Compartmentalization and movement of calcium in the thyroid.

The present work suggests that in dog thyroid tissue Ca2+ is distributed at least in two compartments (A) and (B). Compartment (A) could be the extracellular space, and extracellular binding sites for Ca2+. The uptake of Ca2+ in this compartment is increased and the release is decreased at 0 degrees C. The release is not influenced by the ionophore A23187 or by metabolic inhibitors (NaF, iodoacetate, dinitrophenol) or by thyrotropin (TSH). Compartment (B) is defined functionally as a slowly exchangeable store of Ca2+. Uptake and release from this compartment are temperature dependent. The release is accelerated by A23187 and by antimycin A, this suggests an intracellular, presumably mitochondrial, location. TSH stimulates the efflux of Ca2+ originating from the cellular compartment (B). Our data are compatible with the hypothesis that, as in other tissues, the translocations of stored intracellular calcium may be a crucial step in the activation of the thyroid cell.