, perchlorate, and amiloride depolarized the membrane of cultured cells, whereas furosemide and triamterene had no effect. Ouabain and perchlorate increased intracellular Na+ and Cl- and decreased K+ activities; furosemide and monensin reduced all three ions, but triamterene had no effect. Amiloride decreased intracellular Na+ and increased intracellular Cl- activities, however, its effect on K+ activity could not be determined because of interference by this compound of the K+ ion exchanger. All the agents, except furosemide, inhibited Na+-K+-ATPase activity. These experiments demonstrate that 1) Na+-I- cotransport is responsible for most iodide accumulation in thyroid cells; 2) Na+-I- cotransport system is linked to the Na+-K+ pump; 3) active iodide transport does not always correlate with Na+-K+-ATPase activity; 4) a perchlorate-sensitive iodide transport system is present in thyroid cells; 5) transport processes, not involved in active iodide transport (Na+-Cl- cotransport and Na+-H+ counter transport), are also present in cultured thyroid cells.

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.

A histochemical demonstration of the Na+ + K+-ATPase activity in the thyroid and the effect of cyclic adenosine monophosphate (c-AMP)

With a suitable modification of the Farquhar and Palade technique the Na+ + K+-ATPase activity in guinea-pig thyroid is demonstrated. The addition of c-AMP (5 X 10(-6) M or 1.5 X 10(-5) M) to the incubation media produced an apparent intensification of the Na+ + K+ -ATPase activity in the thyroid.