Chloride and potassium activities in luminal fluid of turtle thyroid follicles as determined by selective ion-exchanger microelectrodes.

Cl(-) and K(+) activities in the follicular lumen and the intraluminal potential of the same lumen were measured simultaneously with specific liquid ion-exchanger and 3 M KCl microelectrodes, respectively, in turtle thyroid glands. The Cl(-) equilibrium potential between the thyroid interstitium and the lumen calculated from the measured Cl(-) activities was higher than the directly measured intraluminal potential. These data indicate that Cl(-) is actively transported out of the follicular lumen in turtle thyroid gland. On the other hand, the calculated K(+) equilibrium potential was not different from the directly measured potential, which indicates that K(+) is probably distributed passively according to the electrochemical gradients between the interstitial and the luminal compartments in turtle thyroid gland. Results obtained from the specific liquid ion-exchanger microelectrode studies correlate well with those obtained from previous radioautographic observations and chemical determinations.