Mechanisms of pH recovery after alkalinization and acidification by exposing or prepulsing turtle thyroid slices with a Hanks' balanced salt solution (HBSS) containing NH4Cl or CO2 were studied by examining the effects of amiloride, 4-acetamido-4′-isocyanostilbene-2,2′-disulphonic acid (SITS), frusemide and acetazolamide, and of reducing the concentration of Na+ or Cl− in the incubation medium. When alkalinization was produced either during exposure to NH4Cl or after a CO2 pulse, the pH in thyroid slices rose rapidly and then recovered gradually. Addition of SITS (0·1 mmol/l) or reduction of the Cl− concentration markedly inhibited pH recovery. However, amiloride (0·1 mmol/l) and low Na+ in the medium had no significant effect on recovery from alkalinization induced by NH4Cl exposure or by a CO2 pulse. These data suggest that pH recovery from alkalinization in turtle thyroid gland is achieved by an exchange of internal HCO3− for external Cl−. When acidification was accomplished by either exposure to CO2 or removal of NH4Cl, the pH of thyroid slices fell rapidly and then recovered gradually. If amiloride was added or the Na+ concentration in the medium was reduced, the pH recovery was greatly attenuated. However, SITS and low Cl− in the medium did not affect the recovery from an acid load in turtle thyroid slices. These results suggest that pH recovery from acidification in turtle thyroid gland is achieved by an exchange of internal H+ for external Na+. Both frusemide and acetazolamide prevented the pH recovery in turtle thyroid slices during exposure to and withdrawal from NH4Cl. These results suggest that besides the Na+-H+ and Cl−-HCO3− exchange processes, other mechanisms may also be involved in pH regulation in turtle thyroid glands.
Simultaneous uptakes into turtle thyroid slices of 125I− and 22Na+ and of 125I− and 36Cl− were studied during and following exposure to NH4Cl in the absence and presence of different transport inhibitors, such as frusemide, amiloride, SITS and acetazolamide. When the thyroid slices were exposed to HBSS containing 30 mmol/l NH4Cl (alkalinization phase), the tissue/medium (T/M) ratios of 125I− increased gradually, reached the highest point in 10 min, and were maintained at this level for the next 20 min. The T/M ratios of 22Na+ and 36Cl− of thyroid slices also slowly increased after exposure to NH4Cl. After withdrawal of NH4Cl (acidification phase), the T/M ratios of 125I− decreased rapidly below the control level (samples not exposed to NH4Cl), reached the lowest point in 2–5 min, and returned to the control level within 20–30 min. During the withdrawal period, the T/M ratios of 22Na+ were either not changed or slightly increased further; the T/M ratios of Cl− were not changed; and the T/M ratios of both 22Na+ and 36Cl− were higher than the controls. Frusemide decreased the T/M ratios of 125I−, 22Na+ and 36Cl− in both the alkalinization and acidification phases. Amiloride decreased the T/M ratios of both 125I− and 22Na+ only in the acidification phase, but had no effect on the T/M ratios of 36Cl− in both the alkalinization and acidification phases. SITS increased the T/M ratios of 125I−, but slightly decreased the T/M ratios of 36Cl− in the alkalinization phase. It had no effect on the T/M ratios of 36Cl− in the acidification phase, nor on the T/M ratios of 22Na+ in both the alkalinization and acidification phases. Acetazolamide increased the T/M ratios of 125I− and 36Cl− in both the alkalinization and acidification phases, and had no effect on the uptake of 22Na+ by the thyroid slices in both phases. Based on the results of these studies, it appears likely that under normal conditions or when cell pH is lower than normal, the Na+-dependent I− transport system acts in concert with the Na+-H+ exchange and the Na+-HCO3− co-transport processes of the pH regulatory system to maintain the accumulation of I− and to increase the pH inside thyroid follicles. When cell pH is higher than normal, the Na+-dependent I− transport system acts in concert with the Cl−-HCO3− exchange process of the pH regulatory system to enhance the accumulation of I− and to decrease the pH in thyroid glands.