Effects of calcium on synthesis and secretion of parathyroid hormone and secretory protein I

R. R. MacGregor, D. A. Hinton and R. D. Ridgeway Department of Anatomy, University of Kansas Medical Center, Kansas City 66103. Bovine parathyroid organoids were cultured for up to 3 wk in medium containing 1.4 or 1.8 mM calcium. Steady-state secretion of parathyroid hormone and secretory protein I was two- to fourfold greater at 1.4 mM. At the end of culture, organoids were incubated 3.5 h in 1 or 2 mM calcium to examine maximum and minimum acute secretory rates. Relative to organoids cultured at 1.8 mM calcium, culture at 1.4 mM induced a hypersecretory state, i.e., both the maximum and minimum acute secretory rates of organoids previously cultured at 1.4 mM calcium were up to threefold greater than those of organoids previously at 1.8 mM calcium. Proparathyroid hormone synthesis was up to 50% greater in organoids cultured at 1.4 mM calcium, whereas secretory protein I and total protein synthesis were unaltered. The results showed that parathyroid hypersecretion can be induced by chronic hypocalcemic conditions in vitro. We conclude that the secretory adaptation to chronic hypocalcemia in vitro involves alterations in both synthesis and degradation of parathyroid hormone.