Protein iodination by the dog thyrocyte (a marker of thyroid hormone synthesis) is stimulated by the Ca(2+)-phosphatidylinositol and cAMP cascades. We have shown previously that H2O2 generation, a limiting step of thyroid hormone synthesis, is modulated by these two cascades. In this work, we show that the I- release from preloaded thyrocytes is also activated by agents activating the Ca(2+)-phosphatidylinositol cascade and by Ca2+ ionophores, especially in synergy with 12-O-tetradecanoylphorbol 13-acetate, a potent activator of protein kinase-C. The effect of carbachol is reduced when the extracellular Ca2+ is depleted. Thus, both arms of the Ca(2+)-phosphatidylinositol cascade, Ca2+ and diacylglycerol, acutely and synergistically activate dog thyrocyte I- release. This I- release was also accelerated by acute and chronic exposure to TSH, forskolin, or (BU)2cAMP. The chronic stimulation of I- release by TSH exposure was diminished by chronic epidermal growth factor treatment (which dedifferentiates the thyrocytes). In addition, the chronic stimulation of I- release by forskolin was not affected by withdrawal of the agent up to 4 h before the experiment, in contrast to the acute effect of forskolin, which vanished within 16 min after forskolin withdrawal. These results suggest that the chronic stimulation of I- release by TSH or forskolin involves a stable mechanism. The I- transport system causing the release of I- from the dog thyrocyte is almost insensitive to inhibition by NaClO4 and KSCN. Hence, the iodide release cannot be due to the action of the basolateral Na+/I- cotransporter. In addition, we show that I- release was less sensitive than I- uptake to the inhibition by dysidenin, a marine toxin isolated from the sponge, Dysidea herbacea, known to inhibit I- uptake by dog thyroid slices. In summary, this work suggests that in a well defined model of the thyroid, the dog thyrocyte in primary culture, an I- transport system distinct from the basolateral Na+/I- cotransporter, is responsible for the observed I- release. The complex modulation of this transport system, involving at least the Ca(2+)-phosphatidylinositol and cAMP cascades, parallels the regulation of protein iodination, which itself reflects thyroid hormone synthesis.
Animals Biological Transport