The disposition of calcium within parathyroid tissue

The disposition of 45Ca was studied in equine parathyroid gland slices. This preparation was shown to be viable by its ability to carry out protein synthesis, by the responsiveness of its respiration to effectors of oxidative phosphorylation, and by its calcium-inhibitable PTH secretion. 45Ca accumulates in the slices as the extracellular calcium is raised through the range in which physiological serum calcium concentration lies. The tissue slices behave as if they have no calcium pump: (a) depriving the cells of energy by treatment with antimycin A does not admit more calcium to the tissue, (b) a significant amount of calcium is not displaced by lanthanum, and (c) a divalent cation ionophore does not expose new calcium binding sites. An energy-dependent sequestration of calclum by the tissue seems unlikely: (a) calcium is very rapidly taken up, (b) more is taken up at 0 degrees C than at 37 degrees C, and (c) antimycin A does not affect the amount of calcium retained. We conclude that the parathyroid gland passively accumulates calcium, and that it is held intacellularly by a substance with a dissociation constant in the millimolar range.