The effects of extracellular calcium (0.5 to 5.0 mM) on the rate of intracellular conversion of proparathyroid hormone to parathyroid hormone were examined. Slices of bovine parathyroid glands were incubated in vitro with radioactive amino acids, and the amounts of radioactive PTH and proPTH in tissue extracts were assayed by electrophoresis on polyacrylamide gels. No changes in the ratio of proPTH to PTH were observed in media containing high concentrations of calcium (2.5 mM), which suppress both the secretion of parathyroid hormone by 80% and specific biosynthesis of prohormone and hormone by 15%. The results indicate that calcium does not regulate the biosynthesis of parathyroid hormone by affecting its conversion rate from the prohormone.
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